URN Syntax
RFC 2141

Document Type RFC - Proposed Standard (May 1997; Errata)
Obsoleted by RFC 8141
Last updated 2014-07-29
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Network Working Group                                           R. Moats
Request for Comments: 2141                                          AT&T
Category: Standards Track                                       May 1997

                               URN Syntax

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.


   Uniform Resource Names (URNs) are intended to serve as persistent,
   location-independent, resource identifiers. This document sets
   forward the canonical syntax for URNs.  A discussion of both existing
   legacy and new namespaces and requirements for URN presentation and
   transmission are presented.  Finally, there is a discussion of URN
   equivalence and how to determine it.

1. Introduction

   Uniform Resource Names (URNs) are intended to serve as persistent,
   location-independent, resource identifiers and are designed to make
   it easy to map other namespaces (which share the properties of URNs)
   into URN-space. Therefore, the URN syntax provides a means to encode
   character data in a form that can be sent in existing protocols,
   transcribed on most keyboards, etc.

2. Syntax

   All URNs have the following syntax (phrases enclosed in quotes are

                     <URN> ::= "urn:" <NID> ":" <NSS>

   where <NID> is the Namespace Identifier, and <NSS> is the Namespace
   Specific String.  The leading "urn:" sequence is case-insensitive.
   The Namespace ID determines the _syntactic_ interpretation of the
   Namespace Specific String (as discussed in [1]).

Moats                       Standards Track                     [Page 1]
RFC 2141                       URN Syntax                      May 1997

   RFC 1630 [2] and RFC 1737 [3] each presents additional considerations
   for URN encoding, which have implications as far as limiting syntax.
   On the other hand, the requirement to support existing legacy naming
   systems has the effect of broadening syntax.  Thus, we discuss the
   acceptable syntax for both the Namespace Identifier and the Namespace
   Specific String separately.

2.1 Namespace Identifier Syntax

   The following is the syntax for the Namespace Identifier. To (a) be
   consistent with all potential resolution schemes and (b) not put any
   undue constraints on any potential resolution scheme, the syntax for
   the Namespace Identifier is:

   <NID>         ::= <let-num> [ 1,31<let-num-hyp> ]

   <let-num-hyp> ::= <upper> | <lower> | <number> | "-"

   <let-num>     ::= <upper> | <lower> | <number>

   <upper>       ::= "A" | "B" | "C" | "D" | "E" | "F" | "G" | "H" |
                     "I" | "J" | "K" | "L" | "M" | "N" | "O" | "P" |
                     "Q" | "R" | "S" | "T" | "U" | "V" | "W" | "X" |
                     "Y" | "Z"

   <lower>       ::= "a" | "b" | "c" | "d" | "e" | "f" | "g" | "h" |
                     "i" | "j" | "k" | "l" | "m" | "n" | "o" | "p" |
                     "q" | "r" | "s" | "t" | "u" | "v" | "w" | "x" |
                     "y" | "z"

   <number>      ::= "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" |
                     "8" | "9"

   This is slightly more restrictive that what is stated in [4] (which
   allows the characters "." and "+").  Further, the Namespace
   Identifier is case insensitive, so that "ISBN" and "isbn" refer to
   the same namespace.

   To avoid confusion with the "urn:" identifier, the NID "urn" is
   reserved and MUST NOT be used.

Moats                       Standards Track                     [Page 2]
RFC 2141                       URN Syntax                      May 1997

2.2 Namespace Specific String Syntax
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