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Rev02: seek clarification on conditional headers

From: Dave Kristol <dmk@research.bell-labs.com>
Date: Wed, 25 Feb 98 14:51:40 EST
Message-Id: <9802251951.AA02758@aleatory.tempo.bell-labs.com>
To: http-wg@cuckoo.hpl.hp.com
I've been trying to figure out how to add If-Match (I-M) and
If-None-Match (I-N-M) to my server.  I'm getting a severe headache as I
read and reread the relevant sections and try to understand the
interactions between them and If-Modified-Since (I-M-S) and
If-Unmodified-Since (I-U-S).  (I haven't even got to If-Range yet!)

The remarks below apply to a simple server that's trying to respond to
a GET.

14.25 If-Match

    If the request would, without the If-Match header field, result in
    anything other than a 2xx status, then the If-Match header MUST be
    ignored.

This paragraph implies to me that I should test I-M-S before I-M,
because it's possible that I-M-S could produce a 304.  (Note that
Apache 1.2.5 does the I-M test first, on the theory that I-M is a more
accurate test.  I'm using Apache as a reference because I'm assuming
that Roy had a strong hand in the code.  So I would expect it to reflect
the desired behavior.)

14.26 If-None-Match

    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. Instead, if the request
    method was GET or HEAD, the server SHOULD respond with a 304 (Not
    Modified) response, including the cache-related entity-header fields
    (particularly ETag) of one of the entities that matched. For all other
    request methods, the server MUST respond with a status of 412
    (Precondition Failed).

Same remarks here.  Same remarks about Apache.

The problem is, if I test I-M-S first, then I-N-M, I can run afoul of this
part of 14.26:

    If none of the entity tags match, then the server MAY perform the
    requested method as if the If-None-Match header field did not exist, but
    MUST also ignore any If-Modified-Since header field(s) in the request.
    That is, if no entity tags match, then the server MUST not return a 304
    (Not Modified) response.

Also, I'm uncertain what the proper behavior is for I-N-M if an Etag
match succeeds and there's no I-M-S, or if there's an I-M-S and the
file has been modified since the I-M-S date.  Return 412 (Precondition
Failed)?  The response isn't well specified.

I'm too confused to ask any further coherent questions, so I'll wait
for responses to these.

Dave Kristol
Received on Wednesday, 25 February 1998 11:54:46 EST

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