From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752308AbdAaNcs (ORCPT ); Tue, 31 Jan 2017 08:32:48 -0500 Received: from sender153-mail.zoho.com ([74.201.84.153]:25053 "EHLO sender153-mail.zoho.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752120AbdAaNck (ORCPT ); Tue, 31 Jan 2017 08:32:40 -0500 DomainKey-Signature: a=rsa-sha1; q=dns; c=nofws; s=zapps768; d=zoho.com; h=date:from:to:cc:message-id:in-reply-to:references:subject:mime-version:content-type:user-agent; b=BAfAsFvnLyPSBpg4DiPTo3JlfR7TKQpg1mNZ2Sy/DeCy6QcEonTWU8xIAcNS2Q1g4UqZAA87klvG +sSxxsHZaIwZlFtL+Im+Wcta8nCh6myBfa+Y6QI0tr80f3J23PTH Date: Tue, 31 Jan 2017 14:32:20 +0100 From: Fredrik Markstrom To: "Eric Dumazet" Cc: "netdev" , "Evan Jones" , "David S. Miller" , "Cong Wang" , "Greg Kroah-Hartman" , "Vijay Pandurangan" , "linux-kernel" , "ebiederm" Message-ID: <159f4b9e4ab.12b79bda8122843.8859202944513905410@gmail.com> In-Reply-To: <1484852027.16328.33.camel@edumazet-glaptop3.roam.corp.google.com> References: <159b79aa979.e23be6a5186096.2767722456700391441@gmail.com> <1484852027.16328.33.camel@edumazet-glaptop3.roam.corp.google.com> Subject: Re: Inconsistency in packet drop due to MTU (eth vs veth) MIME-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-Priority: Medium User-Agent: Zoho Mail X-Mailer: Zoho Mail Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org ---- On Thu, 19 Jan 2017 19:53:47 +0100 Eric Dumazet wrote ---- > On Thu, 2017-01-19 at 17:41 +0100, Fredrik Markstrom wrote: > > Hello, > > > > I've noticed an inconsistency between how physical ethernet and veth handles mtu. > > > > If I setup two physical interfaces (directly connected) with different mtu:s, only the size of the outgoing packets are limited by the mtu. But with veth a packet is dropped if the mtu of the receiving interface is smaller then the packet size. > > > > This seems inconsistent to me, but maybe there is a reason for it ? > > > > Can someone confirm if it's a deliberate inconsistency or just a side effect of using dev_forward_skb() ? > > It looks this was added in commit > 38d408152a86598a50680a82fe3353b506630409 > ("veth: Allow setting the L3 MTU") > > But what was really needed here was a way to change MRU :( Ok, do we consider this correct and/or something we need to be backwards compatible with ? Is it insane to believe that we can fix this "inconsistency" by removing the check ? The commit message reads "For consistency I drop packets on the receive side when they are larger than the MTU", do we know what it's supposed to be consistent with or is that lost in history ? /Fredrik > > > >