John Sheridan is pondering URIs and URNs. I have pondered that a lot too. The idea of having another degree of independence between names (e.g. cites to legislation) and actual dereferencable identifiers makes a lot of sense of course. We don't want to tie ourselves down to implementations or platforms or server hosts if we can avoid it. Especially if the goals is to have very long lived identifiers.
However, a few things worry me about the standard "lets use URNs" reaction to long term identifiers.
1) URL's are already completely and utterly devoid of any direct connection to the underlying assets they point to. The number of levels of indirection present in your average resolution of a URL to a stream of bytes in RAM is already very large and many of them are under our control. I.e. we can change the mappings at will. The days of "static" IP addresses are long gone. So, the notion that URNs help because you can change the resolution process without touching the assets themselves doesn't sit well with me because I can do that with plain old URLs...At many levels from DNS to VLANs to NATting to HOSTS files to http redirects etc. etc.
Given the plethora of mappings already present in the (URL->Resource Representation) resolution process, do we gain much adding another one in the form of a URN mapping?
2) URN schemes need resolvers and in many systems I have seen that use URNs, representations get served up with embedded hyperlinks. The embedded hyperlinks often use URLs to access the URN resolver. I.e. http://.../resolve_urn?urn=foo. But of course, in order to do that, the representation ends up creating a dependency on the URL that accesses the resolver :-) If I save that asset, I now have a rendering that is dependent on the URL resolve - despite the presence of the URNs in the asset. So, have I gained anything?
3) It is true of course that domains are rented not owned and this makes folks uneasy about long term reliance on a namespace that is not fully under their control. However, the world is now so utterly dependent on DNS that a fair amount of caselaw exists to protect entities against cybersquats and loss of access to DNS rental rights. Plus standing up your own DNS inside a firewalled environment is straightfoward. Plus creating local mappings in a hosts file is very staightforward. And so on. Lots of options if you need to take control of the resolution process and re-map it.
4) Finally, a non-technical argument that also plays into my skepticism about URNs. They have been around forever. So have identifier schemes like DOI and SSN etc. The internet seems to have voted with its feet already and subsumed all these into URL based resolvers of various kinds. Witness the recent explosion in link shorterners. They map a URL to another URL...I just don't see market pressure out there for a different way to control de-referencing on the Internet.
All in all, with all the mappings already present and the malleability/configurabilty of same, I don't see the compelling rationale for adding other one in the form of URNs.
What am I missing?