netdev - Re: TCP fast retransmit

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Date:	Mon, 28 Nov 2011 13:17:40 -0800
From:	Yuchung Cheng <ycheng@...gle.com>
To:	"Esztermann, Ansgar" <Ansgar.Esztermann@...-bpc.mpg.de>
Cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: TCP fast retransmit

Hi Ansgar,

On Fri, Nov 25, 2011 at 5:33 AM, Esztermann, Ansgar
<Ansgar.Esztermann@...-bpc.mpg.de> wrote:
> [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.
>

This 2003 paper has the core algorithms for Linux loss recovery.
http://www.cs.helsinki.fi/research/iwtcp/papers/linuxtcp.pdf

We've also detailed latest Linux fast retransmit algorithms in the PRR
paper (section 3.2).
http://research.google.com/pubs/pub37486.html

HTH
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