draft-ietf-sigtran-m2pa-02.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

George, et al                                                [Page 22]


Internet Draft  SS7 MTP2-User Peer-to-Peer Adaptation Layer   Mar 2001

Figure 9 and Table 3 show two associations between the same
endpoints. This is accomplished by using different port numbers for
each association at the client.


                IPSP X                        IPSP Y

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

      IPx    = IP address
      P1, P2 = Pre-selected port numbers for Client
      PW     = Well-known port number for M2PA

                Figure 9: Associations and SLCs -
             Multiple Associations Between Endpoints


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

                Table 3: Associations and SLCs -
             Multiple Associations Between Endpoints


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.


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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.

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 5-50 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.



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Internet Draft  SS7 MTP2-User Peer-to-Peer Adaptation Layer   Mar 2001

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
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.

MTP3 sends a Flush Buffers or Continue command to M2PA. 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. Since SCTP has its own
congestion control, the purpose of the M2PA level 2 flow control is to
monitor the association and decide if it should be aborted.

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
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.

The peer M2PA shall continue transmitting messages to SCTP while its
T6 timer is running, i.e., while the other end is Busy. 

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.

Recommended value of T6 is 1-6 seconds. 





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Internet Draft  SS7 MTP2-User Peer-to-Peer Adaptation Layer   Mar 2001

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.

As long as the SCTP association is up, M2PA shall regularly invoke the
SCTP GETSRTTREPORT primitive to determine the Smooth Round Trip Time
(SRTT) from SCTP. If the the SRTT exceeds the maximum acceptable
value, the link is considered failed and taken out of service. The
interval between successive SRTT requests and the maximum acceptable
SRTT value are determined by the parameters:

   SRTT_measurement_interval   
      Range: 1 - 1000 seconds.  
      Default: 5 seconds.

   SRTT_max
      Range: 0-65,535 milliseconds.  
      Default: 1000 milliseconds.


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
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. See
[13] for a possible extension to SCTP to allow for stream priorities.


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.


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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, Continue

The Flush Buffers and Continue commands allow M2PA to resume normal
operations (i.e., transmission of messages to SCTP and receiving
messages from SCTP) after a processor outage (local and/or remote)
ceases.

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. These messages shall be
       discarded.

   (b) shall discard all messages currently waiting to be passed 
       to MTP3.

If M2PA receives either a Flush Buffers or Continue command from MTP3,
and the processor outage condition ceases, M2PA shall resume receiving
and transmitting messages.


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 use the Congestion Indication primitive to notify its upper
layer MTP3 when transmit buffer occupancy 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.

Note: M2PA does not discard messages because of transmit
congestion. Discarding of messages due to transmit congestion is
performed by MTP3.


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Internet Draft  SS7 MTP2-User Peer-to-Peer Adaptation Layer   Mar 2001


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 required. 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.)

Also, the following MTP3/MTP2 primitives must use the larger sequence
numbers:

   - BSNT Indication

   - Retrieval Request and FSNC


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Internet Draft  SS7 MTP2-User Peer-to-Peer Adaptation Layer   Mar 2001

For data retrieval, MTP3 requests the Backward Sequence Number to be
Transmitted (BSNT) from M2PA through the Retrieve BSNT
request. Normally, SCTP receives messages in order, in which case the
BSNT is the last message received by SCTP. However, because of
congestion or a failure condition, the sequence numbers of the
acknowledged messages may have gaps.  In particular, the SCTP SACK
(selective acknowledgement message) message can have several of these
gaps. In this case, it is necessary to scan through these gaps and
find the sequence number before the first gap. This is the number
considered as the BSNT and communicated to MTP3. M2PA sends the BSNT