netdev - Re: TCP fast retransmit

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Message-ID: <1322239016.5793.12.camel@edumazet-HP-Compaq-6005-Pro-SFF-PC>
Date:	Fri, 25 Nov 2011 17:36:56 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Esztermann, Ansgar" <Ansgar.Esztermann@...-bpc.mpg.de>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: TCP fast retransmit

Le vendredi 25 novembre 2011 à 14:33 +0100, Esztermann, Ansgar a écrit :
> [originally posted to lkml]
> Hello list,
> 
> is there some documentation available on TCP fast retransmit? There
> seem to be quite a lot of descriptions -- from informal to scholarly
> papers -- on the various algorithms available to calculate the proper
> size of the congestion window, but I have been unable so far to find
> out *when* a fast retransmit is triggered. RFC 2581 states the third
> dupACK "should" do it, and this seems to be quoted fairly often.
> However, I can easily produce connections that fail to perform fast
> retransmit even after 5 dupACKs. Some people mention Linux uses a
> different (presumable more sophisticated) algorithm to trigger fast
> retransmits, but no-one seems to elaborate.

Could you send a sample pcap of such problem, but please include full
tcp sesssion, from the first SYN packet, up to packets following
restransmits.

A diff of "netstat -s" taken before your session and after your session
on receiver would help too, if receiver is not a loaded machine of
course.

Also, what version of linux kernel are you using in receiver ?



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