Re: ISSUE-262: guidance regarding server responses and timing

I feel we've let this thread get rather a long ways off of the original topic. ISSUE-262 is an issue raised as a Last Call comment to the TPE specification, regarding the timing of server responses in cases of ad exchanges or other cases where server-to-server communication takes place.

> Accordingly, Rubicon Project requests that the Working Group include some guidance as to how responding servers should deal with such timing issues.
https://www.w3.org/2011/tracking-protection/track/issues/262

I believe we can address this with the existing "?" tracking status value (for use in pre-fetching) and some clarity about Tk header responses being valid only for the request in question. Regarding next steps: we can follow up with the commenter to see if that explanation makes sense to them. We could also consider an editorial note or example in the TPE specification, if we expect this to be a common confusion for implementers.

TPE doesn't require any compliance determinations, but just a response to the question of how to make a response available when different downstream servers are involved. There seems to be some interesting discussion about the different possible models and how that would apply to DNT compliance (as described in the separate Tracking Compliance specification) and our definition of service provider. If someone has a text proposal for changes to the Compliance editor's draft (or even just a concrete description of the problem you see, so that someone else can draft a change proposal) we could continue that discussion with a different issue number.

Thanks,
Nick