[Bug 293227] VNET jail regression on 14-STABLE from 14.3 using bridge and epair

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=293227

--- Comment #6 from Timo Voelker <timo.voelker@fh-muenster.de> ---
Thanks for the script. I was able to reproduce the problem. Here is my test
setup.

 Jail              Jail
sender           receiver
epair0b           epair1b
1::2/64           2::2/64
   \               /
    \             /
     \    Host   /
  epair0a        |
     |        epair1a
   bridge0    2::1/64
   1::1/64    MTU 1420
sysctl net.inet6.ip6.forwarding=1

If I let the sender send a file to 2::2, it sends packets that are too large,
and the Host responds with a Packet Too Big message, which the sender ignores.

The sender ignores the Packet Too Big messages, because the ICMP checksum is
incorrect. The issue seems to be related to transmission checksum offloading
and to the bridge. If I disable the transmission checksum on epair0b with

jexec sender ifconfig epair0b -txcsum6

or if I destroy bridge0 (and configure the IP on epair0a), the issue
disappears.

This is how far I've gotten by now.

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