netdev - Re: [PATCH v1 1/1] veth: Do not drop packets larger then the mtu set on the receiving side

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Message-ID: <CAMJ5cBG8wjesvW=1_o1g6BYa=pfoR+y7pueYSdwNh+ThFSpxRQ@mail.gmail.com>
Date:   Fri, 11 Jan 2019 14:13:01 +0100
From:   Fredrik Gustavsson <gustfred@...il.com>
To:     Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
Cc:     netdev <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [PATCH v1 1/1] veth: Do not drop packets larger then the mtu set
 on the receiving side

Den fre 11 jan. 2019 kl 05:23 skrev Toshiaki Makita
<makita.toshiaki@....ntt.co.jp>:
>
> On 2019/01/10 22:26, Fredrik Gustavsson wrote:
> > commit affede4a779420bd8510ab937251a3796d3228df
> > Author: Fredrik Gustavsson <gustfred@...il.com>
> > Date:   Tue Jan 8 11:21:39 2019 +0100
> >
> > veth: Do not drop packets larger then the mtu set on the receiving side
> >
> > Currently veth drops all packets larger then the mtu set on the receiving
> > end of the pair. This is inconsistent with most hardware ethernet drivers
> > that happily receives packets up the the ethernet MTU independent of the
> > configured MTU.
> >
> > There should not be a need for dropping IP packets at receiver with
> > size > configured IP MTU, IP MTU is for fragmentation at sender side.
> > And IP packets with size > receiver L2 MTU will be dropped at sub-IP layer.
>
> But with current veth behavior setting MTU effectively sets MRU as well.
> This may be being used to drop unexpectedly long packets e.g. from
> containers on container host. If we unconditionally change this behavior
> it can cause regression on some environment. This should be an option at
> least.
>
> BTW there was a similar precedent attempt. You might want to take a look.
>
> https://www.mail-archive.com/netdev@vger.kernel.org/msg167636.html
> https://www.mail-archive.com/netdev@vger.kernel.org/msg167899.html
>
> --
> Toshiaki Makita
>
Good argument, but do you agree that it shouldn't be working like it
does? But not sure if such a case would exist but breaking
compatitbility have been discussed in other threads. So your saying
that people have actually used it as a receiving limit for IP packets?

Nice that you found that older thread it is also availble here:
https://lkml.org/lkml/2017/5/12/254
Were I thought the ending argument was that if it shouldn't be dropped
don't drop it. And that introducing a MRU parameter was just making
things more complicated. But for my use case either way would work.