[TLS] Application-Layer Protocol Settings

[Victor Vasiliev <[e-mail]>]

Hello TLS and HTTP working groups,

(QUIC WG bcc'd as this has been discussed there before)

Currently, we use SETTINGS frames as an extensibility mechanism in HTTP/2
and HTTP/3.  The SETTINGS frame is sent at the very beginning of TLS
application data; this approach, while simple, has some drawbacks.  The
most notable one is that when SETTINGS are used to negotiate extensions,
there is an entire round-trip where the client can send requests, but
doesn't know yet about any server extensions, thus making any
extension-dependant requests take an extra RTT.

The proposed solution to this problem is to move HTTP SETTINGS frame into
the TLS handshake.  Here are some issues where this has been discussed
before:

   - https://github.com/quicwg/base-drafts/issues/3086
   - https://github.com/quicwg/base-drafts/issues/3622
   - https://github.com/WICG/client-hints-infrastructure/pull/30

I wrote up a draft for the TLS extension that would solve this problem:
https://tools.ietf.org/html/draft-vvv-tls-alps-00

I also wrote up a draft that explains how to use that extension with HTTP,
and defines some settings (the ones discussed here
<https://github.com/quicwg/base-drafts/issues/3622>) that would not be
possible without it: https://tools.ietf.org/html/draft-vvv-httpbis-alps-00

I would appreciate feedback on those drafts.

Thanks,
  Victor.