Some confusion about policy application and its effects

[Vint Cerf]

Thanks Erik and Andrew. It would seem that there has to be a sort of  
balance on the subject of local mappings. Plainly, they can lead to  
idiosyncracies. On the other hand, the introduction of such a broad  
range of new scripts in the context of many languages not expressible  
in limited Latin character sets and keyboards does push the DNS in  
the direction of broader scope and somewhat less convenient reliance  
on a limited character set. The "LDH" limitations had many benefits  
including a convenient "compatbility" with the protocol strings  
associated with email addresses, host names, URLs and other WWW  
elements, etc. I think Erik's remarks suggest that there might grow  
up some conventions around particular scripts (if not languages) that  
might become widely practiced. It's an interesting question whether,  
when and how one might codify such conventions.

vint


On Aug 4, 2008, at 5:19 PM, Erik van der Poel wrote:

> On Mon, Aug 4, 2008 at 8:08 PM, Andrew Sullivan  
> <ajs at commandprompt.com> wrote:
>> In IDNA2003 use, we have applications that call for resolution  
>> using a
>> specially-formatted name that otherwise does not perturb the
>> traditional use of DNS at all.  There are restrictions on what may go
>> into the DNS, in the sense that there are rules about characters, and
>> possibly labels when taken as a whole.  There may also be a number of
>> equivalence rules.  All of these restrictions, however, are rules  
>> that
>> are imposed at the registration side.  Applications still just get a
>> result back, and use that.  For convenience and consistency, the
>> application may just use Unicode and expect the transformations to be
>> supplied by some underlying library that's "bolted onto the front" of
>> the traditional resolver.  In some sense, though, we could regard  
>> that
>> approach as logically equivalent to having the library "bolted onto
>> the back" of the application.  The key is that rules about what  
>> counts
>> as a "legal domain name" under IDNA2003 (and under traditional DNS)
>> are applied at the registration end of the DNS.  You get a local  
>> error
>
> IDNA2003 seems to say that client applications must check for
> prohibited characters and must not use the domain name if ToASCII
> fails.
>
>> if you don't have an input label that can be successfully transformed
>> according to IDNA2003 rules; but that's not really different from a
>> typo where you include a "/" in your ASCII-only domain name.
>>
>> IDNA2008 takes a different approach, though, because local
>> mappings are allowed.  This means that there is policy at the
>> registration/DNS side of DNS operation, _but also_ on the client end
>> of the transaction.  I understand the motivation for this innovation.
>> But I think it probably breaks the "lookup and use" model that
>> underlies the way we talk about using the DNS.
>>
>> In my reading, in traditional ASCII-only DNS resolution, two clients
>> C1 and C2 will both get the same result to the same query for a  
>> name N
>> when querying the same server at the same time.  Under IDNA2003,
>> that's also true.  While the scope of N might be different, an
>> IDNA2003-compliant client will perform the same transformation to the
>> "Unicode labels" each time.  The display of the result will be the
>> same, too (as long as both C1 and C2 are both IDNA-aware).  As  
>> near as
>> I can tell, however, the same is _not_ true for IDNA2008, because
>> local mappings may change both the input to the transformation
>> function and the displayed output after the answer is returned.   
>> If C1
>> and C2 have different policies (different locales, for instance?),
>> then at least the meaning of "same query" is not clear to me.
>
> Even though IDNA200X appears to allow local mappings, it would be
> unwise to perform them in certain contexts. For example, it would be
> bad to apply local mappings to domain names found in "href" attributes
> in HTML "a" (anchor) tags. If HTML implementors do not all agree to
> use the same mappings, there would be interoperability problems.
>
> Without putting words into Mark's mouth, I believe he is saying that
> it would be bad to perform local mappings in any context, because
> people often copy text from one place to another. While I can see how
> such local mappings could create interoperability problems if the
> mappings are applied recklessly, it would probably be OK to apply some
> mappings in certain specific contexts, e.g. Turkish dotted/dotless
> letter 'i' when the user is known to be expecting Turkish conventions.
>
> Erik
>
>> If I'm right about the above, I wonder whether it is a (new) layering
>> violation; and if so, whether it's an acceptable one in the face of
>> the alternatives.
>>
>> A
>>
>> --
>> Andrew Sullivan
>> ajs at commandprompt.com
>> +1 503 667 4564 x104
>> http://www.commandprompt.com/
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