Subject: [Tsvwg] SCTP fast retranmit question

From: Ravi Sastry Aysola <[email-masked]>
Date: Tue, 20 Sep 2005 10:47:32 -0400

I have two devices A and B that have an SCTP session up between them.
A sent a burst of 25 *individual* (not bundled) DATA chunks to B.
B sent a SACK to A for every second DATA chunk.

What I have noticed is, after receiving 4 SACKs from B, A was doing
a fast retransmit of remaining unacked TSNs to B. Looks like the
implementation on A is following section 7.2.4 to perform this 
fast retransmit. I am not convinced it should be doing that considering
the fact that there are *no* GAP blocks in any of the SACKs from B.

So the question is, is the behaviour of A correct or, should it just follow
delayed acknowledgment without doing any fast retransmit?
I feel if there are no explicit GAP blocks in the SACKs received, then
section 7.2.4 does not apply. Correct?

I would appreciate your help in clarifying.

thank you.

ravi.
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From: Randall Stewart <[email-masked]>
Date: Tue, 20 Sep 2005 13:10:17 -0400

Ravi:

I believe the implemention is broken :-0

The section header of 7.2.4 is:

7.2.4 Fast Retransmit on Gap Reports

That in itself pretty much covers it but if you want more
there is also:
"
    Whenever an endpoint receives a SACK that indicates some TSN(s)
    missing, it SHOULD wait for 3 further miss indications (via
    subsequent SACK's) on the same TSN(s) before taking action with
    regard to Fast Retransmit.
"

It cannot notice something is missing unless it has gaps
appearing..

R

Ravi Sastry Aysola wrote:
> ------------------------------------------------------------------------
> 

-- 
Randall Stewart
803-345-0369 <or> 815-342-5222(cell)