draft-ietf-sigtran-m2pa-00.txt

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Internet Draft  SS7 MTP2-User Peer-to-Peer Adaptation Layer   Nov 2000

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

This interface is the same as the MTP2/MTP3 interface described in
[2], with the addition of support for larger sequence numbers in
[7]. 

Because SCTP uses larger sequence numbers than MTP, the MTP3
Changeover procedure must use the Extended Changeover Order and
Extended Changeover Acknowledgment messages described in [7]. This
will allow for use of the SCTP stream sequence numbers in the
changeover messages.

1.6.2 Support for peer-to-peer communication

In SS7, MTP Level 2 sends three types of messages, known as signal
units: Message Signal Units (MSUs), Link Status Signal Units (LSSUs),
and Fill-In Signal Units (FISUs).

MSUs originate at a higher level than MTP2, and are destined for a
peer at another node. Likewise, M2PA passes these messages from MTP3
to SCTP as data for transport across a link. These are called User
Data messages in M2PA.

LSSUs allow peer MTP2 layers to exchange status information. Analogous
messages are needed for M2PA. The Link Status message serves this
purpose.

FISUs are sent when no other signal units are waiting to be sent. This
purpose is served by the heartbeat messages in SCTP. FISUs also carry
acknowledgment of messages. This function is performed by
SCTP. Therefore, it is unnecessary for M2PA to provide a protocol unit
like the FISU.

1.7 Functions Provided by M2PA

1.7.1 Mapping

For each IP link, the M2PA layer must maintain a map of the SS7 link
to its SCTP association and its corresponding IP destination.
 
1.7.2 SCTP Stream Management

SCTP allows a user-specified number of streams to be opened during the
initialization.  It is the responsibility of the M2PA layer to ensure
proper management of the streams allowed within each association.

1.7.3  Retention of MTP3 in the SS7 Network 

M2PA allows MTP3 to perform all of its Message Handling and Network
Management functions with IPSPs as with other SS7 nodes.



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1.8 Definition of the M2PA Boundaries

1.8.1 Definition of the M2PA / MTP Level 3 boundary

The upper layer primitives provided by M2PA are the same as those
provided by MTP2 to MTP3 [2].

1.8.2 Definition of the Lower Layer Boundary between M2PA and SCTP

The upper layer primitives provided by SCTP are described in Reference
[5] Section 10 "Interface with Upper Layer".


2.  Protocol Elements

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

All fields in an M2PA message must be transmitted in the network byte
order, i.e., most significant byte first, unless otherwise stated.


2.1 Common Message Header

The protocol messages for M2PA require a message header structure
which contains a version, message type and message length.  This
message header is common among all SCN adaptation layers. The 
header structure is shown in Figure 3.


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Version    |     Spare     |         Message Type          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Message Length                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |


                 Figure 3:  Common Message Header


2.1.1  Version

The version field (vers) contains the version of the M2PA adapation
layer.  The supported versions are:

      01   Release 1.0 of M2PA protocol




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2.1.2  Message Type

The valid message types are defined below and the message contents are
described in Section 2.2.  Each message can contain parameters.

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


     MTP2 User Adaptatation Messages

        Type                     Value (Hex)

        User Data                0601
        Link Status              0602

       
2.1.3  Message Length

The Message length defines the length of the message in octets, not 
including the header.

2.2 M2PA Messages

The following section defines the messages and parameter contents.  An
M2PA message consists of a Common Message Header followed by the data
appropriate to the message.


   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   ...
   |                       Common Message Header                   |
                                   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   ...
   |                           Message Data                        |
                                   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


2.2.1 User Data 

The User Data is the data sent from the MTP3 in the form of the
contiguous LI, SIO, and SIF fields of the MSU ([2] Q.703, section 2.2
Signal Unit Format). The format for the User Data message is as
follows:








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    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   ...
   |                           User Data                           |
                                   ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


No padding is added to the MTP3 message.

Note that the User Data field contains only the LI, SIF, and SIO
octets. The other components of the message transmitted by MTP2 (the
Flag, BSN, BIB, FSN, FIB, CK) are not included in M2PA.

2.2.2  Link Status

The MTP2 Link Status message can be sent between M2PA peers to
indicate link status. This message performs a function similar to the
the Link Status Signal Unit in MTP2.


    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Link Status                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


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

    Value  Description

     1     In Service
     2     Processor Outage
     3     Processor Outage Ended
     4     Busy
     5     Busy Ended


3.  Procedures

3.1 Procedures to Support MTP2 Features

3.1.1 Signal Unit Format, Delimitation, Acceptance

Messages for transmission across the network must follow the format
described in section 2.

SCTP provides reliable, in-sequence delivery. Therefore the related
functionality of MTP2 is not needed. SCTP does not provide functions
related to Link State Control in MTP2. These functions must be

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provided by M2PA.


3.1.2 Link Alignment

Link alignment begins when MTP3 sends the Start command to M2PA.

To begin alignment in M2PA, M2PA sends the ASSOCIATE primitive to SCTP
if the SCTP association is not already established. 

To prevent duplicate associations from being established, it must be
decided in advance which endpoint initiates the establishment of the
association. In a pair of endpoints, the endpoint that initiates the
establishment of the association is called the client. The other
endpoint is the server. An endpoint may be a client in its
relationship with one endpoint, and a server in its relationship with
another endpoint. The designations of client and server are needed
only to decide which endpoint initiates the establishment of the
association. After that, the endpoints function as peers.

The client initiates the association using the server's IP address and
the M2PA well-known port number as the destination endpoint. In order
to allow for multiple links between the two endpoints, the client uses
a different local port number for each link. It must be decided in
advance which local ports are used by the client. Each of these client
ports must be known to the server. Each combination of client IP
address/port and server IP address/port must be mapped to the same
Signaling Link Code (SLC) in the client and server, so that each
endpoint knows which link is being created at the time the SCTP
association is established. However, M2PA does not do any processing
based on the SLC.

An example of the relationships between the associations and the SLCs
is shown in Figure 4 and Table 1. Note that a link is an SCTP
association identified by two endpoints, in this case a client and
server. Each endpoint is identified by an IP address and port
number. Each association is mapped to an SLC. Table 1 is only
conceptual. The actual method for mapping the SCTP associations to the
SLCs is implementation dependent.














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                 Client                       Server

                  IPA                           IPB
            +-------------+               +-------------+
            |             |	SCTP      |             |
            |   SLC = a   | association 1 |   SLC = a   |
            |  port = P1  +---------------+  port = PW  |
            |             |	          |             |
            |             |	          |             |
            |             |	          |             |
            |             |	SCTP      |             |
            |   SLC = b   | association 2 |   SLC = b   |
            |  port = P2  +---------------+  port = PW  |
            |             |	          |             |
            |             |	          |             |
            +-------------+               +-------------+

      IPA    = IP address of Client
      IPB    = IP address of Server
      P1, P2 = Pre-selected port numbers for Client
      PW     = Well-known port number for M2PA

                Figure 4: Associations and SLCs


     +-------------+---------------------------------------+-----+
     | Association |      Client       |      Server       | SLC |
     |             +------------+------+------------+------+     |
     |             | IP address | Port | IP address | Port |     |
     +=============+============+======+============+======+=====+
     |      1      |    IPA     |  P1  |    IPB     |  PW  |  a  |
     +-------------+------------+------+------------+------+-----+
     |      2      |    IPA     |  P2  |    IPB     |  PW  |  b  |
     +-------------+------------+------+------------+------+-----+

                  Table 1: Associations and SLCs


The association shall contain two streams in each direction. Stream 0
is designated for Link Status messages. Stream 1 is designated for
User Data messages.

If SCTP fails to establish the association, and M2PA had received a
Start command from its MTP3, then M2PA shall report to MTP3 that the
link is out of service. If M2PA has an SCTP association ID for that
association, it should ABORT the association. The association ID is a
number provided by the SCTP used to identify an association.

Once the association is established, M2PA invokes the GETSRTTREPORT
primitive to determine the Smooth Round Trip Time (SRTT) from SCTP. If
the SRTT exceeds its maximum allowed value (which is implementation
dependent), M2PA should use the ABORT primitive to end the

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association.  If M2PA had received a Start command from its MTP3, then
M2PA shall report to MTP3 that the link is out of service.

Once M2PA has received a Start from MTP3, the association is
established, the SRTT is determined to be satisfactory, and if MTP3
has not deactivated the link, then:

   (a) If there is no local processor outage condition, M2PA sends a
       Link Status of In Service to its peer. 

   (b) If there is a local processor outage condition, M2PA sends Link
       Status Processor Outage to its peer. When MTP3 sends Local
       Processor Recovered, then M2PA sends Link Status Processor
       Outage Ended to its peer, followed by Link Status In Service.

If M2PA has not received a Link Status In Service from its peer at the
time it sends the Link Status In Service, M2PA starts timer T1. Timer
T1 is stopped when M2PA receives Link Status In Service from its
peer. If M2PA does not receive Link Status In Service from its peer
before T1 expires, then M2PA reports to MTP3 that the link is out of
service. Then M2PA uses the ABORT primitive to end the association.

Recommended value of T1 is 5-150 seconds.

When the association is established, M2PA has sent Link Status In
Service to its peer, and has received Link Status In Service from its
peer, and there is no local processor outage condition, then M2PA
sends Link In Service to its MTP3. 

If M2PA receives a Link Status of Processor Outage during alignment,
and M2PA had received a Start command from its MTP3, M2PA shall report
Remote Processor Outage to MTP3. 

M2PA shall ignore the Emergency and Emergency Ceases commands from
MTP3.

3.1.3 Processor Outage

A processor outage occurs when M2PA cannot transfer messages because
of a condition at a higher layer than M2PA.

When M2PA detects a local processor outage, it sends a Link Status
message to its peer with status Processor Outage. M2PA shall discard
any User Data messages received. M2PA shall also cease sending User
Data messages to SCTP for transmission.

The peer M2PA, upon receiving the Link Status Processor Outage
message, shall report Remote Processor Outage to its MTP3. M2PA ceases
sending User Data messages.

When the processor outage ceases, MTP3 sends a Local Processor
Recovered indication to M2PA. The local M2PA notifies its peer by

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