Need your help: different connection IDs in the same datagram

[Martin Thomson <[e-mail]>]

https://github.com/quicwg/base-drafts/issues/3800 discovered a minor "discrepancy" in our requirements for coalescing packets.  We required that:

* senders not coalesce packets for different connections in the same datagram.

* receivers reject packets with different connection **IDs** in the same datagram.

https://github.com/quicwg/base-drafts/pull/3870/files was the result of a discussion where I was convinced that looser constraints here weren't harmful.  The requirements would identify connections, not connection IDs.

Originally, my inclination was to fix the requirement on the sender to use connection IDs, which would remove some linkability.  But then I was reminded that linkability is already defeated for several reasons.  You can't coalesce outside of the handshake and we require a stable path for the handshake AND long headers include a source connection ID that can be used for linkability.  So the linkability concern is basically lost.

We also learned that some implementations were generating packets while processing incoming packets.  Later they would coalesce these into a datagram that might then have different connection IDs on the packets it contains.

There has been some opposition to the proposed resolution in PR 3870.  

Apparently, for some, having multiple connection IDs in the same datagram complicates processing.  I don't understand this objection.  It seems to me more difficult to retain state across packets than it is to process each atomically.  I was hoping that Christian or Nick can explain more about how this affects them.

Marten also indicated that there was a reason not to allow this, but if I understand that, this is based on some potential for future optimization, which could be conditional on having consistent connection IDs.  If we moved to a scheme that required consistent connection IDs, anyone using that scheme could be required to avoid coalescing different connection IDs.  So this seems more of a theoretical concern.

And - in anticipation of maybe choosing to make senders use a consistent connection ID - if you might be generating coalesced datagrams with different connection IDs in packets, how hard would it be to ensure that these are consistent?  Lars and Ian, I think both of you indicated that you might do this.