From mboxrd@z Thu Jan 1 00:00:00 1970 From: =?ISO-8859-1?Q?Ra=FAl_Hern=E1ndez?= Subject: high rate injection and fragmentation Date: Mon, 9 Feb 2009 16:20:34 +0100 Message-ID: <3f847c820902090720w98e2ca8pc66f55716c030194@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE To: netdev@vger.kernel.org Return-path: Received: from mail-bw0-f161.google.com ([209.85.218.161]:65217 "EHLO mail-bw0-f161.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753938AbZBIPUi convert rfc822-to-8bit (ORCPT ); Mon, 9 Feb 2009 10:20:38 -0500 Received: by bwz5 with SMTP id 5so1424740bwz.13 for ; Mon, 09 Feb 2009 07:20:35 -0800 (PST) Sender: netdev-owner@vger.kernel.org List-ID: Hi there, I am trying to understand the behavior of the IP stack regarding fragmentation when injecting at high speed ratio : so far I am seeing that TCP starts multiplexing the application level protocol (in my case, diameter: previously 1 diameter message =3D 1 TCP message, when the rate increases, multiple diameter messages are coalesced in one TCP messages - similar to a Nagle control -). At that very moment is when we start finding issues regarding packet length checking with ethereal: frames start growing due the multiplex above and ethereal shows captured packets >3000 bytes ! (IP total lenght makes sense as well according to that figure). The thing is that I am working under ethernet law (1500 bytes) and I have checked through iproute this is the MTU set for the link (neither DONT_FRAGMENT ip options for the socket regarding nor MTU modifications have been introduced) so my understanding is that no packet > 1500 bytes should be allowed with my configuration (would need fragmentation). Checking the IP header with ethereal, "dont fragment" is set and "fragment offset"=3D0. IOW, It seems that IP is not fragmenting the packet although > MTU which I am not able to understand. Any idea why Linux tries to work with MTU > 1500 bytes when not configured explicitly to do so (may I have missed more options/checkings) ? Hope not to be cheated by Ethereal ... Thx ! Best regards, Raul --=20 Los viajes son en la juventud una parte de educaci=F3n y, en la vejez, una parte de experiencia -- Francis Bacon