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<pre>Network Working Group                                       K. Morneault
Request for Comments: 4233                                 Cisco Systems
Obsoletes: <a href="./rfc3057">3057</a>                                             S. Rengasami
Category: Standards Track                                   Tridea Works
                                                                M. Kalla
                                                  Telcordia Technologies
                                                           G. Sidebottom
                                                   Signatus Technologies
                                                            January 2006


               <span class="h1">Integrated Services Digital Network (ISDN)</span>
                      <span class="h1">Q.921-User Adaptation Layer</span>

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 (2006).

Abstract

   This document defines a protocol for backhauling of Integrated
   Services Digital Network (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.

   This document obsoletes <a href="./rfc3057">RFC 3057</a>.















<span class="grey">Morneault, et al.           Standards Track                     [Page 1]</span></pre>
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<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


Table of Contents

   <a href="#section-1">1</a>. Introduction ....................................................<a href="#page-3">3</a>
      <a href="#section-1.1">1.1</a>. Scope ......................................................<a href="#page-3">3</a>
      <a href="#section-1.2">1.2</a>. Terminology ................................................<a href="#page-3">3</a>
      <a href="#section-1.3">1.3</a>. IUA Overview ...............................................<a href="#page-5">5</a>
      <a href="#section-1.4">1.4</a>. Services Provided by the IUA Layer .........................<a href="#page-7">7</a>
      <a href="#section-1.5">1.5</a>. Functions Implemented by the IUA Layer ....................<a href="#page-10">10</a>
      <a href="#section-1.6">1.6</a>. Definition of IUA Boundaries ..............................<a href="#page-12">12</a>
   <a href="#section-2">2</a>. Conventions ....................................................<a href="#page-15">15</a>
   <a href="#section-3">3</a>. Protocol Elements ..............................................<a href="#page-15">15</a>
      <a href="#section-3.1">3.1</a>. Common Message Header .....................................<a href="#page-15">15</a>
      <a href="#section-3.2">3.2</a>. IUA Message Header ........................................<a href="#page-19">19</a>
      <a href="#section-3.3">3.3</a>. IUA Messages ..............................................<a href="#page-21">21</a>
   <a href="#section-4">4</a>. Procedures .....................................................<a href="#page-46">46</a>
      <a href="#section-4.1">4.1</a>. Procedures to Support Service in <a href="#section-1.4.1">Section 1.4.1</a> ............<a href="#page-46">46</a>
      <a href="#section-4.2">4.2</a>. Procedures to Support Service in <a href="#section-1.4.2">Section 1.4.2</a> ............<a href="#page-46">46</a>
      <a href="#section-4.3">4.3</a>. Procedures to Support Service in <a href="#section-1.4.3">Section 1.4.3</a> ............<a href="#page-48">48</a>
   <a href="#section-5">5</a>. Examples .......................................................<a href="#page-58">58</a>
      5.1. Establishment of Association and Traffic between
           SGs and ASPs ..............................................<a href="#page-58">58</a>
      <a href="#section-5.2">5.2</a>. ASP Traffic Fail-over Examples ............................<a href="#page-62">62</a>
      <a href="#section-5.3">5.3</a>. Q.921/Q.931 Primitives Backhaul Examples ..................<a href="#page-63">63</a>
      <a href="#section-5.4">5.4</a>. Layer Management Communication Examples ...................<a href="#page-64">64</a>
   <a href="#section-6">6</a>. Security .......................................................<a href="#page-65">65</a>
   <a href="#section-7">7</a>. IANA Considerations ............................................<a href="#page-65">65</a>
      <a href="#section-7.1">7.1</a>. SCTP Payload Protocol Identifier ..........................<a href="#page-65">65</a>
      <a href="#section-7.2">7.2</a>. IUA Protocol Extensions ...................................<a href="#page-65">65</a>
   <a href="#section-8">8</a>. Timer Values ...................................................<a href="#page-67">67</a>
   <a href="#section-9">9</a>. Acknowledgements ...............................................<a href="#page-67">67</a>
   <a href="#section-10">10</a>. References ....................................................<a href="#page-67">67</a>
      <a href="#section-10.1">10.1</a>. Normative References .....................................<a href="#page-67">67</a>
      <a href="#section-10.2">10.2</a>. Informative References ...................................<a href="#page-67">67</a>
   <a href="#section-11">11</a>. Change Log ....................................................<a href="#page-68">68</a>
   <a href="#appendix-A">Appendix A</a> ........................................................<a href="#page-69">69</a>
      <a href="#appendix-A.1">A.1</a>. Signaling Network Architecture ............................<a href="#page-69">69</a>
      <a href="#appendix-A.2">A.2</a>. Application Server Process Redundancy .....................<a href="#page-70">70</a>














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<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   In this document, the term Q.921-User refers to an upper layer that
   uses the services of Q.921, not the user side of ISDN interface [<a href="#ref-1">1</a>].
   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.

<span class="h3"><a class="selflink" id="section-1.1" href="#section-1.1">1.1</a>.  Scope</span>

   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

   Signaling Transport [<a href="#ref-5" title="&quot;Framework Architecture for Signaling Transport&quot;">5</a>].  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 SCTP 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.

<span class="h3"><a class="selflink" id="section-1.2" href="#section-1.2">1.2</a>.  Terminology</span>

   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.





<span class="grey">Morneault, et al.           Standards Track                     [Page 3]</span></pre>
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<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


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

   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.

   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 backup MGC).  Fail-over
   also applies upon the return to service of a previously unavailable
   process.

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

   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.

   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
   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.

   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.

   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.

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







<span class="grey">Morneault, et al.           Standards Track                     [Page 4]</span></pre>
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<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


<span class="h3"><a class="selflink" id="section-1.3" href="#section-1.3">1.3</a>.  IUA Overview</span>

   The architecture that has been defined [<a href="#ref-5" title="&quot;Framework Architecture for Signaling Transport&quot;">5</a>] 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.

<span class="h4"><a class="selflink" id="section-1.3.1" href="#section-1.3.1">1.3.1</a>.  Example: SG to MGC</span>

   In a Signaling Gateway (SG), 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:

            ******   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 [<a href="#ref-4" title="&quot;Stream Control Transmission Protocol&quot;">4</a>,<a href="#ref-8" title="&quot;Stream Control Transmission Protocol (SCTP) Checksum Change&quot;">8</a>])
            IUA  - ISDN User Adaptation Layer Protocol

           Figure 1.  IUA in the SG to MGC Application

   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)





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<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


      -  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 be determined to not be required
   and TCP MAY be used as the underlying common transport protocol.

<span class="h4"><a class="selflink" id="section-1.3.2" href="#section-1.3.2">1.3.2</a>.  Support for the Management of SCTP Associations between the SG</span>
<span class="h4">        and ASPs</span>

   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 states of remote ASP(s) are maintained.  Active ASPs
   are those currently receiving traffic from the SG.

   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.

<span class="h4"><a class="selflink" id="section-1.3.3" href="#section-1.3.3">1.3.3</a>.  ASP Fail-over Model and Terminology</span>

   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).



<span class="grey">Morneault, et al.           Standards Track                     [Page 6]</span></pre>
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<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   The IUA layer supports an n+k redundancy model (active-standby, load
   sharing, broadcast) where n is 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.

<span class="h4"><a class="selflink" id="section-1.3.4" href="#section-1.3.4">1.3.4</a>.  Client/Server Model</span>

   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
   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 TCP Registered User Port Number Assignment for IUA is
   9900.

<span class="h3"><a class="selflink" id="section-1.4" href="#section-1.4">1.4</a>.  Services Provided by the IUA Layer</span>

<span class="h4"><a class="selflink" id="section-1.4.1" href="#section-1.4.1">1.4.1</a>.  Support for Transport of Q.921/Q.931 Boundary Primitives</span>

   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 [<a href="#ref-1">1</a>]:

   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.









<span class="grey">Morneault, et al.           Standards Track                     [Page 7]</span></pre>
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<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   DL-DATA

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

<span class="h4"><a class="selflink" id="section-1.4.2" href="#section-1.4.2">1.4.2</a>.  Support for Communication between Layer Management Modules on SG</span>
<span class="h4">        and MGC</span>

   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 shown below:

   M-TEI STATUS

   The M-TEI STATUS primitives are used to request, confirm, and
   indicate the status (assigned/unassigned) of an ISDN Terminal
   Endpoint Identifier (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).

<span class="h4"><a class="selflink" id="section-1.4.3" href="#section-1.4.3">1.4.3</a>.  Support for Management of Active Associations between SG and MGC</span>

   A set of primitives between the IUA layer and the Layer Management is
   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.






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<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   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).

   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.







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<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   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.

<span class="h3"><a class="selflink" id="section-1.5" href="#section-1.5">1.5</a>.  Functions Implemented by the IUA Layer</span>

<span class="h4"><a class="selflink" id="section-1.5.1" href="#section-1.5.1">1.5.1</a>.  Mapping</span>

   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 Time-
   Division Multiplexing (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
   ISDN Terminal Endpoint Identifiers (TEIs) and Service Access Point
   Identifiers (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
   fail-over 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--------&gt;|IID |-+          +--&gt;| ASP |---&gt;| Assoc |       v
         /      +----+ |  +----+  |   +-----+    +-------+ -+--+--+--+-
        /              +-&gt;| AS |--+                        Streams
       /        +----+ |  +----+   stb+-----+
D chan2--------&gt;|IID |-+              | ASP |
                +----+                +-----+




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<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   where IID = Interface Identifier

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

<span class="h4"><a class="selflink" id="section-1.5.2" href="#section-1.5.2">1.5.2</a>.  Status of ASPs</span>

   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 <a href="#section-3.3.2">Section 3.3.2</a>)
   or reception of indications from the local SCTP association.  ASP
   state transition procedures are described in <a href="#section-4.3.1">Section 4.3.1</a>.

   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 backup 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.

<span class="h4"><a class="selflink" id="section-1.5.3" href="#section-1.5.3">1.5.3</a>.  SCTP Stream Management</span>

   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.

<span class="h4"><a class="selflink" id="section-1.5.4" href="#section-1.5.4">1.5.4</a>.  Seamless Network Management Interworking</span>

   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.






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   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.

<span class="h4"><a class="selflink" id="section-1.5.5" href="#section-1.5.5">1.5.5</a>.  Congestion Management</span>

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

<span class="h3"><a class="selflink" id="section-1.6" href="#section-1.6">1.6</a>.  Definition of IUA Boundaries</span>

<span class="h4"><a class="selflink" id="section-1.6.1" href="#section-1.6.1">1.6.1</a>.  Definition of IUA/Q.921 Boundary</span>

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

<span class="h4"><a class="selflink" id="section-1.6.2" href="#section-1.6.2">1.6.2</a>.  Definition of IUA/Q.931 Boundary</span>

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

<span class="h4"><a class="selflink" id="section-1.6.3" href="#section-1.6.3">1.6.3</a>.  Definition of SCTP/IUA Boundary</span>

   An example of the upper layer primitives provided by SCTP are
   available in <a href="./rfc2960#section-10">Section&nbsp;10 of RFC 2960</a> [<a href="#ref-4" title="&quot;Stream Control Transmission Protocol&quot;">4</a>].

<span class="h4"><a class="selflink" id="section-1.6.4" href="#section-1.6.4">1.6.4</a>.  Definition of IUA/Layer-Management Boundary</span>

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

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

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





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

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

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

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

   M-SCTP STATUS indication
   Direction: IUA -&gt; LM
   Purpose: IUA reports status of SCTP association.

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

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

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

   M-AS-STATUS indication
   Direction: IUA -&gt; LM
   Purpose: SG reports status of AS.

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

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





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

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

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

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

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

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

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

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

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

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





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

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Conventions</span>

   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
   [<a href="#ref-6" title="&quot;Key words for use in RFCs to Indicate Requirement Levels&quot;">6</a>].

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Protocol Elements</span>

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

<span class="h3"><a class="selflink" id="section-3.1" href="#section-3.1">3.1</a>.  Common Message Header</span>

   The protocol messages for Q.921-User Adaptation require a message
   header that 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 2.  Common Header Format

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

<span class="h4"><a class="selflink" id="section-3.1.1" href="#section-3.1.1">3.1.1</a>.  Version</span>

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

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







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<span class="h4"><a class="selflink" id="section-3.1.2" href="#section-3.1.2">3.1.2</a>.  Message Classes and Types</span>

   The following list contains the valid Message Classes:

   Message Class: 8 bits (unsigned integer)

       0       Management (MGMT) Message
       1       Reserved for Other SIGTRAN Adaptation Layer
       2       Reserved for Other SIGTRAN Adaptation Layers
       3       ASP State Maintenance (ASPSM) Messages
       4       ASP Traffic Maintenance (ASPTM) Messages
       5       Q.921/Q.931 Boundary Primitives Transport (QPTM) Messages
       6       Reserved for Other SIGTRAN Adaptation Layer
       7       Reserved for Other SIGTRAN Adaptation Layer
       8       Reserved for Other SIGTRAN Adaptation Layer
     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



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    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

    Management (MGMT) Messages

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

<span class="h4"><a class="selflink" id="section-3.1.3" href="#section-3.1.3">3.1.3</a>.  Reserved</span>

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

<span class="h4"><a class="selflink" id="section-3.1.4" href="#section-3.1.4">3.1.4</a>.  Message Length</span>

   The Message Length defines the length of the message in octets,
   including the Common Header.  The Message Length MUST include
   parameter padding bytes, if any.

   Note: A receiver SHOULD accept the message whether or not the final
   parameter padding is included in the message length.

<span class="h4"><a class="selflink" id="section-3.1.5" href="#section-3.1.5">3.1.5</a>.  Variable-Length Parameter Format</span>

   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
   Type-Length-Value (TLV) format as shown below.










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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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          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.  Common parameters used by adaptation
   layers are in the range of 0x00 to 0x3f.  The parameter Tags defined
   are as follows:

   Common Parameters.  These TLV parameters are common across the
   different adaptation layers:

   Parameter Name                     Parameter ID
   ==============                     ============
   Reserved                              0x0000
   Interface Identifier (integer)        0x0001
   Not Used in IUA                       0x0002
   Interface Identifier (text)           0x0003
   INFO String                           0x0004
   DLCI                                  0x0005
   Not Used in IUA                       0x0006
   Diagnostic Information                0x0007
   Interface Identifier Range            0x0008
   Heartbeat Data                        0x0009
   Not Used in IUA                       0x000a
   Traffic Mode Type                     0x000b
   Error Code                            0x000c
   Status                                0x000d
   Protocol Data                         0x000e
   Release Reason                        0x000f
   TEI Status                            0x0010
   ASP Identifier                        0x0011
   Not Used in IUA                       0x0012 - 0x003f

   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.

<span class="h3"><a class="selflink" id="section-3.2" href="#section-3.2">3.2</a>.  IUA Message Header</span>

   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 3.  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 4.  IUA Message Header (Text-based Interface Identifier)

   The Tag value for the Text-based [<a href="#ref-2">2</a>] Interface Identifier is 0x3.
   The length is variable.

   The DLCI format is shown below in Figure 5.

        most                                     least
     significant                              significant
         bit                                      bit
      +-----+-----+-----+-----+-----+-----+-----+-----+
      |            SAPI                   | SPR |  0  |
      +-----------------------------------------------+
      |            TEI                          |  1  |
      +-----------------------------------------------+

                          Figure 5.  DLCI Format



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

   SAPI: Service Access Point Identifier (6 bits)

   TEI:  Terminal Endpoint Identifier (7 bits)

   As an example, SAPI = 0, TEI = 64, SPR = 0 would be encoded as
   follows:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x5)           |             Length=8          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      0x0      |      0x81     |               0x0             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

<span class="h3"><a class="selflink" id="section-3.3" href="#section-3.3">3.3</a>.  IUA Messages</span>

   The following section defines the messages and parameter contents.
   The IUA messages will use the common message header (Figure 2) and
   the IUA message header (Figure 3 and Figure 4).

<span class="h4"><a class="selflink" id="section-3.3.1" href="#section-3.3.1">3.3.1</a>.  Q.921/Q.931 Boundary Primitives Transport (QPTM) Messages</span>

<span class="h5"><a class="selflink" id="section-3.3.1.1" href="#section-3.3.1.1">3.3.1.1</a>.  Establish Messages (Request, Confirm, Indication)</span>

   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 resend 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 a 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 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



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   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.

<span class="h5"><a class="selflink" id="section-3.3.1.2" href="#section-3.3.1.2">3.3.1.2</a>.  Release Messages (Request, Indication, Confirmation)</span>

   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

   The format for Release 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 (0xf)           |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              Reason                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+












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   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.

<span class="h5"><a class="selflink" id="section-3.3.1.3" href="#section-3.3.1.3">3.3.1.3</a>.  Data Messages (Request, Indication)</span>

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

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

      Protocol Data

   The format for Data 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 (0xe)           |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                        Protocol Data                          /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

<span class="h5"><a class="selflink" id="section-3.3.1.4" href="#section-3.3.1.4">3.3.1.4</a>.  Unit Data Messages (Request, Indication)</span>

   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
   parameter:



<span class="grey">Morneault, et al.           Standards Track                    [Page 23]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-24" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


       Protocol Data

   The format for Unit Data 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 (0xe)           |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                        Protocol Data                          /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

<span class="h4"><a class="selflink" id="section-3.3.2" href="#section-3.3.2">3.3.2</a>.  Application Server Process Maintenance (ASPM) Messages</span>

   The ASPM messages will use only the common message header.

<span class="h5"><a class="selflink" id="section-3.3.2.1" href="#section-3.3.2.1">3.3.2.1</a>.  ASP Up (ASPUP)</span>

   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.

   The ASPUP message contains the following parameters:

     ASP Identifier           (Optional)
     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 = 0x0011          |           Length = 8          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         ASP Identifier                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag = 0x0004          |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                         INFO String                           /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   ASP Identifier: 32-bit unsigned integer






<span class="grey">Morneault, et al.           Standards Track                    [Page 24]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-25" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   The optional ASP Identifier parameter contains a unique value that is
   locally significant among the ASPs that support an AS.  The SG should
   save the ASP Identifier to be used, if necessary, with the Notify
   message (see <a href="#section-3.3.3.2">Section 3.3.3.2</a>).

   The optional INFO String parameter can carry any meaningful 8-bit
   ASCII [<a href="#ref-2">2</a>] 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.

<span class="h5"><a class="selflink" id="section-3.3.2.2" href="#section-3.3.2.2">3.3.2.2</a>.  ASP Up Ack</span>

   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 = 0x0004          |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                         INFO String                           /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The format and description of the optional INFO String parameter are
   the same as for the ASP Up message (see <a href="#section-3.3.2.1">Section 3.3.2.1</a>).

<span class="h5"><a class="selflink" id="section-3.3.2.3" href="#section-3.3.2.3">3.3.2.3</a>.  ASP Down (ASPDN)</span>

   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:

      INFO String (Optional)








<span class="grey">Morneault, et al.           Standards Track                    [Page 25]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-26" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   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 = 0x0004          |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                         INFO String                           /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The format and description of the optional INFO String parameter are
   the same as for the ASP Up message (see <a href="#section-3.3.2.1">Section 3.3.2.1</a>).

<span class="h5"><a class="selflink" id="section-3.3.2.4" href="#section-3.3.2.4">3.3.2.4</a>.  ASP Down Ack</span>

   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:

      INFO String (Optional)

   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 = 0x0004          |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                         INFO String                           /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The format and description of the optional INFO String parameter are
   the same as for the ASP Up message (see <a href="#section-3.3.2.1">Section 3.3.2.1</a>).

<span class="h5"><a class="selflink" id="section-3.3.2.5" href="#section-3.3.2.5">3.3.2.5</a>.  ASP Active (ASPAC)</span>

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








<span class="grey">Morneault, et al.           Standards Track                    [Page 26]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-27" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   The ASPAC 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)












































<span class="grey">Morneault, et al.           Standards Track                    [Page 27]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-28" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   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 = 0x000b          |           Length = 8          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     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 Identifier Parameters         /
   \                  of Tag Type 0x1 or 0x8                       \
   /                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                         INFO String                           /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






<span class="grey">Morneault, et al.           Standards Track                    [Page 28]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-29" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   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 = 0x000b          |           Length = 8          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Traffic Mode Type                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Tag (0x3=string)        |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                     Interface Identifiers                     /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /           Additional Interface Identifier Parameters          /
   \                      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 AS, 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/backup 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




<span class="grey">Morneault, et al.           Standards Track                    [Page 29]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-30" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   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 or ASes in which the
   ASP is provisioned.  If only a subset of Interface Identifiers is
   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, whereas the
   text-formatted Interface Identifier MAY be supported.

   The format and description of the optional INFO String parameter are
   the same as for the ASP Up message (see <a href="#section-3.3.2.1">Section 3.3.2.1</a>.).

   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).

<span class="h5"><a class="selflink" id="section-3.3.2.6" href="#section-3.3.2.6">3.3.2.6</a>.  ASP Active Ack</span>

   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)
















<span class="grey">Morneault, et al.           Standards Track                    [Page 30]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-31" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   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 = 0x000b          |           Length = 8          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     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 Identifier Parameters         /
   \                  of Tag Type 0x1 or 0x8                       \
   /                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                         INFO String                           /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






<span class="grey">Morneault, et al.           Standards Track                    [Page 31]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-32" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   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 = 0x000b          |           Length = 8          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Traffic Mode Type                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Tag (0x3=string)        |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                     Interface Identifiers                     /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /           Additional Interface Identifier Parameters          /
   \                      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 <a href="#section-3.3.2.5">Section</a>
   <a href="#section-3.3.2.5">3.3.2.5</a>).

   The format and description of the optional INFO String parameter are
   the same as for the ASP Up message (see <a href="#section-3.3.2.1">Section 3.3.2.1</a>).

<span class="h5"><a class="selflink" id="section-3.3.2.7" href="#section-3.3.2.7">3.3.2.7</a>.  ASP Inactive (ASPIA)</span>

   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.










<span class="grey">Morneault, et al.           Standards Track                    [Page 32]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-33" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   The ASPIA message contains the following parameters:

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

      INFO String (Optional)

   The format for the ASP Inactive message parameters using integer-
   formatted Interface Identifiers is as follows:









































<span class="grey">Morneault, et al.           Standards Track                    [Page 33]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-34" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


    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=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 Identifier Parameters         /
   \                  of Tag Type 0x1 or 0x8                       \
   /                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                         INFO String                           /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+













<span class="grey">Morneault, et al.           Standards Track                    [Page 34]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-35" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   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 (0x3=string)        |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                     Interface Identifiers                     /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /           Additional Interface Identifier Parameters          /
   \                      of Tag Type 0x3                          \
   /                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                         INFO String                           /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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.

   The format and description of the optional Interface Identifiers and
   INFO String parameters are the same as for the ASP Active message
   (see <a href="#section-3.3.2.5">Section 3.3.2.5</a>).

<span class="h5"><a class="selflink" id="section-3.3.2.8" href="#section-3.3.2.8">3.3.2.8</a>.  ASP Inactive Ack</span>

   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:

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






<span class="grey">Morneault, et al.           Standards Track                    [Page 35]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-36" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   The format for the ASP Inactive Ack 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 (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 Identifier Parameters         /
   \                  of Tag Type 0x1 or 0x8                       \
   /                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                         INFO String                           /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+










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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-37" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   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 (0x3=string)        |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                     Interface Identifiers                     /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /           Additional Interface Identifier Parameters          /
   \                      of Tag Type 0x3                          \
   /                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                         INFO String                           /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The format and description of the optional Interface Identifiers and
   INFO String parameters are the same as for the ASP Active message
   (see <a href="#section-3.3.2.5">Section 3.3.2.5</a>).

<span class="h5"><a class="selflink" id="section-3.3.2.9" href="#section-3.3.2.9">3.3.2.9</a>.  Heartbeat (BEAT)</span>

   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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<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   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 = 0x0009          |            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.

<span class="h5"><a class="selflink" id="section-3.3.2.10" href="#section-3.3.2.10">3.3.2.10</a>.  Heartbeat Ack (BEAT-Ack)</span>

   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.

<span class="h4"><a class="selflink" id="section-3.3.3" href="#section-3.3.3">3.3.3</a>.  Layer Management (MGMT) Messages</span>

<span class="h5"><a class="selflink" id="section-3.3.3.1" href="#section-3.3.3.1">3.3.3.1</a>.  Error (ERR)</span>

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

      Error Code
      Diagnostic Information (Optional)













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<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Tag = 0x000c         |          Length = 8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Error Code                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Tag = 0x0007         |            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
      Refused - Management Blocking                 0x0d
      ASP Identifier Required                       0x0e
      Invalid ASP Identifier                        0x0f

   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 an SG if an
   ASP sends a message with an invalid (unconfigured) Interface
   Identifier value.  For this error, the Diagnostic Information MUST
   contain enough of the offending message to identify the invalid
   Interface Identifier.  For example, in the case of QPTM and TEI
   Status management messages, the Common and IUA message headers of the
   offending message would be placed in the Diagnostic Information at a
   minimum.




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<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   The "Unsupported Traffic Handling Mode" error would be sent by an 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
   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 (e.g., a MGMT message was
   received on a stream other than "0").

   The "Unsupported Interface Identifier Type" error would be sent by an
   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 that has not been assigned or recognized
   for use on the indicated ISDN D-channel.

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

   The "Refused - Management Blocking" error is sent when an ASP Up or
   ASP Active message is received and the request is refused for
   management reasons (e.g., management lockout).





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<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   The "ASP Identifier Required" is sent by an SG in response to an ASP
   Up message that does not contain an ASP Identifier parameter when the
   SG requires one.  The ASP SHOULD resend the ASP Up message with an
   ASP Identifier.

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

   Diagnostic Information: variable length

      When included, the optional Diagnostic information can be any
      information germane to the error condition, to assist in
      identification of the error condition.  The Diagnostic information
      SHOULD contain the offending message.

   Error messages MUST NOT be generated in response to other Error
   messages.

<span class="h5"><a class="selflink" id="section-3.3.3.2" href="#section-3.3.3.2">3.3.3.2</a>.  Notify (NTFY)</span>

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

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

      Status                     (Mandatory)
      ASP Identifier             (Optional)
      Interface Identifiers      (Optional)
      INFO String                (Optional)

   The format for the Notify message with integer-formatted Interface
   Identifiers is as follows:


















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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Tag = 0x000d           |           Length = 8          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Status Type            |    Status Identification      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Tag = 0x0011           |           Length = 8          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        ASP Identifier                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     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 Identifier Parameters         /
   \                  of Tag Type 0x1 or 0x8                       \
   /                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag = 0x0004          |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                         INFO String                           /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-43" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   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 = 0x000d           |           Length = 8          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Status Type            |    Status Identification      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Tag = 0x0011           |           Length = 8          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        ASP Identifier                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Tag (0x3=string)        |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                     Interface Identifiers                     /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /           Additional Interface Identifier Parameters          /
   \                      of Tag Type 0x3                          \
   /                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag = 0x0004          |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                         INFO String                           /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Status Type: 16 bits (unsigned integer)

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

         1     Application Server State Change (AS-State_Change)
         2     Other

   Status Information: 16 bits (unsigned integer)

      The Status Information 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
      Information values are used:





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<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


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

   These notifications are sent from an 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
        3    ASP Failure

   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 ASP-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 ASP Identifier (if available) of the Alternate ASP MUST be placed
   in the message.  For the ASP Failure case, the SG is indicating to
   ASP(s) in the AS that one of the ASPs has transitioned to ASP-DOWN.
   The ASP Identifier (if available) of the failed ASP MUST be placed in
   the message.

   The format and description of the optional ASP Identifier are the
   same as for the ASP Up message (see <a href="#section-3.3.2.1">Section 3.3.2.1</a>).  The format and
   description of the optional Interface Identifiers and INFO String
   parameters are the same as for the ASP Active message (see <a href="#section-3.3.2.5">Section</a>
   <a href="#section-3.3.2.5">3.3.2.5</a>).

<span class="h5"><a class="selflink" id="section-3.3.3.3" href="#section-3.3.3.3">3.3.3.3</a>.  TEI Status Messages (Request, Confirm, and Indication)</span>

   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



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<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   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.

   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 = 0x0010         |          Length = 8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                              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

<span class="h5"><a class="selflink" id="section-3.3.3.4" href="#section-3.3.3.4">3.3.3.4</a>.  TEI Query Message (Request)</span>

   The TEI Query message is sent by the ASP to query the TEI(s).  This
   message consists of the common header and IUA header.  The DLCI in
   the IUA header MUST be ignored by the SG.  The SG will respond to
   this message with TEI Status Indication(s).












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<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Procedures</span>

   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.

<span class="h3"><a class="selflink" id="section-4.1" href="#section-4.1">4.1</a>.  Procedures to Support Service in <a href="#section-1.4.1">Section 1.4.1</a></span>

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

<span class="h4"><a class="selflink" id="section-4.1.1" href="#section-4.1.1">4.1.1</a>.  Q.921 or Q.931 Primitives Procedures</span>

   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).

<span class="h4"><a class="selflink" id="section-4.1.2" href="#section-4.1.2">4.1.2</a>.  QPTM Message Procedures</span>

   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.

<span class="h3"><a class="selflink" id="section-4.2" href="#section-4.2">4.2</a>.  Procedures to Support Service in <a href="#section-1.4.2">Section 1.4.2</a></span>

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

<span class="h4"><a class="selflink" id="section-4.2.1" href="#section-4.2.1">4.2.1</a>.  Layer Management Primitives Procedures</span>

   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 setup
   is complete.  An M-SCTP ESTABLISH indication is sent to Layer
   Management upon successful completion of an incoming SCTP association
   setup from a peer IUA node.






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   An M-SCTP RELEASE request from Layer Management will initiate the
   teardown 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 teardown 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.

   Upon receipt of an 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.

<span class="h4"><a class="selflink" id="section-4.2.2" href="#section-4.2.2">4.2.2</a>.  Receipt of IUA Peer Management Messages</span>

   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.





<span class="grey">Morneault, et al.           Standards Track                    [Page 47]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-48" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


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

<span class="h3"><a class="selflink" id="section-4.3" href="#section-4.3">4.3</a>.  Procedures to Support Service in <a href="#section-1.4.3">Section 1.4.3</a></span>

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

<span class="h4"><a class="selflink" id="section-4.3.1" href="#section-4.3.1">4.3.1</a>.  AS and ASP State Maintenance</span>

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

<span class="h5"><a class="selflink" id="section-4.3.1.1" href="#section-4.3.1.1">4.3.1.1</a>.  ASP States</span>

   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
      *  Local Management intervention

   The ASP state transition diagram is shown in Figure 6.  The possible
   states of an ASP are the following:

   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.











<span class="grey">Morneault, et al.           Standards Track                    [Page 48]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-49" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


                                    +--------------+
             +----------------------|              |
             |   Alternate  +-------|  ASP-ACTIVE  |
             |       ASP    |       +--------------+
             |    Takeover  |           ^     |
             |              |    ASP    |     | ASP Inactive /
             |              |    Active |     | ASP Up
             |              |           |     v
             |              |       +--------------+
             |              |       |              |
             |              +------&gt;| ASP-INACTIVE |
             |                      +--------------+
             |                          ^    |
   ASP Down/ |                     ASP  |    | ASP Down /
   SCTP CDI/ |                     Up   |    | SCTP CDI /
   SCTP RI   |                          |    v SCTP RI
             |                      +--------------+
             +---------------------&gt;|              |
                                    |   ASP-DOWN   |
                                    +--------------+

                  Figure 6.  ASP State Transition Diagram

   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.

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

<span class="h5"><a class="selflink" id="section-4.3.1.2" href="#section-4.3.1.2">4.3.1.2</a>.  AS States</span>

   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:

      *  ASP state transitions
      *  Recovery timer triggers








<span class="grey">Morneault, et al.           Standards Track                    [Page 49]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-50" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   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.

























<span class="grey">Morneault, et al.           Standards Track                    [Page 50]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-51" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


      +----------+ one ASP trans to ASP-ACTIVE +-------------+
      |    AS-   |----------------------------&gt;|     AS-     |
      | INACTIVE |                             |   ACTIVE    |
      |          |&lt;---                         |             |
      +----------+    \                        +-------------+
         ^   |         \ Tr Expiry,                ^    |
         |   |          \ at least one             |    |
         |   |           \ ASP in ASP-INACTIVE     |    |
         |   |            \                        |    |
         |   |             \                       |    |
         |   |              \                      |    |
 one ASP |   | all ASP       \            one ASP  |    | Last ACTIVE
 trans   |   | trans to       \           trans to |    | ASP trans to
 to      |   | ASP-DOWN        -------\   ASP-     |    | ASP-INACTIVE
 ASP-    |   |                         \  ACTIVE   |    | or ASP-DOWN
 INACTIVE|   |                          \          |    |  (start Tr)
         |   |                           \         |    |
         |   |                            \        |    |
         |   v                             \       |    v
      +----------+                          \  +-------------+
      |          |                           --|             |
      | AS-DOWN  |                             | AS-PENDING  |
      |          |                             |  (queueing) |
      |          |&lt;----------------------------|             |
      +----------+    Tr Expiry and no ASP     +-------------+
                     in ASP-INACTIVE state

     Tr = Recovery Timer

                 Figure 7: AS State Transition Diagram

<span class="h4"><a class="selflink" id="section-4.3.2" href="#section-4.3.2">4.3.2</a>.  ASPM Procedures for Primitives</span>

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




<span class="grey">Morneault, et al.           Standards Track                    [Page 51]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-52" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   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 <a href="#section-4.3.3">Section 4.3.3</a>).

   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, it 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.

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

<span class="h4"><a class="selflink" id="section-4.3.3" href="#section-4.3.3">4.3.3</a>.  ASPM Procedures for Peer-to-Peer Messages</span>

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

<span class="h5"><a class="selflink" id="section-4.3.3.1" href="#section-4.3.3.1">4.3.3.1</a>.  ASP Up Procedures</span>

   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 message.  This action MAY be initiated at the ASP by an
   M-ASP_UP request primitive from Layer Management or MAY be initiated
   automatically by an IUA management function.






<span class="grey">Morneault, et al.           Standards Track                    [Page 52]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-53" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


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

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

   If for any local reason (e.g., management lockout) the SG cannot
   respond with an ASP Up Ack message, the SG responds to an ASP Up
   message with an Error message with reason "Refused - Management
   Blocking".

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

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

   The ASP must wait for the ASP Up Ack message before sending any other
   IUA messages (e.g., ASP Active).  If the SG receives any other IUA
   messages before an ASP Up message is received (other than ASP Down;
   see <a href="#section-4.3.3.2">Section 4.3.3.2</a>), the SG MAY discard them.

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

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





<span class="grey">Morneault, et al.           Standards Track                    [Page 53]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-54" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


<span class="h5"><a class="selflink" id="section-4.3.3.2" href="#section-4.3.3.2">4.3.3.2</a>.  ASP Down Procedures</span>

   The ASP will send an ASP Down message to an SG when the ASP wishes to
   be removed from the list of ASPs in all Application Servers that it
   is a member and no longer receive any IUA QPTM or ASPTM messages.
   This action MAY be initiated at the ASP by an M-ASP_DOWN request
   primitive from Layer Management or MAY be initiated automatically by
   an IUA management function.

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

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

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

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

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

<span class="h5"><a class="selflink" id="section-4.3.3.3" href="#section-4.3.3.3">4.3.3.3</a>.  IUA Version Control</span>

   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.

   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.



<span class="grey">Morneault, et al.           Standards Track                    [Page 54]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-55" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


<span class="h5"><a class="selflink" id="section-4.3.3.4" href="#section-4.3.3.4">4.3.3.4</a>.  ASP Active Procedures</span>

   Any time after the ASP has received an ASP Up Ack from the SG, the
   ASP sends an ASP Active message to the SG indicating that the ASP is
   ready to start processing traffic.  This action MAY be initiated at
   the ASP by an M-ASP_ACTIVE request primitive from Layer Management or
   MAY be initiated automatically by an IUA management function.  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.

   If the Application Server can be successfully activated, the SG
   responds to the ASP with an ASPAC Ack message acknowledging that the
   ASPAC message was received and starts sending traffic for the
   Application Server to that ASP.

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

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

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

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

   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.




<span class="grey">Morneault, et al.           Standards Track                    [Page 55]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-56" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   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
   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 an ASP Active Ack message to the ASP.

<span class="h5"><a class="selflink" id="section-4.3.3.5" href="#section-4.3.3.5">4.3.3.5</a>.  ASP Inactive Procedures</span>

   When an ASP wishes to withdraw from receiving traffic within an AS,
   the ASP sends an ASP Inactive message to the SG.  This action MAY be
   initiated at the ASP by an M-ASP_INACTIVE request primitive from
   Layer Management or MAY be initiated automatically by an IUA
   management function.  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
   ASP Inactive message 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.
   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
   message, the ASP that sends the ASPIA message is already considered
   by the SG to be ASP-INACTIVE.  An ASPIA Ack message is sent to the
   ASP, after ensuring that all traffic is stopped to the ASP.




<span class="grey">Morneault, et al.           Standards Track                    [Page 56]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-57" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   In the case of a Load-share mode AS, the SG moves the ASP to the
   ASP-INACTIVE state and the AS traffic is re-allocated across the
   remaining ASP-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 Notify (Insufficient ASPs)
   message MAY be sent to all inactive ASPs, if required.

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

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

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

<span class="h5"><a class="selflink" id="section-4.3.3.6" href="#section-4.3.3.6">4.3.3.6</a>.  Notify Procedures</span>

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





<span class="grey">Morneault, et al.           Standards Track                    [Page 57]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-58" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   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 compel the ASP(s) receiving the message to become
   active.  The ASPs remain in control of what (and when) action is
   taken.

<span class="h5"><a class="selflink" id="section-4.3.3.7" href="#section-4.3.3.7">4.3.3.7</a>.  Heartbeat</span>

   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).

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

   If no Heartbeat Ack message (or any other IUA message) is received
   from the IUA peer within 2*T(beat), the remote IUA peer is considered
   unavailable.  Transmission of Heartbeat messages is stopped and the
   signaling process SHOULD attempt to re-establish communication if it
   is configured as the client for the disconnected 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 6, "ASP State
   Transition Diagram".

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  Examples</span>

<span class="h3"><a class="selflink" id="section-5.1" href="#section-5.1">5.1</a>.  Establishment of Association and Traffic between SGs and ASPs</span>

<span class="h4"><a class="selflink" id="section-5.1.1" href="#section-5.1.1">5.1.1</a>.  Single ASP in an Application Server (1+0 sparing)</span>

   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 setup.




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                SG                       ASP1
                 |
                 |&lt;---------ASP Up----------|
                 |--------ASP Up Ack-------&gt;|
                 |                          |
                 |-----NTFY(AS-INACTIVE)---&gt;|
                 |                          |
                 |&lt;-------ASP Active--------|
                 |------ASP Active Ack-----&gt;|
                 |                          |
                 |------NTFY(AS-ACTIVE)----&gt;|
                 |                          |

<span class="h4"><a class="selflink" id="section-5.1.2" href="#section-5.1.2">5.1.2</a>.  Two ASPs in Application Server (1+1 sparing)</span>

   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
           |                         |                          |
           |&lt;--------ASP Up----------|                          |
           |-------ASP Up Ack-------&gt;|                          |
           |                         |                          |
           |----NTFY(AS-INACTIVE)---&gt;|                          |
           |                         |                          |
           |&lt;-----------------------------ASP Up----------------|
           |----------------------------ASP Up Ack-------------&gt;|
           |                         |                          |
           |                         |                          |
           |&lt;-------ASP Active-------|                          |
           |-----ASP Active Ack-----&gt;|                          |
           |                         |                          |
           |-----NTFY(AS-ACTIVE)----&gt;|                          |
           |----------------------NTFY(AS-ACTIVE)--------------&gt;|









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<span class="h4"><a class="selflink" id="section-5.1.3" href="#section-5.1.3">5.1.3</a>.  Two ASPs in an Application Server (1+1 sparing, load-sharing</span>
<span class="h4">        case)</span>

   This scenario shows a similar case to <a href="#section-5.1.2">Section 5.1.2</a> 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
           |                         |                          |
           |&lt;---------ASP Up---------|                          |
           |--------ASP Up Ack------&gt;|                          |
           |                         |                          |
           |----NTFY(AS-INACTIVE)---&gt;|                          |
           |                         |                          |
           |&lt;------------------------------ASP Up---------------|
           |-----------------------------ASP Up Ack------------&gt;|
           |                         |                          |
           |                         |                          |
           |&lt;--ASP Active (Ldshr)----|                          |
           |----ASP Active Ack------&gt;|                          |
           |                         |                          |
           |-----NTFY(AS-ACTIVE)----&gt;|                          |
           |----------------------NTFY(AS-ACTIVE)--------------&gt;|
           |                         |                          |
           |&lt;----------------------------ASP Active (Ldshr)-----|
           |-----------------------------ASP Active Ack--------&gt;|
           |                         |                          |

<span class="h4"><a class="selflink" id="section-5.1.4" href="#section-5.1.4">5.1.4</a>.  Three ASPs in an Application Server (n+k sparing, load-sharing</span>
<span class="h4">        case)</span>

   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).















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      SG                  ASP1                ASP2                ASP3
       |                    |                   |                   |
       |&lt;------ASP Up-------|                   |                   |
       |-----ASP Up Ack----&gt;|                   |                   |
       |                    |                   |                   |
       |-NTFY(AS-INACTIVE)-&gt;|                   |                   |
       |                    |                   |                   |
       |&lt;--------------------------ASP Up-------|                   |
       |-----------------------ASP Up Ack------&gt;|                   |
       |                    |                   |                   |
       |&lt;---------------------------------------------ASP Up--------|
       |--------------------------------------------ASP Up Ack-----&gt;|
       |                    |                   |                   |
       |                    |                   |                   |
       |&lt;-ASP Act (Ldshr)---|                   |                   |
       |----ASP Act Ack----&gt;|                   |                   |
       |                    |                   |                   |
       |&lt;---------------------ASP Act (Ldshr)---|                   |
       |----------------------ASP Act Ack------&gt;|                   |
       |                    |                   |                   |
       |--NTFY(AS-ACTIVE)--&gt;|                   |                   |
       |---------------NTFY(AS-ACTIVE)---------&gt;|                   |
       |------------------------NTFY(AS-ACTIVE)--------------------&gt;|

<span class="h4"><a class="selflink" id="section-5.1.5" href="#section-5.1.5">5.1.5</a>.  Interface Identifier Configuration Mismatch Example</span>

   This scenario shows the example IUA message flows for the
   establishment of traffic between an SG and an ASP in which some of
   the Interface Identifiers have been misconfigured on the ASP side.
   The SG in this case has Interface Identifiers 1-5 configured for
   ASP1.

                SG                               ASP1
                 |                                |
                 |                                |
                 |&lt;----ASP Active (IIDs 1-10)-----|
                 |---ASP Active Ack (IIDs 1-5)---&gt;|
                 |-------Error (IIDs 6)----------&gt;|
                 |-------Error (IIDs 7)----------&gt;|
                 |-------Error (IIDs 8)----------&gt;|
                 |-------Error (IIDs 9)----------&gt;|
                 |-------Error (IIDs 10)---------&gt;|
                 |                                |








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<span class="h3"><a class="selflink" id="section-5.2" href="#section-5.2">5.2</a>.  ASP Traffic Fail-over Examples</span>

<span class="h4"><a class="selflink" id="section-5.2.1" href="#section-5.2.1">5.2.1</a>.  (1+1 Sparing, withdrawal of ASP, Backup Over-ride)</span>

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

          SG                       ASP1                       ASP2
           |                         |                          |
           |&lt;-----ASP Inactive-------|                          |
           |----ASP Inactive Ack----&gt;|                          |
           |                         |                          |
           |----NTFY(AS-Pending)----&gt;|                          |
           |-------------------NTFY(AS-Pending)----------------&gt;|
           |                         |                          |
           |&lt;------------------------------ ASP Active----------|
           |-----------------------------ASP Active Ack)-------&gt;|
           |                         |                          |
           |----NTFY(AS-ACTIVE)-----&gt;|                          |
           |-------------------NTFY(AS-ACTIVE)-----------------&gt;|

   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.

<span class="h4"><a class="selflink" id="section-5.2.2" href="#section-5.2.2">5.2.2</a>.  (1+1 Sparing, Backup Over-ride)</span>

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

          SG                       ASP1                       ASP2
           |                         |                          |
           |&lt;-------------------------------ASP Active----------|
           |-----------------------------ASP Active Ack--------&gt;|
           |----NTFY( Alt ASP-Act)--&gt;|
           |                         |                          |

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








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<span class="h4"><a class="selflink" id="section-5.2.3" href="#section-5.2.3">5.2.3</a>.  (n+k Sparing, Load-sharing case, withdrawal of ASP)</span>

   Following on from the example in <a href="#section-5.1.4">Section 5.1.4</a>, and ASP1 withdraws
   from service:

     SG                  ASP1                 ASP2                 ASP3
      |                    |                   |                   |
      |&lt;----ASP Inact------|                   |                   |
      |---ASP Inact Ack---&gt;|                   |                   |
      |                    |                   |                   |
      |---------------------------------NTFY(Ins. ASPs)-----------&gt;|
      |                    |                   |                   |
      |&lt;-----------------------------------------ASP Act (Ldshr)---|
      |-------------------------------------------ASP Act (Ack)---&gt;|
      |                    |                   |                   |

   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.

<span class="h3"><a class="selflink" id="section-5.3" href="#section-5.3">5.3</a>.  Q.921/Q.931 Primitives Backhaul Examples</span>

   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

      -  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



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      -  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 (<a href="#section-5.1">Section 5.1</a>) prior to the following message
   flows.

            SG                             ASP

                        &lt;----------- Establish Request
      Establish Confirm  ----------&gt;

                        &lt;----------- Data Request
         Data Indication -----------&gt;
                        &lt;----------- Data Request
         Data Indication -----------&gt;
                        &lt;----------- Data Request
                        &lt;----------- Data Request
         Data Indication -----------&gt;

                        &lt;----------- Release Request (RELEASE_MGMT)
        Release Confirm  ----------&gt;

   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.

            SG                             ASP

                        &lt;----------- Establish Request (ESTABLISH_START)
      Release Indication ----------&gt;
      (RELEASE_PHYS)

<span class="h3"><a class="selflink" id="section-5.4" href="#section-5.4">5.4</a>.  Layer Management Communication Examples</span>

   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 (<a href="#section-5.1">Section 5.1</a>) prior to
   the following message flows.



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                  SG                       ASP

                        &lt;----------- Data Request
        Error Indication ----------&gt;
         (INVALID_TEI)

                        &lt;----------- TEI Status Request
      TEI Status Confirm ----------&gt;
           (Unassigned)

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Security</span>

   The security considerations discussed in "Security Considerations for
   SIGTRAN Protocols", <a href="./rfc3788">RFC 3788</a> [<a href="#ref-3" title="&quot;Security Considerations for Signaling Transport (SIGTRAN) Protocols&quot;">3</a>], apply to this document.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  IANA Considerations</span>

<span class="h3"><a class="selflink" id="section-7.1" href="#section-7.1">7.1</a>.  SCTP Payload Protocol Identifier</span>

   The IANA has assigned an IUA value for the Payload Protocol
   Identifier in SCTP Payload Data chunk.  The following SCTP Payload
   Protocol Identifier has been 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.

<span class="h3"><a class="selflink" id="section-7.2" href="#section-7.2">7.2</a>.  IUA Protocol Extensions</span>

   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.

   The definition and use of new message classes, types, and parameters
   are an integral part of SIGTRAN adaptation layers.  Thus, these
   extensions are assigned by IANA through an IETF Consensus action as
   defined in [<a href="#ref-7" title="">7</a>].




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

<span class="h4"><a class="selflink" id="section-7.2.1" href="#section-7.2.1">7.2.1</a>.  IETF-Defined Message Classes</span>

   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.

<span class="h4"><a class="selflink" id="section-7.2.2" href="#section-7.2.2">7.2.2</a>.  IETF-Defined Message Types</span>

   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.
   (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 that it does not
   support, it MUST respond with an Error (ERR) message with an Error
   Code of Unsupported Message Type.

<span class="h4"><a class="selflink" id="section-7.2.3" href="#section-7.2.3">7.2.3</a>.  IETF-Defined TLV Parameter Extension</span>

   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 <a href="#section-3.1.5">Section 3.1.5</a>.
   (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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<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Timer Values</span>

   The following are suggestions for default timer values.

   T(r)                                    3-5 seconds
   T(ack)                                  2-5 seconds
   T(beat)   Heartbeat Timer               30 seconds

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  Acknowledgements</span>

   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.

<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.   References</span>

<span class="h3"><a class="selflink" id="section-10.1" href="#section-10.1">10.1</a>.  Normative References</span>

   [<a id="ref-1">1</a>]  ITU-T Recommendation Q.920, 'Digital Subscriber signaling System
        No. 1 (DSS1) - ISDN User-Network Interface Data Link Layer -
        General Aspects'

   [<a id="ref-2">2</a>]  Coded Character Set--7-Bit American Standard Code for
        Information Interchange, ANSI X3.4-1986.

   [<a id="ref-3">3</a>]  Loughney, J., Tuexen, M., and J. Pastor-Balbas, "Security
        Considerations for Signaling Transport (SIGTRAN) Protocols", <a href="./rfc3788">RFC</a>
        <a href="./rfc3788">3788</a>, June 2004.

<span class="h3"><a class="selflink" id="section-10.2" href="#section-10.2">10.2</a>.  Informative References</span>

   [<a id="ref-4">4</a>]  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", <a href="./rfc2960">RFC 2960</a>, October 2000.

   [<a id="ref-5">5</a>]  Ong, L., Rytina, I., Garcia, M., Schwarzbauer, H., Coene, L.,
        Lin, H., Juhasz, I., Holdrege, M., and C. Sharp, "Framework
        Architecture for Signaling Transport", <a href="./rfc2719">RFC 2719</a>, October 1999.

   [<a id="ref-6">6</a>]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", <a href="https://www.rfc-editor.org/bcp/bcp14">BCP 14</a>, <a href="./rfc2119">RFC 2119</a>, March 1997.

   [<a id="ref-7">7</a>]  Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
        Considerations Section in RFCs", <a href="https://www.rfc-editor.org/bcp/bcp26">BCP 26</a>, <a href="./rfc2434">RFC 2434</a>, October 1998.




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   [<a id="ref-8">8</a>]  Stone, J., Stewart, R., and D. Otis, "Stream Control
        Transmission Protocol (SCTP) Checksum Change", <a href="./rfc3309">RFC 3309</a>,
        September 2002.

<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>.  Change Log</span>

   Below is a list of the major changes between this document and <a href="./rfc3057">RFC</a>
   <a href="./rfc3057">3057</a>.

   1.  The TEI Query message was added.

   2.  An explanation of the DLCI format (shown in Figure 6) is
       provided.

   3.  Aligned the ASP and AS procedures in <a href="#section-4">Section 4</a> with <a href="./rfc3331">RFC3331</a> and
       <a href="./rfc3332">RFC3332</a>.

   4.  Alinged the format of the ASPSM and ASPTM messages with <a href="./rfc3331">RFC3331</a>
       and <a href="./rfc3332">RFC3332</a>.  These changes include removing the Reason field
       from the ASP Down and ASP Down Ack messages and the Traffic Mode
       Type field from the ASP Inactive and ASP Inactive Ack messages.

   5.  Sections <a href="#section-1.3.3">1.3.3</a> and <a href="#section-1.3.4">1.3.4</a> were moved to <a href="#appendix-A">Appendix A</a>.  A new section
       was added in place of <a href="#section-1.3.3">Section 1.3.3</a>.

   6.  The references have been split between Normative and Informative.

   7.  The new Sigtran security document is referenced and <a href="#section-6">Section 6</a> has
       been updated appropriately.






















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Appendix A

<span class="h3"><a class="selflink" id="appendix-A.1" href="#appendix-A.1">A.1</a>.  Signaling Network Architecture</span>

   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
   Quality of Service (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 8 below:






















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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-70" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


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

                      Figure 8.  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.

<span class="h3"><a class="selflink" id="appendix-A.2" href="#appendix-A.2">A.2</a>.  Application Server Process Redundancy</span>

   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 8, 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






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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-71" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


   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
   are 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.



























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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-72" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


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
   Tridea Works

   Phone: +1-732-512-0969
   EMail: selvam@trideaworks.com


   Greg Sidebottom
   Signatus Technologies
   Kanata, Ontario, Canada

   EMail: greg@signatustechnologies.com
















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<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-73" ></span>
<span class="grey"><a href="./rfc4233">RFC 4233</a>            ISDN Q.921-User Adaptation Layer        January 2006</span>


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</pre>