[quicwg/base-drafts] Client connection IDs are broken (#2844)

[David Schinazi <[e-mail]>]

QUIC supports client connection IDs (CIDs) in order to allow a client to multiplex multiple QUIC connections on the same local IP and port. However, the current spec doesn't allow that to work, as pointed out by @kazuho on #2834 : if a server decides to use zero-length CIDs, then the client cannot establish multiple connections to that server from the same local port. For this to work, the client would need to know out-of-band that the server will not use zero-length CIDs, and that's not viable.

Therefore we have two protocol features that are at odds with one another:
(1) server can share port and use zero-length CIDs (which prohibits migration)
(2) client can share port and use non-zero-length CIDs

We can't have both (1) and (2) because the client wanting to do (2) doesn't know if the server will do (1) or not. If that happens, long headers will be demultiplexed properly because they contain the client's CID, but short headers from client to server do not and therefore cannot be demultiplexed.

@huitema and I have a use-case for (2) which is a QUIC proxy that only uses one port for outbound connections and relies on client CIDs for demultiplexing. I don't see a use for (1).

For client connection IDs to be useful in QUIC, I think we should ban zero-length connection IDs when you're multiplexing multiple connections on a local IP + port.

The current spec states:

> A zero-length connection ID MAY be used when the connection ID is not needed for routing and the address/port tuple of packets is sufficient to identify a connection.

We may want to change it to something like:

> A zero-length connection ID MAY be used when the connection ID is not needed for routing and the destination address/port tuple of incoming packets is sufficient to identify a connection.

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