netdev - Re: [PATCH v3] veth: Drop MTU check when forwarding packets

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Message-ID: <87h6bkb2fn.fsf@toke.dk>
Date: Fri, 16 Aug 2024 23:09:00 +0200
From: Toke Høiland-Jørgensen <toke@...nel.org>
To: Duan Jiong <djduanjiong@...il.com>
Cc: Willem de Bruijn <willemdebruijn.kernel@...il.com>, netdev@...r.kernel.org
Subject: Re: [PATCH v3] veth: Drop MTU check when forwarding packets

Duan Jiong <djduanjiong@...il.com> writes:

> On Tue, Aug 13, 2024 at 7:40 PM Toke Høiland-Jørgensen <toke@...nel.org> wrote:
>>
>> Duan Jiong <djduanjiong@...il.com> writes:
>>
>> >> >
>> >
>> > vm1(mtu 1600)---ovs---ipsec vpn1(mtu 1500)---ipsec vpn2(mtu
>> > 1500)---ovs---vm2(mtu 1600)
>>
>> Where's the veth device in this setup?
>>
>
> The veth device is used for ipsec vpn containers to connect to ovs, and
> traffic before and after esp encapsulation goes to this NIC.
>
>
>> > My scenario is that two vms are communicating via ipsec vpn gateway,
>> > the two vpn gateways are interconnected via public network, the vpn
>> > gateway has only one NIC, single arm mode. vpn gateway mtu will be
>> > 1500 in general, but the packets sent by the vm's to the vpn gateway
>> > may be more than 1500, and at this time, if implemented according to
>> > the existing veth driver, the packets sent by the vm's will be
>> > discarded. If allowed to receive large packets, the vpn gateway can
>> > actually accept large packets then esp encapsulate them and then
>> > fragment so that in the end it doesn't affect the connectivity of the
>> > network.
>>
>> I'm not sure I quite get the setup; it sounds like you want a subset of
>> the traffic to adhere to one MTU, and another subset to adhere to a
>> different MTU, on the same interface? Could you not divide the traffic
>> over two different interfaces (with different MTUs) instead?
>>
>
> This is indeed a viable option, but it's not easy to change our own
> implementation right now, so we're just seeing if it's feasible to skip
> the veth mtu check.

Hmm, well, that basically means asking the kernel community to take on
an extra maintenance burden so that you won't have to do that yourself.
And since there's a workaround, my inclination would be to say that the
use case is not sufficiently compelling that it's worth doing so.

However, I don't feel that strongly about it, and if others think it's
worth adding a knob to disable the MTU check on forward, I won't object.

-Toke