From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andi Kleen Subject: Re: UDP path MTU discovery Date: Thu, 1 Apr 2010 02:55:39 +0200 Message-ID: <20100401005539.GZ20695@one.firstfloor.org> References: <1269561751.2891.8.camel@ilion> <877how25kx.fsf@basil.nowhere.org> <4BB0DCF6.9020401@hp.com> <1270078984.2389.33.camel@ilion> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Rick Jones , Andi Kleen , netdev@vger.kernel.org To: Glen Turner Return-path: Received: from one.firstfloor.org ([213.235.205.2]:59759 "EHLO one.firstfloor.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757179Ab0DAAzo (ORCPT ); Wed, 31 Mar 2010 20:55:44 -0400 Content-Disposition: inline In-Reply-To: <1270078984.2389.33.camel@ilion> Sender: netdev-owner@vger.kernel.org List-ID: On Thu, Apr 01, 2010 at 10:13:04AM +1030, Glen Turner wrote: > On Mon, 2010-03-29 at 10:01 -0700, Rick Jones wrote: > > > But which of the last N datagrams sent by the application should be retained for > > retransmission? It could be scores if not hundreds of datagrams depending on > > the behaviour of the application and the latency to the narrow part of the network. > > We don't need that sort of exotica from the kernel. The applications > have to be prepared to retransmit lost packets in any case. > > What we need is an API for an instant notification that a ICMP Packet > Too Big message has arrived concerning the socket. That already exists of course: IP_RECVERR > As for David Miller's rant, the applications currently have no choice > but to "do it stupidly" as the kernel doesn't pass enough information > for user space to do it intelligently. If the kernel passed user space > the same indication as TCP gets, then we could -- and would -- do it > right. That's wrong. Linux has supported UDP/RAW pmtu discovery since many many years. I have a really old presentation on it (from 2000 or so): http://halobates.de/net-topics/text33.htm http://halobates.de/net-topics/text34.htm http://halobates.de/net-topics/text35.htm http://halobates.de/net-topics/text36.htm It's also in the manpages. However I suspect it's too much work to change a lot of applications to that, so I suspect the IPV6_MIN_MTU workaround is still needed. -Andi -- ak@linux.intel.com -- Speaking for myself only.