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RFC 3332


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

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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 Receipt of Primitives from the Layer Management .............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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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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   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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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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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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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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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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   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].



(page 25 continued on part 2)

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