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From: Martin J. Duerst <mduerst@ifi.unizh.ch>
Date: Fri, 18 Jul 1997 15:40:27 +0200 (MET DST)
To: Maurizio Codogno <mau@beatles.cselt.stet.it>
Cc: Larry Masinter <masinter@parc.xerox.com>, http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
Message-Id: <Pine.SUN.3.96.970717115711.245O-100000@enoshima>
X-Mailing-List: <http-wg@cuckoo.hpl.hp.com> archive/latest/3805
I have been one of those originally discussing this issue,
and I think I should take it up again after it has appeared
on Larry's list.

The question is how the Accept-Language field is matched with
the language information of the documents on the server. In
particular, the question is what should be done in terms of
prefix matching.

Language tags, as to RFC 1766, are strings separated into
components with "-". RFC 1766 does not look at the individual
components, but it is in general very convenient to consider
the components and to do prefix matching.

To give an examlpe, the current situation is:

% Accept-Language      Document        Match?
% en                   en              YES
% en-us                en-us           YES
% en                   en-us           YES
% en-us                en              NO????
% en-us                en-uk           NO!!!!

The NO!!!! in the last case is there because even though it
would make sense to return an en-uk (GB English) document
in request for en-us (US English) [of course if en-uk as
such is not available], this never applies in general.
For example, there could be x-klingon and x-martian, without
any mutual intellegibility. There is zh-cn and zh-tw (mainland
Chinese Chinese and Taiwanese Chinese, usually standing for
simplified and traditional style Chinese), where some people
wouldn't care getting either of these, but others would have
difficulties understanding one or the other. Even for "en",
we could have something like en-ebonics, not easily intellegible
for an average "en" reader.

Once it is understood that we cannot match aa-bb with aa-cc,
the question is whether we can do anything to improve the
situation for getting a match between en-us and en, as marked
with NO???? above. In an earlier mail, I proposed to solve
this by just changing the spec to saying that this case
matches. I found out in the meantime that this has some
subtle undesired consequences. The story goes as follows:

We want to have a way to get en-uk back to an en-us reader
(for this and similar cases, not for the general case).
With the current spec, we know that it's the user side's
responsibility to do something about it, i.e. to have
	Accept-Language: en-us, en
maybe with the necessary q values.

Now if we add to the spec that en-us matches an en document,
it could as well be the responsibility of the server side
to tag the document both as en-uk and en, in order to get
it send back on an en-us request. We don't know anymore
who is responsible, which means that either both will do
it (to make sure it works) or nobody will do it (because
they hope for the other side). This is rather undesired :-(.

The current solution is better in that even if it doesn't
do as much as possible automatically, it assigns clear

The question is whether the responsibilities are on the
right side. In one respect, they are, because it's the
reader who ultimately knows what she can (or wants to)
read and what not. However, in another respect, it's on
the wrong side, because the end user is not aware that
she is responsible, or how she should take up her
responsibility. As a consequence, she might have set up
language preferences as en-us only, and then be very
annoyed when she gets back a variant list saying:

	The document is not available in US English
	as you requested, but it is available in the
	following languages:


	Please click on the language in which you prefer
	to receive the document.

To change the spec and put the responsibility on the server
side might have some advantages (server side generally has
a little more expertise than user side), but also has
disadvantages (because it assumes the server side knows what
users with various prefeneces do understand and what they
don't, which is difficult e.g. in the zh (Chinese) case).
Its biggest disadvantage is of course that we would have
to turn the spec upside down.

To avoid the annoying case above, the best thing that can
be done is that browser implementors help users to get
their choices right. For example, after a user selects
en-us and fr-ca and hits OK, a little dialog could come
up and ask:

	You selected US English, but not general English.
	In order for the browser to obtain all documents
	readable to you, we suggest to add general English.
	Should we do that for you?	[YES] [NO] [CANCEL]

Another question that remains to me is that currently, the
spec assumes that each document has assigned exactly one
language-tag, and that there either is a match or there
is none. On sophisticated servers with database background
and so on, this could look vastly different. How is the
spec supposed to be applied in such a case?

I therefore propose the following:

- Leave the matching mechanism in the spec as is.
- Add some comments to help avoid situations that are
	really annoying to end users.

The current text is:

> 14.4 Accept-Language
>    The Accept-Language request-header field is similar to Accept, but
>    restricts the set of natural languages that are preferred as a
>    response to the request.
>           Accept-Language = "Accept-Language" ":"
>                             1#( language-range [ ";" "q" "=" qvalue ] )
>           language-range  = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" )
>    Each language-range MAY be given an associated quality value which
>    represents an estimate of the user's preference for the languages
>    specified by that range. The quality value defaults to "q=1". For
>    example,
>           Accept-Language: da, en-gb;q=0.8, en;q=0.7
>    would mean: "I prefer Danish, but will accept British English and
>    other types of English." A language-range matches a language-tag if
>    it exactly equals the tag, or if it exactly equals a prefix of the
>    tag such that the first tag character following the prefix is "-".
>    The special range "*", if present in the Accept-Language field,
>    matches every tag not matched by any other range present in the
>    Accept-Language field.
>      Note: This use of a prefix matching rule does not imply that
>      language tags are assigned to languages in such a way that it is
>      always true that if a user understands a language with a certain
>      tag, then this user will also understand all languages with tags
>      for which this tag is a prefix. The prefix rule simply allows the
>      use of prefix tags if this is the case.
>    The language quality factor assigned to a language-tag by the
>    Accept-Language field is the quality value of the longest language-
>    range in the field that matches the language-tag. If no language-
>    range in the field matches the tag, the language quality factor
>    assigned is 0. If no Accept-Language header is present in the
>    request, the server SHOULD assume that all languages are equally
>    acceptable. If an Accept-Language header is present, then all
>    languages which are assigned a quality factor greater than 0 are
>    acceptable.
>    It may be contrary to the privacy expectations of the user to send an
>    Accept-Language header with the complete linguistic preferences of
>    the user in every request. For a discussion of this issue, see
>    section 15.7.
>      Note: As intelligibility is highly dependent on the individual
>      user, it is recommended that client applications make the choice of
>      linguistic preference available to the user. If the choice is not
>      made available, then the Accept-Language header field must not be
>      given in the request.

I propose to add another note at the end of Section 14.4:

Note: When making the choice of linguistic preference available to
the user, implementors should take into account the fact that users
are not familliar with the details of language matching as described
above, and should provide appropriate guidance. As an examlpe, users
may assume that on selecting "en-gb", they will be served any kind
of English document if British English is not available. A user
agent may suggest in such a case to add "en" to get the best
matching behaviour.

I hope that giving the proposed addition in the form above is
sufficient. Please inform me of whatever other action that
might be necessary to move this ISSUE forward.

Regards,	Martin.
Received on Friday, 18 July 1997 06:57:29 UTC

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