Tag URI scheme

The Tag URI scheme is a uniform resource identifier (URI) scheme for the simple production of unique identifiers called tags.[1][2] It was defined by RFC4151 in October 2005.[3]

The definition of the scheme identifies four requirements for tags:

  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.
Hawke and Kindberg, The 'tag' URI Scheme


The following example shows a tag that could indicate the authorship of a resource on the World Wide Web. The semantics of tags do not require the user to have current ownership of the hostname upon which the tag is based, but only have had it at the date specified within the tag.

tag:[email protected],2007-11-02:Tag_URI

A similar tag could indicate the name of a pet belonging to the same individual. The semantics of tags do not require the tagged entity to be accessible electronically.

tag:[email protected],2006:Fluffy

See also


  1. "Tag". Retrieved 2007-11-02.
  2. "YAML specification". Retrieved 2007-11-02.
  3. Kindberg, T.; Hawke, S. (October 2005). "The 'tag' URI Scheme". Internet Engineering Task Force. Retrieved 1 September 2015.

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