Re: [quicwg/base-drafts] QUIC advertisement description (#87)

[Mike Bishop <[e-mail]>]

Perhaps.  Here's the thing:  QUIC version and app-layer protocol aren't the same.  So the question is whether there *needs* to be a hint from the application layer into QUIC's version negotiation about what versions the server supports.

Here are the paths I see that we need to support:

- **Client to server, first time**:  This is inherently a 1-RTT handshake, so spending that same 1-RTT on version negotiation isn't a loss.  However, if reuse of the TLS/TCP connection information is supported, this will likely never happen for an HTTP/2-to-QUIC transition.
- **Client to server, 0-RTT**:  The cached information should contain information about the server's supported versions so the client can come back with the same version.  The scenarios where one side has changed need consideration, though:
    + **Client upgraded:**  This can be solved by caching the server's full set of supported versions and comparing against the client's supported set at connect time, not just caching the selected version.
    + **Server upgraded:**  The current mechanism offers no way for a client to discover that the server *would have accepted* a version it didn't initially offer, whether because the server didn't previously support it or the client is being conservative.  We should think about have a version-upgrade path as well.

Having a v= parameter in Alt-Svc *does* solve all of these. It gives a useful springboard if TLS allows 0-RTT using data obtained over TCP.  The client will cache the list of supported versions with the Alt-Svc header and will learn about new ones when the Alt-Svc entry gets refreshed periodically.

Ideally, I'd like to see these solved for QUIC independent of HTTP, though -- if/when we manage that, the Alt-Svc parameter could become unnecessary.

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