[Iccrg] RE: [conex] ECN in TCP and CWND

[Ingemar Johansson S]

Hi

Thanks, and really sorry for my delayed response, seems like my mailbox and mind is bloated these days :-)
Thanks for the reply, I double checked our TCP implementation and it seems then to be in order even though I find it a bit odd.
I will have a look at  
 http://tools.ietf.org/html/draft-mathis-tcpm-proportional-rate-reduction-00
later on, get the feeling that is is related to DCTCP or ?

Again, thanks for the help.
/Ingemar


> -----Original Message-----
> From: Scheffenegger, Richard [mailto:rs at netapp.com] 
> Sent: den 22 augusti 2011 15:04
> To: Ingemar Johansson S; iccrg at cs.ucl.ac.uk
> Cc: ConEx IETF list
> Subject: RE: [conex] ECN in TCP and CWND
> 
> Hi Ingemar,
> 
> I believe there was a very similar question recently in TCPM, 
> revolving around TCP-NCR, i.e.
> 
> http://www.ietf.org/mail-archive/web/tcpm/current/msg06528.html
> 
> 
> My summary was that RFC5681 does allow cwnd to increase 
> whenever an ACK acks new data (as would be the case with 
> ECN); however, the consensus back then was that cwnd should 
> not increase during "exceptional" events (reordering, loss 
> recovery, ecn rtt), even though that may hinder tcp from 
> throttling up when that may be safe...
> 
> OTOH, if cwnd is held constant during such events, particular 
> loss recovery may not complete without RTO (and subsequent 
> slow start), or cwnd may reduce to (and kept at) 1 segment 
> for extended periods of time...
> 
> But your observation, that many implementations will actually 
> increase cwnd after the cwnd reduction event, was made then too. 
> 
> Further note, that this particular behavior may get addressed 
> with draft-mathis-tcpm-proportional-rate-reduction-00, as the 
> cwnd reduction will be spaced out over ~1RTT -  basically a 
> new ACK will not necessarily trigger the sending of a new 
> data segment (and not increase cwnd), when PRR is active.
> 
> 
> But you may want to take this question to the tcpm group too...
> 
> Best regards,
> 
> 
> 
> Richard Scheffenegger
> 
> > -----Original Message-----
> > From: Ingemar Johansson S [mailto:ingemar.s.johansson at ericsson.com]
> > Sent: Montag, 01. August 2011 19:13
> > To: iccrg at cs.ucl.ac.uk
> > Cc: ConEx IETF list
> > Subject: [conex] ECN in TCP and CWND
> > 
> > Hi
> > 
> > I am digging in an experimental ECN for TCP implemementation. It is 
> > supposed to be implemented according to RFC3168.
> > 
> > As I understand things "TCP should not react to congestion 
> indications 
> > more than once every window of data" (6.1.3 in RFC3168) 
> Sofar so good, 
> > sounds very reasonable.
> > But what about CWND?, should it be allowed to increase during this 
> > time?.
> > 
> > To me it sounds most reasonable to _not_ allow the CWND to increase 
> > during the time that congestion indications are ingored but 
> the code I 
> > am staring at does not behave in this way.
> > 
> > 
> > What is the correct behavior ?
> > 
> > Regards
> > Ingemar
> > =================================
> > INGEMAR JOHANSSON  M.Sc.
> > Senior Researcher
> > 
> > Ericsson AB
> > Wireless Area Networks
> > Labratoriegränd 11
> > 971 28, Luleå, Sweden
> > Phone +46-1071 43042
> > SMS/MMS +46-73 078 3289
> > ingemar.s.johansson at ericsson.com
> > www.ericsson.com
> > Visit http://labs.ericsson.com !
> > =================================
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