From owner-freebsd-net@FreeBSD.ORG Sat Jul 21 17:06:13 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 5695516A418 for ; Sat, 21 Jul 2007 17:06:13 +0000 (UTC) (envelope-from karels@redrock.karels.net) Received: from redrock.karels.net (redrock.karels.net [206.196.45.2]) by mx1.freebsd.org (Postfix) with ESMTP id 12A2013C481 for ; Sat, 21 Jul 2007 17:06:12 +0000 (UTC) (envelope-from karels@redrock.karels.net) Received: from redrock.karels.net (localhost.karels.net [127.0.0.1]) by redrock.karels.net (8.13.8/8.13.6) with ESMTP id l6LH2ukt039317; Sat, 21 Jul 2007 12:02:56 -0500 (CDT) (envelope-from karels@redrock.karels.net) Message-Id: <200707211702.l6LH2ukt039317@redrock.karels.net> To: Artyom Viklenko From: Mike Karels In-reply-to: Your message of Sat, 21 Jul 2007 11:03:42 +0300. <46A1BDDE.5080403@aws-net.org.ua> Date: Sat, 21 Jul 2007 12:02:56 -0500 Sender: karels@karels.net Cc: freebsd-net@freebsd.org, 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 Reply-To: karels@karels.net List-Id: Networking and TCP/IP with FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Sat, 21 Jul 2007 17:06:13 -0000 > 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