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Are IDNs allowed in http IRIs?

From: Adam M. Costello BOGUS address, see signature <BOGUS@BOGUS.nicemice.net>
Date: Fri, 19 Mar 2004 03:42:57 +0000
To: public-iri@w3.org, uri@w3.org
Message-ID: <20040319034257.GA13798~@nicemice.net>

Are IDNs allowed in http IRIs?  It seems like a silly question, but
currently the IRI draft tries to inherit the answer from the URI & HTTP
specs, and the URI draft tries to inherit the answer from the HTTP spec,
and of course the HTTP spec knows nothing of IDNs.  The result is that
the question has no clear answer.  Does it need a clear answer, and if
so, which document is the place to provide that answer?

Consider the purported IRI http://josť.example.net/.  Is that a valid
http IRI?  If so, what does it mean?

There are four places we might hope to find clues to answer those
questions:  The IRI spec, the URI spec, the HTTP spec, and the IDNA

Here's what the IRI spec has to say on the matter:

    IRIs are only valid if they map to syntactically valid URIs.

    the resource that the IRI locates is the same as the one located by
    the URI obtained after converting the IRI

According to the conversion procedure, http://josť.example.net/
may be converted to either http://jos%C3%A9.example.net/ or
http://xn--jos-dma.example.net/.  Maybe we'll be lucky and the answers
to our two questions will be the same for both URIs, or maybe we'll be
unlucky and the answers will be different for the two URIs, in which
case the answers for the IRI are indeterminate.

According to the HTTP and IDNA specs, http://xn--jos-dma.example.net/ is
valid and has a clear meaning, while http://jos%C3%A9.example.net/ is
invalid for two reasons.  First, it does not match the http URI grammar,
which explicitly uses the host component from RFC-2396, which does not
allow percent-escapes.  Second, the IDNA spec requires the ASCII form
(xn--jos-dma.example.net) in IDN-unaware domain name slots, and the host
component of an http URI is an IDN-unaware domain name slot (because
RFC-2616 predates RFC-3490).

The only way for the URI http://jos%C3%A9.example.net/ to become valid
and meaningful is for the HTTP URI spec to be updated, either by a
successor to RFC-2616, or by a sweeping action of the successor to
RFC-2396.  For example, 2396bis could say that it updates all schemes
that use domain names to use IDNs.  Such a sweeping update would of
course explicitly bypass the interoperability protections designed into
the IDNA architecture, and would let things break during the transition
(it amounts to "just send UTF-8"), but I suppose it's an option.

The current 2396bis draft does not make such an update.  It says:

    When a non-ASCII host name represents an internationalized domain
    name intended for resolution via DNS, the name must be transformed
    to the IDNA encoding [RFC3490] prior to name lookup.

It does not tell us when a non-ASCII host name represents an IDN, so
presumably that determination is left up to the scheme.  Existing
schemes like http don't use non-ASCII names to represent IDNs (the IDNA
spec makes that clear).

Getting back to the original questions regarding the purported IRI
http://josť.example.net/, we have not been able to determine whether it
is valid or what it means, because the answers change depending on which
conversion to to URI we happen to use, and the IRI spec blesses both

Of course the same indeterminacy exists for ftp://josť.example.net/ and
mailto:postmaster@josť.example.net etc.

Is this indeterminacy intended?

If not, I can think of two ways to rectify the situation (and maybe
other folks can think of other ways).  One way is for the URI spec to
update all URI schemes along these lines:

    All schemes that use data types that use an ASCII-Compatible
    Encoding (ACE) for internationalization are hereby updated to
    allow the use of equivalent non-ASCII forms, represented as
    percent-encoded UTF-8.  An example of such a data type is domain
    names, see [IDNA].

(Here I've generalized from IDNs to ACEs because there is a possibility
that email address local parts will also use an ACE.)

Alternatively, the URI spec could be left as-is, and the IRI spec could
redefine the validity of IRIs along these lines:

    An IRI is valid if it conforms to the syntax of the corresponding
    URI except for the following two additional freedoms:

       1) Wherever the URI might contain percent-encoded octets
       representing UTF-8, the IRI may instead use non-ASCII characters
       whose UTF-8 encoding is those same octets.

       2) Wherever the URI might contain a data type that uses an
       ASCII-Compatible Encoding (ACE) for internationalization, the IRI
       may instead use an equivalent non-ASCII form.  An example of such
       a data type is domain names, see [IDNA].

    When an IRI is converted to a URI, the conversion MAY use
    scheme-specific knowledge to convert appropriate components to ACE
    forms rather than percent-encoded UTF-8.  Lack of scheme-specific
    knowledge (or failure to use it) can cause valid IRIs to be
    converted to invalid URIs that contain percent-encoded non-ASCII
    text where only the ACE form would be permitted.  Such invalid URIs
    cannot in general be resolved directly, but can always be converted
    back into valid IRIs, which could then be reconverted to valid URIs
    by a scheme-specific converter.

Maybe it would be bothersome to bless a conversion procedure that can
produce invalid output.  If not, disregard the rest of this message.
Otherwise, the possibility of invalid output could be avoided by
creating a new "scheme name registration tree" (BCP-35).  For example,
the "i" tree could be defined as follows:  For any scheme foo, the
scheme i-foo is just like foo except for the following additional

    Wherever the foo: URI might contain a data type that uses an
    ASCII-Compatible Encoding (ACE) for internationalization, the
    i-foo: URI may instead use an equivalent non-ASCII form, represented
    as percent-encoded UTF-8.  An example of such a data type is domain
    names, see [IDNA].

A scheme-agnostic IRI-to-URI converter can avoid the possibility of
invalid output by prefixing i- to the front of the output.  When the URI
eventually makes its way to an agent with scheme-specific knowledge,
that agent can know whether the prefix can simply be discarded, or if
ACE conversions need to be performed first.

A URI-to-IRI converter can always discard the i- prefix, even without
recognizing the scheme.

Received on Thursday, 18 March 2004 22:43:00 UTC

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