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Negotiation in draft 01

From: Jim Seidman <jim@spyglass.com>
Date: Tue, 8 Aug 95 15:20:52 -0500
Message-Id: <9508082020.AA09528@hook.spyglass.com>
To: Roy Fielding <fielding@beach.w3.org>
Cc: http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
I have some concerns with how negotiation works in draft 01 as compared to
draft 00.  Before I list those, when were all of these changes discussed on
the mailing list?  I looked through all of the archives and couldn't find
any discussion of this.  (Nor could I find the minutes from the Danvers
IETF, but even if it was discussed there, it still should have been on the
mailing list.)

Anyway, here are my concerns:

1.  The effect of a request not having an Accept-Encoding or Accept-Charset
has flipped completely from 00 to 01.  This seems to make the doubtful
assumption that current implementations of clients which don't produce these
headers can accept any encoding or character set.

2.  All of the Accept-* headers were defined in 00 as requiring at least one
item.  In 01 they can have 0.  While RFC822 allows this, under 00 every
HTTP-header had a field-body.  I wouldn't be surprised if some header
parsers choke on that.  Requiring empty "Accept-Encoding:" and
"Accept-Charset:" strings to describe the most common case for a client
could break many existing servers.

3.  The spec allows you to specify "Accept: " but doesn't say what the
effect is.  My reading seems to indicate that this means no MIME type is
but it's somewhat ambiguous.

4.  The Accept example in 8.1 shows a "text/html;version=2.0" and a
"text/html;level=3".  Section 3.4 does not specify a list of parameters.
Was the use of both "version" and "level" a deliberate attempt to show that
any parameter name is valid?  It's a little scary to me that someone could
create an arbitrary parameter name and expect the server to parse it.
(Also, in this case we need to specify a list of reserved attributes, like
q, ql, mxb, etc.)  We also need to specify what "more specific" 

5.  If qe and qc default to 0.001 instead of 0, do we provide a client with
any way at all to say that it doesn't want encodings and character sets it
can't handle?  If a web-searching robot, for example, says that it can't
handle compressed files then it probably really means that, and would
probably prefer a "406 none acceptable" to something it can't receive.  Yet
under 01 there's no way for the robot to avoid having such content sent to it.

6.  The URI description in 8.28 still doesn't address the issue I brought up
back in June.  Namely, how would a cache practically use the information
presented in the URI as described?  Since the URI field doesn't have to
enumerate all of the variants that are available, it doesn't help to know
what varies unless the next request has an identical entity header for that

I bring this up because the 8.28 says "When the caching proxy gets a request
for that URI, it must forward the request toward the origin server if the
request profile includes a variant dimension that has not already been
cached."  In practice, as currently speced, the request must be forwarded
unless a requset with an identical profile has been made.

As an example, suppose that someone sends this request to a proxy server:

GET http://www.bar.com/foo HTTP/1.0
Accept: image/gif;q=0.5, image/jpeg

The proxy server sends it along to www.bar.com, which returns

200 OK
Content-type: image/jpeg
URI: <foo>; vary="type"

If someone then comes along and requests of the proxy server:

GET http://www.bar.com/foo HTTP/1.0
Accept: image/gif;q=0.8, image/jpeg

The proxy server needs to send this along to www.bar.com as well, since it
doesn't know whether there's a gif version with a higher qs than the jpeg
version which might be optimal for this request.  Similarly, if someone

GET http://www.bar.com/foo HTTP/1.0
Accept: image/jpeg, image/xbm

the proxy won't know if an xbm version is available.

Jim Seidman, Senior Software Engineer
Spyglass Inc., 1230 E. Diehl Road, Naperville IL 60563
Received on Tuesday, 8 August 1995 13:24:34 EDT

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