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

[David Ahern <dsahern@gmail.com>]

On 7/17/20 6:27 AM, Stefano Brivio wrote:
>>
>>> Note that this doesn't work as it is because of a number of reasons
>>> (skb doesn't have a dst, pkt_type is not PACKET_HOST), and perhaps we
>>> shouldn't be using icmp_send(), but at a glance that looks simpler.  
>>
>> Yes, it also requires that the bridge has IP connectivity
>> to reach the inner ip, which might not be the case.
> 
> If the VXLAN endpoint is a port of the bridge, that needs to be the
> case, right? Otherwise the VXLAN endpoint can't be reached.
> 
>>> Another slight preference I have towards this idea is that the only
>>> known way we can break PMTU discovery right now is by using a bridge,
>>> so fixing the problem there looks more future-proof than addressing any
>>> kind of tunnel with this problem. I think FoU and GUE would hit the
>>> same problem, I don't know about IP tunnels, sticking that selftest
>>> snippet to whatever other test in pmtu.sh should tell.  
>>
>> Every type of bridge port that needs to add additional header on egress
>> has this problem in the bridge scenario once the peer of the IP tunnel
>> signals a PMTU event.
> 
> Yes :(
> 

The vxlan/tunnel device knows it is a bridge port, and it knows it is
going to push a udp and ip{v6} header. So why not use that information
in setting / updating the MTU? That's what I was getting at on Monday
with my comment about lwtunnel_headroom equivalent.