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Re: [ontolog-forum] OWL and lack of identifiers

To: Waclaw Kusnierczyk <Waclaw.Marcin.Kusnierczyk@xxxxxxxxxxx>
Cc: "[ontolog-forum]" <ontolog-forum@xxxxxxxxxxxxxxxx>
From: Ed Barkmeyer <edbark@xxxxxxxx>
Date: Mon, 16 Apr 2007 19:04:36 -0400
Message-id: <46240104.6010606@xxxxxxxx>
Waclaw Kusnierczyk wrote:    (01)

> Ed Barkmeyer wrote:    (02)

>> If the URL doesn't mean that, it is not a "valid" HTTP URL, because it 
>> does not satisfy the requirements of RFC 2616.  So the example URI 
>> that Waclaw offers:
>>    'http://www.nonsense.no'
>> is NOT a "valid" URL.  It may satisfy the grammar requirements, but it 
>> probably does not satisfy the syntactic requirement for 
>> www.nonsense.no to be a valid domain name (i.e., a sequence of 
>> characters registered as a domain name through the Internet cascading 
>> directory scheme).  And it surely doesn't satisfy the requirement for 
>> it to refer to an accessible resource.  So it is likely to be 
>> *syntactically invalid* and it has *no valid interpretation*.    (03)

> Agreed.  I should have written *syntactically valid* instead of 
> (syntactically) *valid*.  And according to the interpretation of 'valid' 
>  that you advocate here, one can make sure that a string is a valid url 
> only by actually connecting to the host and receiving a response.    (04)

We are splitting some hairs here.  I said, very carefully, that
is "grammatically valid" per RFC 2616 -- it satisfies the production rules.
And your use of "syntactically valid" only means "satisfies the production 
rules".    (05)

The problem is whether 'www.nonsense.no' is a valid instance of 'host', which 
depends on whether 'nonsense.no' is registered with IANA to correspond to an 
allocated IP address.  If it is registered, then I can agree that this is 
syntactically valid.  If is not registered, one can argue that it is not even 
"syntactically valid", because valid domain names are determined by IANA 
registry, not by a production rule.  That is, the requirement is not just that 
it is well-formed, but rather that it is in a particular list of valid 
strings.  By way of analogy, consider a language in which symbols must be 
declared before they are used. If the parser encounters a well-formed lexical 
object that should be a declared symbol but the "symbol table lookup" fails 
(the symbol was never declared), is that a "syntax error"?  Is that text 
"syntactically valid"?  (I don't know that the formal languages community has 
consistent terminology in this area, but I haven't written a compiler in over 
20 years.  Maybe they do now.)    (06)

Technically, an invalid domain name will fail before any connection is 
attempted, because the DNS will find no corresponding entry, and therefore no 
IP address and no host to connect to ("host not found").  A DNS lookup request 
is not a connection request.  The connection attempt will only occur if the 
DNS returns an IP address.    (07)

Finally, the "validity of the URL" is separate from being able to connect to 
the referenced host and access the resource.  The real "validity of the URL" 
depends only on whether the referenced host actually associates a resource 
with that URL.  The connection or access could fail for several reasons even 
when the URL is valid, e.g., the host could be down, or refuse connection to 
your system, or refuse to let you access the resource.  So, if you can access 
a resource using the URL, then it is certainly valid, but the inverse does not 
hold: If you cannot access a resource using the URL, the URL is not 
necessarily invalid.    (08)

These are probably subtleties you never wanted to know about, but they have a 
habit of surfacing in conversations about "validity" of URLs.    (09)

-Ed    (010)

Edward J. Barkmeyer                        Email: edbark@xxxxxxxx
National Institute of Standards & Technology
Manufacturing Systems Integration Division
100 Bureau Drive, Stop 8263                Tel: +1 301-975-3528
Gaithersburg, MD 20899-8263                FAX: +1 301-975-4694    (011)

"The opinions expressed above do not reflect consensus of NIST,
  and have not been reviewed by any Government authority."    (012)

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