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 (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
Table of Contents
1. Introduction ....................................................21.1. Terminology ................................................31.2. Further Information and Discussion of this Document ........42. Tag Syntax and Rules ............................................42.1. Tag Syntax and Examples ....................................42.2. Rules for Minting Tags .....................................52.3. Resolution of Tags .........................................72.4. Equality of Tags ...........................................73. Security Considerations .........................................74. IANA Considerations .............................................85. References ......................................................95.1. Normative References .......................................95.2. Informative References .....................................91. Introduction
A tag is a type of Uniform Resource Identifier (URI)  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.
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 ,  are hard for humans to read.
OIDs ,  and Digital Object Identifiers  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
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
o Entities could rely on purl.org 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 purl.org) 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.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
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 email@example.com.
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 , 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 
DNScomp = alphaNum [*(alphaNum /"-") alphaNum]
emailAddress = 1*(alphaNum /"-"/"."/"_") "@" DNSname
alphaNum = DIGIT / ALPHA
specific = *( pchar / "/" / "?" ) ; pchar from RFC 3986 
fragment = *( pchar / "/" / "?" ) ; same as RFC 3986 
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 "tag:EXAMPLE.com,2000:"
are never equal to those beginning "tag:example.com,2000:", even
though they refer to the same domain name.
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 . 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 .
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
Examples of tag URIs are:
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
that moment. The date is specified using one of the "YYYY",
"YYYY-MM" and "YYYY-MM-DD" formats allowed by the ISO 8601 standard
 (see also RFC 3339 ). 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
"tag:example.com,2000:" are never equal to those beginning
"tag:example.com,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 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 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 example.com
could delegate control over all tags with the prefixes
"tag:example.com,2004:fred:" and "tag:example.com,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
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
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
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 (firstname.lastname@example.org) and Sandro Hawke
Author/Change controller: Tim Kindberg and Sandro Hawke
5.1. Normative References
 Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
 Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
 Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
 "Data elements and interchange formats -- Information
interchange -- Representation of dates and times", ISO
(International Organization for Standardization) ISO 8601:1988,
5.2. Informative References
 Leach, P. and R. Salz, "UUIDs and GUIDs", Work in Progress,
 "Information technology - Open Systems Interconnection - Remote
Procedure Call (RPC)", ISO (International Organization for
Standardization) ISO/IEC 11578:1996, 1996.
 "Specification of abstract syntax notation one (ASN.1)", ITU-T
recommendation X.208, (see also RFC 1778), 1988.
 Mealling, M., "A URN Namespace of Object Identifiers",
RFC 3061, February 2001.
 Paskin, N., "Information Identifiers", Learned Publishing Vol.
10, No. 2, pp. 135-156, (see also www.doi.org), April 1997.
 Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, July 2002.
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