draft-george-sigtran-m2peer-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

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

2.2.1 User Data 

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


    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










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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
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 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. 

An example of the relationships between the associations and the SLCs
is shown in Figure 4 and the following table. 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 corresponds to an SLC.









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                 Client                       Server

                  IPA                           IPB
            +-------------+               +-------------+
            |             |	SCTP      |             |
            |   SLC = a   |  association  |   SLC = a   |
            |  port = P1  +---------------+  port = PW  |
            |             |	          |             |
            |             |	          |             |
            |             |	          |             |
            |             |	SCTP      |             |
            |   SLC = b   |  association  |   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              | SLC |
            +-------------------+-------------------+     |
            |      Client       |      Server       |     |
            +------------+------+------------+------+     |
            | IP address | Port | IP address | Port |     |
            +============+======+============+======+=====+
            |    IPA     |  P1  |    IPB     |  PW  |  a  |
            +------------+------+------------+------+-----+
            |    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 association ID for that
association, it should ABORT the association. 




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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 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.














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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 discards any
messages received.

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

3.1.4 Level 2 Flow Control

Notification of receive congestion from M2PA to MTP3 is implementation
dependent.

3.1.5 Error monitoring

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

3.1.6  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.

3.2 Procedures to Support the MTP3/MTP2 Interface

3.2.1  Sending/receiving messages

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

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

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


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3.2.2  Link activation and restoration

When MTP3 requests that M2PA activate or restore a link by a Start
command, M2PA shall follow the alignment procedure in section 3.1.2.

3.2.3  Link deactivation

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

3.2.4  Flush buffers

Processing of the Flush Buffers request from MTP3 is implementation
dependent.

3.2.5 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.

The US National version of SS7 has congestion levels. For US National
SS7, the Indication primitive for Congestion Onset should report the
congestion level.

3.2.6 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 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 transmitted
over the alternate links. References to stream sequence numbers in
this section refer only to the User Data stream's stream sequence
numbers.




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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
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 its upper layer by stream, and
based on SSNs.

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.



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(Note that any messages with stream sequence number greater than this
BSNT value have been acknowledged by SCTP, but have not been passed up
to 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.

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.


4.  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.


4.1  Link Initialization (Alignment)

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











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    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
     <------------


At this point, MTP3 may begin sending data messages.






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