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Re: If-Range with other conditionals

From: Jeffrey Mogul <Jeff.Mogul@hp.com>
Date: Tue, 24 Sep 2002 19:47:14 -0400 (EDT)
Message-Id: <200209242347.g8ONl5lB026652@wera.hpl.hp.com>
To: Alex Rousskov <rousskov@measurement-factory.com>
cc: ietf-http-wg@w3.org




Alex Rousskov asks:

	Can the author(s) or anybody clarify the intent?

I think that's probably "me", for these excerpts, but it's
possible other people had some say in the wording here.

(1) Alex writes:

    RFC 2616 says:
    
       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.
    
    As far as I can see, a 304 (Not Modified) response code SHOULD NOT be
    generated for an If-Range request. If-Range definition implies that
    only 200 (OK) and 206 (Partial Content) responses are valid. Can a 304
    response be ever "consistent with" an If-Range field?
    
It would have been helpful to note the specific section number from
RFC2616.  This particular text is in 13.3.4 (Rules for When to Use
Entity Tags and Last-Modified Dates) and isn't meant to be normative
with respect to what status codes are valid under other circumstances.
That is, this is an extra restriction (imposed when a request has
multiple conditions), and can't be taken as implying a lack of
restrictions in other cases.

My guess is that we (I?) wrote this without fully thinking through
whether If-Range could ever return ("be consistent with") 304.
But on the "better safe than sorry" principle, I see nothing
really wrong with this paragraph.

    Can a 412 response be ever "consistent with" an If-Range field?

Consider at least this case:

	GET /foo.html HTTP/1.1
	Host: whatever.com
	If-Range: *
	Range: bytes=100-200
    
Remember that section 14.27 (If-Range) informally defines If-Range
as a sort of combination of GET+If-Match+Range, followed by a plain
GET if the If-Match fails.

Section 14.24 (If-Match) says:

   If none of the entity tags match, or if "*" is given and no current
   entity exists, the server MUST NOT perform the requested method, and
   MUST return a 412 (Precondition Failed) response. This behavior is
   most useful when the client wants to prevent an updating method, such
   as PUT, from modifying a resource that has changed since the client
   last retrieved it.

All of the other language that defines what 412 really means
looks a lot like this.

Therefore, I suppose it is plausible that if "/foo.html" simply
doesn't exist, then 412 is ""consistent with" If-Range.  However,
this seems to be a nonsensical special case (why not simply return
404 here, and in any case what does "If-Range: *" mean?) so I think
we can rule it out.  The other reason for returning 412 with a
plain If-Match (none of the entity tags match) is precisely the
case where If-Range wants the server to return 200.

Thus, I don't think If-Range should ever give rise to a 412 response,
except in this one probably nonsensical example for which a 404
response would be more appropropriate.

But again, this is not really the point being addressed in section
13.3.4!

    And a related question regarding proxy behavior; RFC 2616 says:
    
       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.
    
    What does "return a locally cached response" mean exactly? Does it
    include returning a 304 response (cached headers, no content)? Or just
    responses with locally cached _content_ (such as 200 and 206
    responses, but not 412).

(Again, we're in 13.3.4.)  This is one of those places where the
terminology ("response") used in RFC2616 fails us.  (I won't rehash
that debate here.)  I think the most reasonable interpretation of this
paragraph is that a cache entry of any sort (i.e., with any status
code) is useless if it's not consistent with all of the conditional
header fields in the request.

This follows my basic philsophy about caching: if there's any
doubt, don't use a cache entry; contacting the origin server
might be slow but it won't give the wrong content or headers.
    
    I find it curious that all "If-*" sections in the RFC have "the
    behavior when both If-Foo and If-Bar are present is undefined"
    clauses, _except_ for the If-Range section. It sort of implies that
    If-Range can be used in combination with all others, but valid server
    behavior upon receiving certain combinations is not clear to me. Thus
    the questions above.
    
Perhaps an oversight on my part.  On the other hand, we made an
explicit decision (discussed on the mailing list, if my memory
is correct) not to list in the spec all possible combinations of
headers that were not meaningful.  The fact that we did list a
few specific "undefined" combinations does not mean that all
other combinations are reasonable!

Anyway, I can't think of any reasonable interpretation for this
example:

	GET /foo.html HTTP/1.1
	Host: whatever.com
	If-Range: "a"
	Range: bytes=100-200
	If-Match: "b"

Note that unlike "If-Match", which can carry many entity tags,
If-Range is explictly defined to carry only one validator.  The
assumption is that If-Range is defined to fill in holes for
a specific cached instance, not to grab a chunk of some
finite subset of the instances of a resource.

It *might* make sense to do:

	GET /foo.html HTTP/1.1
	Host: whatever.com
	If-Range: "a"
	Range: bytes=100-200
	If-None-Match: "b"

which means
	if the current entity tag is "b" then send 304
	else if the current entity tag is "a" then send 206 (w/bytes=100-200)
	else send 200

but believe me, I haven't thought this through all the way.

-Jeff

P.S.: Alex, I owe you answers for several previous questions
you've posted.  I'm trying to catch up on my email.
Received on Wednesday, 25 September 2002 05:36:05 GMT

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