From owner-freebsd-net@FreeBSD.ORG Fri Sep 7 17:51:43 2007 Return-Path: Delivered-To: freebsd-net@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 8A51A16A417 for ; Fri, 7 Sep 2007 17:51:43 +0000 (UTC) (envelope-from davidch@broadcom.com) Received: from mms2.broadcom.com (mms2.broadcom.com [216.31.210.18]) by mx1.freebsd.org (Postfix) with ESMTP id 67A4313C48D for ; Fri, 7 Sep 2007 17:51:43 +0000 (UTC) (envelope-from davidch@broadcom.com) Received: from [10.10.64.154] by mms2.broadcom.com with ESMTP (Broadcom SMTP Relay (Email Firewall v6.3.1)); Fri, 07 Sep 2007 10:51:33 -0700 X-Server-Uuid: A6C4E0AE-A7F0-449F-BAE7-7FA0D737AC76 Received: by mail-irva-10.broadcom.com (Postfix, from userid 47) id 4BDDE2AF; Fri, 7 Sep 2007 10:51:33 -0700 (PDT) Received: from mail-irva-8.broadcom.com (mail-irva-8 [10.10.64.221]) by mail-irva-10.broadcom.com (Postfix) with ESMTP id 3798C2AE; Fri, 7 Sep 2007 10:51:33 -0700 (PDT) Received: from mail-irva-12.broadcom.com (mail-irva-12.broadcom.com [10.10.64.146]) by mail-irva-8.broadcom.com (MOS 3.7.5a-GA) with ESMTP id FQR79249; Fri, 7 Sep 2007 10:51:28 -0700 (PDT) Received: from NT-IRVA-0750.brcm.ad.broadcom.com ( nt-irva-0750.brcm.ad.broadcom.com [10.8.194.64]) by mail-irva-12.broadcom.com (Postfix) with ESMTP id 06CA569CA3; Fri, 7 Sep 2007 10:51:28 -0700 (PDT) X-MimeOLE: Produced By Microsoft Exchange V6.5 Content-class: urn:content-classes:message MIME-Version: 1.0 Date: Fri, 7 Sep 2007 10:51:27 -0700 Message-ID: <09BFF2FA5EAB4A45B6655E151BBDD903051CBEB6@NT-IRVA-0750.brcm.ad.broadcom.com> In-Reply-To: <46E0A5DB.3080404@elischer.org> Thread-Topic: FreeBSD discarding received packets > MTU Thread-Index: Acfw7GuVhd0rU0HcQxu+nYf4UFN0iwAin/yw References: <46E0632D.8070200@elischer.org> <46E07E74.5020204@elischer.org> <09BFF2FA5EAB4A45B6655E151BBDD9030501D5C0@NT-IRVA-0750.brcm.ad.broadcom.com> <46E0A5DB.3080404@elischer.org> From: "David Christensen" To: "Julian Elischer" X-WSS-ID: 6AFF502F2M81501789-01-01 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable Cc: FreeBSD Net Subject: RE: FreeBSD discarding received packets > MTU X-BeenThere: freebsd-net@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Networking and TCP/IP with FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 07 Sep 2007 17:51:43 -0000 > > It could certainly be argued by some that Cisco is not standards > > compliant in this case for sending an oversized Ethernet frame > > and expecting everyone to accept it. Hardware has limitations > > and assuming that all Ethernet controllers can support frames > > greater than 1522 bytes is not reasonable. Fortunately there is > > a suitable workaround which is setting a larger MTU for the=20 > > interface. What size do you use? How did you arrive at that > > value? >=20 > I use 1550 to make it work in the test harness. >=20 > The trouble is that if I set the mtu to 1550, and the machine=20 > talks to another > such machine with it's mtu also set to 1550 then they=20 > negotiate a maximum sized > packet based on 1550, and the problem hits me again. This is=20 > a web proxy=20 > and that problem occurs when there are two layers of proxy=20 > and one proxy talks to=20 > another. I really just need it to to silently accept a packet some=20 > 32 bytes or so larger than the stated MTU. >=20 > I see no reason for the driver to not do what the em driver=20 > does and allow=20 > itself to receive any packet up to the MCLBYTES size. >=20 > We only hit this problem recently because the data interfaces on our > devices are usually em NICs and we only just recently started=20 > allowing the=20 > users to use the built in (on DELL 2950) bce interfaces for=20 > this purpose. >=20 I'm not completely opposed to making such a change, but I don't want to make a default change in the driver's behavior that other people=20 may be depending upon (whether they are aware of it or not). A tunable driver value could be the answer but I'm not entirely sure how it would fare in the hardware at the high end of MTU values such=20 as 9000. Dave