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Re: i37: Vary and non-existant headers

From: Mark Nottingham <mnot@mnot.net>
Date: Thu, 4 Jun 2009 17:06:46 +1000
Cc: "Roy T. Fielding" <fielding@gbiv.com>, Brian Smith <brian@briansmith.org>, 'HTTP Working Group' <ietf-http-wg@w3.org>
Message-Id: <19AC8559-09F9-44AE-90C5-5BA95ECCC3AA@mnot.net>
To: Julian Reschke <julian.reschke@gmx.de>
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/
Received on Thursday, 4 June 2009 07:07:28 GMT

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