If URNs are to be successful in the long term, it seems likely that both centralized and distributed models will have their place. Essentially we're just re-writing the source URL in a way which avoids the normal URL syntax. It is often the case that the same files are mirrored by different names. This may be not be immediately evident to the computer, but is usually obvious when the filenames involved are scanned by a human. Of a given directory hierarchy on a mirrored FTP server, it is often the case that there are a number of mirror sites each of which mirrors different areas.
Some mirrors appear to be very much out of date.
This can be a major problem for the end user, unless they know the currency of the resource which they are dealing with - and so will have some idea whether they are picking up a stale version. We haven't yet been able to deploy the URNs discussed here in a production environment as yet. In practice, 1.2 beta releases have been too unstable for serious production use. There is the question of how best to make use of the FTP mirrors database which has been constructed. There have been many lists of archive sites and mirror sites at various times in the past, but these have typically been maintained by a single person or a small group of people.