From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Templin, Fred L" Subject: RE: UDP path MTU discovery Date: Tue, 30 Mar 2010 08:58:32 -0700 Message-ID: References: <1269561751.2891.8.camel@ilion> <877how25kx.fsf@basil.nowhere.org> <4BB0DCF6.9020401@hp.com> <20100329201431.GH20695@one.firstfloor.org> <20100329205035.GA32656@laped.iglesias.mooo.com> <4BB11510.9000302@hp.com> <1269898152.1958.86.camel@edumazet-laptop> <20100330052044.GJ20695@one.firstfloor.org> <20100330061627.GA22436@laped.iglesias.mooo.com> <20100330061952.GO20695@one.firstfloor.org> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 8BIT Cc: Eric Dumazet , Rick Jones , Glen Turner , "netdev@vger.kernel.org" To: Andi Kleen , "Edgar E. Iglesias" Return-path: Received: from stl-smtpout-01.boeing.com ([130.76.96.56]:44526 "EHLO stl-smtpout-01.boeing.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752790Ab0C3Qcc convert rfc822-to-8bit (ORCPT ); Tue, 30 Mar 2010 12:32:32 -0400 In-Reply-To: <20100330061952.GO20695@one.firstfloor.org> Content-Language: en-US Sender: netdev-owner@vger.kernel.org List-ID: > -----Original Message----- > From: Andi Kleen [mailto:andi@firstfloor.org] > Sent: Monday, March 29, 2010 11:20 PM > To: Edgar E. Iglesias > Cc: Andi Kleen; Templin, Fred L; Eric Dumazet; Rick Jones; Glen Turner; netdev@vger.kernel.org > Subject: Re: UDP path MTU discovery > > > If you don't want to hassle with all of that, the app can stick to > > 1280 (or I guess for the extreme/lazy cases turn on fragmentation).. > > See the early mails in this thread. This is about apps who can't > limit themselves to 1280, but still don't want full blown PMTU. > [They probably should, but it can be a lot of work] Right. Some apps may need to send isolated packets that are larger than the path MTU without invoking path MTU discovery. > The MTU would allow to force fragmentation on the sending host > as a workaround similar to IPv4. Right again. Unlike IPv4, however, IPv6 does not allow in-the-network fragmentation. So when in doubt, apps that need to send isolated packets that may violate the path MTU should really perform host-based fragmentation with a maximum fragment size of 1280. Isn't there a socket option "IPV6_USE_MIN_MTU" that apps can use to force fragmentation on large packets (RFC3542)? Caveat - the app may have no way of knowing whether the destination is capable of reassembling fragmented packets larger than 1500... Fred fred.l.templin@boeing.com > -Andi > -- > ak@linux.intel.com -- Speaking for myself only.