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RFC 4151

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The 'tag' URI Scheme


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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).


   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.


   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.

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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

   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.

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   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
      there is no resource accessible at the "location".

   o  Domain names change hands and the new assignee of a domain name
      can't be sure that they are minting new names.  For example, if is assigned first to a user Smith and then to a user
      Jones, there is no systematic way for Jones to tell whether Smith
      has already used a particular identifier such as

   o  Entities could rely on or a similar service as a
      (first-come, first-served) assigner of unique URIs; but a solution
      without reliance upon another entity such as the Online Computer
      Library Center (OCLC, which runs may be preferable.

   Lastly, many entities -- especially individuals -- are assignees of
   email addresses but not domain names.  It would be preferable to
   enable those entities to mint unique identifiers.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119.

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1.2.  Further Information and Discussion of this Document

   Additional information about the tag URI scheme -- motivation,
   genesis, and discussion -- can be obtained from

   Earlier versions of this document have been discussed on
   The authors welcome further discussion and comments.

2.  Tag Syntax and Rules

   This section first specifies the syntax of tag URIs and gives
   examples.  It then describes a set of rules for minting tags that is
   designed to make them unique.  Finally, it discusses the resolution
   and comparison of tags.

2.1.  Tag Syntax and Examples

   The general syntax of a tag URI, in ABNF [2], is:

      tagURI = "tag:" taggingEntity ":" specific [ "#" fragment ]


      taggingEntity = authorityName "," date
      authorityName = DNSname / emailAddress
      date = year ["-" month ["-" day]]
      year = 4DIGIT
      month = 2DIGIT
      day = 2DIGIT
      DNSname = DNScomp *( "."  DNScomp ) ; see RFC 1035 [3]
      DNScomp = alphaNum [*(alphaNum /"-") alphaNum]
      emailAddress = 1*(alphaNum /"-"/"."/"_") "@" DNSname
      alphaNum = DIGIT / ALPHA
      specific = *( pchar / "/" / "?" ) ; pchar from RFC 3986 [1]
      fragment = *( pchar / "/" / "?" ) ; same as RFC 3986 [1]

   The component "taggingEntity" is the name space part of the URI.  To
   avoid ambiguity, the domain name in "authorityName" (whether an email
   address or a simple domain name) MUST be fully qualified.  It is
   RECOMMENDED that the domain name should be in lowercase form.
   Alternative formulations of the same authority name will be counted
   as distinct and, hence, tags containing them will be unequal (see
   Section 2.4).  For example, tags beginning ",2000:"
   are never equal to those beginning ",2000:", even
   though they refer to the same domain name.

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   Authority names could, in principle, belong to any syntactically
   distinct namespaces whose names are assigned to a unique entity at a
   time.  Those include, for example, certain IP addresses, certain MAC
   addresses, and telephone numbers.  However, to simplify the tag
   scheme, we restrict authority names to domain names and email
   addresses.  Future standards efforts may allow use of other authority
   names following syntax that is disjoint from this syntax.  To allow
   for such developments, software that processes tags MUST NOT reject
   them on the grounds that they are outside the syntax defined above.

   The component "specific" is the name-space-specific part of the URI:
   it is a string of URI characters (see restrictions in syntax
   specification) chosen by the minter of the URI.  Note that the
   "specific" component allows for "query" subcomponents as defined in
   RFC 3986 [1].  It is RECOMMENDED that specific identifiers should be

   Tag URIs may optionally end in a fragment identifier, in accordance
   with the general syntax of RFC 3986 [1].

   In the interests of tractability to humans, tags SHOULD NOT be minted
   with percent-encoded parts.  However, the tag syntax does allow
   percent-encoded characters in the "pchar" elements (defined in RFC
   3986 [1]).

   Examples of tag URIs are:,2001:web/externalHome,2004-05:Sandro,2001-09-15:TimKindberg:presentations:UBath2004-05-19,1999:blog-555,2002:int

2.2.  Rules for Minting Tags

   As Section 2.1 has specified, each tag includes a "tagging entity"
   followed, optionally, by a specific identifier.  The tagging entity
   is designated by an "authority name" -- a fully qualified domain name
   or an email address containing a fully qualified domain name --
   followed by a date.  The date is chosen to make the tagging entity
   globally unique, exploiting the fact that domain names and email
   addresses are assigned to at most one entity at a time.  That entity
   then ensures that it mints unique identifiers.

   The date specifies, according to the Gregorian calendar and UTC, any
   particular day on which the authority name was assigned to the
   tagging entity at 00:00 UTC (the start of the day).  The date MAY be
   a past or present date on which the authority name was assigned at

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   that moment.  The date is specified using one of the "YYYY",
   "YYYY-MM" and "YYYY-MM-DD" formats allowed by the ISO 8601 standard
   [4] (see also RFC 3339 [10]).  The tag specification permits no other
   formats.  Tagging entities MUST ascertain the date with sufficient
   accuracy to avoid accidentally using a date on which the authority
   name was not, in fact, assigned (many computers and mobile devices
   have poorly synchronised clocks).  The date MUST be reckoned from
   UTC, which may differ from the date in the tagging entity's local
   timezone at 00:00 UTC.  That distinction can generally be safely
   ignored in practice, but not on the day of the authority name's
   assignment.  In principle it would otherwise be possible on that day
   for the previous assignee and the new assignee to use the same date
   and, thus, mint the same tags.

   In the interests of brevity, the month and day default to "01".  A
   day value of "01" MAY be omitted; a month value of "01" MAY be
   omitted unless it is followed by a day value other than "01".  For
   example, "2001-07" is the date 2001-07-01 and "2000" is the date
   2000-01-01.  All date formulations specify a moment (00:00 UTC) of a
   single day, and not a period of a day or more such as "the whole of
   July 2001" or "the whole of 2000".  Assignment at that moment is all
   that is required to use a given date.

   Tagging entities should be aware that alternative formulations of the
   same date will be counted as distinct and, hence, tags containing
   them will be unequal.  For example, tags beginning
   ",2000:" are never equal to those beginning
   ",2000-01-01:", even though they refer to the same
   date (see Section 2.4).

   An entity MUST NOT mint tags under an authority name that was
   assigned to a different entity at 00:00 UTC on the given date, and it
   MUST NOT mint tags under a future date.

   An entity that acquires an authority name immediately after a period
   during which the name was unassigned MAY mint tags as if the entity
   were assigned the name during the unassigned period.  This practice
   has considerable potential for error and MUST NOT be used unless the
   entity has substantial evidence that the name was unassigned during
   that period.  The authors are currently unaware of any mechanism that
   would count as evidence, other than daily polling of the "whois"

   For example, Hewlett-Packard holds the domain registration for
   and may mint any tags rooted at that name with a current or past date
   when it held the registration.  It must not mint tags, such as
   ",2001:", under domain names not registered to it.
   It must not mint tags dated in the future, such as

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   ",2999:".  If it obtains assignment of
   "" on 2001-05-01, then it must not
   mint tags under ",2001-04-01" unless
   it has evidence proving that name was continuously unassigned between
   2001-04-01 and 2001-05-01.

   A tagging entity mints specific identifiers that are unique within
   its context, in accordance with any internal scheme that uses only
   URI characters.  Tagging entities SHOULD use record-keeping
   procedures to achieve uniqueness.  Some tagging entities (e.g.,
   corporations, mailing lists) consist of many people, in which case
   group decision-making SHOULD also be used to achieve uniqueness.  The
   outcome of such decision-making could be to delegate control over
   parts of the namespace.  For example, the assignees of
   could delegate control over all tags with the prefixes
   ",2004:fred:" and ",2004:bill:",
   respectively, to the individuals with internal names "fred" and
   "bill" on 2004-01-01.

2.3.  Resolution of Tags

   There is no authoritative resolution mechanism for tags.  Unlike most
   other URIs, tags can only be used as identifiers, and are not
   designed to support resolution.  If authoritative resolution is a
   desired feature, a different URI scheme should be used.

2.4.  Equality of Tags

   Tags are simply strings of characters and are considered equal if and
   only if they are completely indistinguishable in their machine
   representations when using the same character encoding.  That is, one
   can compare tags for equality by comparing the numeric codes of their
   characters, in sequence, for numeric equality.  This criterion for
   equality allows for simplification of tag-handling software, which
   does not have to transform tags in any way to compare them.

3.  Security Considerations

   Minting a tag, by itself, is an operation internal to the tagging
   entity, and has no external consequences.  The consequences of using
   an improperly minted tag (due to malice or error) in an application
   depends on the application, and must be considered in the design of
   any application that uses tags.

   There is a significant possibility of minting errors by people who
   fail to apply the rules governing dates, or who use a shared
   (organizational) authority-name without prior organization-wide
   agreement.  Tag-aware software MAY help catch and warn against these

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   errors.  As stated in Section 2, however, to allow for future
   expansion, software MUST NOT reject tags which do not conform to the
   syntax specified in Section 2.

   A malicious party could make it appear that the same domain name or
   email address was assigned to each of two or more entities.  Tagging
   entities SHOULD use reputable assigning authorities and verify
   assignment wherever possible.

   Entities SHOULD also avoid the potential for malicious exploitation
   of clock skew, by using authority names that were assigned
   continuously from well before to well after 00:00 UTC on the date
   chosen for the tagging entity -- preferably by intervals in the order
   of days.

4.  IANA Considerations

   The IANA has registered the tag URI scheme as specified in this
   document and summarised in the following template:

   URI scheme name: tag

   Status: permanent

   URI scheme syntax: see Section 2

   Character encoding considerations: percent-encoding is allowed in
   'specific' and 'fragment' components (see Section 2)

   Intended usage: see Section 1 and Section 2.3

   Applications and/or protocols that use this URI scheme name: Any
   applications that use URIs as identifiers without requiring
   dereference, such as RDF, YAML, and Atom.

   Interoperability considerations: none

   Security considerations: see Section 3

   Relevant publications: none

   Contact: Tim Kindberg ( and Sandro Hawke

   Author/Change controller: Tim Kindberg and Sandro Hawke

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5.  References

5.1.  Normative References

   [1]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
        Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
        January 2005.

   [2]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.

   [3]  Mockapetris, P., "Domain names - implementation and
        specification", STD 13, RFC 1035, November 1987.

   [4]  "Data elements and interchange formats -- Information
        interchange -- Representation of dates and   times", ISO
        (International Organization for Standardization) ISO 8601:1988,

5.2.  Informative References

   [5]   Leach, P. and R. Salz, "UUIDs and GUIDs", Work in Progress,

   [6]   "Information technology - Open Systems Interconnection - Remote
         Procedure Call (RPC)", ISO (International Organization for
         Standardization) ISO/IEC 11578:1996, 1996.

   [7]   "Specification of abstract syntax notation one (ASN.1)", ITU-T
         recommendation X.208,  (see also RFC 1778), 1988.

   [8]   Mealling, M., "A URN Namespace of Object Identifiers",
         RFC 3061, February 2001.

   [9]   Paskin, N., "Information Identifiers", Learned Publishing Vol.
         10, No. 2, pp. 135-156,  (see also, April 1997.

   [10]  Klyne, G. and C. Newman, "Date and Time on the Internet:
         Timestamps", RFC 3339, July 2002.

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Authors' Addresses

   Tim Kindberg
   Hewlett-Packard Corporation
   Hewlett-Packard Laboratories
   Filton Road
   Stoke Gifford
   Bristol  BS34 8QZ

   Phone: +44 117 312 9920

   Sandro Hawke
   World Wide Web Consortium
   32 Vassar Street
   Building 32-G508
   Cambridge, MA  02139

   Phone: +1 617 253-7288

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Full Copyright Statement

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