Subject: [Sigtran] M2PA: T7 and dropping SCTP association after link failure

From: "TOLGA ASVEREN" <[email-masked]>
Date: Thu, 24 Apr 2003 09:00:02 +0300

Hi Tom,

1) In M2PA 4.2.1
"Because propagation delays in IP networks are more variable than in
traditional SS7 networks, a single T7 timer (excessive delay of
acknowledgement) is inadequate. A T7 timer SHALL be started for each
message in the M2PA retransmit buffer."

I think this explanation might be confusing -or better said not complete-
for people, which did not follow the threads related with T7. IMO, it could
be better to mention about possibility of SCTP delaying acknowledgements
for a specific message. Furthermore, based on what it stands in RFC2960 6.1
"This document is specified as if there is a single retransmission timer
per destination transport address, but implementations MAY have a
retransmission timer for each DATA chunk." , I would think it could be
better to revise the above section of M2PA, reflecting that based on how
the underlying SCTP handles T3, how to use M2PA T7 might be different.

2) A few weeks ago there was a thread about why we don't want to drop
association when  link failure occures. Personally I still think dropping
SCTP association is a better choice, because IMO there is no real reason
-as far as I can see- to not drop it and if not dropped, messages arriving
in the old link context might trigger a new link failure(in this thread
there are also counterarguments expressed by Brian). What do the others
think?

     Tolga

_______________________________________________
Sigtran mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/sigtran

---

From: "TOLGA ASVEREN" <[email-masked]>
Date: Thu, 24 Apr 2003 15:56:50 +0300

Hi Jeff,

Hello Tolga,

I think it is a bad idea to drop the association due to M2PA protocol
events,
for reasons that have be previously stated in several SIGTRAN threads.
[TOLGA]The reasons, which are expressed to support not to drop SCTP
association-at least the ones I could find in the threads- (cwnd shrinking
-cwnd will shrink anyway-, problems with lost ABORT on SCTP level -if this
is a problem, it is related with SCTP not with M2PA-) are not very valid
IMO. Furthermore, as I stated, messages arriving in the old link context
might cause another link failure after link is restarted -2 out of 3 rule
might not help-. So, the arguments presented before are not satisfactory
IMO.

I also have no issues with the current M2PA text pertaining to timer T7.
This
topic has also been thoroughly discussed in prior threads.
[TOLGA]Even if one uses per message T3 on SCTP? In such a case I would like
to use T7 not as a per message timer. And even when T3 is not kept per
message on SCTP, still I think it is really very unlikely to need a per
message T7
(As far as I remember the example was assuming a faulty hardware which
fails CRC checks for a specific value. Furthermore feedback from TSVWG WG
was that one can keep a counter for retransmissions for a specific message
and can drop SCTP association after certain value is exceeded, to handle
such a scenario) What I would prefer is a more balanced and explanatory
text in M2PA.

     Tolga

Regards,

Jeff

                      "TOLGA ASVEREN"

                      <TOLGA.ASVEREN@o         To:      [email-masked]

                      2.com.tr>                cc:

                      Sent by:                 Subject: [Sigtran] M2PA: T7
and dropping SCTP association after link failure
                      sigtran-admin@ie

                      tf.org

                      04/24/03 02:00

                      AM

Hi Tom,

1) In M2PA 4.2.1

     Tolga