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Comments on `Common User Agent Problems' document

From: Koen Holtman <koen@hep.caltech.edu>
Date: Thu, 15 Feb 2001 22:39:42 -0800 (PST)
To: www-talk@w3.org
cc: Koen Holtman <koen@hep.caltech.edu>, http-wg@cuckoo.hpl.hp.com
Message-ID: <Pine.A41.4.10.10102152237480.33242-100000@hep505.cithep.caltech.edu>

First off, thanks to the authors and the W3C for writing documents
like this!

I have some comments on the current version of
http://www.w3.org/TR/2001/NOTE-cuap-20010206 , mainly about the HTTP and  
negotiation parts.  Quoted text below is from this current version of the 

>1.11 Allow the user to bookmark negotiated resources.

This text makes content negotiation in HTTP sound straightforward,
which it is not.  Some more info for browser implementers who want to
support this would be nice.

The text fails to mention that bookmarking a particular version of a
negotiated resource is not always possible under HTTP semantics, because     
a) the particular version may not have its own URI and b) even if it does,
HTTP does not guarantee that the user agent will be informed of this.

HTTP/1.1 defines the Content-Location header field as the way for the
server to indicate the URI of the variant, and some servers do supply
this Content-Location when negotiation took place most of the time.
However, Content-Location is also used for some other things, and its
inclusion in a response does not necessarily mean that content
negotiation took place.  Under plain HTTP/1.x the user agent will have
to use heuristics to guess if negotiation took place.  RFC 2295
defines the `TCN' and `Alternates' header fields, the inclusion of
these in a response is a sure sign that content negotiation did take

RFC 2295 also specifies (section 11.2) that if a browser knows both
the generic URI and the variant URI, the defaults for showing the URI
and bookmarking the URI should be that the generic, not the variant,
URI is used.

>1.12 Allow the user to choose among supported transfer encodings. 

I think that what you _want_ to say here is that user agents should
try to support as many time-saving transfer encoding mechanisms
(compression, delta-encoding) as possible, and send out TE headers
announcing their support.

User control over this is unnecessary.  The HTTP/1.1 transfer encoding
negotiation mechanism was designed to avoid the need for the end user
to get involved.  Using the HTTP protocol, the server, proxy, and
client implementations among themselves will be able to choose and use
the most efficient transfer encoding.  Some power users might have
enough knowledge to fine-tune this process beyond what can be done
automatically, if their browsers provide an interface for this, but
that would be a very small group of users indeed.

>1.13 Use the user interface language as the default value for language

>In case the user does not specify any language, the user agent may use
>the language of its user interface as the value sent out.

This is wrong: a user agent should never automatically configure
itself to send out a single language value without asking for user
approval, because in HTTP semantics sending a single language value
only may very easily block convenient access to content in all other

Case in point: On the Apache mailing list someone recently reported
seeing problems due to user agents being preconfigured to send out

 Accept-language: xy

(for some language xy).  In requests for a page with many language
variants, but not including xy, this (currently) causes Apache to drop
into a `pick a language' 406 error response.  This is the best option
under HTTP semantics but it confuses the user, who is not aware that
the user agent is preconfigured to send out this very restricted
accept-language, who will therefore not have a clue on how to correct
this, and was just expecting to see the English variant if xy were not

So, I propose that the document be changed to read:

In case the user does not specify any language, the user agent may
specify the language of its user interface as the preferred language,
while allowing other languages with a lower preference, for example by

 Accept-Language: dk, *;q=0.5

>3.4 Do not treat HTTP temporary redirects as permanent redirects. 

>The user should be able to bookmark, copy, or link to the original
>(persistent) URI or the result of a temporary redirect.
>Wrong: User agents usually show the user (in the user interface) the
>URI that is the result of a temporary (302 or 307) redirect, as they
>would do for a permanent (301) redirect.

I do not like this advice to depart from current practice in showing
the target URI of a 302 redirect.  Changing current practice creates
usability problems for existing web content.  Here is why: server-side
image maps, the maps with HTML like this:

 <A HREF="http://x.com/cgi-bin/imagemap/worldmap">
 <IMG SRC="worldmap.gif" ISMAP></A>

generate URLs like
http://your.server.name/cgi-bin/imagemap/worldmap?444,33 when clicked
(if I recall the syntax correctly).  With an image map the server side
there is a CGI script (or other component) that takes the coordinates
and translates them to the correct URL (say http://x.com/canada/)
corresponding to the part of the image clicked.  In all
implementations I have ever seen, the script uses a *302 code*
redirect (usually created through the Location: mechanism in the CGI
interface) to direct the user agent to http://x.com/canada/!!!
Following your proposal would mean that the ?444,33 URL, not the
Canada URL, is shown and bookmarked, whereas the Canada URL is both
more informative and generally more permanent!

So at least I would like the document to make an exception to the
above rule for 302 redirection where imagemaps are the source.  But
actually I'd rather see the whole rule dropped and current common
behavior declared correct.  Based on experience with clarifying and
evolving the redirection codes in the http-wg, I take the view that a
departure from existing practice should only be done by adding fresh
codes, not by `fixing' the semantics of existing ones.

>3.6 List only supported media types in an HTTP Accept header. 

See also my comments to 1.13.  There is a similar problem here: in
non-inline image requests the user agent should generally *not* block
the receipt of unsupported types by specifying an overly restrictive
Accept header.

According to HTTP semantics, if the user agent is willing to process
any type (e.g. by saving it, see your point 1.3) then the Accept
header should either be omitted entirely or include */*;q=X for some
low value X.  Omitting this */*;q=X if you are willing to save to disk
as last resort is simply bad HTTP, though most server implementations
will usually let you get away with it.

In Apache the negotiation module has enough workarounds for bad Accept
headers already, so I would not like to to see new classes of bad
Accept headers introduced.  Point 3.6 should be rewritten to talk
about inline image requests only, or extended to also cover the
necessary subtleties in non-image requests.

Received on Friday, 16 February 2001 06:41:07 UTC

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