From owner-freebsd-current@FreeBSD.ORG Sat Aug 2 17:29:10 2003 Return-Path: Delivered-To: freebsd-current@freebsd.org Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125]) by hub.freebsd.org (Postfix) with ESMTP id A03E737B401 for ; Sat, 2 Aug 2003 17:29:10 -0700 (PDT) Received: from stork.mail.pas.earthlink.net (stork.mail.pas.earthlink.net [207.217.120.188]) by mx1.FreeBSD.org (Postfix) with ESMTP id 1299D43FB1 for ; Sat, 2 Aug 2003 17:29:10 -0700 (PDT) (envelope-from tlambert2@mindspring.com) Received: from user-38lc076.dialup.mindspring.com ([209.86.0.230] helo=mindspring.com) by stork.mail.pas.earthlink.net with asmtp (SSLv3:RC4-MD5:128) (Exim 3.33 #1) id 19j6k8-0006LA-00; Sat, 02 Aug 2003 17:29:00 -0700 Message-ID: <3F2C5715.D549B6CB@mindspring.com> Date: Sat, 02 Aug 2003 17:28:05 -0700 From: Terry Lambert X-Mailer: Mozilla 4.79 [en] (Win98; U) X-Accept-Language: en MIME-Version: 1.0 To: Tom Samplonius References: Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-ELNK-Trace: b1a02af9316fbb217a47c185c03b154d40683398e744b8a4f6a7d1375aca4d71bd005e149cef8ad22601a10902912494350badd9bab72f9c350badd9bab72f9c cc: freebsd-current@freebsd.org cc: Boris Kovalenko Subject: Re: bge & vlan stranges X-BeenThere: freebsd-current@freebsd.org X-Mailman-Version: 2.1.1 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: Sun, 03 Aug 2003 00:29:10 -0000 Tom Samplonius wrote: > Probably wouldn't be affective anyhow. L2 switches assume that they can > encapsulate 1500 byte ethernet frames into 802.1q properly. It is part of > the 802.1q standard. If the NIC can't understand the frame because it is > now 1504 bytes, it will be a layer 2 discard. There will be no ICMP > message sent in this case. You could argue that the switch should also be > configured with a 1456 byte MTU to allow for the addition of the 802.1q > encapsulation. But a L2 switch is not going to send a L3 message like a > ICMP "unable to fragment" fragment. So MTU detection buys you nothing. > > The fact of the matter is, if you use 802.1q encapsulation, the total > frame size can be 1504. That is the standard. This is truly evil. I would suggest, though, that L2 switches shouldn't be boundaries for this type of encapsulation, if this is the case. Worst case, though, an attempt at PMTU through a switch that did this anyway should end up with whatever MTU the host has setup. I expect that the 802.1q encapsulation of 1500 MTU packets (yielding 1504 MTU packets on the wire) was actually intended to operate in switch-to-switch trunking, not sitch-to-host trunking. Effectively, this means the machine that's doing it's own VLAN stuff is really a trunk endpoint, not a traditional ethernet endpoint. I have no problem with this. I'm just pointing out that not all cards will be able to support the idea, and that if you depend on arbitrary cards supporting it, you are going to get shot in the foot. You'll agree that a PMTU discovery through an L2 switch that was doing 802.1q to a machine with a card that couldn't handle more than 1500 MTU total, should result in a discovery of "1496" and not "1500", right? -- Terry