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Re: If-Range with other conditionals

From: Jeffrey Mogul <Jeff.Mogul@hp.com>
Date: Tue, 24 Sep 2002 21:04:03 -0400 (EDT)
Message-Id: <200209250103.g8P13wu8029380@wera.hpl.hp.com>
To: Alex Rousskov <rousskov@measurement-factory.com>
cc: ietf-http-wg@w3.org




    > 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"
    
    What, in your opinion, should an origin server return when
    receiving the above request (we can assume that entity has an ETag
    of "c")? It looks like returning 200/206 would violate If-Match
    while returning anything else would violate If-Range SHOULD
    requirements. Should the server return "Bad Request"? Or is this
    simply an "undefined behavior" case?
    
According to the spec, it's "undefined behavior" (because we
don't define it) but I think you are correct that neither 200
nor 206 is allowed in this case, and 304 seems forbidden, too.
So as a server implementor, I would probably go for 400 (Bad
Request).  I'm generally a "be liberal in what you accept"
kind of guy, but returning a 200 response in this case does
violate the intent of If-Match.

    > 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
    
    OK. It looks to me that you are saying that the presence of If-Range
    headers does not limit response codes that can be returned. In other
    words, the existing SHOULD "definition" of If-Range is valid only when
    no other If-* headers are present:
    
       If the entity tag given in the If-Range header matches the current
       entity tag for the entity, then the server SHOULD provide the
       specified sub-range of the entity using a 206 (Partial content)
       response. If the entity tag does not match, then the server SHOULD
       return the entire entity using a 200 (OK) response.

Like I wrote, "believe me, I haven't thought this through all the way."
    
Anyway, "SHOULD" means (per RFC2119):

   that there
   may exist valid reasons in particular circumstances to ignore a
   particular item, but the full implications must be understood and
   carefully weighed before choosing a different course.

so that we could argue that RFC2616 isn't truly self-contradictory :-)

    Here is what we currently do to test the proxy-related MUST in
    question: generate all possible combinations of If-* headers that do
    not violate other MUSTs, make on of the If- header mismatch LMT/tag of
    the cached entity, and then make sure that the proxy under test does
    not return any cached information (including headers). The proxy can
    return any status codes. Do you think that's the best strategy of
    verifying the MUST in question?

This seems plausible, but I'm not an expert on formal verification,
and such experts have found bugs in spec language that I helped
write (see the unfortunately-titled "Safe Composition of Web
Communication Protocols for Extensible Edge Services", by
Adam D. Bradley, Azer Bestavros, Assaf J. Kfoury in the 7th
WCW, which is really about the bugs that arise w.r.t. the
Expect/100 Continue specification).

In general, a proxy "cache" should be able to meet all of the
caching-related parts of RFC2616 by shrinking its cache size to
zero, so a cache that passes your test could be compliant, but
I'm not sure that it's *necessarily* compliant.  That is, I'm
not sure that your approach actually tests all of the MUSTs
in the spec.  There might be some interactions with Cache-Control
headers that you need to test, too.

I'm also assuming that you limit your testing to GET/HEAD
methods, since nobody has really worked out how HTTP caches
deal with PUTs or POSTs in all possible cases (except by
simply forwarding these).

-Jeff
Received on Wednesday, 25 September 2002 05:36:40 GMT

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