Conflicts between Alt-Svc and ALPN

[MikeBishop]

Alt-Svc tells me to use h2, so I open a TLS connection, and the server either doesn’t support ALPN or doesn’t select h2. What now? Weird corner cases ensue where I’m connected to something that may or may not be the alternate I was intending to reach. (Similar issues ensue attempting to connect using h2c -- does it mean a direct TCP connection to the specified port, or that the client should offer Upgrade? What if the server doesn't accept the Upgrade?)

It's simpler to just say TLS is the protocol and require ALPN support in 2.3 as well. Point me to a TLS-enabled endpoint, and the specific protocols we'll use will be negotiated in real-time.

However, this does pose a challenge if you wanted only HTTP/3 traffic going to your alternate.

[martinthomson]

Yes, that's a bad sort of failure, but I don't think that it needs special handling. Any failure in establishing the alternative just results in falling back to the primary.

(BTW, h2c has to mean one thing, and I think that it's clear that this is Upgrade.)

[mnot]

Maybe some security considerations text to the effect that the alternative isn't usable if it doesn't negotiate the advertised protocol.

[martinthomson]

s/security considerations//; just text.

[mnot]

Discussed on list, add text.

[reschke]

in 2.4?

[mnot]

Makes sense to me.

[mnot]

@reschke do you need text to be proposed?

[reschke]

yes