From owner-freebsd-net Wed Aug 8 9:21:33 2001 Delivered-To: freebsd-net@freebsd.org Received: from smtp1.sentex.ca (smtp1.sentex.ca [199.212.134.4]) by hub.freebsd.org (Postfix) with ESMTP id D8D3937B415 for ; Wed, 8 Aug 2001 09:21:25 -0700 (PDT) (envelope-from mike@sentex.net) Received: from simoeon.sentex.net (pyroxene.sentex.ca [199.212.134.18]) by smtp1.sentex.ca (8.11.2/8.11.1) with ESMTP id f78GLO657368; Wed, 8 Aug 2001 12:21:24 -0400 (EDT) (envelope-from mike@sentex.net) Message-Id: <5.1.0.14.0.20010808121005.04473600@marble.sentex.ca> X-Sender: mdtpop@marble.sentex.ca X-Mailer: QUALCOMM Windows Eudora Version 5.1 Date: Wed, 08 Aug 2001 12:15:10 -0400 To: Jeroen Ruigrok/Asmodai From: Mike Tancsa Subject: Re: gif MTU of 1280 ? Cc: freebsd-net@freebsd.org In-Reply-To: <20010808181454.Q2937@daemon.ninth-circle.org> References: <5.1.0.14.0.20010808101139.0277e010@marble.sentex.ca> <5.1.0.14.0.20010808101139.0277e010@marble.sentex.ca> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; format=flowed Sender: owner-freebsd-net@FreeBSD.ORG Precedence: bulk List-ID: List-Archive: (Web Archive) List-Help: (List Instructions) List-Subscribe: List-Unsubscribe: X-Loop: FreeBSD.org Thanks for the clarification. I just had a read of the man pages as well and there is mention of that too. I guess the question I am left with is that can I safely set the MTU to 1500 if I am using it to tunnel IPV4 traffic only, and in another case, IPV4 and IPSEC traffic. When using 1280 in a strict tunnel mode, I have problems with large packets from certain sites. Broken PMTU somewhere ? Not sure, but setting the MTU to 1500 seemed to fix it. ---Mike At 06:14 PM 8/8/01 +0200, Jeroen Ruigrok/Asmodai wrote: >-On [20010808 16:30], Mike Tancsa (mike@sentex.net) wrote: > > > >Just wondering, is there a reason why the MTU of the gif interface defaults > >to 1280 ? Why not 1500 ? > >Per RFC2460: > >"IPv6 requires that every link in the internet have an MTU of 1280 >octets or greater. On any link that cannot convey a 1280-octet packet >in one piece, link-specific fragmentation and reassembly must be >provided at a layer below IPv6. > >Links that have a configurable MTU (for example, PPP links [RFC1661]) >must be configured to have an MTU of at least 1280 octets; it is >recommended that they be configured with an MTU of 1500 octets or >greater, to accommodate possible encapsulations (i.e., tunneling) >without incurring IPv6-layer fragmentation." > >Actually I am wondering about it now myself. X.25 is one of the few >link layer protocols left which has a MTU < 1500 (aside from 802.3's >1492). > >Maybe some IPv6 guru is able to shed some light? > >-- >Jeroen Ruigrok van der Werven/Asmodai asmodai@[wxs.nl|freebsd.org|xmach.org] >Documentation nutter/C-rated Coder, finger asmodai@ninth-circle.dnsalias.net >http://www.freebsd.org/doc/en_US.ISO8859-1/books/developers-handbook/ >Light-in-Darkness, lift me up from here... To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message