Re: [PATCH v2 net 3/3] wireguard: send: account for mtu=0 devices

On 2/14/20 10:15 AM, Jason A. Donenfeld wrote:

On Fri, Feb 14, 2020 at 6:56 PM Eric Dumazet [off-list ref] wrote:

quoted

Oh dear, can you describe what do you expect of a wireguard device with mtu == 0 or mtu == 1

Why simply not allowing silly configurations, instead of convoluted tests in fast path ?

We are speaking of tunnels adding quite a lot of headers, so we better not try to make them
work on networks with tiny mtu. Just say no to syzbot.

The idea was that wireguard might still be useful for the persistent
keepalive stuff. This branch becomes very cold very fast, so I don't
think it makes a difference performance wise, but if you feel strongly
about it, I can get rid of it and set a non-zero min_mtu that's the
smallest thing wireguard's xmit semantics will accept. It sounds like
you'd prefer that?

Well, if you believe that wireguard in persistent keepalive
has some value on its own, I guess that we will have to support this mode.

Some legacy devices can have arbitrary mtu, and this has caused headaches.
I was hoping that for brand new devices, we could have saner limits.

About setting max_mtu to ~MAX_INT, does it mean wireguard will attempt
to send UDP datagrams bigger than 64K ? Where is the segmentation done ?