Re: IPv6 path MTU discovery not working (large number of retransmited TCP data packets)

[Christof Meerwald <cmeerw%cmeerw.org@localhost>]

On Mon, May 26, 2025 at 06:41:47PM +0200, Christof Meerwald wrote:
> Noticed a lot of retransmitted data packets showing up in "netstat
> -ss", and after some experimentation it appears to be some issue with
> path MTU discovery on IPv6 (or the lack of it).

Ok, did some debugging...

It seems to fail to allocate a new route in icmp6_mtudisc_clone:

  error = rtrequest(RTM_ADD, dst, rt->rt_gateway, NULL,
      RTF_GATEWAY | RTF_HOST | RTF_DYNAMIC, &nrt);

where it returns error 51 (ENETUNREACH).

Now I could imagine that the way the default route is set up might
have something to do with it, i.e. I am following what was discussed
here:

https://mail-index.netbsd.org/tech-net/2025/01/19/msg008956.html

so I have

2a03:d9c2:100::1 -connected -ifp vioif0

in /etc/mygate6


And that indeed seems to be causing the issue here. If I change the
network configuration to use a /48 and just put "2a03:d9c2:100::1"
into /etc/mygate6, then it works as expected.


Christof

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