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draft-bidulock-sigtran-isua-00

Description: Request For Comments

You can download source copies of the file as follows:

draft-bidulock-sigtran-isua-00.txt in text format.
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Listed below is the contents of file draft-bidulock-sigtran-isua-00.txt.




Network Working Group                                     Brian Bidulock
INTERNET-DRAFT                                       OpenSS7 Corporation

Expires in six months                                    January 5, 2003

                     SS7 ISUP-User Adaptation Layer
                                  ISUA
                  <draft-bidulock-sigtran-isua-00.txt>

Status of this Memo

     This document is an Internet-Draft and is in full conformance with
  all provisions of Section 10 or RFC 2026.  Internet-Drafts are working
  documents of the Internet Engineering Task Force (IETF), its areas,
  and its working groups.  Note that other groups may also distribute
  working documents as Internet-Drafts.

     Internet-Drafts are draft documents valid for a maximum of six
  months and may be updated, replaced, or obsoleted by other documents
  at any time.  It is inappropriate to use Internet-Drafts as reference
  material or to cite them other than as 'work in progress'.

     The list of current Internet-Drafts can be accessed at
  http://www.ietf.org/ietf/1id-abstracts.txt

     The list of Internet-Draft Shadow Directories can be accessed at
  http://www.ietf.org/shadow.html

     To learn the current status of any Internet-Draft, please check the
  '1id-abstracts.txt' listing contained in the Internet-Drafts Shadow
  Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
  munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or
  ftp.isi.edu (US West Coast).

Abstract

     This document defines a protocol for the transport of any SS7 ISUP-
  User signalling (e.g, Call Control) over IP using the Stream Control
  Transport Protocol [RFC 2960].  The protocol should be modular and
  symmetric, to allow it to work in diverse architectures, such as a
  Signalling Gateway and IP Signalling End-point architecture.  Protocol
  elements are added to allow seamless operation between peers in the
  SS7 and IP domains.

Contents

     A complete table of contents appears the end of this document.

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1.  Introduction

     This draft defines a protocol for the transport of SS7 ISUP [Q.761,
  T1.113] Users (i.e, Call Control) signalling messages over IP using
  the Stream Control Transmission Protocol (SCTP) [RFC 2960].  This
  protocol would be used between a Signalling Gateway (SG) and
  Signalling End-point located in an IP network.  Additionally, the
  protocol can be used to transport SS7 ISUP users between two
  signalling end-points located within an IP network.

1.1.  Scope

     There is on-going integration of SCN networks and IP networks.
  Network service providers are designing all IP architectures that
  include support for SS7 signalling protocols.  IP provides an
  effective way to transport user data and for operators to expand their
  networks and build new services.  In these networks, there is a need
  for interworking between the SS7 and IP domains [RFC 2719].

     This document details the delivery of Call Control messages over IP
  between two signalling end-points.  Consideration is given for the
  transport from an SS7 Signalling Gateway (SG) to an IP signalling node
  (such as an IP-resident Database) as described in the Framework
  Architecture for Signalling Transport [RFC 2719] This protocol can
  also support transport of Call Control messages between two end-points
  wholly contained within and IP network.

  The delivery mechanism addresses the following criteria:

   - Support for transfer of ISUP messages (Call Control)
   - Support for the seamless operation of Call Control protocol peers.
   - Support for the management of SCTP transport associations between
     an SG and one ore more IP-based signalling nodes.
   - Support for distributed IP-based signalling nodes.
   - Support for the asynchronous reporting of status changes to
     management.

1.2.  Change History

    EDITOR'S NOTE:-  This change history section will be deleted if
    and when the draft is advanced.

1.2.1.  Version 0.0

     This is the initial version of this document.

1.3.  Terminology

  Application Server (AS) - a logical entity serving a specific Routing
     Key.  An example of an Application Server is a virtual database
     element handling all HLR or SCP transactions for a particular SS7
     Signalling Point.  The AS contains a set of one or more unique
     Application Server Processes, of which one or more is normally

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     actively processing traffic.  There is a one-to-one relationship
     between an Application Server and a Routing Key.

  Application Server Process (ASP) - a process instance of an
     Application Server.  An Application Server Process serves as an
     active, backup, load-share or broadcast process of an Application
     Server (e.g, part of a distributed signalling node or database
     element).  Examples of ASPs are MGCs, IP SCPs, or IP HLRs.  An ASP
     contains an SCTP end-point and may be configured to process traffic
     within more that one Application Server.

  Association - refers to an SCTP association [RFC 2960].  The
     association provides the transport for the delivery of ISUP
     protocol data units and ISUA layer peer messages.

  Call Control - The layer above the ISDN User Part in the SS7 protocol
     stack that exchanges primitives with the ISUP provider.  Call
     Control has two major functional blocks: Call Processing and
     Circuit Supervision.  Unlike other layers of the SS7 stack, ISUP
     does not have individual "Users" or ISUP-SAPs.  A single Call
     Control entity is responsible for controlling both ISUP and other
     switch signalling stacks at the Application Layer of the ISO
     7-layer model.  for

  Call Processing] - Call Processing is a major functional block of both
     ISUP and Call Control which is responsible for signalling and
     controlling the state of calls (as opposed to circuits).

  Circuit Supervision] - Circuit Supervision is a major functional block
     of both ISUP and Call Control which is responsible for signalling
     and controlling the state of circuits (as opposed to calls).

  Fail-over - the capability to reroute signalling traffic as required
     to an alternate Application Server Process, or group of ASPs,
     within an Application Server in the event of failure or
     unavailability of a currently used Application Server Process.
     Fail-over may apply upon the return to service of a previously
     unavailable Application Server Process.

  Host - the computing platform that the process (SGP, ASP or IPSP) is
     running on.

  IP Server Process (IPSP) - a process instance of an IP-based
     application.  An IPSP is essentially the same as an ASP, except
     that it uses ISUA in a point-to-point fashion.

  ISDN User Part (ISUP) - The Integrated Services Digital Network (ISDN)
     User Part [Q.761, T1.113] of the SS7 protocol.

  Layer Management (LM) - a nodal function that handles the inputs and
     outputs between the ISUA layer and a local management entity.

  Message Transfer Part (MTP) - The Message Transfer Part [Q.701,
     T1.111] of the SS7 protocol.

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  Nodal Interworking Function (NIF) - an implementation dependent
     interworking function present at a Signalling Gateway that
     interworks primitives and procedures between the ISUP and ISUA
     layers in the SG.

  Network Appearance (NA) - a value that identifies the SS7 network
     context of a Routing Key.  The Network Appearance value is of
     significance only within an administrative domain; it is
     coordinated between the SG and ASP.

  Network Byte Order - the ordering of bytes most-significant-byte
     first, also referred to as Big Endian.

  Routing Context (RC) - a value that uniquely identifies a Routing Key
     and an Application Server.  Routing Context values are either
     configured using a configuration management interface, or by using
     the Routing Key Management (RKM) messages and procedures defined
     for ISUA.

  Routing Key (RK) - describes a set of SS7 parameters and parameter
     values that uniquely define the range of signalling traffic to be
     handled by a particular Application Server.

  Signalling Gateway (SG) - a signalling agent that exchanges SCN native
     signalling at the edge of the IP network [RFC 2719].  An SG appears
     to the SS7 network as an SS7 Signalling Point.  An SG contains a
     set of one or more Signalling Gateway Processes, of which one or
     more is normally actively processing traffic.  When an SG contains
     more than one SGP, the SG is a logical entity and the contained
     SGPs are assumed to be coordinated into a single management view
     both toward the SS7 network and toward the supported Application
     Servers.

  Signalling Gateway Process (SGP) - a process instance of a Signalling
     Gateway.  It serves as an active, backup, load-sharing or broadcast
     process of a Signalling Gateway.

  Stream - an SCTP stream; a unidirectional logical channel established
     from one SCTP endpoint to another associated SCTP endpoint, within
     which all user messages are delivered in sequence, except for those
     submitted to the unordered delivery service.

  Circuit Mapping Function (CMF) - an implementation dependent function
     that is responsible for resolving the address and application
     context presented in the incoming ISUA message to the correct SCTP
     association and Routing Context for the desired application.  The
     CMF MAY use routing context or routing key information as selection
     criteria for the appropriate SCTP association.

  Transport Address - an address that serves as a source or destination
     for the unreliable packet transport service used by SCTP.  In IP
     networks, a transport address is defined by the combination of IP
     address and an SCTP port number [1].

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1.4.  ISUA Overview

1.4.1.  Signalling Transport Architecture

     The framework architecture that has been defined for SCN signalling
  transport over IP [RFC 2719] uses multiple components, including an IP
  transport protocol, a signalling common transport protocol and an
  adaptation module to support the services expected by a particular SCN
  signalling protocol from its underlying protocol layer.

     In general terms, the ISUA architecture can be modeled as a peer-
  to-peer architecture.  The first section considers the SS7-to-IP
  interworking architectures for ISUP call control.  For this case, it
  is assumed that the ASP initiates the establishment of the SCTP
  association with the SG.

1.4.2.  Protocol Architecture for Call Control

     In this architecture (illustrated in Figure 1), the ISUP and ISUA
  layers interface in the SG.  A Nodal Interworking Function (NIF)
  provides for interworking between the ISUP and ISUA layers and
  provides for the transfer of the call processing as well as circuit
  supervision messages.

          .........         ...............        .........
          :       :         :             :        :       :
          :  SEP  :   SS7   :             :   IP   :       :
          :   or  :.........:     SG      :........:  ASP  :
          :  STP  :         :             :        :       :
          :.......:         :.............:        :.......:
           _______           _____________          _______
          |       |         |             |        |       |
          |  CC   |         |     NIF     |        |  CC   |
          |-------|         |------ ------|        |-------|
          | ISUP  |         | ISUP | ISUA |        | ISUA  |
          |-------|         |------|------|        |-------|
          | MTP3  |         | MTP3 |      |        |       |
          |-------|         |------| SCTP |        | SCTP  |
          | MTP2  |         | MTP2 |      |        |       |
          |-------|         |------|------|        |-------|
          |  L1   |         |  L1  |  IP  |        |  IP   |
          |_______|         |______|______|        |_______|
              |                |       |               |
              |________________|       |_______________|

                 CC  - Call Control
                 STP - SS7 Signaling Transfer Point
                 NIF - Nodal Interworking Function

                   Figure 1.  Protocol Architecture

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1.4.3.  All IP Architecture

     This architecture, illustrated in Figure 2, can be used to carry a
  protocol which uses the transport services of ISUP, but is contained
  within an IP network.  This allows extra flexibility in developing
  networks, especially when interaction between legacy signalling is not
  needed.  The architecture removes the need for a signalling gateway
  function.

                   ........        ........
                   :      :   IP   :      :
                   :  AS  :........:  AS  :
                   :      :        :      :
                   :......:        :......:
                    ______          ______
                   |      |        |      |
                   |  AP  |        |  AP  |
                   |------|        |------|
                   | ISUA |        | ISUA |
                   |------|        |------|
                   | SCTP |        | SCTP |
                   |------|        |------|
                   |  IP  |        |  IP  |
                   |______|        |______|
                      |                |
                      |________________|

           AP - Application Protocol (e.g. - Call Control)

                    Figure 2.  All IP Architecture

1.4.4.  ASP Fail-over Model and Terminology

     The ISUA protocol supports ASP fail-over functions to support a
  high availability of transaction processing capability.

     An Application Server can be considered as a list of all ASPs
  configured or registered to handled Call Control messages within a
  certain range of routing information, or within a certain set of
  transaction dialogues, known as a `Routing Key.'  One or more ASPs in
  the list may normally be active to handle traffic, while others may be
  inactive but available in the event of failure or unavailability of
  the active ASPs.

     For operational considerations, see Appendix A.

1.4.5.  Services Provided by the ISUA Layer

1.4.5.1.  Support for the transport of Call Control Messages

     The ISUA supports the transfer of Call Control messages.  The ISUA
  layer at the SG and the ASP support the seamless transport of user
  messages between the SG and the ASP.

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1.4.5.1.1.  ISUP Call Control Support

     Depending on the specific implementation of Call Control supported,
  the ISUA shall support Call Control transparently.  Call Control
  consists of two major functional blocks:

     Call Processing is responsible for signalling and control of calls
  (as opposed to circuits).  Call processing functions move a call
  through its life-cycle by providing the following functions:

   - call setup,
   - call suspend/resume,
   - call release,
   - call exception handling.

     Circuit Supervision is responsible for signalling and control of
  circuits (as opposed to calls).  Circuit supervision functions affect
  the management state of circuits and provides the following functions:

   - circuit testing,
   - circuit reset,
   - circuit blocking and unblocking due to hardware failure and
     recovery events,
   - circuit blocking and unblocking maintenance action,
   - circuit state query.

1.4.5.2.  Native Management Functions

     The ISUA layer provides the capability to indicate errors
  associated with the ISUA protocol messages and to provide notification
  to local circuit management and the remote peer as necessary.

1.4.5.3.  Interworking with Circuit Supervision Functions

     The ISUA layer provides interworking with Circuit Supervision
  functions at the SG for seamless inter-operation between the SCN
  network and the IP network.  ISUA provides the following circuit
  supervision functions:

   - Provides an indication or accpets a request to perform a continuity
     check on a circuit.
   - Provides an indication or accepts a request to reset a circuit or
     circuit group.
   - Provides an indication or accepts a request to block a circuit or
     circuit group.
   - Provides an indication or accepts a request to unblock a circuit or
     circuit group.
   - Provides an indication or accepts a request to query a circuit or
     circuit group.

     The interworking with ISUP circuit supervision messages consists of
  CCNT, CCNA, CREP, CRSC, CBLO, CBLA, CUBL, CUBA, CQRY and CQRA messages
  on receipt of circuit supervision events to the appropriate ASPs.  The

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           Table 1. Mapping of Circuit Supervision Primitives

  +---------------------------+---------------------------+------------+
  |           Name            |          Message          |    ISUA    |
  +--------------+------------+-------------+-------------+ Cc't Supv. |
  | Generic [2]  | Specific   | ITU-T Q.764 | ANSI T1.113 |  Message   |
  +--------------+------------+-------------+-------------+------------+
  |CONT RECHECK  | Request    |     CCR     |     CCR     |    CCNT    |
  |              | Indication |             |             |            |
  |              +------------+-------------+-------------+------------+
  |              | Response   |      -      |     LPA     |    CCNA    |
  |              | Confirm    |             |             |            |
  +--------------+------------+-------------+-------------+------------+
  |CONT REPORT   | Request    |     COT     |     COT     |    CREP    |
  |              | Indication |             |             |            |
  +--------------+------------+-------------+-------------+------------+
  |RESET         | Request    |  RSC, GRS   |  RSC, GRS   |    CRSC    |
  |              +------------+-------------+-------------+------------+
  |              | Confirm    |  RLC, GRA   |  RLC, GRA   |    CRSA    |
  +--------------+------------+-------------+-------------+------------+
  |BLOCKING      | Request    |  BLO, CGB   |  BLO, CGB   |    CBLO    |
  |              | Indication |             |             |            |
  |              +------------+-------------+-------------+------------+
  |              | Response   |  BLA, CGBA  |  BLA, CGBA  |    CBLA    |
  |              | Confirm    |             |             |            |
  +--------------+------------+-------------+-------------+------------+
  |UNBLOCKING    | Request    |  UBL, CGU   |  UBL, CGU   |    CUBL    |
  |              | Indication |             |             |            |
  |              +------------+-------------+-------------+------------+
  |              | Response   |  UBA, CGUA  |  UBA, CGUA  |    CUBA    |
  |              | Confirm    |             |             |            |
  +--------------+------------+-------------+-------------+------------+
  |CCT GRP QUERY | Request    |     CQM     |     CQM     |    CQRY    |
  |              | Indication |             |             |            |
  |              +------------+-------------+-------------+------------+
  |              | Response   |     CQR     |     CQR     |    CQRA    |
  |              | Confirm    |             |             |            |
  +--------------+------------+-------------+-------------+------------+

  primitives in Table 1 are sent between the ISUP and ISUA circuit
  supervision functions in the SG to trigger events in the IP and SS7
  domain.

     The ISUA layer provides transparent passing of circuit reset,
  blocking and query primitives (RESET, BLOCKING, UNBLOCKING, CCT GROUP
  QUERY) as provided for in ITU-T Q.724 [Q.724] Q.764 [Q.764], and ANSI
  T1.113 [T1.113].

1.4.5.4.  Support for the Management of SCTP Associations

     The ISUA layer at the SGP maintains the availability state of all
  configured remote ASPs, to manage the SCTP Associations and the

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  traffic between ISUA peers.  As well, the active/inactive and
  congestion state of remote ASPs is maintained.

     The ISUA layer MAY be instructed by local management to establish
  an SCTP association to a peer ISUA node.  This can be achieved using
  the M-SCTP_ESTABLISH primitives to request, indicate and confirm the
  establishment of an SCTP association with a peer ISUA node.  To avoid
  redundant SCTP associations between two ISUA peers, one side (client)
  SHOULD be designated to establish the SCTP association, or ISUA
  configuration information maintained to detect redundant associations
  (e.g, via knowledge of the expected local and remote SCTP endpoint
  addresses).

     Local management MAY request from the ISUA layer the status of the
  underlying SCTP associations using the M-SCTP_STATUS request and
  confirm primitives.  Also, the ISUA MAY autonomously inform local
  management of the reason for the release of an SCTP association,
  determined either locally within the ISUA layer or by a primitive from
  the SCTP.

     Also, the ISUA layer MAY inform the local management of the change
  in status of an ASP or AS.  This MAY be achieved using the M-
  ASP_STATUS request or M-AS_STATUS request primitives.

1.5.  Functional Areas

1.5.1.  Circuit Identifiers, Routing Contexts and Routing Keys

1.5.1.1.  Overview

     The mapping of ISUP messages into calls between the SGP and the
  Application Servers is determined by Circuit Identifiers, Routing Keys
  and their associated Routing Contexts.

     A Routing Key is essentially a set of ISUP parameters used to
  direct ISUP messages; whereas, the Routing Context parameter is a
  4-byte value (unsigned integer) that is associated to that Routing Key
  in a one-to-one relationship.  The Routing Context therefore can be
  viewed as an index into a sending node's Circuit Mapping Function
  tables containing the Routing Key entries.

     Possible ISUP address/routing information that comprise a Routing
  Key entry includes, for example, a local and remote Point Code, and a
  Circuit Identification Code or Call Control specific information such
  as Circuit Group or Trunk Group Identifiers.  The particular
  information used to define a ISUA Routing Key is application and
  network dependent, and none of the above examples are requirements for
  ISUA.

     An Application Server Process (ASP) may be configured to process
  signalling traffic related to more than one Application Server (AS),
  over a single SCTP Association.  ASP Active (ASPAC) and ASP Inactive
  (ASPIA) management messages (see Section 3) use the Routing Context to
  discriminate signalling traffic to be started or stopped.  At an ASP,

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  the Routing Context parameter uniquely identifies the range of
  signalling traffic associated with each Application Server that the
  ASP is configured to receive.

1.5.1.2.  Routing Key Limitations

     Routing Keys SHOULD be unique in the sense that each received ISUP
  message SHOULD have a full or partial match to a single routing
  result.  It is not necessary for the parameter range values within a
  particular Routing Key to be continuous.  For example, an AS could be
  configured to support call processing for multiple ranges of circuits
  that are not represented by contiguous Circuit Identification Codes.

1.5.1.3.  Managing Routing Context and Routing Keys

     There are two ways to provision a Routing Key at an SGP.  A Routing
  Key may be configured statically using an implementation dependent
  management interface, or dynamically managed using the the ISUA
  Routing Key registration procedures.

     When using a management interface to configure Routing Keys, the
  Circuit Mapping Function within the SGP is not limited to the set of
  parameters defined in this document.  Other implementation dependent
  distribution algorithms may be used.

1.5.1.4.  Circuit Mapping Function

     To perform its addressing and relaying capabilities, the ISUA makes
  use of an Circuit Mapping Function (CMF).  This function is considered
  part of ISUA, but the way it is realized is left implementation or
  deployment dependent (local tables, database, etc.)

     The CMF is invoked when a message is received at the incoming
  interface.  The CMF is responsible for resolving the Circuit
  Identification Code (CIC) and any necessary ISUP message parameters
  presented in the incoming ISUP message to SCTP associations and
  destinations within the IP network.  The CMF will select the key
  information available.  The Routing Keys reference an Application
  Server, which will normally have one or more ASPs processing
  transactions for the AS.  The availability and status of the ASPs is
  handled by ISUA ASP management messages.

     Possible SS7 routing information that comprise a Routing Key entry
  includes, for example, ISUP Circuit Identification Code (CIC), Range
  and Status parameters.

     It is expected that the routing keys will be provisioned via a MIB,
  dynamic registration or an external process, such as a database.

1.5.1.4.1.  Circuit Mapping at the SG

     To direct messages received from the SS7 network to the appropriate
  IP destination, the SGP must perform a circuit mapping function using
  information from the received ISUP message.

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     To support this circuit mapping, the SGP might, for example,
  maintain the equivalent of a network address translation table,
  mapping incoming ISUP message information to an Application Server for
  a particular application and set of transactions.  This could be
  accomplished by comparing the circuit identification code and range
  and status portions of the incoming ISUP message to currently defined
  Routing Keys in the SGP.  These Routing Keys could in turn map
  directly to an Application Server that is enabled by one or more ASPs.
  These ASPs proxy dynamic status information regarding their
  availability, call handling capabilities and congestion to the SGP
  using various management messages defined in the ISUA protocol.

     The list of ASPs in the AS is assumed to be dynamic, taking into
  account the availability, call handling capability and congestion
  status of the individual ASPs in the list, as well as configuration
  changes and possible fail-over mechanisms.

     Normally, one or more ASPs are active in the AS (i.e, currently
  processing calls) but in certain failure and transition cases it is
  possible that there may not be an active ASP available.  The SGP will
  buffer the message destined for this AS for a time T(r) or until an
  ASP becomes available.  When no ASP becomes available before expiry of
  T(r), the SGP will flush the buffered messages and initiate the
  appropriate ISUP call clearing procedures.

     If there is no match for an incoming message, a default treatment
  MAY be specified.  Possible solutions are to provide a default
  Application Server to direct all unallocated call processing and
  circuit supervision messges to a (set of) default ASP(s), or to drop
  the messages and provide a notification to management.  The treatment
  of unallocated circuits is implementation dependent.

1.5.1.4.2.  Circuit Mapping at the ASP

     To direct messages to the SS7 network, the ASP MAY perform a
  circuit mapping to choose the proper SGP for the given message.  This
  is accomplished by observing the Circuit Identification Code, Range
  and Status, and other elements of the outgoing message, SS7 network
  status, SGP availability, and Routing Context configuration tables.

     A Signalling Gateway may be composed of one or more SGPs.  There
  is, however, no ISUA messaging to manage the status of an SGP.
  Whenever an SCTP association to an SGP exists, it is assumed to be
  available.  Also, every SGP of one SG communicating with one ASP
  regarding one AS provides identical call control to this ASP.

     In general, an ASP routes responses to the SGP that it received
  messages from; within the routing context which it is currently active
  and receiving transactions.  The routing context itself is used by the
  ASP to select the SGP.

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1.5.1.5.  Signalling Gateway SS7 Layers

     The SG is responsible for terminating up to the Call Control of the
  SS7 protocol, and offering an IP-based extension to its users.

     From an SS7 perspective, it is expected that the Signalling Gateway
  transmits and receives ISUP messages to and from the SS7 Network over
  standard SS7 network interfaces, using the services of the MTP [Q.704]
  to provide transport of the messages.

     Note that it is also possible for the MTP services to be provided
  using the services of the MTP-User Adaptation Layer (M3UA) [M3UA].

     The ISUP-SAP through which ISUA at the SG obtains its services
  could reside at a Signalling Transfer Point (STP) or Signalling End
  Point (SEP) [Q.701].

1.5.1.6.  SS7 and ISUA Interworking at the SG

     The SGP provides a functional interworking of transport functions
  between the SS7 network and the IP network by also supporting the ISUA
  adaptation layer.  It allows the ISUP application to exchange call
  control messages with an IP-based Application Server Process where the
  peer Call Control protocol layer exists.

     To perform ISUP circuit supervision, it is required that the Call
  Control protocols at ASPs receive indications of circuit state, as
  well as call state as they would be expected by an SS7 ISUP
  application.  To accomplish this, the RESET, BLOCKING, UNBLOCKING and
  CCT GROUP QUERY primitives received at the ISUP upper layer interface
  at the SG need to be propagated to the remote Call Control lower layer
  interface at the ASP.

     ISUP call processing and circuit supervision mesages (such as BLO,
  BLA, CGB, CGBA) received from the SS7 network MUST NOT be
  encapsulated.  The SG MUST terminate these messages and generate ISUA
  message as appropriate.

1.5.1.7.  Application Server

     A cluster of Application Servers is responsible for providing the
  overall support for one ore more SS7 upper layers.  From an ISUP
  standpoint, Call Control provides complete support for the upper layer
  service for given Circuits or Trunk Groups.  As an example, Call
  Control could provide complete support for Central Office Call Control
  for a given point code.

1.5.1.8.  SCTP Stream Mapping

     The ISUA supports SCTP streams.  The SG and AS need to maintain a
  list of SCTP and Call Control for mapping purposes.  Call Control
  requiring sequenced message transfer need to be sent over a stream
  using sequenced delivery.

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     ISUA SHOULD NOT use stream 0 for ISUA circuit supervision messages.
  It is OPTIONAL that sequence delivery be used to preserve the order of
  circuit supervision message delivery.

     All ISUA Circuit Supervision (CS) messages MAY select unordered
  delivery, depending on the requirements of Call Control.  Normally one
  stream is used to send ISUA Circuit Supervision (CS) messages between
  peers, regardless of Application Server.

     All Call Processing (CP) messages MUST be sent using ordered
  delivery.  All Call Processing (CP) messages relating to the same call
  MUST be sent on the same stream as other Call Processing (CP) messages
  relating to the same call.  The stream selected is based upon the Call
  Reference given by the Call Control over the primitive interface and
  other traffic information available to the SGP or ASP.

1.5.2.  Redundancy Models

1.5.2.1.  Application Server Redundancy

     All CSET and Circuit Supervision (CS) messages (e.g, SETUP, RESET,
  BLOCKING) which match a provisioned Routing Key at an SGP are mapped
  to an Application Server.

     The Application Server is the set of all ASPs associated with a
  specific Routing Key.  Each ASP in this set may be active, inactive or
  unavailable.  Active ASPs handle traffic; inactive ASPs might be used
  when active ASPs become unavailable.

     The fail-over model supports an "n+k" redundancy model, where "n"
  ASPs is the minimum number of redundant ASPs required to handle
  traffic and "k" ASPs are available to take over for a failed or
  available ASP.  A "1+1" active/backup redundancy is a subset of this
  model.  A simplex "1+0" model is also supported as a subset, with no
  ASP redundancy.

1.5.3.  Flow Control

     Local Management at an ASP may wish to stop traffic across an SCTP
  association to temporarily remove the association from service or to
  perform testing and maintenance activity.  The function could
  optionally be used to control the start of traffic onto a newly
  available SCTP association.

1.5.4.   Congestion Management

     The ISUA layer is informed of local and IP network congestion by
  means of an implementation-dependent function (e.g, an implementation-
  dependent indication from the SCTP of IP network congestion).

     At an ASP or IPSP, the ISUA layer indicates congestion to local
  Call Control by means of an appropriate ISUP primitive, as per current
  ISUP procedures, to invoke appropriate upper layer responses.  When an
  SG determines that the transport of SS7 messages is encountering

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  congestion, the SG might trigger SS7 Congestion messages to
  originating SS7 nodes, per the congestion procedures of the relevant
  ISUP [Q.764, T1.113] or MTP [Q.704, T1.111] standard.  (The triggering
  of SS7 Management messages from an SG is an implementation-dependent
  function.)

1.6.  Definition of ISUA Boundaries

     ISUA has three protocol boundaries: an upper boundary between ISUA
  and Call Control; a lower boundary between ISUA and SCTP; and a layer
  management boundary between ISUA and the Layer Management Function.
  Figure 3 illustrates the ISUA protocol boundaries.

                      ...........
                      :   CC    :
                      :.........:  Layer
            Upper Boundary :       Management
                       ____:____   Boundary   ............
                      |   ISUA  |.............:    LM    :
                      |_________|             :..........:
            Lower Boundary :
                      .....:.....
                      :   SCTP  :
                      :.........:

                 Figure 3.  ISUA Protocol Boundaries

1.6.1.  Definition of Upper Boundary

     The primitives and messages listed in Table 2 are provided between
  the ISUA and Call Control in support of Call Control [Q.761, T1.113].

               Table 2. Mapping of Call Control Primitives

    +-------------+------------+---------------+---------------+------+
    |Generic      | Specific   |  ITU-T Q.764  |  ANSI T1.113  | ISUA |
    |Name         | Name       |    Message    |    Message    | Msg  |
    +-------------+------------+---------------+---------------+------+
    |Call Setup Messages                                              |
    +-------------+------------+---------------+---------------+------+
    |SETUP        | Request    |      IAM      |      IAM      | CSET |
    |             | Indication |               |               |      |
    |             +------------+---------------+---------------+------+
    |             | Response   |   ANM, CON    |      ANM      | CCON |
    |             | Confirm    |               |               |      |
    +-------------+------------+---------------+---------------+------+
    |MORE INFO    | Request    |       -       |       -       | CMOR |
    |             | Indication |               |               |      |
    +-------------+------------+---------------+---------------+------+

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    +-------------+------------+---------------+---------------+------+
    |Generic      | Specific   |  ITU-T Q.764  |  ANSI T1.113  | ISUA |
    |Name         | Name       |    Message    |    Message    | Msg  |
    +-------------+------------+---------------+---------------+------+
    |TIMEOUT      | Indication |       -       |       -       | CTOT |
    +-------------+------------+---------------+---------------+------+
    |INFO         | Request    |      SAM      |      SAM      | CINF |
    |             | Indication |               |               |      |
    +-------------+------------+---------------+---------------+------+
    |PROC         | Request    |   ACM, CPG    |   ACM, CPG    | CPRO |
    |             | Indication |               |               |      |
    +-------------+------------+---------------+---------------+------+
    |ALERT        | Request    |   ACM, CPG    |   ACM, CPG    | CALR |
    |             | Indication |               |               |      |
    +-------------+------------+---------------+---------------+------+
    |PROG         | Request    |   ACM, CPG    |   ACM, CPG    | CPRG |
    |             | Indication |               |               |      |
    +-------------+------------+---------------+---------------+------+
    |Call Established Messages                                        |
    +-------------+------------+---------------+---------------+------+
    |SUSPEND      | Request    |      SUS      |      SUS      | CSUS |
    |             | Indication |               |               |      |
    +-------------+------------+---------------+---------------+------+
    |RESUME       | Request    |      RES      |      RES      | CRES |
    |             | Indication |               |               |      |
    +-------------+------------+---------------+---------------+------+
    |Call Termination Messages                                        |
    +-------------+------------+---------------+---------------+------+
    |REATTEMPT    | Indication |       -       |       -       | CREA |
    +-------------+------------+---------------+---------------+------+
    |CALL FAILURE | Indication | RST, REL, RLC | RST, REL, RLC | CERR |
    +-------------+------------+---------------+---------------+------+
    |IBI          | Request    |   ACM, CPG    |   ACM, CPG    | CIBI |
    |             | Indication |               |               |      |
    +-------------+------------+---------------+---------------+------+
    |RELEASE      | Request    |      REL      |      REL      | CREL |
    |             | Indication |               |               |      |
    |             +------------+---------------+---------------+------+
    |             | Response   |   REL, RLC    |   REL, RLC    | CRLC |
    |             | Confirm    |               |               |      |
    +-------------+------------+---------------+---------------+------+

1.6.2.  Definition of Boundary between ISUA and Layer Management

  M-SCTP_ESTABLISH request
  Direction: LM->ISUA
  Purpose:   LM request ASP to establish an SCTP association with its
             peer.

  M-SCTP_ESTABLISH confirm
  Direction: ISUA -> LM

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  Purpose:   ASP confirms to LM that it has established an SCTP
             association with its peer.

  M-SCTP_ESTABLISH indication
  Direction: ISUA -> LM
  Purpose:   ISUA informs LM that a remote ASP has established an SCTP
             association.

  M-SCTP_RELEASE request
  Direction: LM -> ISUA
  Purpose:   LM requests ASP to release an SCTP association with its
             peer.

  M-SCTP_RELEASE confirm
  Direction: ISUA -> LM
  Purpose:   ASP confirms to LM that it has released SCTP association
             with its peer.

  M-SCTP_RELEASE indication
  Direction: ISUA -> LM
  Purpose:   ISUA informs LM that a remote ASP has released an SCTP
             Association or the SCTP association has failed.

  M-SCTP RESTART indication
  Direction: ISUA -> LM
  Purpose:   ISUA informs LM that an SCTP restart indication has been
             received.

  M-SCTP_STATUS request
  Direction: LM -> ISUA
  Purpose:   LM requests ISUA to report the status of an SCTP
             association.

  M-SCTP_STATUS confirm
  Direction: ISUA -> LM
  Purpose:   ISUA responds with the status of an SCTP association.

  M-SCTP_STATUS indication
  Direction: ISUA -> LM
  Purpose:   ISUA reports the status of an SCTP association.

  M-ASP_STATUS request
  Direction: LM -> ISUA
  Purpose:   LM requests ISUA to report the status of a local or
             remote ASP.

  M-ASP_STATUS confirm
  Direction: ISUA -> LM
  Purpose:   ISUA reports status of local or remote ASP.

  M-AS_STATUS request
  Direction: LM -> ISUA
  Purpose:   LM requests ISUA to report the status of an AS.

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  M-AS_STATUS confirm
  Direction: ISUA -> LM
  Purpose:   ISUA reports the status of an AS.

  M-NOTIFY indication
  Direction: ISUA -> LM
  Purpose:   ISUA reports that it has received a Notify (NTFY) message
             from its peer.

  M-ERROR indication
  Direction: ISUA -> LM
  Purpose:   ISUA reports that it has received an Error (ERR) message
             from its peer or that a local operation has been
             unsuccessful.

  M-ASP_UP request
  Direction: LM -> ISUA
  Purpose:   LM requests ASP to start its operation and send an ASP Up
             (ASPUP) message to its peer.

  M-ASP_UP confirm
  Direction: ISUA -> LM
  Purpose:   ASP reports that is has received an ASP UP Ack (ASPUP
             ACK) message from its peer.  T} ; ls l1lw(5.7i).  M-
             ASP_UP indication Direction:;ISUA -> LM Purpose:;T{ ISUA
             reports it has successfully processed an incoming ASP Up
             (ASPUP) message from its peer.

  M-ASP_DOWN request
  Direction: LM -> ISUA
  Purpose:   LM requests ASP to stop its operation and send an ASP
             Down (ASPDN) message to its peer.

  M-ASP_DOWN confirm
  Direction: ISUA -> LM
  Purpose:   ASP reports that is has received an ASP Down Ack (ASPDN
             ACK) message from its peer.

  M-ASP_DOWN indication
  Direction: ISUA -> LM
  Purpose:   ISUA reports it has successfully processed an incoming
             ASP Down (ASPDN) message from its peer, or the SCTP
             association has been lost or reset.

  M-ASP_ACTIVE request
  Direction: LM -> ISUA
  Purpose:   LM requests ASP to send an ASP Active (ASPAC) message to
             its peer.

  M-ASP_ACTIVE confirm
  Direction: ISUA -> LM

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  Purpose:   ASP reports that is has received an ASP Active Ack (ASPAC
             ACK) message from its peer.

  M-ASP_ACTIVE indication
  Direction: ISUA -> LM
  Purpose:   ISUA reports it has successfully processed an incoming
             ASP Active (ASPAC) message from its peer.

  M-ASP_INACTIVE request
  Direction: LM -> ISUA
  Purpose:   LM requests ASP to send an ASP Inactive (ASPIA) message
             to its peer.

  M-ASP_INACTIVE confirm
  Direction: LM -> ISUA
  Purpose:   ASP reports that is has received an ASP Inactive Ack
             (ASPIA ACK) message from its peer.

  M-ASP_INACTIVE indication
  Direction: ISUA -> LM
  Purpose:   ISUA reports it has successfully processed an incoming
             ASP Inactive (ASPIA) message from its peer.

  M-AS_ACTIVE indication
  Direction: ISUA -> LM
  Purpose:   ISUA reports that an AS has moved to the AS-ACTIVE state.

  M-AS_INACTIVE indication
  Direction: ISUA -> LM
  Purpose:   UA reports that an AS has moved to the AS-INACTIVE state.

  M-AS_DOWN indication
  Direction: ISUA -> LM
  Purpose:   UA reports that an AS has moved to the AS-DOWN state.

  M-RK_REG request
  Direction: LM -> ISUA
  Purpose:   LM requests ASP to register RK(s) with its peer by
             sending Registration Request (REG REQ) message

  M-RK_REG confirm
  Direction: ISUA -> LM
  Purpose:   ASP reports that it has received Registration Response
             (REG RSP) message with registration status as successful
             from its peer.

  M-RK_REG indication
  Direction: ISUA -> LM
  Purpose:   ISUA informs LM that it has successfully processed an
             incoming Registration Request (REG REQ) message.

  M-RK_DEREG request
  Direction: LM -> ISUA

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  Purpose:   LM requests ASP to deregister RK(s) with its peer by
             sending Deregistration Request (DEREG REQ) message.

  M-RK_DEREG confirm
  Direction: ISUA -> LM
  Purpose:   ASP reports that it has received Deregistration Request
             (DEREG REQ) message with deregistration status as
             successful from its peer.

  M-RK_DEREG indication
  Direction: ISUA -> LM
  Purpose:   ISUA informs LM that it has successfully processed an
             incoming Deregistration Request (DEREG REQ) message from
             its peer.

1.6.3.  Definition of the Lower Boundary

  The upper layer primitives provided by the SCTP are provided in the
  SCTP specification "Stream Control Transmission Protocol (SCTP)" [RFC
  2960].

2.  Conventions

     The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
  SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when they
  appear in this document, are to be interpreted as described in [RFC
  2119].

  In this document, the following conventions are used to describe how a
  parameter is used in the message:

    Mandatory     The parameter MUST be present in the message.  A
                  message listing a parameter as Mandatory without
                  containing such a parameter is is incorrectly
                  formatted.

    Conditional   The parameter SHOULD be present in the message
                  under the conditions specified.  A message listing
                  a parameter as Conditional without containing such
                  a parameter under the conditions specified is
                  incorrectly formatted.

    Optional      The parameter MAY be present in the message as
                  specified.  A message listing a parameter as
                  Optional without containing such a parameter is
                  correctly formatted.

3.  Protocol Elements

     The general message format includes a Common Message Header
  together with a list of zero or more parameters as defined by the

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  Message Type.

     For forward compatibility, all Message Types MAY have attached
  parameters even if none are specified in this version.

3.1.  Common Message Header

     The protocol messages for the ISUP-User Adaptation Protocol (ISUA)
  require a message structure that contains a version, message type,
  message length and message contents:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |    Version    |   Reserved    | Message Class | Message Type  |
    +---------------+---------------+---------------+---------------+
    |                        Message Length                         |
    +---------------------------------------------------------------+
    |                         Message Data                          |

  Notes:

   - This message header is common among all signalling protocol
     adaptation layers.
   - The 'data' portion of ISUA messages SHALL contain zero or more ISUA
     parameters, and SHALL NOT contain an encapsulated ISUP message.
   - All fields in the ISUA message MUST be transmitted in the network
     byte order, unless otherwise stated.

3.1.1.  ISUA Protocol Version

  Version: 8-bits (unsigned integer)

    The Version field of the Common Message Header contains the version
    of the ISUA adaptation layer.  The supported versions are:

        1 - ISUA Version 1.0

3.1.2.  Message Classes

  Message Class: 8-bits (unsigned integer)

    The Message Class field of the Common Message Header contains the
    class of the message.  The supported classes are as follows:

      0        Management (MGMT) Message
      7        Reserved for Other Signalling Adaptation Layers
      2        Reserved for Other Signalling Adaptation Layers
      3        ASP State Maintenance (ASPSM) Messages

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      4        ASP Traffic Maintenance (ASPTM) Messages
      5        Reserved for Other Signalling Adaptation Layers
      6        Reserved for Other Signalling Adaptation Layers
      7        Reserved for Other Signalling Adaptation Layers
      8        Reserved for Other Signalling Adaptation Layers
      9        Routing key Management (RKM) Messages
     10        ISUA Call Processing (CP) Messages
     11        ISUA Circuit Supervision (CS) Messages
     12 - 127  Reserved by the IETF
    128 - 255  Reserved for IETF-Defined Message Class Extensions

3.1.3.  Message Types

  Message Type: 8-bits (unsigned integer)

    The Message Type field of the Common Message Header contains the
    type of message within a message class.  The supported types of
    messages within the supported classes are as follows:

    Management (MGMT) Messages
      0         Error (ERR)
      1         Notify (NTFY)
      2 -  127  Reserved by the IETF
    128 -  255  Reserved for IETF-Defined Message Class Extensions

    Application Server Process State Maintenance (ASPSM) Messages
      0         Reserved
      1         ASP Up (UP)
      2         ASP Down (DOWN)
      3         Heartbeat (BEAT)
      4         ASP Up Ack (UP ACK)
      5         ASP Down Ack (DOWN ACK)
      6         Heartbeat Ack (BEAT ACK)
      7 -  127  Reserved by the IETF
    128 -  255  Reserved for IETF-Defined Message Class Extensions

    Application Server Process Traffic Maintenance (ASPTM) Messages
      0         Reserved
      1         ASP Active (ASPAC)
      2         ASP Inactive (ASPIA)
      3         ASP Active Ack (ASPAC ACK)
      4         ASP Inactive Ack (ASPIA ACK)
      5 -  127  Reserved by the IETF
    128 -  255  Reserved for IETF-Defined Message Class Extensions

    Routing Key Management (RKM) Messages
      0         Reserved
      1         Registration Request (REG REQ)
      2         Registration Response (REG RSP)
      3         Deregistration Request (DEREG REQ)

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      4         Deregistration Response (DEREG RSP)
      5 -  127  Reserved by the IETF
    128 -  255  Reserved for IETF-Defined Message Class Extensions

    ISUA Call Processing (CP) Messages
      0         Reserved
      1         Setup (CSET)
      2         More Information (CMOR)
      3         Timeout (CTOT)
      4         Information (CINF)
      5         Proceeding (CPRO)
      6         Alerting (CALR)
      7         Progress (CPRG)
      8         Connect (CCON)
      9         Suspend (CSUS)
     10         Resume (CRES)
     11         Reattempt (CREA)
     12         Failure (CERR)
     13         In Band Information (CIBI)
     14         Release (CREL)
     15         Release Complete (CRLC)
     16 -  127  Reserved by the IETF
    128 -  255  Reserved for IETF-Defined Message Class Extensions

    ISUA Circuit Supervision (CS) Messages
      0         Reserved
      1         Continuity Check (CCNT)
      2         Loopback (CLBK)
      3         Report (CREP)
      4         Reset (CRSC)
      5         Reset Acknowledgement (CRSA)
      6         Block (CBLO)
      7         Block Acknowledgement (CBLA)
      8         Unblock (CUBL)
      9         Unblock Acknowledgement (CUBA)
     10         Query (CQRY)
     11         Query Acknowledgement (CQRA)
     12 -  127  Reserved by the IETF
    128 -  255  Reserved for IETF-Defined Message Class Extensions

3.1.4.  Message Length

  Message Length: 32-bits (unsigned integer)

    The Message Length field of the Common Message Header defines the
    length of the message in octets, including the header.

3.1.5.  Tag-Length-Value Format

     ISUA messages consist of a Common Message Header followed by zero
  or more parameters, as defined by the message type.  The Tag-Length-
  Value (TLV) parameters contained in a message are defined in a Tag-

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  Length-Value format as shown below [2].

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          Parameter Tag        |       Parameter Length        |
    +-------------------------------+-------------------------------+
    \                                                               \
    /                       Parameter Value                         /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  Parameter Tag: 16-bits (unsigned integer)

    The Parameter Tag field is a 16-bit identifier of the type of
    parameter.  It takes a value of 0 to 65534.

  Parameter Length: 16-bits (unsigned integer)

    The Parameter Length field contains the size of the parameter in
    bytes, including the Parameter Tag, Parameter Length, and Parameter
    Value fields.  The Parameter Length does not include any padding
    bytes.  However, composite parameters will contain all padding
    bytes, since all parameters contained within this composite
    parameter will considered multiples of 4 bytes.

  Parameter Value: variable-length

    The Parameter Value field contains the actual information to be
    transferred in the parameter.  The total length of a parameter
    (including Tag, Parameter Length and Value fields) MUST be a
    multiple of 4 bytes.  If the length of the parameter is not a
    multiple of 4 bytes, the sender MUST pad the Parameter at the end
    (i.e., after the Parameter Value field) with all zero bytes.  The
    length of the padding MUST NOT be included in the parameter length
    field.  A sender SHOULD NOT pad with more than 3 bytes.  The
    receiver MUST ignore the padding bytes.

3.2.  ISUA Message Header

     In addition to the Common Message Header, a specific message header
  is included for ISUA messages.  The ISUA message header will
  immediately follow the Common Message Header in ISUA Call Processing
  (CP) and Circuit Supervision (CS) messages.

  The ISUA Message Header is formatted as follows:

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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0006          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                        Routing Context                        |
    +-------------------------------+-------------------------------+
    |         Tag = 0x0013          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                        Correlation Id                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The ISUA Message header can contain the following parameters:

      Parameters
      ---------------------------------------------
      Routing Context             Conditional   *1
      Correlation Id              Conditional   *2

  Note 1: When an ASP is registered or configured for multiple AS with
          an SG, the Routing Context MUST be present in the ISUA Message
          Header.  The Routing Context SHOULD always be placed in the
          ISUA Message Header.  When the Routing Context is present in
          the ISUA Message Header it SHOULD be placed first in the
          header because the context of the Correlation Id depends on
          the Routing Context.

  Note 2: Under some circumstances, the Correlation Id parameter MUST be
          included in the ISUA Message Header.  See sections "3.9.9 -
          Correlation Id" and "4.3.4.3 - ASP Active Procedures".

3.3.  ISUA Call Processing (CP) Messages

     The following section describes the ISUA Call Processing (CP)
  messages and parameter contents.  The general message format includes
  a Common Message Header, the ISUA Message Header and the CP Message
  Header, together with a list of zero or more parameters as defined by
  the Message Type.  For forward compatibility, all Message Types MAY
  have optional attached parameters in addition to the message headers.

  These messages are ISUA Call Processing (CP) messages:

              ISUA Call Processing (CP) Messages
      ----------------------------------------------------
      Message Name                 Message Type   Section
      ----------------------------------------------------
      CP Header                                   3.3.1
      Setup                 CSET         1        3.3.2
      More Information      CMOR         2        3.3.3

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      Timeout               CTOT         3        3.3.4
      Information           CINF         4        3.3.5
      Proceeding            CPRO         5        3.3.6
      Alerting              CALR         6        3.3.7
      Progress              CPRG         7        3.3.8
      Connect               CCON         8        3.3.9
      Suspend               CSUS         9        3.3.10
      Resume                CRES        10        3.3.11
      Reattempt             CREA        11        3.3.12
      Failure               CERR        12        3.3.13
      In Band Information   CIBI        13        3.3.14
      Release               CREL        14        3.3.15
      Release Complete      CRLC        15        3.3.16
      ----------------------------------------------------

3.3.1.  CP Message Header

     In addition to the Common Message Header and ISUA Message Header, a
  specific message header is included for ISUA Call Processing (CP)
  messages.  The CP Message Header will immediately follow the ISUA
  Message header in these messages.

  The CP Message Header is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0520          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                           Circuit Id                          |
    +-------------------------------+-------------------------------+
    |         Tag = 0x0501          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                         Call Reference                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The CP Message Header contains the following parameters:

      Parameters
      ---------------------------------------------
      Circuit Id                  Conditional   *1
      Call Reference              Conditional   *2

  Note 1: The Circuit Id MUST be placed in the ISUA CP Message Header
          for all CP messages sent from the SGP to the ASP, and is
          OPTIONAL in the ISUA CP Message Header for all CP messages
          sent from the ASP to the SGP for which a Circuit Id was
          assigend to the call by the SGP before the message was sent.
          If Circuit Id was not assigned by the SGP before the ASP sends
          a CP message, the ASP MAY include the Circuit Id parameter for

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          simplicity, but it MUST then be coded zero (0).  CP messages
          for which a Circuit Id has not been assigned by the SGP
          include only the Setup (CSET) request message sent from the
          ASP to the SGP.

  Note 2: The Call Reference MUST be placed in the ISUA CP Message
          Header for all CP messages sent from ASP to the SGP, and is
          OPTIONAL in the ISUA CP Message Header for all CP messages
          sent from the SGP to the ASP for which a Call Reference was
          assigned to the call by the ASP before the message was sent.
          If Call Reference was not assigned by the ASP before the SGP
          sends a CP message, the SGP MAY include the Call Reference
          parameter for simplicity, but it MUST then be coded zero (0).
          CP messages for which a Call Reference has not been assigned
          by the ASP include only the Setup (CSET) indication message
          sent from the SGP to the ASP.

3.3.2.  Setup (CSET)

     The Setup (CSET) Request message is sent from an ASP to an SG or
  IPSP to initiate an outgoing ISUP call setup.  The CSET Indication
  message is sent from an SGP to an ASP to indicate an incoming ISUP
  call setup.

     The CSET message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  Call Control `Setup' (Request, Indication) primitive and the ITU-T and
  ANSI ISUP `IAM' message [Q.763, T1.113].

  The CSET message is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0502          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                           Call Type                           |
    +-------------------------------+-------------------------------+
    |         Tag = 0x0503          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                           Call Flags                          |
    +-------------------------------+-------------------------------+
    |         Tag = 0x0504          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                       Called Party Number                     /
    \                                                               \
    +-------------------------------+-------------------------------+
    |         Tag = 0x050E          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                       Optional Parameters                     /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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  The CSET message can contain the following parameters:

      Parameters
      -------------------------------------------
      Call Type                   Mandatory
      Call Flags                  Mandatory
      Called Party Number         Mandatory
      Optional Parameters         Optional    *1

  Note 1: Although the Optional Parameters are optional in the CSET
          message, the specific ISUP variant and network policy in which
          the implementation is operating could require that the
          implementation always place specific parameters in the
          Optional Parameters parameter.  An example of this would be
          the Charge Number of GR-394 networks.

3.3.3.  More Information (CMOR)

     The More Information (CMOR) message is sent from an SGP to an ASP
  to request additional address information for an outgoing ISUP call
  setup.

     The CMOR message does not correspond to a Call Control primitive or
  ISUP message.

     The COMR message has no message-type-specific parameters beyond the
  CP Message Header.

3.3.4.  Timeout (CTOT)

     The Timeout (CTOT) message is sent from an SGP to an ASP to
  indicate that the SG has timed out while waiting for additional
  address information.

     The CTOT message does not correspond to a Call Control primitive or
  ISUP message.

     The CTOT message has no message-type-specific parameters beyond the
  CP Message Header.

3.3.5.  Information (CINF)

     The Information (CINF) message is sent from an ASP to an SGP to
  provide additional address information for an outgoing ISUP call
  setup.  The CINF message is sent from an SGP to an ASP to provide
  additional address information for an incoming ISUP call setup.

     The CINF message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  Call Control `Info' primitive and the ITU-T and ANSI ISUP `SAM'
  message [Q.763, T1.113].

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  The CINF message is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0505          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                        Subsequent Number                      /
    \                                                               \
    +-------------------------------+-------------------------------+
    |         Tag = 0x050E          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                       Optional Parameters                     /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The CINF message can contain the following parameters:

      Parameters
      ------------------------------------------
      Subsequent Number           Mandatory
      Optional Parameters         Optional

  Note 1:

3.3.6.  Proceeding (CPRO)

     The Proceeding (CPRO) message is sent from an ASP to an SG to
  indicate that an outgoing call setup is proceeding.  The CPRO message
  is sent from an SGP to an ASP to indicate that an incoming call setup
  is proceeding.

     The CPRO message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  `Proceeding' primitive and the ITU-T and ANSI ISUP `ACM' and `CPG'
  message [Q.763, T1.113].

  The CPRO message is formatted as follows:

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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0508          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                        Proceeding Flags                       |
    +-------------------------------+-------------------------------+
    |         Tag = 0x050E          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                       Optional Parameters                     /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The CPRO message can contain the following parameters:

      Parameters
      -------------------------------------------
      Proceeding Flags            Mandatory
      Optional Parameters         Optional
                                              *1

  Note 1:

3.3.7.  Alerting (CALR)

     The Alerting (CALR) message is sent from an ASP to an SG to
  indicate that the terminating access on a incoming call setup is being
  alerted.  The CALR message is sent from an SGP to an ASP to indicate
  that the terminating access on an outgoing call setup is being
  alerted.

     The CALR message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  `Alerting' primitive and the ITU-T and ANSI `IAM' message [Q.763,
  T1.113].

  The CALR message is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x050E          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                       Optional Parameters                     /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The CALR message can contain the following parameters:

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      Parameters
      ------------------------------------------
      Optional Parameters         Optional
                                             *1

  Note 1:

3.3.8.  Progress (CPRG)

     The Progress (CPRG) message is sent from an ASP to an SG to
  indicate that an incoming call setup is in progress.  The CPRG message
  is sent from an SGP to an ASP to indicate that an outgoing call setup
  is in progress.

     The CPRG message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  `Progress' primitive and the ITU-T and ANSI ISUP `ACM' and `CPG'
  message [Q.763, T1.113].

  The CPRG message is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0509          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                        Progress Event                         |
    +-------------------------------+-------------------------------+
    |         Tag = 0x050A          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                        Progress Flags                         |
    +-------------------------------+-------------------------------+
    |         Tag = 0x050E          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                       Optional Parameters                     /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The CPRG message can contain the following parameters:

      Parameters
      -------------------------------------------
      Progress Event              Mandatory
      Progress Flags              Mandatory
      Optional Parameters         Optional
                                              *1

  Note 1:

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3.3.9.  Connect (CCON)

     The Connect (CCON) message is sent from an ASP to an SG to indicate
  that an incoming ISUP call has been connected.  The CCON message is
  sent from an SGP to an ASP to indicate that an outgoing ISUP call has
  ben connected.

     The CCON message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  Call Control `Setup' (Response and Confirmation) primitive and the
  ITU-T `ANM' and `CON' and ANSI `ANM' message [Q.763, T1.113].

  The CCON message is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x050E          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                       Optional Parameters                     /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The CCON message can contain the following parameters:

      Parameters
      ------------------------------------------
      Optional Parameters         Optional
                                             *1

  Note 1:

3.3.10.  Suspend (CSUS)

     The Suspend (CSUS) message is sent from the ASP to an SG or from
  the SGP to the ASP to indicate that an established call has been
  suspended.

     The CSUS message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  Call Control `Suspend' primitive and the ITU-T and ANSI `SUS' message
  [Q.763, T1.113].

  The CSUS message is formatted as follows:

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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x050B          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                     Suspend/Resume Flags                      |
    +-------------------------------+-------------------------------+
    |         Tag = 0x050E          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                       Optional Parameters                     /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The CSUS message can contain the following parameters:

      Parameters
      ------------------------------------------
      Suspend/Resume Flags        Mandatory
      Optional Parameters         Optional

  Note 1:

3.3.11.  Resume (CRES)

     The Resume (CRES) message is sent from the ASP to an SG or from the
  SGP to the ASP to indicate that a previously suspended established
  call has been resumed.

     The CRES message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  Call Control `Resume' primitive and the ITU-T and ANSI `RES' message
  [Q.763, T1.113].

  The CRES message is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x050B          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                     Suspend/Resume Flags                      |
    +-------------------------------+-------------------------------+
    |         Tag = 0x050E          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                       Optional Parameters                     /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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  The CRES message can contain the following parameters:

      Parameters
      ------------------------------------------
      Suspend/Resume Flags        Mandatory
      Optional Parameters         Optional

  Note 1:

3.3.12.  Reattempt (CREA)

     The Reattempt (CREA) Indication message is sent from an SGP to an
  ASP to indicate that a call attempt on a circuit should be reattempted
  on an alternate circuit.

     If the ASP selected the outgoing circuit in the corresponding CSET,
  then the ASP is responsible for selecting another circuit and issuing
  a new CSET message.  If the ASP did not select the outgoing circuit in
  the corresponding CSET message, then the SGP is responsible for
  performing an automatic reattempt on a new circuit or subsequently
  indicating call failure with a CERR message.

     The CREA message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  Call Control `Reattempt' primitive and does not correspond to an ISUP
  message.

  The CREA message is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0506          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                        Reattempt Reason                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The CREA message can contain the following parameters:

      Parameters
      ------------------------------------------
      Reattempt Reason            Mandatory

3.3.13.  Failure (CERR)

     The Failure (CERR) message is sent from an ASP to an SG to indicate
  the failure of an incoming call setup.  The CERR message is sent from
  an SGP to an ASP to indicate the failure of an outgoing call setup.

     The CERR message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  Call Failure `Call Failure' primitive and the ITU-T and ANSI `RST,'

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  `REL' and `RLC' message [Q.763, T1.113].

  The CERR message is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x050C          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                        Failure Reason                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The CERR message can contain the following parameters:

      Parameters
      -------------------------------------------
      Failure Reason              Mandatory
                                              *1

  Note 1:

3.3.14.  In Band Information (CIBI)

     The In Band Information (CIBI) message is sent from an ASP to an SG
  to indicate that in band information is now available for an incoming
  call.  The CIBI message is sent from an SGP to an ASP to indicate that
  in band information is now available for an outgoing call.

     The CIBI message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  Call Setup `In Band Information' primitive and the ITU-T and ANSI
  `ACM' and `CPG' message [Q.763, T1.113].

  The CIBI message is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x050E          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                       Optional Parameters                     /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The CIBI message can contain the following parameters:

      Parameters
      ------------------------------------------

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      Optional Parameters         Optional
                                             *1

3.3.15.  Release (CREL)

     The Release (CREL) message is sent from an ASP to an SG or from the
  SGP to an ASP to release a call during the setup or established phase.

     The CREL message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  Call Control `Release' (Request, Indication) primitive and the ITU-T
  and ANSI `REL' message [Q.763, T1.113].

  The CREL message is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x050D          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                             Cause                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The CREL message can contain the following parameters:

      Parameters
      ------------------------------------------
      Cause                       Mandatory

3.3.16.  Release Complete (CRLC)

     The Release Complete (CRLC) message is sent from an ASP to an SG or
  from an SGP to an ASP to confrim the release of a call.

     The CRLC message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  Call Control `Release' (Response, Confirmation) primitive and the ITU-
  T and ANSI `REL' and `RLC' message [Q.763, T1.113].

  The CRLC message is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x050D          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                             Cause                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The CRLC message can contain the following parameters:

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      Parameters
      ------------------------------------------
      Cause                       Mandatory

3.4.  ISUA Circuit Supervision (CS) Messaegs

     ISUA Circuit Supervision (CS) Messages are used to convey circuit
  management information to Call Control.  Theses messages correspond to
  specific RESET, BLOCKING, UNBLOCKING and CCT GROUP QUERY primitives.
  The general message format includes a Common Message Header, the ISUA
  Message Header, and the CS Message Header, together with a list of
  zero or more parameters as defined by the Message Type.  For forward
  compatibility, all Message Types MAY have optional attached parameters
  in addition to the message headers.

  These messages are ISUA Circuit Supervision (CS) Messages:

              ISUA Circuit Supervision (CS) Messages
      --------------------------------------------------------
      Message Name                     Message Type   Section
      --------------------------------------------------------
      CS Header                                       3.4.1
      Continuity Check          CCNT        6         3.4.2
      Loopback                  CLBK        7         3.4.3
      Report                    CREP        8         3.4.4
      Reset                     CRSC        1         3.4.5
      Reset Acknowledgement     CRSA        2         3.4.6
      Block                     CBLO        3         3.4.7
      Block Acknowledgement     CBLA        4         3.4.8
      Unblock                   CUBL        5         3.4.9
      Unblock Acknowledgement   CUBA        6         3.4.10
      Query                     CQRY        7         3.4.11
      Query Acknowledgement     CQRA        8         3.4.12
      --------------------------------------------------------

3.4.1.  CS Message Header

     In addition the the Common Message Header and ISUA Message Header,
  a specific message header is included for ISUA Circuit Supervision
  (CS) messages.  The CS Message Header will immediately follow the ISUA
  Message Header in these messages.

  The CS Message Header is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0520          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                           Circuit Id                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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  The CS Message Header contains the following parameters:

      Parameters
      ------------------------------------------
      Circuit Id                  Mandatory

3.4.2.  Continuity Check (CCNT)

     The Continuity Check (CCNT) message is sent from an ASP to an SGP
  to request an continuity check on a specified circuit.  The CCNT
  message is sent from an SGP to an ASP to indicate an a continuity
  check request on the specified circuit.

     The CCNT message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  Call Control `Continuity Recheck' (Request) primitive and the ITU-T
  and ANSI ISUP `CCR' message [Q.763, T1.113].

     The CCNT message has no message-type-specific parameters beyond the
  CS Message Header.

3.4.3.  Loopback (CLBK)

     The Loopback (CLBK) message is sent from an ASP to an SGP to
  indicate that a loopback has been established on the local end of the
  specified circuit.  The CLBK message is sent from an ASP to an SGP to
  indicate that a loopback has been establish on the remote end of the
  specified circuit.

     The CLBK message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  Call Control `Continuity Recheck' (Confirmation) primitive and the
  ITU-T and ANSI ISUP `LPA' message [Q.763, T1.113].

     The CLBK message has no message-type-specific parameters beyond the
  CS Message Header.

3.4.4.  Report (CREP)

     The Report (CREP) Request message is sent from an ASP to SG or from
  an SGP to an ASP to indicate the success or failure of a continuity
  test operation on the specified circuit.

     The CREP message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  `Continuity Report' primitive and the ITU-T and ANSI ISUP `COT'
  message [Q.763, T1.113].

  The CREP message is formatted as follows:

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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0507          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                         Check Result                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The CREP message can contain the following parameters:

      Parameters
      ------------------------------------------
      Check Result                Mandatory

3.4.5.  Reset (CRSC)

     The Reset (CRSC) message is sent from an ASP to an SG to request
  the reset of the specified circuit(s).  The CRSC message is sent from
  the SGP to an ASP to indicate the reset reset of the specified
  circuit(s).

     The CRSC message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  `Reset' (Request) primitive and the ITU-T and ANSI ISUP `RSC' and
  `GRS' message [Q.763, T1.113].

  The CRSC message is formatted as follows:

     0                     1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0523          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                          Circuit Range                        /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The CRSC message can contain the following parameters:

      Parameters
      ---------------------------------------------
      Circuit Range               Conditional   *1

  Note 1: When the Circuit Range parameter is included in the message,
          the CRSC message corresponds to the `GRS' message.  When the
          Circuit Range is not present in the message, the CRSC message
          corresponds to the `RSC' message.

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3.4.6.  Reset Acknowledgement (CRSA)

     The Reset Acknowledgement (CRSA) message is sent from an SGP to an
  ASP to confirm the reset of the specified circuit(s).

     The CRSA message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  `Reset' (Confirmation) primitive and the ITU-T and ANSI ISUP `RLC' and
  'GRA' message.

  The CRSA message is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0523          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                          Circuit Range                        /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The CRSA message can contain the following parameters:

      Parameters
      ---------------------------------------------
      Circuit Range               Conditional   *1

  Note 1: When the Circuit Range parameter is included in the message,
          the CRSA message corresponds to the `GRA' message and the
          Circuit Range parameter SHOULD match the corresponding
          parameter in the CRSC request message.  When the Circuit Range
          is not present in the message, the CRSA message corresponds to
          the `RLC' message.

3.4.7.  Block (CBLO)

     The CBLO Request message is sent from an ASP to an SG or IPSP to
  perform a blocking request.  The CBLO Indication message is sent from
  the SGP to an ASP to indicate the blocking indication.

     The CBLO message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  `BLOCKING' primitive and the ITU-T and ANSI `BLO' and `CGB' message.

  The CBLO message is formatted as follows:

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     0                     1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0523          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                          Circuit Range                        /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The CBLO message can contain the following parameters:

      Parameters
      ---------------------------------------------
      Circuit Range               Conditional   *1

  Note 1: When the Circuit Range parameter is included in the message,
          the CBLO message corresponds to the `CGB' message.  When the
          Circuit Range is not present in the message, the CBLO message
          corresponds to the `BLO' message.

3.4.8.  Block Acknowledgement (CBLA)

     The Block Acknowledgement (CBLA) Request message is sent from an
  ASP to an SG or IPSP to perform a blocking response.  The CBLA
  Indication message is sent from the SGP to an ASP to indicate the
  blocking confirmation.

     The CBLA message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  `BLOCKING' primitive and the ITU-T and ANSI `BLA' and `CGBA' message.

  The CBLA message is formatted as follows:

     0                     1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0523          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                          Circuit Range                        /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The CBLA message can contain the following parameters:

      Parameters
      ---------------------------------------------
      Circuit Range               Conditional   *1

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  Note 1: When the Circuit Range parameter is included in the message,
          the CBLA message corresponds to the `CGBA' message and the
          Circuit Range parameter SHOULD match the corresponding
          parameter in the CBLO request message.  When the Circuit Range
          is not present in the message, the CBLA message corresponds to
          the `BLA' message.

3.4.9.  Unblock (CUBL)

     The Unblock (CUBL) Request message is sent from an ASP to an SG or
  IPSP to perform a unblocking request.  The CUBL Indication message is
  sent from the SGP to an ASP to indicate the unblocking indication.

     The CUBL message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  `UNBLOCKING' primitive and the ITU-T and ANSI `UBL' and `CGU' message.

  The CUBL message is formatted as follows:

     0                     1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0523          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                          Circuit Range                        /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The CUBL message can contain the following parameters:

      Parameters
      ---------------------------------------------
      Circuit Range               Conditional   *1

  Note 1: When the Circuit Range parameter is included in the message,
          the CUBL message corresponds to the `CGU' message.  When the
          Circuit Range is not present in the message, the CUBL message
          corresponds to the `UBL' message.

3.4.10.  Unblock Acknowledgement (CUBA)

     The Unblock Acknowledgement (CUBA) Request message is sent from an
  ASP to an SG or IPSP to perform a unblocking response.  The CUBA
  Indication message is sent from the SGP to an ASP to indicate the
  unblocking confirmation.

     The CUBA message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  `UNBLOCKING' primitive and the ITU-T and ANSI `UBA' and `CGUA'
  message.

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  The CUBA message is formatted as follows:

     0                     1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0523          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                          Circuit Range                        /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The CUBA message can contain the following parameters:

      Parameters
      ---------------------------------------------
      Circuit Range               Conditional   *1

  Note 1: When the Circuit Range parameter is included in the message,
          the CUBA message corresponds to the `CGUA' message and the
          Circuit Range parameter SHOULD match the corresponding
          parameter in the CUBL request message.  When the Circuit Range
          is not present in the message, the CUBA message corresponds to
          the `UBA' message.

3.4.11.  Query (CQRY)

     The Query (CQRY) Request message is sent from an ASP to an SG or
  IPSP to perform a query request.  The CQRY Indication message is sent
  from the SGP to an ASP to indicate the query indication.

     The CQRY message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  `CCT GROUP QUERY' primitive and the ITU-T and ANSI `CQM' message.

  The CQRY message is formatted as follows:

     0                     1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0523          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                          Circuit Range                        /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The CQRY message can contain the following parameters:

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      Parameters
      ------------------------------------------
      Circuit Range               Mandatory   1

3.4.12.  Query Acknowledgement (CQRA)

     The Query Acknowledgement (CQRA) Request message is sent from an
  ASP to an SG or IPSP to perform a query response.  The CQRA Indication
  message is sent from the SGP to an ASP to indicate the query
  confirmation.

     The CQRA message corresponds to the ITU-T [Q.764] and ANSI [T1.113]
  `CCT GROUP QUERY' primitive and the ITU-T and ANSI `CQMA' message.

  The CQRA message is formatted as follows:

     0                     1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x05XX          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                          Circuit Status                       /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The CQRA message can contain the following parameters:

      Parameters
      -------------------------------------------
      Circuit Status              Mandatory   *1

  Note 1: The Circuit Status parameter SHOULD contain a circuit status
          for each of the circuit identifiers present in the
          corresponding CQRY message.

3.5.  Application Server Process State Maintenance (ASPSM) Messages

3.5.1.  ASP Up (UP)

     The ASP Up (UP) message is used to indicate to a remote ISUA peer
  that the Adaptation layer is up and running.

  The ASP UP message is formatted as follows:

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     0                     1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0011          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                        ASP Identifier                         |
    +-------------------------------+-------------------------------+
    |         Tag = 0x0004          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                          Info String                          /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The ASP UP message can contain the following parameters:

      Parameters
      ---------------------------------------------
      ASP Identifier              Conditional   *1
      Info String                 Optional

  Note 1: ASP Identifier MUST be used where the IPSP/SGP cannot identify
          the ASP by pre-configured address/port number information
          (e.g, where an ASP is resident on a Host using dynamic
          address/port number assignment).

3.5.2.  ASP Up Ack (UP ACK)

     The ASP Up Ack (UP ACK) message is used to acknowledge an ASP UP
  message received from a remote ISUA peer.

  The ASP UP ACK message is formatted as follows:

     0                     1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0004          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                          Info String                          /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The ASP UP ACK message can contain the following parameters:

      Parameters
      -----------------------------------------
      Info String                 Optional

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3.5.3.  ASP Down (DOWN)

     The ASP Down (DOWN) message is used to indicate to a remote ISUA
  peer that the adaptation layer is not running.

  The ASP DOWN message is formatted as follows:

     0                     1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0004          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                          Info String                          /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The ASP DOWN message can contain the following parameters:

      Parameters
      -----------------------------------------
      Info String                 Optional

3.5.4.  ASP Down Ack (DOWN ACK)

     The ASP Down Ack (DOWN ACK) message is used to acknowledge an ASP
  DOWN message received from a remote ISUA peer.

  The ASP DOWN ACK message is formatted as follows:

     0                     1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0004          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                          Info String                          /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The ASP DOWN ACK message can contain the following parameters:

      Parameters
      -----------------------------------------
      Info String                 Optional

  Note:
     The ASP DOWN ACK message will always be sent to acknowledge an ASP
     DOWN message.

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3.5.5.  Heartbeat (BEAT)

     The Heartbeat (BEAT) message is optionally used to ensure that the
  ISUA peers are still available to each other.

  The BEAT message is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0009          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                         Heartbeat Data                        /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The BEAT message can contain the following parameters:

      Parameters
      -----------------------------------------
      Heartbeat Data              Optional

3.5.6.  Heartbeat Ack (BEAT ACK)

     The Heartbeat ACK (BEAT ACK) message is sent in response to a BEAT
  message.  A peer MUST send a BEAT ACK in response to a BEAT message.
  It includes all the parameters of the received BEAT message, without
  any change.

  The BEAT ACK message is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0009          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                         Heartbeat Data                        /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The BEAT ACK message can contain the following parameters:

      Parameters
      -----------------------------------------
      Heartbeat Data              Optional

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3.6.  Application Server Process Traffic Maintenance (ASPTM) Messages

3.6.1.  ASP Active (ASPAC)

     The ASP Active (ASPAC) message is sent by an ASP to indicate to a
  remote ISUA peer that it is Active and ready to process signalling
  traffic for a particular Application Server.

  The ASPAC message is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0006          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                       Routing Context                         /
    \                                                               \
    +-------------------------------+-------------------------------+
    |         Tag = 0x000B          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                       Traffic Mode Type                       |
    +-------------------------------+-------------------------------+
    |         Tag = 0x0004          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                          Info String                          /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The ASPAC message can contain the following parameters:

      Parameters
      ---------------------------------------------
      Routing Context             Conditional   *1
      Traffic Mode Type           Optional      *2
      Info String                 Optional

  Note 1: When an ASP is registered or configured for multiple AS with
          an SG, the Routing Context associated with the AS whose
          activation is being requested MUST be placed in the ASPAC
          message.

  Note 2: The Traffic Mode Type parameter is not necessary in the ASPAC
          message when both peers are aware of the traffic mode of the
          AS by configuration or registration.

3.6.2.  ASP Active Ack (ASPAC ACK)

     The ASP Active Ack (ASPAC) Ack message is used to acknowledge an
  ASPAC message received from a remote ISUA peer.

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  The ASPAC ACK message is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0006          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                       Routing Context                         /
    \                                                               \
    +-------------------------------+-------------------------------+
    |         Tag = 0x000B          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                       Traffic Mode Type                       |
    +-------------------------------+-------------------------------+
    |         Tag = 0x0004          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                          Info String                          /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The ASPAC ACK message can contain the following parameters:

      Parameters
      ---------------------------------------------
      Routing Context             Conditional   *1
      Traffic Mode Type           Optional
      Info String                 Optional

  Note 1: When an ASP is registered or configured for multiple AS with
          an SG, the Routing Context associated with the AS whose
          activation is being acknowledged MUST be placed in the ASPAC
          ACK message.

3.6.3.  ASP Inactive (ASPIA)

     The ASP Inactive (ASPIA) message is sent by an ASP to indicate to a
  remote ISUA peer that it is no longer processing signalling traffic
  within a particular Application Server.

  The ASPIA message is formatted as follows:

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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0006          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                       Routing Context                         /
    \                                                               \
    +-------------------------------+-------------------------------+
    |         Tag = 0x0004          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                          INFO String                          /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The ASPIA message can contain the following parameters:

      Parameters
      ---------------------------------------------
      Routing Context             Conditional   *1
      INFO String                 Optional

  Note 1: When an ASP is registered or configured for multiple AS with
          an SG, the Routing Context associated with the AS whose
          deactivation is being requested MUST be placed in the ASPIA
          message.

3.6.4.  ASP Inactive Ack (ASPIA ACK)

     The ASP Inactive Ack (ASPIA ACK) message is used to acknowledge an
  ASPIA message received from a remote ISUA peer.

  The ASPIA message is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0006          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                       Routing Context                         /
    \                                                               \
    +-------------------------------+-------------------------------+
    |         Tag = 0x0004          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                          INFO String                          /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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  The ASPIA message can contain the following parameters:

      Parameters
      ---------------------------------------------
      Routing Context             Conditional   *1
      INFO String                 Optional

  Note 1: When an ASP is registered or configured for multiple AS with
          an SG, the Routing Context associated with the AS whose
          deactivation is being acknowledged MUST be placed in the ASPIA
          ACK message.

3.7.  Management (MGMT) Messages

3.7.1.  Error (ERR)

     The Error (ERR) message is used by a ISUA peer to indicate an error
  situation.  ERR messages MUST NOT be generated in response to other
  ERR messages.

  The ERR message is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x000C          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                          Error Code                           |
    +-------------------------------+-------------------------------+
    |         Tag = 0x0521          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                       Network Appearance                      |
    +-------------------------------+-------------------------------+
    |         Tag = 0x0006          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                         Routing Context                       /
    \                                                               \
    +-------------------------------+-------------------------------+
    |         Tag = 0x0520          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                           Circuit Id                          |
    +-------------------------------+-------------------------------+
    |         Tag = 0x0501          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                         Call Reference                        |
    +-------------------------------+-------------------------------+
    |         Tag = 0x0007          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                         Diagnostic Info                       /
    \                                                               \

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    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The ERR message can contain the following parameters:

      Parameters
      ---------------------------------------------
      Error Code                  Mandatory
      Routing Context             Conditional   *1
      Call Reference              Conditional   *2
      Circuit Id                  Conditional   *3
      Network Appearance          Conditional   *4
      Diagnostic Info             Conditional   *5

  Note 1: When the Error Code is "Invalid Routing Context," the Routing
          Context parameter MUST contain the invalid routing context
          value(s).

  Note 2: When the Error Code is "Call Reference Unknown," the Call
          Reference parameter MUST contain the call reference for which
          status is unknown or unauthorized.

  Note 3: When the Error Code is "Circuit Status Unknown," the Circuit
          Id parameter MUST contain the circuit for which status is
          unknown or unauthorized.

  Note 4: When the Error Code is "Invalid Network Appearance," the
          Network Appearance parameter MUST contains the invalid network
          appearance value.

  Note 5: The Diagnostic Info parameter SHOULD contain at least the
          first 40 bytes of the message that caused the ERR message to
          be sent.

3.7.2.  Notify (NTFY)

     The Notify message is used to provide an autonomous indication of
  ISUA events at an SG or IPSP to an ASP.

  The NTFY message is formatted as follows:

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     0                     1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x000D          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                           Status                              |
    +-------------------------------+-------------------------------+
    |         Tag = 0x0011          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                        ASP Identifier                         |
    +-------------------------------+-------------------------------
    |         Tag = 0x0006          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                       Routing Context                         /
    \                                                               \
    +-------------------------------+-------------------------------+
    |         Tag = 0x0004          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                          Info String                          /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The NTFY message can contain the following parameters:

      Parameters
      ---------------------------------------------
      Status                      Mandatory
      ASP Identifier              Conditional   *1
      Routing Context             Conditional   *2
      Info String                 Optional

  Note 1: ASP Identifier MUST be used where the IPSP/SGP cannot identify
          the ASP by pre-configured address/port number information
          (e.g, where an ASP is resident on a Host using dynamic
          address/port number assignment) and the Status parameter is
          set to "Alternate ASP Active" or "ASP Failure".

  Note 2: When an ASP is registered or configured for multiple AS with
          an SG, to identify the Application Server, the Routing Context
          associated with the AS whose state is being notified MUST be
          placed in the NTFY message when the Status parameter is set to
          "AS_State_Change".

3.8.  Routing Key Management (RKM) Messages

     Routing Key Management (RKM) messages are used to manage the
  Routing Keys that are used by an SG to direct traffic toward an
  Application Server.

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3.8.1.  Registration Request (REG REQ)

     The Registration Request (REG REQ) message is sent by an ASP to
  indicate to a remote ISUA peer that it wishes to register one or more
  given Routing Keys with the remote peer.  Typically, an ASP would send
  this message to an SGP, and expects to receive a REG RSP message in
  return with an associated Routing Context value.

  The REG REQ message is formatted as follows:

     0                     1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0522          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                          Routing Key 1                        /
    \                                                               \
    +-------------------------------+-------------------------------+
    \                                                               \
    /                              ...                              /
    \                                                               \
    +-------------------------------+-------------------------------+
    |         Tag = 0x0522          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                          Routing Key n                        /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The REG REQ message can contain the following parameters:

      Parameters
      -------------------------------------------
      Routing Key                 Mandatory   *1

  Note 1: One or more Routing Key parameters MAY be included in a single
          REG REQ message.  Whereas it is OPTIONAL for an implementation
          to be able to generate a REG REQ message with more than one
          Routing Key parameter, it is REQUIRED that the implementation
          be able to receive multiple Routing Key parameters in a single
          REG REQ message.

3.8.2.  Registration Response (REG RSP)

     The Registration Response (REG RSP) message is sent by an SG to an
  ASP to indicate the result of a previous REG REQ from an ASP.  When
  successful, the REG RSP message contains the Routing Context assigned
  to the one or more Routing Keys that were presented in the REG REQ
  message.

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  The REG RSP message is formatted as follows:

     0                     1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0014          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                     Registration Result 1                     /
    \                                                               \
    +-------------------------------+-------------------------------+
    \                                                               \
    /                              ...                              /
    \                                                               \
    +-------------------------------+-------------------------------+
    |         Tag = 0x0014          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                     Registration Result n                     /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The REG RSP message can contain the following parameters:

      Parameters
      -------------------------------------------
      Registration Result         Mandatory   *1

  Note 1: REG RSP message.  Whereas it is OPTIONAL for an implementation
          to be able to generate a REG RSP message with more than one
          Routing Key parameter, it is REQUIRED that the implementation
          be able to receive multiple Routing Key parameters in a single
          REG RSP message.

3.8.3.  Deregistration Request (DEREG REQ)

     The Deregistration Request (DEREG REQ) message is sent by an ASP to
  indicate to a remote ISUA peer that it wishes to deregister a given
  Routing Key as identified by the given Routing Context.  Typically, an
  ASP would send this message to an SGP, and expects to receive a DEREG
  RSP message in return with the associated Routing Context value.

  The DEREG REQ message is formatted as follows:

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     0                     1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0006          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                        Routing Context                        /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The DEREG REQ message contains the following parameters:

      Parameters
      -------------------------------------------
      Routing Context             Mandatory   *1

  Note 1: One or more Routing Context values MAY be included in the
          Routing Context parameter.  Whereas it is OPTIONAL for an
          implementation to be able to generate a DEREG REQ message with
          multiple Routing Context values in the Routing Context
          parameter, it is REQUIRED that an implementation be able to
          receive multiple Routing Context values in the Routing Context
          parameter of the DEREG REQ message.

3.8.4.  Deregistration Response (DEREG RSP)

     The Deregistration Response (DEREG RSP) message is used as a
  response to the DEREG REQ message from a remote ISUA peer.

  The DEREG REQ message is formatted as follows:

     0                     1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0015          |            Length = 12        |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                   Deregistration Result 1                     /
    \                                                               \
    +-------------------------------+-------------------------------+
    \                                                               \
    /                              ...                              /
    \                                                               \
    +-------------------------------+-------------------------------+
    |         Tag = 0x0015          |            Length = 12        |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                   Deregistration Result n                     /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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  The DEREG REQ message contains the following parameters:

      Parameters
      -------------------------------------------
      Deregistration Result       Mandatory   *1

  Note 1: One or more Deregistration Result parameters MAY be included
          in one DEREG RSP message.  Whereas it is OPTIONAL for an
          implementation to be able to generate a DEREG RSP message with
          multiple Deregistration Result parameters, it is REQUIRED that
          an implementation be able to receive multiple Deregistration
          Result parameters in a single DEREG RSP message.

3.9.  Common Parameters

  These TLV parameters are common across the different adaptation
  layers:

      Parameter Name                Parameter ID   Section
      -----------------------------------------------------
      Reserved                         0x0000         -
      Not used in ISUA                 0x0001         -
      Not used in ISUA                 0x0002         -
      Not used in ISUA                 0x0003         -
      Info String                      0x0004      3.9.1
      Not used in ISUA                 0x0005         -
      Routing Context                  0x0006      3.9.2
      Diagnostic Info                  0x0007      3.9.3
      Not used in ISUA                 0x0008         -
      Heartbeat Data                   0x0009      3.9.4
      Not used in ISUA                 0x000A         -
      Traffic Mode Type                0x000B      3.9.5
      Error Code                       0x000C      3.9.6
      Status                           0x000D      3.9.7
      Not used in ISUA                 0x000E         -
      Not used in ISUA                 0x000F         -
      Not used in ISUA                 0x0010         -
      ASP Identifier                   0x0011      3.9.8
      Not used in ISUA                 0x0012         -
      Correlation Id                   0x0013      3.9.9
      Registration Result              0x0014      3.9.10
      Deregistration Result            0x0015      3.9.11
      Registration Status              0x0016      3.9.12
      Deregistration Status            0x0017      3.9.13
      Local Routing Key Identifier     0x0018      3.9.14

3.9.1.  Info String

     The Info String parameter is optionally included in all MGMT, ASPSM
  and ASPTM messages to provide additional debugging or diagnostic
  information.

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  The Info String parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0004          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                          Info String                          /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Info String parameter contains the following fields:

  Info String field: variable (ASCII string)

    The Info String field can carry any meaningful UTF-8 [RFC 2279]
    character string along with the message.  Length of the Info String
    field is from 0 to 255 characters.  No procedures are presently
    identified for its use but implementations may use the Info String
    for debugging purposes.

3.9.2.  Routing Context

     The Routing Context parameter is included in all ISUA CP and CS
  messages as well as in MGMT, ASPTM, ASPSM that reference one or more
  Application Servers.  The Routing Context parameter is used to
  uniquely identify an Application Server and Routing Key within an
  association between an SGP and ASP.

  The Routing Context parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0006          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                      Routing Context(s)                       /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Routing Context parameter can contain the following fields:

  Routing Context field: list of 32-bit (unsigned integer)

    The Routing Context field contains (a list of) 32-bit unsigned
    integers indexing the Application Server traffic that the sending
    ASP is configured or registered to receive.  There is one-to-one
    relationship between a Routing Context value, an SG Routing Key and
    an Application Server [3].  If the Routing Context parameter is
    present, it SHOULD be the first parameter in the message as it

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    defines the format and/or interpretation of the parameters
    containing a PC or SSN value.

3.9.3.  Diagnostic Information

     The Diagnostic Info parameter is used in the MGMT )Error (ERR)
  message to provide additional information concerning the message that
  generated an ERR message reply.  The Diagnostic Info parameter SHOULD
  contain at least the first 40 bytes of the message that generated the
  error.

  The Diagnostic Info parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0007          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                         Diagnostic Info                       /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Diagnostic Info parameter contains the following fields:

  Diagnostic Info field: variable length (bytes)

    The Diagnostic Info field can contain any information germane to the
    error condition, to assist in the identification of the error
    condition.  The Diagnostic Info SHOULD be the first 40 bytes of the
    offending message.

3.9.4.  Heartbeat Data

     The Heartbeat Data parameter is used in the BEAT and BEAT ACK
  messages and contains whatever information the sender of the BEAT
  message chooses to include.  Some uses for the Heartbeat Data
  parameter are described in Section 4.

  The Heartbeat Data parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0009          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                         Heartbeat Data                        /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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  The Heartbeat Data parameter contains the following fields:

  Heartbeat Data field: variable length (opaque)

    The sending node defines the Heartbeat Data field contents.  It may
    include a Heartbeat Sequence Number or Time-stamp, or other
    implementation specific details.  The receiver of a Heartbeat (BEAT)
    message does not process this field as it is only of significance to
    the sender.  The receiver MUST echo the content of the Heartbeat
    Data in a BEAT ACK message.  The data field can be used to store
    information in the Heartbeat (BEAT) message useful to the sending
    node (e.g. the data field can contain a time stamp, a sequence
    number, etc.).

3.9.5.  Traffic Mode Type

     The Traffic Mode Type parameter indicates the fail-over and traffic
  distribution algorithm and procedures that will be used for an
  Application Server Process serving an Application Server.  Each
  Application Server has associated with it only one Traffic Mode Type.

  The Traffic Mode Type parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x000B          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                       Traffic Mode Type                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Traffic Mode Type parameter contains the following fields:

  Traffic Mode Type field: 32-bits (unsigned integer)

    The Traffic Mode Type field identifies the traffic mode of operation
    of an ASP within an AS.  The valid values for the Traffic Mode Type
    field are as follows:

        1   Override
        2   Load-share
        3   Broadcast

    Within a Routing Context, Override, Load-share Types and Broadcast
    cannot be mixed.  The Override value indicates that the ASP is
    operating in Override mode, and that when the ASP becomes active for
    the Application Server, it will take over all traffic for the AS
    (i.e, primary/back-up operation), overriding any currently active
    ASP in the AS.  In Load-share mode, when the ASP becomes active for
    the AS, the ASP will share in the traffic distribution with any
    other active ASPs.  In Broadcast mode, when the ASP becomes active

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    for the AS, the ASP will receive the same traffic as any other
    active ASPs.

3.9.6.  Error Code

     The Error Code parameter is used in the Error (ERR) message to
  indicate the reason that the ERR message was generated and, along with
  the other parameters in the ERR message, help to locate the problem
  that generated the error condition.

  The Error Code parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x000C          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                          Error Code                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Error Code parameter contains the following fields:

  Error Code field: 32-bit (unsigned integer)

    The Error Code field indicates the reason for the Error Message.
    The Error Code field value can be one of the following values:

         1   Invalid Version
         3   Unsupported Message Class
         4   Unsupported Message Type
         5   Unsupported Traffic Handling Mode
         6   Unexpected Message
         7   Protocol Error
         9   Invalid Stream Identifier
        13   Refused - Management Blocking
        14   ASP Identifier Required
        15   Invalid ASP Identifier
        17   Invalid Parameter Value
        18   Parameter Field Error
        19   Unexpected Parameter
        21   Invalid Network Appearance
        22   Missing Parameter
        23   Routing Key Change Refused
        25   Invalid Routing Context
        26   No Configured AS for ASP
        34   Circuit Status Unknown
        35   Call Reference Status Unknown

     The "Invalid Version" error is sent if a message was received with
  an invalid or unsupported version.  The ERR message contains the
  supported version in the Common header.  The ERR message could

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  optionally provide the supported version in the Diagnostic parameter.

     The "Unsupported Message Class" error is sent if a message with an
  unexpected or unsupported Message Class is received.

     The "Unsupported Message Type" error is sent if a message with an
  unexpected or unsupported Message Type is received.

     The "Unsupported Traffic Handling Mode" error is sent by a SGP if
  an ASP sends an ASP Active (ASPAC) message with an unsupported Traffic
  Mode Type or a Traffic Mode Type that is inconsistent with the
  presently configured mode for the Application Server.  An example
  would be a case in which the SGP did not support load-sharing.

     The "Unexpected Message" error MAY be sent if a defined and
  recognized message is received that is not expected in the current
  state (in some cases the ASP may optionally silently discard the
  message and not send an ERR message).  For example, silent discard is
  used by an ASP if it received a ISUA CP message from an SGP while it
  was in the ASP-INACTIVE state.  If the Unexpected message contained
  Routing Context(s), the Routing Context(s) SHOULD be included in the
  ERR message.

     The "Protocol Error" error is sent for any protocol anomaly (i.e.,
  reception of a parameter that is syntactically correct but unexpected
  in the current situation.

     The "Invalid Stream Identifier" error is sent if a message is
  received on an unexpected SCTP stream (e.g, a Management message was
  received on a stream other than "0", or a ISUA CP message was received
  on stream "0").

     The "Refused - Management Blocking" error is sent when an ASP Up
  (ASPUP) or ASP Active (ASPAC) message is received and the request is
  refused for management reasons (e.g, management lockout").  If this
  error is in response to an ASP Active (ASPAC) message, the Routing
  Context(s) in the ASP Active (ASPAC) message SHOULD be included in the
  ERR message.

     The "ASP Identifier Required" is sent by a SGP in response to an
  ASP Up (ASPUP) message which does not contain an ASP Identifier
  parameter when the SGP requires one.  The ASP SHOULD resend the ASP Up
  (ASPUP) message with an ASP Identifier.

     The "Invalid ASP Identifier" is send by a SGP in response to an ASP
  Up (ASPUP) message with an invalid (i.e., non-unique) ASP Identifier.

     The "Invalid Parameter Value" error is sent if a message is
  received with an invalid parameter value (e.g, a DUPU message was
  received with a Mask value other than "0").

     The "Parameter Field Error" would be sent if a message is received
  with a parameter having a wrong length field.

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     The "Unexpected Parameter" error would be sent if a message
  contains an invalid parameter.

     The "Invalid Network Appearance" error is sent by a SGP if an ASP
  sends a message with an invalid (not configured) Network Appearance
  value.  For this error, the invalid (not configured) Network
  Appearance MUST be included in the Network Appearance parameter in the
  ERR message.

     The "Missing Parameter" error is sent if a mandatory parameter was
  not included in a message.

     The "Routing Key Change Refused" error is sent when an SG refuses a
  change in the Routing Key parameters.

     The "Invalid Routing Context" error is sent if a message is
  received from a peer with an invalid (not configured) Routing Context
  value, or if a message is received from a peer without a Routing
  Context parameter and it is not known by configuration data which
  Application Servers are referenced.  For this error, the invalid
  Routing Context(s) MUST be included in the ERR message.

     The "No Configured AS for ASP" error is sent if a message is
  received from a peer without a Routing Context parameter and it is not
  known by configuration data which Application Servers are referenced.

     The "Circuit Status Unknown" Error MAY be sent it a CQRY is receive
  at an SG inquiring of the status of a circuit or circuits and the SG
  does not wish to provide the status (e.g. the sender is not authorized
  to know the status).  For this error, the invalid or unauthorized
  Circuit Id MUST be included along with any Network Appearance or
  Routing Context associated with the Circuit Id from the CQRY message.

     The "Call Reference Status Unknown" Error MAY be sent it a CQRY is
  receive at an SG inquiring of the status of a circuit or circuits and
  the SG does not wish to provide the status (e.g. the sender is not
  authorized to know the status).  For this error, the invalid or
  unauthorized Call ReferenceFR MUST be included along with any Network
  Appearance or Routing Context associated with the Call Reference from
  the CQRY message.

3.9.7.  Status

     The Status parameter identifies the type of the status that is
  being notified in a Notify (NTFY) message and the Status ID.

  The Status parameter is formatted as follows:

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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x000D          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |         Status Type           |            Status ID          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Status parameter contains the following fields:

  Status Type field: 16-bits (unsigned integer)

    The valid values for Status Type field are as follows:

        1   Application Server state change (AS_State_Change)
        2   Other

  Status ID field: 16-bits (unsigned integer)

    The Status ID parameter contains more detailed information for the
    notification, based on the value of the Status Type.

    (1)   If the Status Type is "AS_State_Change", then the Status ID
          values are as follows:

              1   reserved
              2   Application Server Inactive (AS-Inactive)
              3   Application Server Active (AS-Active)
              4   Application Server Pending (AS-Pending)

     These notifications are sent from an SGP to an ASP upon a change in
     status of a particular Application Server.  The value reflects the
     new state of the Application Server.

    (2)   If the Status Type is "Other", then the following Status
          Information values are defined:

              1   Insufficient ASP resources active in AS
              2   Alternate ASP Active
              3   ASP failure

     These notifications are not based on the SGP reporting the state
     change of an ASP or AS.  In the Insufficient ASP Resources case,
     the SGP is indicating to an "Inactive" ASP(s) in the AS that
     another ASP is required to handle the load of the AS (Load-sharing
     mode or Broadcast mode).  For the Alternate ASP Active case, an ASP
     is informed when an alternate ASP transitions to the ASP-Active
     state in Override mode.

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3.9.8.  ASP Identifier

  The ASP Identifier parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0011          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                        ASP Identifier                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The ASP Identifier parameter contains the following fields:

  ASP Identifier field: 32-bits (unsigned integer)

    The ASP Identifier field contains a unique value that is locally
    significant among the ASPs that support an AS.  The SGP should save
    the ASP Identifier to be used, if necessary, with the Notify (NTFY)
    message (see Section 3.7.2).

3.9.9.  Correlation Id

     The Correlation Id parameter is used to tag messages sent to an ASP
  in a Broadcast group as well as during fail-over.

  The Correlation Id parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0013          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                         Correlation Id                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Correlation Id parameter can contain the following fields:

  Correlation Id field: 32-bits (unsigned integer)

    The Correlation Id field contains a Correlation Id.  The Correlation
    Id is a 32-bit identifier that is attached to the ISUA Message
    Header to indicate to a newly entering ASP in a Broadcast AS where
    in the traffic flow of ISUA messages the ASP is joining.  It is
    attached to the ISUA Message Header of the first CP message sent to
    an ASP by an SG after sending an ASP Active Ack or otherwise
    starting traffic to an ASP.  The Correlation Id is only significant
    within a Routing Context [4].

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3.9.10.  Registration Result

     The Registration Result parameter is used to indicate the result of
  a successful or unsuccessful registration operation for a specific
  Routing Key.

  The Registration Result parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0014          |             Length            |
    +-------------------------------+-------------------------------+
    |                  Local Routing Key Identifier                 |
    +---------------------------------------------------------------+
    |                       Registration Status                     |
    +---------------------------------------------------------------+
    |                         Routing Context                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Registration Result parameter can contain the following fields:

  Local Routing Key Identifier: TLV

    The Local Routing Key Identifier field is mandatory in the
    Registration Result parameter.  The Local Routing Key Identifier
    field contains the same TLV formatted parameter value as found in
    the corresponding Routing Key parameter in the Registration Request
    (REG REQ) message.

  Registration Status: TLV

    The Registration Status field is mandatory in the Registration
    Result parameter.  The Registration Status field indicates the
    success or reason for failure of the corresponding registration
    request.  For details on the format of the Registration Status
    parameter, see Section 3.9.12.

  Routing Context: TLV

    The Routing Context field is mandatory in the Registration Result
    parameter.  The Routing Context field contains the TLV formatted
    Routing Context parameter for the associated Routing Key if the
    registration was successful.  If the registration was not
    successful, it is set to zero (0).

3.9.11.  Deregistration Result

     The Deregistration Result parameter is used to indicate the result
  of a successful or unsuccessful deregistration operation for a
  specific Routing Key.

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  The Deregistration Result parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0015          |             Length            |
    +-------------------------------+-------------------------------+
    |                         Routing Context                       |
    +---------------------------------------------------------------+
    |                      Deregistration Status                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Deregistration Result parameter can contain the following fields:

  Routing Context: TLV

    The Routing Context field is mandatory in the Deregistration Result
    parameter.  The Routing Context field contains the same TLV
    formatted Routing Context parameter as found in the corresponding
    Deregistration Request (DEREG REQ) message.

  Deregistration Status: TLV

    The Deregistration Status field is mandatory in the Deregistration
    Result parameter.  The Deregistration Status field indicates the
    success or reason for failure of the corresponding deregistration
    request.  For details on the format of the Deregistration Status
    parameter, see Section 3.9.13.

3.9.12.  Registration Status

     The Registration Status parameter is used to indicate the success
  or failure of a registration operation.

  The Registration Status parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0016          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                       Registration Status                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Registration Status parameter can contain the following fields:

  Registration Status: 32-bits (unsigned integer)

    The Registration Status field indicates the success or the reason
    for failure of a registration request.

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    Its values can be:

         0   Successfully Registered
         1   Error - Unknown
         2   Error - Invalid Circuit Identifier
         3   Error - Invalid Network Appearance
         4   Error - Invalid Routing Key
         5   Error - Permission Denied
         6   Error - Cannot Support Unique Routing
         7   Error - Routing Key not Currently Provisioned
         8   Error - Insufficient Resources
         9   Error - Unsupported RK parameter Field
        10   Error - Unsupported/Invalid Traffic Mode Type
        11   Error - Routing Context Registration Refused

3.9.13.  Deregistration Status

     The Deregistration Status parameter is used to indicate the success
  or failure of a deregistration operation.

  The Deregistration Status parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0017          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                      Deregistration Status                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Deregistration Status parameter can contain the following fields:

  Deregistration Status: 32-bits (unsigned integer)

    The Deregistration Status field indicates the success or the reason
    for failure of a deregistration request.

    Its values can be:

        0   Successfully Deregistered
        1   Error - Unknown
        2   Error - Invalid Routing Context
        3   Error - Permission Denied
        4   Error - Not Registered
        5   Error - ASP Currently Active for Routing Context

3.9.14.  Local Routing Key Identifier

     The Local Routing Key Identifier parameter is used for correlating
  the Routing Key parameter in a specific Registration Request (REG REQ)

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  message with the Registration Result parameter in the corresponding
  Registration Response (REG RSP) message.

  The Local Routing Key Identifier parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0018          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                  Local Routing Key Identifier                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Local Routing Key Identifier parameter can contain the following
  fields:

  Local Routing Key Identifier: 32-bits (unsigned integer)

    The Local Routing Key Identifier value is assigned by the ASP and is
    used to correlate the response in a Registration Response (REG RSP)
    message with the original registration request from the Registration
    Request (REG REQ) message.  The Local Routing Key Identifier value
    must remain unique until the REG RSP message is received.

3.10.  ISUA-Specific parameters

  These TLV parameters are specific to the ISUA protocol:

               Parameters used in CP Messages
        ----------------------------------------------
        Parameter Name        Parameter ID   Section
        ----------------------------------------------
        Call Reference           0x0501     3.10.1.1
        Call Type                0x0502     3.10.1.2
        Call Flags               0x0503     3.10.1.3
        Called Party Number      0x0504     3.10.1.4
        Subsequent Number        0x0505     3.10.1.5
        Reattempt Reason         0x0506     3.10.1.6
        Check Result             0x0507     3.10.1.7
        Proceeding Flags         0x0508     3.10.1.8
        Progress Event           0x0509     3.10.1.9
        Progress Flags           0x050A     3.10.1.10
        Suspend/Resume Flags     0x050B     3.10.1.11
        Failure Reason           0x050C     3.10.1.12
        Cause                    0x050D     3.10.1.13
        Optional Parameters      0x050E     3.10.1.14
        ----------------------------------------------

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            Parameters used in CS Messages
        ---------------------------------------
        Parameter Name  Parameter ID  Section
        ---------------------------------------
        Circuit Status     0x0510     3.10.2.1
        ---------------------------------------

                     Other Parameters
        -------------------------------------------
        Parameter Name      Parameter ID  Section
        -------------------------------------------
        Circuit Id             0x0520     3.10.3.1
        Network Appearance     0x0521     3.10.3.2
        Routing Key            0x0522     3.10.3.3
        Circuit Range          0x0523     3.10.3.4
        Local Point Code       0x0524     3.10.3.5
        Remote Point Code      0x0525     3.10.3.5
        -------------------------------------------

3.10.1.  Parameters used in CP Messages

3.10.1.1.  Call Reference

     The Call Reference parameter is used in the ISUA Message Header to
  identify the call within the Application Server indicated by the
  Routing Context (also in the ISUA Message Header).

  The Call Reference parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0501          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                         Call Reference                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Call Reference parameter contains the following fields:

  Call Reference field: 32-bits (unsigned integer)

    The Call Reference field contains an identifier that is used both at
    the SG and the ASP to identify a call within an Application Server.
    The Call Reference value must be unique within the scope of a given
    Application Server and Routing Context.

    For a given AS and Routing Context, either the SG or the ASP is
    responsible for assigning Call Reference, but not both.

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3.10.1.2.  Call Type

  The Call Type parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0502          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                           Call Type                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Call Type parameter contains the following fields:

  Call Type field: 32-bits (unsigned integer)

    The Call Type field can take on the following values:

        0   Speech
        1   64 kbit/s unrestricted digital information
        2   3.1 kHZ audio
        3   64 kbit/s preferred
        4   2 x 64 kbit/s unrestricted digital information
        5   284 kbit/s unrestricted digital information
        6   1536 kbit/s unrestricted digital information
        7   1920 kbit/s unrestricted digital information

3.10.1.3.  Call Flags

  The Call Flags parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0503          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                           Call Flags                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Call Flags parameter contains the following fields:

  Call Flags field: 32-bits (bit field)

    The Call Flags field consists of the following fields:

    Satellite Indicator: 2-bits (bits 30-31)

      The Satellite Indicator field corresponds to the Nature of Address
      Indicators of ITU-T ISUP [Q.763] and indicate the number of
      satellites present in the ISUP connection.  The Satellite

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      Indicator field can take on the following values:

          0   no satellite in circuit
          1   one satellite in circuit
          2   two satellites in circuit
          3   (reserved)

    Continuity Check Indicator: 2-bits (bits 28-29)

      The Continuity Check Indicator field corresponds to the Nature of
      Address Indicators of ITU-T ISUP [Q.763] and indicates whether a
      continuity check is required on the circuit, whether a check has
      previously been performed, or which a check is not required on the
      circuit.  The Continuity Check Indicator field can take on the
      following values:

          0   no continuity check required
          1   continuity check performed on previous circuit
          2   continuity check required
          3   (reserved)

    Outgoing Half Echo Control Device: 1-bit (bit 27)

      The Outgoing Half Echo Control Device field corresponds to the
      Nature of Address Indicator of ITU-T ISUP [Q.763] and indicates
      whether an outgoing half echo control device is included on the
      circuit.  The Outgoing Half Echo Control Device field can take on
      the following values:

          0   no outgoing half echo control device included
          1   outgoing half echo control device included

    International/National: 1-bit (bit 26)

      The Internation/National field corresponsds to the Forward Call
      Indicators of ITU-T ISUP [Q.763] and indicates whether the call is
      an International or National call.  The International/National
      field can take on the following values:

          0   National call
          1   International call

    End to End Method: 2-bits (bit 24-25)

      The End to End Method field corresponds to the Forward Call
      Indicators of ITU-T ISUP [Q.763] and indicates which end to end
      methods are available.  The End to End Method field can take on
      the following values:

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          0   link by link method only
          1   pass along method available
          2   SCCP end to end method available
          3   both methods available

    Interworking Encountered: 1-bit (bit 23)

      The Interworking Encountered field corresponds to the Forward Call
      Indicators of ITU-T ISUP [Q.763] and indicates whether
      interworking was encountered on the call.  The Interworking
      Encountered field can take on the following values:

          0   no interworking encountered
          1   interworking encountered

    End to End Information Available: 1-bit (bit 22)

      The End to End Information Available field corresponds to the
      Forward Call Indicators of ITU-T ISUP [Q.763] and indicates
      whether end to end information is now available.  The End to End
      Information Available field can take on the following values:

          0   no end to end information available
          1   end to end information available

    ISUP All the Way: 1-bit (bit 21)

      The ISUP All the Way field corresponds to the Forward Call
      Indicators of the ITU-T ISUP [Q.763] and indicates whether ISDN
      User Part is used all the way.  The ISUP All the Way field can
      take on the following values:

          0   ISDN User Part not used all the way
          1   ISDN User Part used all the way

    Originating Access ISDN: 1-bit (bit 20)

      The Originating Access ISDN field corresponds to the Forward Call
      Indicators of the ITU-T ISUP [Q.763] and indicates whether the
      originating access is ISDN.  The Originating Access ISDN field can
      take on the following values:

          0   originating access is not ISDN
          1   originating access is ISDN

    SCCP Methods Available: 2-bits (bit 18-19)

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      The SCCP Methods Available field corresponds to the Forward Call
      Indicators of the ITU-T ISUP [Q.763] and indicates the SCCP method
      available.  The SCCP Methods Available field can take on the
      following values:

          0   no SCCP method available
          1   connectionless SCCP method available
          2   connection oriented SCCP method available
          3   both methods available

3.10.1.4.  Called Party Number

  The Called Party Number parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0504          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                       Called Party Number                     /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Called Party Number parameter contains the following fields:

  Called Party Number field: 32-bits (unsigned integer)

3.10.1.5.  Subsequent Number

  The Subsequent Number parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0505          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                        Subsequent Number                      /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Subsequent Number parameter contains the following fields:

  Subsequent Number field: 32-bits (unsigned integer)

3.10.1.6.  Reattempt Reason

  The Reattempt Reason parameter is formatted as follows:

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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0506          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                        Reattempt Reason                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Reattempt Reason parameter contains the following fields:

  Reattempt Reason field: 32-bits (unsigned integer)

    The Reattempt Reason field indicates the reason that a call
    reattempt is indicated.  The Reattempt Reason field can take on one
    of the following values:

        1   dual sizeure
        2   reset
        3   blocking
        4   T24 timeout
        5   unexpected message
        6   continuity check failure
            all other values reserved

    The Reattempt Reason values are interpreted as follows:

    The "dual sizeure" reason indicates that the selected circuit was
      siezed by a controlling exchange during the initial setup of the
      call (i.e. before any backward message was received).

    The "reset" reason indicates that the selected circuit was reset
      during the initial setup of the call (i.e.  before any backward
      message was received).

    The "blocking" reason indicates that the selected circuit was
      blocked during the initial setup of the call (i.e.  before any
      backward message was received).

    The "T24 timeout" reason indicates that continuity check failure
      occured due to timeout on the selected circuit.

    The "unexpected message" reason indicates that an unexpected
      messagew as received for the call during the initial setup of the
      call (i.e. before any backward message was received).

    The "continuity check failure" reason indicates that continuity
      check failed on the selected circuit.

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3.10.1.7.  Check Result

  The Check Result parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0507          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                          Check Result                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Check Result parameter contains the following fields:

  Check Result field: 32-bits (unsigned integer)

    The Check Result field indicates the success of failure of the
    continuity check.  The Check Result field can take on one of the
    following values:

        0   continuity check failed
        1   continuity check successful

3.10.1.8.  Proceeding Flags

  The Proceeding Flags parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0509          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                        Proceeding Flags                       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Proceeding Flags parameter contains the following fields:

  Proceeding Flags field: 32-bits (bit field)

    The Proceeding Flags field contains the following bit fields:

    Charge: 2-bits (bit 30-31)

      The Charge field corresponds to the Backwards Call Indicators of
      ITU-T ISUP [Q.763] and indicates whether the call is to be
      charged.  The Charge field can take on one of the following
      values:

          1   charge

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          2   no charge
              all other values reserved

    Free: 2-bits (bit 28-29)

      The Free field corresponds to the Backwards Call Indicators of
      ITU-T ISUP [Q.763] and indicates whether the call is subscriber
      free or connection free.  The Free field can take on one of the
      following values:

          0   no indication
          1   subscriber free
          2   connection free
              all other values reserved

    Payphone: 2-bits (bit 26-27)

      The Payphone field corresponds to the Backwards Call Indicators of
      ITU-T ISUP [Q.763] and indicates whether the call has terminated
      to an ordinary subscriber or a payphone.  The Payphone field can
      take on one of the following values:

          1   ordinary subscriber
          2   payphone
              all other values reserved

    End to End Method Available: 2-bits (bit 24-25)

      The End to End Method Available field corresponds to the Backwards
      Call Indicators of ITU-T ISUP [Q.763] and indicates which end to
      end methods are available.  The End to End Method Available field
      can take on one of the following values:

          0   link by link method available
          1   pass along method available
          2   SCCP method available
          3   all methods available

    Interworking Encountered: 1-bit (bit 23)

      The Interworking Encountered field corresponds to the Backwards
      Call Indicators of ITU-T ISUP [Q.763] and indicates whether
      interworking was encountered on the call.  The Interworking
      Encountered field can take on one of the following values:

          0   no interworking encountered
          1   interworking encountered

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    End to End Information Available: 1-bit (bit 22)

      The End to End Information Available field corresponds to the
      Backwards Call Indicators of ITU-T ISUP [Q.763] and indicates
      whether end to end information is available.  The End to End
      Information Available field can take on one of the following
      values:

          0   no end to end information available
          1   end to end information available

    ISUP All the Way: 1-bit (bit 21)

      The ISUP All the Way field corresponds to the Backwards Call
      Indicators of ITU-T ISUP [Q.763] and indicates whether ISDN User
      Part was used all the way.  The ISUP All the Way field can take on
      one of the following values:

          0   ISDN user part not used all the way
          1   ISDN user part used all the way

    Holding Requested: 1-bit (bit 20)

      The Holding Requested field corresponds to the Backwards Call
      Indicators of ITU-T ISUP [Q.763] and indicates whether holding was
      requested.  The Holding Requested field can take on one of the
      following values:

          0   holding not requested
          1   holding requested

    Terminating Access ISDN: 1-bit (bit 19)

      The Terminating Access ISDN field corresponds to the Backwards
      Call Indicators of ITU-T ISUP [Q.763] and indicates whether the
      terminating access is ISDN.  The Terminating Access ISDN field can
      take on one of the following values:

          0   terminating access not ISDN
          1   terminating access ISDN

    Incoming Half Echo Control Device: 1-bit (bit 18)

      The Incoming Half Echo Control Device field corresponds to the
      Backwards Call Indicators of ITU-T ISUP [Q.763] and indicates
      whether an incoming half echo control device has been included on
      the call.  The Incoming Half Echo Control Device field can take on
      one of the following values:

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          0   no incoming half echo control device
          1   incoming half echo control device

    SCCP Methods Available: 2-bits (bit 16-17)

      The SCCP Methods Available field corresponds to the Backwards Call
      Indicators of ITU-T ISUP [Q.763] and indicates the SCCP methods
      available.  The SCCP Methods Available field can take on one of
      the following values:

          0   no SCCP method available
          1   connectionless SCCP method available
          2   connection oriented SCCP method available
          3   both SCCP methods available

3.10.1.9.  Progress Event

  The Progress Event parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0509          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                         Progress Event                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Progress Event parameter contains the following fields:

  Progress Event field: 32-bits (unsigned integer)

    The Progress Event field indicates the progress event associated
    with the call.  The Progress Event field can take on one of the
    following values:

        1   alerting
        2   progress
        3   in band information
        4   call forwarded on busy
        5   call forwarded on no answer
        6   call forwarded unconditional
            all other values reserved

    The Progress Event values are interpreted as follows:

    The "alerting" event indicates that the called party is being
      alerted.  This event is indicated only if a CPRO message has
      already been received.

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    The "progress" event indicates that the call is progressing with the
      specified optional parameters.

    The "in band information" event is indicated only via the CIBI
      message and MUST NOT be indicated in the CPRG message.

    The "call forwarded on busy" event indicates that the call has been
      forwarded on busy and the optional parameters (if any) in the
      message contain the attributes of the forwarding (e.g. redirecting
      number).

    The "call forwarded on no answer" event indicates that the call has
      been forwarded on no answer and the optional parameters (if any)
      in the message contain the attributes of the forwarding (e.g.
      redirecting number).

    The "call forwarded unconditional" event indicates that the call has
      been forwarded unconditionally and the optional parameters (if
      any) in the message contain the attributes of the forwarding (e.g.
      redirecting number).

3.10.1.10.  Progress Flags

  The Progress Flags parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x050A          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                         Progress Flags                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Progress Flags parameter contains the following fields:

  Progress Flags field: 32-bits (bit field)

    The Progress Flags field contains the following bit fields:

    Presentation Restricted: 1-bit (bit 31)

      The Presentation Restricted field indicates whether the event (and
      any associated optional parameters, such as redirecting number) is
      presentation restricted.  The Presentation Restricted field can
      take on the following values:

          0   event presentation allowed
          1   event presentation restricted

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3.10.1.11.  Suspend/Resume Flags

  The Suspend/Resume Flags parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x050B          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                      Suspend/Resume Flags                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Suspend/Resume Flags parameter contains the following fields:

  Suspend/Resume Flags field: 32-bits (bit field)

    The Suspend/Resume flags field contains the following bit fields:

    Network Initiated: 1-bit (bit 31)

      The Network Initiated field indicates whether the suspend or
      resume operation was user or network initiated.  The Network
      Initiated field can take on the following values:

          0   user initiated
          1   network initiated

3.10.1.12.  Failure Reason

  The Failure Reason parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x050C          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                         Failure Reason                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Failure Reason parameter contains the following fields:

  Failure Reason field: 32-bits (unsigned integer)

    The Failure Reason indicates the reason for call setup failure and
    can take on the following values:

        1   continuity check failure
        2   received release complete

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        3   blocking
        4   T6 timeout
        5   T7 timeout
        6   T8 timeout
        7   T9 timeout
        8   T35 timeout
        9   T38 timeout
            all other values reserved

    The values of the Failure Reason field are interpreted as follows:

    The "continuity check failure" reason indicates that continuity
      check on the circuit failed.  The applies to incoming calls only.

    The "received release complete" reason indicates that the selected
      circuit was not completely released by the distant end.  The
      applies to incoming calls only.

    The "blocking" reason indicates that the circuit was blocked during
      call setup.  The applies to incoming calls only.

    The "T6 timeout" reason indicates that the call was suspended beyond
      the allowable period.  The applies to all established calls.

    The "T7 timeout" reason indicates that there was no response to the
      call setup request.  The applies to outgoing calls only.

    The "T8 timeout" reason indicates that the call failed waiting for a
      continuity check report from the distant end.  The applies to
      incoming calls only.

    The "T9 timeout" reason indicates that the call failed while waiting
      for the distant end to answer.  The applies to outgoing calls
      only.

    The "T35 timeout" reason indicates that additional information
      (digits) were not received from the caller within a sufficient
      period.  The applies to incoming calls only.

    The "T38 timeout" reason indicates that the call was suspended
      beyond the allowable period.  The applies to all established
      calls.

3.10.1.13.  Cause

  The Cause parameter is formatted as follows:

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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x050D          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                             Cause                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Cause parameter contains the following fields:

  Cause field: 32-bits (unsigned integer)

    The Cause field indicates the reason for call release and can take
    on the following values:

          ITU-T                            ANSI
    --------------------------------------------------------------------
      1   unalloc. no.
      2   no route to transit ntwk
      3   no route to dest
      4   send special info tone
      5   misdialled trunk prefix
      8   preemption
      9   preemption cc't reserved
     16   normal call clearing
     17   user busy
     18   no user responding
     19   no answer
     20   subscriber absent
     21   call rejected
     22   no. changed
     23   redirect                         unalloc. dest no.
     24   ------------------------------   unknown business group
     25   ------------------------------   exchange routing error
     26   ------------------------------   misrouted call to ported no.
     27   out of order                     LNP QoR no. not found
     28   address incomplete
     29   facility rejected
     31   normal unspecified
     34   no cc't available
     38   ntwk out of order
     41   temporary failure
     42   switching equip cong
     43   access info discarded
     44   cc't unavailable
     45   ------------------------------   resource preemption
     46   precedence call blocked
     47   resource unavailable
     50   not subscribed
     51   ------------------------------   call type incompatible

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          ITU-T                            ANSI
    --------------------------------------------------------------------
     53   og call barred in CUG
     54   ------------------------------   group restrictions
     55   ic call barred in CUG
     57   bearer cap not authorized
     58   bearer cap not available
     62   inconsistency
     63   service opt not available
     65   bearer cap not impl.
     69   facility not impl.
     70   restricted bearer cap only
     79   service opt not impl.
     87   user not member of CUG
     88   incompatible dest
     90   non-existent CUG
     91   invalid transit ntwk selection
     95   invalid message
     97   message type not impl.
     99   parameter not impl.
    102   recovery on timer expiry
    103   parameter passed on
    110   message discarded
    111   protocol error
    127   interworking
          all other values reserved
    --------------------------------------------------------------------

3.10.1.14.  Optional Parameters

  The Optional Parameters parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x050E          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                                                               \
    /                       Optional Parameters                     /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Optional Parameters parameter contains the following fields:

  Optional Parameters field: (ISUP Optional Parameters)

    The Optional Parameters field is formatted according to the format
    of the ISUP Optional Parameters Part [Q.763, T1.113] of the ISUP
    message, starting with the first byte of the first optional
    parameter in the ISUP Optional Parameters Part of the message and
    continuing through and including the ISUP End of Optional Parameters

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    parameter [Q.763, T1.113].

    The ISUP Optional Parameters from the ISUP message MUST be placed
    transparently in this fashion into the ISUA Optional Parameters
    parameter.

3.10.2.  Parameters used in CS Messages

     The sections (below) provide the format of the parameters used in
  ISUA Circuit Supervision (CS) messages.

3.10.2.1.  Circuit Status

     The Circuit Status parameter indicates the state of a circuit.  The
  state of a circuit is maintained and obtained by the SG and
  communicated to the ASP.

  The Circuit Status parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0510          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |         Circuit Id #1         |        Circuit State #1       |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                               .                               \
    /                               .                               /
    \                               .                               \
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |         Circuit Id #n         |        Circuit State #n       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Circuit Status parameter contains (a list of) the following
  fields:

  Circuit Id field: 16 bits (unsigned integer)

    The Circuit Id field  contains the circuit identifier for one
    circuit.  This is the least significant bit aligned Circuit
    Identification Code (CIC) [Q.763, T1.113] associated with the
    circuit.  Unused bits are coded zero (0).

    For example, a 12-bit Circuit Identification Code (CIC) is formatted
    into the Circuit Id field as follows:

       0                   1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0 0 0 0|          CIC          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              |MSB-----------------LSB|

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  Circuit State field: 32-bits (integer)

    The Circuit State field contains the least significant bit aligned
    Circuit State Indicator (CSI) [Q.763, T1.113] indicating the status
    of the circuit.  Unused bits are coded zero (0).

    For example, the ITU-T Circuit State Indicator (CSI) is formatted
    into the Circuit State field as follows:

       1       2                   3
       6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0 0 0 0 0 0 0 0 0 0|    CSI    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                          |MSB-----LSB|

    The ITU-T Circuit State Indicator (CSI) [Q.763] can take on the
    following values:

        +---------+------------------------+-------------------+
        |         |         State          |  Blocking|State   |
        |  CSI    +------------+-----------+----------+--------+
        |         |   Maint    | Call Proc | Hardware | Maint  |
        +---------+------------+-----------+----------+--------+
        |XX 00 00 | transient  |     -     |    -     |   -    |
        +---------+------------+           |          |        |
        |XX 00 11 | unequipped |           |          |        |
        +---------+------------+-----------+----------+--------+
        |00 01 00 |  equipped  |  ic busy  |  active  | active |
        |00 01 01 |            |           |          | local  |
        |00 01 10 |            |           |          | remote |
        |00 01 11 |            |           |          |  both  |
        |         |            +-----------+          +--------+
        |00 10 00 |            |  og busy  |          | active |
        |00 10 01 |            |           |          | local  |
        |00 10 10 |            |           |          | remote |
        |00 10 11 |            |           |          |  both  |
        |         |            +-----------+          +--------+
        |00 11 00 |            |   idle    |          | active |
        |00 11 01 |            |           |          | local  |
        |00 11 10 |            |           |          | remote |
        |00 11 11 |            |           |          |  both  |
        |         |            |           +----------+--------+
        |01 11 00 |            |           |  local   | active |
        |01 11 01 |            |           |          | local  |
        |01 11 10 |            |           |          | remote |
        |01 11 11 |            |           |          |  both  |
        |         |            |           +----------+--------+
        |10 11 00 |            |           |  remote  | active |
        |10 11 01 |            |           |          | local  |
        +---------+------------+-----------+----------+--------+

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        +---------+------------------------+-------------------+
        |         |         State          |  Blocking|State   |
        |  CSI    +------------+-----------+----------+--------+
        |         |   Maint    | Call Proc | Hardware | Maint  |
        +---------+------------+-----------+----------+--------+
        |10 11 10 |            |           |          | remote |
        |10 11 11 |            |           |          |  both  |
        |         |            |           +----------+--------+
        |11 11 00 |            |           |   both   | active |
        |11 11 01 |            |           |          | local  |
        |11 11 10 |            |           |          | remote |
        |11 11 11 |            |           |          |  both  |
        +---------+------------+-----------+----------+--------+

3.10.3.  Other Parameters

3.10.3.1.  Circuit Id

     The Circuit Id parameter is used in the ISUA CP and CS Message
  Header to identify one or more circuits within the Application Server
  indicated by the Routing Context parameter (in the ISUA Message
  Header).

  The Circuit Id parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0520          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                           Circuit Id                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Circuit Id parameter can contain the following fields:

  Circuit Id field: 32-bit (unsigned integer)

    The Circuit Id field contains the circuit identifier for the circuit
    within an Application Server that the sending ASP ro SGP is
    configured or registered to control and manage.  This is the least
    significant bit aligned Circuit Identification Code (CIC) [Q.763,
    T1.113] associated with the circuit.  Unused bits are coded zero
    (0).

    For example, a 12-bit Circuit Identification Code (CIC) is formatted
    into the Circuit Id field as follows:

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       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|          CIC          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                              |MSB-----------------LSB|

    a list of one or more 32-bit unsigned integers indexing the circuits
    within an Application Server that the sending ASP is configured or
    registered to control and manage.

     If the Circuit Id parameter is present, it SHOULD be the first
  parameter in the message following the Routing Context as it defines
  the format and/or interpretation of the parameters which follow.

3.10.3.2.  Network Appearance

     The Network Appearance parameter is used as a parameter in the
  Registration Request (REG REQ) message to indicate the network context
  in which the remainder of the Routing Key parameters are to be
  interpreted.  The Network Appearance parameter is also used in the
  Error (ERR) message in response to a REG REQ message when a received
  Network Appearance parameter contains an invalid value.

  The Network Appearance parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0521          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                        Network Appearance                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Network Appearance parameter can contain the following fields:

  Network Appearance field: 32-bits (unsigned integer)

    The Network Appearance field identifies the SS7 network context for
    the Routing Key.  The Network Appearance value is of local
    significance only, coordinated between the SG and ASP.  Therefore,
    in the case where the ASP is connected to more than one SG, the same
    SS7 Network context may be identified by a different Network
    Appearance value depending upon to which SG the ASP is registering.

    In the Routing Key, the Network Appearance identifies the SS7 Point
    Code format used, and the ISUP and Call Control protocol (type,
    variant and version) used within the specific SS7 network.

3.10.3.3.  Routing Key

     The Routing Key parameter is used in the REG REQ message to list
  and identify the Routing Keys that are being registered.

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  The Routing Key parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0522          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                  Local Routing Key Identifier                 |
    +- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -+
    \                                                               \
    /                        Key parameter(s)                       /
    \                                                               \
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Routing Key parameter can contain the following fields:

  Local Routing Key Identifier: TLV

    The Local Routing Key Identifier parameter is used to uniquely
    identify the registration request.  The identifier value is assigned
    by the ASP and is used to correlate the response in a REG RSP
    message with the original registration request.  The identifier
    value must remain unique until the REG RSP (or ERR) message is
    received.

  Key field: variable (TLV parameters)

    The key field can contain the following parameters:

        Parameters
        ---------------------------------------------
        Traffic Mode Type           Optional
        Network Appearance          Conditional   *1
        Local Point Code            Mandatory
        Remote Point Code           Mandatory
        Circuit Id                  Conditional   *2
        Circuit Range               Conditional   *2

    Note 1: The Network Appearance parameter MUST be included in the
            Routing Key when the ASP is able to register in multiple SS7
            Network contexts.

    Note 2: One of the Circuit Id or Circuit Range parameters MUST be
            present in the Key parameters.

3.10.3.4.  Circuit Range

  The Circuit Range parameter is formatted as follows:

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     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0523          |            Length             |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |      Circuit Id Beg #1        |       Circuit Id End #1       |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    \                               .                               \
    /                               .                               /
    \                               .                               \
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |      Circuit Id Beg #n        |       Circuit Id End #n       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Circuit Range parameter can contain (a list of) the following
  fields:

  Circuit Id Beg field: 16-bits (unsigned integer)

    The Circuit Id Beg field contains the circuit identifier for the
    circuit at the beginning of the range (inclusive).  This is the
    least significant bit aligned Circuit Identification Code (CIC)
    [Q.763, T1.113] associated with the first circuit in the range.
    Unused bits are coded zero (0).  The first and last circuit in the
    range MAY be the same circuit.

    For example, a 12-bit Circuit Identification Code (CIC) is formatted
    into the Circuit Id Beg field as follows:

       0                   1
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0 0 0 0|          CIC          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              |MSB-----------------LSB|

  Circuit Id End field: 16-bits (unsigned integer)

    The Circuit Id End field contains the circuit identifier for the
    circuit at the end of the range (inclusive).  This is the least
    significant bit aligned Circuit Identification Code (CIC) [Q.763,
    T1.113] associated with the last circuit in the range.  Unused bits
    are coded zero (0).  The first and last circuit in the range MAY be
    the same circuit.

    For example, a 12-bit Circuit Identification Code (CIC) is formatted
    into the Circuit Id End field as follows:

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       1       2                   3
       6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |0 0 0 0|          CIC          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              |MSB-----------------LSB|

3.10.3.5.  Local Point Code

     The Local Point Code parameter appears in the Routing Key parameter
  in the REG REQ message.  It is used in conjunction with an implied or
  specified Network Appearance parameter which also appears in the
  Routing Key to identify the local ISUP switch for which an ASP is
  registering.

  The Local Point Code parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0524          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                          Point Code                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Local Point Code parameters contains the following fields:

  Point Code field: 32-bits (unsigned integer)

    The Point Code field contains an SS7 signalling point code.  Point
    codes that are less than 32-bits are padded on the left to the
    32-bit boundary.  The following examples show ANSI and ITU-T point
    codes:

    ANSI 24-bit Point Code:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 0 0 0 0 0 0 0|    Network    |    Cluster    |     Member    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    |MSB-----------------------------------------LSB|

    ITU-T, ETSI 14-bit Point Code:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0|Zone |     Region    | SP  |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                        |MSB---------------------LSB|

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3.10.3.6.  Remote Point Code

     The Remote Point Code parameter appears in the Routing Key
  parameter in the REG REQ message.  It is used in conjunction with an
  implied or specified Network Appearance parameter which also appears
  in the Routing Key to identify the ISUP switch at the remote end of
  the ISUP circuits for which an ASP is registering.

  The Remote Point Code parameter is formatted as follows:

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Tag = 0x0525          |            Length = 8         |
    +- - - - - - - - - - - - - - - -+- - - - - - - - - - - - - - - -+
    |                          Point Code                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  The Remote Point Code parameters contains the following fields:

  Point Code field: 32-bits (unsigned integer)

    The Point Code field contains an SS7 signalling point code.  Point
    codes that are less than 32-bits are padded on the left to the
    32-bit boundary.  For examples of point codes, see the Local Point
    Code parameter description.

4.  Procedures

     The ISUA layer needs to respond to various local primitives it
  receives from other layers as well as the messages that it receives
  from the peer ISUA layer.  This section describes the ISUA procedures
  in response to these events.

4.1.  Procedures to Support Call Control

4.1.1.  Receipt of Primitives from Call Control

     Upon receiving a ISUP request or response primitive from the upper
  layer at an ASP or IPSP, the ISUA layer sends a corresponding ISUA
  Call Processing (CP) message (see Section 3) to its ISUA peer.  The
  ISUA peer receiving the CP message delivers the corresponding ISUP
  primitive to Call Control at the IPSP or Nodal Interworking Function
  at the SG as illustrated in Figure 4.  The mapping of ISUP primitives
  to ISUA CP Messages is listed in Table 2 (see Section 1.6.1).

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      _______________         _______       _______         _______
     |               |       |       |     |       |       |       |
     |    Nodal      |       |       |     |       |       |       |
     | Interworking  |       I   CC  |     I   CC  |       |   CC  |
     |   Function    |       |       |     |       |       |       |
     |  ___________  |       |_______|     |_______|       |_______|
     | |    ___    | |         |   ^         |   ^           |   ^
     | |   |   |   | |         |   |         |   |           |   |
     |_v___|___v___|_|         |   |         |   |           |   |
       |   ^   |   ^           |   |         |   |           |   |
       |   |   |   | Call Cntl |   |         |   | Call Cntl |   |
     - + - + - + - + - - - - - + - + - - - - + - + - - - - - + - + - -
       |   |   |   |  Boundary |   |         |   |  Boundary |   |
      _v___|_ _v___|_         _v___|_       _v___|_         _v___|_
     |       |       |       |       |     |       |       |       |
     |       |       |       |       |     |       |       |       |
     |  ISUP |  ISUA |       |  ISUA |     |  ISUA |       |  ISUA |
     |       |       |       |       |     |       |       |       |
     |_______|_______|       |_______|     |_______|       |_______|
     |       | |   ^           |   ^         |   ^           |   ^
     |       | |   |           |   |         |   |           |   |
     |       | |   |    _      |   |         |   |    _      |   |
     |  SS7  | |   |___/_\_____|   |         |   |____/_\____|   |
     |       | |______|___|________|         |_______|___|_______|
     |///////|         \_/                            \_/
     |       |         /                              /
     |       |        /                              /
                 SCTP Association               SCTP Association

     \______  ______/        \___  ___/    \___  ___/     \___  ___/
            \/                   \/            \/             \/
            SG                    ASP         IPSP           IPSP

                     Figure 4.  ISUA Layer Model

4.1.2.  Receipt of Primitives from ISUP

     Upon receiving a ISUP indication or confirmation primitive from
  ISUP at an SG, the Nodal Interworking Function passes the primitive to
  ISUA.  The ISUA layer sends a corresponding ISUA Call Processing (CP)
  message (see Section 3) to its ISUA peer at the ASP.

     The ISUA peer receiving the CP message delivers the corresponding
  ISUP primitive to Call Control at the ASP as illustrated in Figure 5.
  The mapping of ISUP primitives to ISUA CP Messages is listed in Table
  2 (see Section 1.6.1).

     The ISUA Circuit Mapping Function (see Section 1.5.1.4)

     For SETUP indications, the ISUA Circuit Mapping Function (CMF)
  determines the Application Server (AS) based on comparing the circuit
  information in the primitive with a provisioned Routing Key.

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     From the list of ASPs within an AS table, an ASP in the ASP-ACTIVE
  state is selected and a CSET message is constructed and issued on the
  corresponding SCTP association.  The ISUA at the SG is also
  responsible for assigning and managing a Circuit Identifier which is
  sent to the ASP in the CSET message to identify the newly created call
  to the ASP.  Information associated with the dialogue is stored in the
  SG in an implementation dependent manner; however, the SG must be
  capable of associating further ISUA messages with the correct Dialogue
  at the SG.  The SG will have to access this stored information to
  continue processing the dialogue.

     The ISUA Circuit Mapping Function (CMF) determines the Application
  Server (AS) based on comparing the information in the primitive with a
  provisioned Routing Key.

4.1.2.1.  Receipt of Management Primitives from ISUP

     When ISUP Circuit Management indications are received (RESET,
  BLOCKING, UNBLOCKING, CCT GROUP QUERY), ISUP Management determines
  whether there are concerned local Call Control.  When these local Call
  Control are in fact Application Servers, serviced by ASPs, ISUA
  circuit supervision is transparently informed with the RESET,
  BLOCKING, UNBLOCKING and CCT GROUP QUERY indication primitive upon
  which it formats and transfers the applicable CS message (CRES, CBLO,
  CUBL or CQRY) to the list of concerned ASPs.

     The ISUA message distribution function determines the Application
  Server (AS) based on comparing the information in the ISUP primitive
  with a provisioned Routing Key.

     From the list of ASPs within the AS table, an ASP in the ASP-ACTIVE
  state is selected and Call Processing (CP) messages are constructed
  and issued on the corresponding SCTP association.  If more than one
  ASP is in the ASP-ACTIVE state (i.e., traffic is to be load-shared
  across more than one ASP), one of the ASPs in the ASP-ACTIVE state is
  selected from the list.  (If the ASPs are in Broadcast Mode, all
  active ASPs will be selected and the message sent to each of the
  active ASPs.)  The selection algorithm is implementation dependent but
  could, for example, be round robin or based on the SLS.  The
  appropriate selection algorithm must be chosen carefully as it is
  dependent on application assumptions and understanding of the degree
  of state coordination between the ASP-ACTIVE ASPs in the AS.

     In addition, the message needs to be sent on the appropriate SCTP
  stream, again taking care to meet the message sequencing needs of the
  signalling application.  Call Processing (CP) messages SHOULD be sent
  on an SCTP stream other than stream `0'.

     When there is no Routing Key match, or only a partial match, for an
  incoming SS7 message, a default treatment MAY be specified.  Possible
  solutions are to provide a default Application Server at the SGP that
  directs all unallocated traffic to a (set of) default ASP(s), or to
  drop the message and provide a notification to Layer Management in an
  M-ERROR indication primitive.  The treatment of unallocated traffic is

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  implementation dependent.

4.1.3.  Receipt of Primitive from the Layer Management

     On receiving primitives from the local Layer Management, the ISUA
  layer will take the requested action and provide an appropriate
  response primitive to Layer Management.

     An M-SCTP_ESTABLISH request primitive from Layer Management at an
  ASP or IPSP will initiate the establishment of an SCTP association.
  The ISUA layer will attempt to establish an SCTP association with the
  remote ISUA peer by sending an SCTP-ASSOCIATE primitive to the local
  SCTP layer.

     When an SCTP association has been successfully established, the
  SCTP will send an SCTP-COMMUNICATION_UP notification primitive to the
  local ISUA layer.  At the SGP or IPSP that initiated the request, the
  ISUA layer will send an M-SCTP_ESTABLISH confirm primitive to Layer
  Management when the association setup is complete.  At the peer ISUA
  layer, an M-SCTP_ESTABLISH indication primitive is sent to Layer
  Management upon successful completion of an incoming SCTP association
  setup.

     An M-SCTP_RELEASE request primitive from Layer Management initiates
  the shutdown of an SCTP association.  The ISUA layer accomplishes a
  graceful shutdown of the SCTP association by sending an SCTP-SHUTDOWN
  primitive to the SCTP layer.

     When the graceful shutdown of the SCTP association has been
  accomplished, the SCTP layer returns an SCTP-SHUTDOWN_COMPLETE
  notification primitive to the local ISUA layer.  At the ISUA Layer
  that initiated the request, the ISUA layer will send an M-SCTP_RELEASE
  confirm primitive to Layer Management when the association shutdown is
  complete.  At the peer ISUA Layer, an M-SCTP_RELEASE indication
  primitive is sent to Layer Management upon abort or successful
  shutdown of an SCTP association.

     An M-SCTP_STATUS request primitive supports a Layer Management
  query of the local status of a particular SCTP association.  The ISUA
  layer simply maps the M-SCTP_STATUS request primitive to an SCTP-
  STATUS primitive to the SCTP layer.  When the SCTP responds, the ISUA
  layer maps the association status information to an M-SCTP_STATUS
  confirm primitive.  No peer protocol is invoked.

     Similar LM-to-ISUA-to-SCTP and SCTP-to-ISUA-to-LM primitive
  mappings can be described for the various other SCTP Upper Layer
  primitives in RFC 2960 [2960] such as INITIALIZE, SET PRIMARY, CHANGE
  HEARTBEAT, REQUEST HEARTBEAT, GET SRTT REPORT, SET FAILURE THRESHOLD,
  SET PROTOCOL PARAMETERS, DESTROY SCTP INSTANCE, SEND FAILURE, AND
  NETWORK STATUS CHANGE.  Alternatively, these SCTP Upper Layer
  primitives (and Status as well) can be considered for modeling
  purposes as a Layer Management interaction directly with the SCTP
  Layer.

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     M-NOTIFY indication and M-ERROR indication primitives indicate to
  Layer Management the notification or error information contained in a
  received ISUA Notify (NTFY) or Error (ERR) message respectively.
  These indications can also be generated based on local ISUA events.

     An M-ASP_STATUS request primitive supports a Layer Management query
  of the status of a particular local or remote ASP.  The ISUA layer
  responds with the status in an M-ASP_STATUS confirm primitive.  No
  ISUA peer protocol is invoked.  An M-AS_STATUS request supports a
  Layer Management query of the status of a particular AS.  The ISUA
  responds with an M-AS_STATUS confirm primitive.  No ISUA peer protocol
  is invoked.

     M-ASP_UP request, M-ASP_DOWN request, M-ASP_ACTIVE request and M-
  ASP_INACTIVE request primitives allow Layer Management at an ASP to
  initiate state changes.  Upon successful completion, a corresponding
  confirm primitive is provided by the ISUA layer to Layer Management.
  If an invocation is unsuccessful, an Error indication primitive is
  provided in the primitive.  These requests result in outgoing ASP Up
  (ASPUP), ASP Down (ASPDN), ASP Active (ASPAC) and ASP Inactive (ASPIA)
  messages to the remote ISUA peer at an SGP or IPSP.

4.2.  Procedures to Support the Management of SCTP Associations

4.2.1.  Receipt of ISUA Peer Management Messages

     Upon successful state changes resulting from reception of ASP Up
  (ASPUP), ASP Down (ASPDN), ASP Active (ASPAC) and ASP Inactive (ASPIA)
  messages from a peer ISUA, the ISUA layer MAY invoke corresponding M-
  ASP_UP, M-ASP_DOWN, M-ASP_ACTIVE and M-ASP_INACTIVE, M-AS_ACTIVE, M-
  AS_INACTIVE, and M-AS_DOWN indication primitives to the local Layer
  Management.

     M-NOTIFY indication and M-ERROR indication primitives indicate to
  Layer Management the notification or error information contained in a
  received ISUA Notify (NTFY) or Error (ERR) message.  These indications
  can also be generated based on local ISUA events.

     All MGMT, ASPSM, ASPTM and RKM messages, except BEAT, BEAT ACK and
  NTFY, SHOULD be sent with sequenced delivery to ensure ordering.  All
  MGMT, ASPSM and RKM messages, with the exception of BEAT, BEAT ACK and
  NTFY messages MUST be sent on SCTP stream '0'.  ASPTM messages MAY be
  sent on one of the streams used to carry data traffic related to the
  Routing Context(s), to minimize possible message loss.  BEAT, BEAT
  ACK, and NTFY messages MAY be sent using out-of-order delivery, and
  MAY be sent on any stream.

4.3.  AS and ASP State Maintenance

     The ISUA layer on the SGP maintains the state of each remote ASP,
  in each Application Server that the ASP is configured to receive
  traffic, as input to the ISUA message distribution function.
  Similarly, where IPSPs use ISUA in a point-to-point fashion, the ISUA
  layer in an IPSP maintains the state of remote IPSPs.  For the

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  purposes of the following procedures, only the SGP and ASP case is
  described but the SGP side of the procedures also apply to an IPSP
  sending traffic to an AS consisting of a set of remote IPSPs.

4.3.1.  ASP States

     The state of each remote ASP, in each AS that it is configured to
  operate, is maintained in the ISUA layer in the SGP.  The state of a
  particular ASP in a particular AS changes due to events.  The events
  include: .bu reception of messages from the peer ISUA layer at the
  ASP;

   - reception of some messages from the peer ISUA layer at other ASPs
     in the AS (e.g, ASP Active message indicating "Override");
   - reception of indications from the SCTP layer; or,
   - Local Management intervention.

     The ASP state transition diagram is shown in Figure 5.  The
  possible states of an ASP are:

  ASP-DOWN:     The remote ISUA peer at the ASP is unavailable or the
                related SCTP association is down.  Initially all ASPs
                will be in this state.  An ASP in this state SHOULD NOT
                be sent any ISUA messages, with the exception of
                Heartbeat (BEAT), ASP Down Ack (ASPDN ACK) and Error
                (ERR) messages.

                                         +--------------+
                                         |              |
                  +----------------------|  ASP-ACTIVE  |
                  |      Other   +-------|              |
                  |   ASP in AS  |       +--------------+
                  |   Overrides  |           ^     |
                  |              |    ASP    |     | ASP
                  |              |    Active |     | Inactive
                  |              |           |     v
                  |              |       +--------------+
                  |              |       |              |
                  |              +------>| ASP-INACTIVE |
                  |                      +--------------+
                  |                          ^     |
        ASP Down/ |                     ASP  |     | ASP Down /
        SCTP CDI/ |                     Up   |     | SCTP CDI/
        SCTP RI   |                          |     v SCTP RI
                  |                      +--------------+
                  |                      |              |
                  +--------------------->|   ASP-DOWN   |
                                         |              |
                                         +--------------+

            Figure 5.  ASP State Transition Diagram (Per AS)

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  ASP-INACTIVE: The remote ISUA peer at the ASP is available (and the
                related SCTP association is up) but application traffic
                is stopped.  In this state, the ASP SHOULD NOT be sent
                any CP or CS messages for the AS for which the ASP is
                inactive.

  ASP-ACTIVE:   The remote ISUA peer at the ASP is available and
                application traffic is active (for a particular Routing
                Context or set of Routing Contexts).

  SCTP CDI:     The SCTP CDI denotes the local SCTP layer's
                Communication Down Indication to the Upper Layer
                Protocol (ISUA) on an SGP.  The local SCTP layer will
                send this indication when it detects the loss of
                connectivity to the ASPs peer SCTP layer.  SCTP CDI is
                understood as either a SHUTDOWN_COMPLETE notification or
                COMMUNICATION_LOST notification from the SCTP layer.

  SCTP RI:      The local SCTP layer's Restart indication to the upper
                layer protocol (ISUA) on an SG.  The local SCTP will
                send this indication when it detects a restart from the
                ASPs peer SCTP layer.

4.3.2.  AS States

     The state of the AS is maintained in the ISUA layer on the SGP.
  The state of an AS changes due to events.  These events include:

   - ASP state transitions
   - Recovery timer triggers

     The possible states of an AS are:

  AS-DOWN:      The Application Server is unavailable.  This state
                implies that all related ASPs are in the ASP-DOWN state
                for this AS.  Initially the AS will be in this state.
                An Application Server is in the AS-DOWN state when it is
                removed from a configuration.

  AS-INACTIVE:  The Application Server is available but no application
                traffic is active (i.e., one or more related ASPs are in
                the ASP-INACTIVE state, but none in the ASP-ACTIVE
                state).  The recovery timer T(r) is not running or has
                expired.

  AS-ACTIVE:    The Application Server is available and application
                traffic is active.  This state implies that at least one
                ASP is in the ASP-ACTIVE state.

  AS-PENDING:   An active ASP has transitioned to ASP-INACTIVE or ASP-
                DOWN and it was the last remaining active ASP in the AS.
                A recovery timer T(r) SHOULD be started and all incoming
                signalling messages SHOULD be queued by the SGP.  If an
                ASP becomes ASP-ACTIVE before T(r) expires, the AS is

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                moved to the AS-ACTIVE state and all the queued messages
                will be sent to the ASP.

     If T(r) expires before an ASP becomes ASP-ACTIVE, and the SGP has
  no other alternative, the SGP may stop queuing messages and discard
  all previously queued messages.  The AS will move to the AS-INACTIVE
  state if at least one ASP is in ASP-INACTIVE state, otherwise it will
  move to AS-DOWN state.

         +----------+   one ASP trans to ACTIVE   +-------------+
         |    AS-   |---------------------------->|     AS-     |
         | INACTIVE |                             |   ACTIVE    |
         |          |<---                         |             |
         +----------+    \                        +-------------+
            ^   |         \ Tr Expiry,                ^    |
            |   |          \ at least one             |    |
            |   |           \ ASP in ASP-INACTIVE     |    |
            |   |            \                        |    |
            |   |             \                       |    |
            |   |              \                      |    |
    one ASP |   | all ASP       \            one ASP  |    | Last ACTIVE
    trans   |   | trans to       \           trans to |    | ASP trans to
    to      |   | ASP-DOWN        -------\   ASP-     |    | ASP-INACTIVE
    ASP-    |   |                         \  ACTIVE   |    | or ASP-DOWN
    INACTIVE|   |                          \          |    | (start Tr)
            |   |                           \         |    |
            |   |                            \        |    |
            |   v                             \       |    v
         +----------+                          \  +-------------+
         |          |                           --|             |
         | AS-DOWN  |                             | AS-PENDING  |
         |          |                             |  (queuing) |
         |          |<----------------------------|             |
         +----------+    Tr Expiry and no ASP     +-------------+
                         in ASP-INACTIVE state

           Tr = Recovery Timer

                Figure 6.  AS State Transition Diagram

     Figure 6 shows an example AS state machine for the case where the
  AS data is pre-configured.  For other cases where the ASP
  configuration data is created dynamically, there would be differences
  in the state machine, especially at creation of the AS.

     For example, where the AS configuration data is not created until
  Registration of the first ASP, the AS-INACTIVE state is entered
  directly upon the first successful REG REQ from an ASP.  Another
  example is where the AS configuration data is not created until the
  first ASP successfully enters the ASP-ACTIVE state.  In this case the
  AS-ACTIVE state is entered directly.

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4.3.2.1.  IPSP Considerations

     The AS state diagram for the AS-SG case is applicable for IPSP
  communication.

4.3.3.  ISUA Management Procedures for Primitives

     Before the establishment of an SCTP association the ASP state at
  both the SGP and ASP is assumed to be in the state ASP-DOWN.

     Once the SCTP association is established (see Section 4.2.1) and
  assuming that local Call Control is ready, the local ISUA ASP
  Maintenance (ASPM) function will initiate the relevant procedures,
  using the ASP Up, ASP Down, ASP Active and ASP Inactive messages to
  convey the ASP state to the SGP (see Section 4.3.4).

     If the ISUA layer subsequently receives an SCTP-COMMUNICATION_DOWN
  or SCTP-RESTART indication primitive from the underlying SCTP layer,
  it will inform the Layer Management by invoking the M-SCTP_STATUS
  indication primitive.  The state of the ASP will be moved to ASP-DOWN.

     At an ASP, Call Control will be informed of the status of any
  affected ISUP circuit through the use of RESET, BLOCKING and
  UNBLOCKING indication primitives.

     In the case of SCTP-COMMUNICATION_DOWN, the SCTP client MAY try to
  re-establish the SCTP association.  This MAY be done by the ISUA layer
  automatically, or Layer Management MAY re-establish using the M-
  SCTP_ESTABLISH request primitive.

     In the case of an SCTP-RESTART indication at an ASP, the ASP is now
  considered by its ISUA peer to be in the ASP-DOWN state.  The ASP, if
  it is to recover, must begin any recovery with the ASP-Up procedure.

4.3.4.  ASPM Procedures for Peer-to-Peer Messages

4.3.4.1.  ASP Up Procedures

     After an ASP has successfully established an SCTP association to an
  SGP, the SGP waits for the ASP to send an ASP Up (ASPUP) message,
  indicating that the ASP ISUA peer is available.  The ASP is always the
  initiator of the ASP Up (ASPUP) message.  This action MAY be initiated
  at the ASP by an M-ASP_UP request primitive from Layer Management or
  MAY be initiated automatically by an ISUA management function.

     When an ASP Up (ASPUP) message is received at an SGP and internally
  the remote ASP is in the ASP-DOWN state and not considered locked-out
  for local management reasons, the SGP marks the remote ASP in the
  state ASP-INACTIVE and informs Layer Management with an M-ASP_Up
  indication primitive.  If the SGP is aware, via current configuration
  data, which Application Servers the ASP is configured to operate in,
  the SGP updates the ASP state to ASP-INACTIVE in each AS that it is a
  member.

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     Alternatively, the SGP may move the ASP into a pool of Inactive
  ASPs available for future configuration within Application Server(s),
  determined in a subsequent Registration Request or ASP Active
  procedure.  If the ASP Up (ASPUP) message contains an ASP Identifier,
  the SGP should save the ASP Identifier for that ASP.  The SGP MUST
  send an ASP Up Ack (ASPUP ACK) message in response to a received ASP
  Up (ASPUP) message even if the ASP is already marked as ASP-INACTIVE
  at the SGP.

     If for any local reason (e.g, management lock-out) the SGP cannot
  respond with an ASP Up Ack (ASPUP ACK) message, the SGP responds to an
  ASP Up (ASPUP) message with an Error (ERR) message with Reason
  "Refused - Management Blocking".

     At the ASP, the ASP Up Ack (ASPUP ACK) message received is not
  acknowledged.  Layer Management is informed with an M-ASP_UP confirm
  primitive.

     When the ASP sends an ASP Up (ASPUP) message it starts timer
  T(ack).  If the ASP does not receive a response to an ASP Up (ASPUP)
  message within T(ack), the ASP MAY restart T(ack) and resend ASP Up
  (ASPUP) messages until it receives an ASP Up Ack (ASPUP ACK) message.
  T(ack) is provisionable, with a default of 2 seconds.  Alternatively,
  retransmission of ASP Up (ASPUP) messages MAY be put under control of
  Layer Management.  In this method, expiry of T(ack) results in an M-
  ASP_UP confirm primitive carrying a negative indication.

     The ASP must wait for the ASP Up Ack (ASPUP ACK) message before
  sending any other ISUA messages (e.g, ASP Active or REG REQ).  If the
  SGP receives any other ISUA messages before ASPUP message is received
  (other than ASPDN - see Section 4.3.4.2), the SGP SHOULD discard them.

     If an ASP Up (ASPUP) message is received and internally the remote
  ASP is in the ASP-ACTIVE state, an ASP Up Ack (ASPUP ACK) message is
  returned, as well as an Error (ERR) message ("Unexpected Message), and
  the remote ASP state is changed to ASP-INACTIVE in all relevant
  Application Servers.

     If an ASP Up (ASPUP) message is received and internally the remote
  ASP is already in the ASP-INACTIVE state, an ASP Up Ack (ASPUP ACK)
  message is returned and no further action is taken.

4.3.4.1.1.  ISUA Version Control

     If an ASP Up (ASPUP) message with an unsupported version is
  received, the receiving end responds with an Error (ERR) message,
  indicating the version the receiving node supports and notifies Layer
  Management.

     This is useful when protocol version upgrades are being performed
  in a network.  A node upgraded to a newer version should support the
  older versions used on other nodes it is communicating with.  Because
  ASPs initiate the ASP Up procedure it is assumed that the Error (ERR)
  message would normally come from the SGP.

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4.3.4.1.2.  IPSP Considerations

     An IPSP may be considered in the ASP-INACTIVE state after and ASPUP
  or ASPUP Ack has been received from it.  An IPSP can be considered in
  the ASP-DOWN state after an ASPDN or ASPDN Ack has been received from
  it.  The IPSP may inform Layer Management of the change in state of
  the remote IPSP using M-ASP_UP or M-ASP_DN indication or confirmation
  primitives.

     Alternatively, an interchange of ASPUP messages from each end can
  be performed.  This option follows the ASP state transition diagram.
  It would need four messages for completion.

     If for any local reason (e.g, management lock-out) and IPSP cannot
  respond to an ASP Up (ASPUP) message with an ASP Up Ack (ASPUP ACK)
  message, it responds to an ASP Up (ASPUP) message with an Error (ERR)
  message with Reason "Refused - Management Blocking" and leaves the
  remote IPSP in the ASP-DOWN state.

4.3.4.2.  ASP Down Procedures

     The ASP will send an ASP Down (ASPDN) message to an SGP when the
  ASP wishes to be removed from service in all Application Servers that
  it is a member and no longer receive any CP, CS or ASPTM messages.
  This action MAY be initiated at the ASP by an M-ASP_DOWN request
  primitive from Layer Management or MAY be initiated automatically by
  an ISUA management function.

     Whether the ASP is permanently removed from any AS is a function of
  configuration management.  Whenever the ASP previously used the
  Registration procedures (see Section 4.4.1) to register within
  Application Servers but has not deregistered from all of them prior to
  sending the ASP Down (ASPDN) message, the SGP MUST consider the ASP as
  Deregistered in all Application Servers that it is still a member.

     The SGP marks the ASP as ASP-DOWN, informs Layer Management with an
  M-ASP_Down indication primitive, and returns an ASP Down Ack (ASPDN
  ACK) message to the ASP.

     The SGP MUST send an ASP Down Ack (ASPDN ACK) message in response
  to a received ASP Down (ASPDN) message from the ASP even if the ASP is
  already marked as ASP-DOWN at the SGP.

     At the ASP, the ASP Down Ack (ASPDN ACK) message received is not
  acknowledged.  Layer Management is informed with an M-ASP_DOWN confirm
  primitive.  If the ASP receives an ASP Down Ack without having sent an
  ASP Down (ASPDN) message, the ASP should now consider itself as in the
  ASP-DOWN state.  If the ASP was previously in the ASP-ACTIVE or
  ASP_INACTIVE state, the ASP should then initiate procedures to return
  itself to its previous state.

     When the ASP sends an ASP Down (ASPDN) message it starts timer
  T(ack).  If the ASP does not receive a response to an ASP Down (ASPDN)
  message within T(ack), the ASP MAY restart T(ack) and resend ASP Down

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  (ASPDN) messages until it receives an ASP Down Ack (ASPDN ACK)
  message.  T(ack) is provisionable, with a default of 2 seconds.
  Alternatively, retransmission of ASP Down (ASPDN) messages MAY be put
  under control of Layer Management.  In this method, expiry of T(ack)
  results in an M-ASP_DOWN confirm primitive carrying a negative
  indication.

4.3.4.3.  ASP Active Procedures

     Anytime after the ASP has received an ASP Up Ack (ASPUP ACK)
  message from the SGP or IPSP, the ASP MAY send an ASP Active (ASPAC)
  message to the SGP indicating that the ASP is ready to start
  processing traffic.  This action MAY be initiated at the ASP by an M-
  ASP_ACTIVE request primitive from Layer Management or MAY be initiated
  automatically by an ISUA management function.  Whenever an ASP wishes
  to process the traffic for more than one Application Server across a
  common SCTP association, the ASP Active (ASPAC) message(s) SHOULD
  contain a list of one or more Routing Contexts to indicate for which
  Application Servers the ASP Active (ASPAC) message applies.  It is not
  necessary for the ASP to include all Routing Contexts of interest in a
  single ASP Active (ASPAC) message, thus requesting to become active in
  all Routing Contexts at the same time.  Multiple ASP Active (ASPAC)
  messages MAY be used to activate within the Application Servers
  independently, or in sets.  Whenever an ASP Active (ASPAC) message
  does not contain a Routing Context parameter, the receiver must know,
  via configuration data, which Application Server(s) the ASP is a
  member.

     For the Application Servers that the ASP can successfully activate,
  the SGP or IPSP responds with one or more ASP Active Ack (ASPAC ACK)
  messages, including the associated Routing Context(s) and reflecting
  any Traffic Mode Type values present in the related ASP Active (ASPAC)
  message.  The Routing Context parameter MUST be included in the ASP
  Active Ack (ASPAC ACK) message(s) if the received ASP Active (ASPAC)
  message contained any Routing Contexts.  Depending on any Traffic Mode
  Type request in the ASP Active (ASPAC) message or local configuration
  data if there is no request, the SGP moves the ASP to the correct ASP
  traffic state within the associated Application Server(s).  Layer
  Management is informed with an M-ASP_Active indication.  If the SGP or
  IPSP receives any CP messages before an ASP Active (ASPAC) message is
  received, the SGP or IPSP MAY discard them.  By sending an ASP Active
  Ack (ASPAC ACK) message, the SGP or IPSP is now ready to receive and
  send traffic for the related Routing Context(s).  The ASP SHOULD NOT
  send CP messages for the related Routing Context(s) before receiving
  an ASP Active Ack (ASPAC ACK) message, or it will risk message loss.

     Multiple ASP Active Ack (ASPAC ACK) messages MAY be used in
  response to an ASP Active (ASPAC) message containing multiple Routing
  Contexts, allowing the SGP or IPSP to independently acknowledge the
  ASP Active (ASPAC) message for different (sets of) Routing Contexts.
  The SGP or IPSP MUST send an Error (ERR) message ("Invalid Routing
  Context") for each Routing Context value that cannot be successfully
  activated.

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     Whenever an "out-of-the-blue" ASP Active (ASPAC message is received
  (i.e., the ASP has not registered with the SG or the SG has no static
  configuration data for the ASP), the message MAY be silently
  discarded.

     The SGP MUST send an ASP Active Ack (ASPAC ACK) message in response
  to a received ASP Active (ASPAC) message from the ASP, if the ASP is
  already marked in the ASP-ACTIVE state at the SGP.

     At the ASP, the ASP Active Ack (ASPAC ACK) message received is not
  acknowledged.  Layer Management is informed with an M-ASP_ACTIVE
  confirm primitive.  It is possible for the ASP to receive CP
  message(s) before the ASP Active Ack (ASPAC ACK) message as the ASP
  Active Ack and CP messages from an SG or IPSP may be sent on different
  SCTP streams.  Message loss is possible, as the ASP does not consider
  itself in the ASP-ACTIVE state until reception of the ASP Active Ack
  (ASPAC ACK) message.

     When the ASP sends an ASP Active (ASPAC) message it starts timer
  T(ack).  If the ASP does not receive a response to an ASP Active
  (ASPAC) message within T(ack), the ASP MAY restart T(ack) and resend
  ASP Active (ASPAC) messages until it receives an ASP Active Ack (ASPAC
  ACK) message.  T(ack) is provisionable, with a default of 2 seconds.
  Alternatively, retransmission of ASP Active (ASPAC) messages MAY be
  put under control of Layer Management.  In this method, expiry of
  T(ack) results in an M-ASP_ACTIVE confirm primitive carrying a
  negative indication.

     There are three modes of Application Server traffic handling in the
  SGP ISUA layer: Override, Load-share and Broadcast.  When included,
  the Traffic Mode Type parameter in the ASP Active (ASPAC) message
  indicates the traffic-handling mode to be used in a particular
  Application Server.  If the SGP determines that the mode indicated in
  an ASP Active (ASPAC) message is unsupported or incompatible with the
  mode currently configured for the AS, the SGP responds with an Error
  (ERR) message ("Unsupported/Invalid Traffic Handling Mode").  If the
  traffic- handling mode of the Application Server is not already known
  via configuration data, then the traffic-handling mode indicated in
  the first ASP Active (ASPAC) message causing the transition of the
  Application Server state to AS-ACTIVE MAY be used to set the mode.

     In the case of an Override mode AS, reception of an ASP Active
  (ASPAC) message at an SGP causes the (re)direction of all traffic for
  the AS to the ASP that sent the ASP Active (ASPAC) message.  Any
  previously active ASP in the AS is now considered to be in state ASP-
  INACTIVE and SHOULD no longer receive traffic from the SGP within the
  AS.  The SGP or IPSP then MUST send a Notify (NTFY) message
  ("Alternate ASP Active") to the previously active ASP in the AS, and
  SHOULD stop traffic to or from that ASP.  The ASP receiving this
  Notify MUST consider itself now in the ASP-INACTIVE state, if it is
  not already aware of this via inter- ASP communication with the
  Overriding ASP.

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     In the case of a Load-share mode AS, reception of an ASP Active
  (ASPAC) message at an SGP or IPSP causes the direction of traffic to
  the ASP sending the ASP Active (ASPAC) message, in addition to all the
  other ASPs that are currently active in the AS.  The algorithm at the
  SGP for load-sharing traffic within an AS to all the active ASPs is
  implementation dependent.  The algorithm could, for example, be round
  robin or based on information in the CCmessage.

     An SGP or IPSP, upon reception of an ASP Active (ASPAC) message for
  the first ASP in a Load-share AS, MAY choose not to direct traffic to
  a newly active ASP until it determines that there are sufficient
  resources to handle the expected load (e.g, until there are "n" ASPs
  in state ASP-ACTIVE in the AS).

     All ASPs within a load-sharing mode AS must be able to process any
  CP message received for the AS, to accommodate any potential fail-over
  or re-balancing of the offered load.

     In the case of a Broadcast mode AS, reception of an ASP Active
  (ASPAC) message at an SGP or IPSP causes the direction of traffic to
  the ASP sending the ASP Active (ASPAC) message, in addition to all the
  other ASPs that are currently active in the AS.  The algorithm at the
  SGP for broadcasting traffic within an AS to all the active ASPs is a
  simple broadcast algorithm, where every message is sent to each of the
  active ASPs.  An SGP or IPSP, upon reception of an ASP Active (ASPAC)
  message for the first ASP in a Broadcast AS, MAY choose not to direct
  traffic to a newly active ASP until it determines that there are
  sufficient resources to handle the expected load (e.g, until there are
  "n" ASPs in state ASP-ACTIVE in the AS).

     Whenever an ASP in a Broadcast mode AS becomes ASP-ACTIVE, the SGP
  MUST tag the first CP message broadcast in each SCTP stream with a
  unique Correlation Id parameter.  The purpose of this Correlation Id
  is to permit the newly active ASP to synchronize it's processing of
  traffic in each ordered stream with the other ASPs in the broadcast
  group.

4.3.4.3.1.  IPSP Considerations

     Either of the IPSPs can initiate communication.  When an IPSP
  receives an ASP Active, it should mark the peer as ASP-ACTIVE and
  return an ASP Active Ack (ASPAC ACK) message.  An ASP receiving an ASP
  Active Ack (ASPAC ACK) message may mark the peer as ASP-Active, if it
  is not already in the ASP- ACTIVE state.

     Alternatively, an interchange of ASPAC messages from each end can
  be performed.  This option follows the ASP state transition diagram
  and gives the additional advantage of selecting a particular AS to be
  activated from each end.  It is especially useful when an IPSP is
  serving more than one AS.  It would need four messages for completion.

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4.3.4.4.  ASP Inactive Procedures

     When an ASP wishes to withdraw from receiving traffic within an AS,
  the ASP sends an ASP Inactive (ASPIA) message to the SGP or IPSP.
  This action MAY be initiated at the ASP by an M-ASP_INACTIVE request
  primitive from Layer Management or MAY be initiated automatically by
  an ISUA management function.  Whenever an ASP is processing the
  traffic for more than one Application Server across a common SCTP
  association, the ASP Inactive (ASPIA) message contains one or more
  Routing Contexts to indicate for which Application Servers the ASP
  Inactive (ASPIA) message applies.  Whenever an ASP Inactive (ASPIA)
  message does not contain a Routing Context parameter, the receiver
  must know, via configuration data, which Application Servers the ASP
  is a member and move the ASP to the ASP-INACTIVE state in each all
  Application Servers.  In the case of an Override mode AS, where
  another ASP has already taken over the traffic within the AS with an
  ASP Active (ASPAC) message, the ASP that sends the ASP Inactive
  (ASPIA) message is already considered by the SGP to be in state ASP-
  INACTIVE.  An ASP Inactive Ack (ASPIA ACK) message is sent to the ASP,
  after ensuring that all traffic is stopped to the ASP.

     In the case of a Load-share mode AS, the SGP moves the ASP to the
  ASP-INACTIVE state and the AS traffic is re-allocated across the
  remaining ASPs in the state ASP-ACTIVE, as per the load-sharing
  algorithm currently used within the AS.  A Notify (NTFY) message
  ("Insufficient ASP resources active in AS") MAY be sent to all
  inactive ASPs, if required.  An ASP Inactive Ack (ASPIA ACK) message
  is sent to the ASP after all traffic is halted and Layer Management is
  informed with an M-ASP_INACTIVE indication primitive.

     In the case of a Broadcast mode AS, the SGP moves the ASP to the
  ASP- INACTIVE state and the AS traffic is broadcast only to the
  remaining ASPs in the state ASP-ACTIVE.  A Notify (NTFY) message
  ("Insufficient ASP resources active in AS") MAY be sent to all
  inactive ASPs, if required.  An ASP Inactive Ack (ASPIA ACK) message
  is sent to the ASP after all traffic is halted and Layer Management is
  informed with an M-ASP_INACTIVE indication primitive.

     Multiple ASP Inactive Ack (ASPIA ACK) messages MAY be used in
  response to an ASP Inactive (ASPIA) message containing multiple
  Routing Contexts, allowing the SGP or IPSP to independently
  acknowledge for different (sets of) Routing Contexts.  The SGP or IPSP
  sends an Error (ERR) ("Invalid Routing Context") message for each
  invalid or not configured Routing Context value in a received ASP
  Inactive (ASPIA) message.

     The SGP MUST send an ASP Inactive Ack (ASPIA ACK) message in
  response to a received ASP Inactive (ASPIA) message from the ASP and
  the ASP is already marked as ASP-INACTIVE at the SGP.

     At the ASP, the ASP Inactive Ack (ASPIA ACK) message received is
  not acknowledged.  Layer Management is informed with an M-ASP_INACTIVE
  confirm primitive.  If the ASP receives an ASP Inactive Ack without
  having sent an ASP Inactive (ASPIA) message, the ASP should now

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  consider itself as in the ASP-INACTIVE state.  If the ASP was
  previously in the ASP-ACTIVE state, the ASP should then initiate
  procedures to return itself to its previous state.  When the ASP sends
  an ASP Inactive (ASPIA) message it starts timer T(ack).  If the ASP
  does not receive a response to an ASP Inactive (ASPIA) message within
  T(ack), the ASP MAY restart T(ack) and resend ASP Inactive (ASPIA)
  messages until it receives an ASP Inactive Ack (ASPIA ACK) message.
  T(ack) is provisionable, with a default of 2 seconds.  Alternatively,
  retransmission of ASP Inactive (ASPIA) messages MAY be put under
  control of Layer Management.  In this method, expiry of T(ack) results
  in a M-ASP_Inactive confirm primitive carrying a negative indication.

     If no other ASPs in the Application Server are in the state ASP-
  ACTIVE, the SGP MUST send a Notify (NTFY) message ("AS-Pending") to
  all of the ASPs in the AS which are in the state ASP-INACTIVE.  The
  SGP SHOULD start buffering the incoming messages for T(r) seconds,
  after which messages MAY be discarded.  T(r) is configurable by the
  network operator.  If the SGP receives an ASP Active (ASPAC) message
  from an ASP in the AS before expiry of T(r), the buffered traffic is
  directed to that ASP and the timer is canceled.  If T(r) expires, the
  AS is moved to the AS-INACTIVE state.

4.3.4.4.1.  IPSP Considerations

     An IPSP may be considered in the ASP-INACTIVE state by a remote
  IPSP after an ASP Inactive or ASP Inactive Ack (ASPIA ACK) message has
  been received from it.

     Alternatively, an interchange of ASPIA messages from each end can
  be performed.  This option follows the ASP state transition diagram
  and gives the additional advantage of selecting a particular AS to be
  deactivated from each end.  It is especially useful when an IPSP is
  serving more than one AS.  It would need four messages for completion.

4.3.4.5.  Notify Procedures

     A Notify (NTFY) message reflecting a change in the AS state MUST be
  sent to all ASPs in the AS, except those in the ASP-DOWN state, with
  appropriate Status Information and any ASP Identifier of the failed
  ASP.  At the ASP, Layer Management is informed with an M- NOTIFY
  indication primitive.  The Notify (NTFY) message must be sent whether
  the AS state change was a result of an ASP failure or reception of an
  ASP State management (ASPSM) or ASP Traffic Management (ASPTM)
  message.  In the second case, the Notify (NTFY) message MUST be sent
  after any ASP State or Traffic Management related acknowledgments
  messages (e.g, ASP Up Ack, ASP Down Ack, ASP Active Ack, or ASP
  Inactive Ack).

     Whenever a Notify (NTFY) ("AS-PENDING") message is sent by an SGP
  that now has no ASPs active to service the traffic, or where a Notify
  NTFY ("Insufficient ASP resources active in AS") message MUST be sent
  in the Load-share or Broadcast mode, the Notify (NTFY) message does
  not explicitly compel the ASP(s) receiving the message to become
  active.  The ASPs remain in control of what (and when) traffic action

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  is taken.

     Whenever a Notify (NTYF) message does not contain a Routing Context
  parameter, the receiver must know, via configuration data, of which
  Application Servers the ASP is a member and take the appropriate
  action in each AS.

4.3.4.5.1.  IPSP Considerations (NTFY)

     Notify works in the same manner as in the SG-AS case.  One of the
  IPSPs can send this message to any remote IPSP that is not in the ASP-
  DOWN state.

4.3.4.6.  Heartbeat Procedures

     The optional Heartbeat procedures MAY be used when operating over
  transport layers that do not have their own heartbeat mechanism for
  detecting loss of the transport association (i.e., other than SCTP).

     Either ISUA peer may optionally send Heartbeat (BEAT) messages
  periodically, subject to a provisionable timer T(beat).  Upon
  receiving a Heartbeat (BEAT) message, the ISUA peer MUST respond with
  a Heartbeat Ack (BEAT ACK) message.

     If no Heartbeat Ack (BEAT ACK) message (or any other ISUA message)
  is received from the ISUA peer within 2*T(beat), the remote ISUA peer
  is considered unavailable.  Transmission of Heartbeat (BEAT) messages
  is stopped and the signalling process SHOULD attempt to re-establish
  communication if it is configured as the client for the disconnected
  ISUA peer.

     The Heartbeat (BEAT) message may optionally contain an opaque
  Heartbeat Data parameter that MUST be echoed back unchanged in the
  related Heartbeat Ack (BEAT ACK) message.  The sender, upon examining
  the contents of the returned Heartbeat Ack (BEAT ACK) message, MAY
  choose to consider the remote ISUA peer as unavailable.  The contents
  and format of the Heartbeat Data parameter is implementation-dependent
  and only of local interest to the original sender.  The contents may
  be used, for example, to support a Heartbeat sequence algorithm (to
  detect missing Heartbeats), or a time-stamp mechanism (to evaluate
  delays).

     Note: Heartbeat related events are not shown in Figure 4 "ASP state
  transition diagram".

4.4.  Routing Key Management Procedures

4.4.1.  Registration

     An ASP MAY dynamically register with an SGP as an ASP within an
  Application Server using the REG REQ message.  A Routing Key parameter
  in the REG REQ message specifies the parameters associated with the
  Routing Key.

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     The SGP examines the contents of the received Routing Key parameter
  and compares it with the currently provisioned Routing Keys.  If the
  received Routing Key matches an existing SGP Routing Key entry, and
  the ASP is not currently included in the list of ASPs for the related
  Application Server, the SGP MAY authorize the ASP to be added to the
  AS.  Or, if the Routing Key does not currently exist and the received
  Routing Key data is valid and unique, an SGP supporting dynamic
  configuration MAY authorize the creation of a new Routing Key and
  related Application Server and add the ASP to the new AS.  In either
  case, the SGP returns a Registration Response (REG RSP) message to the
  ASP, containing the same Local-RK-Identifier as provided in the
  initial request, and a Registration Result "Successfully Registered".
  A unique Routing Context value assigned to the SGP Routing Key is
  included.  The method of Routing Context value assignment at the SGP
  is implementation dependent but must be guaranteed to be unique for
  each Application Server or Routing Key supported by the SGP.   If the
  SGP determines that the received Routing Key data is invalid, or
  contains invalid parameter values, the SGP returns a Registration
  Response (REG RSP) message to the ASP, containing a Registration
  Result "Error - Invalid Routing Key", "Error - Invalid DPC", "Error -
  Invalid Network Appearance" as appropriate.

     If the SGP does not support the registration procedure, the SGP
  returns an Error (ERR) message to the ASP, with an error code of
  "Unsupported Message Type".

     If the SGP determines that a unique Routing Key cannot be created,
  the SGP returns a Registration Response (REG RSP) message to the ASP,
  with a Registration Status of "Error - "Cannot Support Unique
  Routing." An incoming signalling message received at an SGP should not
  match against more than one Routing Key.

     If the SGP does not authorize the registration request, the SGP
  returns a REG RSP message to the ASP containing the Registration
  Result "Error - Permission Denied".

     If an SGP determines that a received Routing Key does not currently
  exist and the SGP does not support dynamic configuration, the SGP
  returns a Registration Response (REG RSP) message to the ASP,
  containing a Registration Result "Error - Routing Key not Currently
  Provisioned".

     If an SGP determines that a received Routing Key does not currently
  exist and the SGP supports dynamic configuration but does not have the
  capacity to add new Routing Key and Application Server entries, the
  SGP returns a Registration Response (REG RSP) message to the ASP,
  containing a Registration Result "Error - Insufficient Resources".

     If an SGP determines that one or more of the Routing Key parameters
  are not supported for the purpose of creating new Routing Key entries,
  the SGP returns a Registration Response (REG RSP) message to the ASP,
  containing a Registration Result "Error - Unsupported RK parameter
  field".  This result MAY be used if, for example, the SGP does not
  support RK Address parameter.

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     A Registration Response "Error - Unsupported Traffic Handling Mode"
  is returned if the Routing Key in the REG REQ contains a Traffic
  Handling Mode that is inconsistent with the presently configured mode
  for the matching Application Server.

     An ASP MAY register multiple Routing Keys at once by including a
  number of Routing Key parameters in a single REG REQ message.  The SGP
  MAY respond to each registration request in a single REG RSP message,
  indicating the success or failure result for each Routing Key in a
  separate Registration Result parameter.  Alternatively the SGP MAY
  respond with multiple REG RSP messages, each with one or more
  Registration Result parameters.  The ASP uses the Local-RK-Identifier
  parameter to correlate the requests with the responses.

     An ASP MAY modify an existing Routing Key by including a Routing
  Context parameter in the REG REQ.  If the SGP determines that the
  Routing Context applies to an existing Routing Key, the SG MAY adjust
  the existing Routing Key to match the new information provided in the
  Routing Key parameter.  A Registration Response "Routing Context
  Registration Refused" is returned if the SGP does not accept the
  modification of the Routing Key.

     Upon successful registration of an ASP in an AS, the SGP can now
  send related SS7 Signalling Network Management messaging, if this did
  not previously start upon the ASP transition to state ASP-INACTIVE

4.4.2.  Deregistration

     An ASP MAY dynamically deregister with an SGP as an ASP within an
  Application Server using the DEREG REQ message.  A Routing Context
  parameter in the DEREG REQ  message specifies which Routing Keys to
  deregister.  An ASP SHOULD move to the ASP-INACTIVE state for an
  Application Server before attempting to deregister the Routing Key
  (i.e., deregister after receiving an ASP Inactive Ack).  Also, an ASP
  SHOULD deregister from all Application Servers that it is a member
  before attempting to move to the ASP-Down state.

     The SGP examines the contents of the received Routing Context
  parameter and validates that the ASP is currently registered in the
  Application Server(s) related to the included Routing Context(s).  If
  validated, the ASP is deregistered as an ASP in the related
  Application Server.

     The deregistration procedure does not necessarily imply the
  deletion of Routing Key and Application Server configuration data at
  the SGP.  Other ASPs may continue to be associated with the
  Application Server, in which case the Routing Key data MUST NOT be
  deleted.  If a Deregistration results in no more ASPs in an
  Application Server, an SGP MAY delete the Routing Key data.

     The SGP acknowledges the deregistration request by returning a
  DEREG RSP message to the requesting ASP.  The result of the
  deregistration is found in the Deregistration Result parameter,
  indicating success or failure with cause.

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     An ASP MAY deregister multiple Routing Contexts at once by
  including a number of Routing Contexts in a single DEREG REQ message.
  The SGP MAY respond to each deregistration request in a single DEREG
  RSP message, indicating the success or failure result for each Routing
  Context in a separate Deregistration Result parameter.

4.4.3.  IPSP Considerations (REG/DEREG)

     The Registration and Deregistration procedures work in the IPSP
  cases in the same way as in AS-SG cases.  An IPSP may register an RK
  in the remote IPSP.  An IPSP is responsible for deregistering the RKs
  that it has registered.

4.5.  Procedures to Support Circuit and Call State

4.5.1.  At an SGP

     Upon receiving a RESET, BLOCKING, UNBLOCKING, CCT GROUP QUERY
  indication primitive from the nodal inter-working function at an SGP,
  the SGP ISUA layer will send a corresponding ISUA Circuit Supervision
  (CS) CRES, CBLO, CUBL or CQRY message (see Section 3) to the ISUA
  peers at concerned ASPs.  The ISUA layer must fill in various fields
  of the CS messages consistently with the information received in the
  primitives.

     CS messages SHOULD NOT be sent on stream "0" and MAY use ordered
  delivery.

4.5.2.  At an ASP

4.5.2.1.  Single SG Configurations

     At an ASP, upon receiving an ISUA Circuit Supervision (CS) message
  from the remote ISUA Peer, the ISUA layer invokes the appropriate
  primitive indications to the resident Call Control.  Local management
  is informed.

     Whenever a local event has caused the change in state of ISUP
  circuits, the ISUA layer at the ASP SHOULD pass up appropriate
  indications in the primitives to the ISUA User, as though equivalent
  CS messages were received.  For example, the loss of an SCTP
  association to an SGP may cause the software blocking of a set of ISUP
  circuits.  BLOCKING indication primitives to the ISUA User are
  appropriate.

4.5.2.2.  Multiple SG Configurations

     At an ASP, upon receiving an ISUA Circuit Supervision (CS) message
  from the remote ISUA Peer, the ISUA layer updates the status of the
  affected circuit(s) via the originating SG and determines, whether or
  not the overall status of the affected circuits(s) has changed.  If
  so, the ISUA layer invokes the appropriate primitive indications to
  the resident Call Control [5].  Local management is informed.

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4.5.3.  ASP Auditing

     An ASP may optionally initiate an audit procedure to inquire of an
  SG the status of a circuit or circuit(s).  A Circuit Query (CQRY)
  message is sent from the ASP to the SGP requesting the current status
  of one or more circuits.

  The CQRY message MAY be sent with unordered delivery.  The ASP MAY
  send the CQRY in the following cases:

  - Periodic:   A Timer originally set upon reception of a CBLO message
                has expired without a subsequent CUBL or CQRY message
                updating the circuit status of the affected circuits.
                The Timer is reset upon issuing a CQRY.  In this case
                the CQRY is sent to the SGP that originally sent the CS
                message.

  - Isolation:  The ASP is newly ASP-ACTIVE or has been isolated from an
                SG for an extended period.  The ASP MAY request status
                of one or more ISUP circuits for which it expects to
                communicate.

     The SGP SHOULD either respond to a CQRY messages with CS messages
  indicating the status of the circuit, or SHOULD respond with an ERR
  ("Circuit Status Unknown") or ERR ("Call Reference Status Unknown")
  message for each Circuit Id or Call Reference requested in the CQRY
  message.

     The status of each ISUP circuit requested is indicated in a CQRY
  response message.  If the SGP cannot return information on the status
  of the ISUP circuit or call reference, the SGP responds with an ERR
  ("Circuit Status Unknown") or ERR ("Call Reference Status Unknown")
  with a list of all the Circuit Ids and Call References for which the
  SGP cannot provide information.

     In some cases, the SGP MAY chose not to respond to a CQRY message
  or a component of a CQRY message on the basis of policy [6].

     Any CQRY message in response to a CQRY message MAY contain a list
  of Call References.

4.5.4.  ISUP - ISUA Interworking at the SG

     On the SG, the ISUP routing or interworking function determines
  that the message must be sent to an AS via the ISUA stack, based on
  information in the incoming message.  The ISUA outgoing mapping
  function identifies the appropriate Application Server (AS) and
  selects an active ASP from the list of ASPs servicing this AS.  The
  appropriate ASP can be determined based on the routing information in
  the incoming message, local load sharing information, etc.  The
  appropriate ISUA message is then constructed and sent to the
  appropriate endpoint, via the correct SCTP association and stream.

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4.5.4.1.  Primitives received from the local Call Control

     These support the ISUA transport of Call Control boundary
  primitives.  The same services as supported by ISUP are to be provided
  by ISUA.  Call Control at the SG should be able to use the same
  primitive interface to ISUP/ISUA without any changes.  The ISUP-ISUA
  interworking function takes care of selecting the appropriate stack.

     The ISUA needs to setup and maintain the appropriate SCTP
  association to the selected endpoint.  ISUA also manages the usage of
  SCTP streams.  The address information passed by the ISUA-user at an
  ASP must contain:

   (1)   a valid circuit identifier to specify an ISUP circuit in the
         SS7 network via the appropriate SCTP association to a SG

   (2)   a valid IP address or host name to reach another ASP in the IP
         network via the appropriate SCTP association.

5.  Examples of ISUA Procedures

5.1.  Establishment of Association and Traffic between SGPs and ASPs

5.1.1.1.  Single ASP in an Application Server ("1+0" sparing)

     This scenario shows the example ISUA message flows for the
  establishment of traffic between an SG and an ASP, where only one ASP
  is configured within an AS (no backup).  It is assumed that the SCTP
  association is already set-up.

                 SG                              ASP
                  |                                |
                  |<-------------ASP Up------------|
                  |-----------ASP-Up Ack---------->|
                  |                                |
                  |<------- ASP Active-------------|
                  |-----ASP Active Ack------------>|
                  |                                |

5.1.1.2.  Two ASPs in Application Server ("1+1" sparing)

     This scenario shows the example ISUA message flows for the
  establishment of traffic between an SG and two ASPs in the same
  Application Server, where ASP1 is configured to be "active" and ASP2 a
  "standby" in the event of communication failure or the withdrawal from
  service of ASP1.  ASP2 may act as a hot, warm, or cold standby
  depending on the extent to which ASP1 and ASP2 share call or
  transaction state or can communicate call state under failure or
  withdrawal events.  The example message flow is the same whether the
  ASP Active (ASPAC) messages are Override or Load-share mode although
  typically this example would use an Override mode.

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       SG                       ASP1                       ASP2
        |                         |                          |
        |<--------ASP Up----------|                          |
        |-------ASP-Up Ack------->|                          |
        |                         |                          |
        |<-----------------------------ASP Up----------------|
        |-----------------------------ASP-Up Ack------------>|
        |                         |                          |
        |                         |                          |
        |<-------ASP Active-------|                          |
        |------ASP-Active Ack---->|                          |
        |                         |                          |

5.1.1.3.  Two ASPs in an Application Server ("1+1" sparing, load-sharing
case)

     This scenario shows the example ISUA message flows for the
  establishment of traffic between an SG and two ASPs in the same
  Application Server, where the two ASPs are brought to "active" and
  load-share the traffic load.  In this case, one ASP is sufficient to
  handle the total traffic load.

       SG                       ASP1                       ASP2
        |                         |                          |
        |<---------ASP Up---------|                          |
        |--------ASP-Up Ack------>|                          |
        |                         |                          |
        |<------------------------------ASP Up---------------|
        |-----------------------------ASP Up Ack------------>|
        |                         |                          |
        |                         |                          |
        |<--ASP Active -----------|                          |
        |-----ASP-Active Ack----->|                          |
        |                         |                          |
        |<----------------------------ASP Active ------------|
        |-------------------------------ASP-Active Ack------>|
        |                         |                          |

5.1.1.4.  Three ASPs in an Application Server ("n+k" sparing, load-
sharing case)

     This scenario shows the example ISUA message flows for the
  establishment of traffic between an SG and three ASPs in the same
  Application Server, where two of the ASPs are brought to "active" and
  share the load.  In this case, a minimum of two ASPs are required to
  handle the total traffic load (2+1 sparing).

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    SG                  ASP1                 ASP2                ASP3
     |                    |                   |                   |
     |<------ASP Up-------|                   |                   |
     |-----ASP-Up Ack---->|                   |                   |
     |                    |                   |                   |
     |<--------------------------ASP Up-------|                   |
     |-------------------------ASP-Up Ack---->|                   |
     |                    |                   |                   |
     |<---------------------------------------------ASP Up--------|
     |---------------------------------------------ASP-Up Ack---->|
     |                    |                   |                   |
     |                    |                   |                   |
     |<--ASP Act ---------|                   |                   |
     |----ASP-Act Ack---->|                   |                   |
     |                    |                   |                   |
     |<--------------------ASP Act  ----------|                   |
     |-----------------------ASP-Act Ack----->|                   |
     |                    |                   |                   |

5.1.2.  ASP Traffic Fail-over Examples

5.1.2.1.  (1+1 Sparing, withdrawal of ASP, Back-up Override)

     ASP1 withdraws from service:

       SG                       ASP1                       ASP2
        |                         |                          |
        |<-----ASP Inactive-------|                          |
        |----ASP Inactive Ack---->|                          |
        |-----------------------NTFY(ASP-Inact.)(Optional)-->|
        |                         |                          |
        |<------------------------------ ASP Active----------|
        |------------------------------ASP-Active Ack------->|
        |                                                    |

  Note: If the SG detects loss of the ISUA peer (ISUA heartbeat loss or
        detection of SCTP failure), the initial SG-ASP1 ASP Inactive
        (ASPIA) message exchange would not occur.

5.1.2.2.  (1+1 Sparing, Back-up Override)

     ASP2 wishes to override ASP1 and take over the traffic:

       SG                       ASP1                       ASP2
        |                         |                          |
        |<------------------------------ ASP Active----------|
        |-------------------------------ASP-Active Ack------>|
        |----NTFY(Alt ASP-Act)--->|
        |                         |                          |

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5.1.2.3.  (n+k Sparing, Load-sharing case, withdrawal of ASP)

     ASP1 withdraws from service:

    SG                  ASP1                 ASP2                 ASP3
     |                    |                   |                   |
     |<----ASP Inact.-----|                   |                   |
     |---ASP-Inact Ack--->|                   |                   |
     |                    |                   |                   |
     |---------------------------------NTFY(Ins. ASPs)(Optional)->|
     |                    |                   |                   |
     |<-----------------------------------------ASP Act ----------|
     |-------------------------------------------ASP Act (Ack)--->|
     |                    |                   |                   |

     The Notify (NTFY) message to ASP3 is optional, as well as the ASP-
  Active from ASP3.  The optional Notify can only occur if the SG
  maintains knowledge of the minimum ASP resources required - for
  example if the SG knows that "n+k" = "2+1" for a load-share AS and "n"
  currently equals "1".

  Note: If the SG detects loss of the ASP1 ISUA peer (ISUA heartbeat
        loss or detection of SCTP failure), the first SG-ASP1 ASP
        Inactive (ASPIA) message exchange would not occur.

5.1.3.  ISUP/CC Service Translation Examples

  When the ISUA layer on the ASP has a CP message to send to the SG, it
  will do the following:

   (1)   Determine the correct SGP

   (2)   Find the SCTP association to the chosen SGP

   (3)   Determine the correct stream in the SCTP association based on
         the DID

   (4)   Build the CP message, fill ISUA Message Header, fill Common
         Header

   (5)   Send the CP message to the remote ISUA peer in the SG, over the
         SCTP association

  When the ISUA layer on the SG has a CP message to send to the ASP, it
  will do the following:

   (1)   Determine the AS

   (2)   Determine the Active ASP (SCTP association) within the AS

   (3)   Determine the correct stream in the SCTP association based on
         the DID

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   (4)   Build the CP message, fill in ISUA Message Header, fill in
         Common Header

   (5)   Send the CP message to the remote ISUA peer in the ASP, over
         the SCTP association

  An example of the message flows for establishing a dialogue service is
  shown below.  An active association between ASP and SG is established
  (Section 5.1) prior to the following message flows.

               SG                             ASP

                          <----------- Invoke Request
                          <----------- Query(Begin) Request

    Conversation(Continue)
        Indication        ---------->
    Result Indication     ---------->

                          <----------- Invoke Request
                          <----------- Conversation(Continue) Request
                              .
                              .
                              .
    End(response)Indication ----------->

        Result Indication ----------->

     An example of the message flows for a failed attempt to establish a
  dialogue on the signalling channel is shown below.  In this case, the
  gateway has a problem with its physical connection , so it cannot
  establish a dialogue on the signalling channel.

               SG                             ASP

                            <----------- Invoke Request
                            <----------- Query(Begin) Request

         Abort Indication   ---------->

5.2.  IP-IP Architecture

     The sequences below outline logical steps for a variety of
  scenarios within an IP-IP architecture.  Please note that these
  scenarios cover a Primary/Backup configuration.  Where there is a
  load-sharing configuration then the AS can declare availability when 1
  ASP issues ASPAC but can only declare unavailability when all ASPs
  have issued ASPIA.

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5.2.1.  Establishment of ISUA connectivity

     The following shows an example establishment of ISUA connectivity.
  In this example, each IP SP consists of a Management Instance (MI) and
  two ASPs.  The Management Instance handles the address mapping
  mechanisms and monitors the states of the remote peer.  For
  simplicity, the Management Instances and ASPs are considered as a
  separate entity.  This is not a requirement, as they can be collocated
  with an ASP.

     The following must be established before ISUA traffic can flow.  A
  connection-less flow is shown for simplicity.

     Each node is configured (via MIB, for example) with the connections
  that need to be setup

    IP SEP A                                                  IP SEP B
    ASP-a1     ASP-a2     MI a             MI b       ASP-b2    ASP-b1
    (Primary) (Backup)                               (Backup) (Primary)

                Establish SCTP Connectivity

                          |-- Est. SCTP Ass.--|

    |------ Establish SCTP Association -------|
    |------------- Establish SCTP Association -------------|
    |------------------ Establish SCTP Association ------------------|

               |--- Establish SCTP Assoc. ----|
               |------- Establish SCTP Association --------|
               |------------ Establish SCTP Association -------------|

                          |-- Establish SCTP Association -|
                          |------- Establish SCTP Association ------|

                Establish ISUA Connectivity

    +---------------ASP Up------------------->
    <---------------ASP Up Ack---------------+

               +------------ASP Up----------->
               <------------ASP Up Ack-------+

                         <--------------ASP Up-------------+
                         +--------------ASP Up Ack--------->

                         <----------------ASP Up---------------------+
                         +----------------ASP Up Ack----------------->

    +---------------ASP Act------------------>
    <---------------ASP Act Ack--------------+

                         <----------------ASP Act--------------------+
                         +----------------ASP Act Ack---------------->

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    Traffic can now flow directly between ASPs.

    +-------------------------------ISUP_User Message------------------>

5.2.2.  Fail-over scenarios

     The following sequences address fail-over of ASP

5.2.2.1.  Successful ASP Fail-over scenario

     The following is an example of a successful fail-over scenario,
  where there is a fail-over from ASP-a1 to ASP-a2, i.e, Primary to
  Backup.  Since data transfer passes directly between peer ASPs, ASP-b1
  is notified of the fail-over of ASP-a1 and must buffer outgoing data
  messages until ASP-a2 becomes available.

    IP SEP A                                                  IP SEP B
    ASP-a1     ASP-a2     MI a             MI b       ASP-b2    ASP-b1
    (Primary) (Backup)                               (Backup) (Primary)
    +--------------ASP Inact----------------->
    <--------------ASP Inact Ack-------------+

               <----NTFY (ASP-a1 Inactive)---+

               +----------ASP Act------------>
               <----------ASP Act Ack--------+

5.2.2.2.  Unsuccessful ASP Fail-over scenario

     The sequence is the same as 5.2.2.1 except that, since the backup
  fails to come in then, the Notify (NTFY) messages declaring the
  availability of the backup are not sent.

6.  Security

6.1.  Introduction

     ISUA is designed to carry signalling messages for telephone
  services.  As such, ISUA involves the security needs of several
  parties: the end users of the services; the network providers and the
  applications involved.  Additional security requirements may come from
  local regulation.  While having some overlapping security needs, any
  security solution should fulfill all of the different parties' needs.

6.2.  Threats

  There is no quick fix, one-size-fits-all solution for security.  As a
  transport protocol, ISUA has the following security objectives:

   - Availability of reliable and timely user data transport.
   - Integrity of user data transport.

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   - Confidentiality of user data.

     ISUA runs on top of SCTP.  SCTP provides certain transport related
  security features, such as:

   - Blind Denial of Service Attacks
   - Flooding
   - Masquerade
   - Improper Monopolization of Services

     When ISUA is running in professionally managed corporate or service
  provider network, it is reasonable to expect that this network include
  an appropriate security policy framework.  The "Site Security
  Handbook" [RFC 2196] should be consulted for guidance.

     SS7 networks have a different security model that IP networks.
  Traditionally, the PSTN has been a private and closed network, where
  in many cases, to get connectivity, one would need to be a service
  provider and negotiate physical connections to the PSTN.

     The Internet has a slightly different security mode, on which
  connectivity is a primary goal.  When signalling protocols are run
  over IP, one must be aware that it is impossible to guarantee that the
  IP network will be physically separate from another IP network.
  Firewalls and gateways may create an illusion of separateness, but do
  not guarantee this.  One mis-configured parameter in a firewall could
  leave a dangerous security hole.

     The most reasonable security model for ISUA is to assume a virtual
  private network (VPN) type of security, where TLS of IPsec are used to
  encrypt traffic between nodes.

6.3.  Protecting Confidentiality

     Particularly for mobile users, the requirement for confidentiality
  may include the masking of IP addresses and ports.  In this case
  application level encryption is not sufficient; IPSEC ESP should be
  used instead.  Regardless of which level performs the encryption, the
  IPSEC ISAKMP service should be used for key management.

6.4.  IPsec Usage

     All ISUA implementations MUST support IPsec ESP [RFC 2406] in
  transport mode with non-null encryption and authentication algorithms
  to provide per-packet authentication, integrity protection and
  confidentiality, and MUST support the replay protection mechanisms of
  IPsec.

     ISUA implementations MUST support IKE for peer authentication,
  negotiation of security associations, and key management, using IPsec
  DOI [RFC 2407].  ISUA implementations MUST support peer authentication
  using a pre-shared key, and MAY support certificate-authentication
  using the public key encryption methods outlined in IKE sections 5.2
  and 5.3 [RFC 2409] SHOULD NOT be used.

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     Conforming implementations MUST support both IDE Main Mode and
  Aggressive Mode.  When pre-shared keys are used for authentication,
  IKE Aggressive Mode SHOULD be used, and IKE Main Mode SHOULD NOT be
  used.  When digital signatures are used for authentication, either IKE
  Main Mode or IKE Aggressive Mode MAY be used.

     When digital signatures are used to achieve authentication, an IKE
  negotiator SHOULD use IKE Certificate Request Payload(s) to specify
  the certificate authority (or authorities) that are trusted in
  accordance with it local policy.  IKE negotiators SHOULD use pertinent
  certificate revocation checks before accepting a PKI certificate for
  use in IKE's authentication procedures.

     The Phase 2 Quick Mode exchanges used to negotiate protection for
  ISUA connections MUST explicitly carry the Identity Payload fields
  (IDci and IDcr).  The DOI provides for several types of
  implementations, each ID Payload MUST carry a single IP address and a
  single non-zero port number, and MUST NOT use the IP Subnet or IP
  Address Range formats.  This allows the Phase 2 security association
  to correspond to specific TCP and SCTP connections.

     Since IPsec acceleration hardware may only be able to handle a
  limited number of active IKE Phase 2 SAs, Phase 2 delete messages may
  be sent for idle SAs, as a means of keeping the number of active Phase
  2 SAs to a minimum.  The receipt of an IKE Phase 2 delete message
  SHOULD NOT be interpreted as a reason for tearing down a ISUA
  connection.  Rather, it is preferable to leave the connection up, and
  if additional traffic is sent on it, to bring up another IKE Phase 2
  SA to protect it.  This avoids the potential for continually bringing
  connections up and down.

6.5.  TLS Usage

     A ISUA peer that initiates a connection to another ISUA peer acts
  as a TLS client according to TLS [RFC 2246, RFC 3436], and a ISUA peer
  that accepts a connection acts as a TLS server.  ISUA peers
  implementing TLS for security MUST mutually authenticate as part of
  TLS session establishment.  To ensure mutual authentication, the ISUA
  node acting as TLS server must request a certificate from the ISUA
  node acting as TLS client, and the ISUA node acting as TLS client MUST
  be prepared to supply a certificate on request.

  ISUA peers supporting TLS MUST be able to negotiate the following TLS
  cipher suites:

    TLS_RSA_WITH_RC4_128_MD5
    TLS_RSA_WITH_RC4_128_SHA
    TLS_RSA_WITH_3DES_EDE_CBC_SHA

     ISUA nodes MAY negotiate other TLS cipher suites.

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6.6.  Peer-to-Peer Considerations

     As with any peer-to-peer protocol, proper configuration of the
  trust model within a ISUA peer is essential to security.  When
  certificates are used, it is necessary to configure the root
  certificate authorities trusted by the ISUA peer.  These root CAs are
  likely to be unique to ISUA usage and distinct from the root CAs that
  might be trusted for other purposes such as Web browsing.  In general,
  it is expected that those root CAs will be configured to reflect the
  business relationships between the organization hosting the ISUA peer
  and other organizations.  Therefore, a ISUA peer will typically not be
  configured to allow connectivity with any arbitrary peer.  With
  certificate authentication, ISUA peers might not be known beforehand,
  and therefore peer discovery may be required.

     Note that IPsec is considerably less flexible than TLS when it
  comes to configuring root CAs.  Since use of Port identifiers is
  prohibited within IKE Phase 1, within IPsec it is not possible to
  uniquely configure trusted root CAs for each application individually;
  the same policy must be used for all applications.  This implies, for
  example, that a root CA trusted for use with ISUA must also be trusted
  to protect SNMP.  These restrictions can be awkward at best.  Since
  TLS supports application-level granularity in certificate policy, TLS
  SHOULD be used to protect ISUA connections between administrative
  domains.  IPsec is most appropriate for intra-domain usage when pre-
  shared keys are used as a security mechanism.

     When pre-shared key authentication is used with IPsec to protect
  ISUA, unique pre-shared keys are configured with ISUA peers, who are
  identified by their IP address (Main Mode), or possibly their FQDN
  (Aggressive Mode).  As a result, it is necessary for the set of ISUA
  peers to be known beforehand.  Therefore, peer discovery is typically
  not necessary.

  The following is intended to provide some guidance on the issue.

     It is recommended that a ISUA peer implement the same security
  mechanism (IPsec or TLS) across all its peer-to-peer connections.
  Inconsistent use of security mechanisms can result in redundant
  security mechanisms being used (e.g. TLS over IPsec) or worse,
  potential security vulnerabilities.  When IPsec is used with ISUA, a
  typical security policy for outbound traffic is "Initiate IPsec, from
  me to any, destination port ISUA"; for inbound traffic, the policy
  would be "Require IPsec, from any to me, destination port ISUA".

     This policy causes IPsec to be used whenever a ISUA peer initiates
  a connection to another ISUA peer, and to be required whenever an
  inbound ISUA connection occurs.  This policy is attractive, since it
  does not require policy to be set for each peer or dynamically
  modified each time a new ISUA connection is created; an IPsec SA is
  automatically created based on a simple static policy.  Since IPsec
  extensions are typically not available to the sockets API on most
  platforms, and IPsec policy functionality is implementation dependent,
  use of a simple static policy is the often the simplest route to

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  IPsec-enabling a ISUA implementation.

     One implication of the recommended policy is that if a node is
  using both TLS and IPsec, there is not a convenient way in which to
  use either TLS or IPsec, but not both, without reserving an additional
  port for TLS usage.  Since ISUA uses the same port for TLS and non-TLS
  usage, where the recommended IPsec policy is put in place, a TLS-
  protected connection will match the IPsec policy, and both IPsec and
  TLS will be used to protect the ISUA connection.  To avoid this, it
  would be necessary to plumb peer-specific policies either statically
  or dynamically.

     If IPsec is used to secure ISUA peer-to-peer connections, IPsec
  policy SHOULD be set so as to require IPsec protection for inbound
  connections, and to initiate IPsec protection for outbound
  connections.  This can be accomplished via use of inbound and outbound
  filter policy.

7.  IANA Considerations

7.1.  SCTP Payload Protocol ID

  IANA has assigned a ISUA value for the Payload Protocol Identifier in
  the SCTP DATA chunk.  The following SCTP Payload Protocol Identifier
  is registered:

                 ISUA "5"

     The SCTP Payload Protocol Identifier value "5" SHOULD be included
  in each SCTP DATA chunk, to indicate that the SCTP is carrying the
  ISUA protocol.  The value "0" (unspecified) is also allowed but any
  other values MUST NOT be used.  This Payload Protocol Identifier is
  not directly used by SCTP but MAY be used by certain network entities
  to identify the type of information being carried in a DATA chunk.

      EDITOR'S NOTE:-  The value shown above as "5" is to be
      assigned by IANA an may change in future versions of this
      document.

     The User Adaptation peer MAY use the Payload Protocol Identifier,
  as a way of determining additional information about the data being
  presented to it by SCTP.  A request will be made to IANA to assign CTP
  Payload Protocol IDs.

7.2.  Port Number

     IANA has registered SCTP Port Number 14001 for ISUA.  It is
  recommended that SGPs use this SCTP port number for listening for new
  connections.  SGPs MAY also use statically configured SCTP port
  numbers instead.

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7.3.  Protocol Extensions

     This protocol may also be extended through IANA in three ways:

   - Through definition of additional message classes.
   - Through definition of additional message types.
   - Through definition of additional message parameters.

     The definition and use of new message classes, types and parameters
  is an integral part of SIGTRAN adaptation layers.  Thus, these
  extensions are assigned by IANA through an IETF Consensus action as
  defined in [RFC 2434].

     The proposed extension MUST in no way adversely affect the general
  working of the protocol.

     A new registry will be created by IANA to allow

7.3.1.  IETF Defined Message Classes

     The documentation for a new message class MUST include the
  following information:

   (1)   A long and short name for the message class;

   (2)   A detailed description of the purpose of the message class.

7.3.2.  IETF Defined Message Types

     Documentation of the message type MUST contain the following
  information:

   (1)   A long and short name for the new message type;

   (2)   A detailed description of the structure of the message.

   (3)   A detailed definition and description of intended use of each
         field within the message.

   (4)   A detailed procedural description of the use of the new message
         type within the operation of the protocol.

   (5)   A detailed description of error conditions when receiving this
         message type.

     When an implementation receives a message type which it does not
  support, it MUST respond with an Error (ERR) message, with an Error
  Code = Unsupported Message Type.

7.3.3.  IETF-defined TLV Parameter Extension

     Documentation of the message parameter MUST contain the following
  information:

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   (1)   Name of the parameter type.

   (2)   Detailed description of the structure of the parameter field.
         This structure MUST conform to the general type-length-value
         format described earlier in the document.

   (3)   Detailed definition of each component of the parameter value.

   (4)   Detailed description of the intended use of this parameter
         type, and an indication of whether and under what circumstances
         multiple instances of this parameter type may be found within
         the same message type.

8.  Timer Values

     Following are the RECOMMENDED timer values for ISUA timers:

      Timer     Description                  Value
      ------------------------------------------------
      Ta        -                           2 seconds
      Tr        -                           2 seconds
      T(ack)    Inactivity Send Timer       7 minutes
      T(ias)    Inactivity Receive Timer   15 minutes
      T(beat)   Heartbeat Timer            30 seconds

Acknowledgments

     The authors would like to thank Jianxing Hou, Min Lin for their
  original input to this document, and to the authors of M2UA, M3UA and
  SUA for the large sections of text which apply also to ISUA and was
  included here.

End Notes

  [1]  IMPLEMENTATION NOTE:-  Only one SCTP port may be defined for each
       endpoint, but each SCTP endpoint may have multiple IP addresses
       [RFC 2960].

  [2]  IMPLEMENTATION NOTE:-  The use of TLV in principle allows the
       parameters to be placed in a random order in the message.
       However, some guidelines should be considered for easy processing
       in the following order:

        - parameters needed to correctly process other message
          parameters, preferably should precede these parameters (such
          as Routing Context).
        - Mandatory parameters preferably SHOULD precede any optional
          parameters.
        - The data parameter will normally be the final one in the
          message.

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        - The receiver SHOULD accept parameters in any order, except
          where explicitly mandated.

  [3]  IMPLEMENTATION NOTE:-  An Application Server Process may be
       configured to process traffic for more than one logical
       Application Server.  From the perspective of an ASP, a Routing
       Context defines a range of signalling traffic that the ASP is
       currently configured to receive from the SG.

       Additionally, the Routing Context parameter identifies the SS7
       network context for the message, for the purposes of logically
       separating the signalling traffic between the SGP and the
       Application Server Process over a common SCTP Association, when
       needed.  An example is where an SGP is logically partitioned to
       appear as an element in several different national SS7 networks.
       It implicitly defines the SS7 Point Code format used, the SS7
       Network Indicator value and ISUP protocol type/variant/version
       used within a separate SS7 network.  It also defines the network
       context for the PC and SSN values.  Where an SGP operates in the
       context of a single SS7 network, or individual SCTP associations
       are dedicated to each SS7 network context, this functionality is
       not needed.

  [4]  IMPLEMENTATION NOTE:-  Correlation Id parameter can be used for
       features like Synchronization of ASPs and SGPs in a Broadcast
       Mode AS or SG; avoid message duplication and mis-sequencing in
       case of fail-over of association from one ASP or SGP to another
       ASP or SGP, etc.

       For application of the Correlation Id parameter see CORID
       [CORID].

  [5]  IMPLEMENTATION NOTE:-  To accomplish the handling of CS messages
       from multiple SGs in a multiple SG configuration, the ISUA layer
       at an ASP maintains the status of circuits via each SG.

  [6]  IMPLEMENTATION NOTE:-  For example, an SGP MAY chose to not
       respond to a request for the circuit status of a specific circuit
       in the CQRY message because the ASP that issued the CQRY message
       is not authorized to obtain information concerning the status of
       the circuit as requested.

References

  RFC 2960.
       R. Stewart, Q. Xie, K. Morneault, C. Sharp, H. J. Schwarzbauer,
       T. Taylor, I. Rytina, H. Kalla, L. Zhang and V. Paxson, "Stream
       Control Transmission Protocol (SCTP)," RFC 2960, The Internet
       Society (February 2000).  [Normative]

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  Q.761.
       ITU, "Signalling System No. 7 - Functional Description of the
       ISDN User Part," ITU-T Recommendation Q.761, ITU-T
       Telecommunication Standardization Sector of ITU, Geneva (March
       1993).  [Informative]

  T1.113.
       ANSI, "Signalling System No. 7 - ISDN User Part," ANSI T1.113,
       American National Standards Institue (1992).  [Informative]

  RFC 2719.
       L. Ong, I. Rytina, M. Holdrege, L. Coene, M.-A. Garcia, C. Sharp,
       I. Juhasz, H. P. Lin and HannsJ. Schwarzbauer, "Framework
       Architecture for Signaling Transport," RFC 2719, The Internet
       Society (October, 1999).  [Informative]

  Q.701.
       ITU, "Functional Description of the Message Transfer Part (MTP)
       of Signalling System No. 7," ITU-T Recommendation Q.701, ITU-T
       Telecommunication Standardization Sector of ITU, Geneva (March
       1993).  [Informative]

  T1.111.
       ANSI, "Signalling System No. 7 - Message Transfer Part," ANSI
       T1.111, American National Standards Institue (1992).
       [Informative]

  Q.724.
       ITU, "Signalling System No. 7 - Telephone User Part - Signalling
       Procedures," ITU-T Recommendation Q.724, ITU-T Telecommunication
       Standardization Sector of ITU, Geneva (November 1988).
       (Previously "CCITT Recommendation")

  Q.764.
       ITU, "Signalling System No. 7 - ISDN User Part Signalling
       Procedures," ITU-T Recommendation Q.764, ITU-T Telecommunication
       Standardization Sector of ITU, Geneva (March 1993).  (Previously
       "CCITT Recommendation")

  Q.704.
       ITU, "Message Transfer Part - Signalling Network Functions and
       Messages," ITU-T Recommendation Q.704, ITU-T Telecommunication
       Standardization Sector of ITU, Geneva (March 1993).
       [Informative]

  M3UA.
       G. Sidebottom, K. Morneault and J. Pastor-Balbas, (eds),
       "Signaling System 7 (SS7) Message Transfer Part 3 (MTP3) - User
       Adaptation Layer (M3UA)," RFC 3332, Internet Engineering Task
       Force - Signalling Transport Working Group (September, 2002).

  RFC 2119.
       S. Bradner, "Key words for use in RFCs to Indicate Requirement
       Levels," RFC 2119 - BCP 14, Internet Engineering Task Force

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       (March 1997).  [Normative]

  Q.763.
       ITU, "Signalling System No. 7 - Formats and Codes of the ISDN
       User Part," ITU-T Recommendation Q.763, ITU-T Telecommunication
       Standardization Sector of ITU, Geneva (March 1993).  (Previously
       "CCITT Recommendation")

  RFC 2279.
       F. Yergenau, "UTF-8, a transformation format of ISO 10646," RFC
       2279, Internet Engineering Task Force (January 1998).
       [Normative]

  CORID.
       B. Bidulock, "Correlation Id and Heartbeat Procedures Supporting
       Lossless Fail-Over," <draft-bidulock-sigtran-corid-01.txt>,
       Internet Engineering Task Force - Signalling Transport Working
       Group (January 2, 2003).  [Informative]

  RFC 2196.
       B. Y. Frazer, "Site Security Handbook," RFC 2196, Internet
       Engineering Task Force (September 1997).  [Normative]

  RFC 2406.
       S. Kent, R. Atkinson, "IP Encapsulating Security Payload (ESP),"
       RFC 2406, Internet Engineering Task Force (November 1998).
       [Normative]

  RFC 2407.
       D. Piper, "The Internet IP Security Domain of Interpretation for
       ISAKMP," RFC 2407, Internet Engineering Task Force (November
       1998).  [Normative]

  RFC 2409.
       D. Harkins, D. Carrel, "The Internet Key Exchange (IKE)," RFC
       2409, Internet Engineering Task Force (November 1998).
       [Normative]

  RFC 2246.
       T. Dierke, C. Allen, "The TLS Protocol - Version 1.0," RFC 2246,
       The Internet Society (January 1999).  [Normative]

  RFC 3436.
       A. Jungmaier, E. Rescorla and M. Tuxen, "Transport Layer Security
       over Stream Control Transmission Protocol," RFC 3436, The
       Internet Society (December 2002).  [Normative]

  RFC 2434.
       T. Narten, H. T. Alvestrand, "Guidelines for Writing an IANA
       Considerations Section in RFCs," RFC 2434, The Internet Society
       (October, 1998).  [Normative]

Internet Draft       SS7 ISUP-User Adaptation Layer      January 5, 2003

Author's Addresses

  Brian Bidulock                                  Phone: +1-780-490-1141
  OpenSS7 Corporation                        Email: bidulock@openss7.org
  1469 Jeffreys Crescent                    URL: http://www.openss7.org/
  Edmonton, AB  T6L 6T1
  Canada

  This draft expires July 2003.

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Appendices

A.  Operational Considerations

A.1.  Signalling Network Architecture

     A Signalling Gateway is used to support the transport of Call
  Control signalling traffic received from the SS7 network to multiple
  distributed ASPs (e.g., MGCs and IP Databases).  Clearly, the ISUA
  protocol is not designed to meet the performance and reliability
  requirements for such transport by itself.  However, the conjunction
  of distributed architecture and redundant networks provides support
  for reliable transport of signalling traffic over IP.  The ISUA
  protocol is flexible enough to allow its operation and management in a
  variety of physical configurations, enabling Network Operators to meet
  their performance and reliability requirements.

     To meet the stringent SS7 signalling reliability and performance
  requirements for carrier grade networks, Network Operators might
  require that no single point of failure is present in the end-to-end
  network architecture between an SS7 node and an IP-based application.
  This can typically be achieved through the use of redundant SGPs or
  SGs, redundant hosts, and the provision of redundant QOS-bounded IP
  network paths for SCTP Associations between SCTP End Points.
  Obviously, the reliability of the SG, the MGC and other IP-based
  functional elements also needs to be taken into account.  The
  distribution of ASPs and SGPs within the available Hosts MAY also be
  considered.  As an example, for a particular Application Server, the
  related ASPs could be distributed over at least two Hosts.

     One example of a physical network architecture relevant to SS7
  carrier-grade operation in the IP network domain is shown in Figure 7.

            SGs                                     MGCs
           ..............                          ..............
    Host#1 :   ______   :                          :   ______   : Host#3
           :  |      |__:__________________________:__|      |  :   =
           :  |SGP1.1|__:_____      _______________:__| ASP1 |  :  MGC1
           :  |______|  :     \    /               :  |______|  :
           :  |      |__:______\__/________________:__|      |  :
           :  |SGP2.1|__:_______\/______      _____:__| ASP2 |  :
           :  |______|  :       /\      |    |     :  |______:  :
           :   __:___   :  -   /  \     |    |  -  :   ___:__   :
           :  |      |  :  -  /    \    |    |  -  :  |      |  :
           :  | SGPn |  :  -  |    |    |    |  -  :  | ASPn |  :
           :  |______|  :     |    |    |    |     :  |______|  :
           :............:     |    |    |    |     :............:
           ..............     |    |    \    /     ..............
    Host#2 :   ______   :     |    |     \  /      :   ______   : Host#4
           :  |      |__:_____|    |______\/_______:__|      |  :   =
           :  |SGP1.2|__:_________________/\_______:__| ASP1 |  :  MGC2
           :  |______|  :                /  \      :  |______|  :
           :  |      |__:_______________/    \_____:__|      |  :
           :  |SGP2.2|__:__________________________:__| ASP2 |  :

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           :  |______|  :                          :  |______|  :
           :   __:___   :  -    SCTP            -  :   ___:__   :
           :  |      |  :  -    Associations    -  :  |      |  :
           :  | SGPn |  :  -                    -  :  | ASPn |  :
           :  |______|  :                          :  |______|  :
           :............:                          :............:

     SGP1.1 and SGP1.2 are part of SG1
     SGP2.1 and SGP2.2 are part of SG2

                      Figure 7.  Physical Model

     In this model, each host MAY have many application processes.  In
  the case of the MGC, an ASP may provide service to one or more
  Application Servers, and is identified as an SCTP end point. One or
  more Signalling Gateway Processes make up a single Signalling Gateway.

     This example model can also be applied to IPSP-IPSP signalling.  In
  this case, each IPSP MAY have its services distributed across 2 hosts
  or more, and may have multiple server processes on each host.

     In the example above, each signalling process (SGP, ASP or IPSP) is
  the end point to more than one SCTP association, leading to more than
  one other signalling processes.  To support this, a signalling process
  must be able to support distribution of ISUA messages to many
  simultaneous active associations.  This message distribution function
  is based on the status of provisioned Routing Keys, the status of the
  signalling routes to signalling points in the SS7 network , and the
  redundancy model (override, load-sharing, broadcast) of the remote
  signalling processes.

     For carrier grade networks, the failure or isolation of a
  particular signalling process should not cause transactions to be
  lost.  This implies that signalling processes need, in some cases, to
  share the transaction state or be able to pass the transaction state
  information between each other.  However, this sharing or
  communication of transaction state information is outside the scope of
  this document.

     This model serves as an example.  ISUA imposes no restrictions as
  to the exact layout of the network elements, the message distribution
  algorithms and the distribution of the signalling processes.  Instead,
  it provides a framework and a set of messages that allow for a
  flexible and scalable signalling network architecture, aiming to
  provide reliability and performance.

A.2.  Redundancy Models

A.2.1.  Application Server Redundancy

     At the SGP, an Application Server list contains active and inactive
  ASPs to support ASP broadcast, load-sharing and override procedures.
  The list of ASPs within a logical Application Server is kept updated

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  in the SGP to reflect the active Application Server Processes.

  For example, in the network shown in Figure 7, all messages to SSN x
  could be sent to ASP1 in Host3 or ASP1 in Host4.  The AS list at SGP1
  in Host 1 might look like the following:

      Routing Key {SSN=x) - "Application Server #1"
             ASP1/Host3     -   State = Active
             ASP1/Host4     -   State = Inactive

     In this "1+1" redundancy case, ASP1 in Host3 would be sent any
  incoming message with SSN=x.  ASP1 in Host4 would normally be brought
  to the "active" state upon failure of, or loss of connectivity to,
  ASP1/Host1.

     The AS List at SGP1 in Host1 might also be set up in load-share
  mode:

      Routing Key {SSN=x) - "Application Server #1"
             ASP1/Host3      -   State = Active
             ASP1/Host4      -   State = Active

     In this case, both the ASPs would be sent a portion of the traffic.
  For example the two ASPs could together form a database, where
  incoming queries may be sent to any active ASP.

     Care might need to be exercised by a Network Operator in the
  selection of the routing information to be used as the Routing Key for
  a particular AS.

     For example, where Application Servers are defined using ranges of
  Circuit Identification Codes (CICs), the Operator is implicitly
  splitting up control of the related circuit groups.  Some CIC value
  range assignments may interfere with ISUP circuit supervision
  procedures.

     In the process of fail-over, it is recommended that in the case of
  ASPs that transactions do not fail.  For example, the two ASPs may
  share transaction state via shared memory, or may use an ASP to ASP
  protocol to pass transaction state information.  Any ASP-to-ASP
  protocol to support this function is outside the scope of this
  document.

A.2.2.  Signalling Gateway Redundancy

     Signalling Gateways may also be distributed over multiple hosts.
  Much like the AS model, SGs may comprise one or more SG Processes
  (SGPs), distributed over one or more hosts, using an override, load-
  share or broadcast model.  Should an SGP lose all or partial SS7
  connectivity and other SGPs exist, the SGP may terminate the SCTP
  associations to the concerned ASPs or send an unsolicited ASPIA ACK

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  for the concerned Application Servers.

     It is possible for an ASP to route signalling messages destined to
  the SS7 network using more than one SGP.  In this model, a Signalling
  Gateway is deployed as a cluster of hosts acting as a single SG.  An
  override redundancy model is possible, where the unavailability of the
  SCTP association to a primary SGP could be used to reroute affected
  traffic to an alternate SGP.  A load-sharing model is possible, where
  the signalling messages are load-shared between multiple SGPs.  A
  broadcast model is also possible, where signalling messages are sent
  to each active SGP in the SG. The distribution of the Call Control
  messages over the SGPs should be done in such a way to minimize
  message mis-sequencing, as required by the SS7 User Parts.

     It may also be possible for an ASP to use more than one SG to
  access a specific SS7 end point, in a model that resembles an SS7 STP
  mated pair.  Typically, SS7 STPs are deployed in mated pairs, with
  traffic load-shared between them.  Other models are also possible,
  subject to the limitations of the local SS7 network provisioning
  guidelines.

     From the perspective of the ISUA layer at an ASP, a particular SG
  is capable of transferring traffic to a provisioned SS7 destination,
  subsystem or application X if an SCTP association with at least one
  SGP of the SG is established, the SGP has returned an acknowledgment
  to the ASP to indicate that the ASP is actively handling traffic for
  that destination, subsystem or application X, and the SGP has not
  indicated that the destination, subsystem or application X is
  inaccessible.  When an ASP is configured to use multiple SGPs for
  transferring traffic to the SS7 network, the ASP must maintain
  knowledge of the current capability of the SGPs to handle traffic to
  destinations, subsystems and applications of interest.  This
  information is crucial to the overall reliability of the service, for
  override, load-sharing and broadcast models, in the event of failures,
  recovery and maintenance activities.  The ASP ISUA may also use this
  information for congestion avoidance purposes.  The distribution of
  the Call Control messages over the SGPs should be done in such a way
  to minimize message mis-sequencing, as required by the some ISUP
  applications.

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                            List of Tables

  Table 1 Mapping of Circuit Supervision Primitives .............    8
  Table 2 Mapping of Call Control Primitives ....................   14

                        List of Illustrations

  Figure 1 Protocol Architecture ................................    5
  Figure 2 All IP Architecture ..................................    6
  Figure 3 ISUA Protocol Boundaries .............................   14
  Figure 4 ISUA Layer Model .....................................   92
  Figure 5 ASP State Transition Diagram (Per AS) ................   96
  Figure 6 AS State Transition Diagram ..........................   98
  Figure 7 Physical Model .......................................  130

                          Table of Contents

  Status of this Memo ...........................................    1
  Abstract ......................................................    1
  Contents ......................................................    1
  1 Introduction ................................................    2
  1.1 Scope .....................................................    2
  1.2 Change History ............................................    2
  1.2.1 Version 0.0 .............................................    2
  1.3 Terminology ...............................................    2
  1.4 ISUA Overview .............................................    5
  1.4.1 Signalling Transport Architecture .......................    5
  1.4.2 Protocol Architecture for Call Control ..................    5
  1.4.3 All IP Architecture .....................................    6
  1.4.4 ASP Fail-over Model and Terminology .....................    6
  1.4.5 Services Provided by the ISUA Layer .....................    6
  1.5 Functional Areas ..........................................    9
  1.5.1 Circuit Identifiers, Routing Contexts and Routing Keys ..    9
  1.5.2 Redundancy Models .......................................   13
  1.5.3 Flow Control ............................................   13
  1.5.4  Congestion Management ..................................   13
  1.6 Definition of ISUA Boundaries .............................   14
  1.6.1 Definition of Upper Boundary ............................   14
  1.6.2 Definition of Boundary between ISUA and Layer Management
     ............................................................   15
  1.6.3 Definition of the Lower Boundary ........................   19
  2 Conventions .................................................   19
  3 Protocol Elements ...........................................   19
  3.1 Common Message Header .....................................   20
  3.1.1 ISUA Protocol Version ...................................   20
  3.1.2 Message Classes .........................................   20
  3.1.3 Message Types ...........................................   21

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  3.1.4 Message Length ..........................................   22
  3.1.5 Tag-Length-Value Format .................................   22
  3.2 ISUA Message Header .......................................   23
  3.3 ISUA Call Processing (CP) Messages ........................   24
  3.3.1 CP Message Header .......................................   25
  3.3.2 Setup (CSET) ............................................   26
  3.3.3 More Information (CMOR) .................................   27
  3.3.4 Timeout (CTOT) ..........................................   27
  3.3.5 Information (CINF) ......................................   27
  3.3.6 Proceeding (CPRO) .......................................   28
  3.3.7 Alerting (CALR) .........................................   29
  3.3.8 Progress (CPRG) .........................................   30
  3.3.9 Connect (CCON) ..........................................   31
  3.3.10 Suspend (CSUS) .........................................   31
  3.3.11 Resume (CRES) ..........................................   32
  3.3.12 Reattempt (CREA) .......................................   33
  3.3.13 Failure (CERR) .........................................   33
  3.3.14 In Band Information (CIBI) .............................   34
  3.3.15 Release (CREL) .........................................   35
  3.3.16 Release Complete (CRLC) ................................   35
  3.4 ISUA Circuit Supervision (CS) Messaegs ....................   36
  3.4.1 CS Message Header .......................................   36
  3.4.2 Continuity Check (CCNT) .................................   37
  3.4.3 Loopback (CLBK) .........................................   37
  3.4.4 Report (CREP) ...........................................   37
  3.4.5 Reset (CRSC) ............................................   38
  3.4.6 Reset Acknowledgement (CRSA) ............................   39
  3.4.7 Block (CBLO) ............................................   39
  3.4.8 Block Acknowledgement (CBLA) ............................   40
  3.4.9 Unblock (CUBL) ..........................................   41
  3.4.10 Unblock Acknowledgement (CUBA) .........................   41
  3.4.11 Query (CQRY) ...........................................   42
  3.4.12 Query Acknowledgement (CQRA) ...........................   43
  3.5  Application Server Process State Maintenance (ASPSM) Mes-
     sages ......................................................   43
  3.5.1 ASP Up (UP) .............................................   43
  3.5.2 ASP Up Ack (UP ACK) .....................................   44
  3.5.3 ASP Down (DOWN) .........................................   45
  3.5.4 ASP Down Ack (DOWN ACK) .................................   45
  3.5.5 Heartbeat (BEAT) ........................................   46
  3.5.6 Heartbeat Ack (BEAT ACK) ................................   46
  3.6 Application Server  Process  Traffic  Maintenance  (ASPTM)
     Messages ...................................................   47
  3.6.1 ASP Active (ASPAC) ......................................   47
  3.6.2 ASP Active Ack (ASPAC ACK) ..............................   47
  3.6.3 ASP Inactive (ASPIA) ....................................   48
  3.6.4 ASP Inactive Ack (ASPIA ACK) ............................   49
  3.7 Management (MGMT) Messages ................................   50
  3.7.1 Error (ERR) .............................................   50
  3.7.2 Notify (NTFY) ...........................................   51
  3.8 Routing Key Management (RKM) Messages .....................   52
  3.8.1 Registration Request (REG REQ) ..........................   53
  3.8.2 Registration Response (REG RSP) .........................   53
  3.8.3 Deregistration Request (DEREG REQ) ......................   54

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  3.8.4 Deregistration Response (DEREG RSP) .....................   55
  3.9 Common Parameters .........................................   56
  3.9.1 Info String .............................................   56
  3.9.2 Routing Context .........................................   57
  3.9.3 Diagnostic Information ..................................   58
  3.9.4 Heartbeat Data ..........................................   58
  3.9.5 Traffic Mode Type .......................................   59
  3.9.6 Error Code ..............................................   60
  3.9.7 Status ..................................................   62
  3.9.8 ASP Identifier ..........................................   64
  3.9.9 Correlation Id ..........................................   64
  3.9.10 Registration Result ....................................   65
  3.9.11 Deregistration Result ..................................   65
  3.9.12 Registration Status ....................................   66
  3.9.13 Deregistration Status ..................................   67
  3.9.14 Local Routing Key Identifier ...........................   67
  3.10 ISUA-Specific parameters .................................   68
  3.10.1 Parameters used in CP Messages .........................   69
  3.10.2 Parameters used in CS Messages .........................   84
  3.10.3 Other Parameters .......................................   86
  4 Procedures ..................................................   91
  4.1 Procedures to Support Call Control ........................   91
  4.1.1 Receipt of Primitives from Call Control .................   91
  4.1.2 Receipt of Primitives from ISUP .........................   92
  4.1.3 Receipt of Primitive from the Layer Management ..........   94
  4.2  Procedures to Support the Management of SCTP Associations
     ............................................................   95
  4.2.1 Receipt of ISUA Peer Management Messages ................   95
  4.3 AS and ASP State Maintenance ..............................   95
  4.3.1 ASP States ..............................................   96
  4.3.2 AS States ...............................................   97
  4.3.3 ISUA Management Procedures for Primitives ...............   99
  4.3.4 ASPM Procedures for Peer-to-Peer Messages ...............   99
  4.4 Routing Key Management Procedures .........................  107
  4.4.1 Registration ............................................  107
  4.4.2 Deregistration ..........................................  109
  4.4.3 IPSP Considerations (REG/DEREG) .........................  110
  4.5 Procedures to Support Circuit and Call State ..............  110
  4.5.1 At an SGP ...............................................  110
  4.5.2 At an ASP ...............................................  110
  4.5.3 ASP Auditing ............................................  111
  4.5.4 ISUP - ISUA Interworking at the SG ......................  111
  5 Examples of ISUA Procedures .................................  112
  5.1 Establishment of Association and Traffic between SGPs  and
     ASPs .......................................................  112
  5.1.2 ASP Traffic Fail-over Examples ..........................  114
  5.1.3 ISUP/CC Service Translation Examples ....................  115
  5.2 IP-IP Architecture ........................................  116
  5.2.1 Establishment of ISUA connectivity ......................  117
  5.2.2 Fail-over scenarios .....................................  118
  6 Security ....................................................  118
  6.1 Introduction ..............................................  118
  6.2 Threats ...................................................  118
  6.3 Protecting Confidentiality ................................  119

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  6.4 IPsec Usage ...............................................  119
  6.5 TLS Usage .................................................  120
  6.6 Peer-to-Peer Considerations ...............................  121
  7 IANA Considerations .........................................  122
  7.1 SCTP Payload Protocol ID ..................................  122
  7.2 Port Number ...............................................  122
  7.3 Protocol Extensions .......................................  123
  7.3.1 IETF Defined Message Classes ............................  123
  7.3.2 IETF Defined Message Types ..............................  123
  7.3.3 IETF-defined TLV Parameter Extension ....................  123
  8 Timer Values ................................................  124
  Acknowledgments ...............................................  124
  End Notes .....................................................  124
  References ....................................................  125
  Author's Addresses ............................................  128
  Appendices ....................................................  129
  A Operational Considerations ..................................  129
  A.1 Signalling Network Architecture ...........................  129
  A.2 Redundancy Models .........................................  130
  A.2.1 Application Server Redundancy ...........................  130
  A.2.2 Signalling Gateway Redundancy ...........................  131
  List of Tables ................................................  133
  List of Illustrations .........................................  133

B. Bidulock                    Version 0.0                      Page 136

Internet Draft       SS7 ISUP-User Adaptation Layer      January 5, 2003

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B. Bidulock                    Version 0.0                      Page 137


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