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Re: i37: Vary and non-existant headers

From: Mark Nottingham <mnot@mnot.net>
Date: Thu, 25 Jun 2009 16:04:12 +1000
Cc: Julian Reschke <julian.reschke@gmx.de>, Roy Fielding <fielding@gbiv.com>, Brian Smith <brian@briansmith.org>
Message-Id: <7BB34C68-4832-43C8-A6FE-9D23748D2C71@mnot.net>
To: HTTP Working Group <ietf-http-wg@w3.org>
I haven't seen any comment on this, so my inclination is to  
incorporate into -07 and see how it stands in context. I'll leave the  
issue open for now, and close once people have a chance to see that.


On 04/06/2009, at 5:06 PM, Mark Nottingham wrote:

> I think the right way to fix this is to move the language about  
> validation into section 2.4 (validation model) and carefully  
> updating that and section 2.7 (combining responses) to indicate how  
> validation affects the response, whether or not there are selecting  
> headers. This requires some non-trivial changes to the text, but  
> we've already done that, and I think the result is worth it...
>
> Note that 2.4 already says:
>> When a stored response includes one or more validators, such as the  
>> field values of an ETag or Last-Modified header field, then a  
>> validating request SHOULD be made conditional to those field values.
>
> Proposal for updated section 2.4:
>
> """
> 2.4. Validation Model
> Checking with the origin server to see if a stale or otherwise  
> unusable cached response can be reused is called "validating" or  
> "revalidating." Doing so potentially avoids the overhead of  
> retransmitting the response body when a stored response is valid.
> When a cache has one or more stored responses for a URI, but cannot  
> serve any of them (see section 2.2), it can use the conditional  
> request mechanism (part 4) in the forwarded request to select a  
> valid stored response to be used, and update it.
>
> When sending such a conditional request, the cache SHOULD add an If- 
> Modified-Since header whose value is that of the Last-Modified  
> header from the selected (see section 2.6) stored response, if  
> present.
>
> Additionally, the cache SHOULD add an If-None-Match header whose  
> value is that of the ETag header(s) from all responses stored for  
> that URI (see section 2.6), if present. However, if any of the  
> stored responses contains only partial content, its entity-tag  
> SHOULD NOT be included in the If-None-Match header field unless the  
> request is for a range that would be fully satisfied by that stored  
> response.
>
> A 304 (Not Modified) response status code indicates that a stored  
> response should be updated and used to satisfy the request; see  
> Section 2.7.
>
> A full response (i.e., one with a response body) indicates that none  
> of the stored responses nominated in the conditional request is  
> suitable. Instead, the full response is used both to satisfy the  
> request and replace the stored response. [[anchor8: Should there be  
> a requirement here?]]
> If a cache receives a 5xx response while attempting to validate a  
> response, it MAY either forward this response to the requesting  
> client, or act as if the server failed to respond. In the latter  
> case, it MAY return a stale stored response if allowed (see Section  
> 2.3.3).
> If a cache receives a successful response whose Content-Location  
> field matches that of an existing stored response for the same  
> Request-URI, whose entity-tag differs from that of the existing  
> stored response, and whose Date is more recent than that of the  
> existing response, the existing response SHOULD NOT be returned in  
> response to future requests and SHOULD be deleted from the cache. 
> [[anchor13: DISCUSS: Not sure if this is necessary.]]
> """
>
> Proposal for updated section 2.6:
>
> """
> 2.6.  Caching Negotiated Responses
>
> When a cache receives a request that can be satisfied by a stored  
> response with a Vary header field (Section 3.5), it MUST NOT use  
> that response unless all of the selecting request-headers nominated  
> by the Vary header match in both the original request (i.e., that  
> associated with the stored response), and the presented request.
>
> The selecting request-headers from two requests are defined to match  
> if and only if the selecting request-headers in the first request  
> can be transformed to the selecting request-headers in the second  
> request by adding or removing linear white space [[anchor11: [ref]]]  
> at places where this is allowed by the corresponding ABNF, and/or  
> combining multiple message-header fields with the same field name  
> following the rules about message headers in Section 4.2 of [Part1].
>
> If a header field is absent from a a request, it can only match  
> another request if it is also absent there.
>
> A Vary header field-value of "*" always fails to match, and  
> subsequent requests to that resource can only be properly  
> interpreted by the origin server.
>
> If no stored response matches, the cache MAY forward the presented  
> request to the origin server as a conditional request; see Section  
> 2.4.
> """
>
>
> Proposal for updated section 2.7 (there's some duplication with p5- 
> range section 4 that we need to take care of, btw):
>
> """
> 2.7.  Combining Responses
>
> When a cache receives a 304 (Not Modified) response or a 206  
> (Partial Content) response (collectively, the "new" response), it  
> needs to update the stored response with the new one, so that the  
> updated response can be used to satisfy the request.
>
> If the new response contains an ETag, it identifies the stored  
> response to use.  [[ note: may need language about Content-Location  
> here ]]
>
> If the status code is 206 (partial content), both the stored and new  
> responses MUST have ETags, and those ETags MUST match using the  
> strong comparison function (see Section 4 of [Part4]). Otherwise,  
> the responses MUST NOT be combined.
>
> The stored response headers are used as those of the updated  
> response, except that
>
>   o Any stored Warning headers with warn-code 1xx (see Section 3.6)  
> MUST be deleted from the stored response and the updated response.
>
>   o Any stored Warning headers with warn-code 2xx MUST be retained  
> in the stored response and the updated response.
>
>   o Any headers provided in the new response MUST replace the  
> corresponding headers from the stored response.
>
> If a header field-name in the new response matches more than one  
> header in the stored response, all such old headers MUST be replaced.
>
> The updated response can [[requirement?]] be used to replace the  
> stored response in cache. In the case of a 206 response, the  
> combined entity-body MAY be stored.
>
>   [[anchor14: ISSUE: discuss how to handle HEAD updates]]
> """
>
>
>
> On 08/05/2009, at 7:29 AM, Julian Reschke wrote:
>
>> Roy T. Fielding wrote:
>>> On May 7, 2009, at 12:43 PM, Julian Reschke wrote:
>>>> Brian Smith wrote:
>>>>> Julian Reschke wrote:
>>>>>> "When a cache receives a request that can be satisfied by a  
>>>>>> stored
>>>>>> response that includes a Vary header field (Section 3.5), it  
>>>>>> MUST NOT
>>>>>> use that response unless all of the selecting request-headers  
>>>>>> nominated
>>>>>> by the *stored* Vary header match in both the original request
>>>>>> associated with the stored response, and the presented request."
>>>>> This is wrong, but not for the reason Jamie stated.
>>>>> Consider this:
>>>>> 1. Client requests "GET /foo"
>>>>> 2. Cache forwards the request to the server "GET /foo" with
>>>>>  an If-None-Match: "A", "B", "C"
>>>>> 3. Server returns "304 Not Modified" with ETag "A".
>>>>> According to the statement above, the cache could only return  
>>>>> the cached
>>>>> response with ETag "A" if the Vary'd headers match; otherwise,  
>>>>> it couldn't
>>>>> return any response at all. However, the cache must be able to  
>>>>> return the
>>>>> response with ETag "A" regardless of whether the Vary'd headers  
>>>>> match.
>>>>
>>>> Why?
>>>>
>>>> What you describe sounds like a bug. If the selecting headers  
>>>> that resulted in the response with entity tag "A" had a "Accept- 
>>>> Encoding: gzip", then the response can't be used for a request  
>>>> without "Accept-Encoding", even if it may be fresh.
>>> Because the origin server just told the cache to deliver "A" in (3).
>>> The origin server is authoritative.
>>> We've had this discussion before.
>>
>> Right, sorry.
>>
>> So the text is only needs to consider the case where the origin  
>> server gives its ok.
>>
>> ""When a cache receives a request that can be satisfied by a stored
>> response that includes a Vary header field (Section 3.5), it MUST NOT
>> use that response unless all of the selecting request-headers  
>> nominated
>> by the *stored* Vary header match in both the original request  
>> associated with the stored response, and the presented request, *or  
>> after validating with the origin server*".
>>
>> ?
>>
>> BR, Julian
>>
>>
>>
>>
>>
>
>
> --
> Mark Nottingham     http://www.mnot.net/
>
>


--
Mark Nottingham     http://www.mnot.net/
Received on Thursday, 25 June 2009 06:04:58 GMT

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