From mboxrd@z Thu Jan 1 00:00:00 1970 From: Thomas Graf Subject: Re: [ovs-dev] [PATCH net 0/2] vxlan: Set a large MTU on ovs-created vxlan devices Date: Thu, 7 Jan 2016 18:21:37 +0100 Message-ID: <20160107172137.GA24672@pox.localdomain> References: <1452087186-12926-1-git-send-email-david@weave.works> <20160106.155950.1007160228570301281.davem@davemloft.net> <8660z6qohn.fsf@weave.works> <568DADEE.1050206@stressinduktion.org> <20160107114935.GJ32456@pox.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Hannes Frederic Sowa , David Wragg , David Miller , dev@openvswitch.org, Linux Kernel Network Developers To: Jesse Gross Return-path: Received: from mail-wm0-f46.google.com ([74.125.82.46]:36466 "EHLO mail-wm0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751220AbcAGRVk (ORCPT ); Thu, 7 Jan 2016 12:21:40 -0500 Received: by mail-wm0-f46.google.com with SMTP id l65so105767967wmf.1 for ; Thu, 07 Jan 2016 09:21:39 -0800 (PST) Content-Disposition: inline In-Reply-To: Sender: netdev-owner@vger.kernel.org List-ID: On 01/07/16 at 08:35am, Jesse Gross wrote: > On Thu, Jan 7, 2016 at 3:49 AM, Thomas Graf wrote: > > A simple start could be to add a new return code for > MTU drops in > > the dev_queue_xmit() path and check for NET_XMIT_DROP_MTU in > > ovs_vport_send() and emit proper ICMPs. > > That could be interesting. The problem in the past was making sure > that ICMPs that are generated fit in the virtual network appropriately > - right addresses, etc. This requires either spoofing addresses or > some additional knowledge about the topology that we don't currently > have in the kernel. Are you worried about emitting an ICMP with a source which is not a local host address? Can't we just use icmp_send() in the context of the inner header and feed it to the flow table to send it back? It should be the same as for ip_forward(). skb->dev or skb->dst should lead us to the real MTU which can be included in the ICMP frag needed. It's a bit tricky because we would have to know whether it was encapsulated or not and adjust accordingly.