Thing-to-Thing Research Group K. Hartke
Internet-Draft Ericsson
Intended status: Informational October 22, 2018
Expires: April 25, 2019
CoRE Applications
draft-hartke-core-apps-08
Abstract
The application programmable interfaces of RESTful, hypermedia-driven
Web applications consist of a number of reusable components such as
Internet media types and link relation types. This document proposes
"CoRE Applications", a convention for application designers to build
the interfaces of their applications in a structured way, so that
implementers can easily build interoperable clients and servers, and
other designers can reuse the components in their own applications.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. CoRE Applications . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Communication Protocols . . . . . . . . . . . . . . . . . 4
2.1.1. URI Schemes . . . . . . . . . . . . . . . . . . . . . 4
2.2. Representation Formats . . . . . . . . . . . . . . . . . 5
2.2.1. Internet Media Types . . . . . . . . . . . . . . . . 5
2.3. Links . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.3.1. Link Relation Types . . . . . . . . . . . . . . . . . 8
2.3.2. Template Variable Names . . . . . . . . . . . . . . . 8
2.4. Forms . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.4.1. Form Relation Types . . . . . . . . . . . . . . . . . 9
2.4.2. Form Field Names . . . . . . . . . . . . . . . . . . 9
2.5. Well-Known Locations . . . . . . . . . . . . . . . . . . 10
3. CoRE Application Descriptions . . . . . . . . . . . . . . . . 10
3.1. Template . . . . . . . . . . . . . . . . . . . . . . . . 11
4. URI Design Considerations . . . . . . . . . . . . . . . . . . 12
5. Security Considerations . . . . . . . . . . . . . . . . . . . 14
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. Normative References . . . . . . . . . . . . . . . . . . 14
7.2. Informative References . . . . . . . . . . . . . . . . . 15
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
Representational State Transfer (REST) [16] is an architectural style
for distributed hypermedia systems. Over the years, REST has gained
popularity not only as an approach for large-scale information
dissemination, but also as the basic principle for designing and
building Internet-based applications in general.
In the coming years, the size and scope of the Internet is expected
to increase greatly as physical-world objects become smart enough to
communicate over the Internet -- a phenomenon known as the Internet
of Things (IoT). As things learn to speak the languages of the net,
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the idea of applying REST principles to the design of IoT application
architectures suggests itself. To this end, the Constrained
Application Protocol (CoAP) [23] was created, an application-layer
protocol that enables RESTful applications in constrained-node
networks [10], giving rise to a new setting for Internet-based
applications: the Constrained RESTful Environment (CoRE).
To realize the full benefits and advantages of the REST architectural
style, a set of constraints needs to be maintained when designing
applications and their application programming interfaces (APIs).
One of the fundamental principles of REST is that "REST APIs must be
hypertext-driven" [17]. However, this principle is often ignored by
application designers. Instead, APIs are specified out-of-band in
terms of fixed URI patterns (e.g., in the API documentation or in a
machine-readable format that facilitates code generation). Although
this approach may appear easy for clients to use, the fixed resource
names and data formats lead to a tight coupling between client and
server implementations and make the system less flexible [5].
Violations of REST design principles like this result in APIs that
may not be as scalable, extensible, and interoperable as promised by
REST.
REST is intended for network-based applications that are long-lived
and span multiple organizations [17]. Principled REST APIs require
some design effort, since application designers do not only have to
take current requirements into consideration, but also have to
anticipate changes that may be required in the future -- years or
even decades after the application has been deployed for the first
time. The reward is long-term stability and evolvability, both of
which are very desirable features in the Internet of Things.
To aid application designers in the design process, this document
proposes "CoRE Applications", a convention for building the APIs of
RESTful, hypermedia-driven Web applications. The goal is to help
application designers avoid common mistakes by focusing almost all of
the descriptive effort on defining the Internet media type(s) that
are used for representing resources and driving application state.
A template for a "CoRE Application Description" provides a consistent
format for the description of APIs so that implementers can easily
build interoperable clients and servers, and other application
designers can reuse the components in their own applications.
2. CoRE Applications
A CoRE Application API is a named set of reusable components. It
describes a contract between a server hosting an instance of the
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described application and clients that wish to interface with that
instance.
The API is generally comprised of:
o communication protocols, identified by URI schemes,
o representation formats, identified by Internet media types,
o link relation types,
o form relation types,
o template variables in templated links,
o form field names in forms, and
o well-known locations.
Together, these components provide the specific, in-band instructions
to a client for interfacing with a given application.
2.1. Communication Protocols
The foundation of a hypermedia-driven REST API are the communication
protocol(s) spoken between a client and a server. Although HTTP/1.1
[14] is by far the most common communication protocol for REST APIs,
a REST API should typically not be dependent on any specific
communication protocol.
2.1.1. URI Schemes
The usage of a particular protocol by a client is guided by URI
schemes [7]. URI schemes specify the syntax and semantics of URI
references [1] that the server includes in hypermedia controls such
as links and forms.
A URI scheme refers to a family of protocols, typically distinguished
by a version number. For example, the "http" URI scheme refers to
the two members of the HTTP family of protocols: HTTP/1.1 [14] and
HTTP/2 [8] (as well as some predecessors). The specific HTTP version
used is negotiated between a client and a server in-band using the
version indicator in the HTTP request-line or the TLS Application-
Layer Protocol Negotiation (ALPN) extension [18].
IANA maintains a list of registered URI schemes at
<http://www.iana.org/assignments/uri-schemes>.
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2.2. Representation Formats
In RESTful applications, clients and servers exchange representations
that capture the current or intended state of a resource and that 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.
Representation formats should generally allow clients with different
goals, so they can do different things with the same data. The
specification of a representation format "describes a problem space,
not a prescribed relationship between client and server. Client and
server must share an understanding of the representations they're
passing back and forth, but they don't need to have the same idea of
what the problem is that needs to be solved." [21]
Representation formats and their specifications frequently evolve
over time. It is part of the responsibility of the designer of a new
version to insure both forward and backward compatibility: new
representations should work reasonably (with some fallback) with old
processors and old representations should work reasonably with new
processors [20].
Representation formats enable hypermedia-driven applications when
they support the expression of hypermedia controls such as links
(Section 2.3) and forms (Section 2.4).
2.2.1. Internet Media Types
One of the most important aspect of hypermedia-driven communications
is the concept of Internet media types [2]. Media types are used to
label representations so that it is known how the representation
should be interpreted and how it is encoded. The centerpiece of a
CoRE Application Description should be one or more media types.
Note: The terms media type and representation format are often used
interchangeably. In this document, the term "media type" refers
specifically to a string of characters such as "application/xml"
that is used to label representations; the term "representation
format" refers to the definition of the syntax and semantics of
representations, such as XML 1.0 [12] or XML 1.1 [13].
A media type identifies a versioned series of representation formats
(Section 2.2): a media type does not identify a particular version of
a representation format; rather, the media type identifies the
family, and includes provisions for version indicator(s) embedded in
the representations themselves to determine more precisely the nature
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of how the data is to be interpreted [20]. A new media type is only
needed to designate a completely incompatible format [20].
Media types consist of a top-level type and a subtype, structured
into trees [2]. Optionally, media types can have parameters. For
example, the media type "text/plain; charset=utf-8" is a subtype for
plain text under the "text" top-level type in the standards tree and
has a parameter "charset" with the value "utf-8".
Media types can be further refined by
o structured type name suffixes (e.g., "+xml" appended to the base
subtype name; see Section 4.2.8 of RFC 6838 [2]),
o a "profile" parameter (see Section 3.1 of RFC 6906 [24]),
o subtype information embedded in the representations themselves
(e.g., "xmlns" declarations in XML documents [11]),
or a similar annotation. An annotation directly in the media type is
generally preferable, since subtype information embedded in
representations can typically not be negotiated during content
negotiation (e.g., using the CoAP Accept option).
In CoAP, media types are paired with a content coding [15] to
indicate the "content format" [23] of a representation. Each content
format is assigned a numeric identifier that can be used instead of
the (more verbose) textual name of the media type in representation
formats with size constraints. The flat number space loses the
structural information that the textual names have, however.
The media type of a representation 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>.
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2.3. Links
As defined in RFC 8288 [6], a link is a typed connection between two
resources. Additionally, a link is the primary means for a client to
navigate from one resource to another.
A link is comprised of:
o a link context,
o a link relation type that identifies the semantics of the link
(see Section 2.3.1),
o a link target, identified by a URI, and
o optionally, target attributes that further describe the link or
the link target.
A link can be viewed as a statement of the form "{link context} has a
{link relation type} resource at {link target}, which has {target
attributes}" [6]. For example, the resource <http://example.com/>
could have a "terms-of-service" resource at <http://example.com/tos>,
which has a representation with the media type "text/html".
There are two special kinds of links:
o An embedding link is a link with an additional hint: when the link
is processed, it should be substituted with the representation of
the referenced resource rather than cause the client to navigate
away from the current resource. Thus, traversing an embedding
link adds to the current state rather than replacing it.
The most well known example for an embedding link is the HTML
<img> element. When a Web browser processes this element, it
automatically dereferences the "src" and renders the resulting
image in place of the <img> element.
o A templated link is a link where the client constructs the link
target URI from provided in-band instructions. The specific rules
for such instructions are described by the representation format.
URI Templates [3] provide a generic way to construct URIs through
variable expansion.
Templated links allow a client to construct resource URIs without
being coupled to the resource structure at the server, provided
that the client learns the template from a representation sent by
the server and does not have the template hard-coded.
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2.3.1. Link Relation Types
A link relation type identifies the semantics of a link [6]. For
example, a link with the relation type "copyright" indicates that the
resource identified by the target URI is a statement of the copyright
terms applying to the link context.
Relation types are not to be confused with media types; they do not
identify the format of the representation that results when the link
is dereferenced [6]. Rather, they only describe how the link context
is related to another resource [6].
IANA maintains a list of registered link relation types at
<http://www.iana.org/assignments/link-relations>.
Applications that don't wish to register a link relation type can use
an extension link relation type [6]: a URI that uniquely identifies
the link relation type. For example, an application can use the
string "http://example.com/foo" as link relation type without having
to register it. Using a URI to identify an extension link relation
type, rather than a simple string, reduces the probability of
different link relation types using the same identifiers.
2.3.2. Template Variable Names
A templated link enables clients to construct the target URI of a
link, for example, when the link refers to a space of resources
rather than a single resource. The most prominent mechanisms for
this are URI Templates [3] and the HTML <form> element with a
submission method of GET.
To enable an automated client to construct an URI reference from a
URI Template, the name of the variable in the template can be used to
identify the semantics of the variable. For example, when retrieving
the representation of a collection of temperature readings, a
variable named "threshold" could indicate the variable for setting a
threshold of the readings to retrieve.
Template variable names are scoped to link relation types, i.e., two
variables with the same name can have different semantics if they
appear in links with different link relation types.
2.4. Forms
A form is the primary means for a client to submit information to a
server, typically in order to change resource state.
A form is comprised of:
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o a form context,
o a form relation type that identifies the semantics of the form
(see Section 2.4.1),
o a request method (e.g., PUT, POST, DELETE),
o a submission URI,
o a description of a representation that the server expects as part
of the form submission, and
o optionally, target attributes that further describe the form or
the form target.
A form can be viewed as an instruction of the form "To perform a
{form relation type} operation on {form context}, make a {request
method} request to {submission URI}, which has {target attributes}".
For example, to "update" the resource <http://example.com/config>, a
client would make a PUT request to <http://example.com/config>. (In
many cases, the target of a form is the same resource as the context,
but this is not required.)
The description of the expected representation can be a set of form
fields (see Section 2.4.2) or simply a list of acceptable media
types.
Note: A form with a submission method of GET is, strictly speaking,
a templated link, since it provides a way to construct a URI and
does not submit a representation to the server.
2.4.1. Form Relation Types
A form relation type identifies the semantics of a form. For
example, a form with the form relation type "create" indicates that a
new item can be created within the form context by making a request
to the resource identified by the target URI.
Similarly to extension link relation types, applications can use
extension form relation types when they don't wish to register a form
relation type.
2.4.2. Form Field Names
Forms can have a detailed description of the representation expected
by the server as part of form submission. This description typically
consists of a set of form fields where each form field is comprised
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of a field name, a field type, and optionally a number of attributes
such as a default value, a validation rule or a human-readable label.
To enable an automated client to fill out a form, the field name can
be used to identify the semantics of the form field. For example,
when controlling a smart light bulb, the field name "brightness"
could indicate the field for setting the desired brightness of the
light bulb.
Field names are scoped to form relation types, i.e., two form fields
with the same name can have different semantics if they appear in
forms with different form relation types.
The type of a form field is a data type such as "an integer between 1
and 100" or "an RGB color". The type is orthogonal to the field
name, i.e., the type should not be determined from the field name
even though the client can identify the semantics of the field from
the name. This separation makes it easy to change the set of
acceptable values in the future.
2.5. Well-Known Locations
Some applications may require the discovery of information about a
host, known as "site-wide metadata" in RFC 5785 [4]. For example,
RFC 6415 [19] defines a metadata document format for describing a
host; similarly, RFC 6690 [22] defines a link format for the
discovery of resources hosted by a server.
Applications that need to define a resource for this kind of metadata
can register new "well-known locations". RFC 5785 [4] defines the
path prefix "/.well-known/" in "http" and "https" URIs for this
purpose. RFC 7252 [23] extends this convention to "coap" and "coaps"
URIs.
IANA maintains a list of registered well-known URIs at
<http://www.iana.org/assignments/well-known-uris>.
3. CoRE Application Descriptions
As applications are implemented and deployed, it becomes important to
describe them in some structured way. This section provides a simple
template for CoRE Application Descriptions. A uniform structure
allows implementers to easily determine the components that make up
the interface of an application.
The template below lists all components of applications that both the
client and the server implementation of the application need to
understand in order to interoperate. Crucially, items not listed in
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the template are not part of the contract between clients and servers
-- they are implementation details. This includes in particular the
URIs of resources (see Section 4).
CoRE Application Descriptions are intended to be published in human-
readable format by designers of applications and by operators of
deployed application instances. Application designers may publish an
application description as a general specification of all application
instances, so that implementers can create interoperable clients and
servers. Operators of application instances may publish an
application description as part of the API documentation of the
service, which should also include instructions how the service can
be located and which communication protocols and security modes are
used.
3.1. Template
The fields of the template are as follows:
Application name:
Name of the application. The name is not used to negotiate
capabilities; it is purely informational. A name may include a
version number or, for example, refer to a living standard that is
updated continuously.
URI schemes:
URI schemes identifying the communication protocols that need to
be understood by clients and servers. This information is mostly
relevant for deployed instances of the application rather than for
the general specification of the application.
Media types:
Internet media types that identify the representation formats that
need to be understood by clients and servers. An application
description must comprise at least one media type. Additional
media types may be required or optional.
Link relation types:
Link relation types that identify the semantics of links. An
application description may comprise IANA-registered link relation
types and extension link relation types. Both may be required or
optional.
Template variable names:
For each link relation type, variable names that identify the
semantics of variables in templated links with that link relation
type. Whether a template variable is required or optional is
indicated in-band inside the templated link.
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Form relation types:
Form relation types that identify the semantics of forms and, for
each form relation type, the submission method(s) to be used. An
application description may comprise IANA-registered form relation
types and extension form relation types. Both may be required or
optional.
Form field names:
For each form relation type, form field names that identify the
semantics of form fields in forms with that form relation type.
Whether a form field is required or optional is indicated in-band
inside the form.
Well-known locations:
Well-known locations in the resource identifier space of servers
that clients can use to discover information given the DNS name or
IP address of a server.
Interoperability considerations:
Any issues regarding the interoperable use of the components of
the application should be given here.
Security considerations:
Security considerations for the security of the application must
be specified here.
Contact:
Person (including contact information) to contact for further
information.
Author/Change controller:
Person (including contact information) authorized to change this
application description.
Each field should include full citations for all specifications
necessary to understand the application components.
4. URI Design Considerations
URIs [1] are a cornerstone of RESTful applications. They enable
uniform identification of resources via URI schemes [7] and are used
every time a client interacts with a particular resource or when a
resource representation references another resource.
URIs often include structured application data in the path and query
components, such as paths in a filesystem or keys in a database. It
is common for many RESTful applications to use these structures not
only as an implementation detail but also make them part of the
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public REST API, prescribing a fixed format for this data. However,
there are a number of problems with this practice [5], in particular
if the application designer and the server owner are not the same
entity.
In hypermedia-driven applications, URIs are therefore not included in
the application interface. A CoRE Application Description must not
mandate any particular form of URI substructure.
RFC 7320 [5] describes the problematic practice of fixed URI
structures in detail and provides some acceptable alternatives.
Nevertheless, the design of the URI structure on a server is an
essential part of implementing a RESTful application, even though it
is not part of the application interface. The server implementer is
responsible for binding the resources identified by the application
designer to URIs.
A good RESTful URI is:
o Short. Short URIs are easier to remember and cause less overhead
in requests and representations.
o Meaningful. A URI should describe the resource in a way that is
meaningful and useful to humans.
o Consistent. URIs should follow a consistent pattern to make it
easy to reason about the application.
o Bookmarkable. Cool URIs don't change [9]. However, in practice,
application resource structures do change. That should cause URIs
to change as well so they better reflect reality. Implementations
should not depend on unchanging URIs.
o Shareable. A URI should not be context sensitive, e.g., to the
currently logged-in user. It should be possible to share a URI
with third parties so they can access the same resource.
o Extension-less. Some applications return different data for
different extensions, e.g., for "contacts.xml" or "contacts.json".
But different URIs imply different resources. RESTful URIs should
identify a single resource. Different representations of the
resource can be negotiated (e.g., using the CoAP Accept option).
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5. Security Considerations
The security considerations of RFC 3986 [1], RFC 5785 [4], RFC 6570
[3], RFC 6838 [2], RFC 7320 [5], RFC 7595 [7], and RFC 8288 [6] are
inherited.
All components of an application description are expected to contain
clear security considerations. CoRE Application Descriptions should
furthermore contain security considerations that need to be taken
into account for the security of the overall application.
6. IANA Considerations
This document has no IANA actions.
7. References
7.1. Normative References
[1] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[2] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[3] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570,
DOI 10.17487/RFC6570, March 2012,
<https://www.rfc-editor.org/info/rfc6570>.
[4] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785,
DOI 10.17487/RFC5785, April 2010,
<https://www.rfc-editor.org/info/rfc5785>.
[5] Nottingham, M., "URI Design and Ownership", BCP 190,
RFC 7320, DOI 10.17487/RFC7320, July 2014,
<https://www.rfc-editor.org/info/rfc7320>.
[6] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/info/rfc8288>.
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[7] Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
and Registration Procedures for URI Schemes", BCP 35,
RFC 7595, DOI 10.17487/RFC7595, June 2015,
<https://www.rfc-editor.org/info/rfc7595>.
7.2. Informative References
[8] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
[9] Berners-Lee, T., "Cool URIs don't change", 1998,
<http://www.w3.org/Provider/Style/URI.html>.
[10] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>.
[11] Bray, T., Hollander, D., Layman, A., Tobin, R., and H.
Thompson, "Namespaces in XML 1.0 (Third Edition)", World
Wide Web Consortium Recommendation REC-xml-names-20091208,
December 2009,
<http://www.w3.org/TR/2009/REC-xml-names-20091208>.
[12] Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E., and
F. Yergeau, "Extensible Markup Language (XML) 1.0 (Fifth
Edition)", World Wide Web Consortium Recommendation REC-
xml-20081126, November 2008,
<http://www.w3.org/TR/2008/REC-xml-20081126>.
[13] Bray, T., Paoli, J., Sperberg-McQueen, M., Maler, E.,
Yergeau, F., and J. Cowan, "Extensible Markup Language
(XML) 1.1 (Second Edition)", World Wide Web Consortium
Recommendation REC-xml11-20060816, August 2006,
<http://www.w3.org/TR/2006/REC-xml11-20060816>.
[14] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[15] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
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[16] 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>.
[17] Fielding, R., "REST APIs must be hypertext-driven",
October 2008, <http://roy.gbiv.com/untangled/2008/
rest-apis-must-be-hypertext-driven>.
[18] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/info/rfc7301>.
[19] Hammer-Lahav, E., Ed. and B. Cook, "Web Host Metadata",
RFC 6415, DOI 10.17487/RFC6415, October 2011,
<https://www.rfc-editor.org/info/rfc6415>.
[20] Masinter, L., "MIME and the Web", draft-masinter-mime-web-
info-02 (work in progress), January 2011.
[21] Richardson, L. and M. Amundsen, "RESTful Web APIs",
O'Reilly Media, ISBN 978-1-4493-5806-8, September 2013.
[22] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[23] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[24] Wilde, E., "The 'profile' Link Relation Type", RFC 6906,
DOI 10.17487/RFC6906, March 2013,
<https://www.rfc-editor.org/info/rfc6906>.
Acknowledgements
Jan Algermissen, Mike Amundsen, Mike Kelly, Julian Reschke, and Erik
Wilde provided valuable input on link and form relation types.
Thanks to Olaf Bergmann, Carsten Bormann, Stefanie Gerdes, Ari
Keranen, Michael Koster, Matthias Kovatsch, Teemu Savolainen, and
Bilhanan Silverajan for helpful comments and discussions that have
shaped the document.
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Some of the text in this document has been borrowed from [5], [6],
[17], and [20]. All errors are my own.
This work was funded in part by Nokia.
Author's Address
Klaus Hartke
Ericsson
Torshamnsgatan 23
Stockholm SE-16483
Sweden
Email: klaus.hartke@ericsson.com
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