From owner-freebsd-current@FreeBSD.ORG Wed Mar 10 07:41:55 2004 Return-Path: Delivered-To: freebsd-current@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 2B4D716A4CE; Wed, 10 Mar 2004 07:41:55 -0800 (PST) Received: from odin.ac.hmc.edu (Odin.AC.HMC.Edu [134.173.32.75]) by mx1.FreeBSD.org (Postfix) with ESMTP id 0647843D46; Wed, 10 Mar 2004 07:41:55 -0800 (PST) (envelope-from brdavis@odin.ac.hmc.edu) Received: from odin.ac.hmc.edu (IDENT:brdavis@localhost.localdomain [127.0.0.1]) by odin.ac.hmc.edu (8.12.10/8.12.3) with ESMTP id i2AFffRH019119; Wed, 10 Mar 2004 07:41:41 -0800 Received: (from brdavis@localhost) by odin.ac.hmc.edu (8.12.10/8.12.3/Submit) id i2AFffVq019114; Wed, 10 Mar 2004 07:41:41 -0800 Date: Wed, 10 Mar 2004 07:41:41 -0800 From: Brooks Davis To: Harti Brandt Message-ID: <20040310154139.GA14892@Odin.AC.HMC.Edu> References: <20040309214205.3EE2D5D07@ptavv.es.net> <20040309160821.P705@odysseus.silby.com> <20040310123237.V61186@beagle.fokus.fraunhofer.de> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="WIyZ46R2i8wDzkSu" Content-Disposition: inline In-Reply-To: <20040310123237.V61186@beagle.fokus.fraunhofer.de> User-Agent: Mutt/1.5.4i X-Virus-Scanned: by amavisd-milter (http://amavis.org/) on odin.ac.hmc.edu cc: Mike Silbersack cc: freebsd-current@freebsd.org cc: freebsd-net@freebsd.org Subject: Re: Who wants SACK? (Re: was My planned work on networking stack) X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.1 Precedence: list List-Id: Discussions about the use of FreeBSD-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 10 Mar 2004 15:41:55 -0000 --WIyZ46R2i8wDzkSu Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Wed, Mar 10, 2004 at 12:37:11PM +0100, Harti Brandt wrote: > On Tue, 9 Mar 2004, Mike Silbersack wrote: >=20 > MS> > MS>On Tue, 9 Mar 2004, Kevin Oberman wrote: > MS> > MS>> Selective ACKnowledgment (SACK) allows acknowledgment of received > MS>> packets in a TCP window so that only the missing/damaged packet need= s to > MS>> be re-transmitted. This is normally of little value on a LAN where A= CKs > MS>> arrive quickly and windows are smaller and no use on slow circuits. = On > MS>> fat pipes with latency and big windows it is a huge win as it allows= you to > MS>> recover much faster from a packet drop. If you don't have SACK, you = need > MS>> to re-transmit all of the packets in flight within the window while > MS>> with SACK, you need only retransmit the dropped packet(s). If you ha= ve a > MS>> 10 or 20 MB window, this is a big deal. > MS> > MS>That's not correct. Non-SACK TCP doesn't drop any additional packets = vs > MS>SACK. The difference is that SACK allows the transmitter to transmit = the > MS>packet which fills the "hole" and then immediately start transmitting = new > MS>data (or fill other holes.) Non-SACK senders have to wait to receive = an > MS>ACK after retransmitting the hole in order to find out if there are ot= her > MS>holes which must be filled or if new data can be transmitted. > MS> > MS>SACK itself really doesn't do much, it's all the new congestion control > MS>schemes (FACK, Rate Halving, etc) that come shipped with most SACK > MS>implementations that do the work and contain most of the complexity. >=20 > For satellite pipes with drops that are not the result of congestion, but > of transmission errors SACK helps. With congestion control only you get no > throughput no matter what you do (I did some tests with a simulated > 50MBit/sec GEO link with errors). But this is a rather limited > application. For that matter, there are sufficent drops on 10GbE from data errors to insure that two boxes connected back to back won't achieve line speed on a single TCP session. -- Brooks --=20 Any statement of the form "X is the one, true Y" is FALSE. PGP fingerprint 655D 519C 26A7 82E7 2529 9BF0 5D8E 8BE9 F238 1AD4 --WIyZ46R2i8wDzkSu Content-Type: application/pgp-signature Content-Disposition: inline -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.2.1 (GNU/Linux) iD8DBQFATzcxXY6L6fI4GtQRAqTDAKDlCGkr03SBdrHmE6clLj4KjiLuIQCdFTg7 hsqoox+1LkIhef/yYr4dFvc= =t/mZ -----END PGP SIGNATURE----- --WIyZ46R2i8wDzkSu--