Username / Password : Request For Comments

RFC Number : 2079

Title : Definition of an X.

Network Working Group M. Smith
Request for Comments: 2079 Netscape Communications
Category: Standards Track January 1997

Definition of an X.500 Attribute Type and an Object Class to Hold
Uniform Resource Identifiers (URIs)

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the 'Internet
Official Protocol Standards' (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.


Uniform Resource Locators (URLs) are being widely used to specify the
location of Internet resources. There is an urgent need to be able
to include URLs in directories that conform to the LDAP and X.500
information models, and a desire to include other types of Uniform
Resource Identifiers (URIs) as they are defined. A number of
independent groups are already experimenting with the inclusion of
URLs in LDAP and X.500 directories. This document builds on the
experimentation to date and defines a new attribute type and an
auxiliary object class to allow URIs, including URLs, to be stored in
directory entries in a standard way.

Background and Intended Usage

Uniform Resource Locators (URLs) as defined by [1] are the first of
several types of Uniform Resource Identifiers (URIs) being defined by
the IETF. URIs are widely used on the Internet, most notably within
Hypertext Markup Language [2] documents. This document defines an
X.500 [3,4] attribute type called labeledURI and an auxiliary object
class called labeledURIObject to hold all types of URIs, including
URLs. These definitions are designed for use in LDAP and X.500
directories, and may be used in other contexts as well.

Smith Standards Track [Page 1]

RFC 2079 URI Attribute Type and Object Class January 1997

Schema Definition of the labeledURI Attribute Type

Name: labeledURI
ShortName: None
Description: Uniform Resource Identifier with optional label
OID: umichAttributeType.57 (
Syntax: caseExactString
SizeRestriction: None
SingleValued: False

Discussion of the labeledURI Attribute Type

The labeledURI attribute type has the caseExactString syntax (since
URIs are case-sensitive) and it is multivalued. Values placed in the
attribute should consist of a URI (at the present time, a URL)
optionally followed by one or more space characters and a label.
Since space characters are not allowed to appear un-encoded in URIs,
there is no ambiguity about where the label begins. At the present
time, the URI portion must comply with the URL specification [1].
Multiple labeledURI values will generally indicate different
resources that are all related to the X.500 object, but may indicate
different locations for the same resource.

The label is used to describe the resource to which the URI points,
and is intended as a friendly name fit for human consumption. This
document does not propose any specific syntax for the label part. In
some cases it may be helpful to include in the label some indication
of the kind and/or size of the resource referenced by the URI.

Note that the label may include any characters allowed by the
caseExactString syntax, but that the use of non-IA5 (non-ASCII)
characters is discouraged as not all directory clients may handle
them in the same manner. If non-IA5 characters are included, they
should be represented using the X.500 conventions, not the HTML
conventions (e.g., the character that is an 'a' with a ring above it
should be encoded using the T.61 sequence 0xCA followed by an 'a'
character; do not use the HTML escape sequence 'å').

Examples of labeledURI Attribute Values

An example of a labeledURI attribute value that does not include a

Smith Standards Track [Page 2]

RFC 2079 URI Attribute Type and Object Class January 1997

An example of a labeledURI attribute value that contains a tilde
character in the URL (special characters in a URL must be encoded as
specified by the URL document [1]). The label is 'LDAP Home Page': LDAP Home Page

Another example. This one includes a hint in the label to help the
user realize that the URL points to a photo image. Rennes [photo]

Schema Definition of the labeledURIObject Object Class

Name: labeledURIObject
Description: object that contains the URI attribute type
OID: umichObjectClass.15 (
SubclassOf: top
MayContain: labeledURI

Discussion of the labeledURIObject Object Class

The labeledURIObject class is a subclass of top and may contain the
labeledURI attribute. The intent is that this object class can be
added to existing directory objects to allow for inclusion of URI
values. This approach does not preclude including the labeledURI
attribute type directly in other object classes as appropriate.

Security Considerations

Security considerations are not discussed in this memo, except to
note that blindly inserting the label portion of a labeledURI
attribute value into an HTML document is not recommended, as this may
allow a malicious individual to include HTML tags in the label that
mislead viewers of the entire document in which the labeledURI value
was inserted.


Paul-Andre Pays, Martijn Koster, Tim Howes, Rakesh Patel, Russ
Wright, and Hallvard Furuseth provided invaluable assistance in the
creation of this document.

This material is based in part upon work supported by the National
Science Foundation under Grant No. NCR-9416667.

Smith Standards Track [Page 3]

RFC 2079 URI Attribute Type and Object Class January 1997

Appendix: The labeledURL Attribute Type (Deprecated)

An earlier draft of this document defined an additional attribute
type called labeledURL. This attribute type is deprecated, and
should not be used when adding new values to directory entries. The
original motivation for including a separate attribute type to hold
URLs was that this would better enable efficient progammatic access
to specific types of URIs. After some deliberation, the IETF-ASID
working group concluded that it was better to simply have one
attribute than two.

The schema definition for labeledURL is included here for historical
reference only. Directory client software may want to support this
schema definition (in addition to labeledURI) to ease the transition
away from labeledURL for those sites that are using it.

Name: labeledURL
ShortName: None
Description: Uniform Resource Locator with optional label
OID: umichAttributeType.41 (
Syntax: caseExactString
SizeRestriction: None
SingleValued: False
OID: umichAttributeType.41 (


[1] Berners-Lee, T., Masinter, L., and M. McCahill, 'Uniform
Resource Locators (URL)', RFC 1738, CERN, Xerox Corporation,
University of Minnesota, December 1994.

[2] Berners-Lee, T., and D. Connolly, 'Hypertext Markup Language -
2.0', RFC 1866,

[3] The Directory: Overview of Concepts, Models and Service. CCITT
Recommendation X.500, 1988.

[4] Information Processing Systems -- Open Systems Interconnection --
The Directory: Overview of Concepts, Models and Service. ISO/IEC JTC
1/SC21; International Standard 9594-1, 1988.

Smith Standards Track [Page 4]

RFC 2079 URI Attribute Type and Object Class January 1997

Author's Address

Mark Smith
Netscape Communications Corp.
501 E. Middlefield Rd.
Mountain View, CA 94043, USA

Phone: +1 415 937-3477

Smith Standards Track [Page 5]

Site Hosted By Digital Environments, Inc. This Website was Created with DE-Web Version,
The Fast, Web Based - Website Design Tool, Groupware and Web Hosting System by Digital Environments, Inc.
Groupware:Project Management, Sales Tracking, Web Site Design and News / Blogger all in one package.