Tuesday, April 28, 2009

URNs, URCs, and URIs

URNs, URCs, and URIs

Talking about what a document is rather than where it is makes sense when you consider how information is organized outside the Internet. Nobody talks about which library carries a particular book, or what shelf it is on. The relevant information is the title of the book, it’s author, and perhaps some other information. But what happens if two or more books have the same title, or two authors have the same name? this actually is quite common. Generally, a book should have a unique identifier such as an ISBN number that, when combined with other descriptive information, such as the author, publisher, and publication date, uniquely describes the book. This naming scheme enables people to specify a particular book and then hunt it down.
The Web, however, isn’t as ordered as a library. On the Web, people name their documents whatever they like, and search robots organize their indexes however they like. Categorizing things is difficult. The only unique item for documents is the URL, which simply says where the document lives. But how many URLs does the HTML 4 specification have? A document might exist in many places. Even worse than a document with multiple locations, what happens when the content at the location changes? perhaps a particular URL address points to information about dogs one day and cats the next. This is how the Web really is. However, a great deal of research is being done to address some of the shortcomings of the Web and its addressing schemes.

A new set of addressing ideas, including URNs, URCs and URIs, are emerging to remedy some of the Web’s shortcomings. A uniform resource name (URN) can locate a resource by giving it a unique symbolic name rather than a unique address. Network services analogous to the current DND services will transparently translate a URN into the URL (server IP address, directory path, and filename) needed to actually locate a resource. This translation could be used to select the closet server, to improve document delivery speed, or try various backup servers in case a server is unavailable. The benefit of the abstraction provided by URNs should be obvious from this simple idea alone.
To better understand the logic behind URNs, consider domain names, such as www.democompany.com. These name are already translated into numeric IP addresses, such as, all the time. This mapping provides the ability to change a machines numeric address and location without seriously disturbing access to it because the name stays the same. Furthermore, numeric addresses provide no meaning to a user, whereas domain names provide some indication of the entity in question. Obviously, the level of abstraction provided by a system such as DNS would make sense on the Web. Rather than typing some unwieldy URL, a URN would be issued that would be translated to an underlying URL. Some experts worry that using a resolving system to translate URNs to URLs is inherently flawed and will not scale well. Because the DNS system is fairly fragile, there might be some truth to this concern. Another problem is that, in reality, URNs probably won’t be something easy to remember, such as urn; book title, but will instead be something more difficult, such as urn:isbn: 0-12-518408-5.

A uniform resource characteristic (URC), also known as a uniform resource citation, describes a set of attributes/ value pairs that defines some aspect of an information resource. For example, in the case of a book, a URC might describe a publication date, number of pages, author, and so on. The form of a URC is still under discussion; however, logically what they would provide is already being used often in the form of simple meta tags, with <> on the sides of it.
combined, a URL, URN and collection of URCs describe an information resource. For example, the document “Demo Company Corporate Summary” might have a unique URN such as urn://corpid:55127.

Note; the syntax of the preceding URN is fictional. It simply shows that URNs have easily remembered names and that many naming schemes can be used, such as ISBN numbers or corporate Ids.

The “Demo Company Corporate Summary” also would have a set of URNs that describes the rating of the file, the author, the publisher, and so on. In addition, the document would have a locate(s) on the Web where the document lives, such as one of the following traditional URLs;


Taken all together, a particular information resource has been identified. The collection of information, which is used to identify this document specifically, is termed s uniform resource identifier (URI).

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