- From: Ron Daniel Jr. <rdaniel@acl.lanl.gov>
- Date: Fri, 21 Feb 1997 09:43:27 -0700
- To: Dan Connolly <connolly@w3.org>
- Cc: uri@bunyip.com
At 01:40 AM 2/21/97 -0600, Dan Connolly wrote: >Ron Daniel Jr. wrote: > >> Well, there is at least one place where I think a useful technical >> distinction can be made. URNs are to be location-independent. Not >> only is the notion of a "location-independent locator" a bit >> tortured, I would have a hard time calling >> http://www.foo.com/whatever >> a URN since it clearly has a preferred location for resolution. > >OK, I'll bite: how is it that "location-dependent" vs. >"location-independent" is a technical distinction? Let me start with an example and set some background... Once upon a time there was a site called info.cern.ch. Essentially all the seminal web documents were accesible there. You accessed those documents using URLs of the form http://info.cern.ch/pub/WWW/something-or-the-other All those old URLs are now broken because all the web documents moved to www.w3.org - a different location. For awhile CERN ran a redirection service, but they don't seem to do that anymore. Those identifiers were, and still are, location dependent. I'll be the first to agree that the notion of the "location" of a domain name does not have to map onto a unique location in physical space. We can use something like Cisco's Director to hide lots of machines behind the same IP address. We can provide multiple A records for the same domain name. Now that the SRV record exists we can map one domain name to several. However, in the space of possible domain names, one domain name identifies a particular "location". That location is *strongly* correlated to the structure of organizations. Further, it is correlated (but not so strongly) to the physical spaces under the control of those organizations and the organizations with whom they have established agreements. (I'm not sure what kind of a metric one can use in organization-space, but domain name space has a definite metric due to its hierarchical organization. Its just a strange space since CNAMEs, SRVs, and MXs make so many "wormholes". But I digress.) With all that background in place, let me answer your question: >how is it that "location-dependent" vs. >"location-independent" is a technical distinction? http://www.foo.com/whatever identifies one location in domain name space, just as info.cern.ch does. Those locations are under the control of particular organizations who can decide if they do or do not want to keep those locations distinct. In the case of info.cern.ch and www.w3.org, they were kept distinct. A location-independent identifier does not carry around resolution-system specific information, such as a domain name. >What mechanism depends on or uses the distinction in >any way? Current URL resolution mechanisms. All the major ones are defined to contain a domain name, which is a location in the space of domain names. Their specifications say what to do with that domain name. (Lookup an A record or an MX record, etc.). >What's the test for "location dependent"? Does the identifier embed resolution-system-specific hints, such as a domain name? >For 20 points: tell me the location of http://www.w3.org/. In domain-same space, that is a point. It maps in organizational space to the parts of MIT, INRIA, ... that have a particular business relation with W3C. Those organizations will have provided a set of machines to fulfill their part of that business relation, and we can discover the IP addresses for those machines through DNS. Physically, I would expect those IP addresses to route to about 20 machines in Cambridge and France that are under direct control of W3C staff. There maybe a few others scattered around. Those domain names cannot legally route to, for example, any of the machines on the floor below me. I have no way of conforming to the specification for various URL schemes and doing the resolution on the machines downstairs. Those identifiers are location-dependent. This is a technical distinction. >The evidence you give -- that you would have a hard time calling >it a URN -- is exactly the sort of _non-technical_ difference >in perspective that I'm talking about. Well, I disagree. I'll admit that one *could* resolve current URLs in a location-independent fashion. However, one cannot do it in a manner conformant to existing standards. We can do anything if we change the definitions of our terms. Regards, Ron Daniel Jr. voice:+1 505 665 0597 Advanced Computing Lab fax:+1 505 665 4939 MS B287 email:rdaniel@lanl.gov Los Alamos National Lab http://www.acl.lanl.gov/~rdaniel Los Alamos, NM, USA, 87545
Received on Friday, 21 February 1997 11:44:15 UTC