From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1756654AbdEKTpE (ORCPT <rfc822;w@1wt.eu>);
        Thu, 11 May 2017 15:45:04 -0400
Received: from mail-pf0-f179.google.com ([209.85.192.179]:36710 "EHLO
        mail-pf0-f179.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1751853AbdEKTpC (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Thu, 11 May 2017 15:45:02 -0400
Date: Thu, 11 May 2017 12:44:54 -0700
From: Stephen Hemminger <stephen@networkplumber.org>
To: Fredrik =?UTF-8?B?TWFya3N0csO2bQ==?= <fredrik.markstrom@gmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, bridge@lists.linux-foundation.org
Subject: Re: [PATCH v2 1/2] net: Added mtu parameter to dev_forward_skb
 calls
Message-ID: <20170511124454.473dd56e@xeon-e3>
In-Reply-To: <CAKdL+dS_-UiHA5tr7zP-5LtwDd0aXpZq5tpMYf9kr7hu2SwdGQ@mail.gmail.com>
References: <20170511134629.139528-1-fredrik.markstrom@gmail.com>
        <20170511134629.139528-2-fredrik.markstrom@gmail.com>
        <20170511090132.79fdbf12@xeon-e3>
        <CAKdL+dS_-UiHA5tr7zP-5LtwDd0aXpZq5tpMYf9kr7hu2SwdGQ@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Transfer-Encoding: 8bit
X-MIME-Autoconverted: from quoted-printable to 8bit by mail.home.local id v4BJjDdk007993

On Thu, 11 May 2017 21:10:11 +0200
Fredrik Markström <fredrik.markstrom@gmail.com> wrote:

> On Thu, May 11, 2017 at 6:01 PM, Stephen Hemminger
> <stephen@networkplumber.org> wrote:
> > On Thu, 11 May 2017 15:46:27 +0200
> > Fredrik Markstrom <fredrik.markstrom@gmail.com> wrote:
> >  
> >> From: Fredrik Markström <fredrik.markstrom@gmail.com>
> >>
> >> is_skb_forwardable() currently checks if the packet size is <= mtu of
> >> the receiving interface. This is not consistent with most of the hardware
> >> ethernet drivers that happily receives packets larger then MTU.  
> >
> > Wrong.  
> 
> What is "Wrong" ? I was initially skeptical to implement this patch,
> since it feels odd to have different MTU:s set on the two sides of a
> link. After consulting some IP people and the RFC:s I kind of changed
> my mind and thought I'd give it a shot. In the RFCs I couldn't find
> anything that defined when and when not a received packet should be
> dropped.
> 
> >
> > Hardware interfaces are free to drop any packet greater than MTU (actually MTU + VLAN).
> > The actual limit is a function of the hardware. Some hardware can only limit by
> > power of 2; some can only limit frames larger than 1500; some have no limiting at all.  
> 
> Agreed. The purpose of these patches is to be able to configure an
> veth interface to mimic these different behaviors. Non of the Ethernet
> interfaces I have access to drops packets due to them being larger
> then the configured MTU like veth does.
> 
> Being able to mimic real Ethernet hardware is useful when
> consolidating hardware using containers/namespaces.
> 
> In a reply to a comment from David Miller in my previous version of
> the patch I attached the example below to demonstrate the case in
> detail.
> 
> This works with all ethernet hardware setups I have access to:
> 

Why not just use an iptables rule to enforce what ever semantic you
want?