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Re: If-None-Match and IMS (new Issue IMS_INM_MISMATCH)

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Wed, 12 Nov 97 12:22:53 PST
Message-Id: <9711122022.AA14851@acetes.pa.dec.com>
To: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
The question was:
    Is there a consensus on the correct behavior for a server when it
    receives a request with conflicting If-None-Match and
    If-Modified-Since headers, for instance where the I-N-M ETag is
    outdated but the I-M-S date is good?
    
This is specifically answered for proxy servers in section 13.3.4
(Rules for When to Use Entity Tags and Last-modified Dates).  The text
there says:

   An HTTP/1.1 cache, upon receiving a request, MUST use the most
   restrictive validator when deciding whether the client's cache entry
   matches the cache's own cache entry. This is only an issue when the
   request contains both an entity tag and a last-modified-date
   validator (If-Modified-Since or If-Unmodified-Since).

(Several uses of the word "cache" in that paragraph should probably
be "caching proxy", since end-client caches presumably do not
receive cache-conditional request messages.)

This is followed by:

     A note on rationale: The general principle behind these rules is
     that HTTP/1.1 servers and clients should transmit as much non-
     redundant information as is available in their responses and
     requests. HTTP/1.1 systems receiving this information will make the
     most conservative assumptions about the validators they receive.

I think the most straightforward extension of "these rules" to
the correct behavior for an origin server is to do the same:
use the most restrictive validator.  But, as Roy implied when
he wrote "the RFC is less than clear", the term "most restrictive"
isn't really defined.  So I would replace the first paragraph quoted
above with these two paragraphs:

   An HTTP/1.1 origin server, upon receiving a conditional request that
   includes both a Last-modified date (e.g., in an If-Modified-Since or
   If-Unmodified-Since header field) and one or more entity tags (e.g.,
   in an If-Match, If-None-Match, or If-Range header field) as cache
   validators, MUST NOT return a response status of 304 (Not Modified)
   unless doing so is consistent with all of the conditional header fields
   in the request.

   An HTTP/1.1 caching proxy, upon receiving a conditional request that
   includes both a Last-modified date and one or more entity tags as
   cache validators, MUST NOT return a locally cached response to the
   client unless that cached response is consistent with all of the
   conditional header fields in the request.

Dave Morris wrote:
	My recollection of intent matches the implementation Roy
	described.  If-None-Match supercedes IMS when both are
	present.

This is not quite the same thing; allowing INM to supersede IMS
unconditionally might be wrong if the INM contained a "weak"
entity tag.  However, with a strong entity tag, there should be
no practical difference between Dave's interpretation and the
more conservative one ... except in those rare cases where a
resource's modification date changes, but its value does not.

Henry's question continues:

    For example, take the following sequence
    
    GET /foo.txt HTTP/1.1
    Host: server.company.com
    
    HTTP/1.1 200 OK
    Last-Modified: Wed, 05 Nov 1997 22:10:48 GMT
    ETag: "12345"
    Content-Length: ...
    
    <data>
    
    GET /foo.txt HTTP/1.1
    Host: server.company.com
    If-None-Match: "12344"
    If-Modified-Since: Wed, 05 Nov 1997 22:10:48 GMT
    
    Assuming the Last-Modified date hasn't changed, what should the server
    send back as a response to the 2nd request, 304 or 200? I would have
    thought 200 is correct, since the ETag is invalid or out of date.
    However the spec indicates that the I-N-M header is to be treated as if
    it isn't present if the ETag doesn't match, and then the I-M-S would
    lead to a 304 response.

Section 14.26 (If-None-Match) doesn't exactly say "ignored"; it contains
an apparent contradiction, because it says both:

   If any of the entity tags match the entity tag of the entity that
   would have been returned in the response to a similar GET request
   (without the If-None-Match header) on that resource, or if "*" is
   given and any current entity exists for that resource, then the
   server MUST NOT perform the requested method. [...]

and

   If the request would, without the If-None-Match header field, result
   in anything other than a 2xx status, then the If-None-Match header
   MUST be ignored.

These excerpts both contradict the existing language in 13.3.4 (as it
applies to caching proxies) and the revised language I proposed above.
I.e., if the entity tags don't match, but the dates do match, 13.3.4
says "return 200", but the latter of these two excerpts implies "return
304".

I think that's a real bug.

If the entity tags do match, but the dates don't match, 13.3.4 also
says "return 200", but the former of these two excerpts implies
"return 304".  I think that's also a bug, but this one is probably
not of much practical consequence unless the server is using
weak entity tags.

I would change the first sentence quoted above to:

   If any of the entity tags match the entity tag of the entity that
   would have been returned in the response to a similar GET request
   (without the If-None-Match header) on that resource, or if "*" is
   given and any current entity exists for that resource, then the
   server MUST NOT perform the requested method, unless required
   to do so because the resource's modification date fails to
   match that supplied in an If-Modified-Since header field in the
   request. [...]

and the second sentence to:

   If the request would, without the If-None-Match header field, result
   in anything other than a 2xx or 304 status, then the If-None-Match
   header MUST be ignored.  (See section 13.3.4 for a discussion of
   server behavior when both If-Modified-Since and If-None-Match
   appear in the same request.)

-Jeff
Received on Wednesday, 12 November 1997 12:27:47 EST

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