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Re: Question on byte ranges

From: Jeffrey Mogul <mogul@pa.dec.com>
Date: Tue, 04 Nov 97 13:47:18 PST
Message-Id: <9711042147.AA07731@acetes.pa.dec.com>
To: http working group <http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com>
John Franks writes:

    If a server receives a byte range requests for a 100 byte file like
    
    a)   Range: bytes = 0-10, 130-140
    or
    b)   Range: bytes = 30-20
    or
    c)   Range: bytes = 0-10, 30-20, 40-50
    
    
    what is the correct response?  
    
    In case a) one of the two ranges is invalid, but the spec says send
    a 416 only if *all* are invalid.  If the valid ones are returned with
    a 206 then there is no way to signal an error.  
    
    In case b) the spec says
    
		If the last-byte-pos value is present, it must be greater
		than or equal to the first-byte-pos in that byte-range-spec,
		or the byte-range-spec is invalid. The recipient of an
		invalid byte-range-spec must ignore it.
    
    If the server ignores it then it should send what?
    
    Case c) is like b) except it is possible to send the valid ranges.

I think the confusion here arises because, when I drafted this part
of the spec, I failed to make a clear distinction between "syntactically
invalid" byte-range-specs, and what I probably should have called
"unsatisfiable" byte-range-specs.

A "syntactically invalid" byte-range-spec is one for which there are no
circumstances in which it would ever be valid.  E.g., "bytes = 30-20"
and "bytes = 10-20-30" are both syntactically invalid
byte-range-specs.  Basically, this is a coding bug in the client.

An "unsatisfiable" byte-range-spec is one that is syntactically
valid, but which cannot be satisfied (even partially) given the
current state of the resource.  The fact that the client made
such an unsatisfiable request might not be the result of a
coding bug; it might just be because the client's information
about the resource's current length is either out-of-date or
"optimistic".

Note that some of this terminology already appears in section
14.17, but it isn't used consistently in the entire document.

The philosophy behind the entire byte-range design is that
requests that have "syntactically invalid" byte-range-specs
should be treated as buggy, and the server should simply
ignore the Range header and treat the request just as it
would have treated a regular GET without the buggy Range.

However, when a request contains a Range header which is
syntactically valid, the server should provide as much
as it can to the client, consistent with the request.
That means that if any of the byte-range-specs are even
partially satisfiable, then the server should send a 206
(Partial content) response; if none of the byte-range-specs
are satisfiable, then the server should return a 416 response.

This is basically an application of the Robustness Principle
("be conservative in what you send, be liberal in what you accept").

I think this also clears up Dave Kristol's concern.  Dave wrote:
    However, I have a problem with section 10.4.17, 416 Requested range
    not found.  The helpful hint, "(For byte-ranges, this means that
    the first-byte-pos of all of the byte-range-spec values were
    greater than the current length of the selected resource.)" is
    incorrect.  The byte-range-spec "30-20" (as in (b) above) is
    invalid, but the first-byte-pos is valid.

I think the "helpful hint" is accurate if you treat 416 as
meaning "Requested range not satisfiable" rather than "Requested
range not valid".    

Here are my proposed rewrites:

(1) Section 10.4.17 416 "Requested range not valid" should be
retitled "Requested range not satisfiable", and in section
6.1.1, the BNF for Status-Code should be changed from

                                  | "416"   ; Requested range not valid

to

                                  | "416"   ; Requested range not satisfiable

This name change should also be applied at three places in section 14.17
where this status code is mentioned.

(2) In section 14.36.1 Byte Ranges, change

            If the last-byte-pos value is present, it must be greater
            than or equal to the first-byte-pos in that byte-range-spec,
            or the byte-range-spec is invalid. The recipient of an
            invalid byte-range-spec must ignore it.

to

	    If the last-byte-pos value is present, it must be greater
	    than or equal to the first-byte-pos in that
	    byte-range-spec, or the byte-range-spec is syntactically
	    invalid.  The recipient of a byte-range-set that includes
	    one or more syntactically invalid byte-range-spec values
	    MUST ignore the header field that includes that
	    byte-range-set.

(3) Also in section 14.36.1 Byte Ranges, before the paragraph that
starts "Examples of byte-ranges-specifier values ...", insert this
paragraph:

	If a syntactically valid byte-range-set includes at least one
	byte-range-spec whose first-byte-pos is less than the current
	length of the entity-body, or at least one
	suffix-byte-range-spec with a non-zero suffix-length, then the
	byte-range-set is satisfiable.  Otherwise, the byte-range-set
	is unsatisfiable.  If the byte-range-set is unsatisfiable, the
	server SHOULD return a response with a status of 416 (Requested
	range not satisfiable).  Otherwise, the server SHOULD return a
	response with a status of 206 (Partial Content) containing the
	satisfiable ranges of the entity-body.

-Jeff
Received on Tuesday, 4 November 1997 13:53:29 EST

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