Intent to implement: alternative services

[Bence Béky]

== Contact e-mails ==

"Bence Béky" <b...@chromium.org>

"Ryan Hamilton" <r...@chromium.org>

== Spec ==

Published drafts: https://tools.ietf.org/wg/httpbis/draft-ietf-httpbis-alt-svc/

Editor's copy: https://httpwg.github.io/http-extensions/alt-svc.html

== Summary ==

Alternative services allow an origin serving an http:// or https:// resource to

nominate additional servers that the client can choose to request the resource

from instead of the origin when making subsequent requests.

== Motivation ==

The most important application is enhanced connection pooling.  For example, if

the user visits mail.example.com in a browser, that server might let the browser

know that the same service can also be requested from www.example.com.  Next

time, the user might first open a connection to www.example.com.  When they open

mail.example.com in the same browser session, the browser can pool to the

existing connection to www.example.com to request the same service.  This can

cut down on a number of round trips, for example, the ones that would normally

be required for DNS resolution, TCP connection establishment, and TLS

negotiation.

Other uses may include load balancing, or offering a service with a different

protocol which the client may prefer given user settings or local network

characteristics.

== Compatibility risk ==

Alternative service advertisements are sent by the server.  From the client

side, there is no compatibility risk, because a server not supporting

alternative services will not send such advertisements.  From the server side,

it has to be considered that clients not supporting alternative services but

otherwise compliant with the HTTP or HTTP/2 protocol specification will ignore

such advertisements.

== Tracking bug ==

https://crbug.com/392575

-- Bence

[Chris Bentzel]

+net...@chromium.org

For other browser support, Mozilla actually shipped Alt-Svc support in 37.0, although they did remove in 37.0.1 due to a security issue.

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[David Benjamin]

To that end, we should make sure to be very very careful with this code. Mixing hostnames around like this, particularly with all the pooling that we do, has a large potential to produce similar kinds of problems. Ideally strive for not just "I checked all the cases I could think of and it works" but "the codepaths and APIs contracts are so clear it's obvious that it works and how future extensions would ensure it stays working".

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