The 'tag' URI Scheme
RFC 4151

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Network Working Group                                        T. Kindberg
Request for Comments: 4151                   Hewlett-Packard Corporation
Category: Informational                                         S. Hawke
                                               World Wide Web Consortium
                                                            October 2005

                          The 'tag' URI Scheme

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Disclaimer

   The views and opinions of authors expressed herein do not necessarily
   state or reflect those of the World Wide Web Consortium, and may not
   be used for advertising or product endorsement purposes.  This
   proposal has not undergone technical review within the Consortium and
   must not be construed as a Consortium recommendation.

Abstract

   This document describes the "tag" Uniform Resource Identifier (URI)
   scheme.  Tag URIs (also known as "tags") are designed to be unique
   across space and time while being tractable to humans.  They are
   distinct from most other URIs in that they have no authoritative
   resolution mechanism.  A tag may be used purely as an entity
   identifier.  Furthermore, using tags has some advantages over the
   common practice of using "http" URIs as identifiers for
   non-HTTP-accessible resources.

Kindberg & Hawke             Informational                      [Page 1]
RFC 4151                        Tag URIs                    October 2005

Table of Contents

   1. Introduction ....................................................2
      1.1. Terminology ................................................3
      1.2. Further Information and Discussion of this Document ........4
   2. Tag Syntax and Rules ............................................4
      2.1. Tag Syntax and Examples ....................................4
      2.2. Rules for Minting Tags .....................................5
      2.3. Resolution of Tags .........................................7
      2.4. Equality of Tags ...........................................7
   3. Security Considerations .........................................7
   4. IANA Considerations .............................................8
   5. References ......................................................9
      5.1. Normative References .......................................9
      5.2. Informative References .....................................9

1.  Introduction

   A tag is a type of Uniform Resource Identifier (URI) [1] designed to
   meet the following requirements:

   1.  Identifiers are likely to be unique across space and time, and
       come from a practically inexhaustible supply.

   2.  Identifiers are relatively convenient for humans to mint
       (create), read, type, remember etc.

   3.  No central registration is necessary, at least for holders of
       domain names or email addresses; and there is negligible cost to
       mint each new identifier.

   4.  The identifiers are independent of any particular resolution
       scheme.

   For example, the above requirements may apply in the case of a user
   who wants to place identifiers on their documents:

   a.  The user wants to be reasonably sure that the identifier is
       unique.  Global uniqueness is valuable because it prevents
       identifiers from becoming unintentionally ambiguous.

   b.  The identifiers should be tractable to the user, who should, for
       example, be able to mint new identifiers conveniently, to
       memorise them, and to type them into emails and forms.

   c.  The user does not want to have to communicate with anyone else in
       order to mint identifiers for their documents.

Kindberg & Hawke             Informational                      [Page 2]
RFC 4151                        Tag URIs                    October 2005

   d.  The user wants to avoid identifiers that might be taken to imply
       the existence of an electronic resource accessible via a default
       resolution mechanism, when no such electronic resource exists.

   Existing identification schemes satisfy some, but not all, of the
   requirements above.  For example:

   UUIDs [5], [6] are hard for humans to read.

   OIDs [7], [8] and Digital Object Identifiers [9] require entities to
   register as naming authorities, even in cases where the entity
   already holds a domain name registration.

   URLs (in particular, "http" URLs) are sometimes used as identifiers
   that satisfy most of the above requirements.  Many users and
   organisations have already registered a domain name, and the use of
   the domain name to mint identifiers comes at no additional cost.  But
   there are drawbacks to URLs-as-identifiers:

   o  An attempt may be made to resolve a URL-as-identifier, even though