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Network Working Group                                        T. Kindberg
Internet-Draft                               Hewlett-Packard Corporation
Expires: October 30, 2004                                       S. Hawke
                                               World Wide Web Consortium
                                                                May 2004

                          The 'tag' URI scheme

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Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.


   This document describes the "tag" Uniform Resource Identifier (URI)
   scheme, for identifiers that are unique across space and time.  Tag
   URIs (also known as "tags") are distinct from most other URIs in that
   there is no authoritative resolution mechanism.  A tag may be used
   purely as an entity identifier.  Unlike UUIDs or GUIDs such as "uuid"
   URIs and "urn:oid" URIs, tags are designed to be tractable to humans.
   Furthermore, using tags has some advantages over the common practice
   of using "http" URIs as identifiers for non-HTTP-accessible

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


   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.

Further Information and Discussion of this Document

   Information about the tag URI scheme additional to this document --
   motivation, genesis and discussion -- can be obtained from

   Earlier drafts of this document have been discussed on uri@w3.org.
   The authors welcome further discussion and comments.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Tag Syntax and Rules . . . . . . . . . . . . . . . . . . . . .  5
     2.1   Tag Syntax and Examples  . . . . . . . . . . . . . . . . .  5
     2.2   Rules for Minting Tags . . . . . . . . . . . . . . . . . .  6
     2.3   Resolution of Tags . . . . . . . . . . . . . . . . . . . .  7
     2.4   Equality of Tags . . . . . . . . . . . . . . . . . . . . .  7
   3.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   4.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   4.1   Normative References . . . . . . . . . . . . . . . . . . . .  8
   4.2   Informative References . . . . . . . . . . . . . . . . . . .  8
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .  9
       Intellectual Property and Copyright Statements . . . . . . . . 10

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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 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.  They want to be reasonably sure that the identifier is unique.
       Global uniqueness is valuable because it prevents identifiers
       from becoming unintentionally ambiguous.
   b.  It is useful for the identifier to be tractable to humans: they
       should be able to mint new identifiers conveniently, and to type
       them into emails and forms.
   c.  They do not want to have to communicate with anyone else in order
       to mint identifiers for their documents.
   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
   general requirements above.  For example:

   UUIDs [4], [5] are hard for humans to read.

   OIDs [6], [7] and Digital Object Identifiers [8] require naming
   authorities to register themselves, even if they already hold a
   domain name registration.

   URLs (in particular, "http" URLs) are sometimes used as identifiers
   that satisfy most of our 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

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      can't be sure that they are minting new names.  For example, if
      example.org 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

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 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, is:

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


      taggingEntity = authorityName "," date
      authorityName = DNSname / emailAddress
      date          = 4*dig ["-" 2*dig ["-" 2*dig ]] ; see ISO8601 [2]
      DNSname       = DNScomp / DNSname "." DNScomp  ; see RFC1035 [3]
      DNScomp       = lowAlphaNum [*(lowAlphaNum /"-") lowAlphaNum]
      emailAddress  = 1*(lowAlphaNum /"-"/"."/"_") "@" DNSname
      lowAlphaNum   = dig / lowAlpha
      specific      = 1*(URIchars)  ; URIchars defined in RFC2396 [1]
      lowAlpha      = %x61-7A ; any char in the range "a" through "z"
      dig           = %x30-39 ; any char in the range "0" through "9"

   The component "taggingEntity" is the name space part of the URI.  To
   avoid ambiguity, this MUST be expressed in lower case; the domain
   name in "authorityName" (whether an email address or a simple domain
   name) MUST be fully qualified.

   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 be 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 for
   authorityName defined above.

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   The component "specific" is the name-space-specific part of the URI:
   it is any string of valid URI characters [1] chosen by the minter of
   the URI.  It is RECOMMENDED that specific identifiers should be

   Examples of tag URIs are:


2.2  Rules for Minting Tags

   As Section 2.1 has specified, each tag consists of 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 is
   specified using one of the "YYYY", "YYYY-MM" and "YYYY-MM-DD" formats
   allowed by the ISO 8601 standard [2].  The tag specification permits
   no other formats.  The date MUST be reckoned from UTC -- which may
   differ from the date in the tagging entity's local timezone at 00:00

   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 dates
   specify a moment (00:00) of a single day; they MUST NOT be taken as
   periods of a day or more, such as "the whole of July 2001" or "the
   whole of 2000".

   It is RECOMMENDED that tagging entities use only one formulation for
   a given date, since alternative formulations of the same date will be
   counted as distinct and hence tags containing them will be unequal.
   For example, tags beginning "tag:hp.com,2000:" are never equal to
   those beginning "tag:hp.com,2000-01-01:", even though they refer to
   the same date (see Section 2.4).

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   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
   was 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 hp.com
   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
   "tag:champignon.net,2001:" under domain names not registered to it.
   It must not mint tags dated in the future, such as
   "tag:hp.com,2999:".  If it obtains assignment of
   "extremelyunlikelytobeassigned.org" on 2001-05-01, then it must not
   mint tags under "extremelyunlikelytobeassigned.org,2001-04-01" unless
   it has evidence proving that 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.  Some tagging entities (e.g.  corporations, mailing
   lists) consist of many people, in which case group decision-making
   and record-keeping procedures SHOULD be used to achieve uniqueness.

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.  That is, one can compare tags for equality by
   comparing the numeric codes of their characters, in sequence, for
   numeric equality.  This equality-criterion allows for simplification
   of tag-handling software, which does not have to transform tags in
   any way to compare them.

3.  Security Considerations

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   Minting a tag, by itself, is an operation internal to the tagging
   entity with 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
   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.  References

4.1  Normative References

   [1]  Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
        Identifiers (URI): Generic Syntax", RFC 2396, August 1998.

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

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

4.2  Informative References

   [4]  Leach, P. and R. Salz, "UUIDs and GUIDs", draft-leach-uuids-01
        (work in progress), 1997.

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

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

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

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

Authors' Addresses

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

   Phone: +44 117 312 9920
   EMail: timothy@hpl.hp.com

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

   Phone: +1 617 253-7288
   EMail: sandro@w3.org

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