    Network Working Group                                       K. Morneault
    Request for Comments: 3057                                 Cisco Systems
    Category: Standards Track                                   S. Rengasami
                                                                    M. Kalla
                                                      Telcordia Technologies
                                                               G. Sidebottom
                                                             Nortel Networks
                                                               February 2001

                        ISDN Q.921-User Adaptation Layer

    Status of this Memo

       This document specifies an Internet standards track protocol for the
       Internet community, and requests discussion and suggestions for
       improvements.  Please refer to the current edition of the "Internet
       Official Protocol Standards" (STD 1) for the standardization state
       and status of this protocol.  Distribution of this memo is unlimited.

    Copyright Notice

       Copyright (C) The Internet Society (2001).  All Rights Reserved.

    Abstract

       This document defines a protocol for backhauling of ISDN Q.921 User
       messages over IP using the Stream Control Transmission Protocol
       (SCTP).  This protocol would be used between a Signaling Gateway (SG)
       and Media Gateway Controller (MGC).  It is assumed that the SG
       receives ISDN signaling over a standard ISDN interface.

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    Table of Contents

       1.  Introduction.................................................  2
         1.1  Scope.....................................................  2
         1.2  Terminology...............................................  3
         1.3  IUA Overview..............................................  4
         1.4  Services Provided by the IUA Layer........................  9
         1.5  Functions Implemented by the IUA Layer.................... 12
         1.6  Definition of IUA Boundaries.............................. 14
       2.  Conventions.................................................. 16
       3.  Protocol Elements............................................ 17
         3.1  Common Message Header..................................... 17
         3.2  IUA Message Header........................................ 20
         3.3  Description of Messages................................... 22
       4.  Procedures................................................... 45
         4.1  Procedures to Support Service in Section 1.4.1............ 45
         4.2  Procedures to Support Service in Section 1.4.2............ 46
         4.3  Procedures to Support Service in Section 1.4.3............ 47
       5. Examples...................................................... 56
         5.1 Establishment of associations between SG and MGC examples.. 56
         5.2 ASP Traffic Fail-over Examples............................. 58
         5.3 Q.921/Q.931 primitives backhaul Examples................... 59
         5.4 Layer Management Communication Examples.................... 61
       6.  Security..................................................... 61
         6.1 Threats.................................................... 61
         6.2 Protecting Confidentiality ................................ 62
       7.  IANA Considerations.......................................... 62
         7.1 SCTP Payload Protocol Identifier........................... 62
         7.2 IUA Protocol Extensions.................................... 62
       8.  Acknowledgements............................................. 64
       9.  References................................................... 64
       10. Authors' Addresses........................................... 65
       11. Full Copyright Statement..................................... 66

    1.  Introduction

       In this document, the term Q.921-User refers to an upper layer which
       uses the services of Q.921, not the user side of ISDN interface [1].
       Examples of the upper layer would be Q.931 and QSIG.

       This section describes the need for ISDN Q.921-User Adaptation (IUA)
       layer protocol as well as how this protocol shall be implemented.

    1.1 Scope

       There is a need for Switched Circuit Network (SCN) signaling protocol
       delivery from an ISDN Signaling Gateway (SG) to a Media Gateway
       Controller (MGC) as described in the Framework Architecture for

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       Signaling Transport [4].  The delivery mechanism SHOULD meet the
       following criteria:

       *  Support for transport of the Q.921 / Q.931 boundary primitives
       *  Support for communication between Layer Management modules on SG
          and MGC
       *  Support for management of active associations between SG and MGC

       This document supports both ISDN Primary Rate Access (PRA) as well as
       Basic Rate Access (BRA) including the support for both point-to-point
       and point-to-multipoint modes of communication.  This support
       includes Facility Associated Signaling (FAS), Non-Facility Associated
       Signaling (NFAS) and NFAS with backup D channel.  QSIG adaptation
       layer requirements do not differ from Q.931 adaptation layer, hence;
       the procedures described in this document are also applicable for a
       QSIG adaptation layer.  For simplicity, only Q.931 will be mentioned
       in the rest of this document.

    1.2 Terminology

       Interface - For the purposes of this document an interface supports
       the relevant ISDN signaling channel.  This signaling channel MAY be a
       16 kbps D channel for an ISDN BRA as well as 64 kbps primary or
       backup D channel for an ISDN PRA.  For QSIG, the signaling channel is
       a Qc channel.

       Q.921-User - Any protocol normally using the services of the ISDN
       Q.921 (e.g., Q.931, QSIG, etc.).

       Backhaul - A SG terminates the lower layers of an SCN protocol and
       backhauls the upper layer(s) to MGC for call processing.  For the
       purposes of this document the SG terminates Q.921 and backhauls Q.931
       to MGC.

       Association - An association refers to a SCTP association.  The
       association will provide the transport for the delivery of Q.921-User
       protocol data units and IUA adaptation layer peer messages.

       Stream - A stream refers to an SCTP stream; a uni-directional 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 un-ordered delivery service.

       Interface Identifier - The Interface Identifier identifies the
       physical interface at the SG for which the signaling messages are
       sent/received. The format of the Interface Identifier parameter can
       be text or integer, the values of which are assigned according to

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       network operator policy. The values used are of local significance
       only, coordinated between the SG and ASP.  Significance is not
       implied across SGs served by an AS.

       Application Server (AS) - A logical entity serving a specific
       application instance.  An example of an Application Server is a MGC
       handling the Q.931 and call processing for D channels terminated by
       the Signaling Gateways.  Practically speaking, an AS is modeled at
       the SG as an ordered list of one or more related Application Server
       Processes (e.g., primary, secondary, tertiary).

       Application Server Process (ASP) - A process instance of an
       Application Server.  Examples of Application Server Processes are
       primary or backup MGC instances.

       Fail-over - The capability to re-route signaling traffic as required
       between related ASPs in the event of failure or unavailability of the
       currently used ASP (e.g., from primary MGC to back-up MGC).  Fail-
       over also applies upon the return to service of a previously
       unavailable process.

       Layer Management - Layer Management is a nodal function that handles
       the inputs and outputs between the IUA layer and a local management
       entity.

       Network Byte Order - Most significant byte first, a.k.a Big Endian.

       Host - The computing platform that the ASP process is running on.

    1.3 IUA Overview

       The architecture that has been defined [4] for SCN signaling
       transport over IP uses multiple components, including an IP transport
       protocol, a signaling common transport protocol and an adaptation
       module to support the services expected by a particular SCN signaling
       protocol from its underlying protocol layer.

       This document defines an adaptation module that is suitable for the
       transport of ISDN Q.921-User (e.g., Q.931) messages.

    1.3.1  Example - SG to MGC

       In a Signaling Gateway, it is expected that the ISDN signaling is
       received over a standard ISDN network termination.  The SG then
       provides interworking of transport functions with IP Signaling
       Transport, in order to transport the Q.931 signaling messages to the
       MGC where the peer Q.931 protocol layer exists, as shown below:

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                ******   ISDN        ******      IP      *******
                * EP *---------------* SG *--------------* MGC *
                ******               ******              *******

                +-----+                                  +-----+
                |Q.931|              (NIF)               |Q.931|
                +-----+           +----------+           +-----+
                |     |           |     | IUA|           | IUA |
                |     |           |     +----+           +-----+
                |Q.921|           |Q.921|SCTP|           |SCTP |
                |     |           |     +----+           +-----+
                |     |           |     | IP |           | IP  |
                +-----+           +-----+----+           +-----+

                NIF  - Nodal Interworking Function
                EP   - ISDN End Point
                SCTP - Stream Control Transmission Protocol (Refer to [3])
                IUA  - ISDN User Adaptation Layer Protocol

       It is recommended that the IUA use the services of the Stream Control
       Transmission Protocol (SCTP) as the underlying reliable common
       signaling transport protocol.  The use of SCTP provides the following
       features:

          -  explicit packet-oriented delivery (not stream-oriented)
          -  sequenced delivery of user messages within multiple streams,
             with an option for order-of-arrival delivery of individual user
             messages,
          -  optional multiplexing of user messages into SCTP datagrams,
          -  network-level fault tolerance through support of multi-homing
             at either or both ends of an association,
          -  resistance to flooding and masquerade attacks, and
          -  data segmentation to conform to discovered path MTU size

       There are scenarios without redundancy requirements and scenarios in
       which redundancy is supported below the transport layer.  In these
       cases, the SCTP functions above MAY NOT be a requirement and TCP can
       be used as the underlying common transport protocol.

    1.3.2  Support for the management of SCTP associations between the SG
           and ASPs

       The IUA layer at the SG maintains the availability state of all
       dynamically registered remote ASPs, in order to manage the SCTP
       Associations and the traffic between the SG and ASPs.  As well, the
       active/inactive state of remote ASP(s) are also maintained.  Active
       ASPs are those currently receiving traffic from the SG.

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       The IUA layer MAY be instructed by local management to establish an
       SCTP association to a peer IUA node.  This can be achieved using the
       M-SCTP ESTABLISH primitive to request, indicate and confirm the
       establishment of an SCTP association with a peer IUA node.

       The IUA layer MAY also need to inform local management of the status
       of the underlying SCTP associations using the M-SCTP STATUS request
       and indication primitive.  For example, the IUA MAY inform local
       management of the reason for the release of an SCTP association,
       determined either locally within the IUA layer or by a primitive from
       the SCTP.

    1.3.3  Signaling Network Architecture

       A Signaling Gateway is used to support the transport of Q.921-User
       signaling traffic to one or more distributed ASPs (e.g., MGCs).
       Clearly, the IUA 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 does
       allow for a sufficiently reliable transport of signaling traffic over
       IP.  The IUA 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 ISDN signaling reliability and performance requirements
       for carrier grade networks, Network Operators SHOULD ensure that
       there is no single point of failure provisioned in the end-to-end
       network architecture between an ISDN node and an IP ASP.

       Depending of course on the reliability of the SG and ASP functional
       elements, this can typically be met by the provision of redundant
       QOS-bounded IP network paths for SCTP Associations between SCTP End
       Points, and redundant Hosts, and redundant SGs.  The distribution of
       ASPs within the available Hosts is also important.  For a particular
       Application Server, the related ASPs SHOULD be distributed over at
       least two Hosts.

       An example logical network architecture relevant to carrier-grade
       operation in the IP network domain is shown in Figure 1 below:

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                                                              Host1
         ********                                         **************
         *      *_________________________________________*  ********  *
         *      *                                _________*  * ASP1 *  *
         *  SG1 *   SCTP Associations           |         *  ********  *
         *      *_______________________        |         *            *
         ********                       |       |         **************
                                        |       |
         ********                       |       |
         *      *_______________________________|
         *      *                       |
         *  SG2 *    SCTP Associations  |
         *      *____________           |
         *      *            |          |                     Host2
         ********            |          |                 **************
                             |          |_________________*  ********  *
                             |____________________________*  * ASP1 *  *
                                                          *  ********  *
                                                          *            *
                                                          **************
                                                                  .
                                                                  .
                                                                  .

                           Figure 2 - Logical Model Example

       For carrier grade networks, the failure or isolation of a particular
       ASP SHOULD NOT cause stable calls to be dropped.  This implies that
       ASPs need, in some cases, to share the call state or be able to pass
       the call state between each other.  However, this sharing or
       communication of call state information is outside the scope of this
       document.

    1.3.4 ASP Fail-over Model and Terminology

       The IUA layer supports ASP fail-over functions in order to support a
       high availability of call processing capability.  All Q.921-User
       messages incoming to an SG are assigned to a unique Application
       Server, based on the Interface Identifier of the message.

       The Application Server is, in practical terms, a list of all ASPs
       configured to process Q.921-User messages from certain Interface
       Identifiers.  One or more ASPs in the list are normally active (i.e.,
       handling traffic) while any others MAY be unavailable or inactive, to
       be possibly used in the event of failure or unavailability of the
       active ASP(s).

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

       To avoid a single point of failure, it is recommended that a minimum
       of two ASPs be in the list, resident in separate hosts and therefore
       available over different SCTP Associations.  For example, in the
       network shown in Figure 2, all messages from a particular D Channel
       (Interface Identifier) could be sent to ASP1 in Host1 or ASP1 in
       Host2. The AS list at SG1 might look like the following:

          Interface Identifier(s) - Application Server #1
              ASP1/Host1  - State=Up, Active
              ASP1/Host2  - State=Up, Inactive

       In this 1+1 redundancy case, ASP1 in Host1 would be sent any incoming
       message for the Interface Identifiers registered.  ASP1 in Host2
       would normally be brought to the active state upon failure of, or
       loss of connectivity to, ASP1/Host1.  In this example, both ASPs are
       Up, meaning that the related SCTP association and far-end IUA peer is
       ready.

       The AS List at SG1 might also be set up in load-share mode as shown
       below:

          Interface Identifier(s) - Application Server #1
              ASP1/Host1 - State=Up, Active
              ASP1/Host2 - State=Up, Active

       In this case, both the ASPs would be sent a portion of the traffic.

       In the process of fail-over, it is recommended that in the case of
       ASPs supporting call processing, stable calls do not get released.
       It is possible that calls in transition MAY fail, although measures
       of communication between the ASPs involved can be used to mitigate
       this problem.  For example, the two ASPs MAY share call state via
       shared memory, or MAY use an ASP to ASP protocol to pass call state
       information.  The ASP to ASP protocol is outside the scope of this
       document.

    1.3.5  Client/Server Model

       It is recommended that the SG and ASP be able to support both client
       and server operation.  The peer endpoints using IUA SHOULD be
       configured so that one always takes on the role of client and the

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       other the role of server for initiating SCTP associations.  The
       default orientation would be for the SG to take on the role of server
       while the ASP is the client.  In this case, ASPs SHOULD initiate the
       SCTP association to the SG.

       The SCTP (and UDP/TCP) Registered User Port Number Assignment for IUA
       is 9900.

    1.4  Services Provided by the IUA Layer

    1.4.1  Support for transport of Q.921/Q.931 boundary primitives

       In the backhaul scenario, the Q.921/Q.931 boundary primitives are
       exposed.  IUA layer needs to support all of the primitives of this
       boundary to successfully backhaul Q.931.

       This includes the following primitives [1]:

       DL-ESTABLISH

       The DL-ESTABLISH primitives are used to request, indicate and confirm
       the outcome of the procedures for establishing multiple frame
       operation.

       DL-RELEASE

       DL-RELEASE primitives are used to request, indicate, and confirm the
       outcome of the procedures for terminating a previously established
       multiple frame operation, or for reporting an unsuccessful
       establishment attempt.

       DL-DATA

       The DL-DATA primitives are used to request and indicate layer 3
       (Q.931) messages which are to be transmitted, or have been received,
       by the Q.921 layer using the acknowledged information transfer
       service.

       DL-UNIT DATA

       The DL-UNIT DATA primitives are used to request and indicate layer 3
       (Q.931) messages which are to be transmitted, by the Q.921 layer
       using the unacknowledged information transfer service.

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    1.4.2  Support for communication between Layer Management modules on SG
           and MGC

       It is envisioned that the IUA layer needs to provide some services
       that will facilitate communication between Layer Management modules
       on the SG and MGC.  These primitives are pointed out in [2], which
       are shown below:

       M-TEI STATUS

       The M-TEI STATUS primitives are used to request, confirm and indicate
       the status (assigned/unassigned) of a TEI.

       M-ERROR

       The M-ERROR primitive is used to indicate an error with a received
       IUA message (e.g., interface identifier value is not known to the
       SG).

    1.4.3 Support for management of active associations between SG and MGC

       A set of primitives between the IUA layer and the Layer Management
       are defined below to help the Layer Management manage the SCTP
       association(s) between the SG and MGC.  The IUA layer can be
       instructed by the Layer Management to establish an SCTP association
       to a peer IUA node.  This procedure can be achieved using the M-SCTP
       ESTABLISH primitive.

       M-SCTP ESTABLISH

       The M-SCTP ESTABLISH primitives are used to request, indicate, and
       confirm the establishment of an SCTP association to a peer IUA node.

       M-SCTP RELEASE

       The M-SCTP RELEASE primitives are used to request, indicate, and
       confirm the release of an SCTP association to a peer IUA node.

       The IUA layer MAY also need to inform the status of the SCTP
       associations to the Layer Management.  This can be achieved using the
       M-SCTP STATUS primitive.

       M-SCTP STATUS

       The M-SCTP STATUS primitives are used to request and indicate the
       status of the underlying SCTP association(s).

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       The Layer Management MAY need to inform the IUA layer of an AS/ASP
       status (i.e., failure, active, etc.), so that messages can be
       exchanged between IUA layer peers to stop traffic to the local IUA
       user.  This can be achieved using the M-ASP STATUS primitive.

       M-ASP STATUS

       The ASP status is stored inside IUA layer on both the SG and MGC
       sides.  The M-ASP STATUS primitive can be used by Layer Management to
       request the status of the Application Server Process from the IUA
       layer.  This primitive can also be used to indicate the status of the
       Application Server Process.

       M-ASP-UP

       The M-ASP-UP primitive can be used by Layer Management to send a ASP
       Up message for the Application Server Process.  It can also be used
       to generate an ASP Up Acknowledgement.

       M-ASP-DOWN

       The M-ASP-DOWN primitive can be used by Layer Management to send a
       ASP Down message for the Application Server Process.  It can also be
       used to generate an ASP Down Acknowledgement.

       M-ASP-ACTIVE

       The M-ASP-UP primitive can be used by Layer Management to send a ASP
       Active message for the Application Server Process.  It can also be
       used to generate an ASP Active Acknowledgement.

       M-ASP-INACTIVE

       The M-ASP-UP primitive can be used by Layer Management to send a ASP
       Inactive message for the Application Server Process.  It can also be
       used to generate an ASP Inactive Acknowledgement.

       M-AS STATUS

       The M-AS STATUS primitive can be used by Layer Management to request
       the status of the Application Server.  This primitive can also be
       used to indicate the status of the Application Server.

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    1.5 Functions Implemented by the IUA Layer

    1.5.1 Mapping

       The IUA layer MUST maintain a map of the Interface Identifier to a
       physical interface on the Signaling Gateway.  A physical interface
       would be a T1 line, E1 line, etc., and could include the TDM
       timeslot. In addition, for a given interface the SG MUST be able to
       identify the associated signaling channel.  IUA layers on both SG and
       MGC MAY maintain the status of TEIs and SAPIs.

       The SG maps an Interface Identifier to an SCTP association/stream
       only when an ASP sends an ASP Active message for a particular
       Interface Identifier.  It MUST be noted, however, that this mapping
       is dynamic and could change at any time due to a change of ASP state.
       This mapping could even temporarily be invalid, for example during
       failover of one ASP to another.  Therefore, the SG MUST maintain the
       states of AS/ASP and reference them during the routing of an messages
       to an AS/ASP.

       One example of the logical view of relationship between D channel,
       Interface Identifier, AS and ASP in the SG is shown below:

              /---------------------------------------------------+
             /   /------------------------------------------------|--+
            /   /                                                 v  |
           /   /    +----+             act+-----+    +-------+ -+--+-|+--+-
    D chan1-------->|IID |-+          +-->| ASP |--->| Assoc |       v
             /      +----+ |  +----+  |   +-----+    +-------+ -+--+--+--+-
            /              +->| AS |--+                        Streams
           /        +----+ |  +----+   stb+-----+
    D chan2-------->|IID |-+              | ASP |
                    +----+                +-----+

       where IID = Interface Identifier

       Note that an ASP can be in more than one AS.

    1.5.2 Status of ASPs

       The IUA layer on the SG MUST maintain the state of the ASPs it is
       supporting.  The state of an ASP changes because of reception of
       peer-to-peer messages (ASPM messages as described in Section 3.3.2)
       or reception of indications from the local SCTP association.  ASP
       state transition procedures are described in Section 4.3.1.

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       At a SG, an Application Server list MAY contain active and inactive
       ASPs to support ASP load-sharing and fail-over procedures.  When, for
       example, both a primary and a back-up ASP are available, IUA peer
       protocol is required to control which ASP is currently active.  The
       ordered list of ASPs within a logical Application Server is kept
       updated in the SG to reflect the active Application Server
       Process(es).

       Also the IUA layer MAY need to inform the local management of the
       change in status of an ASP or AS.  This can be achieved using the M-
       ASP STATUS or M-AS STATUS primitives.

    1.5.3 SCTP Stream Management

       SCTP allows a user specified number of streams to be opened during
       the initialization.  It is the responsibility of the IUA layer to
       ensure proper management of these streams.  Because of the
       unidirectional nature of streams, an IUA layer is not aware of the
       stream number to Interface Identifier mapping of its peer IUA layer.
       Instead, the Interface Identifier is in the IUA message header.

       The use of SCTP streams within IUA is recommended in order to
       minimize transmission and buffering delay, therefore improving the
       overall performance and reliability of the signaling elements.  It is
       recommended that a separate SCTP stream is used for each D channel.

    1.5.4 Seamless Network Management Interworking

       The IUA layer on the SG SHOULD pass an indication of unavailability
       of the IUA-User (Q.931) to the local Layer Management, if the
       currently active ASP moves from the ACTIVE state.  The Layer
       Management could instruct Q.921 to take some action, if it deems
       appropriate.

       Likewise, if an SCTP association fails, the IUA layer on both the SG
       and ASP sides MAY generate Release primitives to take the data links
       out-of-service.

    1.5.5 Congestion Management

       If the IUA layer becomes congested (implementation dependent), it MAY
       stop reading from the SCTP association to flow control from the peer
       IUA.

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    1.6 Definition of IUA Boundaries

    1.6.1 Definition of IUA/Q.921 boundary

       DL-ESTABLISH
       DL-RELEASE
       DL-DATA
       DL-UNIT DATA

    1.6.2 Definition of IUA/Q.931 boundary

       DL-ESTABLISH
       DL-RELEASE
       DL-DATA
       DL-UNIT DATA

    1.6.3 Definition of SCTP/IUA Boundary

       An example of the upper layer primitives provided by SCTP are
       available in Reference [3] section 10.

    1.6.4 Definition of IUA/Layer-Management Boundary

       M-SCTP ESTABLISH request
       Direction: LM -> IUA
       Purpose: LM requests ASP to establish an SCTP association with an SG.

       M-STCP ESTABLISH confirm
       Direction: IUA -> LM
       Purpose: ASP confirms to LM that it has established an SCTP
                association with an SG.

       M-SCTP ESTABLISH indication
       Direction: IUA -> LM
       Purpose: SG informs LM that an ASP has established an SCTP
                association.

       M-SCTP RELEASE request
       Direction: LM -> IUA
       Purpose: LM requests ASP to release an SCTP association with SG.

       M-SCTP RELEASE confirm
       Direction: IUA -> LM
       Purpose: ASP confirms to LM that it has released SCTP association
                with SG.

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       M-SCTP RELEASE indication
       Direction: IUA -> LM
       Purpose: SG informs LM that ASP has released an SCTP association.

       M-SCTP STATUS request
       Direction: LM -> IUA
       Purpose: LM requests IUA to report status of SCTP association.

       M-SCTP STATUS indication
       Direction: IUA -> LM
       Purpose: IUA reports status of SCTP association.

       M-ASP STATUS request
       Direction: LM -> IUA
       Purpose: LM requests SG to report status of remote ASP.

       M-ASP STATUS indication
       Direction: IUA -> LM
       Purpose: SG reports status of remote ASP.

       M-AS-STATUS request
       Direction: LM -> IUA
       Purpose: LM requests SG to report status of AS.

       M-AS-STATUS indication
       Direction: IUA -> LM
       Purpose: SG reports status of AS.

       M-NOTIFY indication
       Direction: IUA -> LM
       Purpose: ASP reports that it has received a NOTIFY message
                from its peer.

       M-ERROR indication
       Direction: IUA -> LM
       Purpose: ASP or SG reports that it has received an ERROR
                message from its peer.

       M-ASP-UP request
       Direction: LM -> IUA
       Purpose: LM requests ASP to start its operation and send an ASP UP
                message to the SG.

       M-ASP-UP confirm
       Direction: IUA -> LM
       Purpose: ASP reports that is has received an ASP UP Acknowledgement
                message from the SG.

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       M-ASP-DOWN request
       Direction: LM -> IUA
       Purpose: LM requests ASP to stop its operation and send an ASP DOWN
                message to the SG.

       M-ASP-DOWN confirm
       Direction: IUA -> LM
       Purpose: ASP reports that is has received an ASP DOWN
                Acknowledgement message from the SG.

       M-ASP-ACTIVE request
       Direction: LM -> IUA
       Purpose: LM requests ASP to send an ASP ACTIVE message to the SG.

       M-ASP-ACTIVE confirm
       Direction: IUA -> LM
       Purpose: ASP reports that is has received an ASP ACTIVE
                Acknowledgement message from the SG.

       M-ASP-INACTIVE request
       Direction: LM -> IUA
       Purpose: LM requests ASP to send an ASP INACTIVE message to the SG.

       M-ASP-INACTIVE confirm
       Direction: IUA -> LM
       Purpose: ASP reports that is has received an ASP INACTIVE
                Acknowledgement message from the SG.

       M-TEI STATUS request
       Direction: LM -> IUA
       Purpose: LM requests ASP to send a TEI status request to the SG.

       M-TEI STATUS indication
       Direction: IUA -> LM
       Purpose: ASP reports that is has received a TEI status indication
                from the SG.

       M-TEI STATUS confirm
       Direction: IUA -> LM
       Purpose: ASP reports that is has received a TEI status confirm from the
                SG.

    2.0 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
       [RFC2119].

    Morneault, et al.           Standards Track                    [Page 16]

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    3.0 Protocol Elements

       This section describes the format of various messages used in this
       protocol.

    3.1 Common Message Header

       The protocol messages for Q.921-User Adaptation require a message
       header which contains the adaptation layer version, the message type,
       and message length.

        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                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 3 Common Header Format

       All fields in an IUA message MUST be transmitted in the network byte
       order, unless otherwise stated.

    3.1.1 Version

       The version field contains the version of the IUA adaptation layer.
       The supported versions are the following:

          Value    Version
          -----    -------
            1      Release 1.0

    3.1.2  Message Classes and Types

       The following List contains the valid Message Classes:

       Message Class: 8 bits (unsigned integer)

         0      Management (MGMT) Message [IUA/M2UA/M3UA/SUA]
         1      Transfer Messages [M3UA]
         2      SS7 Signalling Network Management (SSNM) Messages [M3UA/SUA]
         3      ASP State Maintenance (ASPSM) Messages [IUA/M2UA/M3UA/SUA]
         4      ASP Traffic Maintenance (ASPTM) Messages [IUA/M2UA/M3UA/SUA]
         5      Q.921/Q.931 Boundary Primitives Transport (QPTM)
                Messages [IUA]
         6      MTP2 User Adaptation (MAUP) Messages [M2UA]
         7      Connectionless Messages [SUA]

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         8      Connection-Oriented Messages [SUA]
      9 to 127  Reserved by the IETF
    128 to 255  Reserved for IETF-Defined Message Class extensions

       The following list contains the message names for the defined
       messages.

        Q.921/Q.931 Boundary Primitives Transport (QPTM) Messages

           0        Reserved
           1        Data Request Message
           2        Data Indication Message
           3        Unit Data Request Message
           4        Unit Data Indication Message
           5        Establish Request
           6        Establish Confirm
           7        Establish Indication
           8        Release Request
           9        Release Confirm
          10        Release Indication
        11 to 127   Reserved by the IETF
       128 to 255   Reserved for IETF-Defined QPTM 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        Heatbeat Ack (BEAT ACK)
         7 to 127   Reserved by the IETF
       128 to 255   Reserved for IETF-Defined ASPSM extensions

        Application Server Process Traffic Maintenance (ASPTM) messages

           0        Reserved
           1        ASP Active (ACTIVE)
           2        ASP Inactive (INACTIVE)
           3        ASP Active Ack (ACTIVE ACK)
           4        ASP Inactive Ack (INACTIVE ACK)
         5 to 127   Reserved by the IETF
       128 to 255   Reserved for IETF-Defined ASPTM extensions

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        Management (MGMT) Messages

           0        Error (ERR)
           1        Notify (NTFY)
           2        TEI Status Request
           3        TEI Status Confirm
           4        TEI Status Indication
         5 to 127   Reserved by the IETF
       128 to 255   Reserved for IETF-Defined MGMT extensions

    3.1.3  Reserved

       The Reserved field is 8-bits.  It SHOULD be set to all '0's and
       ignored by the receiver.

    3.1.4  Message Length

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

    3.1.5  Variable-Length Parameter Format

       IUA messages consist of a Common Header followed by zero or more
       variable-length parameters, as defined by the message type.  The
       variable-length parameters contained in a message are defined in a
       Tag-Length-Value format as shown below.

        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                         /
       \                                                               \
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Mandatory parameters MUST be placed before optional parameters in a
       message.

       Parameter Tag: 16 bits (unsigned integer)

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

       The value of 65535 is reserved for IETF-defined extensions.  Values
       other than those defined in specific parameter description are
       reserved for use by the IETF.

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       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.

       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 pads the Parameter
       at the end (i.e., after the Parameter Value field) with all zero
       bytes. The length of the padding is NOT included in the parameter
       length field.  A sender SHOULD NEVER pad with more than 3 bytes.  The
       receiver MUST ignore the padding bytes.

    3.2 IUA Message Header

       In addition to the common message header, there will be a specific
       message header for QPTM and the TEI Status MGMT messages.  The IUA
       message header will immediately follow the Common header in these
       messages.

       This message header will contain the Interface Identifier and Data
       Link Connection Identifier (DLCI).  The Interface Identifier
       identifies the physical interface terminating the signaling channel
       at the SG for which the signaling messages are sent/received.  The
       format of the Interface Identifier parameter can be text or integer.
       The Interface Identifiers are assigned according to network operator
       policy.  The integer values used are of local significance only,
       coordinated between the SG and ASP.

       The integer formatted Interface Identifier MUST be supported.  The
       text formatted Interface Identifier MAY optionally be supported.

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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 (0x1)           |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 Interface Identifier (integer)                |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Tag (0x5)           |             Length=8          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            DLCI               |              Spare            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

        Figure 4 IUA Message Header (Integer-based Interface Identifier)

       The Tag value for the Integer-based Interface Identifier is 0x1.  The
       length is always set to a value of 8.

        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 (0x3)           |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                   Interface Identifier (text)                 |

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Tag (0x5)           |             Length=8          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |            DLCI               |             Spare             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Figure 5  IUA Message Header (Text-based Interface Identifier)

       The Tag value for the Text-based Interface Identifier is 0x3.  The
       length is variable.

       The DLCI format is shown below in Figure 6.

          0     1     2     3     4     5     6     7
       +-----+-----+-----+-----+-----+-----+-----+-----+
       |  0  | SPR |      SAPI                         |
       +-----------------------------------------------+
       |  1  |            TEI                          |
       +-----------------------------------------------+

                  Figure 6  DLCI Format

       SPR:  Spare 2nd bit in octet 1, (1 bit)

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       SAPI: Service Access Point Identifier, 3rd through 8th bits in octet
          1 (6 bits)

       TEI:  Terminal Endpoint Identifier, 2nd through 8th bits in octet 2
          (7 bits)

       The DLCI field (including the SAPI and TEI) is coded in accordance
       with Q.921.

    3.3 IUA  Messages

       The following section defines the messages and parameter contents.
       The IUA messages will use the common message header (Figure 3) and
       the IUA message header (Figure 4 and Figure 5).

    3.3.1 Q.921/Q.931 Boundary Primitives Transport (QPTM) Messages

    3.3.1.1  Establish Messages (Request, Confirm, Indication)

       The Establish Messages are used to establish a data link on the
       signaling channel or to confirm that a data link on the signaling
       channel has been established.  The MGC controls the state of the D
       channel.  When the MGC desires the D channel to be in-service, it
       will send the Establish Request message.

       When the MGC sends an IUA Establish Request message, the MGC MAY
       start a timer.  This timer would be stopped upon receipt of an IUA
       Establish Confirm or Establish Indication.  If the timer expires, the
       MGC would re-send the IUA Establish Request message and restart the
       timer.  In other words, the MGC MAY continue to request the
       establishment of the data link on periodic basis until the desired
       state is achieved or take some other action (notify the Management
       Layer).

       When the SG receives an IUA Establish Request from the MGC, the SG
       shall send the Q.921 Establish Request primitive to the its Q.921
       entity.  In addition, the SG shall map any response received from the
       Q.921 entity to the appropriate message to the MGC.  For example, if
       the Q.921 entity responds with a Q.921 Establish Confirm primitive,
       the IUA layer shall map this to an IUA Establish Confirm message.  As
       another example, if the IUA Layer receives a Q.921 Release Confirm or
       Release Indication as an apparent response to the Q.921 Establish
       Request primitive, the IUA Layer shall map these to the corresponding
       IUA Release Confirm or Release Indication messages.

       The Establish messages contain the common message header followed by
       IUA message header.  It does not contain any additional parameters.

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    3.3.1.2  Release Messages (Request, Indication, Confirmation)

       The Release Request message is used to release the data link on the
       signaling channel.  The Release Confirm and Indication messages are
       used to indicate that the data link on the signaling channel has been
       released.

       If a response to the Release Request message is not received, the MGC
       MAY resend the Release Request message.  If no response is received,
       the MGC can consider the data link as being released.  In this case,
       signaling traffic on that D channel is not expected from the SG and
       signaling traffic will not be sent to the SG for that D channel.

       The Release messages contain the common message header followed by
       IUA message header.  The Release confirm message is in response to a
       Release Request message and it does not contain any additional
       parameters.  The Release Request and Indication messages contain the
       following parameter:

         REASON

        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 (0xf)           |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              Reason                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       The valid values for Reason are shown in the following table.

          Define     Value           Description
       RELEASE_MGMT   0x0     Management layer generated release.
       RELEASE_PHYS   0x1     Physical layer alarm generated release.
       RELEASE_DM     0x2     Specific to a request.  Indicates Layer 2
                              SHOULD release and deny all requests from
                              far end to establish a data link on the
                              signaling channel (i.e., if SABME is
                              received send a DM)
       RELEASE_OTHER  0x3     Other reasons

       Note:  Only RELEASE_MGMT, RELEASE_DM and RELEASE_OTHER are valid
       reason codes for a Release Request message.

    3.3.1.3 Data Messages (Request, Indication)

       The Data message contains an ISDN Q.921-User Protocol Data Unit (PDU)
       corresponding to acknowledged information transfer service.

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       The Data messages contain the common message header followed by IUA
       message header.  The Data message contains the following parameters:

         PROTOCOL DATA

        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 (0xe)           |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                          Protocol Data                        |

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

       The protocol data contains upper layer signaling message e.g.  Q.931,
       QSIG.

    3.3.1.4 Unit Data Messages (Request, Indication)

       The Unit Data message contains an ISDN Q.921-User Protocol Data Unit
       (PDU) corresponding to unacknowledged information transfer service.

       The Unit Data messages contain the common message header followed by
       IUA message header.  The Unit Data message contains the following
       parameters

         PROTOCOL DATA

        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 (0xe)           |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                          Protocol Data                        |

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

    3.3.2  Application Server Process Maintenance (ASPM) Messages

       The ASPM messages will only use the common message header.

    3.3.2.1  ASP Up (ASPUP)

       The ASP Up (ASPUP) message is sent by an ASP to indicate to an SG
       that it is ready to receive traffic or maintenance messages.

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       The ASPUP message contains the following parameters:

         Info String (optional)

       The format for ASPUP Message parameters is 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 (0x4)           |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                          INFO String*                         |

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

       The optional INFO String parameter can carry any meaningful 8-bit
       ASCII character string along with the message.  Length of the INFO
       String parameter is from 0 to 255 characters.  No procedures are
       presently identified for its use but the INFO String MAY be used for
       debugging purposes.

    3.3.2.2 ASP Up Ack

       The ASP Up Ack message is used to acknowledge an ASP Up message
       received from a remote IUA peer.

       The ASPUP Ack message contains the following parameters:

          INFO String (optional)

       The format for ASPUP Ack Message parameters is 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 (0x4)           |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                          INFO String*                         |

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

       The format and description of the optional Info String parameter is
       the same as for the ASP Up message (See Section 3.3.3.1).

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    3.3.2.3  ASP Down (ASPDN)

       The ASP Down (ASPDN) message is sent by an ASP to indicate to an SG
       that it is NOT ready to receive traffic or maintenance messages.

       The ASPDN message contains the following parameters:

          Reason
          INFO String (Optional)

       The format for the ASPDN message parameters is 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 (0xa)           |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              Reason                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Tag (0x4)           |            Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                         INFO String*                          |

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

       The format and description of the optional Info String parameter is
       the same as for the ASP Up message (See Section 3.3.3.1.).

       The Reason parameter indicates the reason that the remote IUA
       adaptation layer is unavailable.  The valid values for Reason are
       shown in the following table.

          Value         Description
          0x1          Management Inhibit

       If a ASP is removed from Management Inhibit, the ASP will send an ASP
       Up message.

    3.3.2.4  ASP Down Ack

       The ASP Down Ack message is used to acknowledge an ASP Down message
       received from a remote IUA peer.

       The ASP Down Ack message contains the following parameters:

          Reason
          INFO String (Optional)

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       The format for the ASP Down Ack message parameters is 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 (0xa)           |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              Reason                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Tag (0x4)           |            Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                         INFO String*                          |

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

       The format and description of the optional Info String parameter is
       the same as for the ASP Up message (See Section 3.3.2.1.).

       The format of the Reason parameter is the same as for the ASP Down
       message (See Section 3.3.2.3).

    3.3.2.5  ASP Active (ASPAC)

       The ASPAC message is sent by an ASP to indicate to an SG that it is
       Active and ready to be used.

       The ASPAC message contains the following parameters

          Traffic Mode Type (Mandatory)
          Interface Identifier (Optional)
             - Combination of integer and integer ranges, OR
             - string (text formatted)
          INFO String (Optional)

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       The format for the ASPAC message using integer formatted Interface
       Identifiers is 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 (0xb)           |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Traffic Mode Type                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Tag (0x1=integer)         |            Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                     Interface Identifiers*                    |

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Tag (0x8=integer range)    |            Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 Interface Identifier Start1*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                  Interface Identifier Stop1*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 Interface Identifier Start2*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                  Interface Identifier Stop2*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               .                                                           .
               .                                                           .
               .                                                           .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 Interface Identifier StartN*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                  Interface Identifier StopN*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |              Additional Interface Identifiers                 |
       |                    of Tag Type 0x1 or 0x8                     |

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Tag (0x4)             |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                          INFO String*                         |

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

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       The format for the ASPAC message using text formatted (string)
       Interface Identifiers is 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 (0xb)           |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Traffic Mode Type                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       Tag (0x3=string)        |            Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                     Interface Identifier*                     |

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

       |              Additional Interface Identifiers                 |
       |                        of Tag Type 0x3                        |

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Tag (0x4)             |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                          INFO String*                         |

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

       The Traffic Mode Type parameter identifies the traffic mode of
       operation of the ASP within an AS.  The valid values for Type are
       shown in the following table:

         Value          Description
          0x1            Over-ride
          0x2            Load-share

       Within a particular Interface Identifier, only one Traffic Mode Type
       can be used.  The Over-ride value indicates that the ASP is operating
       in Over-ride mode, where the ASP takes over all traffic in an
       Application Server (i.e., primary/back-up operation), over-riding any
       currently active ASPs in the AS.  In Load-share mode, the ASP will
       share in the traffic distribution with any other currently active
       ASPs.

       The optional Interface Identifiers parameter contains a list of
       Interface Identifier integers (Type 0x1 or Type 0x8) or text strings
       (Type 0x3) indexing the Application Server traffic that the sending
       ASP is configured/registered to receive.  If integer formatted

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       Interface Identifiers are being used, the ASP can also send ranges of
       Interface Identifiers (Type 0x8).  Interface Identifier types Integer
       (0x1) and Integer Range (0x8) are allowed in the same message.  Text
       formatted Interface Identifiers (0x3) cannot be used with either
       Integer (0x1) or Integer Range (0x8) types.

       If no Interface Identifiers are included, the message is for all
       provisioned Interface Identifiers within the AS(s) in which the ASP
       is provisioned.  If only a subset of Interface Identifiers are
       included, the ASP is noted as Active for all the Interface
       Identifiers provisioned for that AS.

       Note:  If the optional Interface Identifier parameter is present, the
       integer formatted Interface Identifier MUST be supported, while the
       text formatted Interface Identifier MAY be supported.

       The format and description of the optional Info String parameter is
       the same as for the ASP Up message (See Section 3.3.2.1.).

       An SG that receives an ASPAC with an incorrect Traffic Mode Type for
       a particular Interface Identifier will respond with an Error Message
       (Cause: Unsupported Traffic Handling Mode).

    3.3.2.6 ASP Active Ack

       The ASPAC Ack message is used to acknowledge an ASP-Active message
       received from a remote IUA peer.

       The ASPAC Ack message contains the following parameters:

          Traffic Mode Type (Mandatory)
          Interface Identifier (Optional)
             - Combination of integer and integer ranges, OR
             - string (text formatted)
          INFO String (Optional)

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       The format for the ASPAC Ack message with Integer-formatted Interface
       Identifiers is 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 (0xb)           |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      Traffic Mode Type                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Tag (0x1=integer)         |            Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                     Interface Identifiers*                    |

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Tag (0x8=integer range)    |            Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 Interface Identifier Start1*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                  Interface Identifier Stop1*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 Interface Identifier Start2*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                  Interface Identifier Stop2*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               .                                                           .
               .                                                           .
               .                                                           .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 Interface Identifier StartN*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                  Interface Identifier StopN*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |              Additional Interface Identifiers                 |
       |                    of Tag Type 0x1 or 0x8                     |

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Tag (0x4)             |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                          INFO String*                         |

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

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       The format for the ASP Active Ack message using text formatted
       (string) Interface Identifiers is 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 (0xb)           |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Traffic Mode Type                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       Tag (0x3=string)        |            Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                     Interface Identifier*                     |

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

       |              Additional Interface Identifiers                 |
       |                        of Tag Type 0x3                        |

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Tag (0x4)             |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                          INFO String*                         |

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

       The format of the Traffic Mode Type and Interface Identifier
       parameters is the same as for the ASP Active message (See Section
       3.3.2.5).

       The format and description of the optional Info String parameter is
       the same as for the ASP Up message (See Section 3.3.2.1.).

    3.3.2.7  ASP Inactive (ASPIA)

       The ASPIA message is sent by an ASP to indicate to an SG that it is
       no longer an active ASP to be used from within a list of ASPs.  The
       SG will respond with an ASPIA Ack message and either discard incoming
       messages or buffer for a timed period and then discard.

       The ASPIA message contains the following parameters

          Traffic Mode Type (Mandatory)
          Interface Identifiers (Optional)
             - Combination of integer and integer ranges, OR
             - string (text formatted)

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          INFO String (Optional)

       The format for the ASP Inactive message parameters using Integer
       formatted Interface Identifiers is 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 (0xb)           |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Traffic Mode Type                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Tag (0x1=integer)         |            Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                     Interface Identifiers*                    |

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Tag (0x8=integer range)    |            Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 Interface Identifier Start1*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                  Interface Identifier Stop1*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 Interface Identifier Start2*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                  Interface Identifier Stop2*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               .                                                           .
               .                                                           .
               .                                                           .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 Interface Identifier StartN*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                  Interface Identifier StopN*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |              Additional Interface Identifiers                 |
       |                    of Tag Type 0x1 or 0x8                     |

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Tag (0x4)           |            Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                          INFO String*                         |

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

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       The format for the ASP Inactive message using text formatted (string)
       Interface Identifiers is 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 (0xb)           |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Traffic Mode Type                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       Tag (0x3=string)        |            Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                     Interface Identifier*                     |

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

       |              Additional Interface Identifiers                 |
       |                        of Tag Type 0x3                        |

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Tag (0x4)             |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                          INFO String*                         |

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

       The Traffic Mode Type parameter identifies the traffic mode of
       operation of the ASP within an AS.  The valid values for Traffic Mode
       Type are shown in the following table:

          Value          Description
           0x1            Over-ride
           0x2            Load-share

       The format and description of the optional Interface Identifiers and
       Info String parameters is the same as for the ASP Active message (See
       Section 3.3.2.3.).

       The optional Interface Identifiers parameter contains a list of
       Interface Identifier integers or text strings indexing the
       Application Server traffic that the sending ASP is
       configured/registered to receive, but does not want to receive at
       this time.

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    3.3.2.8  ASP Inactive Ack

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

       The ASPIA Ack message contains the following parameters:

          Traffic Mode Type (Mandatory)
          Interface Identifiers (Optional)
             - Combination of integer and integer ranges, OR
             - string (text formatted)
          INFO String (Optional)

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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 (0xb)           |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Traffic Mode Type                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Tag (0x1=integer)         |            Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                     Interface Identifiers*                    |

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Tag (0x8=integer range)    |            Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 Interface Identifier Start1*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                  Interface Identifier Stop1*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 Interface Identifier Start2*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                  Interface Identifier Stop2*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               .                                                           .
               .                                                           .
               .                                                           .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 Interface Identifier StartN*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                  Interface Identifier StopN*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |              Additional Interface Identifiers                 |
       |                    of Tag Type 0x1 or 0x8                     |

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Tag (0x4)             |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                          INFO String*                         |

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

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       The format for the ASP Inactive Ack message using text formatted
       (string) Interface Identifiers is 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 (0xb)           |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                     Traffic Mode Type                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       Tag (0x3=string)        |            Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                     Interface Identifier*                     |

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

       |              Additional Interface Identifiers                 |
       |                        of Tag Type 0x3                        |

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Tag (0x4)             |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                          INFO String*                         |

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

       The format of the Traffic Mode Type and Interface Identifier
       parameters is the same as for the ASP Inactive message (See Section
       3.3.2.7).

       The format and description of the optional Info String parameter is
       the same as for the ASP Up message (See Section 3.3.2.1).

    3.3.2.9  Heartbeat (BEAT)

       The Heartbeat message is optionally used to ensure that the IUA peers
       are still available to each other.  It is recommended for use when
       the IUA runs over a transport layer other than the SCTP, which has
       its own heartbeat.

       The BEAT message contains the following parameters:

          Heartbeat Data         Optional

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       The format for the BEAT message is 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 = 9            |            Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       \                                                               \
       |                       Heartbeat Data *                        |
       \                                                               \
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       The Heartbeat Data parameter contents are defined by the sending
       node. The Heartbeat Data could include, for example, a Heartbeat
       Sequence Number and, or Timestamp.  The receiver of a Heartbeat
       message does not process this field as it is only of significance to
       the sender. The receiver MUST respond with a Heartbeat Ack message.

    3.3.2.10  Heartbeat Ack (BEAT-Ack)

       The Heartbeat Ack message is sent in response to a received Heartbeat
       message.  It includes all the parameters of the received Heartbeat
       message, without any change.

    3.3.3  Layer Management (MGMT) Messages

    3.3.3.1  Error (ERR)

       The Error message is used to notify a peer of an error event
       associated with an incoming message.  For example, the message type
       might be unexpected given the current state, or a parameter value
       might be invalid.

       The Error message will only have the common message header.  The
       Error message contains the following parameters:

          Error Code
          Diagnostic Information (optional)

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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 (0xc)           |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          Error Code                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |           Tag (0x7)           |            Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                     Diagnostic Information*                   |

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

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

          Invalid Version                               0x01
          Invalid Interface Identifier                  0x02
          Unsupported Message Class                     0x03
          Unsupported Message Type                      0x04
          Unsupported Traffic Handling Mode             0x05
          Unexpected Message                            0x06
          Protocol Error                                0x07
          Unsupported Interface Identifier Type         0x08
          Invalid Stream Identifier                     0x09
          Unassigned TEI                                0x0a
          Unrecognized SAPI                             0x0b
          Invalid TEI, SAPI combination                 0x0c

       The "Invalid Version" error would be sent if a message was received
       with an invalid or unsupported version.  The Error message would
       contain the supported version in the Common header.  The Error
       message could optionally provide the supported version in the
       Diagnostic Information area.

       The "Invalid Interface Identifier" error would be sent by a SG if an
       ASP sends a message with an invalid (unconfigured) Interface
       Identifier value.

       The "Unsupported Traffic Handling Mode" error would be sent by a SG
       if an ASP sends an ASP Active with an unsupported Traffic Handling
       Mode.  An example would be a case in which the SG did not support
       load-sharing.

       The "Unexpected Message" error would be sent by an ASP if it received
       a QPTM message from an SG while it was in the Inactive state (the ASP
       could optionally drop the message and not send an Error).  It would

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       also be sent by an ASP if it received a defined and recognized
       message that the SG is not expected to send (e.g., if the MGC
       receives an IUA Establish Request message).

       The "Protocol Error" error would be sent for any protocol anomaly
       (i.e., a bogus message).

       The "Invalid Stream Identifier" error would be sent if a message was
       received on an unexpected SCTP stream (i.e., a MGMT message was
       received on a stream other than "0").

       The "Unsupported Interface Identifier Type" error would be sent by a
       SG if an ASP sends a Text formatted Interface Identifier and the SG
       only supports Integer formatted Interface Identifiers.  When the ASP
       receives this error, it will need to resend its message with an
       Integer formatted Interface Identifier.

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

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

       The "Unassigned TEI" error may be used when the SG receives an IUA
       message that includes a TEI which has not been assigned or recognized
       for use on the indicated ISDN D-channel.

       The "Unrecognized SAPI" error would handle the case of using a SAPI
       that is not recognized by the SG.  The "Invalid TEI, SAPI
       combination" error identify errors where the TEI is assigned and the
       the SAPI is recognized, but the combination is not valid for the
       interface (e.g., on a BRI the MGC tries to send Q.921 Management
       messages via IUA when Layer Management at the SG SHOULD be performing
       this function).

       The optional Diagnostic information can be any information germane to
       the error condition, to assist in identification of the error
       condition.  To enhance debugging, the Diagnostic information could
       contain the first 40 bytes of the offending message.

    3.3.3.2  Notify (NTFY)

       The Notify message used to provide an autonomous indication of IUA
       events to an IUA peer.

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       The Notify message will only use the common message header.  The
       Notify message contains the following parameters:

          Status Type
          Status Identification
          Interface Identifiers (Optional)
          INFO String (Optional)

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       The format for the Notify message with Integer-formatted Interface
       Identifiers is 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 (0xd)           |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |        Status Type            |    Status Identification      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Tag (0x1=integer)         |            Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                     Interface Identifiers*                    |

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Tag (0x8=integer range)    |            Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 Interface Identifier Start1*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                  Interface Identifier Stop1*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 Interface Identifier Start2*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                  Interface Identifier Stop2*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               .                                                           .
               .                                                           .
               .                                                           .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 Interface Identifier StartN*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                  Interface Identifier StopN*                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |              Additional Interface Identifiers                 |
       |                    of Tag Type 0x1 or 0x8                     |

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Tag (0x4)             |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                          INFO String*                         |

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

    Morneault, et al.           Standards Track                    [Page 42]

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       The format for the Notify message with Text-formatted Interface
       Identifiers is 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 (0xd)           |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |        Status Type            |    Status Identification      |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |       Tag (0x3=string)        |            Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                     Interface Identifier*                     |

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

       |              Additional Interface Identifiers                 |
       |                        of Tag Type 0x3                        |

       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         Tag (0x4)             |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       |                          INFO String*                         |

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

       The Status Type parameter identifies the type of the Notify message.
       The following are the valid Status Type values:

          Value          Description
           0x1   Application Server state change (AS_State_Change)
           0x2   Other

       The Status Identification parameter contains more detailed
       information for the notification, based on the value of the Status
       Type.  If the Status Type is AS_State_Change the following Status
       Identification values are used:

          Value          Description
            1    Application Server Down (AS_Down)
            2    Application Server Inactive (AS_Inactive)
            3    Application Server Active (AS_Active)
            4    Application Server Pending (AS_Pending)

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

       If the Status Type is Other, then the following Status Information
       values are defined:

          Value          Description
            1    Insufficient ASP resources active in AS
            2    Alternate ASP Active

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

       The format and description of the optional Interface Identifiers and
       Info String parameters is the same as for the ASP Active message (See
       Section 3.3.2.3.).

    3.3.3.3 TEI Status Messages (Request, Confirm and Indication)

       The TEI Status messages are exchanged between IUA layer peers to
       request, confirm and indicate the status of a particular TEI.

       The TEI Status messages contain the common message header followed by
       IUA message header.  The TEI Status Request message does not contain
       any additional parameters.

       In the integrated ISDN Layer 2/3 model (e.g., in traditional ISDN
       switches), it is assumed that the Layer Management for the Q.921
       Layer and the Q.931 layer are co-located.  When backhauling ISDN,
       this assumption is not necessarily valid.  The TEI status messages
       allow the two Layer Management entities to communicate the status of
       the TEI.  In addition, knowing that a TEI is in service allows the
       ASP to request the SG to establish the datalink to the terminal (via
       the IUA Establish message) for signaling if the ASP wants to be in
       control of data link establishment.  Another use of the TEI status
       procedure is where the Layer Management at the ASP can prepare for
       send/receive signaling to/from a given TEI and confirm/verify the
       establishment of a datalink to that TEI.  For example, if a datalink
       is established for a TEI that the ASP did not know was assigned, the
       ASP can check to see whether it was assigned or whether there was an
       error in the signaling message.  Also, knowing that a TEI is out of
       service, the ASP need not request the SG to establish a datalink to
       that TEI.

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       The TEI Status Indication, and Confirm messages contain the following
       parameter:

         STATUS

        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 (0x10)           |             Length            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                              Status                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       The valid values for Status are shown in the following table.

          Define     Value           Description
       ASSIGNED       0x0        TEI is considered assigned by Q.921
       UNASSIGNED     0x1        TEI is considered unassigned by Q.921

    4.0  Procedures

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

    4.1  Procedures to support service in section 1.4.1

       These procedures achieve the IUA layer's "Transport of Q.921/Q.931
       boundary" service.

    4.1.1  Q.921 or Q.931 primitives procedures

       On receiving these primitives from the local layer, the IUA layer
       will send the corresponding QPTM message (Data, Unit Data, Establish,
       Release) to its peer.  While doing so, the IUA layer needs to fill
       various fields of the common and specific headers correctly.  In
       addition the message needs to be sent on the SCTP stream that
       corresponds to the D channel (Interface Identifier).

    4.1.2  QPTM message procedures

       On receiving QPTM messages from a peer IUA layer, the IUA layer on an
       SG or MGC needs to invoke the corresponding layer primitives (DL-
       ESTABLISH, DL-DATA, DL-UNIT DATA, DL-RELEASE) to the local Q.921 or
       Q.931 layer.

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    4.2  Procedures to support service in section 1.4.2

       These procedures achieve the IUA layer's "Support for Communication
       between Layer Managements" service.

    4.2.1 Layer Management primitives procedures

       On receiving these primitives from the local Layer Management, the
       IUA layer will provide the appropriate response primitive across the
       internal local Layer Management interface.

       An M-SCTP ESTABLISH request from Layer Management will initiate the
       establishment of an SCTP association.  An M-SCTP ESTABLISH confirm
       will be sent to Layer Management when the initiated association set-
       up is complete.  An M-SCTP ESTABLISH indication is sent to Layer
       Management upon successful completion of an incoming SCTP association
       set-up from a peer IUA node

       An M-SCTP RELEASE request from Layer Management will initiate the
       tear-down of an SCTP association.  An M-SCTP RELEASE confirm will be
       sent by Layer Management when the association teardown is complete.
       An M-SCTP RELEASE indication is sent to Layer Management upon
       successful tear-down of an SCTP association initiated by a peer IUA.

       M-SCTP STATUS request and indication support a Layer Management query
       of the local status of a particular SCTP association.

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

       M-ASP STATUS request/indication and M-AS-STATUS request/indication
       support a Layer Management query of the local status of a particular
       ASP or AS.  No IUA peer protocol is invoked.

       M-ASP-UP request, M-ASP-DOWN request, M-ASP-INACTIVE request and M-
       ASP-ACTIVE request allow Layer Management at an ASP to initiate state
       changes.  These requests result in outgoing IUA ASP UP, ASP DOWN, ASP
       INACTIVE and ASP ACTIVE messages.

       M-ASP-UP confirmation, M-ASP-DOWN confirmation, M-ASP-INACTIVE
       confirmation and M-ASP-ACTIVE confirmation indicate to Layer
       Management that the previous request has been confirmed.

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       Upon receipt of a M-TEI Status primitive from Layer Management, the
       IUA will send the corresponding MGMT message (TEI Status) to its
       peer.  While doing so, the IUA layer needs to fill various fields of
       the common and specific headers correctly.

       All MGMT messages are sent on a sequenced stream to ensure ordering.
       SCTP stream '0' SHOULD be used.

    4.2.2  Receipt of IUA Peer Management messages

       Upon receipt of IUA Management messages, the IUA layer MUST invoke
       the corresponding Layer Management primitive indications (e.g., M-AS
       Status ind., M-ASP Status ind., M-ERROR ind., M-TEI STATUS...) to the
       local layer management.

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

       All MGMT messages are sent on a sequenced stream to ensure ordering.
       SCTP stream '0' SHOULD be used.

    4.3 Procedures to support service in section 1.4.3

       These procedures achieve the IUA layer's "Support for management of
       active associations between SG and MGC" service.

    4.3.1 AS and ASP State Maintenance

       The IUA layer on the SG needs to maintain the states of each ASP as
       well as the state of the AS.

    4.3.1.1  ASP States

       The state of the each ASP, in each AS that it is configured, is
       maintained in the IUA layer on the SG.  The state of an ASP changes
       due to the following type of events:

          *  Reception of messages from peer IUA layer at that ASP
          *  Reception of some messages from the peer IUA layer at other
             ASPs in the AS
          *  Reception of indications from SCTP layer

       The ASP state transition diagram is shown in Figure 7.  The possible
       states of an ASP are the following:

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       ASP-DOWN: Application Server Process is unavailable and/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 IUA messages.

       ASP-INACTIVE: The remote IUA peer at the ASP is available (and the
       related SCTP association is up) but application traffic is stopped.
       In this state the ASP can be sent any non-QPTM IUA messages (except
       for TEI Status messages).

       ASP-ACTIVE: The remote IUA peer at the ASP is available and
       application traffic is active.

                       Figure 7  ASP State Transition Diagram

                                        +-------------+
                 +----------------------|             |
                 |   Alternate  +-------| ASP-ACTIVE  |
                 |       ASP    |       +-------------+
                 |    Takeover  |           ^     |
                 |              |    ASP    |     | ASP
                 |              |    Active |     | Inactive
                 |              |           |     v
                 |              |       +-------------+
                 |              |       |             |
                 |              +------>|  ASP-INACT  |
                 |                      +-------------+
                 |                          ^    |
       ASP Down/ |                     ASP  |    | ASP Down /
       SCTP CDI  |                     Up   |    | SCTP CDI
                 |                          |    v
                 |                      +-------------+
                 +--------------------->|             |
                                        |  ASP-DOWN   |
                                        +-------------+

       SCTP CDI:  The local SCTP layer's Communication Down Indication to
       the Upper Layer Protocol (IUA) on an SG.  The local SCTP will send
       this indication when it detects the loss of connectivity to the ASP's
       peer SCTP layer.  SCTP CDI is understood as either a SHUTDOWN
       COMPLETE notification and COMMUNICATION LOST notification from the
       SCTP.

    4.3.1.2  AS States

       The state of the AS is maintained in the IUA layer on the SG.

       The state of an AS changes due to events.  These events include the
       following:

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          *  ASP state transitions
          *  Recovery timer triggers

       The possible states of an AS are the following:

       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.

       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 from active to inactive or
       down and it was the last remaining active ASP in the AS.  A recovery
       timer T(r) will be started and all incoming SCN messages will be
       queued by the SG.  If an ASP becomes active before T(r) expires, the
       AS will move to AS-ACTIVE state and all the queued messages will be
       sent to the active ASP.

       If T(r) expires before an ASP becomes active, the SG stops queuing
       messages and  discards all previously queued messages.  The AS will
       move to AS-INACTIVE if at least one ASP is in ASP-INACTIVE state,
       otherwise it will move to AS-DOWN state.

    Morneault, et al.           Standards Track                    [Page 49]

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                     Figure 8  AS State Transition Diagram

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

          Tr = Recovery Timer

    4.3.2 ASPM procedures for primitives

       Before the establishment of an SCTP association the ASP state at both
       the SG and ASP is assumed to be "Down".

       As the ASP is responsible for initiating the setup of an SCTP
       association to an SG, the IUA layer at an ASP receives an M-SCTP
       ESTABLISH request primitive from the Layer Management, the IUA layer
       will try to establish an SCTP association with the remote IUA peer at
       an SG.  Upon reception of an eventual SCTP-Communication Up confirm
       primitive from the SCTP, the IUA layer will invoke the primitive M-
       SCTP ESTABLISH confirm to the Layer Management.

       At the SG, the IUA layer will receive an SCTP Communication Up
       indication primitive from the SCTP.  The IUA layer will then invoke
       the primitive M-SCTP ESTABLISH indication to the Layer Management.

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       Once the SCTP association is established and assuming that the local
       IUA-User is ready, the local ASP IUA Application Server Process
       Maintenance (ASPM) function will initiate the ASPM procedures, using
       the ASP Up/-Down/-Active/-Inactive messages to convey the ASP state
       to the SG - see Section 4.3.3.

       The Layer Management and the IUA layer on SG can communicate the
       status of the application server using the M-AS STATUS primitives.
       The Layer Management and the IUA layer on both the SG and ASP can
       communicate the status of an SCTP association using the M-SCTP STATUS
       primitives.

       If the Layer Management on SG or ASP wants to bring down an SCTP
       association for management reasons, they would send M-SCTP RELEASE
       request primitive to the local IUA layer.  The IUA layer would
       release the SCTP association and upon receiving the SCTP
       Communication Down indication from the underlying SCTP layer, it
       would inform the local Layer Management using M-SCTP RELEASE confirm
       primitive.

       If the IUA layer receives an SCTP-Communication Down indication from
       the underlying SCTP layer, it will inform the Layer Management by
       invoking the M-SCTP RELEASE indication primitive.  The state of the
       ASP will be moved to "Down" at both the SG and ASP.

       At an ASP, the Layer Management MAY try to reestablish the SCTP
       association using M-SCTP ESTABLISH request primitive.

    4.3.3 ASPM procedures for peer-to-peer messages

       All ASPM messages are sent on a sequenced stream to ensure ordering.
       SCTP stream '0' SHOULD be used.

    4.3.3.1 ASP Up

       After an ASP has successfully established an SCTP association to an
       SG, the SG waits for the ASP to send an ASP Up message, indicating
       that the ASP IUA peer is available.  The ASP is always the initiator
       of the ASP Up exchange.

       When an ASP Up message is received at an SG and internally the remote
       ASP is not considered locked-out for local management reasons, the SG
       marks the remote ASP as "Inactive".  The SG responds with an ASP Up
       Ack message in acknowledgement.  The SG sends an ASP-Up Ack message
       in response to a received ASP Up message even if the ASP is already
       marked as "Inactive" at the SG.

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       If for any local reason the SG cannot respond with an ASP Up, the SG
       responds to a ASP Up with a with an ASP-Down Ack message with Reason
       "Management Blocking".

       At the ASP, the ASP Up Ack message received from the SG is not
       acknowledged by the ASP.  If the ASP does not receive a response from
       the SG, or an ASP Down Ack is received, the ASP MAY resend ASP Up
       messages every 2 seconds until it receives a ASP Up Ack message from
       the SG.  The ASP MAY decide to reduce the frequency (say to every 5
       seconds) if an ASP Up Ack is not received after a few tries.

       The ASP MUST wait for the ASP Up Ack message from the SG before
       sending any ASP traffic control messages (ASPAC or ASPIA) or Data
       messages or it will risk message loss.  If the SG receives QPTM, ASP
       Active or ASP Inactive messages before an ASP Up is received, the SG
       SHOULD discard these messages.

    4.3.3.2 ASP Down

       The ASP will send an ASP Down to an SG when the ASP is to be removed
       from the list of ASPs in all Application Servers that it is a member
       and no longer receive any IUA traffic or management messages.

       Whether the ASP is permanently removed from an AS is a function of
       configuration management.

       The SG marks the ASP as "Down" and returns an ASP Down Ack message to
       the ASP if one of the following events occur:

          -  to acknowledge an ASP Down message from an ASP,
          -  to reply to ASPM messages from an ASP which is locked out for
             management reasons.

       The SG sends an ASP Down Ack message in response to a received ASP
       Down message from the ASP even if the ASP is already marked as "Down"
       at the SG.

       If the ASP does not receive a response from the SG, the ASP MAY send
       ASP Down messages every 2 seconds until it receives an ASP Down Ack
       message from the SG or the SCTP association goes down.  The ASP MAY
       decide to reduce the frequency (say to every 5 seconds) if an ASP
       Down Ack is not received after a few tries.

    4.3.3.3 IUA Version Control

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

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       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
       message would normally come from the SG.

    4.3.3.4 ASP Active

       Any time after the ASP has received a ASP Up Ack from the SG, the ASP
       sends an ASP-Active (ASPAC) to the SG indicating that the ASP is
       ready to start processing traffic.  In the case where an ASP is
       configured/registered to process the traffic for more than one
       Application Server across an SCTP association, the ASPAC contains one
       or more Interface Identifiers to indicate for which Application
       Servers the ASPAC applies.

       When an ASP Active (ASPAC) message is received, the SG responds to
       the ASP with a ASPAC Ack message acknowledging that the ASPAC was
       received and starts sending traffic for the associated Application
       Server(s) to that ASP.

       The ASP MUST wait for the ASP-Active Ack message from the SG before
       sending any Data messages or it will risk message loss.  If the SG
       receives QPTM messages before an ASP Active is received, the SG
       SHOULD discard these messages.

       There are two modes of Application Server traffic handling in the SG
       IUA - Over-ride and Load-sharing.  The Type parameter in the ASPAC
       message indicates the mode used in a particular Application Server.
       If the SG determines that the mode indicates in an ASPAC is
       incompatible with the traffic handling mode currently used in the AS,
       the SG responds with an Error message indicating Unsupported Traffic
       Handling Mode.

       In the case of an Over-ride mode AS, reception of an ASPAC message at
       an SG causes the redirection of all traffic for the AS to the ASP
       that sent the ASPAC.  The SG responds to the ASPAC with an ASP-Active
       Ack message to the ASP.  Any previously active ASP in the AS is now
       considered Inactive and will no longer receive traffic from the SG
       within the AS.  The SG sends a Notify (Alternate ASP-Active) to the
       previously active ASP in the AS, after stopping all traffic to that
       ASP.

       In the case of a load-share mode AS, reception of an ASPAC message at
       an SG causes the direction of traffic to the ASP sending the ASPAC,
       in addition to all the other ASPs that are currently active in the
       AS. The algorithm at the SG for load-sharing traffic within an AS to
       all the active ASPs is implementation dependent.  The algorithm

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       could, for example be round-robin or based on information in the Data
       message, such as Interface Identifier, depending on the requirements
       of the application and the call state handling assumptions of the
       collection of ASPs in the AS.  The SG responds to the ASPAC with a
       ASP-Active Ack message to the ASP.

    4.3.3.5 ASP Inactive

       When an ASP wishes to withdraw from receiving traffic within an AS,
       the ASP sends an ASP Inactive (ASPIA) to the SG.  In the case where
       an ASP is configured/registered to process the traffic for more than
       one Application Server across an SCTP association, the ASPIA contains
       one or more Interface Identifiers to indicate for which Application
       Servers the ASPIA applies.

       There are two modes of Application Server traffic handling in the SG
       IUA when withdrawing an ASP from service - Over-ride and Load-
       sharing. The Type parameter in the ASPIA message indicates the mode
       used in a particular Application Server.  If the SG determines that
       the mode indicates in an ASPAC is incompatible with the traffic
       handling mode currently used in the AS, the SG responds with an Error
       message indicating Unsupported Traffic Handling Mode.

       In the case of an Over-ride mode AS, where normally another ASP has
       already taken over the traffic within the AS with an Over-ride ASPAC,
       the ASP which sends the ASPIA is already considered by the SG to be
       "Inactive".  An 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 SG moves the ASP to the
       "Inactive" state and the AS traffic is re-allocated across the
       remaining "active" ASPs per the load-sharing algorithm currently used
       within the AS.  An ASPIA Ack message is sent to the ASP after all
       traffic is halted to the ASP.  A NTFY (Insufficient ASPs) MAY be sent
       to all inactive ASPs, if required.

       If no other ASPs are Active in the Application Server, the SG sends a
       NTFY (AS-Pending) to all inactive ASPs of the AS and either discards
       all incoming messages for the AS or starts buffering the incoming
       messages for T(r)seconds, after which messages will be discarded.
       T(r) is configurable by the network operator.  If the SG receives an
       ASPAC from an ASP in the AS before expiry of T(r), the buffered
       traffic is directed to the ASP and the timer is cancelled.  If T(r)
       expires, the AS is moved to the "Inactive" state.

    Morneault, et al.           Standards Track                    [Page 54]

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    4.3.3.6  Notify

       A Notify message reflecting a change in the AS state is sent to all
       ASPs in the AS, except those in the "Down" state, with appropriate
       Status Identification.

       In the case where a Notify (AS-Pending) message is sent by an SG that
       now has no ASPs active to service the traffic, or a NTFY
       (Insufficient ASPs) is sent in the Load-share mode, the Notify does
       not explicitly force the ASP(s) receiving the message to become
       active.  The ASPs remain in control of what (and when) action is
       taken.

    4.3.3.7  Heartbeat

       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 the
       SCTP).

       After receiving an ASP Up Ack message from the SG in response to an
       ASP Up message, the ASP MAY optionally send Beat messages
       periodically, subject to a provisionable timer T(beat).  The SG IUA,
       upon receiving a BEAT message from the ASP, responds with a BEAT ACK
       message.  If no BEAT message (or any other IUA message) is received
       from the SG within the timer 2*T(beat), the SG will consider the
       remote IUA as "Down".  The SG will also send an ASP Down Ack message
       to the ASP.

       At the ASP, if no BEAT ACK message (or any other IUA message) is
       received from the SG within 2*T(beat), the SG is considered
       unavailable.  Transmission of BEAT messages is stopped and ASP Up
       procedures are used to re-establish communication with the SG IUA
       peer.

       The BEAT message MAY optionally contain an opaque Heartbeat Data
       parameter that MUST be echoed back unchanged in the related Beat Ack
       message.  The ASP upon examining the contents of the returned BEAT
       Ack message MAY choose to consider the remote ASP as unavailable.
       The contents/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), and/or a
       timestamp mechanism (to evaluate delays).

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

    Morneault, et al.           Standards Track                    [Page 55]

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    5.0 Examples

    5.1 Establishment of Association and Traffic between SGs and ASPs

    5.1.1 Single ASP in an Application Server (1+0 sparing)

       This scenario shows the example IUA 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                       ASP1
                     |
                     |<---------ASP Up----------|
                     |--------ASP Up Ack------->|
                     |                          |
                     |<-------ASP Active--------|
                     |------ASP Active Ack----->|
                     |                          |

    5.1.2 Two ASPs in Application Server (1+1 sparing)

       This scenario shows the example IUA 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 state or
       can communicate call state under failure/withdrawal events.  The
       example message flow is the same whether the ASP-Active messages are
       Over-ride or Load-share mode although typically this example would
       use an Over-ride mode.

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

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

       This scenario shows a similar case to Section 5.1.2 but 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 (Ldshr)----|                          |
               |----ASP Active Ack------>|                          |
               |                         |                          |
               |<----------------------------ASP Active (Ldshr)-----|
               |-----------------------------ASP Active Ack-------->|
               |                         |                          |

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

       This scenario shows the example IUA 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).

    Morneault, et al.           Standards Track                    [Page 57]

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          SG                  ASP1                ASP2                ASP3
           |                    |                   |                   |
           |<------ASP Up-------|                   |                   |
           |-----ASP Up Ack---->|                   |                   |
           |                    |                   |                   |
           |<--------------------------ASP Up-------|                   |
           |------------------------ASPUp Ack)----->|                   |
           |                    |                   |                   |
           |<---------------------------------------------ASP Up--------|
           |--------------------------------------------ASP Up Ack----->|
           |                    |                   |                   |
           |                    |                   |                   |
           |<-ASP Act (Ldshr)---|                   |                   |
           |----ASP Act Ack---->|                   |                   |
           |                    |                   |                   |
           |<---------------------ASP Act (Ldshr)---|                   |
           |----------------------ASP Act Ack------>|                   |
           |                    |                   |                   |

    5.2 ASP Traffic Fail-over Examples

    5.2.1 (1+1 Sparing, withdrawal of ASP, Back-up Over-ride)

       The following example shows a case in which an ASP withdraws from
       service:

              SG                       ASP1                       ASP2
               |                         |                          |
               |<-----ASP Inactive-------|                          |
               |----ASP Inactive Ack---->|                          |
               |-------------------NTFY(AS-Pending) --------------->|
               |                         |                          |
               |<------------------------------ ASP Active----------|
               |-----------------------------ASP Active Ack)------->|
               |                                                    |

       In this case, the SG notifies ASP2 that the AS has moved to the Down
       state.  The SG could have also (optionally) sent a Notify message
       when the AS moved to the Pending state.

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

    5.2.2 (1+1 Sparing, Back-up Over-ride)

       The following example shows a case in which ASP2 wishes to over-ride
       ASP1 and take over the traffic:

    Morneault, et al.           Standards Track                    [Page 58]

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

       In this case, the SG notifies ASP1 that an alternative ASP has
       overridden it.

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

       Following on from the example in Section 5.1.4, and ASP1 withdraws
       from service

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

       In this case, the SG has knowledge of the minimum ASP resources
       required (implementation dependent) 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 IUA peer (IUA heartbeat
       loss or detection of SCTP failure), the first SG-ASP1 ASP Inactive
       message exchange would not occur.

    5.3 Q.921/Q.931 primitives backhaul Examples

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

          -  Determine the correct SG

          -  Find the SCTP association to the chosen SG

          -  Determine the correct stream in the SCTP association based on
             the D channel

          -  Fill in the QPTM message, fill in IUA Message Header, fill in
             Common Header

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          -  Send the QPTM message to the remote IUA peer in the SG, over
             the SCTP association

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

          -  Determine the AS for the Interface Identifier

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

          -  Determine the correct stream in the SCTP association based on
             the D channel

          -  Fill in the QPTM message, fill in IUA Message Header, fill in
             Common Header

          -  Send the QPTM message to the remote IUA peer in the ASP, over
             the SCTP association

       An example of the message flows for establishing a data link on a
       signaling channel, passing PDUs and releasing a data link on a
       signaling channel is shown below.  An active association between MGC
       and SG is established (Section 5.1) prior to the following message
       flows.

                SG                             ASP

                            <----------- Establish Request
          Establish Confirm  ---------->

                            <----------- Data Request
             Data Indication ----------->
                            <----------- Data Request
             Data Indication ----------->
                            <----------- Data Request
                            <----------- Data Request
             Data Indication ----------->

                            <----------- Release Request (RELEASE_MGMT)
            Release Confirm  ---------->

       An example of the message flows for a failed attempt to establish a
       data link on the signaling channel is shown below.  In this case, the
       gateway has a problem with its physical connection (e.g., Red Alarm),
       so it cannot establish a data link on the signaling channel.

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                SG                             ASP

                            <----------- Establish Request (ESTABLISH_START)
          Release Indication ---------->
          (RELEASE_PHYS)

    5.4 Layer Management Communication Examples

       An example of the message flows for communication between Layer
       Management modules between SG and ASP is shown below.  An active
       association between ASP and SG is established (Section 5.1) prior to
       the following message flows.

                      SG                       ASP

                            <----------- Data Request
            Error Indication ---------->
             (INVALID_TEI)

                            <----------- TEI Status Request
          TEI Status Confirm ---------->
               (Unassigned)

    6.0 Security

       IUA is designed to carry signaling messages for telephony services.
       As such, IUA MUST involve the security needs of several parties the
       end users of the services; the network providers and the applications
       involved.  Additional requirements MAY come from local regulation.
       While having some overlapping security needs, any security solution
       SHOULD fulfill all of the different parties' needs.

    6.1 Threats

       There is no quick fix, one-size-fits-all solution for security.  As a
       transport protocol, IUA has the following security objectives:

          *  Availability of reliable and timely user data transport.
          *  Integrity of user data transport.
          *  Confidentiality of user data.

       IUA runs on top of SCTP.  SCTP [3] provides certain transport related
       security features, such as

          *  Blind Denial of Service Attacks
          *  Flooding
          *  Masquerade
          *  Improper Monopolization of Services

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       When IUA is running in professionally managed corporate or service
       provider network, it is reasonable to expect that this network
       includes an appropriate security policy framework.  The "Site
       Security Handbook" [5] SHOULD be consulted for guidance.

       When the network in which IUA runs in involves more than one party,
       it MAY NOT be reasonable to expect that all parties have implemented
       security in a sufficient manner.  In such a case, it is recommended
       that IPSEC is used to ensure confidentiality of user payload.
       Consult [6] for more information on configuring IPSEC services.

    6.2 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.

    7.0 IANA Considerations

    7.1 SCTP Payload Protocol Identifier

       A request will be made to IANA to assign an IUA value for the Payload
       Protocol Identifier in SCTP Payload Data chunk.  The following SCTP
       Payload Protocol Identifier will be registered:

             IUA    "1"

       The SCTP Payload Protocol Identifier is included in each SCTP Data
       chunk, to indicate which protocol the SCTP is carrying.  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.

       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.

    7.2  IUA 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, and
          -- through definition of additional message parameters.

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       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 [RFC2434].

       The proposed extension must in no way adversely affect the general
       working of the protocol.

    7.2.1 IETF Defined Message Classes

       The documentation for a new message class MUST include the following
       information:

       (a) A long and short name for the message class.
       (b) A detailed description of the purpose of the message class.

    7.2.2 IETF Defined Message Types

       Documentation of the message type MUST contain the following
       information:

       (a) A long and short name for the new message type.
       (b) A detailed description of the structure of the message.
       (c) A detailed definition and description of intended use of each
           field within the message.
           ti3 (d) A detailed procedural description of the use of the new
           message type within the operation of the protocol.
       (e) 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 of Unsupported Message Type.

    7.2.3 IETF-defined TLV Parameter Extension

       Documentation of the message parameter MUST contain the following
       information:

       (a) Name of the parameter type.
       (b) Detailed description of the structure of the parameter field.
           This structure MUST conform to the general type-length-value
           format described in Section 3.1.5.
       (c) Detailed definition of each component of the parameter value.
       (d) 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.

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    8.0 Acknowledgements

       The authors would like to thank Alex Audu, Maria Sonia Vazquez
       Arevalillo, Ming-te Chao, Keith Drage, Norm Glaude, Nikhil Jain,
       Bernard Kuc, Ming Lin, Stephen Lorusso, John Loughney, Barry
       Nagelberg, Neil Olson, Lyndon Ong, Heinz Prantner, Jose Luis Jimenez
       Ramirez, Ian Rytina, Michael Tuexen and Hank Wang for their valuable
       comments and suggestions.

    9.0  References

       [1] ITU-T Recommendation Q.920, 'Digital Subscriber signaling System
           No. 1 (DSS1) - ISDN User-Network Interface Data Link Layer -
           General Aspects'

       [2] T1S1.7/99-220 Contribution, 'Back-hauling of DSS1 protocol in a
           Voice over Packet Network'

       [3] Stewart, R., Xie, Q., Morneault, K., Sharp, C., Schwarzbauer, H.,
           Taylor, T., Rytina, I., Kalla, M., Zhang, L. and V. Paxson,
           "Stream Control Transmission Protocol", RFC 2960, October 2000.

       [4] Ong, L., Rytina, I., Garcia, M., Schwarzbauer, H., Coene, L.,
           Lin, H., Juhasz, I., Holdrege, M., and C. Sharp, "Architectural
           Framework for Signaling Transport", RFC 2719, October 1999.

       [5] Fraser, B., "Site Security Handbook", FYI 8, RFC 2196, September
           1997.

       [6] Kent, S. and R. Atkinson, "Security Architecture for the Internet
           Protocol", RFC 2401, November 1998.

       [7] Bradner, s., "Key words for use in RFCs to Indicate Requirement
           Levels", BCP 14, RFC 2119, March 1997.

       [8] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
           Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.

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    10.0 Authors' Addresses

       Ken Morneault
       Cisco Systems Inc.
       13615 Dulles Technology Drive
       Herndon, VA. 20171
       USA

       Phone: +1-703-484-3323
       EMail: kmorneau@cisco.com

       Malleswar Kalla
       Telcordia Technologies
       PYA 2J-341
       3 Corporate Place
       Piscataway, NJ 08854
       USA

       Phone: +1-732-699-3728
       EMail: mkalla@telcordia.com

       Selvam Rengasami
       Telcordia Technologies
       NVC-2Z439
       331 Newman Springs Road
       Red Bank, NJ 07701
       USA

       Phone: +1-732-758-5260
       EMail: srengasa@telcordia.com

       Greg Sidebottom
       Nortel Networks
       3685 Richmond Road
       Nepean, Ontario
       Canada  K2H5B7

       Phone: +1-613-763-7305
       EMail: gregside@nortelnetworks.com

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    10. Full Copyright Statement

       Copyright (C) The Internet Society (2001).  All Rights Reserved.

       This document and translations of it may be copied and furnished to
       others, and derivative works that comment on or otherwise explain it
       or assist in its implementation may be prepared, copied, published
       and distributed, in whole or in part, without restriction of any
       kind, provided that the above copyright notice and this paragraph are
       included on all such copies and derivative works.  However, this
       document itself may not be modified in any way, such as by removing
       the copyright notice or references to the Internet Society or other
       Internet organizations, except as needed for the purpose of
       developing Internet standards in which case the procedures for
       copyrights defined in the Internet Standards process must be
       followed, or as required to translate it into languages other than
       English.

       The limited permissions granted above are perpetual and will not be
       revoked by the Internet Society or its successors or assigns.

       This document and the information contained herein is provided on an
       "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
       TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
       BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
       HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
       MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

    Acknowledgement

       Funding for the RFC Editor function is currently provided by the
       Internet Society.

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