Movatterモバイル変換

Network Working Group                                           J. KunzeRequest for Comments: 1736                             IS&T, UC BerkeleyCategory: Informational                                    February 1995Functional Recommendations for Internet Resource LocatorsStatus 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 theirKunze                                                           [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 19952.1.3.  ...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 beKunze                                                           [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 aKunze                                                           [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 insection 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 19954.2 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 19954.7 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 theseKunze                                                           [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 MinnesotaKunze                                                           [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, Bunyip8. 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.eduKunze                                                          [Page 10]