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charset= history, proposal

From: Daniel W. Connolly <connolly@hal.com>
Date: Fri, 06 Jan 1995 17:47:47 -0600
Message-Id: <9501062347.AA11063@ulua.hal.com>
To: Bob Jung <bobj@mcom.com>
Cc: www-mling@square.ntt.jp, html-wg@oclc.org, http-wg%cuckoo.hpl.hp.com@hplb.hpl.hp.com
In message <199501062216.OAA00888@neon.mcom.com>, Bob Jung writes:
>In the HTML2 spec in  Section 2.4, sub-heading "Character sets"
>(http://www.ics.uci.edu/pub/ietf/html/html2/htmlspec281194_9.html#HEADING16),
>there is the statement that the charset parameter is reserved for future use:
>
>        Character sets
>                The charset parameter is reserved for future use. See Section
>                2.16 for a discussion of character sets and encodings in HTML.
>
>Does someone know the history behind this paragraph?

I suppose I do. If you could be more specific, I might be able to
give a better answer.

But the gist of it is: MIME (rfc1521) defines some semantics for
text/* charset=... which make a certain amount of sense for the web,
but aren't widely supported.

So the 2.0 can't say "do what MIME says" cuz then it wouldn't be
descriptive of current practice. So it says "don't use charset= at
all. Just use ISO-8859-1 implicitly all the time". That was the only
semantics we were going to standardize.

The theory was that we wanted to get the 2.0 document out in short
order, and that this issue could wait until later.

Now it's later, and this issue clearly needs addressing.

>How does this affect our discussions about using the charset parameter?

It means that we're now figuring out the "reserved for future use"
semantics.

Regarding your proposal <199501060510.VAA15861@neon.mcom.com>...

First: it looks good in general.

But I'd like to take a look at it from a few perspectives:

	(1) the language lawyer/formal specs perspective
	(2) the information provider perspective
	(3) the information consumer perspective

rom a formal perspective, I think whatever we come up with should be
consistent with MIME. But we'll have to see what that costs...

Let's take a close look at rfc1521 and see how it constrains the semantics
we want to define:

|7.1.1.     The charset parameter
|
|	[...]
|
|   The default character set, which
|   must be assumed in the absence of a charset parameter, is US-ASCII.

This conflicts somewhat with your proposal. However, the RFC goes on
to say...

|   The specification for any future subtypes of "text" must specify
|   whether or not they will also utilize a "charset" parameter, and may
|   possibly restrict its values as well.

I wonder if changing the default from "US-ASCII" to
"implementation-dependent" can be considered "restricting the values"
of the charset parameter.

I suppose the relevant scenario is where an info provider serves up
an ISO2022-JP document with a plain old:
	Content-Type: text/plain
header. I gather that this is current practice.

The intent of MIME is that a mail user agent, on seeing text/* with no
charset= parameter, can reasonably blast that document to any device
capable of handling US-ASCII.

That intent is already mucked up somewhat by the fact that normal html
documents are allowed to have bytes>127, which are normally
interpreted as per ISO8559-1. So we already have the situation where a
conforming HTTP client, say on a DOS box might retreive a text/html
document and pass it over to a conforming MIME user agent, which would
then blast it to the screen. The user would lose, cuz the bytes>127
would get all fouled up.

There was some talk about making the default content type for text/*
in HTTP be ISO8859-1. In that case, the conforming HTTP client should
slap on "charset=ISO8559-1" when it passes the data to the MIME user
agent, to make up for the impedence mismatch. Then the user wouldn't
lose, because the MIME user agent would be obliged to fix up the
bytes>127 as per ISO8559-1.

But... back to the case of ISO2022-JP encoded data tagged as plain
"text/html". The business of slapping "charset=ISO8559-1" on the end
would muck things up. So where do we assign fault?


My vote is to assign fault at the information provider for serving up
-JP encoded data without tagging it as such.

So all those information providers serving up ISO2022-JP data without
tagging it as such are violating the protocol. This doesn't prevent
NetScape and other vendors from hacking in some heuristic way to
handle such a protocol violation. But the spec shouldn't condone this
behaviour.


Ok... so now let's suppose all the information providers agree to
clean up their act. Somehow, they have to get their HTTP servers to
tag -JP documents as such.

How do they do this? File extension mappings? It's not relavent to the
HTML or HTTP specs, but I think the overall proposal is incomplete
until we have a workable proposal for how to enhance the major httpd
implementations to correctly label non-ISO8559-1 documents.


Then web clients will start seeing:

	Content-Type: text/html; charset="ISO2022-JP"

Many of them will balk at this and punt to "save to file?" mode.

Is that a bad thing? For standard NCSA, Mosaic 2.4, no, because
it can't do any reasonable rendering of these documents anyway.

But what about the multi-localized version of Mosaic? Does it handle
charset=... reasonably? What's the cost of enhancing it to do so and
deploying the enhanced version?

The proposal says that the server should not give the charset=
parameter unless the client advertises support for it. I think that
will cause more trouble than its worth (see the above scenario of
untagged -JP documents being passed from HTTP clients to MIME user
agents on a DOS box.)

One outstanding question is: does text/html include all charset=
variations or just latin1? That is, when a client says:

	Accept: text/html

is it implying acceptance of all variations of html, or just latin1?

To be precise, if a client only groks latin1, and it says accept:
text/html, and the server sends ISO2022-JP encoded text, and the user
loses, is the fault in the client for not supporting ISO2022-JP, or at
the server for giving something the client didn't ask for?

First, "text/html" is just shorthand for "text/html; charset=ISO8859-1"
so the client didn't advertise support for -JP data.

But "giving somethign the client didn't ask for" is _not_ an HTTP
protocol viloation (at least not if you ask me; the ink still isn't
dry on the HTTP 1.0 RFC though...). It's something that the client
should be prepared for.

As above, the server is bound to give "charset=ISO2022-JP" if it is
not returning latin1 data. So the client does know that it's not
getting latin1 data. It has the responsibility to interpret the
charset correctly, or save the data to a file or report "sorry, I
don't grok this data" to the user. If it blindly blasts ISO2022-JP
tagged data to an ASCII/Latin1 context, then it's broken.


Does this mean that charset negociation is completely unnecessary?
No. It's not necessary in any of the above scenarios, but it would be
necessary in the case where information can be provided in, for
example, unicode UCS-2, UTF-8, UTF-7, or ISO2022-JP, but the client
only groks UTF-8.

In that case, something like:

	Accept-Charset: ISO8859-1, ISO2022-JP

or perhaps

	Accept-Parameter: charset=ISO8859-1, charset=ISO2022-JP

I'm not convinced of the need for the generality of the latter syntax.
Besides: we ought to allow preferences to be specified ala:

	Accept-Charset: ISO8859-1; q=1
	Accept-Charset: Unicode-UCS-2; q=1
	Accept-Charset: Unicode-UTF-8; q=0.5
	Accept-Charset: Unicode-UTF-7; q=0.4
	Accept-Charset: ISO2022-JP; q=0.2

which says "if you've got latin1 or UCS2, I like that just fine. If
you have UTF-8, UTF-7, or -JP, I'll take it, but I won't like it as
much."

I'm still not sure this is exactly the right syntax: it's does allow
you to sayt that you'll take text/plain in several different charsets,
but text/html in only one, for example.

Of course the bandwidth necessary to express the common cases should
be minimized...


Daniel W. Connolly        "We believe in the interconnectedness of all things"
Software Engineer, Hal Software Systems, OLIAS project   (512) 834-9962 x5010
<connolly@hal.com>                             http://www.hal.com/%7Econnolly
Received on Friday, 6 January 1995 16:02:58 EST

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