Re: Comments on Extensible Prioritization Scheme for HTTP

[Willy Tarreau <[e-mail]>]

Hello Glenn,

On Thu, Mar 31, 2022 at 01:21:42AM -0400, Glenn Strauss wrote:
> Comments on Extensible Prioritization Scheme for HTTP
> https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-priority-12
> 
> HTTP/2 SETTINGS frame in the original RFC7540 spec contains
>   SETTINGS_ENABLE_PUSH
> This is a logical, boolean flag to enable an optional-use feature.
> 
> draft-ietf-httpbis-priority-12 has had multiple revisions with name
> changes for a new setting, and the current proposed name is
>   SETTINGS_NO_RFC7540_PRIORITIES
> with an *inverted boolean value*, which if set to *true* results in
> indicating a desire to *disable* use of RFC7540 HTTP/2 PRIORITY frame.

It's not as much a desire to "disable" as it is an indication that
it will neither emit nor process them.

> I am a developer mocking up draft-ietf-httpbis-priority.
> From my perspective, in order to follow a logical opt-in to enable use
> of optional features, I prefer the following proposal: new SETTINGS
>   SETTINGS_ENABLE_PRIORITY
>   SETTINGS_ENABLE_PRIORITY_UPDATE
> each named after the respective HTTP/2 frame type PRIORITY
> and (proposed) PRIORITY_UPDATE.  These are named in similar pattern to
> SETTINGS_ENABLE_PUSH feature, associated with HTTP/2 frame PUSH_PROMISE.

But that wouldn't work with existing deployments! Right now support for
priorities is implied by H2. Both a client and a server willing to use
them have nothing to say, and any RFC7540-compliant implementation will
see it like this. Thus a client may expect that the server will deliver
important objects first and may possibly announce certain priorities
based on what it thinks it needs. In fact both sides may act in a way
that is supposed to help the other one based on the assumption that it
will use it and benefit from this.

Explicitly saying "no need to make efforts for me" and "I will not
bother you with this" can be a useful indication to the partner that
it ought to fall back to alternate mechanisms instead.

Hoping this helps,
Willy