linux-kernel - Re: [PATCH v2 1/2] net: Added mtu parameter to dev_forward

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Date:   Thu, 11 May 2017 09:01:32 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Fredrik Markstrom <fredrik.markstrom@...il.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, bridge@...ts.linux-foundation.org
Subject: Re: [PATCH v2 1/2] net: Added mtu parameter to dev_forward_skb
 calls

On Thu, 11 May 2017 15:46:27 +0200
Fredrik Markstrom <fredrik.markstrom@...il.com> wrote:

> From: Fredrik Markström <fredrik.markstrom@...il.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.

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.
Any application that should:
  * not expect packets larger than MTU to be received
  * not send packets larger than MTU
  * check actual receive size. IP protocols will do truncation of padded packets