From owner-freebsd-net@FreeBSD.ORG Sun Jul 22 18:09:46 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 DACD816A417 for ; Sun, 22 Jul 2007 18:09:46 +0000 (UTC) (envelope-from netslists@gmail.com) Received: from ag-out-0708.google.com (ag-out-0708.google.com [72.14.246.241]) by mx1.freebsd.org (Postfix) with ESMTP id 9A4D613C457 for ; Sun, 22 Jul 2007 18:09:46 +0000 (UTC) (envelope-from netslists@gmail.com) Received: by ag-out-0708.google.com with SMTP id 35so1283555aga for ; Sun, 22 Jul 2007 11:09:45 -0700 (PDT) DKIM-Signature: a=rsa-sha1; c=relaxed/relaxed; d=gmail.com; s=beta; h=domainkey-signature:received:received:message-id:date:from:user-agent:mime-version:to:cc:subject:references:in-reply-to:content-type:content-transfer-encoding; b=hLgS1oP6nCvJ200Mo08i9XlylXimHJvnxW/dO3a2yeSm4o6uolZqi0J0gQRypKqETcFBRfWXqpjrYO9qDfMrIllQWEb9LOtssvwZ9CKIwRIWx1BQZKNjtvPboIWLE9uTEpfAqv7fPDM5U2rNTcbWsIQvNEZXN1Bcxx8E2cNWGOg= DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=beta; h=received:message-id:date:from:user-agent:mime-version:to:cc:subject:references:in-reply-to:content-type:content-transfer-encoding; b=QX9EDAHWP75XsYn/6SBGvZdyF+IlOOYIKl1ueKuTBF48v+EVq7jpNgYj6yFzs18F6GejjrgmbKHUu7Z74JOMQktjOQwx1mlvS6Rt/ZfqMR8LegaQanHxg8TGfuDzUzYd1ENydqNpSr1raXdAgVB3w7+xUhnJLSTFJIGAUG2UgG8= Received: by 10.70.87.11 with SMTP id k11mr4282565wxb.1185127785072; Sun, 22 Jul 2007 11:09:45 -0700 (PDT) Received: from ?192.168.12.8? ( [72.189.172.75]) by mx.google.com with ESMTPS id r28sm3887068ele.2007.07.22.11.09.44 (version=SSLv3 cipher=RC4-MD5); Sun, 22 Jul 2007 11:09:44 -0700 (PDT) Message-ID: <46A39D67.5030900@gmail.com> Date: Sun, 22 Jul 2007 20:09:43 +0200 From: Sten Daniel Soersdal User-Agent: Thunderbird 2.0.0.5 (Windows/20070716) MIME-Version: 1.0 To: karels@karels.net References: <200707211702.l6LH2ukt039317@redrock.karels.net> In-Reply-To: <200707211702.l6LH2ukt039317@redrock.karels.net> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: freebsd-net@freebsd.org, Artyom Viklenko , Artem Belevich , Julian Elischer Subject: Re: 6.2 mtu now limits size of incomming packet 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: Sun, 22 Jul 2007 18:09:46 -0000 Mike Karels wrote: >> Any two hosts, connected to single Layer2 network MUST use >> same MTU. Any other cases lead to hard-to-solve problems. > > I'd have to disagree. In fact, I'd say that any two hosts on the > same L2 network must use the same MRU. In particular, if a host > choses to use a lower MTU, if that also lowers the MRU, *that* is > the cause of interoperability problems. > > David DeSimone wrote: > } You are correct about misconfigured networks. In my experience, > } the only reason to ever reduce the MTU is to work around a problem > } discovered in someone else's network (not my local segment). Fixing > } the problem by getting someone else to fix their network is generally > } too hard. If MTU == MRU was forced behavior, the viability of this > } workaround would be removed, one less tool in the toolbag, so to speak. > > Exactly. In our local labs, we also reduce the MTU to test PMTU discovery. > Requiring MRU == MTU makes this more difficult. True, it's a contrived > situation, but as you say, one less tool in the toolbag. > > Mike To leave this tool in the toolbag one could set interface mtu to the upperbounds of what one needs (e.g. 1500) and use pr route --mtu setting to work around those broken networks. That shouldn't break the inbound oversized frames (frames smaller than 1500) but lower the outbound mtu. -- Sten Daniel Soersdal