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Network Working Group                                      G. Sidebottom
Request for Comments: 3332                         Signatus Technologies
Category: Standards Track                                   K. Morneault
                                                                   Cisco
                                                        J. Pastor-Balbas
                                                                Ericsson
                                                                 Editors
                                                          September 2002


       Signaling System 7 (SS7) Message Transfer Part 3 (MTP3) -
                      User Adaptation Layer (M3UA)

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 memo defines a protocol for supporting the transport of any SS7
   MTP3-User signalling (e.g., ISUP and SCCP messages) over IP using the
   services of the Stream Control Transmission Protocol.  Also,
   provision is made for protocol elements that enable a seamless
   operation of the MTP3-User peers in the SS7 and IP domains. This
   protocol would be used between a Signalling Gateway (SG) and a Media
   Gateway Controller (MGC) or  IP-resident Database, or between two
   IP-based applications.  It is assumed that the SG receives SS7
   signalling over a standard SS7 interface using the SS7 Message
   Transfer Part (MTP) to provide transport.

Table of Contents

   1.  Introduction..................................................3
   1.1 Scope.........................................................3
   1.2 Terminology...................................................4
   1.3 M3UA Overview.................................................6
   1.4 Functional Areas.............................................10
   1.5 Sample Configurations........................................18
   1.6 Definition of M3UA Boundaries................................21
   2.  Conventions..................................................25



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   3.  M3UA Protocol Elements.......................................25
   3.1 Common Message Header........................................26
   3.2 Variable Length Parameter....................................29
   3.3 Transfer Messages............................................31
   3.4 SS7 Signalling Network Management (SSNM) Messages............35
   3.5 ASP State Maintenance (ASPSM) Messages.......................45
   3.6 Routing Key Management (RKM) Messages........................48
   3.7 ASP Traffic Maintenance (ASPTM) Messages.....................59
   3.8 Management (MGMT) Messages...................................63
   4.  Procedures...................................................69
   4.1 Procedures to Support the M3UA-User .........................69
   4.2 Procedures to Support the Management of SCTP Associations ...70
   4.3 AS and ASP State Maintenance.................................72
   4.4 Routing Key Management Procedures............................87
   4.5 Procedures to Support the Availability or Congestion Status
       of SS7 Destination...........................................89
   4.6 MTP3 Restart.................................................92
   5.  Examples of M3UA Procedures..................................93
   5.1 Establishment of Association and Traffic
       Between SGs and ASPs.........................................93
   5.2 ASP traffic Failover Examples................................99
   5.3 Normal Withdrawal of an ASP from an Application Server
       and Teardown of an Association..............................100
   5.4 M3UA/MTP3-User Boundary Examples............................101
   5.5 Examples of IPSP communication..............................105
   6.  Security Considerations.....................................108
   6.1 Introduction................................................108
   6.2 Threats.....................................................108
   6.3 Protecting Confidentiality..................................108
   7.  IANA Considerations.........................................109
   7.1 SCTP Payload Protocol Identifier............................109
   7.2 M3UA Port Number............................................109
   7.3 M3UA Protocol Extensions....................................109
   8. References...................................................111
   8.1 Normative References........................................111
   8.2 Informative References......................................111
   9. Acknowledgements.............................................113
   10. Document Contributors.......................................113
   Appendix A......................................................114
   A.1 Signalling Network Architecture.............................114
   A.2 Redundancy Models...........................................117
   Editors' Addresses..............................................119
   Full Copyright Statement........................................120








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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


1.  Introduction

   This memo defines a protocol for supporting the transport of any SS7
   MTP3-User signalling (e.g., ISUP and SCCP messages) over IP using the
   services of the Stream Control Transmission Protocol [17]. Also,
   provision is made for protocol elements that enable a seamless
   operation of the MTP3-User peers in the SS7 and IP domains.  This
   protocol would be used between a Signalling Gateway (SG) and a Media
   Gaway Controller (MGC) or  IP-resident Database [11], or between two
   IP-based applications.

1.1 Scope

   There is a need for Switched Circuit Network (SCN) signalling
   protocol delivery from an SS7 Signalling Gateway (SG) to a Media
   Gateway Controller (MGC) or IP-resident Database as described in the
   Framework Architecture for Signalling Transport [11].  The delivery
   mechanism should meet the following criteria:

   *  Support for the transfer of all SS7 MTP3-User Part messages (e.g.,
      ISUP [1,2,3], SCCP [4,5,6], TUP [12], etc.)
   *  Support for the seamless operation of MTP3-User protocol peers
   *  Support for the management of SCTP transport associations and
      traffic between an SG and one or more MGCs or IP-resident
      Databases
   *  Support for MGC or IP-resident Database process failover and load
      sharing
   *  Support for the asynchronous reporting of status changes to
      management

   In simplistic transport terms, the SG will terminate SS7 MTP2 and
   MTP3 protocol layers [7,8,9] and deliver ISUP, SCCP and/or any other
   MTP3-User protocol messages, as well as certain MTP network
   management events, over SCTP transport associations to MTP3-User
   peers in MGCs or IP-resident Databases.
















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1.2 Terminology

   Application Server (AS) - A logical entity serving a specific Routing
   Key.  An example of an Application Server is a virtual switch element
   handling all call processing for a unique range of PSTN trunks,
   identified by an SS7 SIO/DPC/OPC/CIC_range.  Another example is a
   virtual database element, handling all HLR transactions for a
   particular SS7 DPC/OPC/SCCP_SSN combination.  The AS contains a set
   of one or more unique Application Server Processes, of which one or
   more is normally actively processing traffic.  Note that there is a
   1:1 relationship between an AS and a Routing Key.

   Application Server Process (ASP) - A process instance of an
   Application Server. An Application Server Process serves as an active
   or backup process of an Application Server (e.g., part of a
   distributed virtual switch or database).  Examples of ASPs are
   processes (or process instances) of MGCs, IP SCPs or IP HLRs.  An ASP
   contains an SCTP endpoint and may be configured to process signalling
   traffic within more than one Application Server.

   Association - An association refers to an SCTP association.  The
   association provides the transport for the delivery of MTP3-User
   protocol data units and M3UA adaptation layer peer messages.

   IP Server Process (IPSP) - A process instance of an IP-based
   application.  An IPSP is essentially the same as an ASP, except that
   it uses M3UA in a point-to-point fashion.  Conceptually, an IPSP does
   not use the services of a Signalling Gateway node.

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

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

   Layer Management - Layer Management is a nodal function that handles
   the inputs and outputs between the M3UA layer and a local management
   entity.









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   Linkset - A number of signalling links that directly interconnect two
   signalling points, which are used as a module.

   MTP - The Message Transfer Part of the SS7 protocol.

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

   MTP3-User - Any protocol normally using the services of the SS7 MTP3
   (e.g., ISUP, SCCP, TUP, etc.).

   Network Appearance - The Network Appearance is a M3UA local reference
   shared by SG and AS (typically an integer) that together with an
   Signaling Point Code uniquely identifies an SS7 node by indicating
   the specific SS7 network it belongs to. It can be used to distinguish
   between signalling traffic associated with different networks being
   sent between the SG and the ASP over a common SCTP association. An
   example scenario is where an SG appears as an element in multiple
   separate national SS7 networks and the same Signaling Point Code
   value may be reused in different networks.

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

   Routing Key: A Routing Key describes a set of SS7 parameters and
   parameter values that uniquely define the range of signalling traffic
   to be handled by a particular Application Server. Parameters within
   the Routing Key cannot extend across more than a single Signalling
   Point Management Cluster.

   Routing Context - A value that uniquely identifies a Routing Key.
   Routing Context values are either configured using a configuration
   management interface, or by using the routing key management
   procedures defined in this document.

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

   Signalling Gateway - An SG is a signaling agent that receives/sends
   SCN native signaling at the edge of the IP network [11].  An SG
   appears to the SS7 network as an SS7 Signalling Point.  An SG
   contains a set of one or more unique Signalling Gateway Processes, of
   which one or more is normally actively processing traffic.  Where an
   SG contains more than one SGP, the SG is a logical entity and the
   contained SGPs are assumed to be coordinated into a single management
   view to the SS7 network and to the supported Application Servers.






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   Signalling Process - A process instance that uses M3UA to communicate
   with other signalling processes.  An ASP, an SGP and an IPSP are all
   signalling processes.

   Signalling Point Management Cluster (SPMC) - The complete set of
   Application Servers represented to the SS7 network under a single MTP
   entity (Signalling Point) in one specific Network Appearance.  SPMCs
   are used to aggregate the availability, congestion, and user part
   status of an MTP entity (Signalling Point) that is distributed in the
   IP domain, for the purpose of supporting MTP3 management procedures
   towards the SS7 network.  In some cases, the SG itself may also be a
   member of the SPMC.  In this case, the SG availability /congestion
   /User_Part status should also be taken into account when considering
   any supporting MTP3 management actions.

   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 M3UA Overview

1.3.1 Protocol Architecture

   The framework architecture that has been defined for SCN signalling
   transport over IP [11] uses multiple components, including a common
   signalling transport protocol and an adaptation module to support the
   services expected by a particular SCN signalling protocol from its
   underlying protocol layer.

   Within the framework architecture, this document defines an MTP3-User
   adaptation module suitable for supporting the transfer of messages of
   any protocol layer that is identified to the MTP Level 3 as an MTP
   User.  The list of these protocol layers includes, but is not limited
   to, ISDN User Part (ISUP) [1,2,3], Signalling Connection Control Part
   (SCCP) [4,5,6] and Telephone User Part (TUP) [12].  TCAP [13,14,15]
   or RANAP [16] messages are transferred transparently by the M3UA
   protocol as SCCP payload, as they are SCCP-User protocols.

   It is recommended that M3UA use the services of the Stream Control
   Transmission Protocol (SCTP) [17] as the underlying reliable common
   signalling transport protocol. This is to take advantage of various
   SCTP features such as:








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      - Explicit packet-oriented delivery (not stream-oriented),
      - Sequenced delivery of user messages within multiple streams,
        with an option for order-of-arrival delivery of individual
        user messages,
      - Optional multiplexing of user messages into SCTP datagrams,
      - Network-level fault tolerance through support of multi-homing
        at either or both ends of an association,
      - Resistance to flooding and masquerade attacks, and
      - Data segmentation to conform to discovered path MTU size.

   Under certain scenarios, such as back-to-back connections without
   redundancy requirements, the SCTP functions above might not be a
   requirement and TCP MAY be used as the underlying common transport
   protocol.

1.3.2 Services Provided by the M3UA Layer

   The M3UA Layer at an ASP or IPSP provides the equivalent set of
   primitives at its upper layer to the MTP3-Users as provided by the
   MTP Level 3 to its local MTP3-Users at an SS7 SEP.  In this way, the
   ISUP and/or SCCP layer at an ASP or IPSP is unaware that the expected
   MTP3 services are offered remotely from an MTP3 Layer at an SGP, and
   not by a local MTP3 layer.  The MTP3 layer at an SGP may also be
   unaware that its local users are actually remote user parts over
   M3UA.  In effect, the M3UA extends access to the MTP3 layer services
   to a remote IP-based application.  The M3UA layer does not itself
   provide the MTP3 services. However, in the case where an ASP is
   connected to more than one SG, the M3UA layer at an ASP should
   maintain the status of configured SS7 destinations and route messages
   according to the availability and congestion status of the routes to
   these destinations via each SG.

   The M3UA layer may also be used for point-to-point signalling between
   two IP Server Processes (IPSPs).  In this case, the M3UA layer
   provides the same set of primitives and services at its upper layer
   as the MTP3. However, in this case the expected MTP3 services are not
   offered remotely from an SGP.  The MTP3 services are provided but the
   procedures to support these services are a subset of the MTP3
   procedures due to the simplified point-to-point nature of the IPSP to
   IPSP relationship.











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1.3.2.1 Support for the Transport of MTP3-User Messages

   The M3UA layer provides the transport of MTP-TRANSFER primitives
   across an established SCTP association between an SGP and an ASP or
   between IPSPs.

   At an ASP, in the case where a destination is reachable via multiple
   SGPs, the M3UA layer must also choose via which SGP the message is to
   be routed or support load balancing across the SGPs, minimizing
   missequencing.

   The M3UA layer does not impose a 272-octet signalling information
   field (SIF) length limit as specified by the SS7 MTP Level 2 protocol
   [7,8,9]. Larger information blocks can be accommodated directly by
   M3UA/SCTP, without the need for an upper layer segmentation/re-
   assembly procedure as specified in recent SCCP or ISUP versions.
   However, in the context of an SG, the maximum 272-octet block size
   must be followed when interworking to a SS7 network that does not
   support the transfer of larger information blocks to the final
   destination.  This avoids potential ISUP or SCCP fragmentation
   requirements at the SGPs.  The provisioning and configuration of the
   SS7 network determines the restriction placed on the maximum block
   size.  Some configurations (e.g., Broadband MTP [21]) may permit
   larger block sizes.

1.3.2.2 Native Management Functions

   The M3UA layer provides the capability to indicate errors associated
   with received M3UA messages and to notify, as appropriate, local
   management and/or the peer M3UA.

1.3.2.3 Interworking with MTP3 Network Management Functions

   At the SGP, the M3UA layer provides interworking with MTP3 management
   functions to support seamless operation of the user SCN signalling
   applications in the SS7 and IP domains.  This includes:

   - Providing an indication to MTP3-Users at an ASP that a destination
     in the SS7 network is not reachable.

   - Providing an indication to MTP3-Users at an ASP that a destination
     in the SS7 network is now reachable.

   - Providing an indication to MTP3-Users at an ASP that messages to a
     destination in the SS7 network are experiencing SS7 congestion.






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   - Providing an indication to the M3UA layer at an ASP that the routes
     to a destination in the SS7 network are restricted.

   - Providing an indication to MTP3-Users at an ASP that a MTP3-User
     peer is unavailable.

   The M3UA layer at an ASP keeps the state of the routes to remote SS7
   destinations and may initiate an audit of the availability, the
   restricted or the congested state of remote SS7 destinations.  This
   information is requested from the M3UA layer at the SGP.

   The M3UA layer at an ASP may also indicate to the SG that the M3UA
   layer itself or the ASP or the ASP's Host is congested.

1.3.2.4 Support for the Management of SCTP Associations between the SGP
        and ASPs.

   The M3UA layer at the SGP maintains the availability state of all
   configured remote ASPs, to manage the SCTP Associations and the
   traffic between the M3UA peers.  As well, the active/inactive and
   congestion state of remote ASPs is maintained.

   The M3UA layer MAY be instructed by local management to establish an
   SCTP association to a peer M3UA node.  This can be achieved using the
   M-SCTP_ESTABLISH primitives (See Section 1.6.3 for a description of
   management primitives.) to request, indicate and confirm the
   establishment of an SCTP association with a peer M3UA node.  In order
   to avoid redundant SCTP associations between two M3UA peers, one side
   (client) SHOULD be designated to establish the SCTP association, or
   M3UA configuration information maintained to detect redundant
   associations (e.g., via knowledge of the expected local and remote
   SCTP endpoint addresses).

   Local management MAY request from the M3UA layer the status of the
   underlying SCTP associations using the M-SCTP_STATUS request and
   confirm primitives.  Also, the M3UA MAY autonomously inform local
   management of the reason for the release of an SCTP association,
   determined either locally within the M3UA layer or by a primitive
   from the SCTP.

   Also the M3UA layer MAY inform the local management of the change in
   status of an ASP or AS.  This MAY be achieved using the M-ASP_STATUS
   request or M-AS_STATUS request primitives.








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1.3.2.5 Support for the Management of Connections to Multiple SGPs

   As shown in Figure 1 an ASP may be connected to multiple SGPs. In
   such a case a particular SS7 destination may be reachable via more
   than one SGP and/or SG, i.e., via more than one route. As MTP3 users
   only maintain status on a destination and not on a route basis, the
   M3UA layer must maintain the status (availability, restriction,
   and/or congestion of route to destination) of the individual routes,
   derive the overall availability or congestion status of the
   destination from the status of the individual routes, and inform the
   MTP3 users of this derived status whenever it changes.

1.4 Functional Areas

1.4.1 Signalling Point Code Representation

   For example, within an SS7 network, a Signalling Gateway might be
   charged with representing a set of nodes in the IP domain into the
   SS7 network for routing purposes.  The SG itself, as a signalling
   point in the SS7 network, might also be addressable with an SS7 Point
   Code for MTP3 Management purposes. The SG Point Code might also be
   used for addressing any local MTP3-Users at the SG such as a local
   SCCP layer.

   An SG may be logically partitioned to operate in multiple SS7 network
   appearances.  In such a case, the SG could be addressable with a
   Point Code in each network appearance, and represents a set of nodes
   in the IP domain into each SS7 network.  Alias Point Codes [8] may
   also be used within an SG network appearance.

   Where an SG contains more than one SGP, the MTP3 routeset, SPMC and
   remote AS/ASP states of each SGP SHOULD be coordinated across all the
   SGPs.  Rerouting of traffic between the SGPs MAY also be supported.

   Application Servers can be represented under the same Point Code of
   the SG, their own individual Point Codes or grouped with other
   Application Servers for Point Code preservation purposes.  A single
   Point Code may be used to represent the SG and all the Application
   Servers together, if desired.

   If an ASP or group of ASPs is available to the SS7 network via more
   than one SG, each with its own Point Code, the ASP(s) will typically
   be represented by a Point Code that is separate from any SG Point
   Code. This allows, for example, these SGs to be viewed from the SS7
   network as "STPs", each having an ongoing "route" to the same ASP(s).
   Under failure conditions where the ASP(s) become(s) unavailable from
   one of the SGs, this approach enables MTP3 route management messaging
   between the SG and SS7 network, allowing simple SS7 rerouting through



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   an alternate SG without changing the Destination Point Code Address
   of SS7 traffic to the ASP(s).

   Where a particular AS can be reached via more than one SGP, the
   corresponding Routing Keys in the SGPs should be identical.  (Note:
   It is possible for the SGP Routing Key configuration data to be
   temporarily out-of-sync during configuration updates).

                              +--------+
                              |        |
                 +------------+  SG 1  +--------------+
     +-------+   |  SS7 links | "STP"  |  IP network  |     ----
     |  SEP  +---+            +--------+              +---/      \
     |   or  |                    |*                      | ASPs  |
     |  STP  +---+            +--------+              +---\      /
     +-------+   |            |        |              |     ----
                 +------------+  SG 2  +--------------+
                              | "STP"  |
                              +--------+

                    Figure 1  Example with mated SGs

   * Note:.  SG-to-SG communication (i.e., "C-links") is recommended for
   carrier grade networks, using an MTP3 linkset or an equivalent, to
   allow rerouting between the SGs in the event of route failures. Where
   SGPs are used, inter-SGP communication might be used.  Inter-SGP
   protocol is outside of the scope of this document.

   The following example shows a signalling gateway partitioned into two
   network appearances.

                                  SG
     +-------+              +---------------+
     |  SEP  +--------------| SS7 Ntwk |M3UA|              ----
     +-------+   SS7 links  |   "A"    |    |            /      \
                            |__________|    +-----------+  ASPs  |
                            |          |    |            \      /
     +-------+              | SS7 Ntwk |    |              ----
     |  SEP  +--------------+   "B"    |    |
     +-------+              +---------------+

                    Figure 2  Example with multiple Network









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1.4.2 Routing Contexts and Routing Keys

1.4.2.1 Overview

   The distribution of SS7 messages between the SGP and the Application
   Servers is determined by the Routing Keys and their associated
   Routing Contexts.  A Routing Key is essentially a set of SS7
   parameters used to filter SS7 messages, whereas the Routing Context
   parameter is a 4-byte value (integer) that is associated to that
   Routing Key in a 1:1 relationship.  The Routing Context therefore can
   be viewed as an index into a sending node's Message Distribution
   Table containing the Routing Key entries.

   Possible SS7 address/routing information that comprise a Routing Key
   entry includes, for example, the OPC, DPC, SIO found in the MTP3
   routing label, or MTP3-User specific fields (such as the ISUP CIC,
   SCCP subsystem number).  Some example Routing Keys are: the DPC
   alone, the DPC/OPC combination, the DPC/OPC/CIC combination, or the
   DPC/SSN combination.  The particular information used to define an
   M3UA Routing Key is application and network dependent, and none of
   the above examples are mandated.

   An Application Server Process may be configured to process signalling
   traffic related to more than one Application Server, over a single
   SCTP Association.  In ASP Active and ASP Inactive management
   messages, the signalling traffic to be started or stopped is
   discriminated by the Routing Context parameter.  At an ASP, the
   Routing Context parameter uniquely identifies the range of signalling
   traffic associated with each Application Server that the ASP is
   configured to receive.

1.4.2.2 Routing Key Limitations

   Routing Keys SHOULD be unique in the sense that each received SS7
   signalling message SHOULD have a full or partial match to a single
   routing result. It is not necessary for the parameter range values
   within a particular Routing Key to be contiguous.  For example, an AS
   could be configured to support call processing for multiple ranges of
   PSTN trunks that are not represented by contiguous CIC values.

1.4.2.3 Managing Routing Contexts and Routing Keys

   There are two ways to provision a Routing Key at an SGP.  A Routing
   Key may be configured statically using an implementation dependent
   management interface, or dynamically using the M3UA Routing Key
   registration procedure.





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

1.4.2.4 Message Distribution at the SGP

   To direct messages received from the SS7 MTP3 network to the
   appropriate IP destination, the SGP must perform a message
   distribution function using information from the received MTP3-User
   message.

   To support this message distribution, the SGP might, for example,
   maintain the equivalent of a network address translation table,
   mapping incoming SS7 message information to an Application Server for
   a particular application and range of traffic.  This could be
   accomplished by comparing elements of the incoming SS7 message to
   currently defined Routing Keys in the SGP.

   These Routing Keys could in turn map directly to an Application
   Server that is enabled by one or more ASPs.  These ASPs provide
   dynamic status information regarding their availability, traffic
   handling capability and congestion to the SGP using various
   management messages defined in the M3UA protocol.

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

   Normally, one or more ASPs are active (i.e., currently processing
   traffic) in the AS but in certain failure and transition cases it is
   possible that there may be no active ASP available.  Broadcast,
   loadsharing and backup scenarios are supported.

   When there is no matching Routing Key entry for an incoming SS7
   message, a default treatment MAY be specified.  Possible solutions
   are to provide a default Application Server at the SGP that directs
   all unallocated traffic to a (set of) default ASP(s), or to drop the
   message and provide a notification to layer management.  The
   treatment of unallocated traffic is implementation dependent.










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1.4.2.5 Message Distribution at the ASP

   The ASP must choose an SGP to direct a message to the SS7 network.
   This is accomplished by observing the Destination Point Code (and
   possibly other elements of the outgoing message such as the SLS
   value). The ASP must also take into account whether the related
   Routing Context is active or not (See Section 4.3.4.3).

   Implementation Note: Where more than one route (or SGP) is possible
   for routing to the SS7 network, the ASP could, for example, maintain
   a dynamic table of available SGP routes for the SS7 destinations,
   taking into account the SS7 destination
   availability/restricted/congestion status received from the SGP(s),
   the availability status of the individual SGPs and configuration
   changes and failover mechanisms. There is, however, no M3UA messaging
   to manage the status of an SGP (e.g., SGP-Up/Down/Active/Inactive
   messaging).

   Whenever an SCTP association to an SGP exists, the SGP is assumed to
   be ready for the purposes of responding to M3UA ASPSM messages (Refer
   to Section 3).

1.4.3 SS7 and M3UA Interworking

   In the case of SS7 and M3UA interworking, the M3UA adaptation layer
   is designed to provide an extension of the MTP3 defined user
   primitives.

1.4.3.1 Signalling Gateway SS7 Layers

   The SG is responsible for terminating MTP Level 3 of the SS7
   protocol, and offering an IP-based extension to its users.

   From an SS7 perspective, it is expected that the Signalling Gateway
   transmits and receives SS7 Message Signalling Units (MSUs) to and
   from the PSTN over a standard SS7 network interface, using the SS7
   Message Transfer Part (MTP) [7,8,9] to provide reliable transport of
   the messages.

   As a standard SS7 network interface, the use of MTP Level 2
   signalling links is not the only possibility.  ATM-based High Speed
   Links can also be used with the services of the Signalling ATM
   Adaptation Layer (SAAL) [18,19].

   Note: It is also possible for IP-based interfaces to be present,
   using the services of the MTP2-User Adaptation Layer (M2UA) [27] or
   M2PA [28].




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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


   These could be terminated at a Signalling Transfer Point (STP) or
   Signalling End Point (SEP).  Using the services of MTP3, the SG could
   be capable of communicating with remote SS7 SEPs in a quasi-
   associated fashion, where STPs may be present in the SS7 path between
   the SEP and the SG.

1.4.3.2 SS7 and M3UA Interworking at the SG

   The SGP provides a functional interworking of transport functions
   between the SS7 network and the IP network by also supporting the
   M3UA adaptation layer.  It allows the transfer of MTP3-User
   signalling messages to and from an IP-based Application Server
   Process where the peer MTP3-User protocol layer exists.

   For SS7 user part management, it is required that the MTP3-User
   protocols at ASPs receive indications of SS7 signalling point
   availability, SS7 network congestion, and remote User Part
   unavailability as would be expected in an SS7 SEP node.  To
   accomplish this, the MTP-PAUSE, MTP-RESUME and MTP-STATUS indication
   primitives received at the MTP3 upper layer interface at the SG need
   to be propagated to the remote MTP3-User lower layer interface at the
   ASP.

   MTP3 management messages (such as TFPs or TFAs received from the SS7
   network) MUST NOT be encapsulated as Data message Payload Data and
   sent either from SG to ASP or from ASP to SG.  The SG MUST terminate
   these messages and generate M3UA messages as appropriate.

1.4.3.3 Application Server

   A cluster of application servers is responsible for providing the
   overall support for one or more SS7 upper layers.  From an SS7
   standpoint, a Signalling Point Management Cluster (SPMC) provides
   complete support for the upper layer service for a given point code.
   As an example, an SPMC providing MGC capabilities could provide
   complete support for ISUP (and any other MTP3 user located at the
   point code of the SPMC) for a given point code.

   In the case where an ASP is connected to more than one SGP, the M3UA
   layer must maintain the status of configured SS7 destinations and
   route messages according to availability/congestion/restricted status
   of the routes to these SS7 destinations.

1.4.3.4 IPSP Considerations

   Since IPSPs use M3UA in a point-to-point fashion, there is no concept
   of routing of messages beyond the remote end.  Therefore, SS7 and
   M3UA interworking is not necessary for this model.



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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


1.4.4 Redundancy Models

1.4.4.1 Application Server Redundancy

   All MTP3-User messages (e.g., ISUP, SCCP) which match a provisioned
   Routing Key at an SGP are mapped to an Application Server.

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

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

1.4.5 Flow Control

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

1.4.6 Congestion Management

   The M3UA layer is informed of local and IP network congestion by
   means of an implementation-dependent function (e.g., an
   implementation dependent indication from the SCTP of IP network
   congestion).

   At an ASP or IPSP, the M3UA layer indicates congestion to local
   MTP3-Users by means of an MTP-STATUS primitive, as per current MTP3
   procedures, to invoke appropriate upper layer responses.

   When an SG determines that the transport of SS7 messages to a
   Signalling Point Management Cluster (SPMC) is encountering
   congestion, the SG MAY trigger SS7 MTP3 Transfer Controlled
   management messages to originating SS7 nodes, per the congestion
   procedures of the relevant MTP3 standard. The triggering of SS7 MTP3
   Management messages from an SG is an implementation-dependent
   function.






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   The M3UA layer at an ASP or IPSP MAY indicate local congestion to an
   M3UA peer with an SCON message.  When an SG receives a congestion
   message (SCON) from an ASP, and the SG determines that an SPMC is now
   encountering congestion, it MAY trigger SS7 MTP3 Transfer Controlled
   management messages to concerned SS7 destinations according to
   congestion procedures of the relevant MTP3 standard.

1.4.7 SCTP Stream Mapping.

   The M3UA layer at both the SGP and ASP also supports the assignment
   of signalling traffic into streams within an SCTP association.
   Traffic that requires sequencing SHOULD be assigned to the same
   stream.  To accomplish this, MTP3-User traffic may be assigned to
   individual streams based on, for example, the SLS value in the MTP3
   Routing Label or the ISUP CIC assignment, subject of course to the
   maximum number of streams supported by the underlying SCTP
   association.

1.4.8 Client/Server Model

   It is recommended that the SGP and ASP be able to support both client
   and server operation. The peer endpoints using M3UA 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.

   In the case of IPSP to IPSP communication, the peer endpoints using
   M3UA 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 SCTP and TCP Registered User Port Number Assignment for M3UA is
   2905.
















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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


1.5 Sample Configuration

1.5.1 Example 1: ISUP Message Transport

   ********   SS7   *****************   IP   ********
   * SEP  *---------*      SGP      *--------* ASP  *
   ********         *****************        ********

   +------+         +---------------+        +------+
   | ISUP |         |     (NIF)     |        | ISUP |
   +------+         +------+ +------+        +------+
   | MTP3 |         | MTP3 | | M3UA |        | M3UA |
   +------|         +------+-+------+        +------+
   | MTP2 |         | MTP2 | | SCTP |        | SCTP |
   +------+         +------+ +------+        +------+
   |  L1  |         |  L1  | |  IP  |        |  IP  |
   +------+         +------+ +------+        +------+
       |_______________|         |______________|


   SEP - SS7 Signalling End Point
   SCTP - Stream Control Transmission Protocol
   NIF - Nodal Interworking Function

   In this example, the SGP provides an implementation-dependent nodal
   interworking function (NIF) that allows the MGC to exchange SS7
   signalling messages with the SS7-based SEP.  The NIF within the SGP
   serves as the interface within the SGP between the MTP3 and M3UA.
   This nodal interworking function has no visible peer protocol with
   either the MGC or SEP.  It also provides network status information
   to one or both sides of the network.

   For internal SGP modeling purposes, at the NIF level, SS7 signalling
   messages that are destined to the MGC are received as MTP-TRANSFER
   indication primitives from the MTP Level 3 upper layer interface,
   translated to MTP-TRANSFER request primitives, and sent to the local
   M3UA-resident message distribution function for ongoing routing to
   the final IP destination.  Messages received from the local M3UA
   network address translation and mapping function as MTP-TRANSFER
   indication primitives are sent to the MTP Level 3 upper layer
   interface as MTP-TRANSFER request primitives for ongoing MTP Level 3
   routing to an SS7 SEP.  For the purposes of providing SS7 network
   status information the NIF also delivers MTP-PAUSE, MTP-RESUME and
   MTP-STATUS indication primitives received from the MTP Level 3 upper
   layer interface to the local M3UA-resident management function. In
   addition, as an implementation and network option, restricted
   destinations are communicated from MTP network management to the
   local M3UA-resident management function.



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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


1.5.2  Example 2: SCCP Transport between IPSPs

         ********    IP    ********
         * IPSP *          * IPSP *
         ********          ********

         +------+          +------+
         |SCCP- |          |SCCP- |
         | User |          | User |
         +------+          +------+
         | SCCP |          | SCCP |
         +------+          +------+
         | M3UA |          | M3UA |
         +------+          +------+
         | SCTP |          | SCTP |
         +------+          +------+
         |  IP  |          |  IP  |
         +------+          +------+
             |________________|

   This example shows an architecture where no Signalling Gateway is
   used. In this example, SCCP messages are exchanged directly between
   two IP-resident IPSPs with resident SCCP-User protocol instances,
   such as RANAP or TCAP.  SS7 network interworking is not required,
   therefore there is no MTP3 network management status information for
   the SCCP and SCCP-User protocols to consider.  Any MTP-PAUSE, MTP-
   RESUME or MTP-STATUS indications from the M3UA layer to the SCCP
   layer should consider the status of the SCTP Association and
   underlying IP network and any congestion information received from
   the remote site.





















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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


1.5.3 Example 3: SGP Resident SCCP Layer, with Remote ASP

      ********   SS7   *****************   IP   ********
      * SEP  *---------*               *--------*      *
      *  or  *         *      SGP      *        * ASP  *
      * STP  *         *               *        *      *
      ********         *****************        ********

      +------+         +---------------+        +------+
      | SCCP-|         |     SCCP      |        | SCCP-|
      | User |         +---------------+        | User |
      +------+           |   _____   |          +------+
      | SCCP |           |  |     |  |          | SCCP |
      +------+         +------+-+------+        +------+
      | MTP3 |         | MTP3 | | M3UA |        | M3UA |
      +------|         +------+ +------+        +------+
      | MTP2 |         | MTP2 | | SCTP |        | SCTP |
      +------+         +------+ +------+        +------+
      |  L1  |         |  L1  | |  IP  |        |  IP  |
      +------+         +------+ +------+        +------+
          |_______________|         |______________|

   STP - SS7 Signalling Transfer Point

   In this example, the SGP contains an instance of the SS7 SCCP
   protocol layer that may, for example, perform the SCCP Global Title
   Translation (GTT) function for messages logically addressed to the SG
   SCCP.  If the result of a GTT for an SCCP message yields an SS7 DPC
   or DPC/SSN address of an SCCP peer located in the IP domain, the
   resulting MTP-TRANSFER request primitive is sent to the local M3UA-
   resident network address translation and mapping function for ongoing
   routing to the final IP destination.

   Similarly, the SCCP instance in an SGP can perform the SCCP GTT
   service for messages logically addressed to it from SCCP peers in the
   IP domain.  In this case, MTP-TRANSFER indication primitives are sent
   from the local M3UA-resident network address translation and mapping
   function to the SCCP for GTT.  If the result of the GTT yields the
   address of an SCCP peer in the SS7 network then the resulting MTP-
   TRANSFER request primitive is given to the MTP3 for delivery to an
   SS7-resident node.

   It is possible that the above SCCP GTT at the SGP could yield the
   address of an SCCP peer in the IP domain and the resulting MTP-
   TRANSFER request primitive would be sent back to the M3UA layer for
   delivery to an IP destination.





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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


   For internal SGP modeling purposes, this may be accomplished with the
   use of an implementation-dependent nodal interworking function within
   the SGP that effectively sits below the SCCP and routes MTP-TRANSFER
   request/indication messages to/from both the MTP3 and the M3UA layer,
   based on the SS7 DPC or DPC/SSN address information.  This nodal
   interworking function has no visible peer protocol with either the
   ASP or SEP.

   Note that the services and interface provided by the M3UA layer are
   the same as in Example 1 and the functions taking place in the SCCP
   entity are transparent to the M3UA layer.  The SCCP protocol
   functions are not reproduced in the M3UA protocol.

1.6 Definition of M3UA Boundaries

1.6.1 Definition of the Boundary between M3UA and an MTP3-User.

   From ITU Q.701 [7]:

      MTP-TRANSFER request
      MTP-TRANSFER indication
      MTP-PAUSE indication
      MTP-RESUME indication
      MTP-STATUS indication

1.6.2 Definition of the Boundary between M3UA and SCTP

   An example of the upper layer primitives provided by the SCTP are
   provided in Reference [17] Section 10.

1.6.3 Definition of the Boundary between M3UA and Layer Management

   M-SCTP_ESTABLISH request
   Direction: LM -> M3UA
   Purpose: LM requests ASP to establish an SCTP association with its
   peer.

   M-STCP_ESTABLISH confirm
   Direction: M3UA -> LM
   Purpose: ASP confirms to LM that it has established an SCTP
   association with its peer.

   M-SCTP_ESTABLISH indication
   Direction: M3UA -> LM
   Purpose: M3UA informs LM that a remote ASP has established an SCTP
   association.





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   M-SCTP_RELEASE request
   Direction: LM -> M3UA
   Purpose: LM requests ASP to release an SCTP association with its
   peer.

   M-SCTP_RELEASE confirm
   Direction: M3UA -> LM
   Purpose: ASP confirms to LM that it has released SCTP association
   with its peer.

   M-SCTP_RELEASE indication
   Direction: M3UA -> LM
   Purpose: M3UA informs LM that a remote ASP has released an SCTP
   Association or the SCTP association has failed.

   M-SCTP_RESTART indication
   Direction: M3UA -> LM
   Purpose: M3UA informs LM that an SCTP restart indication has been
   received.

   M-SCTP_STATUS request
   Direction: LM -> M3UA
   Purpose: LM requests M3UA to report the status of an SCTP
   association.

   M-SCTP_STATUS confirm
   Direction: M3UA -> LM
   Purpose: M3UA responds with the status of an SCTP association.


   M-SCTP STATUS indication
   Direction: M3UA -> LM
   Purpose: M3UA reports the status of an SCTP association.

   M-ASP_STATUS request
   Direction: LM -> M3UA
   Purpose: LM requests M3UA to report the status of a local or remote
   ASP.

   M-ASP_STATUS confirm
   Direction: M3UA -> LM
   Purpose: M3UA reports status of local or remote ASP.

   M-AS_STATUS request
   Direction: LM -> M3UA
   Purpose: LM requests M3UA to report the status of an AS.





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   M-AS_STATUS confirm
   Direction: M3UA -> LM
   Purpose: M3UA reports the status of an AS.

   M-NOTIFY indication
   Direction: M3UA -> LM
   Purpose: M3UA reports that it has received a Notify message
   from its peer.

   M-ERROR indication
   Direction: M3UA -> LM
   Purpose: M3UA reports that it has received an Error message from
   its peer or that a local operation has been unsuccessful.

   M-ASP_UP request
   Direction: LM -> M3UA
   Purpose: LM requests ASP to start its operation and send an ASP Up
   message to its peer.

   M-ASP_UP confirm
   Direction: M3UA -> LM
   Purpose: ASP reports that is has received an ASP UP Ack message from
   its peer.

   M-ASP_UP indication
   Direction: M3UA -> LM
   Purpose: M3UA reports it has successfully processed an incoming ASP
   Up message from its peer.

   M-ASP_DOWN request
   Direction: LM -> M3UA
   Purpose: LM requests ASP to stop its operation and send an ASP Down
   message to its peer.

   M-ASP_DOWN confirm
   Direction: M3UA -> LM
   Purpose: ASP reports that is has received an ASP Down Ack message
   from its peer.

   M-ASP_DOWN indication
   Direction: M3UA -> LM
   Purpose: M3UA reports it has successfully processed an incoming ASP
   Down message from its peer, or the SCTP association has
   been lost/reset.







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   M-ASP_ACTIVE request
   Direction: LM -> M3UA
   Purpose: LM requests ASP to send an ASP Active message to its peer.

   M-ASP_ACTIVE confirm
   Direction: M3UA -> LM
   Purpose: ASP reports that is has received an ASP Active
   Ack message from its peer.

   M-ASP_ACTIVE indication
   Direction: M3UA -> LM
   Purpose: M3UA reports it has successfully processed an incoming ASP
   Active message from its peer.

   M-ASP_INACTIVE request
   Direction: LM -> M3UA
   Purpose: LM requests ASP to send an ASP Inactive message to its
   peer.

   M-ASP_INACTIVE confirm
   Direction: LM -> M3UA
   Purpose: ASP reports that is has received an ASP Inactive
   Ack message from its peer.

   M-ASP_INACTIVE indication
   Direction: M3UA -> LM
   Purpose: M3UA reports it has successfully processed an incoming ASP
   Inactive message from its peer.

   M-AS_ACTIVE indication
   Direction: M3UA -> LM
   Purpose: M3UA reports that an AS has moved to the AS-ACTIVE state.

   M-AS_INACTIVE indication
   Direction: M3UA -> LM
   Purpose: M3UA reports that an AS has moved to the AS-INACTIVE state.

   M-AS_DOWN indication
   Direction: M3UA -> LM
   Purpose: M3UA reports that an AS has moved to the AS-DOWN state.

   If dynamic registration of RK is supported by the M3UA layer, the
   layer MAY support the following additional primitives:

   M-RK_REG request
   Direction: LM -> M3UA
   Purpose: LM requests ASP to register RK(s) with its peer by sending
   REG REQ message



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   M-RK_REG confirm
   Direction: M3UA -> LM
   Purpose: ASP reports that it has received REG RSP message with
   registration status as successful from its peer.

   M-RK_REG indication
   Direction: M3UA -> LM
   Purpose: M3UA informs LM that it has successfully processed an
   incoming REG REQ message.

   M-RK_DEREG request
   Direction: LM -> M3UA
   Purpose: LM requests ASP to deregister RK(s) with its peer by
   sending DEREG REQ message.

   M-RK_DEREG confirm
   Direction: M3UA -> LM
   Purpose: ASP reports that it has received DEREG REQ message with
   deregistration status as successful from its peer.

   M-RK_DEREG indication
   Direction: M3UA -> LM
   Purpose: M3UA informs LM that it has successfully processed an
   incoming DEREG REQ from its peer.

2. Conventions

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when
   they appear in this document, are to be interpreted as described in
   [20].

3. M3UA Protocol Elements

   The general M3UA message format includes a Common Message Header
   followed by zero or more parameters as defined by the Message Type.
   For forward compatibility, all Message Types may have attached
   parameters even if none are specified in this version.













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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


3.1 Common Message Header

   The protocol messages for MTP3-User Adaptation require a message
   header which contains the adaptation layer version, the message type,
   and message length.

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

   All fields in an M3UA message MUST be transmitted in the network byte
   order, unless otherwise stated.

3.1.1 M3UA Protocol Version: 8 bits (unsigned integer)

   The version field contains the version of the M3UA adaptation layer.

   The supported versions are the following:

         1      Release 1.0

3.1.2  Message Classes and Types

   The following list contains the valid Message Classes:

   Message Class: 8 bits (unsigned integer)

   The following list contains the valid Message Type Classes:

        0     Management (MGMT) Messages
        1     Transfer Messages
        2     SS7 Signalling Network Management (SSNM) Messages
        3     ASP State Maintenance (ASPSM) Messages
        4     ASP Traffic Maintenance (ASPTM) Messages
        5     Reserved for Other Sigtran Adaptation Layers
        6     Reserved for Other Sigtran Adaptation Layers
        7     Reserved for Other Sigtran Adaptation Layers
        8     Reserved for Other Sigtran Adaptation Layers
        9     Routing Key Management (RKM) Messages
       10 to 127 Reserved by the IETF
      128 to 255 Reserved for IETF-Defined Message Class extensions




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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


      Message Type: 8 bits (unsigned integer)

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

      Management (MGMT) Messages (See Section 3.8)

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

      Transfer Messages (See Section 3.3)

        0        Reserved
        1        Payload Data (DATA)
     2 to 127    Reserved by the IETF
   128 to 255    Reserved for IETF-Defined Transfer extensions

      SS7 Signalling Network Management (SSNM) Messages (See Section
      3.4)

        0        Reserved
        1        Destination Unavailable (DUNA)
        2        Destination Available (DAVA)
        3        Destination State Audit (DAUD)
        4        Signalling Congestion (SCON)
        5        Destination User Part Unavailable (DUPU)
        6        Destination Restricted (DRST)
     7 to 127    Reserved by the IETF
   128 to 255    Reserved for IETF-Defined SSNM extensions

      ASP State Maintenance (ASPSM) Messages (See Section 3.5)

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








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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


      ASP Traffic Maintenance (ASPTM) Messages (See Section 3.7)

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

      Routing Key Management (RKM) Messages (See Section 3.6)

        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 RKM extensions

3.1.3  Reserved: 8 bits

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

3.1.4  Message Length: 32-bits (unsigned integer)

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

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


















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3.2 Variable Length Parameter Format

   M3UA messages consist of a Common Header followed by zero or more
   variable length parameters, as defined by the message type.  All the
   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                         /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where more than one parameter is included in a message, the
   parameters may be in any order, except where explicitly mandated.  A
   receiver SHOULD accept the parameters in any order.

   Parameter Tag: 16 bits (unsigned integer)

      The Tag field is a 16-bit identifier of the type of parameter. It
      takes a value of 0 to 65534.  Common parameters used by adaptation
      layers are in the range of 0x00 to 0x3f.   M3UA-specific
      parameters have Tags in the range 0x0200 to 0x02ff.  The parameter
      Tags defined are as follows:

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

        Parameter Name                     Parameter ID
        ==============                     ============
        Reserved                              0x0000
        Not Used in M3UA                      0x0001
        Not Used in M3UA                      0x0002
        Not Used in M3UA                      0x0003
        INFO String                           0x0004
        Not Used in M3UA                      0x0005
        Routing Context                       0x0006
        Diagnostic Information                0x0007
        Not Used in M3UA                      0x0008
        Heartbeat Data                        0x0009
        Not Used in M3UA                      0x000a
        Traffic Mode Type                     0x000b
        Error Code                            0x000c
        Status                                0x000d



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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


        Not Used in M3UA                      0x000e
        Not Used in M3UA                      0x000f
        Not Used in M3UA                      0x0010
        ASP Identifier                        0x0011
        Affected Point Code                   0x0012
        Correlation ID                        0x0013

      M3UA-Specific parameters.  These TLV parameters are specific to
      the M3UA protocol:

        Network Appearance                    0x0200
        Reserved                              0x0201
        Reserved                              0x0202
        Reserved                              0x0203
        User/Cause                            0x0204
        Congestion Indications                0x0205
        Concerned Destination                 0x0206
        Routing Key                           0x0207
        Registration Result                   0x0208
        Deregistration Result                 0x0209
        Local_Routing Key Identifier          0x020a
        Destination Point Code                0x020b
        Service Indicators                    0x020c
        Reserved                              0x020d
        Originating Point Code List           0x020e
        Circuit Range                         0x020f
        Protocol Data                         0x0210
        Reserved                              0x0211
        Registration Status                   0x0212
        Deregistration Status                 0x0213

        Reserved by the IETF             0x0214 to 0xffff

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

   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.







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   Parameter Value: variable length.

      The Parameter Value field contains the actual information to be
      transferred in the parameter.

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

3.3 Transfer Messages

   The following section describes the Transfer messages and parameter
   contents.

3.3.1 Payload Data Message (DATA)

   The DATA message contains the SS7 MTP3-User protocol data, which is
   an MTP-TRANSFER primitive, including the complete MTP3 Routing Label.
   The DATA message contains the following variable length parameters:

      Network Appearance       Optional
      Routing Context          Optional
      Protocol Data            Mandatory
      Correlation Id           Optional























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   The following format MUST be used for the Data 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 = 0x0200           |          Length = 8           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Network Appearance                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Tag = 0x0006           |          Length = 8           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Routing Context                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Tag = 0x0210           |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                        Protocol Data                          /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Tag = 0x0013           |          Length = 8           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Correlation Id                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Network Appearance: 32-bits (unsigned integer)

      The Network Appearance parameter identifies the SS7 network
      context for the message and implicitly identifies the SS7 Point
      Code format used, the SS7 Network Indicator value, and the MTP3
      and possibly the MTP3-User protocol type/variant/version used
      within the specific SS7 network.  Where an SG operates in the
      context of a single SS7 network, or individual SCTP associations
      are dedicated to each SS7 network context, the Network Appearance
      parameter is not required.  In other cases the parameter may be
      configured to be present for the use of the receiver.

      The Network Appearance parameter value is of local significance
      only, coordinated between the SGP and ASP. Therefore, in the case
      where an ASP is connected to more than one SGP, the same SS7
      network context may be identified by different Network Appearance
      values depending over which SGP a message is being
      transmitted/received.

      Where the optional Network Appearance parameter is present, it
      must be the first parameter in the message as it defines the
      format of the Protocol Data field.





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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


      IMPLEMENTATION NOTE: For simplicity of configuration it may be
      desirable to use the same NA value across all nodes sharing a
      particular network context.

   Routing Context: 32-bits (unsigned integer)

      The Routing Context parameter contains the Routing Context value
      associated with the DATA message.   Where a Routing Key has not
      been coordinated between the SGP and ASP, sending of Routing
      Context is not required.  Where multiple Routing Keys and Routing
      Contexts are used across a common association, the Routing Context
      MUST be sent to identify the traffic flow, assisting in the
      internal distribution of Data messages.

   Protocol Data: variable length

      The Protocol Data parameter contains the original SS7 MTP3
      message, including the Service Information Octet and Routing
      Label.

      The Protocol Data parameter contains the following fields:

         Service Indicator,
         Network Indicator,
         Message Priority.

         Destination Point Code,
         Originating Point Code,

         Signalling Link Selection Code (SLS).

         User Protocol Data.  Includes:

            MTP3-User protocol elements (e.g., ISUP, SCCP, or TUP
            parameters).
















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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


   The Protocol Data parameter is encoded 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Originating Point Code                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Destination Point Code                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       SI      |       NI      |      MP       |      SLS      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                     User Protocol Data                        /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Originating Point Code: 32 bits (unsigned integer)
   Destination Point Code: 32 bits (unsigned integer)

   The Originating and Destination Point Code fields contains the OPC
   and DPC from the routing label of the original SS7 message in Network
   Byte Order, justified to the least significant bit.  Unused bits are
   coded `0'.

   Service Indicator: 8 bits (unsigned integer)

   The Service Indicator field contains the SI field from the original
   SS7 message justified to the least significant bit.  Unused bits are
   coded `0'.

   Network Indicator: 8-bits (unsigned integer)

   The Network Indicator contains the NI field from the original SS7
   message justified to the least significant bit.  Unused bits are
   coded `0'.

   Message Priority: 8 bits (unsigned integer)

   The Message Priority field contains the MP bits (if any) from the
   original SS7 message, both for ANSI-style and TTC-style [29] message
   priority bits. The MP bits are aligned to the least significant bit.
   Unused bits are coded `0'.









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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


   Signalling Link Selection: 8 bits (unsigned integer)

   The Signalling Link Selection field contains the SLS bits from the
   routing label of the original SS7 message justified to the least
   significant bit and in Network Byte Order.  Unused bits are coded
   `0'.

   User Protocol Data: (byte string)

   The User Protocol Data field contains a byte string of MTP-User
   information from the original SS7 message starting with the first
   byte of the original SS7 message following the Routing Label.

   Correlation Id: 32-bits (unsigned integer)

   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 M3UA.

3.4 SS7 Signalling Network Management (SSNM) Messages

3.4.1 Destination Unavailable (DUNA)

   The DUNA message is sent from an SGP in an SG to all concerned ASPs
   to indicate that the SG has determined that one or more SS7
   destinations are unreachable.  It is also sent by an SGP in response
   to a message from the ASP to an unreachable SS7 destination.  As an
   implementation option the SG may suppress the sending of subsequent
   "response" DUNA messages regarding a certain unreachable SS7
   destination for a certain period to give the remote side time to
   react.  If there is no alternate route via another SG, the MTP3-User
   at the ASP is expected to stop traffic to the affected destination
   via the SG as per the defined MTP3-User procedures.


















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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


   The DUNA message contains the following parameters:

      Network Appearance      Optional
      Routing Context         Optional
      Affected Point Code     Mandatory
      INFO String             Optional

   The format for DUNA 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 = 0x0200          |          Length = 8           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Network Appearance                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Tag = 0x0006           |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                       Routing Context                         /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Tag = 0x0012          |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Mask      |                 Affected PC 1                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                              ...                              /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Mask      |                 Affected PC n                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Tag = 0x0004         |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                          INFO String                          /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Network Appearance: 32-bit unsigned integer

      See Section 3.3.1

   Routing Context: n x 32-bits (unsigned integer)

      The optional Routing Context parameter contains the Routing
      Context values associated with the DUNA message.  Where a Routing
      Key has not been coordinated between the SGP and ASP, sending of



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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


      Routing Context is not required.  Where multiple Routing Keys and
      Routing Contexts are used across a common association, the Routing
      Context(s) MUST be sent to identify the concerned traffic flows
      for which the DUNA message applies, assisting in outgoing traffic
      management and internal distribution of MTP-PAUSE indications to
      MTP3-Users at the receiver.

   Affected Point Code: n x 32-bits

      The Affected Point Code parameter contains a list of Affected
      Destination Point Code fields, each a three-octet parameter to
      allow for 14-, 16- and 24-bit binary formatted SS7 Point Codes.
      Affected Point Codes that are less than 24-bits, are padded on the
      left to the 24-bit boundary.  The encoding is shown below for ANSI
      and ITU Point Code examples.

   ANSI 24-bit Point Code:

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

                      |MSB-----------------------------------------LSB|

   ITU 14-bit Point Code:

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

                                           |MSB--------------------LSB|

      It is optional to send an Affected Point Code parameter with more
      than one Affected PC but it is mandatory to receive it.  Including
      multiple Affected PCs may be useful when reception of an MTP3
      management message or a linkset event simultaneously affects the
      availability status of a list of destinations at an SG.










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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


   Mask: 8-bits (unsigned integer)

      The Mask field can be used to identify a contiguous range of
      Affected Destination Point Codes.  Identifying a contiguous range
      of Affected DPCs may be useful when reception of an MTP3
      management message or a linkset event simultaneously affects the
      availability status of a series of destinations at an SG.

      The Mask parameter is an integer representing a bit mask that can
      be applied to the related Affected PC field.  The bit mask
      identifies how many bits of the Affected PC field are significant
      and which are effectively "wildcarded".  For example, a mask of
      "8" indicates that the last eight bits of the PC is "wildcarded".
      For an ANSI 24-bit Affected PC, this is equivalent to signalling
      that all PCs in an ANSI Cluster are unavailable.  A mask of "3"
      indicates that the last three bits of the PC is "wildcarded".  For
      a 14-bit ITU Affected PC, this is equivalent to signaling that an
      ITU

      Region is unavailable.  A mask value equal (or greater than) the
      number of bits in the PC indicates that the entire network
      appearance is affected - this is used to indicate network
      isolation to the ASP.

   INFO String: variable length

      The optional INFO String parameter can carry any meaningful UTF-8
      [10] 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.

3.4.2 Destination Available (DAVA)

   The DAVA message is sent from an SGP to all concerned ASPs to
   indicate that the SG has determined that one or more SS7 destinations
   are now reachable (and not restricted), or in response to a DAUD
   message if appropriate. If the ASP M3UA layer previously had no
   routes to the affected destinations the ASP MTP3-User protocol is
   informed and may now resume traffic to the affected destination.  The
   ASP M3UA layer now routes the MTP3-user traffic through the SG
   initiating the DAVA message.









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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


   The DAVA message contains the following parameters:

      Network Appearance       Optional
      Routing Context          Optional
      Affected Point Code      Mandatory
      INFO String              Optional

   The format and description of the Network Appearance, Routing
   Context, Affected Point Code and INFO String parameters is the same
   as for the DUNA message (See Section 3.4.1).

3.4.3 Destination State Audit (DAUD)

   The DAUD message MAY be sent from the ASP to the SGP to audit the
   availability/congestion state of SS7 routes from the SG to one or
   more affected destinations.

   The DAUD message contains the following parameters:

      Network Appearance      Optional
      Routing Context         Optional
      Affected Point Code     Mandatory
      INFO String             Optional

   The format and description of DAUD Message parameters is the same as
   for the DUNA message (See Section 3.4.1).

3.4.4 Signalling Congestion (SCON)

   The SCON message can be sent from an SGP to all concerned ASPs to
   indicate that an SG has determined that there is congestion in the
   SS7 network to one or more destinations, or to an ASP in response to
   a DATA or DAUD message as appropriate.  For some MTP protocol
   variants (e.g., ANSI MTP) the SCON message may be sent when the SS7
   congestion level changes.  The SCON message MAY also be sent from the
   M3UA layer of an ASP to an M3UA peer indicating that the M3UA layer
   or the ASP is congested.

   The SCON message contains the following parameters:

      Network Appearance       Optional
      Routing Context          Optional
      Affected Point Code      Mandatory
      Concerned Destination    Optional
      Congestion Indications   Optional
      INFO String              Optional





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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


   The format for SCON 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 = 0x0200          |           Length = 8          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Network Appearance                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Tag = 0x0006           |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                       Routing Context                         /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Tag = 0x0012          |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Mask     |                 Affected PC 1                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                              ...                              /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Mask     |                 Affected PC n                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Tag = 0x0206          |             Length = 8        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    reserved   |                 Concerned DPC                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Tag = 0x0205          |             Length = 8        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Reserved                    |  Cong. Level  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Tag = 0x0004       |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                         INFO String                           /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+












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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


   The format and description of the Network Appearance, Routing
   Context, Affected Point Code, and INFO String parameters is the same
   as for the DUNA message (See Section 3.4.1).

   The Affected Point Code parameter can be used to indicate congestion
   of multiple destinations or ranges of destinations.

   Concerned Destination: 32-bits

      The optional Concerned Destination parameter is only used if the
      SCON message is sent from an ASP to the SGP. It contains the point
      code of the originator of the message that triggered the SCON
      message. The Concerned Destination parameter contains one
      Concerned Destination Point Code field, a three-octet parameter to
      allow for 14-, 16- and 24-bit binary formatted SS7 Point Codes.  A
      Concerned Point Code that is less than 24-bits is padded on the
      left to the 24-bit boundary.  Any resulting Transfer Controlled
      (TFC) message from the SG is sent to the Concerned Point Code
      using the single Affected DPC contained in the SCON message to
      populate the (affected) Destination field of the TFC message

   Congested Indications: 32-bits

      The optional Congestion Indications parameter contains a
      Congestion Level field.  This optional parameter is used to
      communicate congestion levels in national MTP networks with
      multiple congestion thresholds, such as in ANSI MTP3.  For MTP
      congestion methods without multiple congestion levels (e.g., the
      ITU international method) the parameter is not included.

   Congestion Level field: 8-bits (unsigned integer)

      The Congestion Level field, associated with all of the Affected
      DPC(s) in the Affected Destinations parameter, contains one of the
      following values:

         0     No Congestion or Undefined
         1     Congestion Level 1
         2     Congestion Level 2
         3     Congestion Level 3











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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


      The congestion levels are defined in the congestion method in the
      appropriate national MTP recommendations [7,8].

3.4.5 Destination User Part Unavailable (DUPU)

   The DUPU message is used by an SGP to inform concerned ASPs that a
   remote peer MTP3-User Part (e.g., ISUP or SCCP) at an SS7 node is
   unavailable.

   The DUPU message contains the following parameters:

      Network Appearance       Optional
      Routing Context          Optional
      Affected Point Code      Mandatory
      User/Cause               Mandatory
      INFO String              Optional

   The format for DUPU 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 = 0x0200          |             Length = 8        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Network Appearance                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Tag = 0x0006           |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                       Routing Context                         /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Tag = 0x0012          |          Length = 8           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Mask = 0    |                  Affected PC                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Tag = 0x0204          |          Length = 8           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Cause             |            User               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Tag = 0x0004          |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                          INFO String                          /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


   User/Cause: 32-bits

      The Unavailability Cause and MTP3-User Identity fields, associated
      with the Affected PC in the Affected Point Code parameter, are
      encoded as follows:

   Unavailability Cause field: 16-bits (unsigned integer)

      The Unavailability Cause parameter provides the reason for the
      unavailability of the MTP3-User.  The valid values for the
      Unavailability Cause parameter are shown in the following table.
      The values agree with those provided in the SS7 MTP3 User Part
      Unavailable message.  Depending on the MTP3 protocol used in the
      Network Appearance, additional values may be used - the
      specification of the relevant MTP3 protocol variant/version
      recommendation is definitive.

         0         Unknown
         1         Unequipped Remote User
         2         Inaccessible Remote User

   MTP3-User Identity field: 16-bits (unsigned integer)

      The MTP3-User Identity describes the specific MTP3-User that is
      unavailable (e.g., ISUP, SCCP, ...).  Some of the valid values for
      the MTP3-User Identity are shown below.  The values align with
      those provided in the SS7 MTP3 User Part Unavailable message and
      Service Indicator.  Depending on the MTP3 protocol variant/version
      used in the network appearance, additional values may be used.
      The relevant MTP3 protocol variant/version recommendation is
      definitive.

          0 to 2   Reserved
             3     SCCP
             4     TUP
             5     ISUP
          6 to 8  Reserved
             9     Broadband ISUP
            10     Satellite ISUP
            11     Reserved
            12     AAL type 2 Signalling
            13     Bearer Independent Call Control (BICC)
            14     Gateway Control Protocol
            15     Reserved

      The format and description of the Affected Point Code parameter is
      the same as for the DUNA message (See Section 3.4.1.) except that
      the Mask field is not used and only a single Affected DPC is



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      included.  Ranges and lists of Affected DPCs cannot be signaled in
      a DUPU message, but this is consistent with UPU operation in the
      SS7 network.  The Affected Destinations parameter in an MTP3 User
      Part Unavailable message (UPU) received by an SGP from the SS7
      network contains only one destination.

      The format and description of the Network Appearance, Routing
      Context, and INFO String parameters is the same as for the DUNA
      message (See Section 3.4.1).

3.4.6 Destination Restricted (DRST)

   The DRST message is optionally sent from the SGP to all concerned
   ASPs to indicate that the SG has determined that one or more SS7
   destinations are now restricted from the point of view of the SG, or
   in response to a DAUD message if appropriate.  The M3UA layer at the
   ASP is expected to send traffic to the affected destination via an
   alternate SG with route(s) of equal priority, but only if such an
   alternate route exists and is available.  If the affected destination
   is currently considered unavailable by the ASP, The MTP3-User should
   be informed that traffic to the affected destination can be resumed.
   In this case, the M3UA layer should route the traffic through the SG
   initiating the DRST message.

   This message is optional for the SG to send and it is optional for
   the ASP to act on any information received in the message.  It is for
   use in the "STP" case described in Section 1.4.1.

   The DRST message contains the following parameters:

      Network Appearance       Optional
      Routing Context          Optional
      Affected Point Code      Mandatory
      INFO String              Optional

   The format and description of the Network Appearance, Routing
   Context, Affected Point Code and INFO String parameters is the same
   as for the DUNA message (See Section 3.4.1).













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3.5 ASP State Maintenance (ASPSM) Messages

3.5.1 ASP Up

   The ASP Up message is used to indicate to a remote M3UA peer that the
   adaptation layer is ready to receive any ASPSM/ASPTM messages for all
   Routing Keys that the ASP is configured to serve.

   The ASP Up message contains the following parameters:

      ASP Identifier                Optional
      INFO String                   Optional

   The format for ASP Up message parameters is as follows:

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

   ASP Identifier: 32-bit unsigned integer

      The optional ASP Identifier parameter contains a unique value that
      is locally significant among the ASPs that support an AS.  The SGP
      should save the ASP Identifier to be used, if necessary, with the
      Notify message (see Section 3.8.2).

      The format and description of the optional INFO String parameter
      is the same as for the DUNA message (See Section 3.4.1).













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3.5.2 ASP Up Acknowledgement (ASP Up Ack)

   The ASP UP Ack message is used to acknowledge an ASP Up message
   received from a  remote M3UA peer.

   The ASP Up Ack message contains the following parameters:

   INFO String (optional)

   The format for ASP Up Ack message parameters is as follows:

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

   The format and description of the optional INFO String parameter is
   the same as for the DUNA message (See Section 3.4.1).  The INFO
   String in an ASP Up Ack message is independent from the INFO String
   in the ASP Up message (i.e., it does not have to echo back the INFO
   String received).

3.5.3 ASP Down

   The ASP Down message is used to indicate to a remote M3UA peer that
   the adaptation layer is NOT ready to receive DATA, SSNM, RKM or ASPTM
   messages.

   The ASP Down message contains the following parameters:

   INFO String    Optional















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   The format for the ASP Down message parameters is as follows:

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

   The format and description of the optional INFO String parameter is
   the same as for the DUNA message (See Section 3.4.1).

3.5.4 ASP Down Acknowledgement (ASP Down Ack)

   The ASP Down Ack message is used to acknowledge an ASP Down message
   received from a remote M3UA peer.

   The ASP Down Ack message contains the following parameters:

   INFO String     Optional

   The format for the ASP Down Ack message parameters is as follows:

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

   The format and description of the optional INFO String parameter is
   the same as for the DUNA message (See Section 3.4.1).

   The INFO String in an ASP Down Ack message is independent from the
   INFO String in the ASP Down message (i.e., it does not have to echo
   back the INFO String received).









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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


3.5.5 Heartbeat (BEAT)

   The BEAT message is optionally used to ensure that the M3UA peers are
   still available to each other.  It is recommended for use when the
   M3UA runs over a transport layer other than the SCTP, which has its
   own heartbeat.

   The BEAT message contains the following parameters:

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

3.5.6 Heartbeat Acknowledgement (BEAT Ack)

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

3.6 Routing Key Management (RKM) Messages [Optional]

3.6.1 Registration Request (REG REQ)

   The REG REQ message is sent by an ASP to indicate to a remote M3UA
   peer that it wishes to register one or more given Routing Keys with
   the remote peer.  Typically, an ASP would send this message to an
   SGP, and expects to receive a REG RSP message in return with an
   associated Routing Context value.







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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


   The REG REQ message contains the following parameters:

   Routing Key           Mandatory

   One or more Routing Key parameters MAY be included.  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 = 0x0207         |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                         Routing Key 1                         /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                              ...                              /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Tag = 0x0207         |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                         Routing Key n                         /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Routing Key: variable length

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

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














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   The format of the Routing 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-RK-Identifier                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  Traffic Mode Type (optional)                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Destination Point Code                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  Network Appearance (optional)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  Service Indicators (optional)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              Originating Point Code List (optional)           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Circuit Range List (optional)               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                              ...                              /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Destination Point Code                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  Service Indicators (optional)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |              Originating Point Code List (optional)           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Circuit Range List (optional)               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Note: The Destination Point Code, Service Indicators, Originating
      Point Code List and Circuit Range List parameters MAY be repeated
      as a grouping within the Routing Key parameter, in the structure
      shown above.

   Local-RK-Identifier: 32-bit unsigned integer

      The mandatory Local-RK-Identifier field is used to uniquely
      identify the registration request.  The Identifier value is
      assigned by the ASP, and is used to correlate the response in an
      REG RSP message with the original registration request.  The
      Identifier value must remain unique until the REG RSP message is
      received.






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   The format of the Local-RK-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 = 0x020a          |         Length = 8            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  Local-RK-Identifier value                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Traffic Mode Type: 32-bit (unsigned integer)

   The optional Traffic Mode Type parameter identifies the traffic mode
   of operation of the ASP(s) within an Application Server.  The format
   of the Traffic Mode Type Identifier is as follows:

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Tag = 0x000b          |         Length = 8            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Traffic Mode Type                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The valid values for Traffic Mode Type are shown in the following
   table:

         1     Override
         2     Loadshare
         3     Broadcast

   Destination Point Code:

      The Destination Point Code parameter is mandatory, and identifies
      the Destination Point Code of incoming SS7 traffic for which the
      ASP is registering.  The format is the same as described for the
      Affected Destination parameter in the DUNA message (See Section
      3.4.1).  Its format is:

       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 = 0x020b          |         Length = 8            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Mask = 0   |            Destination Point Code             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


   Network Appearance:

      The optional Network Appearance parameter field identifies the SS7
      network context for the Routing Key, and has the same format as in
      the DATA message (See Section 3.3.1).  The absence of the Network
      Appearance parameter in the Routing Key indicates the use of any
      Network Appearance value.  Its format is:

       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 = 0x0200          |         Length = 8            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Network Appearance                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Service Indicators (SI): n X 8-bit integers

      The optional SI [7,8] field contains one or more Service
      Indicators from the values as described in the MTP3-User Identity
      field of the DUPU message.  The absence of the SI parameter in the
      Routing Key indicates the use of any SI value, excluding of course
      MTP management.  Where an SI parameter does not contain a multiple
      of four SIs, the parameter is padded out to 32-byte alignment.

      The SI format is:

       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 = 0x020c          |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      SI #1    |     SI #2     |    SI #3      |    SI #4      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      /                              ...                              /
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      SI #n    |             0 Padding, if necessary           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   OPC List:

      The Originating Point Code List parameter contains one or more SS7
      OPC entries, and its format is the same as the Destination Point
      Code parameter.  The absence of the OPC List parameter in the
      Routing Key indicates the use of any OPC value,






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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 = 0x020e          |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Mask = 0   |          Origination Point Code #1            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Mask = 0   |          Origination Point Code #2            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      /                              ...                              /
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Mask = 0   |          Origination Point Code #n            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Circuit Range:

      An ISUP controlled circuit is uniquely identified by the SS7 OPC,
      DPC and CIC value.  For the purposes of identifying Circuit Ranges
      in an M3UA Routing Key, the optional Circuit Range parameter
      includes one or more circuit ranges, each identified by an OPC and
      Upper/Lower CIC value.  The DPC is implicit as it is mandatory and
      already included in the DPC parameter of the Routing Key.  The
      absence of the Circuit Range parameter in the Routing Key
      indicates the use of any Circuit Range values, in the case of
      ISUP/TUP traffic.  The Origination Point Code is encoded the same
      as the Destination Point Code parameter, while the CIC values are
      16-bit integers.
























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   The Circuit Range 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 = 0x020f        |              Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Mask = 0   |          Origination Point Code #1            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       Lower CIC Value #1      |      Upper CIC Value #1       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Mask = 0   |          Origination Point Code #2            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       Lower CIC Value #2      |      Upper CIC Value #2       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      /                              ...                              /
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Mask = 0   |          Origination Point Code #n            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |       Lower CIC Value #n      |      Upper CIC Value #n       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.6.2 Registration Response (REG RSP)

   The REG RSP message is used as a response to the REG REQ message from
   a remote M3UA peer.  It contains indications of success/failure for
   registration requests and returns a unique Routing Context value for
   successful registration requests, to be used in subsequent M3UA
   Traffic Management protocol.

   The REG RSP message contains the following parameters:

   Registration Result   Mandatory

   One or more Registration Result parameters MUST be included.  The
   format for the REG RSP message 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 = 0x0208         |              Length           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Registration Result 1                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                              ...                              /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Tag = 0x0208        |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Registration Result n                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Registration Results:

      The Registration Result parameter contains the registration result
      for a single Routing Key in an REG REQ message.  The number of
      results in a single REG RSP message MUST be anywhere from one to
      the total number of number of Routing Key parameters found in the
      corresponding REG REQ message.  Where multiple REG RSP messages
      are used in reply to REG REQ message, a specific result SHOULD be
      in only one REG RSP 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Tag = 0x020a        |          Length = 8             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  Local-RK-Identifier value                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Tag = 0x0212      |          Length = 8             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Registration Status                      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Tag = 0x0006      |          Length = 8             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Routing Context                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Local-RK-Identifier: 32-bit integer

      The Local-RK-Identifier contains the same value as found in the
      matching Routing Key parameter found in the REG REQ message (See
      Section 3.6.1).



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RFC 3332             SS7 MTP3-User Adaptation Layer       September 2002


   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:

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

   Routing Context: 32-bit integer

      The Routing Context field contains the Routing Context value for
      the associated Routing Key if the registration was successful.  It
      is set to "0" if the registration was not successful.

3.6.3 Deregistration Request (DEREG REQ)

   The DEREG REQ message is sent by an ASP to indicate to a remote M3UA
   peer that it wishes to deregister a given Routing Key.  Typically, an
   ASP would send this message to an SGP, and expects to receive a DEREG
   RSP message in return with the associated Routing Context value.

   The DEREG REQ message contains the following parameters:

      Routing Context       Mandatory

   The format for the DEREG REQ message 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 = 0x0006          |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                       Routing Context                         /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Routing Context: n X 32-bit integers

      The Routing Context parameter contains (a list of) integers
      indexing the Application Server traffic that the sending ASP is
      currently registered to receive from the SGP but now wishes to
      deregister.

3.6.4 Deregistration Response (DEREG RSP)

   The DEREG RSP message is used as a response to the DEREG REQ message
   from a remote M3UA peer.

   The DEREG RSP message contains the following parameters:

      Deregistration Result    Mandatory

   One or more Deregistration Result parameters MUST be included.  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 = 0x0209         |               Length          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Deregistration Result 1                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                              ...                              /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Tag = 0x0209        |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   Deregistration Result n                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+







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   Deregistration Results:

      The Deregistration Result parameter contains the deregistration
      status for a single Routing Context in a DEREG REQ message.  The
      number of results in a single DEREG RSP message MAY be anywhere
      from one to the total number of number of Routing Context values
      found in the corresponding DEREG REQ message.

      Where multiple DEREG RSP messages are used in reply to DEREG REQ
      message, a specific result SHOULD be in only one DEREG RSP
      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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Tag = 0x0006          |          Length = 8           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Routing Context                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Tag = 0x0213          |          Length = 8           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                     Deregistration Status                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Routing Context: 32-bit integer

      The Routing Context field contains the Routing Context value of
      the matching Routing Key to deregister, as found in the DEREG REQ
      message.

      Deregistration Status: 32-bit integer

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

      Its values may be:

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








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3.7 ASP Traffic Maintenance (ASPTM) Messages

3.7.1 ASP Active

   The ASP Active message is sent by an ASP to indicate to a remote M3UA
   peer that it is ready to process signalling traffic for a particular
   Application Server.  The ASP Active message affects only the ASP
   state for the Routing Keys identified by the Routing Contexts, if
   present.

   The ASP Active message contains the following parameters:

      Traffic Mode Type     Optional
      Routing Context       Optional
      INFO String           Optional

   The format for the ASP Active 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 = 0x000b          |          Length = 8           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Traffic Mode Type                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Tag = 0x0006          |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                       Routing Context                         /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Tag = 0x0004          |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                          INFO String                          /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Traffic Mode Type: 32-bit (unsigned integer)

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

         1         Override
         2         Loadshare
         3         Broadcast




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      Within a particular Routing Context, Override, Loadshare and
      Broadcast SHOULD NOT be mixed.  The Override value indicates that
      the ASP is operating in Override mode, and the ASP takes over all
      traffic in an Application Server (i.e., primary/backup operation),
      overriding any currently active ASPs in the AS.  In Loadshare
      mode, the ASP will share in the traffic distribution with any
      other currently active ASPs.  In Broadcast mode, the ASP will
      receive the same messages as any other currently active ASP.

   Routing Context: n X 32-bit integers

      The optional Routing Context parameter contains (a list of)
      integers indexing the Application Server traffic that the sending
      ASP is configured/registered to receive.

      There is one-to-one relationship between an index entry and an SGP
      Routing Key or AS Name.  Because an AS can only appear in one
      Network Appearance, the Network Appearance parameter is not
      required in the ASP Active message.

      An Application Server Process may be configured to process traffic
      for more than one logical Application Server.  From the
      perspective of an ASP, a Routing Context defines a range of
      signalling traffic that the ASP is currently configured to receive
      from the SGP.  For example, an ASP could be configured to support
      call processing for multiple ranges of PSTN trunks and therefore
      receive related signalling traffic, identified by separate SS7
      DPC/OPC/CIC ranges.

   The format and description of the optional INFO String parameter is
   the same as for the DUNA message (See Section 3.4.1).

3.7.2 ASP Active Acknowledgement (ASP Active Ack)

   The ASP Active Ack message is used to acknowledge an ASP Active
   message received from a remote M3UA peer.

   The ASP Active Ack message contains the following parameters:

      Traffic Mode Type     Optional
      Routing Context       Optional
      INFO String           Optional









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   The format for the ASP Active 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 = 0x000b        |          Length = 8           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Traffic Mode Type                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |            Tag = 0x0006       |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                       Routing Context                         /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Tag = 0x0004        |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                          INFO String                          /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The format and description of the optional INFO String parameter is
   the same as for the DUNA message (See Section 3.4.1).

   The INFO String in an ASP Active Ack message is independent from the
   INFO String in the ASP Active message (i.e., it does not have to echo
   back the INFO String received).

   The format of the Traffic Mode Type and Routing Context parameters is
   the same as for the ASP Active message. (See Section 3.7.1).

3.7.3  ASP Inactive

   The ASP Inactive message is sent by an ASP to indicate to a remote
   M3UA peer that it is no longer an active ASP to be used from within a
   list of ASPs.  The ASP Inactive message affects only the ASP state in
   the Routing Keys identified by the Routing Contexts, if present.

   The ASP Inactive message contains the following parameters:

      Routing Context         Optional
      INFO String             Optional








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   The format for the ASP Inactive 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 = 0x0006          |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                       Routing Context                         /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Tag = 0x0004          |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                          INFO String                          /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The format and description of the optional Routing Context and INFO
   String parameters is the same as for the ASP Active message (See
   Section 3.5.5.)

3.7.4 ASP Inactive Acknowledgement (ASP Inactive Ack)

   The ASP Inactive Ack message is used to acknowledge an ASP Inactive
   message received from a remote M3UA peer.

   The ASP Inactive Ack message contains the following parameters:

      Routing Context       Optional
      INFO String           Optional




















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   The format for the ASP Inactive 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 = 0x0006          |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                       Routing Context                         /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Tag = 0x0004          |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                          INFO String                          /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The format and description of the optional INFO String parameter is
   the same as for the DUNA message (See Section 3.4.1.)

   The INFO String in an ASP Inactive Ack message is independent from
   the INFO String in the ASP Inactive message (i.e., it does not have
   to echo back the INFO String received).

   The format of the Routing Context parameter is the same as for the
   ASP Inactive message. (See Section 3.7.3).

3.8  Management (MGMT) Messages

3.8.1  Error

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

   The Error message contains the following parameters:

      Error Code                 Mandatory
      Routing Context            Mandatory*
      Network Appearance         Mandatory*
      Affected Point Code        Mandatory*
      Diagnostic Information     Optional

      (*) Only mandatory for specific Error Codes





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   The format for the Error 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 = 0x000c         |          Length = 8           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Error Code                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Tag = 0x0006         |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                        Routing Context                        /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Tag - 0x0012         |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Mask      |             Affected Point Code  1            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                                ...                            /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Mask      |             Affected Point Code  n            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Tag = 0x0200        |           Length = 8          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Network Appearance                       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Tag = 0x0007         |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                     Diagnostic Information                    /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Error Code: 32-bits (unsigned integer)

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

      0x01      Invalid Version
      0x02      Not Used in M3UA
      0x03      Unsupported Message Class
      0x04      Unsupported Message Type
      0x05      Unsupported Traffic Mode Type
      0x06      Unexpected Message



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      0x07      Protocol Error
      0x08      Not used in M3UA
      0x09      Invalid Stream Identifier
      0x0a      Not used in M3UA
      0x0b      Not used in M3UA
      0x0c      Not used in M3UA
      0x0d      Refused - Management Blocking
      0x0e      ASP Identifier Required
      0x0f      Invalid ASP Identifier
      0x10      Not Used in M3UA
      0x11      Invalid Parameter Value
      0x12      Parameter Field Error
      0x13      Unexpected Parameter
      0x14      Destination Status Unknown
      0x15      Invalid Network Appearance
      0x16      Missing Parameter
      0x17      Not Used in M3UA
      0x18      Not Used in M3UA
      0x19      Invalid Routing Context
      0x1a      No Configured AS for ASP

   The "Invalid Stream Identifier" error is sent if a message is
   received on an unexpected SCTP stream (e.g., a MGMT message was
   received on a stream other than "0").  Error messages MUST NOT be
   generated in response to other Error messages.

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

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

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

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





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

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

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

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

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

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

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

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

   The "Destination Status Unknown" Error MAY be sent if a DAUD is
   received at an SG enquiring of the availability/congestion status of
   a destination, and the SG does not wish to provide the status (e.g.,
   the sender is not authorized to know the status).  For this error,
   the invalid or unauthorized Point Code(s) MUST be included along with
   the Network Appearance and/or Routing Context associated with the
   Point Code(s).

   The "Invalid Network Appearance" error is sent by a SGP if an ASP
   sends a message with an invalid (unconfigured) Network Appearance
   value. For this error, the invalid (unconfigured) Network Appearance
   MUST be included in the Network Appearance parameter.







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

   The "Invalid Routing Context" error is sent if a message is received
   from a peer with an invalid (unconfigured) Routing Context value.
   For this error, the invalid Routing Context(s) MUST be included in
   the Error message.

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

   Diagnostic Information: variable length

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

3.8.2 Notify

   The Notify message used to provide an autonomous indication of M3UA
   events to an M3UA peer.

   The Notify message contains the following parameters:

      Status                     Mandatory
      ASP Identifier             Optional
      Routing Context            Optional
      INFO String                Optional





















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   The format for the Notify 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 = 0x000d           |          Length = 8           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Status Type            |       Status Information      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Tag = 0x0011           |             Length = 8        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        ASP Identifier                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Tag = 0x0006           |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                       Routing Context                         /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Tag = 0x0004          |             Length            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      \                                                               \
      /                          INFO String                          /
      \                                                               \
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Status Type: 16-bits (unsigned integer)

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

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

   Status Information: 16-bits (unsigned integer)

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

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






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

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

         1    Insufficient ASP Resources Active in AS
         2    Alternate ASP Active
         3    ASP Failure

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

   The format and description of the optional ASP Identifier is the same
   as for the ASP Up message (See Section 3.5.1).  The format and
   description of the Routing Context and Info String parameters is the
   same as for the ASP Active message (See Section 3.7.1)

4. Procedures

   The M3UA layer needs to respond to various local primitives it
   receives from other layers as well as the messages that it receives
   from the peer M3UA layer.  This section describes the M3UA procedures
   in response to these events.

4.1 Procedures to Support the M3UA-User

4.1.1 Receipt of Primitives from the M3UA-User

   On receiving an MTP-TRANSFER request primitive from an upper layer at
   an ASP/IPSP, or the nodal interworking function at an SGP, the M3UA
   layer sends a corresponding DATA message (see Section 3) to its M3UA
   peer.  The M3UA peer receiving the DATA message sends an MTP-TRANSFER
   indication primitive to the upper layer.







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   The M3UA message distribution function (see Section 1.4.2.1)
   determines the Application Server (AS) based on comparing the
   information in the MTP-TRANSFER request primitive with a provisioned
   Routing Key.

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

   In addition, the message needs to be sent on the appropriate SCTP
   stream, again taking care to meet the message sequencing needs of the
   signalling application. DATA messages MUST be sent on an SCTP stream
   other than stream '0'.

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

4.2 Receipt of Primitives from the Layer Management

   On receiving primitives from the local Layer Management, the M3UA
   layer will take the requested action and provide an appropriate
   response primitive to Layer Management.

   An M-SCTP_ESTABLISH request primitive from Layer Management at an ASP
   or IPSP will initiate the establishment of an SCTP association.  The
   M3UA layer will attempt to establish an SCTP association with the
   remote M3UA 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 M3UA layer.  At the SGP or IPSP that initiated the request, the
   M3UA layer will send an M-SCTP_ESTABLISH confirm primitive to Layer
   Management when the association setup is complete.  At the peer M3UA



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   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 teardown of an SCTP association.  The M3UA 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 M3UA layer.  At the M3UA Layer
   that initiated the request, the M3UA layer will send an M-
   SCTP_RELEASE confirm primitive to Layer Management when the
   association shutdown is complete.  At the peer M3UA 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 M3UA layer
   simply maps the M-SCTP_STATUS request primitive to an SCTP-STATUS
   primitive to the SCTP layer.  When the SCTP responds, the M3UA layer
   maps the association status information to an M-SCTP_STATUS confirm
   primitive.  No peer protocol is invoked.

   Similar LM-to-M3UA-to-SCTP and/or SCTP-to-M3UA-to-LM primitive
   mappings can be described for the various other SCTP Upper Layer
   primitives in RFC2960 [17] such as INITIALIZE, SET PRIMARY, CHANGE
   HEARTBEAT, REQUEST HEARTBEAT, GET SRTT REPORT, SET FAILURE THRESHOLD,
   SET PROTOCOL PARAMETERS, DESTROY SCTP INSTANCE, SEND FAILURE, AND
   NETWORK STATUS CHANGE.  Alternatively, these SCTP Upper Layer
   primitives (and Status as well) can be considered for modeling
   purposes as a Layer Management interaction directly with the SCTP
   Layer.

   M-NOTIFY indication and M-ERROR indication primitives indicate to
   Layer Management the notification or error information contained in a
   received M3UA Notify or Error message respectively.  These
   indications can also be generated based on local M3UA events.

   An M-ASP_STATUS request primitive supports a Layer Management query
   of the status of a particular local or remote ASP.  The M3UA layer
   responds with the status in an M-ASP_STATUS confirm primitive.  No
   M3UA peer protocol is invoked.







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   An M-AS_STATUS request supports a Layer Management query of the
   status of a particular AS.  The M3UA responds with an M-AS_STATUS
   confirm primitive.  No M3UA 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 M3UA 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 M3UA
   peer at an SGP or IPSP.

4.2.1 Receipt of M3UA Peer Management Messages

   Upon successful state changes resulting from reception of ASP Up, ASP
   Down, ASP Active and ASP Inactive messages from a peer M3UA, the M3UA
   layer MAY invoke corresponding M-ASP_UP, M-ASP_DOWN, M-ASP_ACTIVE and
   M-ASP_INACTIVE, M-AS_ACTIVE, M-AS_INACTIVE, and M-AS_DOWN indication
   primitives to the local Layer Management.

   M-NOTIFY indication and M-ERROR indication primitives indicate to
   Layer Management the notification or error information contained in a
   received M3UA Notify or Error message.  These indications can also be
   generated based on local M3UA events.

   All non-Transfer and non-SSNM, messages, except BEAT and BEAT Ack,
   SHOULD be sent with sequenced delivery to ensure ordering.  ASPTM
   messages MAY be sent on one of the streams used to carry the data
   traffic related to the Routing Context(s), to minimize possible
   message loss.  BEAT and BEAT Ack messages MAY be sent using out-of-
   order delivery, and MAY be sent on any stream.

4.3 AS and ASP State Maintenance

   The M3UA 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 M3UA message distribution function.
   Similarly, where IPSPs use M3UA in a point-to-point fashion, the M3UA
   layer in an IPSP maintains the state of remote IPSPs. For the
   purposes of the following procedures, only the SGP/ASP case is
   described but the SGP side of the procedures also apply to an IPSP
   sending traffic to an AS consisting of a set of remote IPSPs.








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4.3.1 ASP States

   The state of each remote ASP, in each AS that it is configured to
   operate, is maintained in the M3UA 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 M3UA layer at the ASP;
   * Reception of some messages from the peer M3UA 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 3.  The possible
   states of an ASP are:

   ASP-DOWN: The remote M3UA 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 M3UA messages,
   with the exception of Heartbeat, ASP Down Ack and Error messages.

   ASP-INACTIVE: The remote M3UA peer at the ASP is available (and the
   related SCTP association is up) but application traffic is stopped.
   In this state the ASP SHOULD NOT be sent any DATA or SSNM messages
   for the AS for which the ASP is inactive.

   ASP-ACTIVE: The remote M3UA peer at the ASP is available and
   application traffic is active (for a particular Routing Context or
   set of Routing Contexts).

   SCTP CDI: The SCTP CDI denotes the local SCTP layer's Communication
   Down Indication to the Upper Layer Protocol (M3UA) 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 (M3UA) on an SG.  The local SCTP will send this indication
   when it detects a restart from the ASP's peer SCTP layer.











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              Figure 3: ASP State Transition Diagram, per AS

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

4.3.2 AS States

   The state of the AS is maintained in the M3UA layer on the SGPs.  The
   state of an AS changes due to events. These events include:

      * ASP state transitions
      * Recovery timer triggers

   The possible states of an AS are:

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

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







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   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, and the SGP has no
   alternative, the SGP may 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 ASP-INACTIVE state, otherwise it will move
   to AS-DOWN state.

   Figure 4 shows an example AS state machine for the case where the
   AS/ASP data is preconfigured.  For other cases where the AS/ASP
   configuration data is created dynamically, there would be differences
   in the state machine, especially at creation of the AS.




























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                   Figure 4: 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

   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.

4.3.3 M3UA 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) and
   assuming that the local M3UA-User is ready, the local M3UA 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).



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   If the M3UA layer subsequently receives an SCTP-COMMUNICATION_DOWN or
   SCTP-RESTART indication primitive from the underlying SCTP layer, it
   will inform the Layer Management by invoking the M-SCTP_STATUS
   indication primitive. The state of the ASP will be moved to ASP-DOWN.
   At an ASP, the MTP3-User will be informed of the unavailability of
   any affected SS7 destinations through the use of MTP-PAUSE indication
   primitives.

   In the case of SCTP-COMMUNICATION_DOWN, the SCTP client MAY try to
   re-establish the SCTP Association.  This MAY be done by the M3UA
   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 M3UA 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 M3UA 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 M3UA 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.







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   If for any local reason (e.g., management lockout) 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
   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
   M3UA messages (e.g., ASP Active or REG REQ).  If the SGP receives any
   other M3UA 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 M3UA 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.









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4.3.4.1.2 IPSP Considerations (ASP Up)

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

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

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

4.3.4.2 ASP-Down Procedures

   The ASP will send an ASP Down 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 DATA, SSNM 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 M3UA
   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 4.4.1) to register within
   Application Servers but has not deregistered from all of them prior
   to sending the ASP Down message, the SGP MUST consider the ASP as
   Deregistered in all Application Servers that it is still a member.

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




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   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
   or IPSP, 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 an M3UA
   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 Routing Contexts to indicate for which Application Servers
   the ASP Active message applies. It is not necessary for the ASP to
   include all Routing Contexts of interest in a single ASP Active
   message, thus requesting to become active in all Routing Contexts 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 Routing Context
   parameter, the receiver must know, via configuration data, which
   Application Server(s) the ASP is a member.

   For the Application Servers that the ASP can be successfully
   activated, the SGP or IPSP responds with one or more ASP Active Ack
   messages, including the associated Routing Context(s) and reflecting
   any Traffic Mode Type value present in the related ASP Active
   message. The Routing Context parameter MUST be included in the ASP
   Active Ack message(s) if the received ASP Active message contained
   any Routing Contexts.  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 or IPSP receives
   any Data messages before an ASP Active message is received, the SGP
   or IPSP MAY discard them.  By sending an ASP Active Ack message, the
   SGP or IPSP is now ready to receive and send traffic for the related




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   Routing Context(s).  The ASP SHOULD NOT send Data or SSNM messages
   for the related Routing Context(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 Routing Contexts, allowing the SGP
   or IPSP to independently acknowledge the ASP Active message for
   different (sets of) Routing Contexts.  The SGP or IPSP MUST send an
   Error message ("Invalid Routing Context") for each Routing Context
   value that the ASP 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
   or IPSP may be sent on different SCTP streams.  Message loss is
   possible as the ASP does not consider itself in the ASP-ACTIVE state
   until reception of the ASP Active Ack message.

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

   There are three modes of Application Server traffic handling in the
   SGP M3UA layer: Override, Loadshare 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





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   configuration data, then 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 state ASP-INACTIVE and
   SHOULD no longer receive traffic from the SGP within the AS.  The SGP
   or IPSP 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 Loadshare mode AS, reception of an ASP Active
   message at an SGP or IPSP 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
   loadsharing 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., the
   SLS, SCCP SSN, ISUP CIC value).

   An SGP or IPSP, upon reception of an ASP Active message for the first
   ASP in a Loadshare 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).  In this case, the SGP or IPSP SHOULD withhold
   the Notify (AS-ACTIVE) until there are sufficient resources.

   For the n+k redundancy case, ASPs which are in that AS should
   coordinate among themselves the number of active ASPs in the AS, and
   should start sending traffic only after n ASPs are active.

   All ASPs within a loadsharing mode AS must be able to process any
   Data message received for the AS, to accommodate any potential
   failover or rebalancing of the offered load.

   In the case of a Broadcast mode AS, reception of an ASP Active
   message at an SGP or IPSP 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
   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.





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   An SGP or IPSP, 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).  In this case, the SGP or IPSP SHOULD withhold
   the Notify (AS-ACTIVE) until there are sufficient resources.

   For the n+k redundancy case, ASPs which are in that AS should
   coordinate among themselves the number of active ASPs in the AS, and
   should start sending traffic only after n ASPs are active.

   Whenever an ASP in a Broadcast mode AS becomes ASP-ACTIVE, the SGP
   MUST tag the first DATA message broadcast in each traffic flow with

   a unique Correlation Id parameter.  The purpose of this Id is to
   permit the newly active ASP to synchronize its processing of traffic
   in each traffic flow with the other ASPs in the broadcast group.

4.3.4.3.1 IPSP Considerations (ASP Active)

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

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

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 or IPSP.  This
   action MAY be initiated at the ASP by an M-ASP_INACTIVE request
   primitive from Layer Management or MAY be initiated automatically by
   an M3UA 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 Routing
   Contexts to indicate for which Application Servers the ASP Inactive
   message applies.  In the case where an ASP Inactive message does not
   contain a Routing Context parameter, the receiver must know, via
   configuration data, which Application Servers the ASP is a member and
   move the ASP to the ASP-INACTIVE state in all Application Servers. In
   the case of an Override mode AS, where another ASP has already taken



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   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 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 Loadshare mode AS, the SGP moves the ASP to the
   ASP-INACTIVE state and the AS traffic is reallocated across the
   remaining ASPs in the state ASP-ACTIVE, as per the loadsharing
   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.

   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 Routing Contexts, allowing the
   SGP or IPSP to independently acknowledge for different (sets of)
   Routing Contexts.  The SGP or IPSP sends an Error message ("Invalid
   Routing Context") message for each invalid or unconfigured Routing
   Context 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,




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   retransmission of ASP Inactive messages MAY be put under control of
   Layer Management.  In this method, expiry of T(ack) results in a M-
   ASP_Inactive confirm primitive carrying a negative indication.

   If no other ASPs in the Application Server are in the state ASP-
   ACTIVE, the 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 cancelled. If T(r) expires, the AS is moved
   to the AS-INACTIVE state.

4.3.4.4.1 IPSP Considerations (ASP Inactive)

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

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

4.3.4.5 Notify Procedures

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

   In the case where a Notify message ("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 is sent in
   the Loadshare 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.




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

4.3.4.5.1 IPSP Considerations (NTFY)

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

4.3.4.6 Heartbeat Procedures

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

   Either M3UA peer may optionally send Heartbeat messages periodically,
   subject to a provisionable timer T(beat).  Upon receiving a Heartbeat
   message, the M3UA peer MUST respond with a Heartbeat Ack message.

   If no Heartbeat Ack message (or any other M3UA message) is received
   from the M3UA peer within 2*T(beat), the remote M3UA 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
   M3UA 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 M3UA peer as
   unavailable.  The contents/format of the Heartbeat Data parameter is
   implementation-dependent and only of local interest to the original
   sender.  The contents may be used, for example, to support a
   Heartbeat sequence algorithm (to detect missing Heartbeats), and/or a
   timestamp mechanism (to evaluate delays).

   Note: Heartbeat related events are not shown in Figure 3 "ASP state
   transition diagram".











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4.4 Routing Key Management Procedures [Optional]

4.4.1 Registration

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

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

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

   If the SGP determines that the received Routing 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 Routing Key", "Error - Invalid DPC", "Error - Invalid Network
   Appearance" as appropriate.

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

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






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

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

   If an SGP determines that one or more of the Routing Key parameters
   are not supported for the purpose of creating new Routing Key
   entries, the SGP returns a Registration Response message to the ASP,
   containing a Registration Result "Error - Unsupported RK parameter
   field".  This result MAY be used if, for example, the SGP does not
   support RK Circuit Range Lists in a Routing Key because the SGP does
   not support ISUP traffic, or does not provide CIC range granularity.

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

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

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

4.4.2 Deregistration

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




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

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

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

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

4.4.3 IPSP Considerations (REG/DEREG)

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

4.5 Procedures to Support the Availability or Congestion Status of SS7
    Destination

4.5.1 At an SGP

   On receiving an MTP-PAUSE, MTP-RESUME or MTP-STATUS indication
   primitive from the nodal interworking function at an SGP, the SGP
   M3UA layer will send a corresponding SS7 Signalling Network
   Management (SSNM) DUNA, DAVA, SCON, or DUPU message (see Section 3.4)
   to the M3UA peers at concerned ASPs.  The M3UA layer must fill in
   various fields of the SSNM messages consistently with the information
   received in the primitives.

   The SGP M3UA layer determines the set of concerned ASPs to be
   informed based on the specific SS7 network for which the primitive
   indication is relevant. In this way, all ASPs configured to




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   send/receive traffic within a particular network appearance are
   informed.  If the SGP operates within a single SS7 network
   appearance, then all ASPs are informed.

   DUNA, DAVA, SCON, and DRST messages may be sent sequentially and
   processed at the receiver in the order sent.

   Sequencing is not required for the DUPU or DAUD messages, which MAY
   be sent unsequenced.

4.5.2 At an ASP

4.5.2.1 Single SG Configurations

   At an ASP, upon receiving an SS7 Signalling Network Management (SSNM)
   message from the remote M3UA Peer, the M3UA layer invokes the
   appropriate primitive indications to the resident M3UA-Users.  Local
   management is informed.

   In the case where a local event has caused the unavailability or
   congestion status of SS7 destinations, the M3UA layer at the ASP
   SHOULD pass up appropriate indications in the primitives to the M3UA
   User, as though equivalent SSNM messages were received.  For example,
   the loss of an SCTP association to an SGP may cause the
   unavailability of a set of SS7 destinations.  MTP-PAUSE indication
   primitives to the M3UA User are appropriate.

4.5.2.2 Multiple SG Configurations

   At an ASP, upon receiving a Signalling Network Management message
   from the remote M3UA Peer, the M3UA layer updates the status of the
   affected route(s) via the originating SG and determines, whether or
   not the overall availability or congestion status of the effected
   destination(s) has changed.  If so, the M3UA layer invokes the
   appropriate primitive indications to the resident M3UA-Users.  Local
   management is informed.

   Implementation Note: To accomplish this, the M3UA layer at an ASP
   maintains the status of routes via the SG, much like an MTP3 layer
   maintains route-set status.

4.5.3 ASP Auditing

   An ASP may optionally initiate an audit procedure to enquire of an
   SGP the availability and, if the national congestion method with
   multiple congestion levels and message priorities is used, congestion
   status of an SS7 destination or set of destinations.  A Destination




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   Audit (DAUD) message is sent from the ASP to the SGP requesting the
   current availability and congestion status of one or more SS7
   Destination Point Codes.

   The DAUD message MAY be sent unsequenced. The DAUD MAY be sent by the
   ASP in the following cases:

      - Periodic.  A Timer originally set upon reception of a DUNA, SCON
                   or DRST message has expired without a subsequent
                   DAVA, DUNA, SCON or DRST message updating the
                   availability/congestion status of the affected
                   Destination Point Codes.  The Timer is reset upon
                   issuing a DAUD.  In this case the DAUD is sent to the
                   SGP that originally sent the SSNM message.

      - Isolation. The ASP is newly ASP-ACTIVE or has been
                   isolated from an SGP for an extended period.  The ASP
                   MAY request the availability/congestion status of one
                   or more SS7 destinations to which it expects to
                   communicate.

   IMPLEMENTATION NOTE: In the first of the cases above, the auditing
   procedure must not be invoked for the case of a received SCON message
   containing a congestion level value of "no congestion" or undefined"
   (i.e., congestion Level = "0").  This is because the value indicates
   either congestion abatement or that the ITU MTP3 international
   congestion method is being used.  In the international congestion
   method, the MTP3 layer at the SGP does not maintain the congestion
   status of any destinations and therefore the SGP cannot provide any
   congestion information in response to the DAUD.  For the same reason,
   in the second of the cases above a DAUD message cannot reveal any
   congested destination(s).

   The SGP SHOULD respond to a DAUD message with the MTP3
   availability/congested status of the routeset associated with each
   Destination Point Code(s) in the DAUD message.  The status of each
   SS7 destination requested is indicated in a DUNA message (if
   unavailable), a DAVA message (if available), or a DRST (if restricted
   and the SGP supports this feature).  Where the SGP maintains the
   congestion status of the SS7 destination, and the SS7 destination is
   congested, the SGP MUST additionally respond with an SCON message
   before the DAVA or DRST message.  If the SS7 destination is available
   and congested, the SGP MUST respond with an SCON message and then a
   DAVA message.  If the SS7 destination is restricted and congested,
   the SGP MUST respond with an SCON message immediately followed by a
   DRST message.  If the SGP has no information on the availability





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   status of the SS7 destination, the SGP responds with a DUNA message,
   as it has no routing information to allow it to route traffic to this
   destination.

   Any DUNA or DAVA message in response to a DAUD message MAY contain a
   list of Affected Point Codes.

   An SG MAY refuse to provide the availability or congestion status of
   a destination if, for example, the ASP is not authorized to know the
   status of the destination.  The SG MAY respond with an Error Message
   (Error Code = "Destination Status Unknown")

4.6 MTP3 Restart

   In the case where the MTP3 in the SG undergoes an MTP restart, event
   communication SHOULD be handled as follows:

   When the SG discovers SS7 network isolation, the SGPs send an
   indication to all concerned available ASPs (i.e., ASPs in the ASP-
   ACTIVE state) using DUNA messages for the concerned destinations.

   When the SG has completed the MTP Restart procedure, the M3UA layers
   at the SGPs inform all concerned ASPs in the ASP-ACTIVE state of any
   available/restricted SS7 destinations using the DAVA/DRST messages.
   No message is necessary for those destinations still unavailable
   after the restart procedure.

   When the M3UA layer at an ASP receives a DUNA message indicating SS7
   destination unavailability at an SG, MTP Users will receive an MTP-
   PAUSE indication and will stop any affected traffic to this
   destination.   When the M3UA receives a DAVA/DRST message, MTP Users
   will receive an MTP-RESUME indication and can resume traffic to the
   newly available SS7 destination, provided the ASP is in the ASP-
   ACTIVE state towards this SGP.

   The ASP MAY choose to audit the availability of unavailable
   destinations by sending DAUD messages.  This would be for example the
   case when an AS becomes active at an ASP and does not have current
   destination statuses.  If MTP restart is in progress at the SG, the
   SGP returns a DUNA message for that destination, even if it received
   an indication that the destination became available or restricted.

   In the IPSP case, MTP restart could be considered if the IPSP also
   has connection to an SS7 network. In that case, the same behavior as
   described above for the SGP would apply to the restarting IPSP.  This
   would also be the case if the IPSPs were perceived as exchanging MTP
   Peer PDUs, instead of MTP primitives between MTP User and MTP
   Provider.  In other words, M3UA does not provide the equivalent to



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   Traffic Restart Allowed messages indicating the end of the restart
   procedure between peer IPSPs that would also be connected to an SS7
   network.

5. Examples of M3UA Procedures

   NOTE: Not all the Notify messages that are appropriate per the Notify
   procedures are shown in these examples.

5.1 Establishment of Association and Traffic between SGPs and ASPs

   These scenarios show the example M3UA message flows for the
   establishment of traffic between an SGP and an ASP or between two
   IPSPs.  In all cases it is assumed that the SCTP association is
   already set up.

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

   These scenarios show the example M3UA message flows for the
   establishment of traffic between an SGP and an ASP where only one ASP
   is configured within an AS (no backup).

5.1.1.1 Single ASP in an Application Server ("1+0" sparing),
        No Registration

               SGP                             ASP1
                |                               |
                |<-------------ASP Up-----------|
                |-----------ASP Up Ack--------->|
                |                               |
                |<------- ASP Active(RCn)-------|  RC: Routing Context
                |-----ASP Active Ack (RCn)----->|      (optional)
                |                               |
                |-----NTFY(AS-ACTIVE)(RCn)----->|
                |                               |

   Note: If the ASP Active message contains an optional Routing Context
   parameter, the ASP Active message only applies for the specified RC
   value(s). For an unknown RC value, the SGP responds with an Error
   message.











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5.1.1.2 Single ASP in Application Server ("1+0" sparing),
        Dynamic Registration

   This scenario is the same as for 5.1.1.1 but with the optional
   exchange of registration information.  In this case the Registration
   is accepted by the SGP.

               SGP                             ASP1
                |                               |
                |<------------ASP Up------------|
                |----------ASP Up Ack---------->|
                |                               |
                |<----REGISTER REQ(LRCn,RKn)----|  LRC: Local Routing
                |                               |       Context
                |----REGISTER RESP(LRCn,RCn)--->|   RK: Routing Key
                |                               |   RC: Routing Context
                |                               |
                |<------- ASP Active(RCn)-------|
                |-----ASP Active Ack (RCn)----->|
                |                               |
                |-----NTFY(AS-ACTIVE)(RCn)----->|
                |                               |

   Note: In the case of an unsuccessful registration attempt (e.g.,
   invalid RKn), the Register Response message will contain an
   unsuccessful indication and the ASP will not subsequently send an ASP
   Active message.
























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5.1.1.3 Single ASP in Multiple Application Servers (each
        with "1+0" sparing), Dynamic Registration (Case 1 - Multiple
        Registration Requests)

               SGP                             ASP1
                |                               |
                |<------------ASP Up------------|
                |----------ASP Up Ack---------->|
                |                               |
                |<----REGISTER REQ(LRC1,RK1)----|  LRC: Local Routing
                |                               |       Context
                |----REGISTER RESP(LRC1,RC1)--->|   RK: Routing Key
                |                               |   RC: Routing Context
                |                               |
                |<------- ASP Active(RC1)-------|
                |-----ASP Active Ack (RC1)----->|
                |                               |
                |                               |
                |<----REGISTER REQ(LRCn,RKn)----|
                |                               |
                |----REGISTER RESP(LRCn,RCn)--->|
                |                               |
                |                               |
                |<------- ASP Active(RCn)-------|
                |-----ASP Active Ack (RCn)----->|
                |                               |

   Note: In the case of an unsuccessful registration attempt (e.g.,
   invalid RKn), the Register Response message will contain an
   unsuccessful indication and the ASP will not subsequently send an ASP
   Active message. Each LRC/RK pair registration is considered
   independently.

   It is not necessary to follow a Registration Request/Response message
   pair with an ASP Active message before sending the next Registration
   Request. The ASP Active message can be sent at any time after the
   related successful registration.














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5.1.1.4 Single ASP in Multiple Application Servers (each
        with "1+0" sparing), Dynamic Registration (Case 2 - Single
        Registration Request)

               SGP                             ASP1
                |                               |
                |<------------ASP Up------------|
                |----------ASP Up Ack---------->|
                |                               |
                |<---REGISTER REQ({LRC1,RK1},---|
                |                   ...,        |
                |                 {LRCn,RKn}),--|
                |                               |
                |---REGISTER RESP({LRC1,RC1},-->|
                |                  ...,         |
                |                 (LRCn,RCn})   |
                |                               |
                |<------- ASP Active(RC1)-------|
                |-----ASP Active Ack (RC1)----->|
                |                               |
                :                               :
                :                               :
                |                               |
                |<------- ASP Active(RCn)-------|
                |-----ASP Active Ack (RCn)----->|
                |                               |

   Note: In the case of an unsuccessful registration attempt (e.g.,
   Invalid RKn), the Register Response message will contain an
   unsuccessful indication and the ASP will not subsequently send an ASP
   Active message.  Each LRC/RK pair registration is considered
   independently.

   The ASP Active message can be sent at any time after the related
   successful registration, and may have more than one RC.

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

   This scenario shows the example M3UA message flows for the
   establishment of traffic between an SGP and two ASPs in the same
   Application Server, where ASP1 is configured to be in the ASP-ACTIVE
   state and ASP2 is to be a "backup" in the event of communication
   failure or the withdrawal from service of ASP1.  ASP2 may act as a
   hot, warm, or cold backup depending on the extent to which ASP1 and
   ASP2 share call/transaction state or can communicate call state under
   failure/withdrawal events.  The example message flow is the same





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   whether the ASP Active messages indicate "Override", "Loadshare" or
   "Broadcast" mode, although typically this example would use an
   Override mode.

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

5.1.3 Two ASPs in an Application Server ("1+1" sparing,
      loadsharing case)

   This scenario shows a similar case to Section 5.1.2 but where the two
   ASPs are brought to the state ASP-ACTIVE and subsequently loadshare
   the traffic.  In this case, one ASP is sufficient to handle the total
   traffic load.

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








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5.1.4 Three ASPs in an Application Server ("n+k" sparing,
      loadsharing case)

   This scenario shows the example M3UA message flows for the
   establishment of traffic between an SGP and three ASPs in the same
   Application Server, where two of the ASPs are brought to the state
   ASP-ACTIVE and subsequently share the load.  In this case, a minimum
   of two ASPs are required to handle the total traffic load (2+1
   sparing).

      SGP                 ASP1                ASP2                ASP3
       |                   |                   |                   |
       |<------ASP Up------|                   |                   |
       |-----ASP Up Ack--->|                   |                   |
       |                   |                   |                   |
       |<-------------------------ASP Up-------|                   |
       |------------------------ASP Up Ack---->|                   |
       |                   |                   |                   |
       |<--------------------------------------------ASP Up--------|
       |--------------------------------------------ASP Up Ack---->|
       |                   |                   |                   |
       |                   |                   |                   |
       |<--ASP Act (Ldshr)-|                   |                   |
       |----ASP Act Ack--->|                   |                   |
       |                   |                   |                   |
       |                   |                   |                   |
       |<-------------------ASP Act. (Ldshr)---|                   |
       |----------------------ASP Act Ack----->|                   |
       |                   |                   |                   |
       |---------Notify (AS-ACTIVE)----------->|                   |
       |----------------------Notify (AS-ACTIVE)------------------>|




















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5.2 ASP Traffic Failover Examples

5.2.1 (1+1 Sparing, Withdrawal of ASP, Backup Override)

   Following on from the example in Section 5.1.2, and ASP1 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)------------>|

   Note: If the SGP M3UA layer detects the loss of the M3UA peer (e.g.,
   M3UA heartbeat loss or detection of SCTP failure), the initial ASP
   Inactive message exchange (i.e., SGP to ASP1) would not occur.

5.2.2 (1+1 Sparing, Backup Override)

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

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















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5.2.3 (n+k Sparing, Loadsharing case, Withdrawal of ASP)

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

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

   For the Notify message to be sent, the SG maintains knowledge of the
   minimum ASP resources required (e.g., if the SG knows that "n+k" =
   "2+1" for a Loadshare AS and "n" currently equals "1").

   Note: If the SGP detects loss of the ASP1 M3UA peer (e.g., M3UA
   heartbeat loss or detection of SCTP failure), the initial ASP
   Inactive message exchange (i.e., SGP-ASP1) would not occur.

5.3 Normal Withdrawal of an ASP from an Application Server
    and Teardown of an Association

   An ASP which is now confirmed in the state ASP-INACTIVE (i.e., the
   ASP has received an ASP Inactive Ack message) may now proceed to the
   ASP-DOWN state, if it is to be removed from service.  Following on
   from Section 5.2.1 or 5.2.3, where ASP1 has moved to the "Inactive"
   state:



















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            SGP                            ASP1
             |                              |
             |<-----ASP Inactive (RCn)------|    RC: Routing Context
             |----ASP Inactive Ack (RCn)--->|
             |                              |
             |<-----DEREGISTER REQ(RCn)-----|    See Notes
             |                              |
             |---DEREGISTER RESP(LRCn,RCn)->|
             |                              |
             :                              :
             |                              |
             |<-----------ASP Down----------|
             |---------ASP Down Ack-------->|
             |                              |

   Note: The Deregistration procedure will typically be used if the ASP
   previously used the Registration procedures for configuration within
   the Application Server.  ASP Inactive and Deregister messages
   exchanges may contain multiple Routing Contexts.

   The ASP should be in the ASP-INACTIVE state and should have
   deregistered in all its Routing Contexts before attempting to move to
   the ASP-DOWN state.

5.4  M3UA/MTP3-User Boundary Examples

5.4.1 At an ASP

   This section describes the primitive mapping between the MTP3 User
   and the M3UA layer at an ASP.

5.4.1.1 Support for MTP-TRANSFER Primitives at the ASP

5.4.1.1.1 Support for MTP-TRANSFER Request Primitive

   When the MTP3-User on the ASP has data to send to a remote MTP3-User,
   it uses the MTP-TRANSFER request primitive.  The M3UA layer at the
   ASP will do the following when it receives an MTP-TRANSFER request
   primitive from the M3UA user:

      - Determine the correct SGP;

      - Determine the correct association to the chosen SGP;

      - Determine the correct stream in the association (e.g.,
        based on SLS);





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      - Determine whether to complete the optional fields of the DATA
        message;

      - Map the MTP-TRANSFER request primitive into the Protocol Data
        field of a DATA message;

      - Send the DATA message to the remote M3UA peer at the SGP,
        over the SCTP association.

         SGP                       ASP
          |                         |
          |<-----DATA Message-------|<--MTP-TRANSFER req.
          |                         |

5.4.1.1.2 Support for the MTP-TRANSFER Indication Primitive

   When the M3UA layer on the ASP receives a DATA message from the M3UA
   peer at the remote SGP, it will do the following:

      - Evaluate the optional fields of the DATA message, if present;

      - Map the Protocol Data field of a DATA message into the
        MTP-TRANSFER indication primitive;

      - Pass the MTP-TRANSFER indication primitive to the user part. In
        case of multiple user parts, the optional fields of the Data
        message are used to determine the concerned user part.

         SGP                       ASP
          |                         |
          |------Data Message------>|-->MTP-Transfer ind.
          |                         |

5.4.1.1.3 Support for ASP Querying of SS7 Destination States

   There are situations such as temporary loss of connectivity to the
   SGP that may cause the M3UA layer at the ASP to audit SS7 destination
   availability/congestion states.  Note: there is no primitive for the
   MTP3-User to request this audit from the M3UA layer as this is
   initiated by an internal M3UA management function.











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         SGP                        ASP
          |                          |
          |<----------DAUD-----------|
          |<----------DAUD-----------|
          |<----------DAUD-----------|
          |                          |
          |                          |

5.4.2 At an SGP

   This section describes the primitive mapping between the MTP3-User
   and the M3UA layer at an SGP.

5.4.2.1 Support for MTP-TRANSFER Request Primitive at the SGP

   When the M3UA layer at the SGP has received DATA messages from its
   peer destined to the SS7 network it will do the following:

      - Evaluate the optional fields of the DATA message, if present, to
        determine the Network Appearance;

      - Map the Protocol data field of the DATA message into an
        MTP-TRANSFER request primitive;

      - Pass the MTP-TRANSFER request primitive to the MTP3 of the
        concerned Network Appearance.

                            SGP                        ASP
                             |                          |
        <---MTP-TRANSFER req.|<---------DATA -----------|
                             |                          |

5.4.2.2 Support for MTP-TRANSFER Indication Primitive at the SGP

   When the MTP3 layer at the SGP has data to pass its user parts, it
   will use the MTP-TRANSFER indication primitive.  The M3UA layer at
   the SGP will do the following when it receives an MTP-TRANSFER
   indication primitive:

      - Determine the correct AS using the distribution function;

      - Select an ASP in the ASP-ACTIVE state

      - Determine the correct association to the chosen ASP;

      - Determine the correct stream in the SCTP association (e.g.,
        based on SLS);




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      - Determine whether to complete the optional fields of the DATA
        message;

      - Map the MTP-TRANSFER indication primitive into the Protocol Data
        field of a DATA message;

      - Send the DATA message to the remote M3UA peer in the ASP, over
        the SCTP association

                           SGP                        ASP
                            |                          |
       --MTP-TRANSFER ind.->|-----------DATA --------->|
                            |                          |

5.4.2.3 Support for MTP-PAUSE, MTP-RESUME, MTP-STATUS Indication
        Primitives

   The MTP-PAUSE, MTP-RESUME and MTP-STATUS indication primitives from
   the MTP3 upper layer interface at the SGP need to be made available
   to the remote MTP3 User Part lower layer interface at the concerned
   ASP(s).

5.4.2.3.1 Destination Unavailable

   The MTP3 layer at the SGP will generate an MTP-PAUSE indication
   primitive when it determines locally that an SS7 destination is
   unreachable.  The M3UA layer will map this primitive to a DUNA
   message. The SGP M3UA layer determines the set of concerned ASPs to
   be informed based on internal SS7 network information associated with
   the MTP-PAUSE indication primitive indication.

                   SGP                       ASP
                    |                         |
 --MTP-PAUSE ind.-->|---------DUNA----------->|--MTP-PAUSE ind.-->
                    |                         |

5.4.2.3.2 Destination Available

   The MTP3 at the SGP will generate an MTP-RESUME indication primitive
   when it determines locally that an SS7 destination that was
   previously unreachable is now reachable.  The M3UA layer will map
   this primitive to a DAVA message.  The SGP M3UA determines the set of
   concerned ASPs to be informed based on internal SS7 network
   information associated with the MTP-RESUME indication primitive.







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                   SGP                       ASP
                    |                         |
--MTP-RESUME ind.-->|-----------DAVA--------->|--MTP-RESUME ind.-->
                    |                         |

5.4.2.3.3 SS7 Network Congestion

   The MTP3 layer at the SGP will generate an MTP-STATUS indication
   primitive when it determines locally that the route to an SS7
   destination is congested.  The M3UA layer will map this primitive to
   a SCON message.  It will determine which ASP(s) to send the SCON
   message to, based on the intended Application Server.

                     SGP                       ASP
                      |                         |
  --MTP-STATUS ind.-->|-----------SCON--------->|--MTP-STATUS ind.-->
                      |                         |

5.4.2.3.4 Destination User Part Unavailable

   The MTP3 layer at the SGP will generate an MTP-STATUS indication
   primitive when it receives an UPU message from the SS7 network.  The
   M3UA layer will map this primitive to a DUPU message.  It will
   determine which ASP(s) to send the DUPU based on the intended
   Application Server.

                      SGP                       ASP
                       |                         |
   --MTP-STATUS ind.-->|----------DUPU---------->|--MTP-STATUS ind.-->
                       |                         |

5.5 Examples for IPSP communication.

   These scenarios show a basic example for IPSP communication for the
   three phases of the connection (establishment, data exchange,
   disconnection).  It is assumed that the SCTP association is already
   set up.  Both single exchange and double exchange behavior are
   included for illustrative purposes.













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5.5.1 Single exchange:

               IPSP-A                           IPSP-B
                 |                                |
                 |-------------ASP Up------------>|
                 |<----------ASP Up Ack-----------|
                 |                                |
                 |<------- ASP Active(RCb)--------|  RC: Routing Context
                 |-----ASP Active Ack (RCb)------>|      (optional)
                 |                                |
                 |                                |
                 |<=========  DATA (RCb) ========>|
                 |                                |
                 |<-----ASP Inactive (RCb)--------|  RC: Routing Context
                 |----ASP Inactive Ack (RCb)----->|      (optional)
                 |                                |
                 |<-----------ASP Down------------|
                 |---------ASP Down Ack---------->|
                 |                                |

   Routing Context are previously agreed to be the same in both
   directions.





























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5.5.2 Double exchange:

               IPSP-A                           IPSP-B
                 |                                |
                 |<-------------ASP Up------------|
                 |-----------ASP Up Ack---------->|
                 |                                |
                 |-------------ASP Up------------>|  (optional)
                 |<----------ASP Up Ack-----------|  (optional)
                 |                                |
                 |<------- ASP Active(RCb)--------|  RC: Routing Context
                 |-----ASP Active Ack (RCb)------>|      (optional)
                 |                                |
                 |------- ASP Active(RCa)-------->|  RC: Routing Context
                 |<-----ASP Active Ack (RCa)------|      (optional)
                 |                                |
                 |<=========  DATA (RCa) =========|
                 |==========  DATA (RCb) ========>|
                 |                                |
                 |<-----ASP Inactive (RCb)--------|  RC: Routing Context
                 |----ASP Inactive Ack (RCb)----->|
                 |                                |
                 |------ASP Inactive (RCa)------->|  RC: Routing Context
                 |<----ASP Inactive Ack (RCa)-----|
                 |                                |
                 |<-----------ASP Down------------|
                 |---------ASP Down Ack---------->|
                 |                                |
                 |------------ASP Down----------->|  (optional)
                 |<--------ASP Down Ack-----------|  (optional)
                 |                                |

   In this approach, only one single exchange of ASP Up message can be
   considered as enough since the response by the other peer can be
   considered as a notice that it is in ASP_UP state.

   For the same reason, only one ASP Down message is needed since once
   that an IPSP receives ASP_Down ack message it is itself considered as
   being in the ASP_Down state and not allowed to receive ASPSM
   messages.











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6. Security Considerations

6.1 Introduction

   M3UA is designed to carry signalling messages for telephony services.
   As such, M3UA must involve the security needs of several parties: the
   end users of the services; the network providers and the applications
   involved.  Additional requirements may come from local regulation.
   While having some overlapping security needs, any security solution
   should fulfill all of the different parties' needs.

6.2 Threats

   There is no quick fix, one-size-fits-all solution for security.  As a
   transport protocol, M3UA has the following security objectives:

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

   M3UA is recommended to be transported on SCTP.  SCTP [17] provides
   certain transport related security features, such as some protection
   against:

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

   When M3UA is running in professionally managed corporate or service
   provider network, it is reasonable to expect that this network
   includes an appropriate security policy framework.  The "Site
   Security Handbook" [22] should be consulted for guidance.

   When the network in which M3UA 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 [23] for more information on configuring IPSEC services.

6.3 Protecting Confidentiality

   Particularly for mobile users, the requirement for confidentiality
   may include the masking of IP addresses and ports.  In this case
   application level encryption is not sufficient; IPSEC ESP [24] SHOULD
   be used instead.  Regardless of which level performs the encryption,
   the IPSEC ISAKMP [25] service SHOULD be used for key management.




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7. IANA Considerations

7.1 SCTP Payload Protocol Identifier

   IANA has assigned an M3UA value for the Payload Protocol Identifier
   in the SCTP DATA chunk.  The following SCTP Payload Protocol
   Identifier is registered:

         M3UA    "3"

   The SCTP Payload Protocol Identifier value "3" SHOULD be included in
   each SCTP DATA chunk, to indicate that the SCTP is carrying the M3UA
   protocol.  The value "0" (unspecified) is also allowed but any other
   values MUST not be used.  This Payload Protocol Identifier is not
   directly used by SCTP but MAY be used by certain network entities to
   identify the type of information being carried in a DATA chunk.

   The User Adaptation peer MAY use the Payload Protocol Identifier as a
   way of determining additional information about the data being
   presented to it by SCTP.

7.2 M3UA Port Number

   IANA has registered SCTP (and UDP/TCP) Port Number 2905 for M3UA.  It
   is recommended that SGPs use this SCTP port number for listening for
   new connections.  SGPs MAY also use statically configured SCTP port
   numbers instead.

7.3 M3UA 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 Guidelines for Writing an IANA Considerations Section in
   RFCs (25]

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







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7.3.1 IETF Defined Message Classes

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

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

7.3.2 IETF Defined Message Types

   The documentation for a new message type MUST include 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 for 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 ("Unsupported
   Message Type").

7.3.3 IETF Defined 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 3.2;
      (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.











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8. References

8.1 Normative References

   [1]  ITU-T Recommendations Q.761 to Q.767, "Signalling System No.7
        (SS7) - ISDN User Part (ISUP)"

   [2]  ANSI T1.113 - "Signaling System Number 7 - ISDN User Part"

   [3]  ETSI ETS 300 356-1 "Integrated Services Digital Network (ISDN);
        Signalling System No.7; ISDN User Part (ISUP) version 2 for the
        international interface; Part 1: Basic services"

   [4]  ITU-T Recommendations Q.711 to Q.715, "Signalling System No. 7
        (SS7) - Signalling Connection Control Part (SCCP)"

   [5]  ANSI T1.112 "Signaling System Number 7 - Signaling Connection
        Control Part"

   [6]  ETSI ETS 300 009-1, "Integrated Services Digital Network (ISDN);
        Signalling System No.7; Signalling Connection Control Part
        (SCCP) (connectionless and connection-oriented class 2) to
        support international interconnection; Part 1: Protocol
        specification"

   [7]  ITU-T Recommendations Q.701 to Q.705, "Signalling System No. 7
        (SS7) - Message Transfer Part (MTP)"

   [8] ANSI T1.111 "Signaling System Number 7 - Message Transfer Part"

   [9]  ETSI ETS 300 008-1, "Integrated Services Digital Network (ISDN);
        Signalling System No.7; Message Transfer Part (MTP) to support
        international interconnection; Part 1: Protocol specification"

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

8.2 Informative References

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

   [12] ITU-T Recommendation Q.720, "Telephone User Part"







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   [13] ITU-T Recommendations Q.771 to Q.775 "Signalling System No. 7
        (SS7) - Transaction Capabilities (TCAP)"

   [14] ANSI T1.114 "Signaling System Number 7 - Transaction
        Capabilities Application Part"

   [15] ETSI ETS 300 287-1, "Integrated Services Digital Network (ISDN);
        Signalling System No.7; Transaction Capabilities (TC) version 2;
        Part 1: Protocol specification"

   [16] 3G TS 25.410 V4.0.0 (2001-04) "Technical Specification - 3rd
        Generation partnership Project; Technical Specification Group
        Radio Access Network; UTRAN Iu Interface: General Aspects and
        Principles"

   [17] Stewart, R., Xie, Q., Mornmeault, K., Sharp, H., Taylor, T.,
        Rytina, I., Kalla, M., Zhang, L. and V. Paxson, "Stream Control
        Transport Protocol", RFC 2960, October 2000.

   [18] ITU-T Recommendation Q.2140 "B-ISDN ATM Adaptation Layer -
        Service Specific Coordination Function for signalling at the
        Network Node Interface (SSCF at NNI)"

   [19] ITU-T Recommendation Q.2110 "B-ISDN ATM Adaptation Layer -
        Service Specific Connection Oriented Protocol (SSCOP)"

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

   [21] ITU-T Recommendation Q.2210 "Message Transfer Part Level 3
        functions and messages using the services of ITU Recommendation
        Q.2140"

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

   [23] Ramakrishnan, S., Floyd, S. and D. Black, "Security Architecture
        for the Internet Protocol", RFC 3168, November 1998.

   [24] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
        (ESP)", RFC 2406, November 1998.

   [25] Maughan, D., Schertler, M., Schneider, M. and J. Turner,
        "Internet Security Association and Key Management Protocol", RFC
        2408, November 1998.

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



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   [27] Morneault, K., Dantu, R., Sidebottom, G., Bidulock, B. and J.
        Heitz, "Signaling System 7 (SS7) Message Transfer Part 2 (MTP2)
        - User Adaptation Layer", RFC 3331, August 2002.

   [28] George, T., et. al., "SS7 MTP2-User Peer-to-Peer Adaptation
        Layer", Work in Progress.

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

9. Acknowledgements

   The authors would like to thank Antonio Roque Alvarez, Joyce
   Archibald, Tolga Asveren, Maria-Cruz Bartolome-Rodrigo, Dan Brendes,
   Antonio Caete, Nikhil Jain, Roland Jesske, Joe Keller, Kurt Kite,
   Ming Lin, Steve Lorusso, Naoto Makinae, Howard May, Francois
   Mouillaud, Barry Nagelberg, Neil Olson, Heinz Prantner, Shyamal
   Prasad, Mukesh Punhani, Selvam Rengasami, John Schantz, Ray Singh,
   Michael Tuexen, Nitin Tomar, Gery Verwimp, Tim Vetter, Kazuo
   Watanabe, Ben Wilson and many others for their valuable comments and
   suggestions.

10. Document Contributors

   Ian Rytina - Ericsson
   Guy Mousseau - Nortel Networks
   Lyndon Ong - Ciena
   Hanns Juergen Schwarzbauer - Siemens
   Klaus Gradischnig - Detecon Inc.
   Mallesh Kalla - Telcordia
   Normand Glaude - Performance Technologies
   Brian Bidulock - OpenSS7
   John Loughney - Nokia

















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Appendix A

A.1 Signalling Network Architecture

   A Signalling Gateway is used to support the transport of MTP3-User
   signalling traffic received from the SS7 network to multiple
   distributed ASPs (e.g., MGCs and IP Databases).  Clearly, the M3UA
   protocol is not designed to meet the performance and reliability
   requirements for such transport by itself.  However, the conjunction
   of distributed architecture and redundant networks provides support
   for reliable transport of signalling traffic over IP.  The M3UA
   protocol is flexible enough to allow its operation and management in
   a variety of physical configurations, enabling Network Operators to
   meet their performance and reliability requirements.

   To meet the stringent SS7 signalling reliability and performance
   requirements for carrier grade networks, Network Operators might
   require that no single point of failure is present in the end-to-end
   network architecture between an SS7 node and an IP-based application.
   This can typically be achieved through the use of redundant SGPs or
   SGs, redundant hosts, and the provision of redundant QOS-bounded IP
   network paths for SCTP Associations between SCTP End Points.
   Obviously, the reliability of the SG, the MGC and other IP-based
   functional elements also needs to be taken into account. The
   distribution of ASPs and SGPs within the available Hosts MAY also be
   considered.  As an example, for a particular Application Server, the
   related ASPs could be distributed over at least two Hosts.
























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   One example of a physical network architecture relevant to SS7
   carrier grade operation in the IP network domain is shown in Figure 5
   below:

           SGs                                     MGCs

   Host#1 **************                          ************** Host#3
          *  ********__*__________________________*__********  *   =
          *  *SGP1.1*__*_____      _______________*__* ASP1 *  *  MGC1
          *  ********  *     \    /               *  ********  *
          *  ********__*______\__/________________*__********  *
          *  *SGP2.1*__*_______\/______      _____*__* ASP2 *  *
          *  ********  *       /\      |    |     *  ********  *
          *      :     *      /  \     |    |     *      :     *
          *  ********  *     /    \    |    |     *  ********  *
          *  * SGPn *  *     |    |    |    |     *  * ASPn *  *
          *  ********  *     |    |    |    |     *  ********  *
          **************     |    |    |    |     **************
                             |    |    \    /
   Host#2 **************     |    |     \  /      ************** Host#4
          *  ********__*_____|    |______\/_______*__********  *   =
          *  *SGP1.2*__*_________________/\_______*__* ASP1 *  *  MGC2
          *  ********  *                /  \      *  ********  *
          *  ********__*_______________/    \_____*__********  *
          *  *SGP2.2*__*__________________________*__* ASP2 *  *
          *  ********  *                          *  ********  *
          *      :     *     SCTP Associations    *      :     *
          *  ********  *                          *  ********  *
          *  * SGPn *  *                          *  * ASPn *  *
          *  ********  *                          *  ********  *
          **************                          **************

   SGP1.1 and SGP1.2 are part of SG1
   SGP2.1 and SGP2.2 are part of SG2

                         Figure 5 - Physical Model

   In this model, each host may have many application processes.  In the
   case of the MGC, an ASP may provide service to one or more
   Application Servers, and is identified as an SCTP end point.  One or
   more Signalling Gateway Processes make up a single Signalling
   Gateway.

   This example model can also be applied to IPSP-IPSP signalling.  In
   this case, each IPSP may have its services distributed across 2 hosts
   or more, and may have multiple server processes on each host.





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   In the example above, each signalling process (SGP, ASP or IPSP) is
   the end point to more than one SCTP association, leading to more than
   one other signalling processes.  To support this, a signalling
   process must be able to support distribution of M3UA messages to many
   simultaneous active associations.  This message distribution function
   is based on the status of provisioned Routing Keys, the status of the
   signalling routes to signalling points in the SS7 network, and the
   redundancy model (active-standby, load sharing, broadcast, n+k) of
   the remote signalling processes.

   For carrier grade networks, the failure or isolation of a particular
   signalling process should not cause stable calls or transactions to
   be lost.  This implies that signalling processes need, in some cases,
   to share the call/transaction state or be able to pass the call state
   information between each other.  In the case of ASPs performing call
   processing, coordination may also be required with the related Media
   Gateway to transfer the MGC control for a particular trunk
   termination. However, this sharing or communication of
   call/transaction state information is outside the scope of this
   document.

   This model serves as an example.  M3UA imposes no restrictions as to
   the exact layout of the network elements, the message distribution
   algorithms and the distribution of the signalling processes.
   Instead, it provides a framework and a set of messages that allow for
   a flexible and scalable signalling network architecture, aiming to
   provide reliability and performance.
























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A.2 Redundancy Models

A.2.1 Application Server Redundancy

   At the SGP, an Application Server list contains active and inactive
   ASPs to support ASP broadcast, loadsharing and failover procedures.
   The list of ASPs within a logical Application Server is kept updated
   in the SGP to reflect the active Application Server Process(es).

   For example, in the network shown in Figure 1, all messages to DPC x
   could be sent to ASP1 in Host3 or ASP1 in Host4.  The AS list at SGP1
   in Host 1 might look like the following:

      Routing Key {DPC=x) - "Application Server #1"
         ASP1/Host3  - State = Active
         ASP1/Host4  - State = Inactive

   In this "1+1" redundancy case, ASP1 in Host3 would be sent any
   incoming message with DPC=x.  ASP1 in Host4 would normally be brought
   to the "active" state upon failure of, or loss of connectivity to,
   ASP1/Host1.

   The AS List at SGP1 in Host1 might also be set up in loadshare mode:

      Routing Key {DPC=x) - "Application Server #1"
         ASP1/Host3 - State = Active
         ASP1/Host4 - State = Active

   In this case, both the ASPs would be sent a portion of the traffic.
   For example the two ASPs could together form a database, where
   incoming queries may be sent to any active ASP.

   Care might need to be exercised by a Network Operator in the
   selection of the routing information to be used as the Routing Key
   for a particular AS.

   For example, where Application Servers are defined using ranges of
   ISUP CIC values, the Operator is implicitly splitting up control of
   the related circuit groups.  Some CIC value range assignments may
   interfere with ISUP circuit group management procedures.

   In the process of failover, it is recommended that in the case of
   ASPs supporting call processing, stable calls do not fail.  It is
   possible that calls in "transition" may fail, although measures of
   communication between the ASPs involved can be used to mitigate this.
   For example, the two ASPs may share call state via shared memory, or





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   may use an ASP to ASP protocol to pass call state information.  Any
   ASP-to-ASP protocol to support this function is outside the scope of
   this document.

A.2.2 Signalling Gateway Redundancy

   Signalling Gateways may also be distributed over multiple hosts.
   Much like the AS model, SGs may comprise one or more SG Processes
   (SGPs), distributed over one or more hosts, using an active/backup or
   a loadsharing model.  Should an SGP lose all or partial SS7
   connectivity and other SGPs exist, the SGP may terminate the SCTP
   associations to the concerned ASPs.

   It is therefore possible for an ASP to route signalling messages
   destined to the SS7 network using more than one SGP.  In this model,
   a Signalling Gateway is deployed as a cluster of hosts acting as a
   single SG.  A primary/backup redundancy model is possible, where the
   unavailability of the SCTP association to a primary SGP could be used
   to reroute affected traffic to an alternate SGP.  A loadsharing model
   is possible, where the signalling messages are loadshared between
   multiple SGPs.  A broadcast model is also possible, where signalling
   messages are sent to each active SGP in the SG. The distribution of
   the MTP3-user messages over the SGPs should be done in such a way to
   minimize message missequencing, as required by the SS7 User Parts.

   It may also be possible for an ASP to use more than one SG to access
   a specific SS7 end point, in a model that resembles an SS7 STP mated
   pair.  Typically, SS7 STPs are deployed in mated pairs, with traffic
   loadshared between them.  Other models are also possible, subject to
   the limitations of the local SS7 network provisioning guidelines.

   From the perspective of the M3UA layer at an ASP, a particular SG is
   capable of transferring traffic to a provisioned SS7 destination X if
   an SCTP association with at least one SGP of the SG is established,
   the SGP has returned an acknowledgement to the ASP to indicate that
   the ASP is actively handling traffic for that destination X, the SGP
   has not indicated that the destination X is inaccessible and the SGP
   has not indicated MTP Restart.  When an ASP is configured to use
   multiple SGPs for transferring traffic to the SS7 network, the ASP
   must maintain knowledge of the current capability of the SGPs to

   handle traffic to destinations of interest.  This information is
   crucial to the overall reliability of the service, for active/backup,
   loadsharing and broadcast models, in the event of failures, recovery
   and maintenance activities.  The ASP M3UA may also use this
   information for congestion avoidance purposes.  The distribution of
   the MTP3-user messages over the SGPs should be done in such a way to
   minimize message missequencing, as required by the SS7 User Parts.



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Editors' Addresses

   Greg Sidebottom
   Signatus Technologies
   Kanata, Ontario, Canada

   EMail: greg@signatustechnologies.com


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

   EMail: kmorneau@cisco.com


   Javier Pastor-Balbas
   Ericsson Espana S.A.
   C/ Retama 1
   28045 Madrid - Spain

   EMail: j.javier.pastor@ericsson.com




























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Full Copyright Statement

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Acknowledgement

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



















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