Any scheme for URNs must permit future extensions to the scheme. That is, there is a single algorithm for comparing two URNs that does not require contacting any external server, is well specified and simple. For this reason, URN comparison is insensitive to case, and probably white space and some punctuation marks. 4 Implications For a URN specification to be acceptible, it must meet the previous requirements. To satisfy the requirements of uniqueness and scalability, name assignment is delegated to naming authorities, who may then assign names directly or delegate that authority to sub-authorities.
Uniqueness is guaranteed by requiring each naming authority to guarantee uniqueness.
The names of the naming authorities themselves are persistent and globally unique and top level authorities will be centrally registered. Naming authorities that support scalable naming are encouraged, but not required. Page 5 It is strongly recommended that there be a mapping between the names generated by each naming authority and URLs. At any specific time there will be zero or more URLs into which a particular URN can be mapped. For URNs to be transcribable and transported in mail, it is necessary to limit the character set usable in URNs, although there is not yet consensus on what the limit might be.