CoRE Working Group K. Hartke Internet-Draft Universitaet Bremen TZI Intended status: Informational April 16, 2015 Expires: October 18, 2015 CoRE Application Descriptions draft-hartke-core-apps-00 Abstract This document defines a template for describing the interface of RESTful, hypertext-driven applications in Constrained RESTful Environments (CoRE). Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on October 18, 2015. Copyright Notice Copyright (c) 2015 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 (http://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. Hartke Expires October 18, 2015 [Page 1] Internet-Draft CoRE Application Descriptions April 2015 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Application Descriptions . . . . . . . . . . . . . . . . . . 3 2.1. Communication Protocols and URI Schemes . . . . . . . . . 3 2.2. Internet Media Types . . . . . . . . . . . . . . . . . . 3 2.3. Representation Formats . . . . . . . . . . . . . . . . . 4 2.4. Link Relation Types . . . . . . . . . . . . . . . . . . . 4 2.5. Well-Known Locations . . . . . . . . . . . . . . . . . . 5 2.6. URI Structures . . . . . . . . . . . . . . . . . . . . . 5 3. Template . . . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Security Considerations . . . . . . . . . . . . . . . . . . . 6 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6 7.1. Normative References . . . . . . . . . . . . . . . . . . 6 7.2. Informative References . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7 1. Introduction The Constrained Application Protocol (CoAP) [RFC7252] is designed to enable applications implementing the REST architectural style [REST] in Constrained-Node Networks [RFC7228]. As CoAP applications are implemented and deployed, it becomes increasingly important to be able to describe them in some structured way to promote interoperability and reuse. Previous efforts like WADL [WADL] focus on code generation from machine-readable service descriptions and are not truly RESTful [DWNWADL]. REST APIs must be hypertext-driven [RESTAPI]. This means that the application interface is a description of the common vocabulary between the client and the server, centered around media types and link relation types, rather than a list of resources and the operations supported on them. RESTful applications are often easy to understand, but require some design effort. The reward is long-term stability and evolvability ("design for decades"). REST is intended for long-lived network- based applications that span multiple organizations. This document defines a template for describing the interface of RESTful, hypertext-driven applications in Constrained RESTful Environments (CoRE). Hartke Expires October 18, 2015 [Page 2] Internet-Draft CoRE Application Descriptions April 2015 2. Application Descriptions In this specification, an application description is a named set of reusable, self-descriptive components, and is comprised of: o Communication protocols, o URI schemes, o Internet media types, o link relation types, and o optionally, well-known locations. Together, these components provide the specific, in-band instructions for interfacing with a given service. 2.1. Communication Protocols and URI Schemes The basics of a hypertext-driven REST API are the communication protocol(s) spoken between a client and a server. Although HTTP [RFC7230] is by far the most common communication protocol for REST APIs, a REST API should typically not be dependent on a specific communication protocol. The use of a particular protocol is guided by URI schemes [RFC3986] describing the URI references found in links and forms (Section 2.3). For example, the COAPS URI scheme identifies resources that are accessible over CoAP [RFC7252] using DTLS [RFC6347] for transport security. IANA maintains a list of registered URI schemes at . 2.2. Internet Media Types One of the most important aspect of hypertext-driven communications is the concept of media types. Media types are used to label resource representations so that it is known how the representation should be interpreted, and how it is encoded. The core of an application description should be one or more hypertext media types. Media types consist of a top-level type and a subtype, structured into trees [RFC6838]. Optionally, media types can have parameters. For example, the media type "text/plain; charset=utf-8" is a subtype for generic text under the "text" top-level type in the standards tree, and has a parameter "charset" set to "utf-8". Hartke Expires October 18, 2015 [Page 3] Internet-Draft CoRE Application Descriptions April 2015 Media types can be further refined by structured type name suffixes (e.g., "+xml" appended to the base subtype name; see Section 4.2.8 of [RFC6838]), or by subtype information embedded in the representations themselves (e.g., "xmlns" declarations in XML documents). A media type must be determined from in-band information (e.g., from the CoAP Content-Format option). Clients must not assume a structure from the application context or other out-of-band information. IANA maintains a list of registered Internet media types at . IANA maintains a list of registered structured suffixes at . IANA maintains a list of registered CoAP content formats at . 2.3. Representation Formats In RESTful applications, clients and servers exchange representations that capture the current or intended state of a resource and are labeled with a media type. A representation is a sequence of bytes whose structure and semantics are specified by a representation format, a set of rules for encoding information in digital form. Representation formats enable hypertext-driven applications when they support the expression of links and/or forms. o A link is a typed connection between two resources [RFC5988] and is the primary means for a client to change application state. It is comprised of a context (usually the current resource), a link relation type (Section 2.4), a target resource URI, and, optionally, some attributes that describe the link target. URI Templates [RFC6570] provide a way to describe a range of URIs as the link target through variable expansion. o A form is associated with a single resource and is the primary means for a client to change resource state. It is comprised of a target resource URI, a submission method (PUT, POST, PATCH, or DELETE), and, optionally, a description of a representation that the service accepts as part of the form submission. 2.4. Link Relation Types A link relation type identifies the semantics of a link [RFC5988]. For example, a link with the relation type "copyright" indicates that Hartke Expires October 18, 2015 [Page 4] Internet-Draft CoRE Application Descriptions April 2015 the resource identified by the target URI is a statement of the copyright terms applying to the current context. Relation types are not to be confused with media types [RFC6838]; they do not identify the format of the representation that results when the link is dereferenced. Rather, they only describe how the current context is related to another resource. IANA maintains a list of registered link relation types at . 2.5. Well-Known Locations Some applications may require the discovery of information about a host ("site-wide metadata"). For example, [RFC6415] defines a metadata document format for describing hosts; similarly, [RFC6690] defines a link format for the discovery of resources hosted by a server. Applications that need to define a resource for site-wide metadata can register new "well-known locations". [RFC5785] defines a path prefix in HTTP(S) URIs for this purpose, "/.well-known/"; [RFC7252] extends this concept to COAP(S) URIs. IANA maintains a list of registered well-known URIs at . 2.6. URI Structures Application descriptions must not constrain URI structures in ways that aren't explicitly allowed by [RFC3986]. In particular, mandating particular forms of URI substructure is inappropriate. [RFC7320] describes this problematic practice and provides some acceptable alternatives for use in application descriptions. 3. Template Application name: Protocols: URI schemes: Media types: Link relations: Well-known locations: Hartke Expires October 18, 2015 [Page 5] Internet-Draft CoRE Application Descriptions April 2015 4. Security Considerations This document raises no new security issues beyond those described in the referenced documents. 5. IANA Considerations This document includes no request to IANA. 6. Acknowledgements Thanks to Carsten Bormann, Stefanie Gerdes, Matthias Kovatsch, Teemu Savolainen, and Bilhanan Silverajan for helpful comments and discussions that have shaped the document. Some of the text in this document has been borrowed from [RESTAPI], [RFC5988], and [RFC7320]. All errors are my own. This work was funded in part by Nokia. 7. References 7.1. Normative References [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005. [RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known Uniform Resource Identifiers (URIs)", RFC 5785, April 2010. [RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010. [RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M., and D. Orchard, "URI Template", RFC 6570, March 2012. [RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type Specifications and Registration Procedures", BCP 13, RFC 6838, January 2013. [RFC7320] Nottingham, M., "URI Design and Ownership", BCP 190, RFC 7320, July 2014. Hartke Expires October 18, 2015 [Page 6] Internet-Draft CoRE Application Descriptions April 2015 7.2. Informative References [DWNWADL] Gregorio, J., "Do we need WADL?", June 2007, . [REST] Fielding, R., "Architectural Styles and the Design of Network-based Software Architectures", Ph.D. Dissertation, University of California, Irvine, 2000, . [RESTAPI] Fielding, R., "REST APIs must be hypertext-driven", October 2008, . [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, January 2012. [RFC6415] Hammer-Lahav, E. and B. Cook, "Web Host Metadata", RFC 6415, October 2011. [RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, August 2012. [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for Constrained-Node Networks", RFC 7228, May 2014. [RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, June 2014. [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, June 2014. [WADL] Hadley, M., "Web Application Description Language", W3C Member Submission SUBM-wadl-200908316, August 2009, . Author's Address Klaus Hartke Universitaet Bremen TZI Postfach 330440 Bremen D-28359 Germany Phone: +49-421-218-63905 EMail: hartke@tzi.org Hartke Expires October 18, 2015 [Page 7]