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
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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).
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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
<http://www.iana.org/assignments/uri-schemes>.
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".
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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
<http://www.iana.org/assignments/media-types>.
IANA maintains a list of registered structured suffixes at
<http://www.iana.org/assignments/media-type-structured-suffix>.
IANA maintains a list of registered CoAP content formats at
<http://www.iana.org/assignments/core-parameters>.
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
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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
<http://www.iana.org/assignments/link-relations>.
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
<http://www.iana.org/assignments/well-known-uris>.
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:
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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.
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7.2. Informative References
[DWNWADL] Gregorio, J., "Do we need WADL?", June 2007,
<http://bitworking.org/news/193/Do-we-need-WADL>.
[REST] Fielding, R., "Architectural Styles and the Design of
Network-based Software Architectures", Ph.D. Dissertation,
University of California, Irvine, 2000,
<http://www.ics.uci.edu/~fielding/pubs/dissertation/
fielding_dissertation.pdf>.
[RESTAPI] Fielding, R., "REST APIs must be hypertext-driven",
October 2008, <http://roy.gbiv.com/untangled/2008/
rest-apis-must-be-hypertext-driven>.
[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,
<http://www.w3.org/Submission/2009/SUBM-wadl-20090831>.
Author's Address
Klaus Hartke
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63905
EMail: hartke@tzi.org
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