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Re: Problems with draft-ietf-http-v11-spec-07

From: Roy T. Fielding <fielding@liege.ICS.UCI.EDU>
Date: Sat, 17 Aug 1996 22:03:35 -0700
To: Klaus Weide <kweide@tezcat.com>
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
Message-Id: <9608172203.aa23227@paris.ics.uci.edu>
X-Mailing-List: <http-wg@cuckoo.hpl.hp.com> archive/latest/1393
> I have to disagree that Section 2.2 (or the draft as a whole) is clear
> about character sets.  What is included in parentheses in the quote 
> above may be the intention, but it is not explicit.

The question of character sets/encoding is one that has been embroiled
in controversy for every single draft that has gone through the applications
area in the past three years.  For that reason, Henrik and I chose to
be inclusive of character set possibilities wherever it was safe
(interoperable) to do so, and not add any explicit requirements that
would prevent future use of things like UTF-8 for some key areas of the
protocol.  This was all done with the understanding that HTTP is an
8-bit clean protocol (but all protocol features do have 7-bit alternatives).

> I tried to answer for myself the question "Where, if at all, does
> HTTP 1.1 allow non-US-ASCII characters (other than within message-
> body)?", according to the latest draft.  I ran into several problems
> with figuring out the answer.
> First, I would prefer if the answer to that question was clearly
> and explicitly stated somewhere in the draft.  As it is now, one
> has to work one's way through several layers of BNF.

Yes.  My only defense for this is simply that we didn't want to answer
that question when we didn't need to.  That was a political choice, not
a technical one.

> On to the details:  Unencoded non-US-ASCII octets (octets with the
> most significant bit set, simply called eightbit chars in the following)
> come in to the BNF in two ways:
> 2.2
>        OCTET          = <any 8-bit sequence of data>
> whence TEXT, comment and ctext, quoted-string and qdstring
> are all allowed to have eightbit chars.  (but not quoted-pair.)
> Note that this allows eightbit chars in lots of places, for example
> Etags or MIME parameters (including boundaries).

Yep -- the keyword being "allows".

> 3.2 Uniform Resource Identifiers
> 3.2.1
>        national       = <any OCTET excluding ALPHA, DIGIT,
>                         reserved, extra, safe, and unsafe>
> whence unreserved, uchar and pchar, and therefore nearly all parts
> of a URI (apart from the scheme) are allowed to have eightbit chars.

Yep, as explained below.

> 3.2.1:
>     "The BNF above includes national characters not
> allowed in valid URLs as specified by RFC 1738, since HTTP servers are
> not restricted in the set of unreserved characters allowed to represent
> the rel_path part of addresses, and HTTP proxies may receive requests
> for URIs not defined by RFC 1738."
> I read that as meaning that HTTP application which handle such URIs
> with eightbit chars can conform to the HTTP/1.1 spec, even if those
> URIs don't conform to RFC 1738.

That is correct.  RFC 1738 did not define the syntax for URNs and there
is no reason for HTTP to restrict URI's to the URL syntax exclusively.

> Although the quoted sentence doesn not explicitly speak of generating
> such URIs, there is nothing forbidding it.  (And it seems logical
> that a server accepting requests for such URI's should also be allowed
> to generate Location: headers etc. containing them.)

The spec doesn't say that.  Wherever possible, it leaves the issue of
naming resources in the hands of the origin server.

> On the other hand, in
> 4.2 Message Headers
>        message-header = field-name ":" [ field-value ] CRLF
>        field-name     = token
>        field-value    = *( field-content | LWS )
>        field-content = <the OCTETs making up the field-value
>                         and consisting of either *TEXT or combinations
>                         of token, tspecials, and quoted-string>
> Note that it doesn't say
>        field-content = <the OCTETs making up the field-value
>                         and consisting of either *TEXT or combinations
>                         of token, tspecials, URI, and quoted-string>
> nor does it say
>        field-content = <the OCTETs making up the field-value
>                         and consisting of either *TEXT or combinations
>                         of token, tspecials, quoted-string etc.>
> From this I conclude that
> (a)  an URI in a field-content (which is not within a quoted-string),
>      since it is not defined as arbitrary *TEXT, has to be understood 
>      as being comprised of component tokens, and
> (b)  (since eightbit chars are not allowed in tokens) an URI in a
>      field-content cannot contain unencoded eightbit chars.

Nope -- that is an invalid extrapolation.  The generic syntax for
parsing header fields considers a URI to be *TEXT.  This has no affect
on specific field definitions, because those specific definitions add their
own requirements to the interpretation of the field-content of a specific
field *after* it has been extracted by the message parser.

In practice, you can put 8-bit text in any of the locations where the
spec allows it -- current HTTP/1.0 applications work that way.

> But the BNF for specific headers uses rules which seem to allow
> eightbit chars, for example
> 14.30
>        Location       = "Location" ":" absoluteURI
> I conclude that the draft is far from clear.

Well, look at how the BNF defines messages -- the only fields that are
defined in terms of <field-content> are the extension fields (those not
defined by the specification itself).

Saying that the draft is "far from clear" is not useful.  Do you have
specific wording that could be added, and where it should be added, which
would help clarify the issue without harming the protocol's extensibility?

> Some other (mostly BNF related) weirdnesses:
> SInce URI is comprised of tokens (see (a) above), the following
> seems to apply:

I'll pass on the rest, since the assumption is false.

> Comments (within parentheses) should probably allowed in more
> places - at least, in 19.4.7
>        MIME-Version   = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT
> should probably be
>        MIME-Version   = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT *comment

Why? A comment in that location serves no useful purpose.

> Also,
>       Via =  "Via" ":" 1#( received-protocol received-by [ comment ] )
> in 14.44 should maybe become
>       Via =  "Via" ":" 1#( received-protocol received-by [ *comment ] )

Comments can be nested and we did not wish to encourage multiple
comments in a protocol that is normally only machine-read (unlike mail).

> 10.3.6 305 Use Proxy
> The requested resource MUST be accessed through the proxy given by the
> Location field. The Location field gives the URL of the proxy. The
> recipient is expected to repeat the request via the proxy.
> How exactly does is a proxy "given" by a Location field?
> Location normally contains an URI, and URIs point to resources but
> not (normally) applications (the proxy).  Does the URI have to be
> a http_URL, does the abs_path have to be empty (or is it required
> to be "/"), and what if not?

On the contrary, proxies are normally identified by URL.  The URL
does not need to be an http URL (though it would be in current practice)
and the interpretation of the path (if any) would be dependent on
the method of proxying (http would not use any path).

Please keep in mind that the spec does not prevent people from doing
things that won't work -- it doesn't have to.

> 3.6 Transfer Codings
> ...
>        hex-no-zero    = <HEX excluding "0">
>        chunk-size     = hex-no-zero *HEX
> ...
>        chunk-data     = chunk-size(OCTET)
> Why does this rule use HEX and not DIGIT?  Does this mean the
> chunk-size is hexadecimally encoded?

Yes. It should have said that in the text as well, but fails to.

>                           *   *   *
> A remark regarding 14.1 Accept:
> It's a pity there is a "q=", but not a "mxb=".  An oversight or
> intentional?

The conneg group decided it "wasn't needed" based on the observation
that browsers didn't implement it.  Koen is wrong in that the presence
of Range does absolutely nothing to replace the functionality of mxb.
The only problem with mxb is that it adds complexity to the process
of configuring a browser and there is no convenient way to adjust the
maximum based on the purpose of an individual request.  Given the
lack of enthusiasm about Accept, and the growing complexity of content
negotiation in general, there was not enough reason to restore it to
the specification once it was removed.

 ...Roy T. Fielding
    Department of Information & Computer Science    (fielding@ics.uci.edu)
    University of California, Irvine, CA 92697-3425    fax:+1(714)824-4056
Received on Saturday, 17 August 1996 22:15:56 UTC

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