linux-kernel - Re: [PATCH 0/2] net: Set maximum receive packet size on veth interfaces

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Message-ID: <CAM_iQpVKJp5Dg_x-ymJa-wpLbbgUpn2cQv+--eXRwr2Eh23Vgw@mail.gmail.com>
Date:   Thu, 11 May 2017 20:27:10 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Fredrik Markström <fredrik.markstrom@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        bridge@...ts.linux-foundation.org
Subject: Re: [PATCH 0/2] net: Set maximum receive packet size on veth interfaces

On Tue, May 9, 2017 at 5:42 PM, Fredrik Markström
<fredrik.markstrom@...il.com> wrote:
>
> Maybe I was unclear, the veth implementation drops all packers larger then the
> configured MTU (on the receiving interface).
> Most ethernet drivers accepts packets up to the ethernet MTU no matter the
> configured MTU. As far as I can tell from the RFC:s that is ok.

This is because IP layer does the fragmentation for you. But some drivers,
for example tg3, drop packet larger than its dev->mtu very early too.

>
> A simpler solution would be to only drop packets larger then ethernet MTU (like
> most network drivers), but I guess that will break existing stuff
> already out there.

I wonder why did we introduce that mtu check for veth when IP layer
could either fragment or reject with ICMP?