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Versions: 01 02 rfc1736                                                 
IETF URI Working Group                                      John A. Kunze
Internet-Draft                                          IS&T, UC Berkeley
28 November 1994                                    Expires in six months

          Functional Requirements for Internet Resource Locators

1. Status of this Document

This document is an Internet-Draft.  Internet-Drafts are working documents
of the Internet Engineering Task Force (IETF), its Areas, and its Working
Groups.  Note that other groups may also distribute working documents as

Internet-Drafts are working documents valid for a maximum of six months.
Internet-Drafts may be updated, replaced, or obsoleted by other documents
at any time.  It is not appropriate to use Internet-Drafts as reference
material or to cite them other than as a ``working draft' or ``work in

To learn the current status of any Internet-Draft, please check the
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Distribution of this document is unlimited.  Please send comments to the
author at jak@violet.berkeley.edu or to the discussion list uri@bunyip.com.

2. Introduction

This document specifies a minimum set of requirements for Internet
resource locators, which convey location and access information for
resources.  Typical examples of resources include network accessible
documents, WAIS databases, FTP servers, and Telnet destinations.

Locators may apply to resources that are not always or not ever network
accessible.  Examples of the latter include human beings and physical
objects that have no electronic instantiation (that is, objects without
an existence completely defined by digital objects such as disk files).

A resource locator is a kind of resource identifier.  Other kinds of
resource identifiers allow names and descriptions to be associated with
resources.  A resource name is intended to provide a stable handle
to refer to a resource long after the resource itself has moved or
perhaps gone out of existence.  A resource description comprises a
body of meta-information to assist resource search and selection.

In this document, an Internet resource locator is a locator defined by
an Internet resource location standard.  A resource location standard
in conjunction with resource description and resource naming standards
specifies a comprehensive infrastructure for network based information
dissemination.  Mechanisms for mapping between locators, names, and
descriptive identifiers are beyond the scope of this document.

3. Overview of Problem

Network-based information resource providers require a method of describing
the location of and access to their resources.  Information systems users
require a method whereby client software can interpret resource access and
location descriptions on their behalf in a relatively transparent way.
Without such a method, transparent and widely distributed, open information
access on the Internet would be difficult if not impossible.

3.1 Defining the General Resource Locator

The requirements listed in this document impose restrictions on the
general resource locator.  To better understand what the Internet resource
locator is, the following general locator definition provides some contrast.

        Definition:  A general resource locator is an object
                     that describes the location of a resource.

This definition deliberately allows many degrees of freedom in order
to contain the furthest reaches of the wide-ranging debate on resource
location standards.  Vast as it is, this problem space is a useful
backdrop for discussion of the requirements (later) that generate a
smaller, more manageable problem space.  A resource location standard
shrinks the space again by applying additional requirements.

Consider the definition in four parts: (1) A general resource locator
is an object (2) that describes (3) the location of (4) a resource.

3.1.1.  A general resource locator is an object...

The object could be a complex data structure.  It could be a contiguous
sequence of bytes.  It could be a pair of latitude-longitude coordinates,
or a three-color road map printed on paper.  It could be a sequence of
characters that are capable of being printed on paper.

3.1.2.  ...that describes

In the fully general case, there are many ways that a resource locator
could describe the location.  It could employ a graphical or natural
language description.  It could be heavily encoded or compressed.  It
could be lightly encoded and readily understandable by human beings.
The description could be a multi-level hierarchy with common semantics
at each level.  It could be a multi-level hierarchy with common semantics
at only the first two levels, where semantics below the second level
depend on the value given at the first level.  These are just a few

3.1.3.  ...the location of

A resource locator describes a location but never guarantees that access
may be established.  While access is often desired when clients follow
location instructions given in a conformant resource locator, the resource
need not exist any longer or need not exist yet.  Indeed it may never exist,
even though the locator continues to describe a location where a resource
might exist (e.g., it might be used as a placeholder with resource
availability contingent upon an event such as a payment).

Furthermore, the nature of certain potential resources, especially animate
beings or physical objects with no electronic instantiation, makes network
access meaningless in some cases; such resources have locators that would
imply non-networked access, but again, access is not guaranteed.

3.1.4.  ...a resource.

A resource can be many things.  Besides the non-networked or non-electronic
resources just mentioned, familiar examples are an electronic document,
an image, a server (e.g., FTP, Gopher, Telnet, HTTP), or a collection
of items (e.g., Gopher menu, FTP directory, HTML page).  Other examples
accompany multi-function protocols such as Z39.50, which can perform
single round trip network access, session-oriented search refinement,
and index browsing.

3.2 Producers and Interpreters of Resource Locators

Central to the discussion of locator requirements is the issue of
parsability.  This is the ability of an agent to recognize or understand
a locator in whole or in part.  Discussion may be assisted by clearly
distinguishing the two main actions associated with locators.

Resource locators are both produced and interpreted.  Producers are bound
by the resource location standards that are in turn bound by requirements
listed in this document.  Interpreters of locators are not bound by the
requirements; they are beneficiaries of them.

3.2.1 Resource Locator Interpreters

A resource locator is interpreted by interpreting agents, which in this
document are simply called interpreters.  Interpreters may be either
human beings or software.  Along the way to establishing access based
on information in a locator, one or more interpreters may be employed.
Some examples of multiple interpreters processing a single locator
illustrate the concept that a resource locator may be understandable
only in part by each of several interpreters, but understandable in
its entirety by a combination of interpreters.

In the first example, a software interpreter recognizes enough of a
locator to understand to which external agent it needs to forward it.
Here, the external agent might be a user and the locator a library call
number; the software forwards the locator simply by displaying it. The
agent might be a network software layer specializing in a particular
communications protocol; once the service is recognized, the locator
is forwarded to it along with an access request.

In another example, a human interpreter might also recognize enough
of a locator to understand where to forward it.  Here, the person might
be a user who recognizes a library call number as such but who does not
understand the location information encoded in it; the person forwards
it to a library employee (an external agent) who knows how to establish
access to the library resource.

A prerequisite to interpreting a locator is understanding when an object
in question actually is a locator, or contains one or more locators.  Some
constrained environments make this question easy to answer, for example,
within HTML anchors or Gopher menu items.  Less constrained environments,
such as within running text, make it more difficult to answer without
well-defined assumptions.  A resource location standard needs to make any
such assumptions explicit.

3.2.2 Resource Locator Producers

Resource locators are produced in many ways, often by an agent that also
interprets them.  The provider of a resource may produce a locator for it,
leaving the locator in places where it is intended to be discovered, such
as an HTML page, a Gopher menu, or an announcement to an e-mail list.

Non-providers of resources can be major producers of locators; for example,
WWW client software produces locators by translating foreign resource
locators (e.g., Gopher menu items) to its own format.  Some locator
databases (e.g., Archie) have been maintained by automated processes that
produce locators for hundreds of thousands of FTP resources that they
"discover" on the Internet.

Users are major producers of resource locators.  A user constructing one
to share with others is responsible for conformance with locator standards.
Sometimes a user composes a resource locator based on an educated guess
and submits it to client software with the intent of establishing access.
Such a user is a producer in a sense, but if the locator is purely for
personal consumption the user is not bound by the requirements.  In fact,
some client software may offer as a service to translate abbreviated,
non-conformant locators entered by users into successful access
instructions or into conformant locators (e.g., by adding a domain name
to an unqualified hostname)

3.3 Uniqueness of Resource Locators

The topic of a "uniqueness" requirement for resource locators has been
discussed a great deal.  This document considers the following aspects
of uniqueness, but deliberately rejects them as requirements.  It is
incumbent upon a resource location standard that takes on this topic
to be clear about which aspects it addresses.

3.3.1. Uniqueness and Multiple Copies of a Resource
A uniqueness requirement might dictate that no identical copies of a
resource may exist.  This document makes no such requirement.

3.3.2. Uniqueness and Deterministic Access
A uniqueness requirement might dictate that the same resource accessed
in one attempt will also be the result of any other successful attempt.
This document makes no such requirement, nor does it define "sameness".
It is inappropriate for a resource location standard to define "sameness"
among resources.

3.3.3. Uniqueness and Multiple Locators
A uniqueness requirement might dictate that a resource have no more than
one locator unless all such locators be the same.  This document makes
no such requirement, nor does it define "sameness" among locators
(which a standard might do using, for example, canonicalization rules).

3.3.4. Uniqueness, Ambiguity, and Multiple Objects per Access
A uniqueness requirement might dictate that a resource locator identify
exactly one object as opposed to several objects.  This document makes
no general definition of what constitutes one object, several objects,
or one object consisting of several objects.

4. Resource Access and Availability

A locator never guarantees access, but establishing access is by far the
most important intended application of a resource locator.  While it is
considered ungracious to advertize a locator for a resource that will
never be accessible (whether a "networkable" resource or not), it is
normal for resource access to fail at a rate that increases with the
age of the locator used.

Resource access can fail for many reasons.  Providers fundamentally affect
accessibility by moving, replacing, or deleting resources over time.  The
frequency of such changes depends on the nature of the resource and provider
service practices, among other things.  A locator that conforms to a
location standard but fails for one of these reasons is called "invalid"
for the purposes of this document; the term invalid locator does not apply
to malformed or non-conformant locators.  Resource naming standards
address the problem of invalid locators.

Ordinary provider support policies may cause resources to be inaccessible
during predictable time periods (e.g., certain hours of the day, or days
of the year), or during periods of heavy system loading.  Rights clearance
restrictions impossible to express in a locator also affect accessibility
for certain user populations.  Heavy network load can also prevent access.
In such situations, this document calls a resource "unavailable".
A locator can both be valid and identify a resource that is unavailable.
Resource description standards address, among other things, some aspects
of resource availability.

In general, the probability with which a given resource locator leads
to successful access decreases over time, and depends on conditions such as
the nature of the resource, support policies of the provider, and loading
of the network.

5. Requirements List for Internet Resource Locators

This list of requirements is applied to the set of general locators
defined in section 3.1.  The resulting subset, called Internet locators
in this document, is suitable for further refinement by an Internet
resource location standard.  Some requirements concern locator encoding
while others concern locator function.

One requirement from the original draft list was dropped after extensive
discussion revealed it to be impractical to meet.  It stated that with a
high degree of reliability, software can recognize Internet locators in
certain relatively unstructured environments, such as within running
ASCII text.

5.1 Locators are transient.

The probability with which a given Internet resource locator leads to
successful access decreases over time.  More stable resource identifier
schemes are addressed in resource naming standards and are outside
the scope of a resource location standard.

5.2 Locators have global scope.

The name space of resource locators includes the entire world.
The probability of successful access using an Internet locator
depends in no way, modulo resource availability, on the geographical
or Internet location of the client.

5.3 Locators are parsable.

Internet locators can be broken down into complete constituent parts
sufficient for interpreters (software or human) to attempt access if
desired.  While these requirements do not bind interpreters, three
points bear emphasizing:

5.3.1  A given kind of locator may still be parsable even if a given
       interpreter cannot parse it.
5.3.2  Parsable by users does not imply readily parsable by untrained users.
5.3.3  A given locator need not be completely parsable by any one interpreter
       as long as a combination of interpreters can parse it completely.

5.4 Locators can be readily distinguished from naming and descriptive
    identifiers that may occupy the same name space.

During a transition period (of possibly indefinite length), other kinds
of resource identifier are expected to co-exist in data structures along
with Internet locators.

5.5 Locators are "transport-friendly".

Internet locators can be transmitted from user to user (e.g, via e-mail)
across Internet standard communications protocols without loss or
corruption of information.

5.6 Locators are human transcribable.

Users can copy Internet locators from one medium to another (such as
voice to paper, or paper to keyboard) without loss or corruption of
information.  This process is not required to be comfortable.

5.7 An Internet locator consists of a service and an opaque parameter package.

The parameter package has meaning only to the service with which it is
paired, where a service is an abstract access method.  An abstract access
method might be a software tool, an institution, or a network protocol.
The parameter package might be service-specific access instructions.
In order to protect creative development of new services, there is an
extensible class of services for which no parameter package semantics
common across services may be assumed.

5.8 The set of services is extensible.

New services can be added over time.

5.9 Locators contain no information about the resource other than that
    required by the access mechanism.

The purpose of an Internet locator is only to describe the location of
a resource, not other properties such as its type, size, modification
date, etc.  These and other properties belong in a resource description

6. Security Considerations

While the requirements have no direct security implications, applications
based on standards that fulfill them may need to consider two potential
vulnerabilities.  First, because locators are transient, a client using an
invalid locator might unwittingly gain access to a resource that was not
the intended target.  For example, when a hostname becomes unregistered
for a period of time and then re-registered, a locator that was no longer
valid during that period might once again lead to a resource, but perhaps
to one that only pretends to be the original resource.

Second, because a locator consists of a service and a parameter package,
potentially enormous processing freedom is allowed, depending on the
individual service.  A server is vulnerable unless it suitably restricts
its input parameters.  For example, a server that advertizes locators for
certain local filesystem objects may inadvertently open a door through
which other filesystem objects can be accessed.

A client is also vulnerable unless it understands the limitations of the
service it is using.  For example, a client trusting a locator obtained
from an uncertain source might inadvertently trigger a mechanism that
applies charges to a user account.  Having a clear definition of service
limitations could help alleviate some of these concerns.

7. Conclusion

Resource location standards, which define Internet resource locators,
give providers the means to describe access information for their
resources.  They give client developers the ability to access disparate
resources while hiding access details from users.

Several minimum requirements distinguish an Internet locator from a general
locator.  Internet resource locators are impermanent handles sufficiently
qualified for resource access not to depend in general on client location.
Locators can be recognized and parsed, and can be transmitted unscathed
through a variety of human and Internet communication mechanisms.

An Internet resource locator consists of a service and access parameters
meaningful to that service.  The form of the locator does not discourage
the addition of new services or the migration to other resource identifiers.
A clean distinction between resource location, resource naming, and resource
description standards is preserved by limiting Internet locators to no more
information than what is required by an access mechanism.

8. Acknowledgements

The core requirements of this document arose from a collaboration of the
following people at the November 1993 IETF meeting in Houston, Texas.

Farhad Ankelesaria, University of Minnesota
John Curran, NEARNET
Peter Deutsch, Bunyip
Alan Emtage, Bunyip
Jim Fullton, CNIDR
Kevin Gamiel, CNIDR
Joan Gargano, University of California at Davis
John Kunze, University of California at Berkeley
Clifford Lynch, University of California
Lars-Gunnar Olson, Swedish University of Agriculture
Mark McCahill, University of Minnesota
Michael Mealing, Georgia Tech
Mitra, Pandora Systems
Pete Percival, Indiana University
Margaret St. Pierre, WAIS, Inc.
Rickard Schoultz, KTH
Janet Vratny, Apple Computer Library
Chris Weider, Merit Network

9. Author's Address

John A. Kunze
Information Systems and Technology
293 Evans Hall
Berkeley, CA  94720
Voice: (510) 642-1530
Fax:   (510) 643-5385