Subject: [Tsvwg] SCTP query

From: [email-masked]
Date: Fri, 28 Sep 2001 17:27:30 +0530

Hi

I am having some doubt regarding the RTO calculation in SCTP.

Suppose two SCTP instances SCTP1 and SCTP2 have established
an association. SCTP1 is transferring DATA chunks to SCTP2.

Let DATA chunk with TSN = 49 is sent by SCTP1.
SACK is sent by SCTP2 with cummulative TSN = 49 and arwnd = 0.
Assume RTO at SCTP1 is RTO.min at this point of time.

When arwnd of SCTP2 is 0, SCTP1 is allowed to send one data chunk
to probe any change in the window at SCTP2.
So another chunk is sent by SCTP1 which has TSN = 50.
An SACK again is sent by SCTP2  with Cummulative TSN = 49 arwnd = 0.

When SCTP1 receives this chunk, it discards it as TSN = 49 has already
been acknowledged.

After timeout, TSN = 50 would be retransmitted by SCTP1. As this
happens, exponential backoff algoritm is applied to recalculate the
value of RTO. Let again SACK with Cummulative TSN = 49 and arwnd = 0
arrive. Thus more timeout happen and we end up increasing the RTO
to some value RTO.max.

My understanding about RTO is that it is increased exponentially in the
anticipation that the network has got congested. In this situation, the network
has not got congested, but due to temporary problem at receiving SCTP2,
RTO is being increased incorrectly at sending SCTP1.

Please correct me where I am wrong.

Regards

Vakul

Hughes Software Systems

_______________________________________________
tsvwg mailing list
[email-masked]
http://www1.ietf.org/mailman/listinfo/tsvwg