draft-ietf-sigtran-m2pa-01.txt

No7信令,我需要交换类似的代码, 请店长审核,谢谢了,急着交换,谢谢

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

                 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 7: 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 the SCTP association is not established, the client M2PA sends the
ASSOCIATE primitive to SCTP. The client should attempt to establish
the association periodically until it is successful.

If SCTP fails to establish the association, and M2PA had received a
Start Request 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.


George, et al                                                [Page 18]


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

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

Once M2PA has received a Start Request 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.

Note that if the server M2PA has not received a Start Request from its
MTP3, it will not send the Link Status In Service message to the
client. Eventually the client will ABORT the association. The client
will then attempt to establish the association.

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 Request from its MTP3, M2PA shall report
Remote Processor Outage to MTP3.

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

4.1.3 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

George, et al                                                [Page 19]


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

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
sending a Link Status message with status Processor Outage Ended.  The
peer notifies its MTP3 that the remote processor outage has ceased.

4.1.4 Level 2 Flow Control

Notification of receive congestion from SCTP to M2PA is implementation
dependent. This section assumes that there is some form of receive
congestion notification from SCTP to M2PA.

If M2PA receives notification from its lower layer SCTP that SCTP is
in receive congestion for an association, M2PA shall send a Link
Status Busy message to its peer on that association.

When the peer M2PA receives the Link Status Busy message, it shall
cease transmitting User Data messages from its upper layer MTP3. (User
Data messages already given to the lower layer SCTP are transmitted as
the SCTP protocol allows.) M2PA shall start the Remote Congestion
timer T6. If timer T6 expires, M2PA shall use the ABORT primitive to
end the association and take the link out of service.

If M2PA receives notification from its lower layer SCTP that SCTP is
no longer in receive congestion for the association, M2PA shall send a
Link Status Busy Ended message to its peer on that association.

When the peer M2PA receives the Link Status Busy Ended message, it
shall stop timer T6 and resume transmitting User Data messages from
its upper layer MTP3.

Recommended value of T6 is 1-6 seconds.

4.1.5 Error Monitoring

If M2PA loses the SCTP association for a link, M2PA shall report to
MTP3 that the link is out of service.

4.1.6  Transmission Priorities

In MTP, Link Status messages have priority over User Data messages
([2] Q.703, section 11.2). To achieve this in M2PA, Link Status and
User Data messages shall be sent via SCTP on separate streams. All
messages are sent using the ordered delivery option.

M2PA should give higher priority to reading the Link Status stream

George, et al                                                [Page 20]


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

over the User Data stream.

M2PA should give higher priority to receiving notifications from SCTP
over reading either the Link Status stream or the User Data stream.

Implementation Note: If the SCTP implementation allows streams to have
different priorities for sending messages, then M2PA should set the
Link Status stream to a higher priority than the User Data stream.


4.2 Procedures to Support the MTP3/MTP2 Interface

4.2.1  Sending/receiving messages

When MTP3 sends a message for transmission to M2PA, M2PA passes the
corresponding M2PA message to SCTP using the SEND primitive.

M2PA Link Status messages are passed to SCTP using the SEND primitive.

Link Status and User Data messages shall be sent via SCTP on separate
streams.

When M2PA receives a Data message from SCTP, M2PA passes the message
to MTP3.

4.2.2  Link activation and restoration

When MTP3 requests that M2PA activate or restore a link by a Start
Request, M2PA shall follow the alignment procedure in section 4.1.2.

4.2.3  Link deactivation

When MTP3 requests that M2PA deactivate a link by a Stop command, M2PA
shall send an ABORT primitive to SCTP.

4.2.4  Flush buffers

If M2PA receives a Flush Buffers command from MTP3, M2PA:

   (a) shall not transmit any messages to SCTP that are currently
       waiting to be transmitted to SCTP. 
   (b) shall discard all messages currently waiting to be passed 
       to MTP3.

4.2.5 MTP3 Signaling Link Congestion

Notification of transmit congestion from SCTP to its upper layer
(M2PA) is implementation dependent. Nevertheless, M2PA should receive
notification from SCTP adequate to allow MTP3 to meet its requirements
for signaling link transmit congestion in [2] Q.704, section 3.8.

M2PA shall notify its upper layer MTP3 when transmit buffer occupancy

George, et al                                                [Page 21]


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

crosses the congestion onset, discard, and abatement thresholds. For
national networks with multiple congestion threshold levels, M2PA
shall notify MTP3 when transmit buffer occupancy crosses each level of
the congestion onset, discard, and abatement thresholds.

4.2.6 Changeover

The objective of the changeover is to ensure that signaling traffic
carried by the unavailable signaling link is diverted to the
alternative signaling link(s) as quickly as possible while avoiding
message loss, duplication, or mis-sequencing.  For this purpose, the
changeover procedure includes data retrieval, which is performed
before opening the alternative signaling links to the diverted
traffic.  Data retrieval consists of these steps:

   (1) buffer updating, i.e., identifying all those User Data 
       messages in the retransmission buffer of the unavailable
       signaling link which have not been received by the far end
       SCTP, as well as untransmitted messages, and

   (2) transferring those messages to the transmission buffers of the
       alternate links.

Note that only User Data messages are retrieved and transmitted over
the alternate links. Link Status messages shall not be retrieved and
transmitted over the alternate links. References to stream sequence
numbers in this section refer only to the User Data stream's stream
sequence numbers.

In order to support changeover in M2PA, the SCTP Stream Sequence
Numbers must be used in place of the Forward and Backward Sequence
Numbers (FSN/BSN) of SS7.

Stream Sequence Numbers used by SCTP are 16 bits long.  MTP2's Forward
and Backward Sequence Numbers are only seven bits long.  Hence it is
necessary for MTP3 to accomodate the larger SSNs. This is done through
the use of the Extended Changeover Order (XCO) and Extended Changeover
Acknowledgement (XCA) messages instead of the Changeover Order (COO)
and Changeover Acknowledgement (COA) messages. The XCO and XCA
messages are specified in Reference [7] section 9.8.1. Only the XCO
and XCA messages from [7] are used. These messages have a 24-bit field
for the sequence number. The upper 8 bits of the 24 bit field should
be set to 0, and the SSN placed in the lower 16 bits.

(Note that the Stream Sequence Numbers are used instead of the
Transmission Sequence Numbers. The Transmission Sequence Numbers are
32 bits long, and therefore would not fit in the XCO and XCA
messages.)

For data retrieval, MTP3 requests Backward Sequence Number (BSN) from
M2PA.  This is the sequence number of the last message received by the
local end.  Normally, SCTP delivers ordered messages to the

George, et al                                                [Page 22]


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

application.  However, during congestion or failure condition, the
sequence numbers of the acknowledged messages may have gaps.  In
particular, the SACK (selective acknowledgement message) message can
have several of these gaps.  Hence, it is important to scan through
these gaps and find the sequence number before the first gap.  This is
the number from which the remote end has to transmit the messages.  So
this is the number considered as the Backward Sequence Number and
communicated to the remote end.  In a similar way, the remote end also
detects the BSN and indicates to the local end. As soon as the MTP3 of
the local end receives this BSN, MTP3 retrieves all the unacknowledged
messages starting from BSN.  This is accomplished through a Retrieval
Request and FSNC request.  After all the messages are sent from M2PA
to MTP3, a Retrieval Complete indication is sent.

Note that the sequence numbers and messages requested by MTP3 may be
obtained by M2PA from SCTP via the Communication Lost primitive [5].
Retrieval of messages is an optional feature in SCTP. To perform data
retrieval, it is necessary that this option be implemented, and that
the SSNs of the messages are identified. SCTP must retain the messages
for retrieval by MTP3/M2PA whenever an association is aborted. SCTP
must be able to return messages to M2PA so that M2PA can perform
retrieval for MTP3. There are various ways that this can be
implemented, such as:

   (1) SCTP provides a way for M2PA to request retrieval of messages
       for a specified stream and SSN(s).

   (2) SCTP retrieves all messages and identifies the stream and SSN
       of each message. M2PA then must select the appropriate messages
       to pass up to MTP3.

If M2PA receives a Retrieve BSNT request from MTP3, M2PA responds with
the BSNT indication. The BSNT value is the SCTP stream sequence number
of the last message received by SCTP User Data stream before any gaps
in the stream sequence numbers.

(Note that any messages with stream sequence number greater than this
BSNT value have been acknowledged by the receiving SCTP, but have not
been passed up to M2PA. These messages are discarded by the receiving
SCTP and are not delivered to the upper layer M2PA. Therefore these
messages should be retransmitted by the far end on the alternate
link.)

If M2PA receives a Retrieval Request and FSNC request from MTP3, M2PA
retrieves from SCTP:

       (a) any transmitted messages beginning with the first 
	   unacknowledged message with stream sequence number greater
	   than FSNC, and

       (b) any untransmitted messages in SCTP.


George, et al                                                [Page 23]


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

Each of these messages is sent to MTP3, first (a), then (b). Then M2PA
sends the Retrieval Complete indication to MTP3.

Note: The changeover procedure makes it impossible for M2PA to have
multiple User Data streams in a direction for one link. Buffer
updating would have to be done for each User Data stream separately to
avoid duplication of messages. But MTP3 provides for only one XCO
message for sending the last-received SSN.


5.  Examples of M2PA Procedures

In general, messages passed between MTP3 and M2PA are the same as
those passed between MTP3 and MTP2.  M2PA interprets messages from
MTP3 and sends the appropriate message to SCTP. Likewise, messages
from SCTP are used to generate a meaningful message to MTP3.

Note that throughout this section, the primitives between MTP3 and
M2PA are named using the MTP terminology [1][2]. Communications
between M2PA and SCTP are named using SCTP terminology.


5.1  Link Initialization (Alignment)

An example of the message flow to bring an SS7 link in service is
shown below. Alignment is done by both ends of the link. To simplify the
diagram, alignment is shown on one end only. It is assumed in this
example that SCTP has been initialized.

























George, et al                                                [Page 24]


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

    MTP3        M2PA        SCTP        SCTP        M2PA        MTP3
    ----        ----        ----        ----        ----        ----

     Out of Service
     <------------

     Emergency OR
     Emergency Ceases
     ------------>

     Start
     ------------>

                 Associate
                 ------------>

                             (SCTP Association
                              procedure)

                 Communication Up        Communication Up
                 <------------           ------------>


Even though the SCTP association is established, it is important that
M2PA not send MTP3 data at this point. It must be confirmed that both
ends of the link are ready for traffic. Otherwise, messages could be
lost. The endpoints must exchange In Service messages.


    MTP3        M2PA        SCTP        SCTP        M2PA        MTP3
    ----        ----        ----        ----        ----        ----

                 Get SRTT Report
                 ------------>

                 Link Status In Service
                 ------------------------------------>

                                Link Status In Service
                 <------------------------------------

     In Service                                      In Service
     <------------                                   ------------>


At this point, MTP3 may begin sending data messages.







George, et al                                                [Page 25]


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

5.2  Message Transmission and Reception

Messages are transmitted using the Data Request primitive from MTP3 to
M2PA. The diagram shows the case where the Link is In Service. The
message is passed from MTP3 of the source to MTP3 of the destination.


    MTP3        M2PA        SCTP        SCTP        M2PA        MTP3
    ----        ----        ----        ----        ----        ----

     Message for 
     transmission
     ------------>

                 Send
                 (Data Message)
                 ------------>

                             (SCTP sends message)

                                         Receive
                                         ------------>

                                                  Received message
                                                     ------------>