From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andi Kleen Subject: Re: UDP Fragmentation and DF bit.. Date: Thu, 27 May 2010 11:43:35 +0200 Message-ID: <87bpc11xyw.fsf@basil.nowhere.org> References: Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: "netdev\@vger.kernel.org" To: "Vincent\, Pradeep" Return-path: Received: from one.firstfloor.org ([213.235.205.2]:60900 "EHLO one.firstfloor.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755408Ab0E0Jnl (ORCPT ); Thu, 27 May 2010 05:43:41 -0400 In-Reply-To: (Pradeep Vincent's message of "Wed\, 26 May 2010 18\:45\:47 -0700") Sender: netdev-owner@vger.kernel.org List-ID: "Vincent, Pradeep" writes: > OMan 7 ip=B9 declares that =B3The system-wide default is controll= ed by the > ip_no_pmtu_disc sysctl for SOCK_STREAM sockets, and disabled on= all > others.=B2 which led me to think ODF=B9 bit will not be set for UDP p= ackets. > But.. One should add ip.7 is not really a spec, just documentation how things were quite a few years ago. It unfortunately does often not get updated when things change. > > In a network environment where MTU-big and MTU-small co-exist (and ha= ve > router=B9s fragmentation turned off in favor of PMTU discovery), UDP = packets > that are > MTU-small and < MTU-big find the PMTU effectively but UDP > packets I don't understand your mtu-small/big concept. PMTU is per IP and per flow. So there's only always a single PMTU, not small and big. Or do you refer to a single IP NAT situation where a single IP shares different MTUs? > Is there a reason ODF=B9 bit cannot be set on fragmented packets on U= DP > transmission ? I couldn=B9t find anything in RFC for IP protocol that > prohibited DF bit on fragmented packets. Did I miss > something here ? > Would it be reasonable if PMTU discovery is performed (DF bit set + > appropriate icmp logic) even for locally fragmented packets ? I think > this DF=3D1 on fragments would mean the application has to do pmtu discovery even with fragments for the case when the kernel does not know=20 the path mtu yet. But if the app supports pmtu discovery it's better to not use fragments in the first place. -Andi --=20 ak@linux.intel.com -- Speaking for myself only.