





Network Working Group                                       K. Morneault
Request for Comments: 3331                                 Cisco Systems
Category: Standards Track                                       R. Dantu
                                                                 NetRake
                                                           G. Sidebottom
                                                   Signatus Technologies
                                                             B. Bidulock
                                                                 OpenSS7
                                                                J. Heitz
                                                                  Lucent
                                                          September 2002


       Signaling System 7 (SS7) Message Transfer Part 2 (MTP2) -
                         User Adaptation Layer

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   This document defines a protocol for the backhauling of Signaling
   System 7 Message Transfer Part 2 (SS7 MTP2) User signalling messages
   over IP using the Stream Control Transmission Protocol (SCTP).  This
   protocol would be used between a Signalling Gateway (SG) and Media
   Gateway Controller (MGC).  It is assumed that the SG receives SS7
   signalling over a standard SS7 interface using the SS7 Message
   Transfer Part (MTP) to provide transport.  The Signalling Gateway
   would act as a Signalling Link Terminal.













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Table of Contents

   1.  Introduction.............................................. 2
     1.1  Scope.................................................. 3
     1.2  Terminology............................................ 3
     1.3  M2UA Overview.......................................... 5
     1.4  Services Provided by the M2UA Adaptation Layer......... 7
     1.5  Functions Provided by the M2UA Layer................... 9
     1.6  Definition of the M2UA Boundaries..................... 12
   2.  Conventions.............................................. 16
   3.  Protocol Elements........................................ 16
     3.1  Common Message Header................................. 16
     3.2  M2UA Message Header................................... 22
     3.3  M2UA Messages......................................... 23
   4.  Procedures............................................... 58
     4.1  Procedures to Support the M2UA-User Layer............. 58
     4.2  Receipt of Primitives from the Layer Management....... 59
     4.3  AS and ASP State Maintenance.......................... 61
     4.4  Link Key Management Procedures........................ 73
   5.  Examples of MTP2 User Adaptation (M2UA) Procedures....... 75
     5.1  Establishment of associations between SGP and MGC..... 75
          examples
     5.2  ASP Traffic Fail-over Examples........................ 77
     5.3  SGP to MGC, MTP Level 2 to MTP Level 3 Boundary
          Procedures............................................ 78
   6.  Timer Values............................................. 85
   7.  Security Considerations.................................. 85
     7.1 Threats................................................ 85
     7.2 Protecting Confidentiality............................. 86
   8.  IANA Considerations...................................... 86
     8.1 SCTP Payload Protocol Identifier....................... 86
     8.2 M2UA Protocol Extensions............................... 86
   9.  Acknowledgements......................................... 87
   10. References............................................... 88
   Appendix A: Signalling Network Architecture.................. 90
   Authors' Addresses........................................... 92
   Full Copyright Statement..................................... 94

1.  Introduction

   This document defines a protocol for the backhauling of SS7 [1] MTP2
   User [2] [3] [4] (i.e. MTP3) signalling messages over IP using the
   Stream Control Transmission Protocol (SCTP) [8].  This protocol would
   be used between a Signalling Gateway (SG) and Media Gateway
   Controller (MGC).






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1.1 Scope

   There is a need for Switched Circuit Network (SCN) signalling
   protocol delivery from a Signalling Gateway (SG) to a Media Gateway
   Controller (MGC) [9].  The delivery mechanism addresses the following
   objectives:

   *  Support for MTP Level 2 / MTP Level 3 interface boundary
   *  Support for communication between Layer Management modules on SG
      and MGC
   *  Support for management of SCTP active associations between the SG
      and MGC

   The SG will terminate up to MTP Level 2 and the MGC will terminate
   MTP Level 3 and above.  In other words, the SG will transport MTP
   Level 3 messages over an IP network to a MGC.

1.2 Terminology

   Application Server (AS) - A logical entity serving a specific
   application instance.  An example of an Application Server is a MGC
   handling the MTP Level 3 and call processing for SS7 links terminated
   by the Signalling 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
   active or standby MGC instances.

   Association - An association refers to a SCTP association.  The
   association will provide the transport for the delivery of protocol
   data units for one or more interfaces.

   Backhaul - Refers to the transport of signalling from the point of
   interface for the associated data stream (i.e., SG function in the
   MGU) back to the point of call processing (i.e., the MGCU), if this
   is not local [9].

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

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





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   Interface - For the purposes of this document, an interface is a SS7
   signalling link.

   Interface Identifier - The Interface Identifier identifies the
   physical interface at the SG for which the signalling 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.

   Layer Management - Layer Management is a nodal function in an SG or
   ASP that handles the inputs and outputs between the M2UA layer and a
   local management entity.

   Link Key - The link key is a locally unique (between ASP and SG)
   value that identifies a registration request for a particular
   Signalling Data Link and Signalling Terminal pair.

   MTP - The Message Transfer Part of the SS7 protocol

   MTP2 - MTP Level 2, the signalling data link layer of SS7

   MTP3 - MTP Level 3, the signalling network layer of SS7

   MTP2-User - A protocol that uses the services of MTP Level 2 (i.e.
   MTP3).

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

   Signalling Data Link - An SDL refers to a specific communications
   facility that connects two Signalling Link Terminals.

   Signalling Gateway (SG) - An SG is a signalling agent at the edge of
   the IP network.  An SG appears to the SS7 as one or more Signalling
   Link Terminals that are connected to one or more Signalling Data
   Links in the SS7 network.  An SG contains a set of one or more unique
   Signalling Gateway Processes, on which one or more is normally
   actively processing traffic.  Where an SG contains more than one SGP,
   the SG is a logical entity.

   Signalling Gateway Process (SGP) - A process instance that uses M2UA
   to communicate to and from a Signalling Link Terminal.  It serves as
   an active, backup or load-sharing process of a Signalling Gateway.

   Signalling Link Terminal (SLT) - Refers to the means of performing
   all of the functions defined at MTP level 2 regardless of their
   implementation [2,3].




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

1.3  M2UA Overview

   The framework architecture that has been defined for SCN signalling
   transport over IP [9] uses two components: a signalling common
   transport protocol and an adaptation module to support the services
   expected by a particular SCN signalling protocol from its underlying
   protocol layer.

   Within this framework architecture, this document defines a SCN
   adaptation module that is suitable for the transport of SS7 MTP2 User
   messages.  The only SS7 MTP2 User is MTP3.  The M2UA uses the
   services of the Stream Control Transmission Protocol [8] as the
   underlying reliable signalling common transport protocol.

   In a Signalling Gateway, it is expected that the SS7 MTP2-User
   signalling is transmitted and received from the PSTN over a standard
   SS7 network interface, using the SS7 Message Transfer Part Level 1
   and Level 2 [2,3,4] to provide reliable transport of the MTP3-User
   signalling messages to and from an SS7 Signalling End Point (SEP) or
   Signalling Transfer Point (STP).  The SG then provides an
   interworking of transport functions with the IP transport, in order
   to transfer the MTP2-User signalling messages to and from an
   Application Server Process where the peer MTP2-User protocol layer
   exists.






















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1.3.1  Example - SG to MGC

   In a Signalling Gateway, it is expected that the SS7 signalling is
   received over a standard SS7 network termination, using the SS7
   Message Transfer Part (MTP) to provide transport of SS7 signalling
   messages to and from an SS7 Signalling End Point (SEP) or SS7
   Signalling Transfer Point (STP).  In other words, the SG acts as a
   Signalling Link Terminal (SLT) [2,3].  The SG then provides an
   interworking of transport functions with IP Signalling Transport, in
   order to transport the MTP3 signalling messages to the MGC where the
   peer MTP3 protocol layer exists, as shown below:

       ******    SS7    ******      IP     *******
       *SEP *-----------* SG *-------------* MGC *
       ******           ******             *******

       +----+                              +----+
       |S7UP|                              |S7UP|
       +----+                              +----+
       |MTP +                              |MTP |
       | L3 |            (NIF)             |L3  |
       +----+         +----+----+          +----+
       |MTP |         |MTP |M2UA|          |M2UA|
       |    |         |    +----+          +----+
       |L2  |         |L2  |SCTP|          |SCTP|
       |L1  |         |L1  +----+          +----+
       |    |         |    |IP  |          |IP  |
       +----+         +---------+          +----+

       NIF  - Nodal Interworking Function
       SEP  - SS7 Signalling Endpoint
       IP   - Internet Protocol
       SCTP - Stream Control Transmission Protocol (Reference [8])

           Figure 1  M2UA in the SG to MGC Application

   Note: STPs MAY be present in the SS7 path between the SEP and the SG.

   It is recommended that the M2UA use the services of the Stream
   Control Transmission Protocol (SCTP) [8] as the underlying reliable
   common signalling transport protocol.  The use of SCTP provides the
   following features:

   -  explicit packet-oriented delivery (not stream-oriented)
   -  sequenced delivery of user messages within multiple streams, with
      an option for order-of-arrival delivery of individual user
      messages,
   -  optional multiplexing of user messages into SCTP datagrams,



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   -  network-level fault tolerance through the support of multi-homing
      at either or both ends of an association,
   -  resistance to flooding and masquerade attacks, and
   -  data segmentation to conform to discovered path MTU size

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

1.3.2  ASP Fail-over Model and Terminology

   The M2UA layer supports ASP fail-over functions in order to support a
   high availability of call and transaction processing capability.  All
   MTP2-User messages incoming to a SGP from the SS7 network are
   assigned to the unique Application Server, based on the Interface
   Identifier of the message.

   The M2UA layer supports a 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.

1.3.3  Client/Server Model

   It is recommended that the SGP and ASP be able to support both client
   and server operation.  The peer endpoints using M2UA 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 SGP to take on the role of
   server while the ASP is the client.  In this case, ASPs SHOULD
   initiate the SCTP association to the SGP.

   The SCTP and TCP Registered User Port Number Assignment for M2UA is
   2904.

1.4  Services Provided by the M2UA Adaptation Layer

   The SS7 MTP3/MTP2(MTP2-User) interface is retained at the termination
   point in the IP network, so that the M2UA protocol layer is required
   to provide the equivalent set of services to its users as provided by
   the MTP Level 2 to MTP Level 3.







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1.4.1  Support for MTP Level 2 / MTP Level 3 interface boundary

   M2UA supports a MTP Level 2 / MTP Level 3 interface boundary that
   enables a seamless, or as seamless as possible, operation of the
   MTP2-User peers in the SS7 and IP domains.  An example of the
   primitives that need to be supported can be found in [10].

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

   The M2UA layer needs to provide some messages that will facilitate
   communication between Layer Management modules on the SG and MGC.  To
   facilitate reporting of errors that arise because of the backhauling
   MTP Level 3 scenario, the following primitive is defined:

   M-ERROR

   The M-ERROR message is used to indicate an error with a received M2UA
   message (e.g., an interface identifier value is not known to the SG).

1.4.3  Support for management of active associations between SG and MGC

   The M2UA layer on the SG keeps the state of the configured ASPs.  A
   set of primitives between M2UA layer and the Layer Management are
   defined below to help the Layer Management manage the association(s)
   between the SG and the MGC.  The M2UA layer can be instructed by the
   Layer Management to establish a SCTP association to a peer M2UA node.
   This procedure can be achieved using the M-SCTP ESTABLISH primitive.

   M-SCTP_ESTABLISH

   The M-SCTP_ESTABLISH primitive is used to request, indicate and
   confirm the establishment of a SCTP association to a peer M2UA node.

   M-SCTP_RELEASE

   The M-SCTP_RELEASE primitives are used to request, indicate, and
   confirm the release of a SCTP association to a peer M2UA node.

   The M2UA layer MAY also need to inform the status of the SCTP
   association(s) to the Layer Management.  This can be achieved using
   the following primitive.

   M-SCTP_STATUS

   The M-SCTP_STATUS primitive is used to request and indicate the
   status of underlying SCTP association(s).




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   The Layer Management MAY need to inform the M2UA layer of an AS/ASP
   status (i.e., failure, active, etc.), so that messages can be
   exchanged between M2UA layer peers to stop traffic to the local M2UA
   user.  This can be achieved using the following primitive.

   M-ASP_STATUS

   The ASP status is stored inside the M2UA 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 M2UA
   layer.  This primitive can also be used to indicate the status of the
   Application Server Process.

   M-ASP_MODIFY

   The M-ASP_MODIFY primitive can be used by Layer Management to modify
   the status of the Application Server Process.  In other words, the
   Layer Management on the ASP side uses this primitive to initiate the
   ASPM procedures.

   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.

1.5  Functions Provided by the M2UA Layer

1.5.1  Mapping

   The M2UA layer MUST maintain a map of an Interface ID to a physical
   interface on the Signalling Gateway.  A physical interface would be a
   V.35 line, T1 line/time slot, E1 line/time slot, etc.  The M2UA layer
   MUST also maintain a map of the Interface Identifier to SCTP
   association and to the related stream within the association.

   The SGP 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 SGP MUST maintain
   the states of AS/ASP and reference them during the routing of any
   messages to an AS/ASP.

   Note that only one SGP SHOULD provide Signalling Link Terminal
   services to an SS7 link.  Therefore, within an SG, an Application
   Server SHOULD be active for only one SGP at any given point in time.



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   An example of the logical view of the relationship between an SS7
   link, Interface Identifier, AS and ASP in an SGP is shown below:

               /-------------------------------------------------+
              /   /----------------------------------------------|--+
             /   /                                               v  |
            /   /    +----+             act+-----+   +-------+ -+--+|-+-
   SS7 link1-------->|IID |-+          +-->| ASP |-->| Assoc |      v
              /      +----+ |  +----+  |   +-----+   +-------+ -+--+--+-
             /              +->| AS |--+                        Streams
            /        +----+ |  +----+   stb+-----+
   SS7 link2-------->|IID |-+              | ASP |
                     +----+                +-----+

   where IID = Interface Identifier

   A SGP MAY support more than one AS.  An AS MAY support more than one
   Interface Identifier.

1.5.2  Support for the management of SCTP associations between the SGPs
       and ASPs

   The M2UA layer at the SG maintains the availability state of all
   configured ASPs, in order to manage the SCTP associations and the
   traffic between the SG and ASPs.  As well, the active/inactive state
   of remote ASP(s) are also maintained.  The Active ASP(s) are the
   one(s) currently receiving traffic from the SG.

   The M2UA layer MAY be instructed by local management to establish an
   SCTP association to a peer M2UA 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 M2UA node.

   The M2UA layer MAY also need to inform local management of the status
   of the underlying SCTP associations using the M-SCTP_STATUS request
   and the indication primitive.  For example, the M2UA MAY inform local
   management of the reason for the release of an SCTP association,
   determined either locally within the M2UA layer or by a primitive
   from the SCTP.

   Also the M2UA layer may need to inform the local management of the
   change in status of an ASP or AS.  This may be achieved using the M-
   ASP STATUS request or M-AS_STATUS request primitives.








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1.5.3 Status of ASPs

   The M2UA layer on the SG MUST maintain the state of the ASPs it is
   supporting.  The state of an ASP changes because of the reception of
   peer-to-peer messages (ASPM messages as described in Section 3.3.2)
   or the reception of indications from the local SCTP association.  The
   ASP state transition procedures are described in Section 4.3.1.

   At a SGP, an Application Server list MAY contain active and inactive
   ASPs to support ASP fail-over procedures.  When, for example, both a
   primary and a backup ASP are available, the M2UA 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
   SGP to reflect the active Application Server Process.

   Also the M2UA layer MAY need to inform the local management of the
   change in status of an ASP or AS.  This can be achieved using the M-
   ASP_STATUS or M-AS_STATUS primitives.

1.5.4  SCTP Specifics

1.5.4.1  SCTP Stream Management

   SCTP allows a user specified number of streams to be opened during
   initialization of the association.  It is the responsibility of the
   M2UA layer to ensure proper management of these streams.  Because of
   the unidirectional nature of streams, a M2UA layer is not aware of
   the stream information from its peer M2UA layer.  For this reason,
   the Interface Identifier is in the M2UA message header.

   The use of SCTP streams within M2UA is recommended in order to
   minimize transmission and buffering delay, thereby, improving the
   overall performance and reliability of the signalling elements.  A
   separate SCTP stream can be used for each SS7 link.  Or, an
   implementation may choose to split the SS7 link across several
   streams based on SLS.  This method may be of particular interest for
   high speed SS7 links (MTP3b) since high speed links have a 24-bit
   sequence number and the stream sequence number is 16-bits.

   SCTP Stream '0' SHOULD NOT be used for MTP2 User Adaptation (MAUP)
   messages (see Section 3) since stream '0' SHOULD only be used for ASP
   Management (ASPM) messages (see Section 4.3.3).









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1.5.5  Seamless SS7 Network Management Interworking

   The M2UA layer on the SGP SHOULD pass an indication of unavailability
   of the M2UA-User (MTP3) to the local Layer Management, if the
   currently active ASP moves from the ACTIVE state.  The actions taken
   by M2UA on the SGP with regards to MTP Level 2 should be in
   accordance with the appropriate MTP specifications.

1.5.6  Flow Control / Congestion

   It is possible for the M2UA layer to be informed of the IP network
   congestion onset and abatement by means of an implementation
   dependent function (i.e. an indication from the SCTP).  The handling
   of this congestion indication by M2UA is implementation dependent.
   However, the actions taken by the SG should be in accordance with the
   appropriate MTP specification and should enable SS7 functionality
   (e.g. flow control) to be correctly maintained.

1.5.7  Audit of SS7 Link State

   After a fail-over of one ASP to another ASP, it may be necessary for
   the M2UA on the ASP to audit the current SS7 link state to ensure
   consistency.  The M2UA on the SGP would respond to the audit request
   with information regarding the current state of the SS7 link (i.e.
   in-service, out-of-service, congestion state, LPO/RPO state).

1.6  Definition of the M2UA Boundaries

1.6.1  Definition of the M2UA / MTP Level 3 boundary

   DATA
   ESTABLISH
   RELEASE
   STATE
   DATA RETRIEVAL
   DATA RETRIEVAL COMPLETE

1.6.2  Definition of the M2UA / MTP Level 2 boundary

   DATA
   ESTABLISH
   RELEASE
   STATE
   DATA RETRIEVAL
   DATA RETRIEVAL COMPLETE






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1.6.3  Definition of the Lower Layer Boundary between M2UA and SCTP

   The upper layer and layer management primitives provided by SCTP are
   provided in Reference [8] Section 10.

1.6.4  Definition of Layer Management / M2UA Boundary

   M-SCTP_ESTABLISH request
   Direction: LM -> M2UA
   Purpose: LM requests ASP to establish an SCTP association with an
            SGP.

   M-SCTP_ESTABLISH confirm
   Direction: M2UA -> LM
   Purpose: ASP confirms to LM that it has established an
            SCTP association with an SGP.

   M-SCTP_ESTABLISH indication
   Direction: M2UA -> LM
   Purpose: SGP informs LM that an ASP has established an SCTP
            association.

   M-SCTP_RELEASE request
   Direction: LM -> M2UA
   Purpose: LM requests ASP to release an SCTP association with SGP.

   M-SCTP_RELEASE confirm
   Direction: M2UA -> LM
   Purpose: ASP confirms to LM that it has released SCTP association
            with SGP.

   M-SCTP_RELEASE indication
   Direction: M2UA -> LM
   Purpose: SGP informs LM that ASP has released an SCTP association.

   M-SCTP_RESTART indication
   Direction: M2UA -> LM
   Purpose: M2UA informs LM that a SCTP Restart indication has
            been received.

   M-SCTP_STATUS request
   Direction: LM -> M2UA
   Purpose: LM requests M2UA to report status of SCTP association.

   M-SCTP_STATUS indication
   Direction: M2UA -> LM
   Purpose: M2UA reports status of SCTP association.




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   M-ASP_STATUS request
   Direction: LM -> M2UA
   Purpose: LM requests SGP to report status of remote ASP.

   M-ASP_STATUS indication
   Direction: M2UA -> LM
   Purpose: SGP reports status of remote ASP.

   M-AS_STATUS request
   Direction: LM -> M2UA
   Purpose: LM requests SG to report status of AS.

   M-AS_STATUS indication
   Direction: M2UA -> LM
   Purpose: SG reports status of AS.

   M-NOTIFY indication
   Direction: M2UA -> LM
   Purpose: ASP reports that it has received a NOTIFY message
            from its peer.

   M-ERROR indication
   Direction: M2UA -> LM
   Purpose: ASP or SGP reports that it has received an ERROR
            message from its peer.

   M-ASP_UP request
   Direction: LM -> M2UA
   Purpose: LM requests ASP to start its operation and send an ASP UP
            message to the SGP.

   M-ASP_UP confirm
   Direction: M2UA -> LM
   Purpose: ASP reports that it has received an ASP UP Acknowledgment
            message from the SGP.

   M-ASP_DOWN request
   Direction: LM -> M2UA
   Purpose: LM requests ASP to stop its operation and send an ASP DOWN
            message to the SGP.

   M-ASP_DOWN confirm
   Direction: M2UA -> LM
   Purpose: ASP reports that is has received an ASP DOWN Acknowledgment
            message from the SGP.






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   M-ASP_ACTIVE request
   Direction: LM -> M2UA
   Purpose: LM requests ASP to send an ASP ACTIVE message to the SGP.

   M-ASP_ACTIVE confirm
   Direction: M2UA -> LM
   Purpose: ASP reports that is has received an ASP ACTIVE
            Acknowledgment message from the SGP.

   M-ASP_INACTIVE request
   Direction: LM -> M2UA
   Purpose: LM requests ASP to send an ASP INACTIVE message to the SGP.

   M-ASP_INACTIVE confirm
   Direction: M2UA -> LM
   Purpose: ASP reports that is has received an ASP INACTIVE
            Acknowledgment message from the SGP.

   M-LINK_KEY_REG Request
   Direction:  LM -> M2UA
   Purpose: LM requests ASP to register Link Key with SG by sending REG
            REQ message.

   M-LINK_KEY_REG Confirm
   Direction:   M2UA -> LM
   Purpose: ASP reports to LM that it has successfully received a REG
            RSP message from SG.

   M-LINK_KEY_REG Indication
   Direction:  M2UA -> LM
   Purpose:  SG reports to LM that it has successfully processed an
             incoming REG REQ message from ASP.

   M-LINK_KEY_DEREG Request
   Direction:  LM -> M2UA
   Purpose:  LM requests ASP to de-register Link Key with SG by sending
             DEREG REQ message.

   M-LINK_KEY_DEREG Confirm
   Direction:  M2UA -> LM
   Purpose:  ASP reports to LM that it has successfully received a
             DEREG RSP message from SG.

   M-LINK_KEY_DEREG  Indication
   Direction:  M2UA -> LM
   Purpose:  SG reports to LM that it has successfully processed an
             incoming DEREG REQ message from ASP.




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2.0 Conventions

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when
   they appear in this document, are to be interpreted as described in
   [RFC2119].

3.0  Protocol Elements

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

3.1  Common Message Header

   The protocol messages for MTP2-User Adaptation require a message
   structure that contains a version, message class, message type,
   message length, and message contents.  This message header is common
   among all signalling protocol adaptation layers:

    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    |     Spare     | Message Class | Message Type  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Message Length                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 2  Common Message Header

   All fields in an M2UA message MUST be transmitted in the network byte
   order, unless otherwise stated.

3.1.1  Version

   The version field contains the version of the M2UA adaptation layer.
   The supported versions are:

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

3.1.2  Spare

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






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3.1.3  Message Class

   The following List contains the valid Message Classes:

   Message Class: 8 bits (unsigned integer)

     0      Management (MGMT) Message [IUA/M2UA/M3UA/SUA]
     1      Transfer Messages [M3UA]
     2      SS7 Signalling Network Management (SSNM) Messages [M3UA/SUA]
     3      ASP State Maintenance (ASPSM) Messages [IUA/M2UA/M3UA/SUA]
     4      ASP Traffic Maintenance (ASPTM) Messages [IUA/M2UA/M3UA/SUA]
     5      Q.921/Q.931 Boundary Primitives Transport (QPTM)
            Messages [IUA]
     6      MTP2 User Adaptation (MAUP) Messages [M2UA]
     7      Connectionless Messages [SUA]
     8      Connection-Oriented Messages [SUA]
     9      Routing Key Management (RKM) Messages (M3UA)
    10      Interface Identifier Management (IIM) Messages (M2UA)
 11 to 127  Reserved by the IETF
128 to 255  Reserved for IETF-Defined Message Class extensions

3.1.4  Message Type

   The following List contains the Message Types for the valid Message
   Classes:

   MTP2 User Adaptation (MAUP) Messages

        0      Reserved
        1      Data
        2      Establish Request
        3      Establish Confirm
        4      Release Request
        5      Release Confirm
        6      Release Indication
        7      State Request
        8      State Confirm
        9      State Indication
       10      Data Retrieval Request
       11      Data Retrieval Confirm
       12      Data Retrieval Indication
       13      Data Retrieval Complete Indication
       14      Congestion Indication
       15      Data Acknowledge
    16 to 127  Reserved by the IETF
   128 to 255  Reserved for IETF-Defined MAUP extensions





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   Application Server Process State Maintenance (ASPSM) messages

        0      Reserved
        1      ASP Up (UP)
        2      ASP Down (DOWN)
        3      Heartbeat (BEAT)
        4      ASP Up Ack (UP ACK)
        5      ASP Down Ack (DOWN ACK)
        6      Heartbeat Ack (BEAT ACK)
     7 to 127  Reserved by the IETF
   128 to 255  Reserved for IETF-Defined ASPSM extensions

   Application Server Process Traffic Maintenance (ASPTM) messages

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

   Management (MGMT) Messages

        0      Error (ERR)
        1      Notify (NTFY)
     2 to 127  Reserved by the IETF
   128 to 255  Reserved for IETF-Defined MGMT extensions

   Interface Identifier Management (IIM) Messages

        0        Reserved
        1        Registration Request (REG REQ)
        2        Registration Response (REG RSP)
        3        Deregistration Request (DEREG REQ)
        4        Deregistration Response (DEREG RSP)
     5 to 127    Reserved by the IETF
   128 to 255    Reserved for IETF-Defined IIM extensions

3.1.5  Message Length

   The Message Length defines the length of the message in octets,
   including the header.  The Message Length MUST include parameter
   padding bytes, if any.  The Message Length MUST NOT be longer than a
   MTP3 message [2,3,4,5] plus the length of the common and M2UA message
   headers.





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3.1.6  Variable-Length Parameter Format

   M2UA messages consist of a Common Header followed by zero or more
   variable-length parameters, as defined by the message type.  The
   variable-length parameters contained in a message are defined in a
   Tag-Length-Value format as shown below.

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

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

   Parameter Tag: 16 bits (unsigned integer)

   The Type field is a 16 bit identifier of the type of parameter.  It
   takes a value of 0 to 65534.  The common parameters used by the
   adaptation layers are in the range of 0x00 to 0xff.  The M2UA
   specific parameters have Tags in the range 0x300 to 0x3ff.

























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   The common parameter tags (used by all User Adaptation layers) that
   M2UA uses are defined below:

      Parameter Value     Parameter Name
      ---------------     --------------
            0 (0x00)       Reserved
            1 (0x01)       Interface Identifier (Integer)
            2 (0x02)       Unused
            3 (0x03)       Interface Identifier (Text)
            4 (0x04)       Info String
            5 (0x05)       Unused
            6 (0x06)       Unused
            7 (0x07)       Diagnostic Information
            8 (0x08)       Interface Identifier (Integer Range)
            9 (0x09)       Heartbeat Data
           10 (0x0a)       Unused
           11 (0x0b)       Traffic Mode Type
           12 (0x0c)       Error Code
           13 (0x0d)       Status Type/Information
           14 (0x0e)       Unused
           15 (0x0f)       Unused
           16 (0x10)       Unused
           17 (0x11)       ASP Identifier
           18 (0x12)       Unused
           19 (0x13)       Correlation Id
          18-255           Reserved

























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   The M2UA specific parameter Tags defined are as follows:

      Parameter Value     Parameter Name
      ---------------     --------------
        768 (0x0300)      Protocol Data 1
        769 (0x0301)      Protocol Data 2 (TTC)
        770 (0x0302)      State Request
        771 (0x0303)      State Event
        772 (0x0304)      Congestion Status
        773 (0x0305)      Discard Status
        774 (0x0306)      Action
        775 (0x0307)      Sequence Number
        776 (0x0308)      Retrieval Result
        777 (0x0309)      Link Key
        778 (0x030a)      Local-LK-Identifier
        779 (0x030b)      Signalling Data Terminal (SDT) Identifier
        780 (0x030c)      Signalling Data Link (SDL) Identifier
        781 (0x030d)      Registration Result
        782 (0x030e)      Registration Status
        783 (0x030f)      De-Registration Result
        784 (0x0310)      De-Registration Status

   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.  Thus, a parameter with a zero-length Parameter Value
   field would have a Length field of 4.  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 MUST NOT pad with more than 3 bytes.  The
   receiver MUST ignore the padding bytes.









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3.2  M2UA Message Header

   In addition to the common message header, there will be a M2UA
   specific message header.  The M2UA specific message header will
   immediately follow the common message header, but will only be used
   with MAUP messages.

   This message header will contain the Interface Identifier.  The
   Interface Identifier identifies the physical interface at the SG for
   which the signalling 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.

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

    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=8            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Interface Identifier (integer)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 3  M2UA 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)                 /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Figure 4  M2UA Message Header (Text-based Interface Identifier)

   The Tag value for the Text-based Interface Identifier is 0x3.  The
   encoding of the Identifier is ANSI X3.4-1986 [7].  The maximum string
   length of the text-based Interface Identifier is 255 octets.  The tag
   length is equal to the string length of the Interface Identifier name
   plus four bytes for the Tag and Length fields.



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3.3 M2UA Messages

   The following section defines the messages and parameter contents.
   The M2UA messages will use the common message header (Figure 2) and
   the M2UA message header (Figure 3 and Figure 4).

3.3.1 MTP2 User Adaptation Messages

3.3.1.1 Data

   The Data message contains an SS7 MTP2-User Protocol Data Unit (PDU).
   The Data message contains the following parameter:

      Protocol Data (mandatory)
      Correlation Id (optional)

   The format for the 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 (0x300)           |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                       Protocol Data                           /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x13)            |          Length = 8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Correlation Id                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Protocol Data field contains the MTP2-User application message in
   network byte order starting with the Signalling Information Octet
   (SIO).  The Correlation Id parameter uniquely identifies the MSU
   carried in the Protocol Data within an AS.  This Correlation Id
   parameter is assigned by the sending M2UA.  The purpose of the
   Correlation Id is to permit the newly active ASP to synchronize its
   processing of the traffic in each ordered stream with other ASPs in
   the broadcast group.











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   The format for a Data Message with TTC PDU 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 (0x301)           |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                    TTC Protocol Data                          /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Tag (0x13)           |          Length = 8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Correlation Id                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Protocol Data field contains the MTP2-User application message in
   network byte order starting with the Length Indicator (LI) octet.
   The Japanese TTC variant uses the spare bits of the LI octet for
   priority.

   The length of the Protocol Data and TTC Protocol Data MUST NOT exceed
   the length of a MTP2-User application message [2,3,5].

3.3.1.2  Data Acknowledge Message

   The Data Acknowledge message contains the Correlation Id of the Data
   message that the sending M2UA is acknowledging as successfully
   processed to the peer M2UA.

   The Data Acknowledge message contains the following parameter:

      Correlation Id       Mandatory

   The following format MUST be used for the Data Ack Message:

    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 (0x13)           |          Length = 8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Correlation Id                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Correlation Id parameter of the Data message and the Data Ack
   message provide a mechanism, for those SG implementations capable of
   taking advantage of them, to obtain an acknowledgment that the MSU
   has been transferred to the M2UA peer before acknowledging the MSU to



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   the SS7 peer, removing the risk of losing messages due to association
   failure or SCTP congestion.

   The Data Ack message MUST be sent if a Correlation Id parameter is
   received from the peer.  Otherwise, the Data Ack message MUST NOT be
   sent.

   If the Data Acknowledge is not sent for Correlation Id(s) or is sent
   with Invalid Correlation Id(s), the SS7 link will eventually fail due
   to lack of MTP Level 2 acknowledgments of the SS7 peer's MSUs.

3.3.1.3  Establish (Request, Confirmation)

   The Establish Request message is used to establish the SS7 link or to
   indicate that the channel has been established.  The MGC controls the
   state of the SS7 link.  When the MGC desires the SS7 link to be in-
   service, it will send the Establish Request message.  Note that the
   SGP MAY already have the SS7 link established at its layer.  If so,
   upon receipt of an Establish Request, the SGP takes no action except
   to send an Establish Confirm.

   When the MGC sends an M2UA Establish Request message, the MGC MAY
   start a timer.  This timer would be stopped upon receipt of an M2UA
   Establish Confirm.  If the timer expires, the MGC would resend the
   M2UA 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 some
   other action is taken (notify the Management Layer).

   The mode (Normal or Emergency) for bringing the SS7 link in service
   is defaulted to Normal.  The State Request (described in Section
   3.3.1.5 below) can be used to change the mode to Emergency.

3.3.1.4  Release (Request, Indication, Confirmation)

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

3.3.1.5  State Request

   The State Request message can be sent from a MGC to cause an action
   on a particular SS7 link supported by the Signalling Gateway Process.
   The SGP sends a State Confirm to the MGC if the action has been
   successfully completed.  The State Confirm reflects that state value
   received in the State Request message.





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   The State Request message contains the following parameter:

    State (mandatory)

    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 (0x302)           |          Length = 8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             State                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

            Define           Value        Description
      STATUS_LPO_SET          0x0      Request local processor outage
      STATUS_LPO_CLEAR        0x1      Request local processor outage
                                       recovered
      STATUS_EMER_SET         0x2      Request emergency alignment
      STATUS_EMER_CLEAR       0x3      Request normal alignment (cancel
                                       emergency)
      STATUS_FLUSH_BUFFERS    0x4      Flush or clear receive, transmit
                                       and retransmit queues
      STATUS_CONTINUE         0x5      Continue or Resume
      STATUS_CLEAR_RTB        0x6      Clear the retransmit queue
      STATUS_AUDIT            0x7      Audit state of link
      STATUS_CONG_CLEAR       0x8      Congestion cleared
      STATUS_CONG_ACCEPT      0x9      Congestion accept
      STATUS_CONG_DISCARD     0xa      Congestion discard

3.3.1.6  State Confirm

   The State Confirm message will be sent by the SGP in response to a
   State Request from the MGC.  The State Confirm reflects that state
   value received in the State Request message.

   The State Confirm message contains the following parameter:

    State (mandatory)

    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 (0x302)           |           Length = 8          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             State                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




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   The valid values for State are shown in the following table.  The
   value of the State field SHOULD reflect the value received in the
   State Request message.

            Define           Value        Description
      STATUS_LPO_SET          0x0      Request local processor outage
      STATUS_LPO_CLEAR        0x1      Request local processor outage
                                       recovered
      STATUS_EMER_SET         0x2      Request emergency alignment
      STATUS_EMER_CLEAR       0x3      Request normal alignment (cancel
                                       emergency)
      STATUS_FLUSH_BUFFERS    0x4      Flush or clear receive, transmit
                                       and retransmit queues
      STATUS_CONTINUE         0x5      Continue or Resume
      STATUS_CLEAR_RTB        0x6      Clear the retransmit queue
      STATUS_AUDIT            0x7      Audit state of link
      STATUS_CONG_CLEAR       0x8      Congestion cleared
      STATUS_CONG_ACCEPT      0x9      Congestion accept
      STATUS_CONG_DISCARD     0xa      Congestion discard

3.3.1.7  State Indication

   The MTP2 State Indication message can be sent from a SGP to an ASP to
   indicate a condition on a SS7 link.

   The State Indication message contains the following parameter:

    Event (mandatory)

    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 (0x303)           |          Length = 8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Event                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

          Define            Value          Description
      EVENT_RPO_ENTER        0x1      Remote entered processor outage
      EVENT_RPO_EXIT         0x2      Remote exited processor outage
      EVENT_LPO_ENTER        0x3      Link entered processor outage
      EVENT_LPO_EXIT         0x4      Link exited processor outage







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3.3.1.8  Congestion Indication

   The Congestion Indication message can be sent from a Signalling
   Gateway Process to an ASP to indicate the congestion status and
   discard status of a SS7 link.  When the MSU buffer fill increases
   above an Onset threshold or decreases below an Abatement threshold or
   crosses a Discard threshold in either direction, the SGP SHALL send a
   congestion indication message when it supports SS7 MTP2 variants that
   support multiple congestion levels.

   The SGP SHALL send the message only when there is actually a change
   in either the discard level or the congestion level to report,
   meaning it is different from the previously sent message.  In
   addition, the SGP SHALL use an implementation dependent algorithm to
   limit the frequency of congestion indication messages.

   An implementation may optionally send Congestion Indication messages
   on a "high priority" stream in order to potentially reduce delay.

   The Congestion Indication message contains the following parameters:

    Congestion Status (mandatory)
    Discard Status (optional)

    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 (0x304)           |          Length = 8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Congestion Status                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x305)           |          Length = 8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Discard Status                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

            Define        Value        Description
          LEVEL_NONE       0x0       No congestion
          LEVEL_1          0x1       Congestion Level 1
          LEVEL_2          0x2       Congestion Level 2
          LEVEL_3          0x3       Congestion Level 3







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   For SS7 networks that do not support multiple levels of congestion,
   only the LEVEL_NONE and LEVEL_3 values will be used.  For SS7
   networks that support multiple levels of congestion, it is possible
   for all values to be used.  Refer to [2], [3] and [12] for more
   details on the Congestion and Discard Status of SS7 signalling links.

3.3.1.9  Retrieval Request

   The MTP2 Retrieval Request message is used during the MTP Level 3
   changeover procedure to request the BSN, to retrieve PDUs from the
   transmit and retransmit queues or to flush PDUs from the retransmit
   queue.  Examples of the use of Retrieval Request for SS7 Link
   Changeover are provided in Section 5.3.6.

   The Retrieval Request message contains the following parameters:

    Action (mandatory)
    Sequence Number (optional)

    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 (0x306)           |          Length = 8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Action                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x307)           |          Length = 8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Sequence Number                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

           Define         Value       Description
      ACTION_RTRV_BSN      0x1     Retrieve the backward sequence number
      ACTION_RTRV_MSGS     0x2     Retrieve the PDUs from the transmit
                                   and retransmit queues

   In the Retrieval Request message, the Sequence Number field SHOULD
   NOT be present if the Action field is ACTION_RTRV_BSN.  The Sequence
   Number field contains the Forward Sequence Number (FSN) of the far
   end if the Action is ACTION_RTRV_MSGS.









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3.3.1.10  Retrieval Confirm

   The MTP2 Retrieval Confirm message is sent by the Signalling Gateway
   in response to a Retrieval Request message.  Examples of the use of
   the Retrieval Confirm for SS7 Link Changeover are provided in Section
   5.3.6.

   The Retrieval Confirm message contains the following parameters:

    Action (mandatory)
    Result (mandatory)
    Sequence Number (optional)

    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 (0x306)           |          Length = 8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Action                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x308)           |          Length = 8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Result                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x307)           |          Length = 8           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Sequence Number                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The valid values for Action are the same as in Retrieval Request.

   The values for Result are shown below:

           Define         Value       Description
      RESULT_SUCCESS       0x0     Action successful
      RESULT_FAILURE       0x1     Action failed

   When the Signalling Gateway Process sends a Retrieval Confirm to a
   Retrieval Request, it echos the Action field.  If the Action was
   ACTION_RTRV_BSN and the SGP successfully retrieved the BSN, the SGP
   will put the Backward Sequence Number (BSN) in the Sequence Number
   field and will indicate a success in the Result field.  If the BSN
   could not be retrieved, the Sequence Number field will not be
   included and the Result field will indicate failure.







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   For a Retrieval Confirm with Action of ACTION_RTRV_MSGS, the value of
   the Result field will indicate success or failure.  A failure means
   that the buffers could not be retrieved.  The Sequence Number field
   is not used with ACTION_RTRV_MSGS.

3.3.1.11  Retrieval Indication

   The Retrieval Indication message is sent by the Signalling Gateway
   with a PDU from the transmit or retransmit queue.  The Retrieval
   Indication message does not contain the Action or Sequence Number
   fields, just a MTP3 Protocol Data Unit (PDU) from the transmit or
   retransmit queue.  Examples of the use of the Retrieval Indication
   for SS7 Link Changeover are provided in Section 5.3.6.

    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 (0x300)          |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                       Protocol Data                           /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   For TTC Data messages, the following parameter will be used to
   indicate a TTC PDU which starts at LI.

    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 (0x301)          |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                     TTC Protocol Data                         /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The M2UA implementation MAY consider the use of the bundling feature
   of SCTP for Retrieval Indication messages.

3.3.1.12  Retrieval Complete Indication

   The MTP2 Retrieval Complete Indication message is exactly the same as
   the MTP2 Retrieval Indication message except that it also indicates
   that retrieval is complete.  In addition, it MAY contain a PDU (which
   MUST be the last PDU) from the transmit or retransmit queue.





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3.3.2  Application Server Process Maintenance (ASPM) Messages

   The ASPM messages will only use the common message header.

3.3.2.1  ASP Up (ASPUP)

   The ASP Up (ASPUP) message is used to indicate to a remote M2UA peer
   that the Adaptation layer is ready to receive traffic or maintenance
   messages.

   The ASPUP message contains the following parameters

      ASP Identifier (optional)
      Info String (optional)

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

   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 (0x11)          |             Length = 8        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        ASP Identifier*                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x4)           |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                          INFO String*                         /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The optional ASP Identifier parameter would contain a unique value
   that is locally significant among the ASPs that support an AS.  The
   SGP should save the ASP Identifier to be used, if necessary, with the
   Notify message (see Section 3.3.3.2).

   The optional INFO String parameter can carry any meaningful UTF-8 [6]
   character string along with the message.  Length of the INFO String
   parameter is from 0 to 255 octets.  No procedures are presently
   identified for its use but the INFO String MAY be used for debugging
   purposes.





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3.3.2.2 ASP Up Ack

   The ASP Up Ack message is used to acknowledge an ASP Up message
   received from a remote M2UA peer.

   The ASPUP Ack message contains the following parameters:

      INFO String (optional)

   The format for ASPUP Ack Message parameters is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x4)           |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                          INFO String*                         /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

3.3.2.3  ASP Down (ASPDN)

   The ASP Down (ASPDN) message is used to indicate to a remote M2UA
   peer that the adaptation layer is not ready to receive traffic or
   maintenance messages.

   The ASPDN message contains the following parameters

       INFO String (optional)

   The format for the ASPDN message parameters is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x4)           |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                          INFO String*                         /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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



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3.3.2.4 ASP Down Ack

   The ASP Down Ack message is used to acknowledge an ASP Down message
   received from a remote M2UA peer.

   The ASP Down Ack message contains the following parameters:

       INFO String (optional)

   The format for the ASPDN Ack message parameters is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x4)           |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                          INFO String*                         /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

3.3.2.5  Heartbeat (BEAT)

   The Heartbeat message is optionally used to ensure that the M2UA
   peers are still available to each other.

   The BEAT message contains the following parameter:

       Heartbeat Data           Optional

   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 sending node defines the Heartbeat Data field contents.  It may
   include a Heartbeat Sequence Number and/or time stamp, or other
   implementation specific details.




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   The receiver of a Heartbeat message does not process this field as it
   is only of significance to the sender.  The receiver echoes the
   content of the Heartbeat Data in a BEAT ACK message.

3.3.2.6  Heartbeat Ack (BEAT ACK)

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

   The BEAT ACK message contains the following parameter:

       Heartbeat Data           Optional

   The format for the BEAT ACK 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 sending node defines the Heartbeat Data field contents.  It may
   include a Heartbeat Sequence Number and/or time stamp, or other
   implementation specific details.

   The receiver of a Heartbeat message does not process this field as it
   is only of significance to the sender.  The receiver echoes the
   content of the Heartbeat Data in a BEAT ACK message.

3.3.2.7  ASP Active (ASPAC)

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

   The ASPAC message contains the following parameters:

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





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   The format for the ASPAC message using integer formatted Interface
   Identifiers is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0xb)           |            Length = 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 Identifiers                 /
   /                    of Tag Type 0x1 or 0x8                     \
   \                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                          INFO String*                         /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






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   The format for the ASPAC message using text formatted (string)
   Interface Identifiers is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0xb)           |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Traffic Mode Type                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Tag (0x3=string)        |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                     Interface Identifier*                     /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \              Additional Interface Identifiers                 /
   /                       of Tag Type 0x3                         \
   \                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                          INFO String*                         /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

      Value          Description
       0x1            Override
       0x2            Load-share
       0x3            Broadcast

   Within a particular AS, only one Traffic Mode Type can be used.  The
   Override value indicates that the ASP is operating in Override 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.  In Broadcast
   mode, all of the Active ASPs receive all message traffic in the
   Application Server.






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

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

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

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

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

3.3.2.8  ASP Active Ack

   The ASP Active (ASPAC) Ack message is used to acknowledge an ASP
   Active message received from a remote M2UA peer.

   The ASPAC Ack message contains the following parameters:

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












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   The format for the ASPAC Ack message with Integer-formatted Interface
   Identifiers is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0xb)           |            Length = 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 Identifiers                 /
   /                    of Tag Type 0x1 or 0x8                     \
   \                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                          INFO String*                         /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






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   The format for the ASP Active Ack message using text formatted
   (string) Interface Identifiers is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0xb)           |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Traffic Mode Type                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Tag (0x3=string)        |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                     Interface Identifier*                     /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \              Additional Interface Identifiers                 /
   /                       of Tag Type 0x3                         \
   \                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                          INFO String*                         /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

   The format of the optional Interface Identifier parameter is the same
   as for the ASP Active message (See Section 3.3.2.7).

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

3.3.2.9  ASP Inactive (ASPIA)

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







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   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:

    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 Identifiers                 /
   /                    of Tag Type 0x1 or 0x8                     \
   \                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x4)           |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                          INFO String*                         /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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   The format for the ASP Inactive message using text formatted (string)
   Interface Identifiers is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Tag (0x3=string)        |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                     Interface Identifier*                     /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \              Additional Interface Identifiers                 /
   /                      of Tag Type 0x3                          \
   \                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                          INFO String*                         /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The format of the optional Interface Identifier parameter is the same
   as for the ASP Active message (See Section 3.3.2.7).

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

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

3.3.2.10 ASP Inactive Ack

   The ASP Inactive (ASPIA) Ack message is used to acknowledge an ASP
   Inactive message received from a remote M2UA 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)





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   The format for the ASPIA Ack 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 (0x1=integer)         |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                     Interface Identifiers*                    /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Tag (0x8=integer range)    |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Interface Identifier Start1*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Interface Identifier Stop1*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Interface Identifier Start2*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Interface Identifier Stop2*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    .                                                             .
    .                                                             .
    .                                                             .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Interface Identifier StartN*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Interface Identifier StopN*                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \              Additional Interface Identifiers                 /
   /                    of Tag Type 0x1 or 0x8                     \
   \                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                          INFO String*                         /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+











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   The format for the ASP Inactive Ack message using text formatted
   (string) Interface Identifiers is as follows:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Tag (0x3=string)        |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                     Interface Identifier*                     /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \              Additional Interface Identifiers                 /
   /                      of Tag Type 0x3                          \
   \                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                          INFO String*                         /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The format of the optional Interface Identifier parameter is the same
   as for the ASP Active message (See Section 3.3.2.7).

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

3.3.3  Layer Management (MGMT) Messages

3.3.3.1  Error (ERR)

   The Error (ERR) 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.

   An Error message MUST not be generated in response to other Error
   messages.

   The ERR message contains the following parameters:

      Error Code (mandatory)
      Interface Identifier (optional)
      Diagnostic Information (optional)




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   The format for the ERR 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 (0xc)           |            Length = 8         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Error Code                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Tag (0x1, 0x3, or 0x8)    |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                     Interface Identifier(s)*                  /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x7)           |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                     Diagnostic Information*                   /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

      Invalid Version                        0x1
      Invalid Interface Identifier           0x2
      Unsupported Message Class              0x3
      Unsupported Message Type               0x4
      Unsupported Traffic Handling Mode      0x5
      Unexpected Message                     0x6
      Protocol Error                         0x7
      Unsupported Interface Identifier Type  0x8
      Invalid Stream Identifier              0x9
      Not Used in M2UA                       0xa
      Not Used in M2UA                       0xb
      Not Used in M2UA                       0xc
      Refused - Management Blocking          0xd
      ASP Identifier Required                0xe
      Invalid ASP Identifier                 0xf
      ASP Active for Interface Identifier(s) 0x10
      Invalid Parameter Value                0x11
      Parameter Field Error                  0x12
      Unexpected Parameter                   0x13
      Not Used in M2UA                       0x14
      Not Used in M2UA                       0x15
      Missing Parameter                      0x16




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

   The "Invalid Interface Identifier" error would be sent by a SGP if an
   ASP sends a message (i.e. an ASP Active message) with an invalid (not
   configured) Interface Identifier value.  One of the optional
   Interface Identifier parameters (Integer-based, text-based or integer
   range) MUST be used with this error code to identify the invalid
   Interface Identifier(s) received.

   The "Unsupported Traffic Handling Mode" error would be sent by a SGP
   if an ASP sends an ASP Active with an unsupported Traffic Handling
   Mode.  An example would be a case in which the SGP did not support
   load-sharing.  One of the optional Interface Identifier parameters
   (Integer-based, text-based or integer range) MAY be used with this
   error code to identify the Interface Identifier(s).

   The "Unexpected Message" error would be sent by an ASP if it received
   a MAUP message from an SGP while it was in the Inactive state.

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

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

   The "Unsupported Interface Identifier Type" error would be sent by a
   SGP if an ASP sends a Text formatted Interface Identifier and the SGP
   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 Class" error would be sent if a message with
   an unexpected or unsupported Message Class is received.

   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 lock-out").

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




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

   The "ASP Currently Active for Interface Identifier(s)" error is sent
   by a SGP when a Deregistration request is received from an ASP that
   is active for Interface Identifier(s) specified in the Deregistration
   request.  One of the optional Interface Identifier parameters
   (Integer-based, text-based or integer range) MAY be used with this
   error code to identify the Interface Identifier(s).

   The "Invalid Parameter Value " error is sent if a message is received
   with an invalid parameter value (e.g., a State Request with an an
   undefined State).

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

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

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

   The optional Diagnostic information can be any information germane to
   the error condition, to assist in the identification of the error
   condition.  In the case of an Invalid Version Error Code the
   Diagnostic information includes the supported Version parameter.  In
   the other cases, the Diagnostic information SHOULD be the first 40
   bytes of the offending message.

3.3.3.2  Notify (NTFY)

   The Notify message is used to provide an autonomous indication of
   M2UA events to an M2UA peer.

   The NTFY message contains the following parameters:

      Status Type (mandatory)
      Status Information (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 (0xd)           |            Length = 8         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Status Type            |      Status Information       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x11)          |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        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 Identifiers                 /
   /                    of Tag Type 0x1 or 0x8                     \
   \                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                          INFO String*                         /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





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

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0xd)           |            Length = 8         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Status Type            |      Status Information       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Tag (0x11)          |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        ASP Identifier*                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Tag (0x3=string)        |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                     Interface Identifier*                     /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \              Additional Interface Identifiers                 /
   /                        of Tag Type 0x3                        \
   \                                                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag (0x4)             |             Length            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                                                               \
   \                          INFO String*                         /
   /                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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













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   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:

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

   These notifications are sent from an SGP to an ASP upon a change in
   status of a particular Application Server.  The value reflects the
   new state of the Application Server.  The Interface Identifiers of
   the AS MAY be placed in the message if desired.

   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

   In the Insufficient ASP Resources case, the SGP 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, the formerly Active ASP is informed when an
   alternate ASP transitions to the ASP Active state in Override mode.
   The ASP Identifier (if available) of the Alternate ASP MUST be placed
   in the message.  For the ASP Failure case, the SGP 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.

   For each of the Status Information values in Status Type Other, the
   Interface Identifiers of the affected AS MAY be placed in the message
   if desired.

   The format of the optional Interface Identifier parameter is the same
   as for the ASP Active message (See Section 3.3.2.7).

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







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3.3.4  Interface Identifier Management (IIM) Messages

   The Interface Identifier Management messages are optional.  They are
   used to support the automatic allocation of Signalling Terminals or
   Signalling Data Links [2][3].

3.3.4.1  Registration Request (REG REQ)

   The REG REQ message is sent by an ASP to indicate to a remote M2UA
   peer that it wishes to register one or more given Link Keys with the
   remote peer.  Typically, an ASP would send this message to an SGP,
   and expect to receive a REG RSP in return with an associated
   Interface Identifier value.

   The REG REQ message contains the following parameter:

      Link Key          (mandatory)

   The format for the REG REQ 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 = 0x0309          |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                           Link Key 1                          /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                              ...                              /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag = 0x0309          |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                           Link Key n                          /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Link Key:   fixed length

      The Link Key parameter is mandatory.  The sender of this message
      expects that the receiver of this message will create a Link Key
      entry and assign a unique Interface Identifier value to it, if the
      Link Key entry does not yet exist.





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      The Link Key parameter may be present multiple times in the same
      message.  This is used to allow the registration of multiple Link
      Keys in a single message.

   The format of the Link Key parameter 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Local-LK-Identifier                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 Signalling Data Terminal Identifier           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Signalling Data Link Identifier             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Local-LK-Identifier: 32-bit integer

      The mandatory Local-LK-Identifier field is used to uniquely
      (between ASP and SGP) identify the registration request.  The
      Identifier value is assigned by the ASP, and is used to correlate
      the response in a REG RSP message with the original registration
      request.  The Identifier value MUST remain unique until the REG
      RSP is received.

      The format of the Local-LK-Identifier field 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 = 0x030a          |         Length = 8            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Local-LK-Identifier value                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

















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   Signalling Data Terminal Identifier

      The Signalling Data Terminal Identifier parameter is mandatory.
      It identifies the Signalling Data Terminal associated with the SS7
      link for which the ASP is registering.  The format 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 = 0x030b          |         Length = 8            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Reserved             |        SDT Identifier         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The SDT Identifier is a 32-bit unsigned value which may only be
      significant to 12 or 14 bits depending on the SS7 variant which is
      supported by the MTP Level 3 at the ASP.  Insignificant SDT
      Identifier bits are coded 0.

   Signalling Data Link Identifier

      The Signalling Data Link Identifier parameter is mandatory.  It
      identifies the Signalling Data Link Identifier associated with the
      SS7 link for which the ASP is registering.  The format 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 = 0x030c          |         Length = 8            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Reserved             |        SDL Identifier         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The SDL Identifier is a 32-bit unsigned value which may only be
      significant to 12 or 14 bits depending on the SS7 variant which
      is supported by the MTP Level 3 at the ASP.  Insignificant SDLI
      bits are coded 0.

3.3.4.2  Registration Response (REG RSP)

   The REG RSP message is used as a response to the REG REQ message
   from a remote M2UA peer.  It contains indications of success/failure
   for registration requests and returns a unique Interface Identifier
   value for successful registration requests, to be used in subsequent
   M2UA Traffic Management protocol.





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   The REG RSP message contains the following parameter:

      Registration Results   (mandatory)

   The format for the REG RSP 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 = 0x030d          |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                    Registration Result 1                      /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                              ...                              /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag = 0x030d          |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                    Registration Result n                      /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Registration Results:  fixed length

      The Registration Results parameter contains one or more results,
      each containing the registration status for a single Link Key in
      the REG REQ message.  The number of results in a single REG RSP
      message MAY match the number of Link Key parameters found in the
      corresponding REG REQ message.  The format of each result 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Local-LK-Identifier                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Registration Status                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Interface Identifier                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+







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   Local-LK-Identifier:  32-bit integer

      The Local-LK-Identifier contains the same value as found in the
      matching Link Key parameter found in the REG REQ message.  The
      format of the Local-LK-Identifier is shown in Section 3.3.4.1.

   Registration Status:  32-bit integer

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

      Its values may be one of the following:

            0         Successfully Registered
            1         Error - Unknown
            2         Error - Invalid SDLI
            3         Error - Invalid SDTI
            4         Error - Invalid Link Key
            5         Error - Permission Denied
            6         Error - Overlapping (Non-unique) Link Key
            7         Error - Link Key not Provisioned
            8         Error - Insufficient Resources

      The format of the Registration Status field 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 = 0x030e          |         Length = 8            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Registration Status                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Interface Identifier:  32-bit integer

      The Interface Identifier field contains the Interface Identifier
      for the associated Link Key if the registration is successful.  It
      is set to "0" if the registration was not successful.  The format
      of integer-based and text-based Interface Identifier parameters
      are shown in Section 3.2.

3.3.4.3  De-Registration Request (DEREG REQ)

   The DEREG REQ message is sent by an ASP to indicate to a remote M2UA
   peer that it wishes to de-register a given Interface Identifier.
   Typically, an ASP would send this message to an SGP, and expects to
   receive a DEREG RSP in return reflecting the Interface Identifier and
   containing a de-registration status.



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   The DEREG REQ message contains the following parameter:

      Interface Identifier  (mandatory)

   The format for the DEREG REQ 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 = 0x1 or 0x3          |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                    Interface Identifier 1                     /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                              ...                              /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Tag = 0x1 or 0x3          |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                    Interface Identifier n                     /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Interface Identifier

      The Interface Identifier parameter contains a Interface Identifier
      indexing the Application Server traffic that the sending ASP is
      currently registered to receive from the SGP but now wishes to
      de-register.  The format of integer-based and text-based Interface
      Identifier parameters are shown in Section 3.2.

3.3.4.4  De-Registration Response (DEREG RSP)

   The DEREG RSP message is used as a response to the DEREG REQ message
   from a remote M2UA peer.

   The DEREG RSP message contains the following parameter:

      De-Registration Results   (mandatory)









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   The format for the DEREG RSP 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 = 0x030f          |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                  De-Registration Result 1                     /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                              ...                              /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         Tag = 0x030f          |            Length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   \                                                               \
   /                  De-Registration Result n                     /
   \                                                               \
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   De-Registration Results:  fixed length

      The De-Registration Results parameter contains one or more
      results, each containing the de-registration status for a single
      Interface Identifier in the DEREG REQ message.  The number of
      results in a single DEREG RSP message MAY match the number of
      Interface Identifier parameters found in the corresponding DEREG
      REQ message.  The format of each result 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Interface Identifier                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     De-Registration Status                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Interface Identifier:  32-bit integer

      The Interface Identifier field contains the Interface Identifier
      value of the matching Link Key to de-register, as found in the
      DEREG REQ.  The format of integer-based and text-based Interface
      Identifier parameters are shown in Section 3.2.






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   De-Registration Status:  32-bit integer

      The De-Registration Result Status field indicates the success or
      the reason for failure of the de-registration.

      Its values may be one of the following:

            0         Successfully De-registered
            1         Error - Unknown
            2         Error - Invalid Interface Identifier
            3         Error - Permission Denied
            4         Error - Not Registered

      The format of the De-Registration Status field 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 = 0x0310          |         Length = 8            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    De-Registration Status                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.0  Procedures

   The M2UA layer needs to respond to various primitives it receives
   from other layers as well as messages it receives from the peer-to-
   peer messages.  This section describes various procedures involved in
   response to these events.

4.1 Procedures to Support the M2UA-User Layer

   These procedures achieve the M2UA layer "Transport of MTP Level 2 /
   MTP Level 3 boundary" service.

4.1.1  MTP Level 2 / MTP Level 3 Boundary Procedures

   On receiving a primitive from the local upper layer, the M2UA layer
   will send the corresponding MAUP message (see Section 3) to its peer.
   The M2UA layer MUST fill in various fields of the common and specific
   headers correctly.  In addition the message SHOULD be sent on the
   SCTP stream that corresponds to the SS7 link.

4.1.2  MAUP Message Procedures

   On receiving MAUP messages from a peer M2UA layer, the M2UA layer on
   an SG or MGC needs to invoke the corresponding layer primitives to
   the local MTP Level 2 or MTP Level 3 layer.



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4.2 Receipt of Primitives from the Layer Management

   On receiving primitives from the local Layer Management, the M2UA
   layer will take the requested action and provide an appropriate
   response primitive to Layer Management.

   An M-SCTP_ESTABLISH request primitive from Layer Management at an ASP
   will initiate the establishment of an SCTP association.  The M2UA
   layer will attempt to establish an SCTP association with the remote
   M2UA peer by sending an SCTP-ASSOCIATE primitive to the local SCTP
   layer.

   When an SCTP association has been successfully established, the SCTP
   will send an SCTP-COMMUNICATION_UP notification primitive to the
   local M2UA layer.  At the SGP that initiated the request, the M2UA
   layer will send an M-SCTP_ESTABLISH confirm primitive to Layer
   Management when the association setup is complete.  At the peer M2UA
   layer, an M-SCTP_ESTABLISH indication primitive is sent to Layer
   Management upon successful completion of an incoming SCTP association
   setup.

   An M-SCTP_RELEASE request primitive from Layer Management initiates
   the shutdown of an SCTP association.  The M2UA layer accomplishes a
   graceful shutdown of the SCTP association by sending an SCTP-SHUTDOWN
   primitive to the SCTP layer.

   When the graceful shutdown of the SCTP association has been
   accomplished, the SCTP layer returns an SCTP-SHUTDOWN_COMPLETE
   notification primitive to the local M2UA layer.  At the M2UA Layer
   that initiated the request, the M2UA layer will send an M-
   SCTP_RELEASE confirm primitive to Layer Management when the
   association shutdown is complete.  At the peer M2UA Layer, an M-
   SCTP_RELEASE indication primitive is sent to Layer Management upon
   abort or successful shutdown of an SCTP association.

   An M-SCTP_STATUS request primitive supports a Layer Management query
   of the local status of a particular SCTP association.  The M2UA layer
   simply maps the M-SCTP_STATUS request primitive to an SCTP-STATUS
   primitive to the SCTP layer.  When the SCTP responds, the M2UA layer
   maps the association status information to an M-SCTP_STATUS confirm
   primitive.  No peer protocol is invoked.

   Similar LM-to-M2UA-to-SCTP and/or SCTP-to-M2UA-to-LM primitive
   mappings can be described for the various other SCTP Upper Layer
   primitives in RFC 2960 [8] such as INITIALIZE, SET PRIMARY, CHANGE
   HEARTBEAT, REQUEST HEARTBEAT, GET SRTT REPORT, SET FAILURE THRESHOLD,
   SET PROTOCOL PARAMETERS, DESTROY SCTP INSTANCE, SEND FAILURE, AND
   NETWORK STATUS CHANGE.  Alternatively, these SCTP Upper Layer



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   primitives (and Status as well) can be considered for modeling
   purposes as a Layer Management interaction directly with the SCTP
   Layer.

   M-NOTIFY indication and M-ERROR indication primitives indicate to
   Layer Management the notification or error information contained in a
   received M2UA Notify or Error message respectively.  These
   indications can also be generated based on local M2UA events.

   An M-ASP_STATUS request primitive supports a Layer Management query
   of the status of a particular local or remote ASP.  The M2UA layer
   responds with the status in an M-ASP_STATUS confirm primitive.  No
   M2UA peer protocol is invoked.

   An M-AS_STATUS request supports a Layer Management query of the
   status of a particular AS.  The M2UA responds with an M-AS_STATUS
   confirm primitive.  No M2UA peer protocol is invoked.

   M-ASP_UP request, M-ASP_DOWN request, M-ASP_ACTIVE request and M-
   ASP_INACTIVE request primitives allow Layer Management at an ASP to
   initiate state changes.  Upon successful completion, a corresponding
   confirm primitive is provided by the M2UA layer to Layer Management.
   If an invocation is unsuccessful, an Error indication primitive is
   provided in the primitive.  These requests result in outgoing ASP Up,
   ASP Down, ASP Active and ASP Inactive messages to the remote M2UA
   peer at an SGP.

4.2.1  Receipt of M2UA Peer Management Messages

   Upon successful state changes resulting from reception of ASP Up, ASP
   Down, ASP Active and ASP Inactive messages from a peer M2UA, the M2UA
   layer SHOULD invoke corresponding M-ASP_UP, M-ASP_DOWN, M-ASP_ACTIVE
   and M-ASP_INACTIVE, M-AS_ACTIVE, M-AS_INACTIVE, and M-AS_DOWN
   indication primitives to the local Layer Management.

   M-NOTIFY indication and M-ERROR indication primitives indicate to
   Layer Management the notification or error information contained in a
   received M2UA Notify or Error message.  These indications can also be
   generated based on local M2UA events.

   All MGMT messages, except BEAT and BEAT Ack, SHOULD be sent with
   sequenced delivery to ensure ordering.  All MGMT messages, with the
   exception of ASPTM, BEAT and BEAT Ack messages, SHOULD be sent on
   SCTP stream '0'.  All ASPTM messages SHOULD be sent on the stream
   which normally carries the data traffic to which the message applies.
   BEAT and BEAT Ack messages MAY be sent using out-of-order delivery,
   and MAY be sent on any stream.




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4.3  AS and ASP State Maintenance

   The M2UA layer on the SGP maintains the state of each remote ASP, in
   each Application Server that the ASP is configured to receive
   traffic, as input to the M2UA message distribution function.

4.3.1  ASP States

   The state of each remote ASP, in each AS that it is configured to
   operate, is maintained in the M2UA layer in the SGP.  The state of a
   particular ASP in a particular AS changes due to events.  The events
   include:

   *  Reception of messages from the peer M2UA layer at the ASP;
   *  Reception of some messages from the peer M2UA layer at other ASPs
      in the AS (e.g., ASP Active message indicating "Override");
   *  Reception of indications from the SCTP layer; or
   *  Local Management intervention.

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

   ASP-DOWN: The remote M2UA peer at the ASP 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 M2UA messages,
   with the exception of Heartbeat, ASP Down Ack and Error messages.

   ASP-INACTIVE: The remote M2UA peer at the ASP is available (and the
   related SCTP association is up) but application traffic is stopped.
   In this state the ASP MAY be sent any non-MAUP M2UA messages.

   ASP-ACTIVE: The remote M2UA peer at the ASP is available and
   application traffic is active (for a particular Interface Identifier
   or set of Interface Identifiers).

















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                    Figure 5: ASP State Transition Diagram

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


   SCTP CDI: The SCTP CDI denotes the local SCTP layer's Communication
   Down Indication to the Upper Layer Protocol (M2UA) on an SGP.  The
   local SCTP layer 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 or COMMUNICATION_LOST
   notification from the SCTP layer.

   SCTP RI: The local SCTP layer's Restart indication to the upper layer
   protocol (M2UA) on an SG.  The local SCTP will send this indication
   when it detects a restart from the ASP's peer SCTP layer.

4.3.2  AS States

   The state of the AS is maintained in the M2UA layer on the SGP.  The
   state of an AS changes due to events.  These events include:

      *  ASP state transitions
      *  Recovery timer triggers







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   The possible states of an AS are:

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

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

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

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

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

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

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











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                    Figure 6: AS State Transition Diagram

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

      Tr = Recovery Timer

4.3.3 M2UA Management Procedures for Primitives

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

   Once the SCTP association is established (see Section 4.2.1) and
   assuming that the local M2UA-User is ready, the local M2UA ASP
   Maintenance (ASPM) function will initiate the relevant procedures,
   using the ASP Up/ASP Down/ASP Active/ASP Inactive messages to convey
   the ASP state to the SGP (see Section 4.3.4).

   If the M2UA layer subsequently receives an SCTP-COMMUNICATION_DOWN or
   SCTP-RESTART indication primitive from the underlying SCTP layer, it
   will inform the Layer Management by invoking the M-SCTP_STATUS
   indication primitive.  The state of the ASP will be moved to ASP-
   DOWN.




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   In the case of SCTP-COMMUNICATION_DOWN, the SCTP client MAY try to
   re-establish the SCTP association.  This MAY be done by the M2UA
   layer automatically, or Layer Management MAY re-establish using the
   M-SCTP_ESTABLISH request primitive.

   In the case of an SCTP-RESTART indication at an ASP, the ASP is now
   considered by its M2UA peer to be in the ASP-DOWN state.  The ASP, if
   it is to recover, must begin any recovery with the ASP-Up procedure.

4.3.4 ASPM Procedures for Peer-to-Peer Messages

4.3.4.1 ASP Up Procedures

   After an ASP has successfully established an SCTP association to an
   SGP, the SGP waits for the ASP to send an ASP Up message, indicating
   that the ASP M2UA 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 M2UA management function.

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

   Alternatively, the SGP may move the ASP into a pool of Inactive ASPs
   available for future configuration within Application Server(s),
   determined in a subsequent Registration Request or ASP Active
   procedure.  If the ASP Up message contains an ASP Identifier, the SGP
   should save the ASP Identifier for that ASP.  The SGP 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 SGP.

   If for any local reason (e.g., management lock-out) the SGP cannot
   respond with an ASP Up Ack message, the SGP 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



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   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
   M2UA messages (e.g., ASP Active or REG REQ).  If the SGP receives any
   other M2UA messages before an ASP Up message is received (other than
   ASP Down - see Section 4.3.4.2), the SGP 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.

4.3.4.1.1  M2UA Version Control

   If an 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 SGP.

4.3.4.2 ASP Down Procedures

   The ASP will send an ASP Down message to an SGP when the ASP wishes
   to be removed from service in all Application Servers that it is a
   member and no longer receive any MAUP 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 M2UA
   management function.

   Whether the ASP is permanently removed from any AS is a function of
   configuration management.  In the case where the ASP previously used
   the Registration procedures (see Section 4.4) to register within
   Application Servers but has not unregistered from all of them prior
   to sending the ASP Down message, the SGP MUST consider the ASP as
   unregistered in all Application Servers that it is still a member.




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   The SGP 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 SGP 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 SGP.

   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.

4.3.4.3 ASP Active Procedures

   Anytime after the ASP has received an ASP Up Ack message from the
   SGP, the ASP MAY send an ASP Active message to the SGP indicating
   that the ASP is ready to start processing traffic.  This action MAY
   be initiated at the ASP by an M-ASP_ACTIVE request primitive from
   Layer Management or MAY be initiated automatically by a M2UA
   management function.  In the case where an ASP wishes to process the
   traffic for more than one Application Server across a common SCTP
   association, the ASP Active message(s) SHOULD contain a list of one
   or more Interface Identifiers to indicate for which Application
   Servers the ASP Active message applies.  It is not necessary for the
   ASP to include any Interface Identifiers of interest in a single ASP
   Active message, thus requesting to become active in all Interface
   Identifiers at the same time.  Multiple ASP Active messages MAY be
   used to activate within the Application Servers independently, or in
   sets.  In the case where an ASP Active message does not contain a
   Interface Identifier parameter, the receiver must know, via
   configuration data, of which Application Server(s) the ASP is a
   member.

   For the Application Servers that the ASP can successfully activate,
   the SGP responds with one or more ASP Active Ack messages, including



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   the associated Interface Identifier(s) and reflecting any Traffic
   Mode Type value present in the related ASP Active message.  The
   Interface Identifier parameter MUST be included in the ASP Active Ack
   message(s) if the received ASP Active message contained any Interface
   Identifiers.  Depending on any Traffic Mode Type request in the ASP
   Active message or local configuration data if there is no request,
   the SGP moves the ASP to the correct ASP traffic state within the
   associated Application Server(s).  Layer Management is informed with
   an M-ASP_Active indication.  If the SGP receives any Data messages
   before an ASP Active message is received, the SGP MAY discard them.
   By sending an ASP Active Ack message, the SGP is now ready to receive
   and send traffic for the related Interface Identifier(s).  The ASP
   SHOULD NOT send MAUP messages for the related Interface Identifier(s)
   before receiving an ASP Active Ack message, or it will risk message
   loss.

   Multiple ASP Active Ack messages MAY be used in response to an ASP
   Active message containing multiple Interface Identifiers, allowing
   the SGP to independently acknowledge the ASP Active message for
   different (sets of) Interface Identifiers.  The SGP MUST send an
   Error message ("Invalid Interface Identifier") for each Interface
   Identifier value that cannot be successfully activated.

   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 SGP 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 SGP.

   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 message(s)
   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 the control of
   Layer Management.  In this method, expiry of T(ack) results in an M-
   ASP_ACTIVE confirm primitive carrying a negative indication.



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   There are three modes of Application Server traffic handling in the
   SGP M2UA layer: Override, Load share and Broadcast.  When included,
   the Traffic Mode Type parameter in the ASP Active message indicates
   the traffic handling mode to be used in a particular Application
   Server.  If the SGP determines that the mode indicated in an ASP
   Active message is unsupported or incompatible with the mode currently
   configured for the AS, the SGP responds with an Error message
   ("Unsupported / Invalid Traffic Handling Mode").  If the traffic
   handling mode of the Application Server is not already known via
   configuration data, the traffic handling mode indicated in the first
   ASP Active message causing the transition of the Application Server
   state to AS-ACTIVE MAY be used to set the mode.

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

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

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

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

   In the case of a Broadcast mode AS, reception of an ASP Active
   message at an SGP causes the direction of traffic to the ASP sending
   the ASP Active message, in addition to all the other ASPs that are
   currently active in the AS.  The algorithm at the SGP for




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   broadcasting traffic within an AS to all the active ASPs is a simple
   broadcast algorithm, where every message is sent to each of the
   active ASPs.

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

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

4.3.4.4 ASP Inactive Procedures

   When an ASP wishes to withdraw from receiving traffic within an AS,
   the ASP sends an ASP Inactive message to the SGP.  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 M2UA
   management function.  In the case where an ASP is processing the
   traffic for more than one Application Server across a common SCTP
   association, the ASP Inactive message contains one or more Interface
   Identifiers to indicate for which Application Servers the ASP
   Inactive message applies.  In the case where an ASP Inactive message
   does not contain a Interface Identifier parameter, the receiver must
   know, via configuration data, of which Application Servers the ASP is
   a member and move the ASP to the ASP-INACTIVE state in all
   Application Servers.  In the case of an Override mode AS, where
   another ASP has already taken over the traffic within the AS with an
   ASP Active ("Override") message, the ASP that sends the ASP Inactive
   message is already considered by the SGP to be in the state ASP-
   INACTIVE.  An ASP Inactive Ack message is sent to the ASP, after
   ensuring that all traffic is stopped to the ASP.

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





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

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

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

   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.

   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 the control
   of Layer Management.  In this method, expiry of T(ack) results in a
   M-ASP_Inactive confirm primitive carrying a negative indication.

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






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4.3.4.5 Notify Procedures

   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
   acknowledgment messages (e.g., ASP Up Ack, ASP Down Ack, ASP Active
   Ack, or ASP Inactive Ack).

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

   In the case where a Notify message does not contain a Interface
   Identifier parameter, the receiver must know, via configuration data,
   of which Application Servers the ASP is a member and take the
   appropriate action in each AS.

4.3.4.6 Heartbeat Procedures

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

   Either M2UA peer may optionally send Heartbeat messages periodically,
   subject to a provisionable timer T(beat).  Upon receiving a Heartbeat
   message, the M2UA peer MUST respond with a Heartbeat Ack message.

   If no Heartbeat Ack message (or any other M2UA message) is received
   from the M2UA peer within 2*T(beat), the remote M2UA peer is
   considered unavailable.  Transmission of Heartbeat messages is
   stopped and the signalling process SHOULD attempt to re-establish
   communication if it is configured as the client for the disconnected
   M2UA peer.

   The Heartbeat message may optionally contain an opaque Heartbeat Data
   parameter that MUST be echoed back unchanged in the related Heartbeat
   Ack message.  The sender, upon examining the contents of the returned
   Heartbeat Ack message, MAY choose to consider the remote M2UA peer as
   unavailable.  The contents/format of the Heartbeat Data parameter is



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   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
   time stamp mechanism (to evaluate delays).

   Note: Heartbeat related events are not shown in Figure 5 "ASP state
         transition diagram".

4.4  Link Key Management Procedures

   The Interface Identifier Management procedures are optional.  They
   can be used to support automatic allocation of Signalling Terminals
   or Signalling Data Links [2][3].

4.4.1 Registration

   An ASP MAY dynamically register with an SGP as an ASP within an
   Application Server for individual Interface Identifier(s) using the
   REG REQ message.  A Link Key parameter in the REG REQ specifies the
   parameters associated with the Link Key.

   The SGP examines the contents of the received Link Key parameters
   (SDLI and SDTI) and compares them with the currently provisioned
   Interface Identifiers.  If the received Link Key matches an existing
   SGP Link Key entry, and the ASP is not currently included in the list
   of ASPs for the related Application Server, the SGP MAY authorize the
   ASP to be added to the AS.  Or, if the Link Key does not currently
   exist and the received Link Key data is valid and unique, an SGP
   supporting dynamic configuration MAY authorize the creation of a new
   Interface Identifier and related Application Server and add the ASP
   to the new AS.  In either case, the SGP returns a Registration
   Response message to the ASP, containing the same Local-LK-Identifier
   as provided in the initial request, a Registration Result
   "Successfully Registered" and the Interface Identifier.  A unique
   method of Interface Identifier valid assignment at the SG/SGP is
   implementation dependent but MUST be guaranteed to be unique for each
   Application server or Link Key served by SGP.

   If the SGP determines that the received Link Key data is invalid, or
   contains invalid parameter values, the SGP returns a Registration
   Response message to the ASP, containing a Registration Result "Error
   - Invalid Link Key", "Error - Invalid SDTI", "Error - Invalid SDLI"
   as appropriate.








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   If the SGP determines that the Link Key parameter overlaps with an
   existing Link Key entry, the SGP returns a Registration Response
   message to the ASP, with a Registration Status of "Error -
   Overlapping (Non-Unique) Link Key".  An incoming signalling message
   received at an SGP cannot match against more than one Link Key.

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

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

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

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

4.4.2 Deregistration

   An ASP MAY dynamically de-register with an SGP as an ASP within an
   Application Server for individual Interface Identifier(s) using the
   DEREG REQ message.  A Interface Identifier parameter in the DEREG REQ
   specifies which Interface Identifier to de-register.

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

   The deregistration procedure does not necessarily imply the deletion
   of Link Key and Application Server configuration data at the SGP.
   Other ASPs may continue to be associated with the Application Server,





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   in which case the Link Key data CANNOT be deleted.  If a
   Deregistration results in no more ASPs in an Application Server, an
   SGP MAY delete the Link Key data.

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

   An ASP MAY de-register multiple Interface Identifiers at once by
   including a number of Interface Identifiers in a single DEREG REQ
   message.  The SGP MUST respond to each deregistration request in a
   single DEREG RSP message, indicating the success or failure result
   for each Interface Identifier in a separate Deregistration Result
   parameter.

5.0  Examples of MTP2 User Adaptation (M2UA) Procedures

5.1  Establishment of associations between SGP and MGC examples

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

   This scenario shows the example M2UA message flows for the
   establishment of traffic between an SGP and an ASP, where only one
   ASP is configured within an AS (no backup).  It is assumed that the
   SCTP association is already set-up.

                SGP                       ASP1
                 |
                 |<---------ASP Up----------|
                 |--------ASP Up Ack------->|
                 |                          |
                 |<-------ASP Active--------|
                 |------ASP Active Ack----->|
                 |                          |
                 |------NTFY(AS-ACTIVE)---->|















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5.1.2 Single ASP in an Application Server (1+0 sparing) with Dynamic
      Registration

   This scenario is the same as the one shown in Section 5.1.1 except
   with a dynamic registration (automatic allocation) of an Interface
   Identifier(s).

                SGP                       ASP1
                 |
                 |<---------ASP Up----------|
                 |--------ASP Up Ack------->|
                 |                          |
                 |<--------REG REQ----------|
                 |------REG REQ RESP------->|
                 |                          |
                 |<-------ASP Active--------|
                 |------ASP Active Ack----->|
                 |                          |
                 |------NTFY(AS-ACTIVE)---->|

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

   This scenario shows the example M2UA message flows for the
   establishment of traffic between an SGP and two ASPs in the same
   Application Server, where ASP1 is configured to be active and ASP2 to
   be 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/transaction
   state or can communicate call state under failure/withdrawal events.

          SGP                       ASP1                       ASP2
           |                         |                          |
           |<--------ASP Up----------|                          |
           |-------ASP Up Ack------->|                          |
           |                         |                          |
           |<-----------------------------ASP Up----------------|
           |----------------------------ASP Up Ack------------->|
           |                         |                          |
           |                         |                          |
           |<-------ASP Active-------|                          |
           |-----ASP Active Ack----->|                          |
           |                         |                          |
           |                         |                          |
           |-----NTFY(AS-ACTIVE)---->|                          |
           |                         |                          |
           |------------------NTFY(AS-ACTIVE)------------------>|





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5.2 ASP Traffic Fail-over Examples

5.2.1 (1+1 Sparing, withdrawal of ASP, backup Override)

   Following on from the example in Section 5.1.2, and ASP withdraws
   from service:

         SGP                       ASP1                       ASP2
           |                         |                          |
           |<-----ASP Inactive-------|                          |
           |----ASP Inactive Ack---->|                          |
           |                         |                          |
           |----NTFY(AS-PENDING)---->|                          |
           |------------------NTFY(AS-PENDING)----------------->|
           |                         |                          |
           |<------------------------------ ASP Active----------|
           |-----------------------------ASP Active Ack-------->|
           |                         |                          |
           |-----NTFY(AS-ACTIVE)---->|                          |
           |------------------NTFY(AS-ACTIVE)------------------>|
           |                         |                          |

   In this case, the SGP notifies ASP2 that the AS has moved to the AS-
   PENDING state.  ASP2 sends ASP Active to bring the AS back to the
   AS-ACTIVE state.  If ASP2 did not send the ASP Active message before
   T(r) expired, the SGP would send a NOTIFY (AS-DOWN).

   Note: If the SGP detects loss of the M2UA peer (through a detection
         of SCTP failure), the initial SGP-ASP1 ASP Inactive message
         exchange would not occur.

          SGP                       ASP1                       ASP2
           |                         |                          |
     (detects SCTP failure)
           |------------------NTFY(AS-PENDING)----------------->|
           |                         |                          |
           |<------------------------------ ASP Active----------|
           |-----------------------------ASP Active Ack-------->|
           |                         |                          |
           |------------------NTFY(AS-ACTIVE)------------------>|
           |                         |                          |










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5.2.2 (1+1 Sparing, backup Override)

   Following on from the example in Section 5.1.2, and ASP2 wishes to
   override ASP1 and take over the traffic:

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

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

5.3  SGP to MGC, MTP Level 2 to MTP Level 3 Boundary Procedures

   When the M2UA layer on the ASP has a MAUP message to send to the SGP,
   it will do the following:

      -  Determine the correct SGP

      -  Find the SCTP association to the chosen SGP

      -  Determine the correct stream in the SCTP association based on
         the SS7 link

      -  Fill in the MAUP message, fill in M2UA Message Header, fill in
         Common Header

      -  Send the MAUP message to the remote M2UA peer in the SGP, over
         the SCTP association

   When the M2UA layer on the SGP has a MAUP message to send to the ASP,
   it will do the following:

      -  Determine the AS for the Interface Identifier

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

      -  Determine the correct stream in the SCTP association based on
         the SS7 link

      -  Fill in the MAUP message, fill in M2UA Message Header, fill in
         Common Header

      -  Send the MAUP message to the remote M2UA peer in the ASP, over
         the SCTP association



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5.3.1  SS7 Link Alignment

   The MGC can request that a SS7 link be brought into alignment using
   the normal or emergency procedure [2][3].  An example of the message
   flow to bring a SS7 link in-service using the normal alignment
   procedure is shown below.

       MTP2            M2UA                  M2UA               MTP3
        SGP             SGP                   ASP                ASP

        <----Start Req---|<---Establish Req----|<----Start Req------

        ---In Serv Ind-->|----Establish Cfm--->|----In Serv Ind---->

   An example of the message flow to bring a SS7 link in-service using
   the emergency alignment procedure.

 MTP2            M2UA                               M2UA           MTP3
  SGP             SGP                                ASP            ASP

  <----Emer Req----|<--State Req (STATUS_EMER_SET)----|<----Emer Req---

  -----Emer Cfm--->|---State Cfm (STATUS_EMER_SET)--->|----Emer Cfm---->

  <---Start Req----|<-------Establish Req-------------|<---Start Req----

  ---In Serv Ind-->|--------Establish Cfm------------>|---In Serv Ind-->

5.3.2  SS7 Link Release

   The MGC can request that a SS7 link be taken out-of-service.  It uses
   the Release Request message as shown below.

     MTP2               M2UA                  M2UA                MTP3
      SGP                SGP                   ASP                 ASP

       <-----Stop Req-----|<---Release Req------|<-----Stop Req------

       --Out of Serv Ind->|----Release Cfm----->|--Out of Serv Ind-->












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   The SGP can autonomously indicate that a SS7 link has gone out-of-
   service as shown below.

       MTP2           M2UA                  M2UA              MTP3
        SGP            SGP                   ASP               ASP

         --Out of Serv->|----Release Ind----->|--Out of Serv-->

5.3.3  Set and Clear Local Processor Outage

   The MGC can set a Local Processor Outage condition.  It uses the
   State Request message as shown below.

  MTP2            M2UA                               M2UA           MTP3
   SGP             SGP                                ASP            ASP

   <----LPO Req----|<---State Req (STATUS_LPO_SET)----|<----LPO Req---

   -----LPO Cfm--->|----State Cfm (STATUS_LPO_SET)--->|----LPO Cfm---->

   The MGC can clear a Local Processor Outage condition.  It uses the
   State Request message as shown below.

  MTP2            M2UA                               M2UA          MTP3
   SGP             SGP                                ASP           ASP

   <---LPO Req---|<---State Req (STATUS_LPO_CLEAR)----|<----LPO Req---

   ----LPO Cfm-->|----State Cfm (STATUS_LPO_CLEAR)--->|----LPO Cfm---->

5.3.4  Notification of Remote Processor Outage

   The SGP can indicate that Remote has entered or exited the Processor
   Outage condition for a SS7 link.  It uses the State Indication
   message as shown below.

 MTP2           M2UA                                M2UA           MTP3
  SGP            SGP                                 ASP            ASP

  ----RPO Ind---->|----State Ind (EVENT_RPO_ENTER)-->|-----RPO Ind---->

  -RPO Rcvr Ind-->|----State Ind (EVENT_RPO_EXIT)--->|--RPO Rcvr Ind-->









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5.3.5  Notification of SS7 Link Congestion

   The SGP can indicate that a SS7 link has become congested.  It uses
   the Congestion Indication message as shown below.

 MTP2           M2UA                                M2UA           MTP3
  SGP            SGP                                 ASP            ASP

  ----Cong Ind---->|--------Cong Ind (STATUS)------->|----Cong Ind---->

  -Cong Cease Ind->|--------Cong Ind (STATUS)------->|-Cong Cease Ind->

5.3.6  SS7 Link Changeover

   An example of the message flow for an error free changeover is shown
   below.  In this example, there were three messages in the
   retransmission queue that needed to be retrieved.

  MTP2          M2UA                            M2UA             MTP3
   SGP           SGP                             ASP              ASP

   <-Rtrv BSN Req-|<--Rtrv Req (ACTION_RTRV_BSN)--|<--Rtrv BSN Req---
                               (seq_num = 0)

   -Rtrv BSN Cfm->|---Rtrv Cfm (ACTION_RTRV_BSN)->|---Rtrv BSN Cfm-->
                               (seq_num = BSN)

   <-Rtrv Msg Req-|<-Rtrv Req (ACTION_RTRV_MSGS)--|<--Rtrv Msg Req---
                               (seq_num = FSN)

   -Rtrv Msg Cfm->|--Rtrv Cfm (ACTION_RTRV_MSGS)->|---Rtrv Msg Cfm-->
                               (seq_num = 0)

   -Rtrv Msg Ind->|---------Retrieval Ind ------->|---Rtrv Msg Ind-->
   -Rtrv Msg Ind->|---------Retrieval Ind ------->|---Rtrv Msg Ind-->
   -Rtrv Msg Ind->|---------Retrieval Ind ------->|---Rtrv Msg Ind-->

   -Rtrv Compl Ind->|----Retrieval Compl Ind ---->|-Rtrv Compl Ind-->

      Note: The number of Retrieval Indication is dependent on the
         number of messages in the retransmit queue that have been
         requested.  Only one Retrieval Complete Indication SHOULD be
         sent.








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   An example of a message flow with an error retrieving the BSN is
   shown below.

  MTP2          M2UA                            M2UA             MTP3
   SGP           SGP                             ASP              ASP

   <-Rtrv BSN Req-|<--Rtrv Req (ACTION_RTRV_BSN)--|<--Rtrv BSN Req---

   -BSN Not Rtrv->|---Rtrv Cfm (ACTION_RTRV_BSN)->|---BSN Not Rtrv-->
                               (seq_num = -1)

   An example of a message flow with an error retrieving the messages is
   shown below.

   <-Rtrv BSN Req-|<--Rtrv Req (ACTION_RTRV_BSN)--|<--Rtrv BSN Req---

   -Rtrv BSN Cfm->|---Rtrv Cfm (ACTION_RTRV_BSN)->|---Rtrv BSN Cfm-->
                               (seq_num = BSN)

   <-Rtrv Msg Req-|<-Rtrv Req (ACTION_RTRV_MSGS)--|<--Rtrv Msg Req---
                               (seq_num = FSN)

   -Rtrv Msg Cfm->|--Rtrv Cfm (ACTION_RTRV_MSGS)->|---Rtrv Msg Cfm-->
                               (seq_num = -1)

   An example of a message flow for a request to drop messages (clear
   retransmission buffers) is shown below.

  MTP2          M2UA                            M2UA             MTP3
   SGP           SGP                             ASP              ASP

 -Clr RTB Req----|<-StateReq (STATUS_CLEAR_RTB)--|<--Clr RTB Req-----

 -Clr RTB Req--->|-StateCfm (STATUS_CLEAR_RTB)-->|---Clr RTB Req---->

5.3.7  Flush and Continue

   The following message flow shows a request to flush buffers.

  MTP2            M2UA                              M2UA          MTP3
   SGP             SGP                               ASP           ASP

   <--Flush Req----|<-State Req (STATUS_FLUSH_BUFS)--|<---Flush Req--

   ---Flush Cfm--->|--State Cfm (STATUS_FLUSH_BUFS)->|---Flush Cfm-->






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   The following message flow shows a request to continue.

  MTP2            M2UA                              M2UA          MTP3
   SGP             SGP                               ASP           ASP

   <---Cont Req----|<--State Req (STATUS_CONTINUE)---|<---Cont Req---

   ----Cont Cfm--->|---State Cfm (STATUS_CONTINUE)-->|----Cont Cfm-->

5.3.8  Auditing of SS7 link state

   It may be necessary for the ASP to audit the current state of a SS7
   link.  The flows below show an example of the request and all the
   potential responses.

   Below is an example in which the SS7 link is out-of-service.

 MTP2           M2UA                              M2UA           MGMT
  SGP            SGP                               ASP            ASP

                 |<----State Req (STATUS_AUDIT)----|<----Audit-------

                                                                 MTP3
                                                                  ASP

                 |-----------Release Ind---------->|-Out of Serv Ind->

                                                                 MGMT
                                                                  ASP

                 |-----State Cfm (STATUS_AUDIT)--->|----Audit Cfm--->

   Below is an example in which the SS7 link is in-service.

 MTP2           M2UA                              M2UA           MGMT
  SGP            SGP                               ASP            ASP

                 |<----State Req (STATUS_AUDIT)----|<----Audit-------

                                                                 MTP3
                                                                  ASP

                 |-----------Establish Cfm-------->|---In Serv Ind-->

                                                                 MGMT
                                                                  ASP

                 |-----State Cfm (STATUS_AUDIT)--->|----Audit Cfm--->



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   Below is an example in which the SS7 link is in-service, but
   congested.

 MTP2           M2UA                              M2UA           MGMT
  SGP            SGP                               ASP            ASP

                 |<----State Req (STATUS_AUDIT)----|<----Audit-------

                                                                 MTP3
                                                                  ASP

                 |-----------Establish Cfm-------->|---In Serv Ind-->

                 |----------Congestion Ind-------->|---Cong Ind----->

                                                                 MGMT
                                                                  ASP

                 |-----State Cfm (STATUS_AUDIT)--->|----Audit Cfm--->

   Below is an example in which the SS7 link is in-service, but in
   Remote Processor Outage.

 MTP2           M2UA                              M2UA           MGMT
  SGP            SGP                               ASP            ASP

                 |<----State Req (STATUS_AUDIT)----|<---Audit Req----

                                                                 MTP3
                                                                  ASP

                 |-----------Establish Ind-------->|---In Serv Ind-->

                 |---State Ind (EVENT_RPO_ENTER)-->|----RPO Enter--->

                                                                 MGMT
                                                                  ASP

                 |-----State Cfm (STATUS_AUDIT)--->|----Audit Cfm--->












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6.0 Timer Values

   The recommended default values for M2UA timers are:

      T(r)                                    2 seconds
      T(ack)                                  2 seconds
      T(beat)   Heartbeat Timer               30 seconds

7.0 Security Considerations

   M2UA is designed to carry signalling messages for telephony services.
   As such, M2UA MUST involve the security needs of several parties: the
   end users of the services; the network providers and the applications
   involved.  Additional requirements MAY come from local regulation.
   While having some overlapping security needs, any security solution
   SHOULD fulfill all of the different parties' needs.

7.1 Threats

   There is no quick fix, one-size-fits-all solution for security.  As a
   transport protocol, M2UA has the following security objectives:

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

   M2UA runs on top of SCTP.  SCTP [8] provides certain transport
   related security features, such as:

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

   When M2UA is running in a professionally managed corporate or service
   provider network, it is reasonable to expect that this network
   includes an appropriate security policy framework.  The "Site
   Security Handbook" [13] SHOULD be consulted for guidance.

   When the network in which M2UA runs in involves more than one party,
   it MAY NOT be reasonable to expect that all parties have implemented
   security in a sufficient manner.  In such a case, it is recommended
   that IPSEC is used to ensure confidentiality of user payload.
   Consult [14] for more information on configuring IPSEC services.







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7.2 Protecting Confidentiality

   Particularly for mobile users, the requirement for confidentiality
   MAY include the masking of IP addresses and ports.  In this case
   application level encryption is not sufficient; IPSEC ESP SHOULD be
   used instead.  Regardless of which level performs the encryption, the
   IPSEC ISAKMP service SHOULD be used for key management.

8.0 IANA Considerations

8.1 SCTP Payload Protocol Identifier

   A request will be made to IANA to assign an M2UA value for the
   Payload Protocol Identifier in SCTP Payload Data chunk.  The
   following SCTP Payload Protocol Identifier has been registered:

         M2UA    "2"

   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.

8.2  M2UA Protocol Extensions

   This protocol may also be extended through IANA in three ways:

      -- through definition of additional message classes,
      -- through definition of additional message types, and
      -- through definition of additional message parameters.

   The definition and use of new message classes, types and parameters
   is an integral part of SIGTRAN adaptation layers.  Thus, these
   extensions are assigned by IANA through an IETF Consensus action as
   defined in [RFC2434].

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








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8.2.1 IETF Defined Message Classes

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

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

8.2.2 IETF Defined Message Types

   Documentation of the message type MUST contain the following
   information:

   (a) A long and short name for the new message type.
   (b) A detailed description of the structure of the message.
   (c) A detailed definition and description of intended use of each
       field within the message.
   (d) A detailed procedural description of the use of the new message
       type within the operation of the protocol.
   (e) A detailed description of error conditions when receiving this
       message type.

   When an implementation receives a message type which it does not
   support, it MUST respond with an Error (ERR) message with an Error
   Code of Unsupported Message Type.

8.2.3 IETF-defined TLV Parameter Extension

   Documentation of the message parameter MUST contain the following
   information:

   (a) Name of the parameter type.
   (b) Detailed description of the structure of the parameter field.
       This structure MUST conform to the general type-length-value
       format described in Section 3.1.5.
   (c) Detailed definition of each component of the parameter value.
   (d) Detailed description of the intended use of this parameter type,
       and an indication of whether and under what circumstances
       multiple instances of this parameter type may be found within the
       same message type.

9.0  Acknowledgments

   The authors would like to thank Tom George (Alcatel) for contribution
   of text and effort on the specification.






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   The authors would like to thank John Loughney, Neil Olson, Michael
   Tuexen, Nikhil Jain, Steve Lorusso, Dan Brendes, Joe Keller, Heinz
   Prantner, Barry Nagelberg, Naoto Makinae, Joyce Archibald, Mark
   Kobine, Nitin Tomar, Harsh Bhondwe and Karen King for their valuable
   comments and suggestions.

10.0  References

10.1  Normative

   [1]  ITU-T Recommendation Q.700, 'Introduction To ITU-T Signalling
        System No. 7 (SS7)'

   [2]  ITU-T Recommendation Q.701-Q.705, 'Signalling System No. 7 (SS7)
        - Message Transfer Part (MTP)'

   [3]  ANSI T1.111 'Signalling System Number 7 - Message Transfer Part'

   [4]  Bellcore GR-246-CORE 'Bell Communications Research Specification
        of Signalling System Number 7', Volume 1, December 1995

   [5]  Telecommunication Technology Committee (TTC) Standard JT-Q704,
        Message Transfer Part Signaling Network Functions, April 28,
        1992.

   [6]  Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
        2279, January 1998.

   [7]  Coded Character Set--7-Bit American Standard Code for
        Information Interchange, ANSI X3.4-1986.

10.2  Informative

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

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

   [10] ITU-T Recommendation Q.2140, 'B-ISDN ATM Adaptation Layer',
        February 1995

   [11] ITU-T Recommendation Q.2210, 'Message transfer part level 3
        functions and messages using the services of ITU-T
        Recommendation Q.2140', August 1995




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   [12] ITU-T Recommendation Q.751.1, 'Network Element Management
        Information Model for the Message Transfer Part', October 1995

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

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











































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Appendix A: Signalling Network Architecture

   A Signalling Gateway will support the transport of MTP2-User
   signalling traffic received from the SS7 network to one or more
   distributed ASPs (e.g., MGCs).  Clearly, the M2UA protocol
   description cannot in itself meet any performance and reliability
   requirements for such transport.  A physical network architecture is
   required, with data on the availability and transfer performance of
   the physical nodes involved in any particular exchange of
   information.  However, the M2UA protocol is flexible enough to allow
   its operation and management in a variety of physical configurations
   that will enable Network Operators to meet their performance and
   reliability requirements.

   To meet the stringent SS7 signalling reliability and performance
   requirements for carrier grade networks, these Network Operators
   should ensure that there is no single point of failure provisioned in
   the end-to-end network architecture between an SS7 node and an IP
   ASP.

   Depending of course on the reliability of the SGP and ASP functional
   elements, this can typically be met by spreading SS7 links in a SS7
   linkset [1] across SGPs or SGs, the provision of redundant QoS-
   bounded IP network paths for SCTP Associations between SCTP End
   Points, and redundant Hosts.  The distribution of ASPs within the
   available Hosts is also important.  For a particular Application
   Server, the related ASPs MAY be distributed over at least two Hosts.
























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   An example of logical network architecture relevant to carrier-grade
   operation in the IP network domain is shown in Figure 7 below:

        **************                              **************
        *  ********__*______________________________*__********  * Host1
   SG1  *  * SGP1 *__*________________       _______*__* ASP1 *  *
        *  ********  *                |     |       *  ********  *
        *      .     *                |     |       *            *
        *      .     *                |     |       **************
        **************                |     |
                                      |     |
        **************                |     |
        *  ********__*______________________|
   SG2  *  * SGP2 *__*________        |
        *  ********  *        |       |
        *      .     *        |       |
        *      .     *        |       |
        **************        |       |             **************
                              |       |_____________*__********  * Host2
                              |_____________________*__* ASP2 *  *
               .                                    *  ********  *
               .            SCTP Associations       *            *
               .                                    **************
                                                            .
                                                            .
                                                            .

                     Figure 7: Logical Model Example

   To avoid a single point of failure, it is recommended that a minimum
   of two ASPs be configured in an AS list, resident in separate hosts
   and, therefore, available over different SCTP associations.  For
   example, in the network shown in Figure 7, all messages for the
   Interface Identifiers could be sent to ASP1 in Host1 or ASP2 in
   Host2.  The AS list at SGP1 might look like the following:

         Interface Identifiers - Application Server #1
             ASP1/Host1  - State = Active
             ASP2/Host2  - State = Inactive

   In this 1+1 redundancy case, ASP1 in Host1 would be sent any incoming
   message for the Interface Identifiers registered.  ASP2 in Host2
   would normally be brought to the active state upon failure of
   ASP1/Host1.  In this example, both ASPs are Inactive or Active,
   meaning that the related SCTP association and far-end M2UA peer is
   ready.





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   For carrier grade networks, Operators should ensure that under
   failure or isolation of a particular ASP, stable calls or
   transactions are not lost.  This implies that ASPs need, in some
   cases, to share the call/-transaction state or be able to pass the
   call/transaction state between each other.  Also, in the case of ASPs
   performing call processing, coordination MAY be required with the
   related Media Gateway to transfer the MGC control for a particular
   trunk termination.  However, this sharing or communication is outside
   the scope of this document.

11.0  Authors' Addresses

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

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


   Ram Dantu, Ph.D.
   NetRake Corporation
   3000 Technology Drive
   Plano, TX 75074
   USA

   Phone: +1-214-291-1111
   EMail: rdantu@netrake.com


   Greg Sidebottom
   Signatus Technologies
   Kanata, Ontario, Canada

   EMail: greg@signatustechnologies.com


   Brian Bidulock
   OpenSS7 Corporation
   1469 Jeffreys Crescent
   Edmonton, AB  T6L 6T1
   Canada

   Phone: +1-780-490-1141
   EMail: bidulock@openss7.org




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   Jacob Heitz
   Lucent Technologies
   1701 Harbor Bay Parkway
   Alameda, CA, 94502
   USA

   Phone: +1-510-747-2917
   EMail: jheitz@lucent.com











































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Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an
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   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

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



















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