From owner-freebsd-net@FreeBSD.ORG Tue Mar 9 15:17:34 2004 Return-Path: Delivered-To: freebsd-net@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id 5DB6D16A4CE; Tue, 9 Mar 2004 15:17:34 -0800 (PST) Received: from pd4mo3so.prod.shaw.ca (shawidc-mo1.cg.shawcable.net [24.71.223.10]) by mx1.FreeBSD.org (Postfix) with ESMTP id 4A14B43D1F; Tue, 9 Mar 2004 15:17:34 -0800 (PST) (envelope-from jhanna@shaw.ca) Received: from pd2mr1so.prod.shaw.ca (pd2mr1so-ser.prod.shaw.ca [10.0.141.110])2003)) with ESMTP id <0HUC006AF0CI26@l-daemon>; Tue, 09 Mar 2004 16:09:54 -0700 (MST) Received: from pn2ml8so.prod.shaw.ca (pn2ml8so-qfe0.prod.shaw.ca [10.0.121.152]) by l-daemon (iPlanet Messaging Server 5.2 HotFix 1.18 (built Jul 28 2003)) with ESMTP id <0HUC0006T0CI81@l-daemon>; Tue, 09 Mar 2004 16:09:54 -0700 (MST) Received: from cub.pangolin-systems.com (h24-81-10-24.vc.shawcable.net [24.81.10.24])2003)) with ESMTP id <0HUC00E020CHOI@l-daemon>; Tue, 09 Mar 2004 16:09:54 -0700 (MST) Date: Tue, 09 Mar 2004 15:09:53 -0800 (PST) From: jhanna@shaw.ca In-reply-to: <20040309214205.3EE2D5D07@ptavv.es.net> Sender: jhanna@cub.pangolin-systems.com To: Brad Knowles Message-id: MIME-version: 1.0 X-Mailer: XFMail 1.5.4 on FreeBSD Content-type: text/plain; charset=us-ascii Content-transfer-encoding: 7BIT X-Priority: 3 (Normal) cc: freebsd-net@freebsd.org cc: freebsd-current@freebsd.org cc: Kevin Oberman Subject: Re: Who wants SACK? (Re: was My planned work on networking stack) X-BeenThere: freebsd-net@freebsd.org X-Mailman-Version: 2.1.1 Precedence: list Reply-To: jhanna@shaw.ca List-Id: Networking and TCP/IP with FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 09 Mar 2004 23:17:34 -0000 On 09-Mar-2004 Kevin Oberman wrote: >> Date: Tue, 9 Mar 2004 20:13:11 +0100 >> From: Brad Knowles >> Sender: owner-freebsd-current@freebsd.org >> >> At 3:32 PM -0800 2004/03/08, Jeffrey Hsu wrote: >> >> > What Luigi says is absolutely correct. It doesn't take a lot to >> > get this done. I've talked to a number of companies about implementing >> > SACK for them and while there was interest, no one wanted to fund >> > it all themselves, potentially for the benefit of their competitors. >> >> Out of curiosity, can someone provide some pointers as to where >> SACK really helps? Is this just for high-speed WANs and doesn't help >> on LANs, or is it useful in both contexts? Also, at what >> speeds/packet sizes does SACK start to become really useful? >> >> I'm just wondering if there aren't a lot of people who could >> benefit from something like this, only they don't know it. If they >> were to find out, it might help provide funding and other resources >> to spur development. > > Selective ACKnowledgment (SACK) allows acknowledgment of received > packets in a TCP window so that only the missing/damaged packet needs to > be re-transmitted. This is normally of little value on a LAN where ACKs > arrive quickly and windows are smaller and no use on slow circuits. On > fat pipes with latency and big windows it is a huge win as it allows you to > recover much faster from a packet drop. If you don't have SACK, you need > to re-transmit all of the packets in flight within the window while > with SACK, you need only retransmit the dropped packet(s). If you have a > 10 or 20 MB window, this is a big deal. > > Dynamic window sizing will make it of less significance in LANs as the > windows will not be very large. Radio links as well, with their latency and higher frame drop rates, can benefit considerably. Cell phones and such may account for a large amount of garden variety traffic as time goes on. jhanna@shaw.ca