From owner-freebsd-net Wed Aug 14 22: 2:18 2002 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 9028C37B400 for ; Wed, 14 Aug 2002 22:02:16 -0700 (PDT) Received: from patrocles.silby.com (d25.as3.nwbl0.wi.voyager.net [169.207.92.153]) by mx1.FreeBSD.org (Postfix) with ESMTP id 80AB243E6E for ; Wed, 14 Aug 2002 22:02:11 -0700 (PDT) (envelope-from silby@silby.com) Received: from patrocles.silby.com (localhost [127.0.0.1]) by patrocles.silby.com (8.12.5/8.12.5) with ESMTP id g7F55wB1097923; Thu, 15 Aug 2002 00:05:58 -0500 (CDT) (envelope-from silby@silby.com) Received: from localhost (silby@localhost) by patrocles.silby.com (8.12.5/8.12.5/Submit) with ESMTP id g7F55Zer097920; Thu, 15 Aug 2002 00:05:49 -0500 (CDT) X-Authentication-Warning: patrocles.silby.com: silby owned process doing -bs Date: Thu, 15 Aug 2002 00:05:35 -0500 (CDT) From: Mike Silbersack To: Barney Wolff Cc: Oleg Polyakov , Subject: Re: Initial congestion window increase In-Reply-To: <20020814121701.GA27934@tp.databus.com> Message-ID: <20020814233935.F97690-100000@patrocles.silby.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-freebsd-net@FreeBSD.ORG Precedence: bulk List-ID: List-Archive: (Web Archive) List-Help: (List Instructions) List-Subscribe: List-Unsubscribe: X-Loop: FreeBSD.org On Wed, 14 Aug 2002, Barney Wolff wrote: > You're assuming that the jumbo will be the successful MTU. But at > the start of a connection PMTUD has yet to run, and you could be > sending jumbos into a choppy link somewhere on the path. > > The tcp-impl IETF WG had (and the email list still has) a very smart > bunch of people with decades of experience with TCP. Those RFCs > didn't just come out of somebody's idle thought. > > Slowstart flightsize doesn't matter a whole lot on a lan (as long > as it's at least 2 to compensate for delayed ack) other than for > locker-room comparisons with Linux. But it does matter a lot on > long pipes, whether fat or thin, and that's where the risk of > an overaggressive strategy is that you can congest the Internet. Hrm, I'm not sure that PMTUD is a strong enough argument against X*MSS slowstart for gigabit networks. Think about the following cases: 1. Server with MTU 1500, client with MTU 1480 (they're going over PPPoE or something similar.) - All four 1500 byte packets sent back to back, all 4 bounced with ICMP too big messages. Bandwidth wasted: All 4 packets traversing the net, all 4 icmps coming back across the net. 2. Server with MTU 9000, client with MTU 1500. - All four 9000 byte packets sent back to back, bounced back at local border router with MTU of 1500. Bandwidth wasted: Internal network bandwidth only. Perhaps less than 4 packets, if all data fit into a single 9000 byte packet. Considering this, I don't believe that the gigabit host using jumbo frames would be any more harmful than a 100mbps host using normal ethernet frames. Mike "Silby" Silbersack To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message