Subject: [Sigtran] Repost - More M2PA 7 questions]

From: Andrew Booth <[email-masked]>
Date: Tue, 18 Feb 2003 22:07:40 -0500

Hi,

I posted these a while ago and haven't seen any responses.  Question (2) 
is really the only relevant one.

Thanks for any info,
Andrew

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Signaling Systems Group, Performance Technologies
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Subject: [Sigtran] More M2PA 7 questions
From: Andrew Booth <[email-masked]>
To: [email-masked]
Date: Thu, 06 Feb 2003 12:31:41 -0500
List-Id: Signaling Transport <sigtran.ietf.org>

Hi All,

A few more questions on M2PA draft 7.

1) Does every User Data received from the peer increases the expected FSN
for the subsequent message, even in cases where the received message is 
later
discarded as part of a Flush Buffers or Changeover procedure?

2) Suppose that an M2PA implementation has received messages from a peer
and that some of the messages are buffered at the M2PA layer (perhaps it is
in processor outage).  Suppose now the link transitions to OOS and the
MTP3 sends a Retrieve BSNT to M2PA.

According to 4.2.6,

   "M2PA determines the Forward Sequence Number of the last User Data
   message received from the peer.  This is the BSNT."

In the M2PA situation it seems more appropriate to respond with the FSN
of the last message which was delivered to MTP3.
Am I missing something?

3) How should T7 be handled when remote processor outage abates?  Should 
we immediately check for excessive ack delay, or reset T7 somehow?

4) In section 5.1 we transition to proving without starting T3 and to 
aligned ready without receiving LS Proving.  Are these omissions 
intentional?

5) In section 5.8 there is some introductory text :

"Note that in this example, the sequence numbers and messages requested
by MTP3 are sent from SCTP to M2PA in the Communication Lost
primitive. In general, the retrieval of sequence numbers and messages
is implementation dependent."

Why are messages retrieved from SCTP?  Is this text indicating that the 
M2PA retransmit buffer is implemented in the SCTP?  Just a little confused.

6) In section 4.2.6, is the following correct or should it be rephrased 
in the context of a circular buffer?
"any transmitted User Data messages beginning with the first
       unacknowledged message with FSN greater than FSNC." 

Thanks for any info,
Andrew

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Signaling Systems Group, Performance Technologies
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From: "Brian F. G. Bidulock" <[email-masked]>
Date: Tue, 18 Feb 2003 20:24:49 -0700

Andrew,

The draft is incorrect.

The BSNT is the last acknowledged received DATA (data should not be
acknowledged unless it is locally "deliverable" to MTP3).

Otherwise there is a slim chance that message loss will occur on changeover
resulting from processor outage or busy conditions.

The statement in section 4.2.6 should be changed to reflect this.

--brian

Andrew Booth wrote:                                                                 (Tue, 18 Feb 2003 22:07:40)

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---

From: Tom George <[email-masked]>
Date: Wed, 19 Feb 2003 15:11:29 -0600

Here is my new improved wording in 4.2.6:

For data retrieval, MTP3 requests the Backward Sequence Number to be
Transmitted (BSNT) from M2PA through the Retrieve BSNT request. M2PA
determines the Forward Sequence Number of the last User Data message
received from the peer and delivered to the upper layer MTP3. This
value is the BSNT. 

tomG.

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---

From: Tom George <[email-masked]>
Date: Wed, 19 Feb 2003 16:02:59 -0600

Andrew and others,

Here are some comments on the other items....

tomG.

> discarded as part of a Flush Buffers or Changeover procedure?

I am not sure what you are asking. Are you asking if the FSNs are ever
reset? 

> of the last message which was delivered to MTP3.

This one is discussed in another message.

> we immediately check for excessive ack delay, or reset T7 somehow?

From our earlier discussion of T7 during Busy, I think the answer is
that T7 is not running during PO. Any messages would be flushed, so T7
would not start again until a new message was sent. 

> aligned ready without receiving LS Proving.  Are these omissions 

These were unintentionally omitted. I added those events to the diagram.

> M2PA retransmit buffer is implemented in the SCTP?  Just a little confused.

This is left over from an earlier version. I removed that paragraph.

>        unacknowledged message with FSN greater than FSNC." 

There is a general warning about this in 4.2.1.

---

From: "Brian F. G. Bidulock" <[email-masked]>
Date: Wed, 19 Feb 2003 23:45:02 -0700

Tom,

Well....

MTP2 is allowed to acknowledge messages which have not actually been
delivered to MTP3.  It would be better to say that "the BSNT is the
FSN of the last User Data message received for which an acknowledgement
was sent."  The timing of actual deliveries to MTP3 is implementation
dependent.

--brian

Tom George wrote:                                                                      (Wed, 19 Feb 2003 15:11:29)

---

From: Tom George <[email-masked]>
Date: Thu, 20 Feb 2003 10:45:15 -0600

Brian,

Good point. Now it says:

M2PA
determines the Forward Sequence Number of the last User Data message
received from the peer for which an acknowledgement was sent. 

tomG.

> was sent."  The timing of actual deliveries to MTP3 is implementation
  [...]

---

From: Andrew Booth <[email-masked]>
Date: Thu, 20 Feb 2003 18:28:15 -0500

Tom, Brian,

Is M2PA also allowed to withhold acknowledgements briefly? i.e. can't I 
have delivered a message to MTP3 but not yet sent the acknowledgement? 
 Perhaps

   M2PA determines the Forward Sequence Number of the last User Data message
   received from the peer which has been marked for acknowledgement i.e. which
   is guaranteed to be delivered to MTP3.

Andrew

---

From: Andrew Booth <[email-masked]>
Date: Thu, 20 Feb 2003 19:31:03 -0500

Tom,

Thanks for the info.  One comment below...

>I am not sure what you are asking. Are you asking if the FSNs are ever
In "4.1.4 Processor Outage" as described in M2PA draft 7:

   MTP3 may send a Flush Buffers or Continue command to M2PA as part of
   its processor outage procedure (See section 4.2.4 Flush Buffers and
   Continue). The use of Flush Buffers and Continue is specified in Q.703
   [Q.703] and T1.111.3 [T1.111].

In q.703 there is a reference to "synchronization of the level 2 
sequence numbers" after flushing buffers.  I'm confused about the 
concept of synchronizing sequence numbers in M2PA, and was wondering if 
this reference could be clarified.  Do we need to do anything special 
with sequence numbers after a Flush Buffers?

Andrew