RE: CHARSET considerations

[John C Klensin <[e-mail]>]

Rick, et al.,

I've been pretty quiet on this one.  Frankly, I've been hoping that it
would burn itself out.  But it doesn't seem to be doing that.  So let's
go back and review a couple of things.   I'm posting this to the 822
list, since most of the issues are really MIME ones, not character set
ones, and to the charset list because that is where you posted your
note.  Everyone else has been removed.  People who are already throughly
sick of this discussion should stop reading here. 

(1) MIME, like every single other Internet protocol at or near the
applications layer, describes "on the wire" behavior.  Neither it, nor
any of the others, describes what should be one on a particular machine
once things arrive there beyond hints about what other systems might
expect vis-a-vis things that are going back out on the wire.  The way
you _represent_ a MIME message on your host and in your UA <->user and
UA <-> MTA communications is not the subject of Internet protocols, even
though ease of use with those protocols may influence whether some
decisions are as smart, or smarter, than others.
   In particular, once something gets to your machine, you can dissect
the headers and store them separately in some unique-to-you canonical
form and report whatever you please to your users.  You can also store
the characters in 17-bit reverse-slobbovian notation for all anyone
cares.  The only requirement--of MIME or anything else--is that you be
able to reconstruct network-canonical form (e.g., MIME) if you send the
stuff back out.

(2) MIME defines text/plain in the absense of an explicit charset
indicator to be identical to "text/plain; charset=ascii".  This didn't
happen by accident: it was discussed at tedious and exhaustive length. 
It turned out to be the only option: any other model would have caused a
violation of RFC822 with potential information-loss.  There was fairly
general agreement in the WG that breaking RFC822-conforming
implementations was a terrible idea.
   To claim that it is better to write text/plain without a charset
parameter than with one is justified under only two circumstances that I
can think of:
    -- you intend to violate the protocols and hope that, if the charset
parameter isn't listed, no one will notice.
    -- you have gotten confused, not about a "charset" versus
"character_set" issue, but about the difference between an on-the-wire
mail format protocol specification and a user-level notion of reality on
a particular terminal at a particular time.

If looking at "charset=us-ascii" is offensive to you, take it out at
display time in your UA, just as many of us hide Received lines unless/
until we need them for something.  Whether you just hide that string, or
replace it with "charset=Plutoian-666" is really not an issue for the
network.

(3)  For similar reasons, ISO-8859-1 can't be unqualified text/plain on
the wire; it would violate RFC822 and unextended RFC821.  And, just
incidentally, it would trade an "English exclusive" mail environment for
a "Western European Latin-based language" primary mail environment. 
That wasn't considered to be enough gain to be worth the trouble.  Maybe
that was a wrong decision, but I don't think so.   Note that trading
"ISO-8859-1" (an assertion about coding of characters on the wire) for
"Latin-1" (an abstraction about a particular character repertoire)
doesn't help with this problem--one is as Western European as the
other-- it just costs you knowledge of the coding on the wire.

(4) There is another major piece of confusion associated with the
assumption that BITNET MAIL is RFC822-conforming.  It isn't now.  It
never has been.   RFC822-Mail over NJE is an oxymoron.   RFC822 messages
are in ASCII.  It is useful sometimes to pretend that there is a virtual
document, let's call it B-822, that defines the format of BITNET mail. 
It contains two lines.  One says "On BITNET, we use EBCDIC where RFC822
uses ASCII" and the second one says "except for the changes implied by
line 1, RFC822 is incorporated here by reference".  Now BITNET seems to
be (gradually and without complete agreement) issuing B-822bis, which
changes "EBCDIC" in that first line to "EBCDIC CodePage 1047".  B-822bis
still isn't RFC822.

And, since the MIME of RFC1341bis/RFC1342bis requires RFC822, BITNET
isn't using it unless all of the headers (822 and body-part) are in
ASCII.  That is a firm requirement, no amount of fussing with "charset"
changes it.

So, there is a very interesting question about what the gateways should
do and what near-MIME should look like in a near-RFC822 environment. 
But it isn't really a MIME question, much less a charset question.  See
item 6, below.

(5) Now, there is something that could have been done back when the WG
was making its decisions that would have made the work of the gateways
between RFC822-based mail and B-822-based mail much easier, although it
would have required changes in those gateways.   We could have decided
that this was a transport problem, that we should extend SMTP to
announce, in some fashion, "here comes the MIME" or "here comes the
charset XXX".  If the receiving SMTP in this situation didn't accept
that assertion, the mail would bounce.  But it would be immediately
clear which gateways were able to cope in a useful way, and which ones
couldn't.  And it would make it relatively easy to establish a
B-1341bis/B-1342bis that used different "charset" conventions if that
was appropriate.   The cost would have been that all of the MTAs would
have had to be changed to implement MIME.

Those who have been putting up with this long enough will recall that I
argued fairly strongly for transport involvement in this process.  The
problems, and semantic unpleasantnesses, that Rick is concerned about
now were most of my motivation then.   Given the engineering tradeoffs
between making things easier for some gateways and a little safe overall
versus speed of deployment, I was probably wrong.  I was certainly
soundly outvoted.

(6) Ok, now what should be done with "MIME" over B-822?  There are a
couple of realistic possibilities.  There are also a bunch of
unrealistic ones that depend, to a greater or lesser degree, on getting
agreement from every gateway and every host that sits on an
Internet/EBCDIC boundary to either bounce MIME messages or handle them
in the same specific way.  I don't think those are worth discussing now,
although maybe it is too bad that we didn't try to deploy MIME when the
number of gateways and hosts in this situation was closer to three than
a few hundred.

  (i) You can let the gateways keep blindly converting everything that
they get from the Internet into EBCDIC, as they are doing now.  If you
do this, you need to extend the first sentence of B-822 to say that
anytime "us-ascii" is seen in a message, it means "ebcdic".  In this
scenario, you don't teach the gateways to accept 8bit traffic.  If that
means you end up with a lot of quoted-printable stuff and base64 stuff,
that is just the way it goes.  Fix it in the receiving UAs if they
recognize the MIME value of the "charset" parameter.  If not, life is no
worse for them than for real MIME UAs.

  (ii) You can do the above, but let the gateways access 8bit traffic. 
This means that they have to be a little more activist about potential
conversions.  They might want to convert ISO-8859-1 to an appropriate
Latin-1-oriented EBCDIC.  I strongly recommend that they identify that
they have done this with a new header that they filter on the way in and
out.  That header has to do with which EBCDIC is being used, presumably
with CodePage 1047 as the default.  If you don't do that, you are going
to be in deep trouble when Latin-2 (much less ideographic character sets
and non-Latin-based ones) arrives.  Remember that there are EARN sites
in Poland and Hungary and Russia and Turkey.  It won't take long and
then BITNET will have an EBCDIC-labelling problem even if there were no
MIME.

   (iii) You can teach the smarter gateways to put a new verb into the
BMSTP for sites that want it that identifies unconverted MIME and then
pass the message exactly as received by the gateway (e.g., no character
conversion).  This would move the conversion problem away from the
gateways and to the delivery MTAs and UAs, which may be able to do
smarter things for their users.  This would work well for smart gateways
talking to smart sites.  It might be rational to assume that the others
fall under (i) above, i.e., they are on their own and they are going to
see some things encoded that don't strictly need to be.  As the saying
goes, life is hard sometimes.

(7) There are a couple of other things in your note that are invitations
to interoperability problems.  For example...
>Specifically,  I feel
>(quite strongly)  that the user should be able to specify any old
>charset and have display at least attempted at the other end.
    I feel (quite strongly) that this falls between "looking for
trouble" and a potential real disaster.   Suppose you receive
"text/plain; charset=Mickey-Mouse".  What is the recipient going to try
to display?  We made the mistake with RFC822 of permitting people to put
anything in, subject only to the rule that we might later come along and
assign some semantics to it.  That means that message structures that
had a specific meaning a half-dozen years ago now may have a different
meaning, with no real clues to anyone.  Bad mistake.  If a user wants to
use "any old charset", use an "X-" as a warning that there had better be
a private agreement between sender and receiver.  If the "X-" is
offensive, let it be defined and registered so that there is some hope
that "attempting display" will get it right.

Similarly,

>If you specify  "Latin-1",  then you can  (must;  I'm arguing for a
>definition here,  not an explanation)  assume that  SMTP  will carry it
>as ISO-8859-1,  BUT THE RECEIVING  (or sending)  HOST MIGHT NOT.
>(and yes,  sad but true,  any SMTPs will strip the high bit)
  If you specify "Latin-1", then you cannot make any assumptions at all
about how SMTP will carry it.  "Latin-1" is an ambiguous reference, one
of whose instantiations of ISO 8859-1.  If you specify "iso-8859-1",
together with a content-transfer-encoding, you know exactly what is
being carried on the wire.  In neither case is any assertion made about
what the receiving host might do with it.  That is a MTA-delivery-UA
problem, not a MIME/822/821/etc one.

And
>That's why I ask that
>(today,  1993)  we  NOT LABEL  true plain text as  US-ASCII/ISO-8859-1.
>Just leave it alone and let it default at the receiving end.
  Get into your time machine.  Go back to 1981 or 82.  Make this case
then.  In "today, 1981" we labelled true plain text as ANS X3.4 ASCII. 
We put the label in the protocol document, not in the mail messages, but
the specification is clear, was clear, and will still be clear when your
time machine gets back to 1993.

>(nor the MIME developers;  not mad at anyone,  just trying to push a
>point that I think is important and has been missed).

  For whatever my assurances are worth, it hasn't been missed.  The
questions and options were examined very carefully; nothing above is
really new.   What is missing, I fear, is an understanding of what has
been going on with email for the last decade.

  There is a dirty little secret that many of us have known, or feared,
for many years and that, for better or worse, MIME and the SMTP
extensions have brought to the foreground.  The mail environment has
worked for the last decade not because of a high level of conformance or
understanding of subtle aspects of the protocols but because of a very
high level of robustness and tolerance for nonsense.  Protocol changes
and extensions almost inevitably raise the conformance threshold and the
understanding threshold along with it.  And we are shaking out a lot of
beliefs that are handy but not quite true, e.g., the one that assumes
that BITNET uses RFC822 on NJE-wires.  It has never been true, but
believing it up until recently hasn't been harmful.  Well, it is getting
harmful and that is one of the prices we pay for MIME.

    john