From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753556AbcI0HcC (ORCPT ); Tue, 27 Sep 2016 03:32:02 -0400 Received: from shards.monkeyblade.net ([184.105.139.130]:43854 "EHLO shards.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753269AbcI0HcA (ORCPT ); Tue, 27 Sep 2016 03:32:00 -0400 Date: Tue, 27 Sep 2016 03:31:53 -0400 (EDT) Message-Id: <20160927.033153.1066118008027608891.davem@davemloft.net> To: parameswaran.r7@gmail.com Cc: kleptog@svana.org, jchapman@katalix.com, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, nprachan@brocade.com, rshearma@brocade.com, dfawcus@brocade.com, stephen@networkplumber.org, acme@redhat.com, lboccass@brocade.com Subject: Re: [PATCH net v2] L2TP:Adjust intf MTU,factor underlay L3,overlay L2 From: David Miller In-Reply-To: References: X-Mailer: Mew version 6.7 on Emacs 24.5 / Mule 6.0 (HANACHIRUSATO) Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-Greylist: Sender succeeded SMTP AUTH, not delayed by milter-greylist-4.5.12 (shards.monkeyblade.net [149.20.54.216]); Tue, 27 Sep 2016 00:31:59 -0700 (PDT) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: "R. Parameswaran" Date: Thu, 22 Sep 2016 13:52:43 -0700 (PDT) > From ed585bdd6d3d2b3dec58d414f514cd764d89159d Mon Sep 17 00:00:00 2001 > From: "R. Parameswaran" > Date: Thu, 22 Sep 2016 13:19:25 -0700 > Subject: [PATCH] L2TP:Adjust intf MTU,factor underlay L3,overlay L2 > > Take into account all of the tunnel encapsulation headers when setting > up the MTU on the L2TP logical interface device. Otherwise, packets > created by the applications on top of the L2TP layer are larger > than they ought to be, relative to the underlay MTU, leading to > needless fragmentation once the outer IP encap is added. > > Specifically, take into account the (outer, underlay) IP header > imposed on the encapsulated L2TP packet, and the Layer 2 header > imposed on the inner IP packet prior to L2TP encapsulation. > > Do not assume an Ethernet (non-jumbo) underlay. Use the PMTU mechanism > and the dst entry in the L2TP tunnel socket to directly pull up > the underlay MTU (as the baseline number on top of which the > encapsulation headers are factored in). Fall back to Ethernet MTU > if this fails. > > Signed-off-by: R. Parameswaran > > Reviewed-by: "N. Prachanda" , > Reviewed-by: "R. Shearman" , > Reviewed-by: "D. Fawcus" I have to ask, how do other tunnels over UDP such as VXLAN handle this problem?