Re: Comments on httpbis-client-hints-01

Hi Vasiliy.

Thanks for the feedback, here and on GitHub. WIP branch:

On Tue, May 31, 2016 at 11:15 AM, Vasiliy Faronov <>

> Hi,
> A few comments on draft-ietf-httpbis-client-hints-01:
> 1. It seems to imply that Vary only includes the hints that were
> present in the request ("emit a Vary . . . to indicate which hints
> were used"). But, for example, if the request had *no* DPR field, yet
> the response *would* be influenced by a DPR field, shouldn't the
> response still include "Vary: DPR"?Otherwise a cache cannot know that
> a subsequent request with DPR must be written through to the origin
> server.

Good points, I think that makes sense. Any cache implementers / Vary
experts want to chime in here? What's the best practice to handle this?

> 2. The Accept-CH field seems underspecified. Is it per-resource? (and
> if so, doesn't it duplicate Vary?) Or is it per-origin? Or does it
> just apply to resources linked from the response?

If we're talking about browser as "client", then the behavior is specified
in Fetch API:

Does it duplicate Vary: no, I don't think so, they signal different things
to the client and they don't have same implications for upstream caches.

> 3. Are requirements like "If DPR occurs in a message more than once"
> (ditto for the other fields) useful? Duplicating these fields is
> already invalid per RFC 7230.

Ah, happy drop those if they're no longer needed. Any objections?


Received on Wednesday, 1 June 2016 21:51:52 UTC