Date: Fri, 25 Jun 2004 09:59:24 -0400 (EDT) From: Robert Watson <rwatson@freebsd.org> To: Daniel Lang <dl@leo.org> Cc: current@freebsd.org Subject: Re: Re: HEADS UP: SACK committed to HEAD Message-ID: <Pine.NEB.3.96L.1040625095735.27139E-100000@fledge.watson.org> In-Reply-To: <20040625130952.GG47324@atrbg11.informatik.tu-muenchen.de>
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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
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