Re: [PATCH 0/2] Re: Do piggybacked ACKs work

["Doug Graham" <dgraham@nortel.com>]

>
> Sorry, haven't had a lot of time to play with this until now.  The 
> behaviour for
> small unfragmented message looks fine, but if the message has to be 
> fragmented,
> things don't look so good.  I'm ping-ponging a 1500 byte message 
> around: client
> sends 1500 bytes, server reads that and replies with the same message, 
> client
> reads the reply then sleeps 2 seconds before doing it all over again.  
> I see no
> piggybacking happening at all.  A typical cycle looks like:
>
> 12 2.007226    10.0.0.248    10.0.0.249    SCTP     DATA  (TSN 7376)
> 13 2.007268    10.0.0.248    10.0.0.249    SCTP     DATA  (TSN 7377)
> 14 2.007313    10.0.0.249    10.0.0.248    SCTP     SACK  (TSN 7377)
> 15 2.007390    10.0.0.249    10.0.0.248    SCTP     SACK  (TSN 7377)
> 16 2.007542    10.0.0.249    10.0.0.248    SCTP     DATA
> 17 2.007567    10.0.0.249    10.0.0.248    SCTP     DATA
> 18 2.007615    10.0.0.248    10.0.0.249    SCTP     SACK
> 19 2.007661    10.0.0.248    10.0.0.249    SCTP     SACK
>
> Those back-to-back SACKs look wasteful too.  One should have done the 
> job,
> although I suppose I can't be sure that SACKs aren't crossing DATA
> on the wire.  But the real mystery is why the SACKs were
> sent immediately after the DATA was received.  Looks like delayed SACKs
> might be broken, although they are working for unfragmented messages.
>

It just occurred to me to check the TSNs too, and I've redone the annotation
in the trace above with those.  So the back-to-back SACKs are 
duplicates: both
acknowledge the second data chunk (so they could not have crossed DATA 
on the
wire).

--Doug