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RFC 3094

Informational
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Tekelec's Transport Adapter Layer Interface

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Network Working Group                                         D. Sprague
Request for Comments: 3094                                    R. Benedyk
Category: Informational                                       D. Brendes
                                                               J. Keller
                                                                 Tekelec
                                                              April 2001


              Tekelec's Transport Adapter Layer Interface

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

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

IESG Note:

   Readers should note that this memo presents a vendor's alternative to
   standards track technology being developed by the IETF SIGTRAN
   Working Group.  The technology presented in this memo has not been
   reviewed by the IETF for its technical soundness or completeness.
   Potential users of this type of technology are urged to examine the
   SIGTRAN work before deciding to use the technology described here.

Abstract

   This document proposes the interfaces of a Signaling Gateway, which
   provides interworking between the Switched Circuit Network (SCN) and
   an IP network.  Since the Gateway is the central point of signaling
   information, not only does it provide transportation of signaling
   from one network to another, but it can also provide additional
   functions such as protocol translation, security screening, routing
   information, and seamless access to Intelligent Network (IN) services
   on both networks.

   The Transport Adapter Layer Interface (TALI) is the proposed
   interface, which provides TCAP (Transaction Capability Application
   Part), ISUP (ISDN User Part), and MTP (Mail Transport Protocol)
   messaging over TCP/IP.  In addition, TALI provides SCCP (Signalling
   Connection Control Part) Management (SCMG), MTP Primitives, dynamic
   registration of circuits, and routing of call control messages based
   on circuit location.

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Table of Contents

   1. Introduction                                                    4
   2. Overview of the TALI Protocol                                   6
   2.1 Traditional PSTN SS7 Networks                                  6
   2.2 Converged SS7 Networks                                         8
   2.3 TALI Protocol Stack Overview                                  10
   2.3.1 An Alternate TALI Protocol Stack using the SAAL Layer       13
   2.3.2 An Alternate TALI Protocol Stack using SCTP                 15
   2.4 Inputs to the TALI Version 1.0 State Machine                  15
   3. TALI Version 1.0                                               17
   3.1 Overview of the TALI Message Structure                        17
   3.1.1 Types of TALI Fields                                        19
   3.2 Detailed TALI Message Structure                               20
   3.2.1 TALI Peer to Peer Messages                                  20
   3.2.1.1 Test Message (test)                                       20
   3.2.1.2 Allow Message (allo)                                      21
   3.2.1.3 Prohibit Message (proh)                                   21
   3.2.1.4 Prohibit Acknowledgement Message (proa)                   21
   3.2.1.5 Monitor Message (moni)                                    22
   3.2.1.6 Monitor Acknowledge Message (mona)                        22
   3.2.2 Service Messages                                            23
   3.2.2.1 SCCP Service Message (sccp)                               23
   3.2.2.1.1 SCCP Encapsulation using TALI                           25
   3.2.2.2 ISUP Service Message (isot)                               27
   3.2.2.2.1 ISUP Encapsulation using TALI                           27
   3.2.2.3 MTP3 Service Message (mtp3)                               28
   3.2.2.3.1 MTP3 Encapsulation using TALI                           29
   3.2.2.4 SAAL Service Message (saal)                               30
   3.2.2.4.1 MTP3 and SAAL Peer to Peer Encapsulation using TALI     31
   3.3 TALI Timers                                                   34
   3.3.1 T1 Timer                                                    34
   3.3.2 T2 Timer                                                    34
   3.3.3 T3 Timer                                                    34
   3.3.4 T4 Timer                                                    34
   3.3.5 Recommended Defaults and Ranges for the TALI Timers         35
   3.4 TALI User Events                                              35
   3.4.1 Management Open Socket Event                                35
   3.4.2 Management Close Socket Event                               36
   3.4.3 Management Allow Traffic Event                              36
   3.4.4 Management Prohibit Traffic Event                           36
   3.5 Other Implementation Dependent TALI Events                    37
   3.6 TALI States                                                   37
   3.7 TALI Version 1.0 State Machine                                38
   3.7.1 State Machine Concepts                                      38
   3.7.1.1 General Protocol Rules                                    38
   3.7.1.2 Graceful Shutdown of a Socket                             39
   3.7.1.3 TALI Protocol Violations                                  39

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   3.7.2 The State Machine                                           40
   3.8 TALI 1.0 Implementation Notes                                 42
   3.8.1 Failure on a TCP/IP Socket                                  42
   3.8.2 Congestion on a TCP/IP Socket                               43
   3.9 TALI 1.0 Limitations                                          43
   4. TALI Version 2.0                                               43
   4.1 Overview of TALI Version 2.0 Features                         45
   4.2 TALI Version Identification                                   47
   4.3 Backwards Compatibility                                       50
   4.3.1 Generating Protocol Violations based on Received Messages   53
   4.4 Overview of the TALI Message Structure                        55
   4.4.1 Types of TALI Fields                                        55
   4.5 Detailed TALI Message Structures for New 2.0 Opcodes          58
   4.5.1 Management Message (mgmt)                                   60
   4.5.1.1 Routing Key Registration Primitive (rkrp)                 61
   4.5.1.1.1 RKRP Data Structures                                    65
   4.5.1.1.1.1 Common Fields in all RKRP Messages                    65
   4.5.1.1.1.2 CIC Based Routing Key Operations                      67
   4.5.1.1.1.3 SCCP Routing Key Operations                           71
   4.5.1.1.1.4 DPC-SI, DPC and SI based Routing Key Operations       74
   4.5.1.1.1.5 Default Routing Key Operations                        76
   4.5.1.1.1.6 Support for Multiple RKRP Registration Operations     78
   4.5.1.1.1.6.1 Multiple Registrations Support                      78
   4.5.1.1.1.6.2 Multiple RKRP Operations in a Single Message        80
   4.5.1.2 MTP3 Primitive (mtpp)                                     82
   4.5.1.3 Socket Option Registration Primitive (sorp)               87
   4.5.2 Extended Service Message (xsrv)                             91
   4.5.3 Special Message (spcl)                                      92
   4.5.3.1 Special Messages Not Supported (smns)                     93
   4.5.3.2 Query Message (qury)                                      93
   4.5.3.3 Reply Message (rply)                                      94
   4.5.3.4 Unsolicited Information Message (USIM)                    95
   4.6 TALI Timers                                                   95
   4.7 TALI User Events                                              95
   4.8 TALI States                                                   96
   4.9 TALI Version 2.0 State Machine                                96
   4.9.1 State Machine Concepts                                      96
   4.9.1.1 General Protocol Rules                                    96
   4.9.1.2 Graceful Shutdown of a Socket                             97
   4.9.1.3 TALI Protocol Violations                                  97
   4.9.2 The State Machine                                           97
   4.10 TALI 2.0 Specification Limitations                          101
   5. Success/Failure Codes                                         101
   6. Security Considerations                                       102
   7. References                                                    102
   8. Acknowledgments                                               103
   9. Authors' Addresses                                            104
   10. Full Copyright Statement                                     105

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1. Introduction

   This document is organized into the following 6 sections:

   -  Introduction to the document
   -  Overview of the TALI Protocol
   -  TALI Version 1.0
   -  TALI Version 2.0
   -  Success/Failure Codes
   -  Security Considerations

   The following terms are used throughout this document.

   Circuit Identification Code (CIC):
   A field identifying the circuit being setup or released.  Depending
   on SI and MSU Type, this field can be 12, 14 or 32 bits.

   Changeover/Changeback (co/cb):
   SS7 MTP3 procedure related to link failure and re-establishment.

   Far End (FE):
   The remote endpoint of a socket connection.

   Far End Allowed (FEA):
   The FE is ready to use the socket for service PDUs.

   Far End Prohibited (FEP):
   The FE is not ready to use the socket for service PDUs.

   Intelligent Network (IN):
   A network that allows functionality to be distributed flexibly at a
   variety of nodes on and off the network and allows the architecture
   to be modified to control the services.

   Management ATM Adaptation Layer (MAAL):
   This layer is a component of SAAL.  This layer maps requests and
   indications between the System Management for the SG and the other
   SAAL layers.  MAAL includes interfaces to/from SSCOP, SSCF, and
   system management.  More information can be found in T1.652.

   Media Gateway (MG):
   A MG terminates SCN media streams, packetizes the media data, if it
   is not already packetized, and delivers packetized traffic  to the
   packet network.  It performs these functions in reverse order for
   media streams flowing from the packet network to the SCN.

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   Media Gateway Controller (MGC):
   An MGC handles the registration and management of resources at the
   MG.  The MGC may have the ability to authorize resource usage based
   on local policy.  For signaling transport purposes, the MGC serves as
   a possible termination and origination point for SCN application
   protocols, such as SS7 ISDN User Part and Q.931/DSS1.

   MTP3 Framing (MTP3F):
   TALI does not require full MTP3 procedures support but rather uses
   the MTP3 framing structure (ie: SIO, Routing Label, etc)

   Near End (NE):
   The local endpoint of a socket connection.

   Near End Allowed (NEA):
   The NE is ready to use the socket for service PDUs.

   Near End Prohibited (NEP):
   The NE is not ready to use the socket for service PDUs.

   Q.BICC ISUP:
   An ISUP+ variant that uses 32 bit CIC codes instead of 14/12 bit CIC
   codes.  ISUP+, or Q.BICC ISUP, is based on the Q.765.BICC
   specification currently being developed in ITU Study Group 11.

   Signaling ATM Adaptation Layer (SAAL):
   This layer is the equivalent of MTP-2 for ATM High Speed Links
   carrying SS7 Traffic as described in GR-2878-CORE [8].  SAAL includes
   SSCF, SSCOP and MAAL.

   Signaling Gateway (SG):
   An SG is a signaling agent that receives/sends SCN native signaling
   at the edge of the IP network.  The SG function may relay, translate
   or terminate SS7 signaling in an SS7-Internet Gateway.  The SG
   function may also be co-resident with the MGC/MG functions to process
   SCN signaling associated with line or trunk terminations controlled
   by the MG (e.g., signaling backhaul).

   Service Specific Coordination Function (SSCF):
   This layer is a component of SAAL.  This layer maps the services
   provided by the lower layers of the SAAL to the needs of a specific
   higher layer user.  In the case of the STP, the higher layer user is
   the MTP-3 protocol, and the SSCF required is that as defined by
   T1.645: SSCF for Support of Signaling at the Network Node Interface
   (SSCF at the NNI).  More information can be found in T1.645.  SSCF
   provides the interface between SSCOP and MTP3 and includes the
   following functions:

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   -  Local Retrieve of messages to support link changeover procedures
   -  Flow control with four levels of congestion

   Switched Circuit Network (SCN):
   The term SCN is used to refer to a network that carries traffic
   within channelized bearers of pre-defined sizes.  Examples include
   Public Switched Telephone Networks (PSTNs) and Public Land Mobile
   Networks (PLMNs).  Examples of signaling protocols used in SCN
   include Q.931, SS7 MTP Level 3 and SS7 Application/User parts.

   Service Specific Connection Oriented Protocol (SSCOP):
   This layer is a component of SAAL.  This layer provides reliable
   point to point data transfer with sequence integrity and error
   recovery by selective retransmission.  Protocol layer interfaces are
   described in T1.637.  Aspects of the protocol include flow control,
   connection control, error reporting to layer management, connection
   maintenance in the prolonged absence of data transfer, local data
   retrieval by the user of the SSCOP, error detection of protocol
   control information and status reporting.  SSCOP provides the link
   layer functions that are:

   -  In-Sequence Delivery
   -  Flow Control
   -  Error Detection/Correction
   -  Keep Alive
   -  Local Data Retrieval
   -  Connection Control
   -  Protocol Error Detection and Recovery

   Signaling Transfer Point (STP):
   Packet switches that provide CCS message routing and transport.  They
   are stored programmed switches that use information contained in the
   message in conjunction with information stored in memory to route the
   message to the appropriate destination signaling point.

2. Overview of the TALI Protocol

2.1 Traditional PSTN SS7 Networks

   The traditional PSTN SS7 network consists of 3 types of devices
   connected via dedicated SS7 signaling links.

   The 3 primary device types for PSTN networks are:

   *  SSP:  Signaling Service Point.  These nodes act as endpoints in
      the SS7 network, originating SS7 messages as users attempt to
      place phone calls.  These nodes contain interfaces into the SS7
      data network and the SS7 voice network.

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   *  STP: Signaling Transfer Point.  These nodes act primarily as
      switches, switching SS7 traffic from node to node throughout the
      network until it reaches another endpoint.  An important feature
      of each STP is to provide SS7 network management functionality
      that allows messages to be delivered even when links and devices
      fail.  STPs also sometimes provide database type services, such as
      Global Title Translations and Local Number Portability.

   *  SCP: Signaling Control Point.  These nodes act as databases.
      These nodes contain stored data that is used to turn SS7 Queries
      into SS7 Replies.

   There are 3 primary types of dedicated SS7 signaling links:

   *  56Kbps SS7 (DS0, V35, OCU) links.  These links implement the MTP-1
      and MTP-2 protocols as defined in [1].

   *  DS1 High Speed Links.  These links use the SAAL protocol to
      provide an alternative to 56Kbps SS7 links that is based on newer,
      faster technology.  These links implement the SS7 protocol as
      defined in [8].

   *  E1 Links.

      Figure 1 provides an overview of the traditional PSTN network.  In
      this network, any of the links can be implemented via either 56
      Kbps, DS1, or E1 links.

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                                 ^
                                / \
                               /SCP\
                              /-----\
                                /  \
                               /    \
                              /      \
                             /        \
               /---\      +---+    +---+      /---\
              | SSP |-----|STP|----|STP|-----| SSP |
               \---/  \  /+-+-+\  /+-+-+ \  / \---/
                       \/   |   \/   |    \/
                       /\   |   /\   |    /\
               /---\  /  \+-+-+/  \+-+-+ /  \ /---\
              | SSP |/----|STP|----|STP|/----| SSP |
               \---/      +---+    +---+      \---/
                           \           /
                            \         /
                             \       /
                              \  ^  /
                               \/ \/
                               /SCP\
                              /-----\

              Figure 1: The Traditional PSTN Network

2.2 Converged SS7 Networks

   In the converged SS7 network, SS7 devices will reside on both the
   traditional PSTN network (with dedicated 56 Kbps and DS1 links) and
   on the IP network (with Ethernet links based on IP protocol).  The
   services of SSPs, STPs, and SCPs can be provided by new types of
   devices that reside on IP networks.  The IP network is not intended
   to completely replace the PSTN, rather devices on the 2 types of
   networks must be able to communicate with one another and convert
   from 1 lower layer protocol to the other.

   Signaling Gateways are new devices that may also function as an STP
   in the converged network.  SGs provide interfaces to:

   *  devices on the SCN (traditional SSPs, STPs, and SCPs)

   *  other SGs

   *  new devices on the IP network

   SGs also continue to perform STP functions such as SS7 network
   management and some database services (such as GTT and LNP).

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   New devices on the IP network include:

   *  Media Gateway Controllers.  In addition to other functions, these
      devices control Media Gateways and perform call processing.

   *  Media Gateways.  In addition to other functions, these devices
      control voice circuits that are used to carry telephone calls.
      MGs + MGCs combine to provide the functionality of traditional
      SSPs.

   *  IP based SCPs.  The database services that are related to SS7 can
      be moved onto devices on the IP network.

      Figure 2 provides an overview of the converged SS7 network.

                         -----              +----+
                /\      /     \-------------| SG |
               /  \----|  SCN  |     +----+ +----+
              /SCP \    \     /------| SG |  |
              ------     -----       +----+  |
                         |   |           |   |
                         |   |           |   |
                         |   |           -----
                         |   |          /     \      /\
                         |   |         |  IP   |----/  \
                         |  /---\       \     /    /SCP \
                         | | SSP |       -----     ------
                         |  \---/         /   \
                         |     |         /     \
                       /---\   |        /       \
                      | SSP |  |     +---+    +---+
                       \---/ +----+  |MGC|    |MGC|
                         |   | MG |  +---+    +---+
                         |   +----+\    \     /
                         |          \    \   /
                         |           \   -----
                         |            \ /     \
                       +----+          |  IP   |
                       | MG |-----------\     /
                       +----+            -----

                    Figure 2: The Converged SS7 Network

   In theory, the TALI protocol can be used between 2 nodes to carry SS7
   traffic across TCP/IP.  Some of the areas that TALI could be used
   include:

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   -  For SG to SG communication across IP
   -  For SG to MGC communication across IP
   -  For SG to IP based SCP communication across IP
   -  For communication between multiple IP based SCPs
   -  For communication between multiple MGCs
   -  For communication between MGCs and MGs
   -  For other IP devices such as DNS, Policy Servers, etc.

   In reality, the communication between MGCs, or between MGC and MG is
   probably better suited to using other protocols.  With respect to the
   Signaling Gateway implementation, the TALI protocol is used to carry
   SS7 traffic:

   -  For SG to SG communication
   -  For SG to MGC communication
   -  For SG to IP based SCP communication

2.3 TALI Protocol Stack Overview

   The Transport Adapter Layer Interface is the proposed interface that
   provides SCCP, ISUP, and MTP messaging encapsulation within a TCP/IP
   packet between two switching elements.  In addition, TALI provides
   SCCP Management (SCMG), MTP Primitives, dynamic registration of
   circuits, and routing of call control messages based on circuit
   location.

   The major purpose of the TALI protocol is to provide a bridge between
   the SS7 Signaling Network and applications that reside within an IP
   network.  Figure 3 provides a simple illustration that highlights the
   protocol stacks used for transport of SS7 MSUs on both the SS7 side
   and the IP side of the SG.

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                 SS7 traffic       SS7 traffic
              via 56Kbps links     via TALI
       +-----------+        +----+          +--------+
       |Traditional|        | SG |          |   IP   |
       |SS7 Devices|<------>|    |<-------->| Devices|
       +-----------+        +----+          +--------+


          SS7                          SS7, TALI, TCP/IP
          protocol stack               protocol stack
        +---------------+              +---------------+
        |SS7 application|              |SS7 application|
        |layer          |              |layer          |
        +-------+-------+              +-------+-------+
        | TCAP  | ISUP  |              | TCAP  | ISUP  |
        +-------+       |              +-------+       |
        | SCCP  |       |              | SCCP  |       |
        +-------+-------+              +-------+-------+
        |    MTP3       |              |    MTP3       |
        +---------------+              +---------------+
        |    MTP2       |              |    TALI       |
        +---------------+              +---------------+
        |    MTP1       |              |    TCP        |
        |   (& phy.     |              +---------------+
        |    layer)     |              |    IP         |
        +---------------+              +---------------+
                                       |    MAC        |
                                       |   (& phy.     |
                                       |    layer)     |
                                       +---------------+

       Figure 3: TALI Protocol to carry SS7 over TCP/IP

   From Figure 3, several observations can be made:

   *  The TALI layer is used when transferring SS7 over IP.

   *  When SS7 traffic is carried over a IP network, the MTP2 and MTP1
      layers of a traditional 56 Kbps link are replaced by the TALI,
      TCP, IP, and MAC layers

   *  The TALI layer sits on top of the TCP layer.

   *  The TALI layer sits below the various SS7 layers (MTP3, SCCP/TCAP,
      ISUP, and applications).  The data from these SS7 layers is
      carried as the data portion of TALI service data packets.

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   Some of the facts concerning the TALI protocol which are important to
   understanding how TALI works that are not evident from Figure 3
   include the following:

   *  Each TALI connection is provided over a single TCP socket.

      *  The standard Berkeley sockets interface to the TCP is used by
         the TALI layer to provide connection oriented service from
         endpoint to peer endpoint.

      *  TCP sockets are based on a Client/Server architecture; one end
         of the TALI connection must be defined as the 'server side',
         the other end is a 'client'.

      *  The client/server roles are important only in bringing up the
         TCP connection between the 2 endpoint, once the connection is
         established both ends use the same Berkeley sockets calls
         (send, recv) to transfer data.

      *  The TCP socket must be connected before the 2 TALI endpoints
         can begin communicating.

   *  TALI provides user control over each TALI connection that is
      defined.  This control:

      *  Allows the user to control when each TALI connection will be
         made

      *  Allows the user to control when each TALI connection is allowed
         to carry SS7 traffic

      * Allows the user to control the graceful shutdown of each socket

   *  TALI provides Peer to Peer messages.  These messages originate
      from the TALI layer of one endpoint of the connection and are
      terminated at the TALI layer of the other endpoint.  Peer to Peer
      messages are used:

      *  To provide test and watchdog maintenance messages

      *  To control the ability of each socket to carry SS7 service
         messages

   *  TALI provides Service messages.  These messages originate from the
      layer above the TALI layer of one endpoint of the connection and
      are transferred to and terminated at the layer above the TALI
      layer of the other endpoint.

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      *  The service messages provide several different ways to
         encapsulate the SS7 messages (SCCP/TCAP, ISUP, and other MTP3
         layer data) across the TCP/IP connection.

      *  As we will see later, different Service opcodes are used to
         communicate across the TALI socket exactly how each SS7 message
         has been encapsulated.

   *  A set of TALI timers is defined.  These timers are used to
      correctly implement the TALI state machine.

2.3.1 An Alternate TALI Protocol Stack using the SAAL Layer

   This section presents a different, slightly more complex, TALI
   protocol stack that can be used in place of the protocol stack in the
   previous section.

   Figure 3 in the previous section provided a simple illustration that
   highlighted the basic TALI protocol stack that can be used to
   transport SS7 MSUs between 56 Kbps links on the SS7 side of an SG and
   the IP devices.

   Figure 4 below illustrates an alternate TALI protocol stack that
   includes the SAAL layer as part of the data transferred across the
   TCP/IP connection.

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                    SS7 traffic       SS7 traffic
                    via DS1 links     via TALI
          +-----------+        +----+          +--------+
          |Traditional|        | SG |          |   IP   |
          |SS7 Devices|<------>|    |<-------->| Devices|
          +-----------+        +----+          +--------+


             SS7 DS1                   SS7, TALI, TCP/IP
             protocol stack            protocol stack
           +-----------------+        +-----------------+
           | SS7 application |        | SS7 application |
           | layer           |        | layer           |
           +--------+--------+        +--------+--------+
           |  TCAP  | ISUP   |        |  TCAP  | ISUP   |
           +--------+        |        +--------+        |
           |  SCCP  |        |        |  SCCP  |        |
           +--------+--------+        +--------+--------+
           |      MTP3       |        |      MTP3       |
           +-----------------+        +-----------------+
           |    SAAL         |        |     SAAL        |
           |(SSCF,MAAL,SSCOP)|        |(SSCF,MAAL,SSCOP)|
           +-----------------+        +-----------------+
           |     AAL5        |        |     TALI        |
           +-----------------+        +-----------------+
           |     ATM         |        |     TCP         |
           |    (& phy.      |        +-----------------+
           |     layer)      |        |     IP          |
           +-----------------+        +-----------------+
                                      |     MAC         |
                                      |    (& phy.      |
                                      |     layer)      |
                                      +-----------------+

        Figure 4: An Alternate TALI Protocol Stack with SAAL

   The following bullets provide a discussion regarding the differences
   between these 2 protocol stacks, the reasons for having 2 protocol
   stacks, and the advantages of each:

   *  When the TALI protocol stack is implemented without the SAAL
      layer, as in Figure 3, the SEQUENCE NUMBER of the SS7 MSU is NOT
      part of the data transferred across the TCP/IP connection.  In 56
      Kbps SS7 links, the MTP2 header contains an 8 bit sequence number
      for each MSU.  The sequence number is used to preserve message
      sequencing and to support complex SS7 procedures involving MSU
      retrieval during link changeover and changeback.  As indicated in
      Figure 3, the MTP2 header is NOT part of the data transferred

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      across the TCP/IP connection.  The TALI protocol stack without
      SAAL still guarantees correct sequencing of SS7 data (this
      sequencing is provided by sequence numbers in the TCP layer),
      however that protocol stack can not support SS7 changeover and
      changeback procedures.

   *  When the TALI protocol stack is implemented with the SAAL layer,
      as in Figure 4, the SEQUENCE NUMBER of the SS7 MSU IS part of the
      data transferred across TCP/IP.  In SS7 DS1 links, the SSCOP
      trailer contains a 24 bit sequence number for each MSU.  This 24
      bit sequence number serves the same purposes as the 8 bit SS7
      sequence number.  As indicated in Figure 4, the SSCOP trailer IS
      part of the data transferred across the TCP/IP connection.  The
      protocol stack in Figure 4 can support SS7 changeover and
      changeback procedures.

   *  Implementing the TALI protocol with SAAL therefore provides
      support for SS7 co/cb and data retrieval and can help to minimize
      MSU loss as SS7 links are deactivated.  However, implementing SAAL
      is not a trivial matter.  The SAAL layer consists of 3 sublayers
      (SSCF, SSCOP, and MAAL), one of which (SSCOP) is quite involved.
      It is envisioned that most SS7 to TCP/IP applications will NOT
      choose to implement SAAL.

2.3.2 An Alternate TALI Protocol Stack using SCTP

   The TALI protocol is dependent on a reliable transport layer below
   it.  At the initial design of TALI, TCP was the only reliable, proven
   transport layer.  Simple Control Transport Protocol (SCTP) is
   currently being designed as a transport later specifically for
   signalling.  Once SCTP is a proven and accepted transport protocol,
   SCTP can then be used in place of TCP as shown in Figures 3 and 4.

2.4 Inputs to the TALI Version 1.0 State Machine

   Figure 5 illustrates the inputs that affect the TALI State Machine.
   Inputs to the state machine include:

   *  Management events (ie: requests from the human user of the TALI
      connection) to control the operation of a particular TALI session.

   *  TALI messages received from the Peer.  These messages include peer
      to peer messages as well as service data messages.

   *  Events from the User of the TALI layer.  The user is the layer
      above TALI in the protocol stack, either the SS7 or SAAL layer.

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   *  Implementation Dependent Events.  Each implementation must provide
      inputs into the TALI state machine such as:

      *  Socket Events

      *  TALI protocol violations.  The TALI state machine must detect
         protocol violations and act accordingly.

      * Timer events.

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      +====+                                   +============+
      |    |    +---------+ +-------------+    |            |
      |User|    | Service | | Mgmt. Open  |    | MANAGEMENT |
      |Part|<-->| Message | | Mgmt. Close |<-->|            |
      |    |    |         | | Mgmt. Proh. |    |            |
      |    |    +---------+ | Mgmt. Allow |    +============+
      +====+          ^     +-------------+
                      |            ^
                      |            |
                      v            v
      +========================================================+
      |                 TALI State Machine                     |
      +========================================================+
            ^               ^                 ^             ^
            |               |                 |             |
            |               |                 |             |
            v               |                 |             |
       +---------+  +-----------------+ +-----------+ +------------+
       | Received|  | Connection est. | | Protocol  | | T1 Expired |
       | 'test'  |  | Connection lost | | Violation | | T2 Expired |
       | 'allo'  |  |                 | |           | | T3 Expired |
       | 'proh'  |  +-----------------+ +-----------+ | T4 Expired |
       | 'proa'  |          ^                 ^       +------------+
       | 'moni'  |          |                 |              ^
       | 'mona'  |          |                 |              |
       |    or   |          |                 |              |
       | Service |          |                 |              |
       | Message |    +========================================+
       +---------+    |         IMPLEMENTATION                 |
            ^         |           DEPENDENT                    |
            |         +========================================+
            |
            v
        +============+
        |    PEER    |
        |            |
        +============+

      Figure 5: Overview of Inputs to the TALI 1.0 State Machine


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