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

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Date:   Fri, 12 May 2017 16:35:55 +0200
From:   Fredrik Markström <fredrik.markstrom@...il.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
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, May 11, 2017 at 9:44 PM, Stephen Hemminger
<stephen@...workplumber.org> wrote:
> On Thu, 11 May 2017 21:10:11 +0200
> Fredrik Markström <fredrik.markstrom@...il.com> wrote:
>
>> On Thu, May 11, 2017 at 6:01 PM, Stephen Hemminger
>> <stephen@...workplumber.org> wrote:
>> > 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.
>>
>> 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?
>

I think that would be ok, but I can't find anything but TCPMSS but
that's only for TCP.

/Fredrik