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LC-2084 - purpose of behavior - 4.2.6 Receipt of Vary HTTP Header

From: Francois Daoust <fd@w3.org>
Date: Mon, 24 Nov 2008 15:31:33 +0100
Message-ID: <492ABAC5.4060905@w3.org>
To: public-bpwg-ct <public-bpwg-ct@w3.org>

The example use case for such behavior is below.

Not directly related to the example, but before I get to it, I have two 
small remarks on section 4.1.5.2 that links to 4.2.6:
 
http://www.w3.org/2005/MWI/BPWG/Group/TaskForces/CT/editors-drafts/Guidelines/081107#sec-avoiding-request-unacceptable

1. I don't completely understand what "is prepared to" implies in:

"and is prepared to reissue the request with unaltered headers, and 
alter its subsequent behavior in respect of the Web site so that 
unaltered headers are sent"

I understand it as willing to emphasize the points defined normatively 
in section 4.2.6:
 
http://www.w3.org/2005/MWI/BPWG/Group/TaskForces/CT/editors-drafts/Guidelines/081107#sec-receipt-of-vary-header

"if it makes a request with altered headers in these circumstances, and 
receives a response containing a Vary header referring to one of the 
altered headers then it SHOULD request the resource again with unaltered 
headers. It SHOULD also update whatever heuristics it uses so that 
unaltered headers are presented first in subsequent requests for this 
resource."

I would either replace "is prepared to reissue [...]" by "in particular 
reissue [...]" or remove the end of the sentence altogether.


2. The above text in 4.1.5.2 mentions "subsequent behavior in respect of 
the *Web Site*" whereas 4.2.6 only refers to "subsequent requests for 
*this* resource". I think we need to be consistent here. I'm leaning 
towards "Web Site", and the second part of the example below is such an 
example.


Anyway, the example I had sent in July is available at:
  http://lists.w3.org/Archives/Public/public-bpwg-ct/2008Jul/0028.html

I reworded it slightly to make it more compact and to follow the same 
pattern as the other examples.
It comes in two parts. First part is similar to current Appendix C.3 
Optimization based on Previous Server Interaction, Server has Changed 
its Operation, but introduces the notion of Web Site. Second part shows 
how this affects request on yet another resource of the same Web Site.

-----
Proxy receives a request for resource Q
Proxy determines (by unspecified means) that resource Q is on the same 
Web Site S as resource P, that it has previously encountered as in C.2 
Optimization based on Previous Server Interaction
Proxy forwards request with altered headers
Response is 200 OK containing a Vary: User-Agent, Accept header
Proxy notices that behavior for resource Q is not the same as for 
resource P and re-issues request for resource Q with original headers
Response is 200 OK
Proxy forwards response as designed specifically for the requesting device

Proxy receives a request for resource Z
Proxy determines (by unspecified means) that resource Z is on the same 
Web Site S as resources P and Q, that it has previously encountered with 
different behaviors.
Proxy forwards request with original headers
Response is 200 OK containing a Vary: User-Agent, Accept header
Proxy forwards response as designed specifically for the requesting device
-----

Francois.
Received on Monday, 24 November 2008 14:32:07 GMT

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