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Date: Fri, 25 Jun 2004 09:59:24 -0400 (EDT)
From: Robert Watson <rwatson@freebsd.org>
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To: Daniel Lang <dl@leo.org>
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cc: liamfoy@sepulcrum.org
cc: Paul Saab <ps@freebsd.org>
cc: current@freebsd.org
Subject: Re: Re: HEADS UP: SACK committed to HEAD
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On Fri, 25 Jun 2004, Daniel Lang wrote:

> liamfoy@sepulcrum.org wrote on Fri, Jun 25, 2004 at 11:24:01AM +0200:
> [..]
> > > > I just commited the work done at Yahoo! to implement SACK in our tcp
> > > > stack.  Please report any bugs or problems and we'll work on getting
> > > > them addressed.
> [..]
> > What is SACK anyone?
> [..]
> 
> "Selective Acknowledgement", it allows a host/router to explicitly
> acknowledge TCP segments and retransmit them, such that if a segment
> gets lost, it can be retransmitted from the last hop instead of the
> connection endpoint, which would result in a much larger delay.
> Especially if you have wireless links, SACK can be a huge improvement. 
> 
> Please correct/elaborate, I'm not sure if I got that entirely right,
> except for the idea. ;-) 

Mostly right, except that it's only the end-hosts in the TCP connection. 
Technically, one can be a router, but I'm guessing that's not the common
case.  Basically, the original TCP said "retransmit everything" when it
realized a packet was dropped, and TCP SACK allows it to be more
selective, which conserves bandwidth, which has the effective of reducing
load, reducing latency, etc. 

Robert N M Watson             FreeBSD Core Team, TrustedBSD Projects
robert@fledge.watson.org      Principal Research Scientist, McAfee Research