From owner-freebsd-hackers Fri Jun 19 22:51:43 1998 Return-Path: Received: (from majordom@localhost) by hub.freebsd.org (8.8.8/8.8.8) id WAA14803 for freebsd-hackers-outgoing; Fri, 19 Jun 1998 22:51:43 -0700 (PDT) (envelope-from owner-freebsd-hackers@FreeBSD.ORG) Received: from mail.webspan.net (root@mail.webspan.net [206.154.70.7]) by hub.freebsd.org (8.8.8/8.8.8) with ESMTP id WAA14788 for ; Fri, 19 Jun 1998 22:51:38 -0700 (PDT) (envelope-from opsys@mail.webspan.net) Received: from orion.webspan.net (orion.webspan.net [206.154.70.5]) by mail.webspan.net (WEBSPAN/970608) with SMTP id BAA02167; Sat, 20 Jun 1998 01:45:14 -0400 (EDT) Date: Sat, 20 Jun 1998 01:51:24 -0400 (EDT) From: Open Systems Networking X-Sender: opsys@orion.webspan.net To: spork cc: Joel Ray Holveck , root@bmccane.maxbaud.net, hackers@FreeBSD.ORG Subject: Re: TweakDUN In-Reply-To: Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-freebsd-hackers@FreeBSD.ORG Precedence: bulk X-Loop: FreeBSD.ORG On Fri, 19 Jun 1998, spork wrote: > I can't think of anywhere this is true. I'll use our dialup pools as an > example: > > modem-> dialup PPP 1500 -> term server -> ethernet 1500 -> router -> T1(s) > HDLC 1500 -> core router -> fast ethernet 1500 -> upstream's border > router -> FDDI 40?? -> upstream core router -> ATM/SONET/whatever ? > > Generally, one avoids small MTUs on big links, I beleive. ATM's small > cell size makes *every* packet get fragmented at layer 2, but I'm not sure > that's even relevant. > > Anyone else? I've never heard of the oft quoted "Internet standard MTU of > 576"... Me either, and theres a reason. Its braind dead to do so. Im REALLY tired right now so im not gonna crack open stevens and quote it. Anyone using an MTU of 576 destroys performance on bulk transfers such as FTP and and the like. The ONLY good an MTU of 576 is good for is interactive traffic, like telnet. At least thats how I have always understood it. For an excercise the reader can change his MTU from 1500 to 576 and test FTP, and telnet over it, then switch back to 1500 and do the same. If you do the calculations of an MTU of 576 and figure in latency. On bulk transfers of an MTU of 1500 winds over 576. Because for each 1500 byte packet sent over the MTU link with say a latency of 533ms for dialup, it would take 533ms for a 1500 byte packet. Now if your using an MTU of 576 and transfer the same 1500 byte packet with latency of 533ms, you have to send 3 packets of 536 bytes @ 533ms latency each. It isnt rocket science to see why for anything BUT interactive traffic where all your traffic will fit inside your MTU your gonna loose and loose big. So why in gods name people think an MTU of 576 is the "internet standard" AND their actually believing it astonishes me. My math above maybe be incorrect but you get the idea. Basically I chalk it up to uninformed admins on the loose, who need to be put back in their cages. :) Not to mention by fragmenting all your non-interactive traffic your creating insane ammounts of traffic. 3 packets at an MTU of 576 or 1 packet at 1500. tripple the traffic. Well you can tell im tired cause im ranting and probably making no sense. So im going to bed. Ill probably kick myself for what I just wrote when I read it in the morning but I am sure some of its right. Chris -- "Linux... The choice of a GNUtered generation." ===================================| Open Systems Networking And Consulting. FreeBSD 2.2.6 is available now! | Phone: 316-326-6800 -----------------------------------| 1402 N. Washington, Wellington, KS-67152 FreeBSD: The power to serve! | E-Mail: opsys@open-systems.net http://www.freebsd.org | Consulting-Network Engineering-Security ===================================| http://open-systems.net -----BEGIN PGP PUBLIC KEY BLOCK----- Version: 2.6.2 mQENAzPemUsAAAEH/06iF0BU8pMtdLJrxp/lLk3vg9QJCHajsd25gYtR8X1Px1Te gWU0C4EwMh4seDIgK9bzFmjjlZOEgS9zEgia28xDgeluQjuuMyUFJ58MzRlC2ONC foYIZsFyIqdjEOCBdfhH5bmgB5/+L5bjDK6lNdqD8OAhtC4Xnc1UxAKq3oUgVD/Z d5UJXU2xm+f08WwGZIUcbGcaonRC/6Z/5o8YpLVBpcFeLtKW5WwGhEMxl9WDZ3Kb NZH6bx15WiB2Q/gZQib3ZXhe1xEgRP+p6BnvF364I/To9kMduHpJKU97PH3dU7Mv CXk2NG3rtOgLTEwLyvtBPqLnbx35E0JnZc0k5YkABRO0JU9wZW4gU3lzdGVtcyA8 b3BzeXNAb3Blbi1zeXN0ZW1zLm5ldD4= =BBjp -----END PGP PUBLIC KEY BLOCK----- To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message