W3C home > Mailing lists > Public > public-iri@w3.org > May 2010

Re: Special ordering for BIDI URLs

From: Aharon (Vladimir) Lanin <aharon@google.com>
Date: Tue, 25 May 2010 14:14:01 -0400
Message-ID: <AANLkTilYEm-1HJLw4xcrTTiRCEQC2CsUQ5hTSmNxGSS0@mail.gmail.com>
To: Mark Davis ☕ <mark@macchiato.com>
Cc: public-iri@w3.org, bidi@unicode.org, Shawn Steele <Shawn.Steele@microsoft.com>, Murray Sargent <murrays@exchange.microsoft.com>
> The best way to solve the problem that I can
> think of can be done right now. Any significant
> site that wants to support BIDI languages
> should provide for the ability to have IRIs
> with *all *RTL characters

This does not seem to be practical under the current URL escaping scheme,
since the query string often needs to contain arbitrary-language data, e.g.
a search string. Let's say that data happens to be Latin script. There is
currently no way to encode it into RTL characters. Thus, to stay uniform,
the whole IRI has to become LTR. This is probably a branding issue for the
site, which prides itself on its RTL domain name. And having the URL switch
from all-RTL to all-LTR, with a different domain name, when the user clicks
on some link in the page is probably quite confusing for the user. So, to
truly allow for all-RTL URIs, we need to extend URL escaping (%XX) to the
RTL domain, perhaps by somehow allowing decimal escapes in addition to
hexadecimal ones, or by allowing using the first six letters of the Hebrew
and Arabic alphabets to be used to represent hexadecimal digits 10 through

But even if such hurdles were overcome, and it would become *possible* for a
site to phrase all the IRIs it requires without mixing LTR and RTL
characters, this would only reduce user confusion. Third-party documents
(including those originating with spoofers) would probably continue
formulating mixed-direction IRIs that would display differently in different
directional contexts, and sometimes seem like they belong to the site when
in fact they don't.

So, should sites be encouraged to stop accepting mixed-direction IRIs, so
that they eventually become rare - and automatically suspect - on the web?

(For a site to detect mixed-direction IRIs would not be trivial. For
example, it will be hampered by having to include the domain name in the
check. Non-ASCII domain names are translated into (ASCII) punicode before
they get to the site. So, the site would have to first translate the
punicode back to the original non-ASCII domain name before checking that the
IRI does not contain both LTR and RTL characters.)

> Another alternative would be to use a limited
> set of markup within URLs so as to preserve the
> right ordering. It would suffice to allow RTM
> and LTM characters around the neutral
> characters.

(The intent here is LRM and RLM, I think.)

This approach requires a mechanism for determining which LRMs and RLMs in an
IRI are just optional "visual sugar" for the user, and should thus be
removed before further processing of the IRI, and which are an integral part
of the IRI. Such a mechanism would have to deal with the different nature of
different parts of the IRI (e.g. domain name, path, and query string), and
would likely affect many of the layers involved in the processing of an IRI:
e.g. browsers (for LRMs and RLMs in the domain name before it is translated
into punicode), HTTP web server software (for LRMs and RLMs in the path),
and the site's final code layers that process the query string.

Not trivial...

Furthermore, we still have the same problem as above: that some documents
containing IRIs will bother to use LRMs and RLMs in them does not mean that
*all* documents will. (For example, it is difficult to imagine a user
manually typing an IRI into an e-mail with LRMs or RLMs.) And thus, users
will become used to seeing IRIs being displayed every which way, making
spoofing that much easier. It is not clear to me that allowing the use of
LRMs and RLMs in IRIs would reduce the problem or make it even larger.


On Tue, May 25, 2010 at 3:10 AM, Mark Davis ☕ <mark@macchiato.com> wrote:

> There has been some discussion of having a special ordering for BIDI URLs
> so that they are more understandable to users. (I'll use URL in the broad
> sense, as including non-ASCII characters.) This is a complicated issue, and
> I can't claim to have all the answers, but here are some thoughts on the
> issue.
> In the Unicode consortium, we've been aware of this issue, and have
> considered options a number of times over the years. However, we have not
> yet heard a good case for how supporting uniform field direction in URLs can
> be done without significant compatibility and security problems. There are
> some big stumbling blocks:
>    - Many clients that display URLs will either not be URL aware, or not
>    be aware of the latest standard, or not be able to parse out text as
>    definitively belonging to a URL.
>    - The specs have no termination criteria for parsing URLs in plain
>    text. So http://abc.def#ghi could be "http://abc.def#ghi" or could be "
>    http://abc.def#ghi* could*", since fragments can include spaces. (And
>    in languages that don't use spaces to separate words, this is further
>    complicated.) Different applications have different heuristics for this, but
>    those heuristics don't always agree.
>    - Many applications heuristically recognize fragment URLs, like "
>    google.com". So in a broad sense, people understand a URL as "something
>    that I could paste into an address bar in my browser and will get me to a
>    page", and have the expectation that they will order similarly. That is,
>    ordering "GOOGLE.COM" one way and "http://GOOGLE.COM" another would be
>    confusing.
> Why is ordering a problem? Suppose I have the URL http://ABC.DEF.
> Currently, any application that displays BIDI will do it as either
> http://FED.CBA ( in a LTR environment) or FED.CBA://http in a RTL one. If
> an application starts to display it as http://CBA.FED, then it represents
> a significant security problem, since the user will think it is the
> different URL http://DEF.ABC. As long as there is significant percentage
> of old applications, there will be the opportunity for that problem. The
> same goes for LTR URLs in a RTL environment.
> Moreover, if I paste text between applications, even where the paragraph
> direction is constant, then the labels can flip in arbitrary ways if some
> applications support uniform direction and some don't. The challenge is to
> get all applications to consistently (a) be URL aware, and (b) all switch to
> some new display order in unison. It might be that someone can come up with
> a way to handle this, but we haven't heard of one yet.
> (Had the importance of URL syntax been known at the time the consortium
> came up with the BIDI algorithm, and were the IRI syntax determinant enough
> that the termination could always be recognized, even in the midst of plain
> text, we'd be in a different world.)
> But we're not. The best way to solve the problem that I can think of can be
> done right now. Any significant site that wants to support BIDI languages
> should provide for the ability to have IRIs with *all *RTL characters:
> host name, path, query, fragment. If all the pieces are RTL text (or infixed
> neutrals), than the display has a consistent direction in both RTL and LTR
> environment, no matter whether the application is URL-aware or not, and
> users won't be confused. Now that the TLD can be RTL, I think there will be
> pressure for the sites to do that, since completely-RTL IRIs will work much
> better in all environments.
> [The one real remaining piece is the scheme; the IRI is still
> understandable (though ugly) if it has to be ASCII, but it would be somewhat
> better if it could have a RTL alias.  (Pure digit fields like IP addresses
> are a bit ugly, but seldom used.)]
> Another alternative would be to use a limited set of markup within URLs so
> as to preserve the right ordering. It would suffice to allow RTM and LTM
> characters around the neutral characters. Any BIDI URL could be normalized
> so as to include these characters in all and only the right places, by a
> compliant implementation. And once this was done, then the text can be cut
> and copies between applications with no change in appearance.
> However, one would come up with sufficient constraints on the use of these
> characters so as to prevent *their* being used for spoofing, and could
> have a problem with breakage on older implementations. (Although in a way,
> breaking is better than sending people to the wrong place.)
> Mark
> — Il meglio è l’inimico del bene —
Received on Tuesday, 25 May 2010 18:14:06 UTC

This archive was generated by hypermail 2.3.1 : Tuesday, 6 January 2015 20:39:41 UTC