udp: honor IPV6_TCLASS cmsg for UDP/IPv4 packets

[tuexen]

Summary

Honor the IPPROTO_IPV6-level cmsg of type IPV6_TCLASS when sending an UDP/IPv4 packet on an AF_INET6 socket.

[glebius]

Why don't embed this entire block into udp_v4mapped_pktinfo()?

[tuexen]

In D53347#1218523, @glebius wrote:

Why don't embed this entire block into udp_v4mapped_pktinfo()?

Because we are neither handling the cmsg of type IPV6_2292PKTINFO or IPV6_PKTINFO. There is more work needed to clean things up. But I want bz@ in that loop and he is short of time now. We can have a function which does all the mappings, but I would suggest to do this refactoring separately and also deal with the TTL/hop limit stuff.
I would like to get this in FreeBSD 15. It is used when doing ECN for QUIC.

[bz]

In D53347#1218542, @tuexen wrote:

In D53347#1218523, @glebius wrote:

Why don't embed this entire block into udp_v4mapped_pktinfo()?

Because we are neither handling the cmsg of type IPV6_2292PKTINFO or IPV6_PKTINFO. There is more work needed to clean things up. But I want bz@ in that loop and he is short of time now. We can have a function which does all the mappings, but I would suggest to do this refactoring separately and also deal with the TTL/hop limit stuff.
I would like to get this in FreeBSD 15. It is used when doing ECN for QUIC.

If you want @bz in the loop remove v4mapped sockets, remove IPv4, be done with this. /SCNR

There's also bits in TCP which would need proper dealing with some of this still and just have placeholders. I think it'll all need a rather proper makeover and unification between all bits and it can properly be done nicely enough setting letting a function know which bits we are interested and return them like we do in the IPv6 side.

I believe even if not neat and I don't like any of this, this is right.