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INFORMATIONAL
Internet Engineering Task Force (IETF)                          E. Wilde
Request for Comments: 8631                                     July 2019
Category: Informational
ISSN: 2070-1721


                  Link Relation Types for Web Services

Abstract

   Many resources provided on the Web are part of sets of resources that
   are provided in a context that is managed by one particular service
   provider.  Often, these sets of resources are referred to as "Web
   services" or "Web APIs".  This specification defines link relations
   that represent relationships from Web services or APIs to resources
   that provide documentation, descriptions, metadata, or status
   information for these resources.  Documentation is primarily intended
   for human consumers, whereas descriptions are primarily intended for
   automated consumers.  Metadata provides information about a service's
   context.  This specification also defines a link relation to identify
   status resources that are used to represent information about service
   status.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8631.













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Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Web Services  . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Documenting Web Services  . . . . . . . . . . . . . . . .   5
     3.2.  Describing Web Services . . . . . . . . . . . . . . . . .   5
     3.3.  Unified Documentation/Description . . . . . . . . . . . .   5
   4.  Link Relations for Web Services . . . . . . . . . . . . . . .   5
     4.1.  The service-doc Link Relation Type  . . . . . . . . . . .   6
     4.2.  The service-desc Link Relation Type . . . . . . . . . . .   6
     4.3.  The service-meta Link Relation Type . . . . . . . . . . .   6
   5.  Web Service Status Resources  . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Link Relation Type: service-doc . . . . . . . . . . . . .   7
     6.2.  Link Relation Type: service-desc  . . . . . . . . . . . .   7
     6.3.  Link Relation Type: service-meta  . . . . . . . . . . . .   8
     6.4.  Link Relation Type: status  . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9












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1.  Introduction

   One of the defining aspects of the Web is that it is possible to
   interact with Web resources without any prior knowledge of the
   specifics of the resource.  Following the practices described in Web
   architecture [W3C.REC-webarch-20041215] by using URIs, HTTP, and
   media types, the Web's uniform interface allows interactions with
   resources without the more complex binding procedures often necessary
   with other approaches.

   Many resources on the Web are provided as part of a set of resources
   that are referred to as a "Web service" or a "Web API".  In many
   cases, these services or APIs are defined and managed as a whole, and
   it may be desirable for clients to be able to discover this service
   information.

   Service information that provides information on how to use service
   resources can be broadly separated into two categories: One category
   primarily targets human users and often uses generic representations
   for human readable documents, such as HTML or PDF.  The other
   category is structured information that follows a more formalized
   description model and is primarily intended for consumption by
   machines -- for example, tools and code libraries.

   In the context of this memo, the human-oriented variant is referred
   to as "documentation", and the machine-oriented variant is referred
   to as "description".

   These two categories are not necessarily mutually exclusive, as
   representations have been proposed that are intended for both human
   consumption and interpretation by machine clients.  In addition, a
   typical pattern for service documentation/descriptions is that there
   is human-oriented, high-level documentation that is intended to put a
   service in context and explain the general model, which is
   complemented by machine-level descriptions that are intended as
   detailed technical descriptions of the service.  These two resources
   could be interlinked, but since they are intended for different
   audiences, it can make sense to provide entry points for both of
   them.

   In addition, while both documentation and descriptions may be
   provided as part of a Web service, there may be other information as
   well.  Generally speaking, a Web service may have any metadata/
   resources associated with it (with documentation and descriptions
   being two specific kinds of resources).  If there is a way in which
   all of these metadata/resources can be represented, then it should be
   possible to find such a resource that provides access to general Web
   service metadata.



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   In addition to these resources about mostly static aspects of a Web
   service, this memo also defines a link relation that allows providers
   of a Web service to link to a resource that represents status
   information about the service.  This information often represents
   operational information that allows service consumers to retrieve
   information about "service health" and related issues.

   This memo places no constraints on the specific representations used
   for all of these resources.  It simply allows providers of a Web
   service to make the documentation, descriptions, metadata, and status
   of their services discoverable and defines link relations that serve
   that purpose.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Web Services

   "Web Services" or "Web APIs" (sometimes also referred to as "HTTP
   API" or "REST API") expose information and services on the Web.
   Following the principles of Web architecture
   [W3C.REC-webarch-20041215], they expose URI-identified resources,
   which are then accessed and transferred using a specific
   representation.  Many services use representations that contain
   links, and these links are often typed links.

   Using typed links, resources can identify relationship types to other
   resources.  RFC 8288 [RFC8288] establishes a framework of registered
   link relation types, which are identified by simple strings and
   registered in an IANA registry.  Any resource that supports typed
   links according to RFC 8288 can then use these identifiers to
   represent resource relationships on the Web without having to
   reinvent registered relation types.

   In recent years, Web services, as well as their documentation and
   description languages, have gained popularity due to the general
   popularity of the Web as a platform for providing information and
   services.  However, the design of documentation and description
   languages varies according to a number of factors, such as the
   general application domain, the preferred application data model, and
   the preferred approach for exposing services.





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   This specification allows service providers to use a unified method
   to link to service documentation and/or descriptions.  This link
   should not make any assumptions about the provided type of
   documentation and/or descriptions, so service providers can choose
   those that best fit their services and needs.

   This specification also allows service providers to link to general
   service metadata.  One part of the metadata may have links to
   documentation and/or description as well as other information about a
   service, such as deployment or operational information.

3.1.  Documenting Web Services

   In the context of this specification, "documentation" refers to
   information that is primarily intended for human consumption.
   Typical representations of this kind of documentation are HTML and
   PDF.

   Documentation is often structured, but its structure depends on the
   structure of the service in question as well as the specific way in
   which the authors choose to present it.

3.2.  Describing Web Services

   In the context of this specification, "description" refers to
   information that is primarily intended for machine consumption.
   Typical representations are dictated by the technology underlying the
   service itself, which means that description formats in today's
   technology landscape are based on XML, JSON, Resource Description
   Framework (RDF), and a variety of other structured data models.  In
   each of those technologies, there may be a variety of languages
   defined to serve the same general purpose of describing a Web
   service.

   Descriptions are always structured, but the structuring principles
   depend on the nature of the described service.  For example, one of
   the earlier service description approaches, the Web Services
   Description Language (WSDL), uses "operations" as its core concept,
   which are essentially identical to function calls because the
   underlying model is based on the Remote Procedure Call (RPC) model.
   Other description languages for non-RPC approaches to services will
   use different structuring approaches, such as structuring service
   descriptions by URIs and/or URI patterns.








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3.3.  Unified Documentation/Description

   If service providers use an approach where there is no distinction
   between service documentation (Section 3.1) and service description
   (Section 3.2), then they may not feel the need to use two separate
   links.  In such a case, an alternative approach is to use the
   previously defined "service" link relation type [RFC5023], which does
   not indicate whether it links to documentation or description and
   thus may be a better fit if no such differentiation is required.

4.  Link Relations for Web Services

   In order to allow Web services to represent the relation of
   individual resources to service documentation/description and
   metadata, this specification introduces and registers three new link
   relation types.

4.1.  The service-doc Link Relation Type

   The "service-doc" link relation type is used to represent the fact
   that a resource or a set of resources is documented at a specific
   URI.  The target resource is expected to provide documentation that
   is primarily intended for human consumption.

4.2.  The service-desc Link Relation Type

   The "service-desc" link relation type is used to represent the fact
   that a resource or a set of resources is described at a specific URI.
   The target resource is expected to provide a service description that
   is primarily intended for machine consumption.  In many cases, it is
   provided in a representation that is consumed by tools, code
   libraries, or similar components.

4.3.  The service-meta Link Relation Type

   The "service-meta" link relation type is used to link to available
   metadata for the service context of a resource.  Service metadata is
   any kind of data that may be of interest to existing or potential
   service users, with documentation/description being only two possible
   facets of service metadata.  The target resource is expected to
   provide a representation that is primarily intended for machine
   consumption.  In many cases, it is provided in a representation that
   is consumed by tools, code libraries, or similar components.

   Since service metadata can have many different purposes and use many
   different representations, it may make sense for representations
   using the "service-meta" link relation to offer additional hints
   about the specific kind or format of metadata that is being linked.



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   This definition of the "service-meta" link relation makes no specific
   assumptions about how these link hints will be represented, and the
   specific mechanism will depend on the context where the "service-
   meta" link relation is being used.

   One example is that a "service-desc" link may identify an OpenAPI
   description, which is supposed to be the machine-readable description
   of a Web API.  A "service-meta" link may identify a resource that
   contains additional metadata about the Web API, such as labels that
   classify the API according to a labeling scheme and a privacy policy
   that makes statements about how the Web API manages personally
   identifiable information.

5.  Web Service Status Resources

   Web services providing access to one or more resources often are
   hosted and operated in an environment for which status information
   may be available.  This information may be as simple as confirming
   that a service is operational, or it may provide additional
   information about different aspects of a service and/or a history of
   status information, possibly listing incidents and their resolution.

   The "status" link relation type can be used to link to such a status
   resource, allowing service consumers to retrieve information about a
   Web service's status.  Such a link may not be available for and from
   all resources provided by a Web service -- only from key resources
   such as a Web service's metadata resource and/or a service's home
   resource (i.e., a resource analogous to the home page of a Web site).

   This memo does not restrict the representation of a status resource
   in any way.  It may be primarily focused on human or machine
   consumption or a combination of both.  It may be a simple "traffic
   light" indicator for service health or a more sophisticated
   representation conveying more detailed information, such as service
   subsystems and/or a status history.
















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6.  IANA Considerations

   The link relation types below have been registered by IANA per
   Section 4.2 of RFC 8288 [RFC8288].

6.1.  Link Relation Type: service-doc

   Relation Name:  service-doc

   Description:  Identifies service documentation for the context that
      is primarily intended for human consumption.

   Reference:  RFC 8631

6.2.  Link Relation Type: service-desc

   Relation Name:  service-desc

   Description:  Identifies service description for the context that is
      primarily intended for consumption by machines.

   Reference:  RFC 8631

6.3.  Link Relation Type: service-meta

   Relation Name:  service-meta

   Description:  Identifies general metadata for the context that is
      primarily intended for consumption by machines.

   Reference:  RFC 8631

6.4.  Link Relation Type: status

   Relation Name:  status

   Description:  Identifies a resource that represents the context's
      status.

   Reference:  RFC 8631











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7.  Security Considerations

   Web service providers should be aware that service descriptions and
   documentation may be used by attackers to gain additional information
   about a service (particularly its implementation) and to test for
   known security issues.  It may thus be advisable to restrict service
   descriptions and documentation to aspects of a service that are
   necessary and to exclude any details that are not necessary for using
   the service.

   Another potential security issue for Web service providers is that
   publishing service descriptions and documentation may generally allow
   clients (both malicious and otherwise) more automated and systematic
   access to a service.  It may therefore be possible that more of a
   service's potential vulnerabilities are made easier to find and
   exploit or simply that a service might receive more load because it
   is accessed by automated clients.

   Web service consumers should be aware that service descriptions and
   documentation can be out of sync or simply incorrect.  Blindly
   trusting service descriptions and documentation (particularly when
   descriptions are retrieved and interpreted programmatically) is not a
   safe practice.  Web service consumers SHOULD always assume that
   service descriptions and documentation may be incorrect and SHOULD
   therefore be prepared to handle errors at runtime.

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/info/rfc8288>.









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8.2.  Informative References

   [RFC5023]  Gregorio, J., Ed. and B. de hOra, Ed., "The Atom
              Publishing Protocol", RFC 5023, DOI 10.17487/RFC5023,
              October 2007, <https://www.rfc-editor.org/info/rfc5023>.

   [W3C.REC-webarch-20041215]
              Jacobs, I. and N. Walsh, "Architecture of the World Wide
              Web, Volume One", World Wide Web Consortium
              Recommendation REC-webarch-20041215, December 2004,
              <http://www.w3.org/TR/2004/REC-webarch-20041215>.

Acknowledgements

   Thanks to Mike Amundsen, Ben Campbell, Alissa Cooper, Oliver Gierke,
   Benjamin Kaduk, Sebastien Lambla, Darrell Miller, and Adam Roach for
   their comments and suggestions.

Author's Address

   Erik Wilde

   Email: erik.wilde@dret.net
   URI:   http://dret.net/netdret/



























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