advice on how to disable ECN is problematic when multiplexing multiple connections

[marten-seemann]

The spec says:

If validation fails, then the endpoint MUST disable ECN. It stops setting the ECT codepoint in IP packets that it sends, assuming that either the network path or the peer does not support ECN.

This is fine advice when a single QUIC connection uses a socket. When multiplexing multiple connections on the same socket, it won't be possible to disable ECN on a per-packet basis, unless you want to call setsockopt before every datagram that is sent.

[larseggert]

Not true. You can use ancillary data with sendmsg to specify different ECN codepoints for each packet to be transmitted.

[marten-seemann]

Interesting, I didn't know that. I guess we can close this issue with no action then?

[larseggert]

I would think so.