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RFC 3094

 
 
 

Tekelec's Transport Adapter Layer Interface

Part 2 of 4, p. 18 to 51
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3. TALI Version 1.0

   This chapter provides the states, messages, message exchange rules
   and state machine that must be implemented to provide a TALI version
   1.0 protocol layer.

3.1 Overview of the TALI Message Structure

   Table 2 provides a summary of the messages and message structure used
   in TALI version 1.0.

   +------------------------------------------------------------------+
   | OCTET | DESCRIPTION              | SIZE     | VALUE  |    TYPE   |
   +------------------------------------------------------------------+
   | 0..3  | SYNC                     | 4 Octets |        | 4 byte    |
   |       |                          |          |        | ASCII     |
   +------------------------------------------------------------------+
   |       |   TALI                   |          | 'TALI' |           |
   +------------------------------------------------------------------+
   | 4..7  | OPCODE                   | 4 Octets |        | 4 byte    |
   |       |                          |          |        | ASCII     |
   +------------------------------------------------------------------+
   |       |   Test Service           |          | 'test' |           |
   |       |   Allow Service          |          | 'allo' |           |
   |       |   Prohibit Service       |          | 'proh' |           |
   |       |   Prohibit Service Ack   |          | 'proa' |           |
   |       |   Monitor Socket         |          | 'moni' |           |
   |       |   Monitor Socket Ack     |          | 'mona' |           |
   |       |   SCCP Service           |          | 'sccp' |           |
   |       |   ISUP Service over TALI |          | 'isot' |           |
   |       |   MTP3 Service over TALI |          | 'mtp3' |           |
   |       |   Service over SAAL      |          | 'saal' |           |
   +------------------------------------------------------------------+
   | 8..9  | LENGTH                   | 2 Octets |        | integer   |
   |       |   (least significant     |          |        |           |
   |       |    byte first) non-0     |          |        |           |
   |       |    if Service or         |          |        |           |
   |       |    Socket monitor message|          |        |           |
   +------------------------------------------------------------------+
   | 10..X | DATA PAYLOAD             | variable |        | variable  |
   +------------------------------------------------------------------+

                 Table 2: Message Structure for TALI 1.0

   Table 3 indicates the valid values of the LENGTH field for each
   version 1.0 opcode.  The LENGTH field is always an indication of the
   # of bytes contained in the DATA PAYLOAD portion of a general TALI
   message.

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   +------------------------------------------------------------------+
   | OPCODE | VALID LENGTH VALUES | COMMENTS                          |
   +------------------------------------------------------------------+
   | test   | 0 bytes             |                                   |
   +------------------------------------------------------------------+
   | allo   | 0 bytes             |                                   |
   +------------------------------------------------------------------+
   | proh   | 0 bytes             |                                   |
   +------------------------------------------------------------------+
   | proa   | 0 bytes             |                                   |
   +------------------------------------------------------------------+
   | moni   | 0-200 bytes         | A maximum length is provided so   |
   |        |                     | that the maximum ethernet frame   |
   |        |                     | size is not exceeded.             |
   +------------------------------------------------------------------+
   | mona   | 0-200 bytes         | Mona reply length and content must|
   |        |                     | match the original moni (with the |
   |        |                     | exception of the opcode)          |
   +------------------------------------------------------------------+
   | sccp   | 12-265 bytes        | These are the valid sizes for the |
   |        |                     | SCCP-ONLY portions of SCCP UDT    |
   |        |                     | MSUs                              |
   +------------------------------------------------------------------+
   | isot   | 8-273 bytes         | The length is the number of octets|
   |        |                     | in the MTP3 and higher layer(s) of|
   |        |                     | the SS7 MSU.  This length includes|
   |        |                     | the SIO byte, the MTP3 routing    |
   |        |                     | label, the CIC code, and the      |
   |        |                     | ISUP Message Type field, and any  |
   |        |                     | other bytes that may exist as part|
   |        |                     | of the SIF (Service Information   |
   |        |                     | Field)                            |
   +------------------------------------------------------------------+
   | mtp3   | 5-280 bytes         | The length is the number of octets|
   |        |                     | in the MTP3 and higher layer(s) of|
   |        |                     | the SS7 MSU.  This length includes|
   |        |                     | the SIO byte and the MTP3 routing |
   |        |                     | labeld, and any other bytes that  |
   |        |                     | may exist as part of the SIF      |
   |        |                     | (Service Information Field)       |
   +------------------------------------------------------------------+
   | saal   | 11-280 bytes        | The length is the number of octets|
   |        |                     | in the MTP3 and higher layer(s) of|
   |        |                     | the SS7 MSU.  This length includes|
   |        |                     | the SIO byte and all bytes in the |
   |        |                     | SIF (Service Information Field)   |
   |        |                     | field.  The MTP3 routing label is |
   |        |                     | part of the SIF field.  Seven (7) |

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   |        |                     | octets of SSCOP trailer is added  |
   |        |                     | to the message.  The SSCOP trailer|
   |        |                     | bytes are also included in the    |
   |        |                     | length.                           |
   +------------------------------------------------------------------+

        Table 3: Valid Length Fields for Each Opcode in TALI 1.0

3.1.1 Types of TALI Fields

   Several field types are used in the general TALI message structure.

   +------------------------------------------------------------------+
   |Field Type | Implementation Notes for that Type                   |
   +------------------------------------------------------------------+
   |4 byte     | * 4 byte ASCII text strings are used to define the   |
   |ASCII text |   sync code and the opcode of the basic TALI message.|
   |           | * These fields are case sensitive, the coding for    |
   |           |   each sync and opcode literal needs to match the    |
   |           |   case specified in Table 2.                         |
   |           | * The standard ASCII conversion table is used to     |
   |           |   transform each character into a byte.              |
   |           | * The order of the ASCII characters is important.    |
   |           |   The first character in the string must be the      |
   |           |   first character transmitted across the wire.       |
   |           | * For example, if the string being encoded is 'abCD',|
   |           |   the order of the bytes as they are transferred     |
   |           |   over the wire must be:                             |
   |           |     1st byte: 0x61 ('a')  3rd byte: 0x43 ('C')       |
   |           |     2nd byte: 0x62 ('b')  4th byte: 0x44 ('D')       |
   |           | * The software for each implementation should be     |
   |           |   written in a manner that accounts for the required |
   |           |   byte order of transmission (ie: the Big Endian/    |
   |           |   Little Endian characteristics of the processor     |
   |           |   need to be dealt with in the software.             |
   +------------------------------------------------------------------+
   |Integer    | * A 1, 2 or 4 byte field to be treated as an integer |
   |           |   value.  Integer fields should be transmitted Least |
   |           |   Significant Byte first across the wire.            |
   |           | * The software for each implementation should be     |
   |           |   written in a manner that accounts for the required |
   |           |   byte order of transmission (ie: the Big Endian/    |
   |           |   Little Endian characteristics of the processor     |
   |           |   need to be dealt with in the software.             |
   +------------------------------------------------------------------+
   |Variable   | * The definition of the message structure for this   |
   |           |   field is governed by other specifications.         |
   |           | * For example, when transferring MTP3 service data   |

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   |           |   via a 'mtp3' opcode, the DATA PAYLOAD begins with  |
   |           |   the SIO byte of the MTP3 routing label.  The       |
   |           |   structure for the entire DATA PAYLOAD is governed  |
   |           |   by the MTP3 message structure defined in [1].      |
   +------------------------------------------------------------------+
   |X byte     | * ASCII text fields of sizes other than 4 bytes      |
   |ASCII text |   should be supported according to the same rules    |
   |           |   presented for the 4 byte ASCII text fields.  For   |
   |           |   instance, an 8 byte string such as 'ab01cd23' could|
   |           |   be used, where the 'a' would be the first byte of  |
   |           |   the field transmitted out the wire.                |
   +------------------------------------------------------------------+

         Table 4: Implementation Notes for each Type of TALI field

3.2 Detailed TALI Message Structure

3.2.1 TALI Peer to Peer Messages

   The following subsections provide more information regarding the TALI
   Peer to Peer messages that are implemented in version 1.0.  The TALI
   peer to peer messages originate at the TALI layer of 1 end of the
   socket connection (the near end) and are terminated at the TALI layer
   of the far end of the connection.

3.2.1.1 Test Message (test)

   The 'test' message is used by a TALI implementation to query the
   remote end of the TALI connection with respect to the willingness of
   the remote end to carry SS7 service data.  This message asks the
   other end: are you ready to carry service data?  This message is sent
   periodically by each TALI implementation based on a T1 timer
   interval.  Upon receiving 'test', a TALI implementation must reply
   with either 'proh' or 'allo' to indicate the nodes willingness to
   carry SS7 service data over that TALI connection.

   +------------------------------------------------------------------+
   | Octets | Field Name  | Description                               |
   +------------------------------------------------------------------+
   | 0..3   | SYNC        | 'TALI'                                    |
   +------------------------------------------------------------------+
   | 4..7   | OPCODE      | 'test'                                    |
   +------------------------------------------------------------------+
   | 8..9   | LENGTH      | Length = 0                                |
   +------------------------------------------------------------------+

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3.2.1.2 Allow Message (allo)

   The 'allo' message is sent in reply to a 'test' query, or in response
   to some internal implementation event, to indicate that a TALI
   implementation IS willing to carry SS7 service data over the TALI
   session.  This message informs the far end that SS7 traffic can be
   transmitted on the socket.  'allo' is one of the 2 possible replies
   to a 'test' message.  Before SS7 traffic can be carried over a
   socket, both ends of the connection need to send 'allo' messages.

   +------------------------------------------------------------------+
   | Octets | Field Name  | Description                               |
   +------------------------------------------------------------------+
   | 0..3   | SYNC        | 'TALI'                                    |
   +------------------------------------------------------------------+
   | 4..7   | OPCODE      | 'allo'                                    |
   +------------------------------------------------------------------+
   | 8..9   | LENGTH      | Length = 0                                |
   +------------------------------------------------------------------+

3.2.1.3 Prohibit Message (proh)

   The 'proh' message is sent in reply to a 'test' query, or in response
   to some internal implementation event, to indicate that a TALI
   implementation is NOT willing to carry SS7 service data over the TALI
   session.  This message informs the far end that SS7 traffic can not
   be transmitted on the socket.  'proh' is one of the 2 possible
   replies to a 'test' message.  As long as 1 end of the connection
   remains in the 'prohibited' state, SS7 traffic can not be carried
   over the socket.

   +------------------------------------------------------------------+
   | Octets | Field Name  | Description                               |
   +------------------------------------------------------------------+
   | 0..3   | SYNC        | 'TALI'                                    |
   +------------------------------------------------------------------+
   | 4..7   | OPCODE      | 'proh'                                    |
   +------------------------------------------------------------------+
   | 8..9   | LENGTH      | Length = 0                                |
   +------------------------------------------------------------------+

3.2.1.4 Prohibit Acknowledgement Message (proa)

   The 'proa' message is sent by a TALI implementation each time a
   'proh' is received from the far end.  This message is sent to
   indicate to the far end that his 'prohibit' message was received
   correctly and will be acted on accordingly.

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   +------------------------------------------------------------------+
   | Octets | Field Name  | Description                               |
   +------------------------------------------------------------------+
   | 0..3   | SYNC        | 'TALI'                                    |
   +------------------------------------------------------------------+
   | 4..7   | OPCODE      | 'proa'                                    |
   +------------------------------------------------------------------+
   | 8..9   | LENGTH      | Length = 0                                |
   +------------------------------------------------------------------+

3.2.1.5 Monitor Message (moni)

   The 'moni' message provides a generic ECHO capability that can be
   used by each TALI implementation as that implementation sees fit.  A
   TALI version 1.0 implementation does not have to originate a 'moni'
   message to be compliant with the 1.0 specification.  The primary
   intent of this message is to provide a way for the TALI layer to test
   the round-trip message transfer time on a socket.  A 'mona' message
   must be sent in reply to each received 'moni' message.  The DATA
   portion of a 'moni' message is vendor implementation dependent.  The
   DATA portion of each 'mona' reply must exactly match the DATA portion
   of the 'moni' that is replied to.  Regardless of whether an
   implementation chooses to send 'moni' or not, 'mona' must be sent in
   response to each 'moni' in order to remain compliant with the TALI
   protocol.

   +------------------------------------------------------------------+
   | Octets | Field Name  | Description                               |
   +------------------------------------------------------------------+
   | 0..3   | SYNC        | 'TALI'                                    |
   +------------------------------------------------------------------+
   | 4..7   | OPCODE      | 'moni'                                    |
   +------------------------------------------------------------------+
   | 8..9   | LENGTH      | Length                                    |
   +------------------------------------------------------------------+
   | 10..X  | DATA PAYLOAD| Vendor Dependent                          |
   +------------------------------------------------------------------+

3.2.1.6 Monitor Acknowledge Message (mona)

   As mentioned above, the 'mona' must be sent in reply to each received
   'moni'.  The contents of the 'mona' DATA area must match the DATA
   area of the received 'moni' message.

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   +------------------------------------------------------------------+
   | Octets | Field Name  | Description                               |
   +------------------------------------------------------------------+
   | 0..3   | SYNC        | 'TALI'                                    |
   +------------------------------------------------------------------+
   | 4..7   | OPCODE      | 'mona'                                    |
   +------------------------------------------------------------------+
   | 8..9   | LENGTH      | Length                                    |
   +------------------------------------------------------------------+
   | 10..X  | DATA PAYLOAD| Vendor Dependent                          |
   +------------------------------------------------------------------+

3.2.2 Service Messages

   The following subsections provide more information regarding the TALI
   Service messages that are implemented in version 1.0.  TALI Service
   messages are used to carry SS7 MSUs across the IP network.  The
   information in this section includes details with respect to how to
   encapsulate SS7 MSUs into TCP/IP frames using each of the TALI
   service opcodes.  The TALI service messages originate at the layer
   above TALI, are transported across the IP network via a TALI service
   message, and are delivered to the layer above TALI at the far end of
   the TALI connection.

3.2.2.1 SCCP Service Message (sccp)

   The 'sccp' opcode is used to deliver SS7 MSUs with a Service
   Indicator of 3 (SCCP) over a TALI connection.  This opcode is only
   used on TALI protocol stacks that are implemented without SAAL.  The
   MTP3 layer of the SS7 MSU is NOT part of the data transferred across
   TCP/IP for this opcode; the data portion of the TALI 'sccp' message
   begins with the first byte of the SCCP data area in the SS7 MSU
   (after the MTP3 routing label).  The first byte in the SCCP data area
   is an SCCP message type field.

   Several restrictions on the SCCP messages that this TALI opcode can
   carry exist.  These restrictions are as follows:

   *  SCCP messages contain an SCCP message type field.  The SCCP
      messages that are supported by TALI 1.0 implementations are
      limited to Class 0 and Class 1 SCCP messages with a message type
      field of either:

      *  UDT
      *  UDTS
      *  XUDT
      *  XUDTS

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   *  SCCP messages must contain a Point Code in the 'calling party'
      area in order to be transferred across the TCP/IP connection as a
      'sccp' message.  An implementation may choose to modify the
      original SCCP MSU to add an appropriate calling party point code
      before transmission across TALI if desired.

   *  SCCP messages must contain a Point Code in the 'called party' area
      in order to be transferred across the TCP/IP connection as a
      'sccp' message.  An implementation may choose to modify the
      original SCCP MSU to add an appropriate called party point code
      before transmission across TALI if desired.

   *  The encoding of the SS7 SCCP MSUs, as they are transmitted across
      TALI via 'sccp', should remain compliant with the ANSI
      specifications (T1.112 and T1.114) that apply to the SCCP and TCAP
      portions of the message respectively.

   NOTE 1: SCCP Subsystem Management for the IP based SCP's is supported
   via this 'sccp' opcode.  SS7 SCCP Management messages are controlled
   by an SCMG SS7 process.  SCMG sends the management messages via SCCP
   UNITDATA (UDT) messages.  Therefore, the SCMG messages can be sent
   across the TALI connection.

   NOTE 2: 'sccp' TALI messages will not include the MTP3 header and
   therefore will not retain the original DPC/OPC of the SS7 MSU.  Each
   TALI implementation needs to consider if/how to provide this DPC/OPC
   information in the SCCP portion of the message.  For example the DPC
   can be replicated to the point code in the SCCP Called Party Address
   area and the OPC can be replicated to the point code in the SCCP
   Calling Party Address area.

   +------------------------------------------------------------------+
   | Octets | Field Name  | Description                               |
   +------------------------------------------------------------------+
   | 0..3   | SYNC        | 'TALI'                                    |
   +------------------------------------------------------------------+
   | 4..7   | OPCODE      | 'sccp'                                    |
   +------------------------------------------------------------------+
   | 8..9   | LENGTH      | Length                                    |
   +------------------------------------------------------------------+
   | 10..X  | SCCP Data   | SCCP data starting at the first byte after|
   |        |             | the Layer 3 Routing Label (data does not  |
   |        |             | include the SIO or Routing Label)         |
   +------------------------------------------------------------------+

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3.2.2.1.1 SCCP Encapsulation using TALI

   When an SCCP MSU arrives at an SG from a 56 Kbps or DS1 link and is
   routed within the SG for transmission to an IP device, the SG
   performs the following processing on the SS7 MSU:

   *  discards the MTP Layer 2 information, CRC and flags

   *  places the DPC from MTP Layer 3 into the Called Party Address
      field of the SCCP layer; the Calling Party Address field is
      created if it does not exist and then filled

   *  places the OPC from MTP Layer 3 into the Calling Party Address
      field of the SCCP layer if there is no Calling Party Point Code

   *  places the modified SCCP and unchanged TCAP data in the service
      payload area of the TALI packet

   *  The SYNC field is set

   *  The OPCODE is set to 'sccp'

   * The LENGTH is set to the number of octets in the SERVICE field

   Once the fully formed 'sccp' TALI packet is created, it is handed to
   the TCP socket layer and transmitted.  The transmission process will
   add TCP, IP and MAC header information.

   Since the routing information from MTP Layer 3 is placed in the SCCP
   part of the outgoing message, no routing information needs to be
   saved by the SG.

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                                SS7 MSU

           |          Layer 3          |     Layer 2      |
           |                           |                  |
      +----+---+-----+-----+-------+---+--+---+---+---+---+----+
      |Flag|FCS|TCAP |SCCP |Routing|SIO|LI|FIB|FSN|BIB|BSN|Flag|
      |    |   |Layer|Layer| Label |   |  |   |   |   |   |    |
      +----+---+-----+-----+-------+---+--+---+---+---+---+----+
               |           |
               |           |
               |           |
        TALI   +-----------+---+------+----+
        Packet |  Service  |LEN|Opcode|SYNC|
               +-----------+---+------+----+
               |                           |
               |                           |
               |                           |
               +---------------------------+------+------+------+
        IP     | TALI Packet               |TCP   | IP   | MAC  |
        Packet |                           |Header|Header|Header|
               +---------------------------+------+------+------+

   Figure 6: Encapsulation of SCCP MSUs using the TALI 'sccp' opcode

   When an 'sccp' TALI packet is received on by an SG from an IP device,
   the SG performs the following processing on the 'sccp' packet:

   *  validates the TALI header

   *  Allocates space for a new SS7 message

   *  Regenerates the SIO with the Sub-Service Field set to National
      Network, priority of zero (0), Service Indicator set to SCCP

   *  extracts the SCCP/TCAP data from the SERVICE area and places it in
      the new SS7 message

   *  sets the DPC to the SCCP Called Party Point Code

   *  sets the OPC to the SCCP Calling Party Point Code

   *  randomly generates the SLS

   Once the 'sccp' packet is transformed back into a normal SS7 MSU, the
   MSU is routed within the SG according to the normal SS7 routing
   procedures.

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3.2.2.2 ISUP Service Message (isot)

   The 'isot' opcode is used to deliver SS7 MSUs with a Service
   Indicator of 5 (ISUP) over a TALI connection.  This opcode is only
   used on TALI protocol stacks that are implemented without SAAL.  The
   MTP3 layer of the SS7 MSU IS part of the data transferred across
   TCP/IP for this opcode; the data portion of the TALI 'isot' message
   begins with the SIO byte of the MTP3 header in the SS7 MSU.

   +------------------------------------------------------------------+
   | Octets | Field Name  | Description                               |
   +------------------------------------------------------------------+
   | 0..3   | SYNC        | 'TALI'                                    |
   +------------------------------------------------------------------+
   | 4..7   | OPCODE      | 'isot'                                    |
   +------------------------------------------------------------------+
   | 8..9   | LENGTH      | Length                                    |
   +------------------------------------------------------------------+
   | 10..X  | ISUP Data   | Raw ISUP data starting at the Layer 3 SIO |
   |        |             | field.                                    |
   +------------------------------------------------------------------+

3.2.2.2.1 ISUP Encapsulation using TALI

   When an ISUP MSU arrives at an SG from a 56 Kbps or DS1 link and is
   routed within the SG to a IP device, the SG performs the following
   processing on the SS7 MSU:

   *  discards the MTP Layer 2 information, CRC and flags

   *  places MTP Layer 3 into the SERVICE payload area of the TALI
      packet

   *  The SYNC field is set

   *  The OPCODE is set to 'isot'

   *  The LENGTH is set to the number of octets in the SERVICE field

   Once the fully formed 'isot' TALI packet is created, it is handed to
   the TCP socket layer and transmitted.  The transmission process will
   add TCP, IP and MAC header information.

   Since the routing information is placed in the TALI Packet, no
   routing information needs to be saved by the SG.

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                              SS7 MSU

           |          Layer 3            |     Layer 2      |
           |                             |                  |
      +----+---+----+----+---+-------+---+--+---+---+---+---+----+
      |Flag|FCS|ISUP|Msg.|CIC|Routing|SIO|LI|FIB|FSN|BIB|BSN|Flag|
      |    |   |Part|Type|   |Label  |   |  |   |   |   |   |    |
      +----+---+----+----+---+-------+---+--+---+---+---+---+----+
               |                         /
               |                        /
               |                       |
        TALI   +-----------------------+---+------+----+
        Packet |  Service              |LEN|Opcode|SYNC|
               +-----------------------+---+------+----+
               |                                       /
               |                              ---------
               |                             /
               +----------------------------+------+------+------+
        IP     | TALI Packet                |TCP   | IP   | MAC  |
        Packet |                            |Header|Header|Header|
               +----------------------------+------+------+------+

     Figure 7: Encapsulation of ISUP MSUs using the TALI 'isot' opcode

   When an 'isot' TALI packet is received on an SG from an IP device,
   the SG performs the following processing on the 'isot' packet:

   *  validates the TALI header

   *  Allocates space for a new SS7 message

   *  extracts the MTP Layer 3 data from the SERVICE area and places it
      in the new SS7 message

   Once the 'isot' packet is transformed back into a normal SS7 MSU, the
   MSU is routed within the SG according to the normal SS7 routing
   procedures.

3.2.2.3 MTP3 Service Message (mtp3)

   The 'mtp3' opcode is used to deliver SS7 MSUs with a Service
   Indicator of 0-2, 4, 6-15 (non-SCCP, non-ISUP) over a TALI
   connection.  This opcode is only used on TALI protocol stacks that
   are implemented without SAAL.  The MTP3 layer of the SS7 MSU IS part
   of the data transferred across TCP/IP for this opcode; the data
   portion of the TALI 'mtp3' message begins with the SIO byte of the
   MTP3 header in the SS7 MSU.

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   +------------------------------------------------------------------+
   | Octets | Field Name  | Description                               |
   +------------------------------------------------------------------+
   | 0..3   | SYNC        | 'TALI'                                    |
   +------------------------------------------------------------------+
   | 4..7   | OPCODE      | 'mtp3'                                    |
   +------------------------------------------------------------------+
   | 8..9   | LENGTH      | Length                                    |
   +------------------------------------------------------------------+
   | 10..X  | Layer 3 MSU | Raw MSU data starting at the Layer 3 SIO  |
   |        | Data        | field.                                    |
   +------------------------------------------------------------------+

3.2.2.3.1 MTP3 Encapsulation using TALI

   When an SS7 MSU with SI=0-2,4,6-15 arrives at an SG from a 56 Kbps or
   DS1 link and is routed within the SG to an IP device, the SG performs
   the following processing on the SS7 MSU:

   *  discards the MTP Layer 2 information, CRC and flags

   *  places MTP Layer 3 into the SERVICE payload area of TALI packet

   *  The SYNC field is set

   *  The OPCODE is set to 'mtp3'

   *  The LENGTH is set to the number of octets in the SERVICE field

   Once the fully formed 'mtp3' TALI packet is created, it is handed to
   the TCP socket layer and transmitted.  The transmission process will
   add TCP, IP and MAC header information.

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                             SS7 MSU

           |      Layer 3              |     Layer 2      |
           |                           |                  |
      +----+---+-----------+-------+---+--+---+---+---+---+----+
      |Flag|FCS|Other Layer|Routing|SIO|LI|FIB|FSN|BIB|BSN|Flag|
      |    |   |3 Data     |Label  |   |  |   |   |   |   |    |
      +----+---+-----------+-------+---+--+---+---+---+---+----+
               |                       /
               |                 ------
               |                /
        TALI   +----------------+---+------+----+
        Packet |  Service       |LEN|Opcode|SYNC|
               +----------------+---+------+----+
               |                                /
               |                              --
               |                             /
               +----------------------------+------+------+------+
        IP     | TALI Packet                |TCP   | IP   | MAC  |
        Packet |                            |Header|Header|Header|
               +----------------------------+------+------+------+

      Figure 8: Encapsulation of SS7 MSUs with SI!=3,5,13 using 'mtp3'

   When an 'mtp3' TALI packet is received by an SG from an IP device,
   the SG performs the following processing on the 'mtp3' packet:

   *  validates the TALI header

   *  Allocates space for a new SS7 message

   *  extracts the MTP Layer 3 data from the SERVICE area and places it
      in the new SS7 message

   Once the 'mtp3' packet is transformed back into a normal SS7 MSU, the
   MSU is routed within the SG according to the normal SS7 routing
   procedures.

3.2.2.4 SAAL Service Message (saal)

   The 'saal' opcode is used to deliver SS7 MSUs with any Service
   Indicator over a TALI connection.  This opcode is only used on TALI
   protocol stacks that are implemented with SAAL.  The 'saal' opcode is
   also used to transmit SAAL peer to peer packets (SSCF peer to peer
   packets and SSCOP peer to peer packets other than SS7 service data)
   over a TALI connection.

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   When used to transfer SS7 MSUs, the MTP3 layer of the SS7 MSU IS part
   of the data transferred across TCP/IP for this opcode; the data
   portion of the TALI 'saal' message begins with the SIO byte of the
   MTP3 header in the SS7 MSU and ends with the last byte of the SSCOP
   trailer.

   When used to transfer SSCF/SSCOP peer to peer messages the data
   portion of the TALI 'saal' message includes the entire SSCOP PDU.

   +------------------------------------------------------------------+
   | Octets | Field Name  | Description                               |
   +------------------------------------------------------------------+
   | 0..3   | SYNC        | 'TALI'                                    |
   +------------------------------------------------------------------+
   | 4..7   | OPCODE      | 'saal'                                    |
   +------------------------------------------------------------------+
   | 8..9   | LENGTH      | Length                                    |
   +------------------------------------------------------------------+
   | 10..X  | Layer 3     | Raw MSU data starting at the Layer 3 SIO  |
   |        | Data        | field.                                    |
   +------------------------------------------------------------------+
   | (X+1)  | SSCOP       | Zero (0) to three (3) octets of padding   |
   |  ..Y   | Trailer     | plus 4 octets for the trailer data.  The  |
   |        |             | total length of the Layer 3 Data and the  |
   |        |             | SSCOP trailer must be a multiple of 4.    |
   +------------------------------------------------------------------+

                        or

   +------------------------------------------------------------------+
   | Octets | Field Name  | Description                               |
   +------------------------------------------------------------------+
   | 0..3   | SYNC        | 'TALI'                                    |
   +------------------------------------------------------------------+
   | 4..7   | OPCODE      | 'saal'                                    |
   +------------------------------------------------------------------+
   | 8..9   | LENGTH      | Length                                    |
   +------------------------------------------------------------------+
   | 10..X  | SAAL Peer   | Raw SSCF/SSCOP peer to peer packets are   |
   |        | to Peer     | also transferred over the TALI connection |
   |        | message     | using this 'saal' opcode.                 |
   +------------------------------------------------------------------+

3.2.2.4.1 MTP3 and SAAL Peer to Peer Encapsulation using TALI

   When an SS7 MSU (with any SI) arrives at an SG from a 56 Kbps or DS1
   link and is routed within the SG for transmission to an IP device,
   the SG performs the following processing on the SS7 MSU:

Top      Up      ToC       Page 33 
   *  discards the MTP Layer 2 information, CRC and flags

   *  the MSU is passed from an MTP3 processing software layer to the
      SSCF and SSCOP layers (the SAAL layers).  These layers convert the
      SS7 MSU into an SSCOP PDU.  Part of this conversion includes
      adding an SSCOP trailer.

   *  the SSCOP PDU (whether it is a peer to peer SAAL message or SS7
      MSU in an SSCOP PDU) is copied into the SERVICE payload area of
      the TALI packet

   *  The SYNC field is set

   *  The OPCODE is set to 'saal'

   *  The LENGTH is set to the number of octets in the SERVICE field

   Once the fully formed 'saal' TALI packet is created, it is handed to
   the TCP socket layer and transmitted.  The transmission process will
   add TCP, IP and MAC header information.

   Since the routing information is placed in the TALI Packet, no
   routing information needs to be saved by the SG.

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                               SS7 MSU

           |          Layer 3          |     Layer 2      |
           |                           |                  |
      +----+---+-----------+-------+---+--+---+---+---+---+----+
      |Flag|FCS|Other Layer|Routing|SIO|LI|FIB|FSN|BIB|BSN|Flag|
      |    |   |3 Data     |Label  |   |  |   |   |   |   |    |
      +----+---+-----------+-------+---+--+---+---+---+---+----+
               |                       |
               |                       |
               |                       |
       +-------+-----------------------+
       |SSCOP  |  Service              |
       |Trailer|                       |
       +-------+-----------------------+
       |                               |
       +-------+-----------------------+---+------+----+
       |Service with SSCOP Trailer     |LEN|Opcode|SYNC|
       +-------+-----------------------+---+------+----+
       |                                               /
       |                              -----------------
       |                             /
       +----------------------------+------+------+------+
       | TALI Packet                |TCP   | IP   | MAC  |
       |                            |Header|Header|Header|
       +----------------------------+------+------+------+

   Figure 9: Encapsulation of SAAL PDUs using the TALI 'saal' opcode

   When an 'saal' TALI packet is received at the SG from an IP device,
   the SG performs the following processing on the 'saal' packet:

   *  validates the TALI header

   *  Allocates space for a new SSCOP PDU message

   *  extracts the SSCOP PDU data from the SERVICE area and places it in
      the new SSCOP PDU message

   Once the 'saal' packet is transformed back into a normal DS1 SSCOP
   PDU, the SSCOP PDU is passed to the SAAL layer for receive
   processing.  If the SSCOP PDU is a peer to peer pdu, it is processed
   completely in the appropriate SAAL layer.  If the SSCOP PDU is an SS7
   MSU, the MSU is transformed back to a normal SS7 MSU and is routed
   within the SG according to the normal SS7 routing procedures.

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3.3 TALI Timers

   Version 1.0 of the TALI specification defined 4 TALI timers that are
   used as part of the TALI state machine.  These timers are generically
   named 'T1' through 'T4'.  Brief descriptions of each timer are
   provided in the following subsections.  Timer expiration events for
   each of the T1-T4 timers appear as inputs to the TALI state machine.
   For exact processing of each timer (when to start/stop, how to
   process timer expirations), refer to the TALI state machine.

   Both ends of the TALI connection have there own T1-T4 timers.  The
   T1-T4 timer values can be set on each end of the connection
   independent of the settings on the far end.  For each timer, a
   default value and range is recommended in the following sections.

3.3.1 T1 Timer

   The T1 timer represents the time interval between the origination of
   a 'test' message at each TALI implementation.  Each time T1 expires,
   the TALI implementation should send a 'test'.

3.3.2 T2 Timer

   The T2 timer represents the amount of time that the Peer has to
   return an 'allo' or a 'proh' in response to a 'test'.  If the far end
   fails to reply with 'allo' or 'proh' before T2 expires, the sender of
   the 'test' treats the T2 expiration as a protocol violation.  Note
   that T2 must be < T1 in order for these timers to work as designed.

3.3.3 T3 Timer

   The T3 timer controls how long the near end should continue to
   process Service Data that is received from the far end after a
   Management Prohibit Traffic Event has occurred (at the near end).
   This timer is used when a transition from NEA-FEA (both ends allowed
   to send service data) to NEP-FEA (only far end willing to send
   service data) occurs.  On that transition, it is reasonable to expect
   that the far end needs some amount of time to adjust its TALI state
   machine and divert service data traffic away from this socket.  The
   T3 timer controls the amount of time the far end has to divert
   traffic.

3.3.4 T4 Timer

   The T4 timer represents the time interval between the origination of
   a 'moni' message at each TALI implementation.  Each time T4 expires,
   the TALI implementation should send a 'moni'.

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3.3.5 Recommended Defaults and Ranges for the TALI Timers

   The following table provides the recommended default and configurable
   range for each TALI timer.

   +------------------------------------------------------------------+
   |Name|  Min  |  Max  |Default| Description                         |
   +------------------------------------------------------------------+
   | T1 | 100ms | 60sec | 4 sec | Send test PDU timer                 |
   +------------------------------------------------------------------+
   | T2 | 100ms | 60sec | 3 sec | Response timer for an allo or proh  |
   |    |       |       |       | response to test message.           |
   +------------------------------------------------------------------+
   | T3 | 100ms | 60sec | 5 sec | Timer controls how long to process  |
   |    |       |       |       | rcvd serv data after an NE          |
   |    |       |       |       | transition from NEA to NEP.  System |
   |    |       |       |       | is waiting for a proa response to   |
   |    |       |       |       | the first proh send when NE         |
   |    |       |       |       | transitions from NEA to NEP.        |
   +------------------------------------------------------------------+
   | T4 | 100ms | 60sec |10 sec | Send moni PDU timer                 |
   +------------------------------------------------------------------+

                         Table 5: Timers

   NOTE: The value of T1 must be at least one (1) millisecond greater
   than T2.  This is to prevent the system from a lockup in the T1
   expired condition.  If T1 is equal or less than T2, it will expire
   and restart T2 and not enforce responses to the test message.

   Enforcement of minimum and maximum timer values is implementation
   dependent.

3.4 TALI User Events

   Each TALI implementation must provide several user event controls
   over the behavior of the TALI state machine for each TALI connection.
   The user interface to provide these capabilities is implementation
   specific.

3.4.1 Management Open Socket Event

   The 'mgmt open socket' event, together with the 'mgmt close socket'
   event, allows the user to control when each defined TALI connection
   will form a TCP socket connection.  When 'open socket' for a
   particular TALI connection occurs, the TALI connection should begin
   trying to form a TCP socket connection to the peer.

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   The steps that are taken to connect are dependent on the
   client/server role of that end of the TALI connection.  The exact
   steps to perform these tasks are implementation dependent and may
   differ based on the TCP stack being used.

   In general, TALI clients form socket connections by using the BSD
   sockets calls:

         Socket()
         Bind()
         Connect()

   In general, TALI servers form socket connections by using the BSD
   sockets calls:

         Socket()
         Bind()
         Listen()
         Accept()

3.4.2 Management Close Socket Event

   The 'mgmt close socket' event can be issued by the user when it is
   desired that the TCP socket for a TALI socket, be closed immediately,
   or discontinue its attempts to connect to the peer.  After acting on
   'close socket', the TALI connection will not be established until
   'mgmt open socket' is issued.

3.4.3 Management Allow Traffic Event

   The 'mgmt allow traffic' event, together with the 'mgmt prohibit
   traffic' event, allows the user to control when each defined TALI
   connection will be willing to carry SS7 service data over that
   particular TALI connection.  When 'mgmt allow traffic' is issued, the
   TALI implementation becomes willing to carry service data.  The TALI
   state for the near end should transition to NEA (near end allowed) if
   the connection is already established.

3.4.4 Management Prohibit Traffic Event

   The 'mgmt prohibit traffic' event is the opposite of 'allow traffic'.
   When 'mgmt prohibit traffic' is issued, the TALI implementation
   becomes un-willing to carry SS7 service data over that particular
   TALI connection.  The TALI state for the near end should transition
   to NEP (near end prohibited) if the connection is already
   established.

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3.5 Other Implementation Dependent TALI Events

   In addition to timers, each TALI implementation needs to be able to
   detect, and react accordingly, for the following events:

   *  Connection Established.  When the TCP socket connection is
      initially established the TALI state machine must be notified.

   *  Connection Lost.  When the TCP socket connection is lost, due to
      socket errors during reads/writes, the TALI state machine must be
      notified.

   *  Protocol Violations.  Any violation of the TALI protocol as
      discussed in 3.7.1.3.

3.6 TALI States

   The TALI version 1.0 specification is based on a state machine that
   considers 6 TALI states.  Each end of the TALI connection maintains
   its own TALI state.

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   +------------------------------------------------------------------+
   | Name       | Description                                         |
   +------------------------------------------------------------------+
   | OOS        | The TALI connection is out of service.  This usually|
   |            | corresponds to a user event to 'close' the socket,  |
   |            | or a user event to 'deactivate the SS7 link'.       |
   +------------------------------------------------------------------+
   | Connecting | The TALI layer is attempting to establish a TCP     |
   |            | socket connection to the peer.  Servers are         |
   |            | 'accepting', clients are 'connecting'.              |
   +------------------------------------------------------------------+
   | NEP-FEP    | The TCP socket connection is established.  Neither  |
   |            | side of the connection is ready to use the socket   |
   |            | for service PDUs.                                   |
   +------------------------------------------------------------------+
   | NEP-FEA    | The TCP socket connection is established.  The NE is|
   |            | not ready to use the socket for service PDUs.  The  |
   |            | FE is ready to use the socket for service PDUs.     |
   +------------------------------------------------------------------+
   | NEA-FEP    | The TCP socket connection is established.  The NE is|
   |            | ready to use the socket for service PDUs.  The FE is|
   |            | not ready to use the socket for service PDUs.       |
   +------------------------------------------------------------------+
   | NEA-FEA    | The TCP socket connection is established.  Both     |
   |            | sides are ready to use the socket for service PDUs. |
   |            | This is the only state where normal bi-directional  |
   |            | SS7 data transfer occurs.                           |
   +------------------------------------------------------------------+

                              Table 6: TALI States

3.7 TALI Version 1.0 State Machine

   This section provides the state machine that must be followed by each
   TALI implementation in order to be compliant with this specification.

3.7.1 State Machine Concepts

   Before presenting the actual state machine, several concepts are
   discussed.

3.7.1.1 General Protocol Rules

   1.  Neither side can send service data unless both sides are allowed.

   2.  Each side initializes to the prohibited state for both near end
       and far end.

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   3.  State changes between the NEx-FEx states are signaled with either
       an 'allo' or 'proh'.

   4.  Each side can poll the far end's state with a 'test'.  Upon
       sending 'test', T1 and T2 should always be restarted.

   5.  Each side polls the far end with a 'test' every T1 expiration.

   6.  The reply to a 'test' is based on the state of the near end only.

   7.  The reply to a 'test' is either 'allo' or 'proh'.

   8.  A far end signals the last service PDU has been transmitted with
       either a 'proh' or a 'proa'.

   9.  Upon receiving a 'proh', the receiver must always reply with
       'proa'.

   10. The NE cannot gracefully close a socket unless a 'proh' is sent
       and 'proa' is received.

   11. On the transition from NEA to NEP, after sending a 'proh', the
       near end must continue to process received service data until a
       'proa' is received or until a T3 timer expires.

3.7.1.2 Graceful Shutdown of a Socket

   The state table treats a management request to close the socket as a
   'hard' shutdown.  That is, it will close the socket immediately
   regardless of the current state.  Therefore, the correct steps to
   ensure a graceful shutdown of a socket (from the NEA_FEP or NEA_FEA
   states) is:

   1. Management issues a Management Prohibit Traffic Event on the
      socket.

   2. Management will wait for T3 to expire.

   3. Management can then issue a Close Socket Event on the socket.

3.7.1.3 TALI Protocol Violations

   Each TALI implementation must detect when violations of the TALI
   protocol have occurred and react accordingly.  Protocol violations
   include:

   *  Invalid sync code in a received message

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   *  Invalid opcode in a received message

   *  Invalid length field in a received message

   *  Not receiving an 'allo' or 'proh', in response to the origination
      of a 'test' , before the T2 timer expires

   *  Receiving Service Messages on a prohibited socket.

   *  TCP Socket errors - Connection Lost

   In the state machine that follows, State/Event combinations that
   should be treated as protocol violations are indicated via a 'PV' in
   the state/event cell.  All of the 'PV' events are then processed as
   per the 'Protocol Violation' row in the table.

3.7.2 The State Machine

   Internal Data required for State Machine:

   boolean sock_allowed.  This flag indicates whether the NE is allowed
   to carry Service Messages.

   Initial Conditions:
   sock_allowed = FALSE
   state = OOS
   no timers running

   +------------------------------------------------------------------+
   |   State| OOS  |Connecting| NEP-FEP | NEP-FEA | NEA-FEP | NEA-FEA |
   |Event   |      |          |         |         |         |         |
   +------------------------------------------------------------------+
   |T1 Exp. |      |          |Send test|Send test|Send test|Send test|
   |        |      |          |Start T1 |Start T1 |Start T1 |Start T1 |
   |        |      |          |Start T2 |Start T2 |Start T2 |Start T2 |
   +------------------------------------------------------------------+
   |T2 Exp. |      |          |   PV    |   PV    |   PV    |   PV    |
   +------------------------------------------------------------------+
   |T3 Exp. |      |          |   PV    |   PV    |         |         |
   +------------------------------------------------------------------+
   |T4 Exp. |      |          |Send moni|Send moni|Send moni|Send moni|
   |        |      |          |Start T4 |Start T4 |Start T4 |Start T4 |
   +------------------------------------------------------------------+
   |Rcv test|      |          |Send proh|Send proh|Send allo|Send allo|
   +------------------------------------------------------------------+
   |Rcv allo|      |          | Stop T2 | Stop T2 | Stop T2 | Stop T2 |
   |        |      |          | NEP-FEA |         | NEA-FEA |         |

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   +------------------------------------------------------------------+
   |Rcv proh|      |          | Stop T2 | Stop T2 | Stop T2 | Stop T2 |
   |        |      |          |Send proa|Send proa|Send proa|Flush or |
   |        |      |          |         | NEP-FEP |         | reroute |
   |        |      |          |         |         |         |Send proa|
   |        |      |          |         |         |         | NEA-FEP |
   +------------------------------------------------------------------+
   |Rcv proa|      |          | Stop T3 | Stop T3 |         |         |
   +------------------------------------------------------------------+
   |Rcv moni|      |          |Convert  |Convert  |Convert  |Convert  |
   |        |      |          | to mona | to mona | to mona | to mona |
   |        |      |          |Send mona|Send mona|Send mona|Send mona|
   +------------------------------------------------------------------+
   |Rcv mona|      |          |Implemen-|Implemen-|Implemen-|Implemen-|
   |        |      |          |tation   |tation   |tation   |tation   |
   |        |      |          |dependent|dependent|dependent|dependent|
   +------------------------------------------------------------------+
   |Rcv     |      |          |   PV    |If T3 run|   PV    |Process  |
   | Service|      |          |         | Process |         |         |
   |        |      |          |         |Else PV  |         |         |
   +------------------------------------------------------------------+
   |Connect.|      | Start T1 |         |         |         |         |
   |Estab.  |      | Start T2 |         |         |         |         |
   |        |      | Start T4 |         |         |         |         |
   |        |      |(if non-0)|         |         |         |         |
   |        |      |if sock_  |         |         |         |         |
   |        |      |  allowed |         |         |         |         |
   |        |      |  = TRUE  |         |         |         |         |
   |        |      | send allo|         |         |         |         |
   |        |      | send test|         |         |         |         |
   |        |      | NEA-FEP  |         |         |         |         |
   |        |      |else      |         |         |         |         |
   |        |      | send proh|         |         |         |         |
   |        |      | send test|         |         |         |         |
   |        |      | NEP-FEP  |         |         |         |         |
   +------------------------------------------------------------------+
   |Connect.|      |          |   PV    |   PV    |   PV    |   PV    |
   |Lost    |      |          |         |         |         |         |
   +------------------------------------------------------------------+
   |Protocol|      |          |Stop all |Stop all |Stop all |Stop all |
   |Violat. |      |          | timers  | timers  | timers  | timers  |
   |        |      |          |Close the|Close the|Close the|Close the|
   |        |      |          | socket  | socket  | socket  | socket  |
   |        |      |          |Connect- |Connect- |Connect- |Connect- |
   |        |      |          |  ing    |  ing    |  ing    |  ing    |

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   +------------------------------------------------------------------+
   |Mgmt.   |Open  |          |         |         |         |         |
   |Open    |socket|          |         |         |         |         |
   |Socket  |Conne-|          |         |         |         |         |
   |        | cting|          |         |         |         |         |
   +------------------------------------------------------------------+
   |Mgmt.   |      |Close the |Stop all |Stop all |Stop all |Stop all |
   |Close   |      | socket   | timers  | timers  | timers  | timers  |
   |Socket  |      |OOS       |Close the|Close the|Close the|Close the|
   |        |      |          | socket  | socket  | socket  | socket  |
   |        |      |          |OOS      |OOS      |OOS      |OOS      |
   +------------------------------------------------------------------+
   |Mgmt.   |sock_ |sock_allo-|sock_all-|sock_all-|sock_all-|sock_all-|
   |Prohibit|allow-| wed=FALSE| owed=   | owed=   | owed=   | owed=   |
   |Socket  |ed =  |          | FALSE   | FALSE   | FALSE   | FALSE   |
   |        |FALSE |          |         |         |send proh|send proh|
   |        |      |          |         |         |start t3 |start t3 |
   |        |      |          |         |         | NEP-FEP | NEP-FEA |
   |        |      |          |         |         |         |         |
   +------------------------------------------------------------------+
   |Mgmt.   |sock_ |sock_allo-|sock_all-|sock_all-|sock_all-|sock_all-|
   |Allow   |allow-| wed=TRUE | owed=   | owed=   | owed=   | owed=   |
   |Traffic |ed =  |          | TRUE    | FALSE   | TRUE    | TRUE    |
   |        |TRUE  |          |send allo|send allo|         |         |
   |        |      |          | NEA-FEP | NEA-FEA |         |         |
   +------------------------------------------------------------------+
   |User    |reject| reject   | reject  | reject  | reject  | send    |
   |Part    |data  | data     | data    | data    | data    | data    |
   |Msgs.   |      |          |         |         |         |         |
   +------------------------------------------------------------------+

                     Table 7: TALI 1.0 State Machine

3.8 TALI 1.0 Implementation Notes

   Several aspects of the expected TALI 1.0 implementation have not been
   specifically addressed in the state machine or previous text (or else
   they were presented but will be reiterated here).  These
   implementation notes in some cases have to do with the expected
   behavior of the software layer above the TALI layer.

3.8.1 Failure on a TCP/IP Socket

   *  The failure to read or write from a TCP socket shall be detected
      and generate a connection lost event.

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3.8.2 Congestion on a TCP/IP Socket

   *  Message streams can be monitored for congestion via implementation
      dependent methods.

   *  One possible definition of congestion for the previous requirement
      might be when a TCP socket is blocked.

3.9 TALI 1.0 Limitations

   Several limitations with the TALI 1.0 specification and
   implementation are identified:

   *  For SCCP traffic, only UDT and XUDT Class 0 and Class 1 traffic
      should be managed by this protocol.

   *  When the MTP3 Routing Label is not part of the data transmitted
      across the wire, priority zero (0) traffic is used for all traffic
      when the SIO is regenerated.

4. TALI Version 2.0

   Version 2.0 of the TALI specification provides several additions to
   the Version 1.0 specification.  The 2.0 additions are provided by
   introducing three new TALI opcodes.  The basic functionality and most
   of the details of the TALI 1.0 implementation are NOT changed by the
   2.0 additions.

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   The table below provides a summary of the messages and message
   structure used in TALI version 2.0.

   +------------------------------------------------------------------+
   | OCTET | DESCRIPTION           | SIZE     | VALUE  |    TYPE      |
   +------------------------------------------------------------------+
   | 0..3  | SYNC                  | 4 Octets |        | 4 byte ASCII |
   +------------------------------------------------------------------+
   |       |   TALI                |          | 'TALI' |              |
   +------------------------------------------------------------------+
   | 4..7  | OPCODE                | 4 Octets |        | 4 byte ASCII |
   +------------------------------------------------------------------+
   |       |   Test Service        |          | 'test' |              |
   |       |   Allow Service       |          | 'allo' |              |
   |       |   Prohibit Service    |          | 'proh' |              |
   |       |   Prohibit Service Ack|          | 'proa' |              |
   |       |   Monitor Socket      |          | 'moni' |              |
   |       |   Monitor Socket Ack  |          | 'mona' |              |
   |       |   SCCP Service        |          | 'sccp' |              |
   |       |   ISUP Service o/TALI |          | 'isot' |              |
   |       |   MTP3 Service o/TALI |          | 'mtp3' |              |
   |       |   Service o/SAAL      |          | 'saal' |              |
   |       |   Management Message  |          | 'mgmt' |              |
   |       |   Extended Service Msg|          | 'xsrv' |              |
   |       |   Special Message     |          | 'spcl' |              |
   +------------------------------------------------------------------+
   | 8..9  | LENGTH                | 2 Octets |        | integer      |
   |       |   (least significant  |          |        |              |
   |       |    byte first) non-0  |          |        |              |
   |       |    if Service or      |          |        |              |
   |       |    Socket monitor msg |          |        |              |
   +------------------------------------------------------------------+
   | 10..X | DATA PAYLOAD          | variable |        | variable     |
   +------------------------------------------------------------------+

   Due to the minimal amount of change from 1.0, this chapter will only
   provide:

   *  Detailed information regarding how a TALI implementation can
      identify itself as a 2.0 vs. a 1.0 implementation

   *  Detailed information regarding how to provide backward
      compatibility for a connection to a far end that is only TALI 1.0
      capable

   *  Detailed information regarding the new 2.0 opcodes

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   *  Detailed information regarding any other changes to the
      information presented in previous sections that need to be
      implemented in order to be 2.0 compatible.

   Therefore, readers of this chapter should read this from the point of
   view of modifying an existing TALI 1.0 implementation to support the
   new 2.0 features.

4.1 Overview of TALI Version 2.0 Features

   A small number of changes to a 1.0 TALI implementation are required
   to support 2.0.  Figure 10 illustrates the inputs that affect the 2.0
   TALI State Machine.  The reader may notice that the only differences
   from the inputs for 1.0 are as follows:

   Three new TALI opcodes can be sent/received between a TALI node and
   its peer.  The new opcodes are:

   *  'mgmt'
   *  'xsrv'
   *  'spcl'

   Three new User Part capabilities need to be supported by the layer of
   code above the TALI layer in each implementation.  The user part
   needs to provide support for 'mgmt', 'xsrv', and 'spcl' data.

   More information about the 3 new opcodes is provided in individual
   sections in this chapter.  However, a brief description of the
   purpose of each of these opcodes is as follows:

   *  'mgmt' - This opcode is intended to allow MANAGEMENT data, or data
      that will manage the operation of the device, to pass between the
      TALI endpoints.  Examples of this management data include:

      *  configuration data, such as which SS7 traffic streams a peer
         would like to receive over a specific socket

      *  SS7 Network Management data, such as information regarding
         point code (un)availability and congestion.

      *  Enabling/disabling various socket options, such as options
         regarding which messages are supported, or how to format data.

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   *  'xsrv' - Extended Service Opcodes.  It is envisioned that the TALI
      protocol could be extended to carry other types of traffic that
      are not covered by the 1.0 service data opcodes ('sccp', 'isot',
      'mtp3', or 'saal').  By defining a new 'xsrv' service opcode, the
      TALI protocol is opened up to the possibility of being used for
      other types of data transport.

   *  'spcl' - Special services.  It is envisioned that vendors may want
      to build special services into their TALI implementations that are
      only activated when the implementation is connected to other
      equipment implementing the same special services.  This opcode is
      intended to provide a general means to discover more information
      regarding who the TALI session is connected to, and a means to
      enable special features based on the vendor/implementation on the
      far end.

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

     Figure 10: Overview of Inputs to the TALI 2.0 State Machine

4.2 TALI Version Identification

   The TALI 1.0 specification did not provide a simple means to perform
   TALI version identification.  However, the general purpose 'moni'
   message from 1.0 can be used to solve this problem in 2.0.

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   Recall from 1.0 that the 'moni' message was very loosely defined in
   the 1.0 spec:

   *  The primary purpose of the 'moni' message was to provide a general
      purpose ECHO capability.  It was envisioned that an important task
      that the ECHO capability could provide would be to measure Round
      Trip TALI/TALI processing time.

   *  The data portion of the 'moni' message could be from 0-200 bytes
      long.  The use of the data area was completely implementation
      specific.

   *  There were no requirements that an implementation ever send a
      'moni'.

   *  If an implementation did send 'moni', it should use the T4 timer
      to control the frequency of the outgoing 'moni'.

   *  The receiver of the 'moni' should not make any assumptions as to
      the data portion of the 'moni'.  The receiver should simply
      convert the 'moni' into a 'mona' and return the message with the
      same data portion.

   TALI 2.0 implementations should use the 'moni' message to provide
   version identification as per the following bullets:

   *  The primary purpose of the 'moni' message is now twofold:

   *  To provide version identification

      *  To continue to provide a general purpose ECHO capability that
         can be used to measure Round Trip time or perform other
         implementation specific tasks.

   *  The data portion of the 'moni' message is now divided into 2
      portions

      *  A portion dedicated to version identification, 12 bytes long,
         with a specific format that must be followed

      *  Followed by a free format section that can be used in a
         completely implementation specific manner.

   *  The overall length of the data portion for a 'moni' should still
      not exceed 200 bytes.  This is required to maintain backward
      compatibility with 1.0 implementations that may check for a
      maximum length of 200 bytes on the 'moni' opcode.

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   *  If a TALI implementation wants to identify itself as a version 2.0
      node, it must send a 'moni' encoded as per Table 8.  Every 'moni'
      it sends should conform to the encoding in Table 8.  The version
      label should not change from 'moni' to 'moni'.  The data following
      the version label can change from 'moni' to 'moni' and can
      continue to be used for RTT calculations, or other purposes.

   *  If a TALI implementation is trying to determine if the far end of
      the TALI connection has implemented version 2.0, the
      implementation must examine any received 'moni' messages that
      arrive from the far end and see if they conform to the new
      stricter 'moni' encoding in Table 8.  On receiving 'moni', a TALI
      2.0 node will compare the 12 bytes of data in the VER LABEL field
      with a list of predetermined strings to determine the
      functionality of the TALI node it is connected to.  If the data
      doesn't match any of the predetermined strings, the Far End is
      assumed to be a TALI 1.0 node.

   *  Each TALI implementation must assume that the far end of the
      connection is a 1.0 implementation until an arriving 'moni'
      announces that the far end supports TALI version 2.0.  If a 'moni'
      never arrives, the implementation knows the far end has
      implemented version 1.0 of the specification.

   *  TALI 1.0 implementations can receive newly encoded 'moni' messages
      and simply ignore the data.  The 1.0 implementations will continue
      to operate as if the far end is always a 1.0 node (ignore the data
      portion of the 'moni', convert 'moni' to 'mona', and return the
      'mona').

   *  The next section provides more information regarding backwards
      compatibility (2.0 implementations connected to devices that
      implemented version 1.0 of the specification).

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   +------------------------------------------------------------------+
   | Octets | Field Name  | Description                  | Field Type |
   +------------------------------------------------------------------+
   | 0..3   | SYNC        | 'TALI'                       |4 byte ASCII|
   +------------------------------------------------------------------+
   | 4..7   | OPCODE      | 'moni'                       |4 byte ASCII|
   +------------------------------------------------------------------+
   | 8..9   | LENGTH      | Length (includes the version | Integer    |
   |        |             | label and data fields)       |            |
   +------------------------------------------------------------------+
   | 10..21 | Ver. Label  | 'vers xxx.yyy'               | 12 byte    |
   |        | See note    |                              | ASCII      |
   +------------------------------------------------------------------+
   | 22..X  | DATA        | Vendor Dependent             | Variable   |
   |        |             | Maximum length of this       |            |
   |        |             | message (as coded in octets 8|            |
   |        |             | -9, and stored in bytes 10-X)|            |
   |        |             | should not exceed 200 bytes. |            |
   +------------------------------------------------------------------+

               Table 8: Version Control 'moni' Message

   NOTE: xxx.yyy = provides the Major and Minor release number of the
                   TALI specification being implemented.
         001.000 = Tali version 1.0
         002.000 = Tali version 2.0     // this specification.
         002.001 = Tali version 2.1     // a minor change to 2.0
         003.000 = Tali version 3.0
         and so on.

   The 'vers 002.000' field is an 12 byte field of field type 'ascii
   text'.  As such, 'v' should be the first byte of the field that is
   transmitted out the wire.



(page 51 continued on part 3)

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