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

To: edbark@xxxxxxxx
Cc: "[ontolog-forum]" <ontolog-forum@xxxxxxxxxxxxxxxx>
From: Waclaw Kusnierczyk <Waclaw.Marcin.Kusnierczyk@xxxxxxxxxxx>
Date: Mon, 16 Apr 2007 22:16:26 +0200
Message-id: <4623D99A.2050904@xxxxxxxxxxx>

Ed Barkmeyer wrote:
> With respect to what is actually supported by W3C and IETF standards, there 
> have been a few bits of misinformation bandied about in this discussion.    (01)

> Waclaw Kusnierczyk wrote:
>> Re: John Sowa's post on URIs etc.
>> "This discussion raises some serious issues:    (01)
>>    4. The URLs and URIs of the WWW are based on a naming
>>       scheme that ultimately resolves to physical devices.
>>       It guarantees that an identifier will determine a
>>       unique storage location at a given point in time.    (05)"
>> This is not exactly true.  In w3 specs [1] we read:
>> "A Uniform Resource Identifier (URI) is a compact sequence of characters 
>> that identifies an abstract or physical resource", and further that "in 
>> many cases, URIs are used to denote resources without any intention that 
>> they be accessed" and "a common misunderstanding of URIs is that they 
>> are only used to refer to accessible resources. The URI itself only 
>> provides identification; access to the resource is neither guaranteed 
>> nor implied by the presence of a URI."
>> A URL is a URI with a specialized syntax: "A URI can be further 
>> classified as a locator, a name, or both. The term "Uniform Resource 
>> Locator" (URL) refers to the subset of URIs that, in addition to 
>> identifying a resource, provide a means of locating the resource by 
>> describing its primary access mechanism".  Ultimate resolution to 
>> physical devices is *not* a part of the scheme.
>> "
>>    5. However, the policies of the WWW and of each domain
>>       on the WWW permit the same identifiers to be resolved
>>       to different physical locations at different times.    (06)
>> "
>> Precisely.  URLs, those that do identify locations, identify them in 
>> virtue of there being a mapping from a particular URI to a physical 
>> address, by means of dns services.  And if there is no such mapping, 
>> there is no translation, and in effect the URL does not identify a 
>> location -- try, e.g., 'http://www.nonsense.no', a (syntactically) 
>> *valid* URL.  Some (most?) valid URLs do *not* ultimately resolve to a 
>> physical device.
> This is technically partly correct, but the paragraph is badly misleading.
> What RFC 2396 says is that a URL begins with a prefix that identifies an 
> Internet access protocol, and it is required to have the syntax required by 
> the specification for that protocol and the interpretation associated with 
> that syntax.
> The HTTP standard (RFC 2616) defines the rest of the syntax of URIs beginning 
> http:, and says:
>     3.2.2 HTTP URL
>     The "http" scheme is used to locate network resources via the HTTP
>     protocol.  This section defines the scheme-specific syntax and
>     semantics for http URLs.
>     http_URL = "http:" "//" host [ ":" port ] [ abs_path [ "?" query ]]
>     If the port is empty or not given, port 80 is assumed. The semantics
>     are that the identified resource is located at the server listening
>     for TCP connections on that port of that host, ...
> And, by adoption of text from RFC 2396:
>     The host is a domain name of a network host, or its IPv4 address ...
> That is, the URL *means* that the resource is accessible at the designated 
> host via the HTTP TCP port.  It doesn't refer to a *physical device*, but it 
> does refer to a specific *host*, and indirectly to the physical device that 
> currently responds to that TCP/IP address and provides network services.
> 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 
>    '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*.    (02)

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.    (03)

vQ    (04)

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