netdev - Re: [PATCH net-next 1/3] udp_tunnel: allow to turn off path mtu discovery on encap sockets

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Date:   Wed, 15 Jul 2020 14:42:58 +0200
From:   Florian Westphal <fw@...len.de>
To:     Stefano Brivio <sbrivio@...hat.com>
Cc:     Florian Westphal <fw@...len.de>, David Ahern <dsahern@...il.com>,
        netdev@...r.kernel.org, aconole@...hat.com
Subject: Re: [PATCH net-next 1/3] udp_tunnel: allow to turn off path mtu
 discovery on encap sockets

Stefano Brivio <sbrivio@...hat.com> wrote:
> I would still like the idea I proposed better (updating MTUs down the
> chain), it's simpler and we don't have to duplicate existing
> functionality (generating additional ICMP messages).

It doesn't make this work though.

With your skeleton patch, br0 updates MTU, but the sender still
won't know that unless input traffic to br0 is routed (or locally
generated).

Furthermore, such MTU reduction would require a mechanism to
auto-reconfig every device in the same linklevel broadcast domain,
and I am not aware of any such mechanism.