Subject: [Sigtran] other processor outage race conditions (was "M2PA LS Ready at the end of Processor Outage")

From: "Davidson, Mark" <[email-masked]>
Date: Wed, 10 Aug 2005 08:52:17 -0500

Brian,

What are the other "race conditions that no longer apply" ?
I'm curious in particular about the following text:

    While there is a remote processor outage (RPO) condition:
       ...
       (b) If any User Data messages received from the peer after the
Link
           Status Processor Outage cannot be delivered to MTP3, then
these
           messages MUST NOT be acknowledged and MUST be buffered.

How can User Data be sent after processor outage indication?

Except for requiring an LS Ready for PO recovery, is there any reason
not to 
simply follow the MTP2 spec on this?  Are there other problems being
solved
by the processor outage procedures beyond those that were self inflicted

by sending processor outage indication on stream 0?

Mark Davidson
[email-masked]

>Howard,
>
>Although it is mentioned in the example in Figure 16 that
acknowledgements 
>are discarded, the example was written before we finally decided to
send LS 
>PR and LS Ready on the data stream.  Before these messages were
synchronized 
>with the data it was possible to have several race conditions that not 
>longer apply.  One of them was late arriving acknowledgements that were

>sent before the Link Status messages used for SN synchronization.  This

>can no longer occur, as the pertinent Link Status messages are sent 
>sequenced on Stream 1 with the data an SCTP will ensure proper
ordering.
>
>So, there should be no need to discard acknowledgements.
>
>--brian
>
>
>May, Howard wrote:                           (Mon, 08 Aug 2005
13:59:29)
>
>> During the SN realignment both LPO an RPO may need to discard
incoming 
>> acknowledgements.  I would like clarification of what 'discard 
>> incoming acknowledgements' means.
>> 
>> Does it simply mean that the receiving side must ignore any
indication 
>> that a message has been successfully received by MTP3.
>> 
>> Cheers
>> 
>> Howard May

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---

From: "Brian F. G. Bidulock" <[email-masked]>
Date: Wed, 10 Aug 2005 13:02:30 -0600

Mark,

Davidson, Mark wrote:                         (Wed, 10 Aug 2005 08:52:17)
> What are the other "race conditions that no longer apply" ?

Some of the discussion about what arrives before what in the example in
Figure 16.

>        (b) If any User Data messages received from the peer after the Link
>            Status Processor Outage cannot be delivered to MTP3, then these

   While in local processor outage (LPO) condition:
      ...
      (c) M2PA SHOULD continue to transmit messages that have been sent
          by its upper layer MTP3.

That's how.

> 
> Except for requiring an LS Ready for PO recovery, is there any reason not to 
> simply follow the MTP2 spec on this?  Are there other problems being solved

Yes, the other problem being solved is the subtle difference between
MTP2 and M2PA: M2PA cannot discard messages because they will never be
retransmitted.

The above is an embellishment that we thought worth while: because M2PA
can send a single LSPO where MTP2 sends continuous SIPO, it is possible
for M2PA to clear any backlog of messages to the peer where MTP2 cannot.

This reduces the number of messages to be changed over or flushed from
the LPO end and is a good thing.

--brian

-- 
Brian F. G. Bidulock
[email-masked]
http://www.openss7.org/