From owner-freebsd-current@FreeBSD.ORG Sat Nov 17 06:00:04 2007 Return-Path: Delivered-To: freebsd-current@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 8D1B116A41A for ; Sat, 17 Nov 2007 06:00:04 +0000 (UTC) (envelope-from mandrews@bit0.com) Received: from mindcrime.bit0.com (bit0.com [207.246.88.211]) by mx1.freebsd.org (Postfix) with ESMTP id 44F5913C458 for ; Sat, 17 Nov 2007 06:00:03 +0000 (UTC) (envelope-from mandrews@bit0.com) Received: from localhost (localhost.bit0.com [127.0.0.1]) by mindcrime.bit0.com (Postfix) with ESMTP id 5E4361E3378 for ; Sat, 17 Nov 2007 00:42:56 -0500 (EST) X-Virus-Scanned: amavisd-new at bit0.com Received: from mindcrime.bit0.com ([127.0.0.1]) by localhost (mindcrime.int.bit0.com [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id U3FDCmCwdAaW for ; Sat, 17 Nov 2007 00:42:54 -0500 (EST) Received: from localhost (localhost.bit0.com [127.0.0.1]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by mindcrime.bit0.com (Postfix) with ESMTP for ; Sat, 17 Nov 2007 00:42:54 -0500 (EST) Date: Sat, 17 Nov 2007 00:42:54 -0500 (EST) From: Mike Andrews X-X-Sender: mandrews@mindcrime.int.bit0.com To: freebsd-current@freebsd.org Message-ID: <20071117003504.R31357@mindcrime.int.bit0.com> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed Subject: bizarre em + TSO + MSS issue in RELENG_7 X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.5 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: Sat, 17 Nov 2007 06:00:04 -0000 Has anyone run into problems with MSS not being respected when using TSO, specifically on em cards? I've run into a weird 3-way interaction with TSO, users with small MTU's (like 576), and a Cisco firewall that by default drops packets larger than the MSS for that session. Most users can connect to our web servers behind this firewall just fine, but a very few couldn't since we moved from RELENG_6 to RELENG_7 about two weeks ago. One of them could connect again after removing a Linksys box at his end that was clamping the MSS at 536. Some of the others were on dialup which tends to use 576 MTU / 536 MSS. The firewall was dropping a lot of *outbound* traffic to these users because the MSS was exceeded -- ~1000 byte packets for a 536 MSS connection. All of those users' problems disappeared after disabling TSO on our end. Pretty weird. :) I'm quite happy running without TSO and I really don't expect this to get looked at before 7.0-RELEASE -- I just thought the info that MSS appears (to me anyway) to not always be respected might be useful for those looking at TSO-related throughput issues. In our case throughput isn't the issue, it's certain connections stalling out -- it actually behaves sort of like (but isn't) a PMTUD issue. I've got a lot more detail for whoever wants it (Jack Vogel, probably) including tcpdumps and firewall logs but I thought I'd put it out here first in case it wasn't em-specific rather than clutter the list right away, or if it's a known issue that's already fixed in HEAD or something. This is with em driver version 6.5.3, 7.0-BETA2 from Nov 2.