<P> The maintainers of the DOI system have deliberately not registered a DOI namespace for URNs, stating that: </P> <P> URN architecture assumes a DNS - based Resolution Discovery Service (RDS) to find the service appropriate to the given URN scheme . However no such widely deployed RDS schemes currently exist...DOI is not registered as a URN namespace, despite fulfilling all the functional requirements, since URN registration appears to offer no advantage to the DOI System . It requires an additional layer of administration for defining DOI as a URN namespace (the string urn: doi: 10.1000 / 1 rather than the simpler doi: 10.1000 / 1) and an additional step of unnecessary redirection to access the resolution service, already achieved through either http proxy or native resolution . If RDS mechanisms supporting URN specifications become widely available, DOI will be registered as a URN . </P>

What are the 3 parts of the doi