Re: [Lksctp-developers] SACK for non-existant CTSN?

[Chris L. <cla...@re...>]

Sridhar Samudrala wrote:
> Chris Lalancette wrote:
> 
>> Hey all,
>>      I'm kind of new to SCTP, so forgive me if I ask silly questions. 
>> I am looking at the latest upstream lksctp implementation (I pulled
>> Linus' git tree this morning).  I am running iperf with a minor patch
>> to make it use SCTP for testing (I'll attach details about the patch
>> below).  When I set up iperf to use buffers of length 65536, I am
>> seeing some on-the-wire behavior that I don't quite understand. 
>> Basically, the connection starts as normal:
>>
>> INIT
>> INIT_ACK
>> COOKIE_ECHO
>> COOKIE_ACK
>>
>> After that, the sender sends the first data packet, which in this case
>> has CTSN of 520344188.  The receiver sends a SACK for 520344188.  Then
>> the sender sends two more data chunks, with CTSN of 520344189 and
>> 190.  The receiver then sends a SACK for CTSN 520344190.  Up to here,
>> things seem fine.  After this is when I don't understand.  The next
>> two packets are SACK's from the receiver, changing the a_rwnd (which I
>> believe is allowed by the protocol), but with CTSN of 520344191.  How
>> can the receiver be SACK'ing a CTSN that it hasn't yet received?  Even
>> more strange is that the next data packet that the sender sends is for
>> CTSN 520344192, and as far as I can tell (with searching through
>> ethereal), CTSN 520344191 is *never* sent.
>>   
> 
> It is possible for ethereal to drop some packets under heavy load.
> You must be running into that situation.
> 
> Thanks
> Sridhar
> 
>> Am I missing something obvious protocol-wise here, or is this a bug? 
>> All of the relevant information is here:
>>
>> download/compile iperf with sctp:
>> http://people.redhat.com/clalance/sctp/sctp_iperf.sh
>> ethereal packet capture: 
>> http://people.redhat.com/clalance/sctp/2.6.18.cap
>>
>> Thanks for your help,
>> Chris Lalancette
>>
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> 
> 
> 

Sridhar,
     Grr.  I didn't know that about ethereal.  Thanks for the info.

Chris Lalancette