GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc1736

Network Working Group J. Kunze Request for Comments: 1736 IS&T, UC Berkeley Category: Informational February 1995

     Functional Recommendations for Internet Resource Locators

Status of this Memo

 This memo provides information for the Internet community.  This memo
 does not specify an Internet standard of any kind.  Distribution of
 this memo is unlimited.

1. 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.

2. 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

Kunze [Page 1] RFC 1736 Recommendations for IRLs February 1995

 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.

2.1 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.

2.1.1. 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.

2.1.2. …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 possibilities.

Kunze [Page 2] RFC 1736 Recommendations for IRLs February 1995

2.1.3. …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.

2.1.4. …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.

2.2 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.

2.2.1 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

Kunze [Page 3] RFC 1736 Recommendations for IRLs February 1995

 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.

2.2.2 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

Kunze [Page 4] RFC 1736 Recommendations for IRLs February 1995

 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)

2.3 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.

2.3.1. 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.

2.3.2. 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.

2.3.3. 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).

2.3.4. 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.

3. 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.

Kunze [Page 5] RFC 1736 Recommendations for IRLs February 1995

 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.

4. Requirements List for Internet Resource Locators

 This list of requirements is applied to the set of general locators
 defined in section 2.1.  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.

4.1 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.

Kunze [Page 6] RFC 1736 Recommendations for IRLs February 1995

4.2 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.

4.3 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:

4.3.1 A given kind of locator may still be parsable even if a given

     interpreter cannot parse it.

4.3.2 Parsable by users does not imply readily parsable by untrained

     users.

4.3.3 A given locator need not be completely parsable by any one

     interpreter as long as a combination of interpreters can parse
     it completely.

4.4 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.

4.5 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.

4.6 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.

Kunze [Page 7] RFC 1736 Recommendations for IRLs February 1995

4.7 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.

4.8 The set of services is extensible.

 New services can be added over time.

4.9 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 standard.

5. 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

Kunze [Page 8] RFC 1736 Recommendations for IRLs February 1995

 concerns.
 For services that by nature offer a great deal of user freedom
 (remote login for example), the pre-specification of user commands
 within a locator presents vulnerabilities.  With careful command
 screening, the deleterious effects of unknowingly executing (at the
 client or server) an embedded command such as "rm -fr *" can be
 avoided.

6. 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.

7. 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

Kunze [Page 9] RFC 1736 Recommendations for IRLs February 1995

    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, Bunyip

8. Author's Address

 John A. Kunze
 Information Systems and Technology
 293 Evans Hall
 Berkeley, CA  94720
 Phone: (510) 642-1530
 Fax:   (510) 643-5385
 EMail: jak@violet.berkeley.edu

Kunze [Page 10]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1736.txt · Last modified: 1995/02/08 01:13 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki