Network Working Group A. Keranen
Internet-Draft Ericsson
Intended status: Informational M. Kovatsch
Expires: May 4, 2017 ETH Zurich
K. Hartke
Universitaet Bremen TZI
October 31, 2016
RESTful Design for Internet of Things Systems
draft-keranen-t2trg-rest-iot-03
Abstract
This document gives guidance for designing Internet of Things (IoT)
systems that follow the principles of the Representational State
Transfer (REST) architectural style.
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 May 4, 2017.
Copyright Notice
Copyright (c) 2016 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
Keranen, et al. Expires May 4, 2017 [Page 1]
Internet-Draft RESTful Design for IoT Systems October 2016
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Architecture . . . . . . . . . . . . . . . . . . . . . . 6
3.2. System design . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Resource modeling . . . . . . . . . . . . . . . . . . . . 8
3.4. Uniform Resource Identifiers (URIs) . . . . . . . . . . . 8
3.5. HTTP/CoAP Methods . . . . . . . . . . . . . . . . . . . . 9
3.5.1. GET . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.5.2. POST . . . . . . . . . . . . . . . . . . . . . . . . 9
3.5.3. PUT . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.5.4. DELETE . . . . . . . . . . . . . . . . . . . . . . . 10
3.6. HTTP/CoAP Status/Response Codes . . . . . . . . . . . . . 10
4. Security Considerations . . . . . . . . . . . . . . . . . . . 11
5. Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . 11
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.1. Normative References . . . . . . . . . . . . . . . . . . 12
6.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. Future Work . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
The Representational State Transfer (REST) architectural style [REST]
is a set of guidelines and best practices for building distributed
hypermedia systems.
When REST principles are applied to the design of a system, the
result is often called RESTful and in particular an API following
these principles is called a RESTful API.
Different protocols can be used with RESTful systems, but at the time
of writing the most common protocols are HTTP [RFC7230] and CoAP
[RFC7252].
RESTful design facilitates many desirable features for a system, such
as good scaling properties. RESTful APIs are also often simple and
lightweight and hence easy to use also with various IoT applications.
The goal of this document is to give basic guidance for designing
RESTful systems and APIs for IoT applications and give pointers for
more information.
Keranen, et al. Expires May 4, 2017 [Page 2]
Internet-Draft RESTful Design for IoT Systems October 2016
Design of a good RESTful IoT system has naturally many commonalities
with other Web systems. Compared to other systems, the key
characteristics of many IoT systems include:
o data formats, interaction patterns, and other mechanisms that
minimize, or preferably avoid, the need for human interaction
o preference for compact and simple data formats to facilitate
efficient transfer over (often) constrained networks and
lightweight processing in constrained nodes
2. Terminology
This section explains some of the common terminology that is used in
the context of RESTful design for IoT systems. For terminology of
constrained nodes and networks, see [RFC7228].
Application State: The state kept by a client between requests.
This typically includes the "current" resource, the set of active
requests, the history of requests, bookmarks (URIs stored for
later retrieval) and application-specific state.
Cache: A local store of response messages and the subsystem that
controls storage, retrieval, and deletion of messages in it.
Client: A node that sends requests to servers and receives
responses.
Content Negotiation: The practice of determining the "best"
representation for a client when examining the current state of a
resource. The most common forms of content negotiation are
Proactive Content Negotiation and Reactive Content Negotiation.
Form: A hypermedia control that enables a client to change the state
of a resource.
Forward Proxy: An intermediary that is selected by a client, usually
via local configuration rules, and that can be tasked to make
requests on behalf of the client. This may be useful, for
example, when the client lacks the capability to make the request
itself or to service the response from a cache in order to reduce
response time, network bandwidth and energy consumption.
Gateway: See "Reverse Proxy".
Hypermedia Control: A component embedded in a representation that
identifies a resource for future hypermedia interactions, such as
a link or a form. If the client engages in an interaction with
Keranen, et al. Expires May 4, 2017 [Page 3]
Internet-Draft RESTful Design for IoT Systems October 2016
the identified resource, the result may be a change resource state
and/or application state.
Idempotent Method: A method where multiple identical requests with
that method lead to the same visible resource state as a single
such request. For example, the PUT method replaces the state of a
resource with a new state; replacing the state multiple times with
the same new state still results in the same state for the
resource. However, the response from the server can be different
when the same idenpotent method is used multiple times. For
example when DELETE is used twice on an existing resource, the
first request would remove the association and return success
acknowledgement whereas the second request would likely result in
error response due to non-existing resource.
Link: A hypermedia control that enables a client to navigate between
resources and thereby change the application state.
Media Type: A string such as "text/html" or "application/json" that
is used to label representations so that it is known how the
representation should be interpreted and how it is encoded.
Method: An operation associated with a resource. Common methods
include GET, PUT, POST, and DELETE (see Section 3.5 for details).
Origin Server: A server that is the definitive source for
representations of its resources and the ultimate recipient of any
request that intends to modify its resources. In contrast,
intermediaries (such as proxies caching a representation) can
assume the role of a server, but are not the source for
representations as these are acquired from the origin server.
Proactive Content Negotiation: A content negotiation mechanism where
the server selects a representation based on the expressed
preference of the client. For example, in an IoT application, a
client could send a request with preferred media type
"application/senml+json".
Reactive Content Negotiation: A content negotiation mechanism where
the client selects a representation from a list of available
representations. The list may, for example, be included by a
server in an initial response. If the user agent is not satisfied
by the initial response representation, it can request one or more
of the alternative representations, selected based on metadata
(e.g., available media types) included in the response.
Representation Format: A set of rules for serializing resource
state. On the Web, the most prevalent representation format is
Keranen, et al. Expires May 4, 2017 [Page 4]
Internet-Draft RESTful Design for IoT Systems October 2016
HTML. Other common formats include plain text and formats based
on JSON [RFC7159], XML, or RDF. Within IoT systems, often compact
formats based on JSON, CBOR [RFC7049], and EXI
[W3C.REC-exi-20110310] are used.
Representation: A serialization that represents the current or
intended state of a resource and that can be transferred between
clients and servers. REST requires representations to be self-
describing, meaning that there must be metadata that allows peers
to understand which representation format is used. Depending on
the protocol needs and capabilities, there can be additional
metadata that is transmitted along with the representation.
Representational State Transfer (REST): An architectural style for
Internet-scale distributed hypermedia systems.
Resource: An item of interest identified by a URI. Anything that
can be named can be a resource. A resource often encapsulates a
piece of state in a system. Typical resources in an IoT system
can be, e.g., a sensor, the current value of a sensor, the
location of a device, or the current state of an actuator.
Resource State: A model of a resource's possible states that is
represented in a supported representation type, typically a media
type. Resources can change state because of REST interactions
with them, or they can change state for reasons outside of the
REST model.
Reverse Proxy: An intermediary that appears as a server towards the
client but satisfies the requests by forwarding them to the actual
server (possibly via one or more other intermediaries). A reverse
proxy is often used to encapsulate legacy services, to improve
server performance through caching, and to enable load balancing
across multiple machines.
Safe Method: A method that does not result in any state change on
the origin server when applied to a resource. For example, the
GET method only returns a representation of the resource state but
does not change the resource. Thus, it is always safe for a
client to retrieve a representation without affecting server-side
state.
Server: A node that listens for requests, performs the requested
operation and sends responses back to the clients.
Uniform Resource Identifier (URI): A global identifier for
resources. See Section 3.4 for more details.
Keranen, et al. Expires May 4, 2017 [Page 5]
Internet-Draft RESTful Design for IoT Systems October 2016
3. Basics
3.1. Architecture
The components of a RESTful system are assigned one of two roles:
client or server. User agents are always in the client role and have
the initiative to issue requests. Intermediaries (such as forward
proxies and reverse proxies) implement both roles, but only forward
requests to other intermediaries or origin servers. They can also
translate requests to different protocols, for instance, as CoAP-HTTP
cross-proxies.
Note that the terms "client" and "server" refer only to the roles
that the nodes assume for a particular message exchange. The same
node might act as a client in some communications and a server in
others.
________ _________
| | | |
| User (C)-------------------(S) Origin |
| Agent | | Server |
|________| |_________|
(Browser) (Web Server)
Figure 1: Client-Server Communication
________ __________ _________
| | | | | |
| User (C)---(S) Inter- (C)--------------------(S) Origin |
| Agent | | mediary | | Server |
|________| |__________| |_________|
(Browser) (Forward Proxy) (Web Server)
Figure 2: Communication with Forward Proxy
Reverse proxies are usually imposed by the origin server. In
addition to the features of a forward proxy, they can also provide an
interface for non-RESTful services such as legacy systems or
alternative technologies such as Bluetooth ATT/GATT. This property
is enforced by the layered system constraint of REST, which says that
a client cannot see beyond the server it is connected to.
Keranen, et al. Expires May 4, 2017 [Page 6]
Internet-Draft RESTful Design for IoT Systems October 2016
________ __________ _________
| | | | | |
| User (C)--------------------(S) Inter- (x)---(x) Origin |
| Agent | | mediary | | Server |
|________| |__________| |_________|
(Browser) (Reverse Proxy) (Legacy System)
Figure 3: Communication with Reverse Proxy
Nodes in IoT systems often implement both roles. Unlike
intermediaries, however, they can take the initiative as a client
(e.g., to register with a directory, such as CoRE Resource Directory
[I-D.ietf-core-resource-directory], or to interact with another
thing) and act as origin server at the same time (e.g., to serve
sensor values or provide an actuator interface).
________ _________
| | | |
| Thing (C)-------------------------------------(S) Origin |
| (S) | Server |
|________| \ |_________|
(Sensor) \ ________ (Resource Directory)
\ | |
(C) Thing |
|________|
(Controller)
Figure 4: Constrained RESTful environments
3.2. System design
When designing a RESTful system, the state of the distributed
application must be assigned to the different components. Here, it
is important to distinguish between "session state" and "resource
state".
Session state encompasses the control flow and the interactions
between the components (see Section 2). Following the statelessness
constraint, the session state must be kept only on clients. On the
one hand, this makes requests a bit more verbose since every request
must contain all the information necessary to process it. On the
other hand, this makes servers efficient, since they do not have to
keep any state about their clients. Requests can easily be
distributed over multiple worker threads or server instances. For
the IoT systems, it lowers the memory requirements for server
implementations, which is particularly important for constrained
servers and servers serving large amount of clients.
Keranen, et al. Expires May 4, 2017 [Page 7]
Internet-Draft RESTful Design for IoT Systems October 2016
Resource state includes the more persistent data of an application
(i.e., independent of the application control flow). This can be
static data such as device descriptions, persistent data such as
system configuration, but also dynamic data such as the current value
of a sensor on a thing.
3.3. Resource modeling
Important part of RESTful API design is to model the system as a set
of resources whose state can be retrieved and/or modified and where
resources can be potentially also created and/or deleted.
Resource representations have a media type that tells how the
representation should be interpreted. Typical media types for IoT
systems include "text/plain" for simple UTF-8 text, "application/
octet-stream" for arbitrary binary data, "application/json" for JSON
[RFC7159], "application/senml+json" [I-D.ietf-core-senml] for Sensor
Markup Language (SenML) formatted data, "application/cbor" for CBOR
[RFC7049], "application/exi" for EXI [W3C.REC-exi-20110310]. Full
list of registered internet media types is available at the IANA
registry [IANA-media-types] and media types registered for use with
CoAP are listed at CoAP Content-Formats IANA registry
[IANA-CoAP-media].
3.4. Uniform Resource Identifiers (URIs)
Uniform Resource Identifiers (URIs) are used to indicate a resource
for interaction, to reference a resource from another resource, to
advertise or bookmark a resource, or to index a resource by search
engines.
foo://example.com:8042/over/there?name=ferret#nose
\_/ \______________/\_________/ \_________/ \__/
| | | | |
scheme authority path query fragment
A URI is a sequence of characters that matches the syntax defined in
[RFC3986]. It consists of a hierarchical sequence of five
components: scheme, authority, path, query, and fragment (from most
significant to least significant). A scheme creates a namespace for
resources and defines how the following components identify a
resource within that namespace. The authority identifies an entity
that governs part of the namespace, such as the server
"www.example.org" in the "http" scheme. A host name (e.g., a fully
qualified domain name) or an IP address, potentially followed by a
transport layer port number, are usually used in the authority
component for the "http" and "coap" schemes. The path and query
contain data to identify a resource within the scope of the URI's
Keranen, et al. Expires May 4, 2017 [Page 8]
Internet-Draft RESTful Design for IoT Systems October 2016
scheme and naming authority. The fragment allows to refer to some
portion of the resource, such as a section in an HTML document.
For RESTful IoT applications, typical schemes include "https",
"coaps", "http", and "coap". These refer to HTTP and CoAP, with and
without Transport Layer Security (TLS) [RFC5246]. (CoAP uses
Datagram TLS (DTLS) [RFC6347], the variant of TLS for UDP.) These
four schemes also provide means for locating the resource; using the
HTTP protocol for "http" and "https", and with the CoAP protocol for
"coap" and "coaps". If the scheme is different for two URIs (e.g.,
"coap" vs. "coaps"), it is important to note that even if the rest of
the URI is identical, these are two different resources, in two
distinct namespaces.
The query parameters can be used to parametrize the resource. For
example, a GET request may use query parameters to request the server
to send only certain kind data of the resource (i.e., filtering the
response). Query parameters in PUT and POST requests do not have
such established semantics and are not commonly used. Whether the
order of the query parameters matters in URIs is unspecified and they
can be re-ordered e.g., by proxies. Therefore applications should
not rely on their order; see Section 3.3 of [RFC6943] for more
details.
3.5. HTTP/CoAP Methods
Section 4.3 of [RFC7231] defines the set of methods in HTTP;
Section 5.8 of [RFC7252] defines the set of methods in CoAP. The
following lists the most relevant methods and gives a short
explanation of their semantics.
3.5.1. GET
The GET method requests a current representation for the target
resource. Only the origin server needs to know how each of its
resource identifiers corresponds to an implementation and how each
implementation manages to select and send a current representation of
the target resource in a response to GET.
A payload within a GET request message has no defined semantics.
The GET method is safe and idempotent.
3.5.2. POST
The POST method requests that the target resource process the
representation enclosed in the request according to the resource's
own specific semantics.
Keranen, et al. Expires May 4, 2017 [Page 9]
Internet-Draft RESTful Design for IoT Systems October 2016
If one or more resources has been created on the origin server as a
result of successfully processing a POST request, the origin server
sends a 201 (Created) response containing a Location header field
that provides an identifier for the resource created and a
representation that describes the status of the request while
referring to the new resource(s).
The POST method is not safe nor idempotent.
3.5.3. PUT
The PUT method requests that the state of the target resource be
created or replaced with the state defined by the representation
enclosed in the request message payload. A successful PUT of a given
representation would suggest that a subsequent GET on that same
target resource will result in an equivalent representation being
sent.
The fundamental difference between the POST and PUT methods is
highlighted by the different intent for the enclosed representation.
The target resource in a POST request is intended to handle the
enclosed representation according to the resource's own semantics,
whereas the enclosed representation in a PUT request is defined as
replacing the state of the target resource. Hence, the intent of PUT
is idempotent and visible to intermediaries, even though the exact
effect is only known by the origin server.
The PUT method is not safe, but is idempotent.
3.5.4. DELETE
The DELETE method requests that the origin server remove the
association between the target resource and its current
functionality.
If the target resource has one or more current representations, they
might or might not be destroyed by the origin server, and the
associated storage might or might not be reclaimed, depending
entirely on the nature of the resource and its implementation by the
origin server.
The DELETE method is not safe, but is idempotent.
3.6. HTTP/CoAP Status/Response Codes
Section 6 of [RFC7231] defines a set of Status Codes in HTTP that are
used by application to indicate whether a request was understood and
Keranen, et al. Expires May 4, 2017 [Page 10]
Internet-Draft RESTful Design for IoT Systems October 2016
satisfied, and how to interpret the answer. Similarly, Section 5.9
of [RFC7252] defines the set of Response Codes in CoAP.
The status codes consist of three digits (e.g., "404" with HTTP or
"4.04" with CoAP) where the first digit expresses the class of the
code. Implementations do not need to understand all status codes,
but the class of the code must be understood. Codes starting with 1
are informational; the request was received and being processed.
Codes starting with 2 indicate successful request. Codes starting
with 3 indicate redirection; further action is needed to complete the
request. Codes stating with 4 and 5 indicate errors. The codes
starting with 4 mean client error (e.g., bad syntax in request)
whereas codes starting with 5 mean server error; there was no
apparent problem with the request but server was not able to fulfill
the request.
Responses may be stored in a cache to satisfy future, equivalent
requests. HTTP and CoAP use two different patterns to decide what
responses are cacheable. In HTTP, the cacheability of a response
depends on the request method (e.g., responses returned in reply to a
GET request are cacheable). In CoAP, the cacheability of a response
depends on the response code (e.g., responses with code 2.04 are
cacheable). This difference also leads to slightly different
semantics for the codes starting with 2; for example, CoAP does not
have a 2.00 response code whereas 200 ("OK") is commonly used with
HTTP.
4. Security Considerations
This document does not define new functionality and therefore does
not introduce new security concerns. However, security consideration
from related specifications apply to RESTful IoT design. These
include:
o HTTP security: Section 9 of [RFC7230], Section 9 of [RFC7231],
etc.
o CoAP security: Section 11 of [RFC7252]
o URI security: Section 7 of [RFC3986]
5. Acknowledgement
The authors would like to thank Mert Ocak, Heidi-Maria Back, Tero
Kauppinen, Michael Koster, Robby Simpson, Ravi Subramaniam, Dave
Thaler, and Erik Wilde for the reviews and feedback.
Keranen, et al. Expires May 4, 2017 [Page 11]
Internet-Draft RESTful Design for IoT Systems October 2016
6. References
6.1. Normative References
[I-D.ietf-core-resource-directory]
Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE
Resource Directory", draft-ietf-core-resource-directory-09
(work in progress), October 2016.
[REST] Fielding, R., "Architectural Styles and the Design of
Network-based Software Architectures", Ph.D. Dissertation,
University of California, Irvine , 2000.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, DOI 10.17487/RFC3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/
RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <http://www.rfc-editor.org/info/rfc7049>.
[RFC7230] 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,
<http://www.rfc-editor.org/info/rfc7230>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI
10.17487/RFC7231, June 2014,
<http://www.rfc-editor.org/info/rfc7231>.
[W3C.REC-exi-20110310]
Schneider, J. and T. Kamiya, "Efficient XML Interchange
(EXI) Format 1.0", World Wide Web Consortium
Recommendation REC-exi-20110310, March 2011,
<http://www.w3.org/TR/2011/REC-exi-20110310>.
Keranen, et al. Expires May 4, 2017 [Page 12]
Internet-Draft RESTful Design for IoT Systems October 2016
6.2. Informative References
[I-D.ietf-core-senml]
Jennings, C., Shelby, Z., Arkko, J., Keranen, A., and C.
Bormann, "Media Types for Sensor Markup Language (SenML)",
draft-ietf-core-senml-03 (work in progress), October 2016.
[IANA-CoAP-media]
"CoAP Content-Formats", n.d.,
<http://www.iana.org/assignments/core-parameters/
core-parameters.xhtml#content-formats>.
[IANA-media-types]
"Media Types", n.d., <http://www.iana.org/assignments/
media-types/media-types.xhtml>.
[RFC6943] Thaler, D., Ed., "Issues in Identifier Comparison for
Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May
2013, <http://www.rfc-editor.org/info/rfc6943>.
[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, <http://www.rfc-editor.org/info/rfc7159>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228, DOI 10.17487/
RFC7228, May 2014,
<http://www.rfc-editor.org/info/rfc7228>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252, DOI 10.17487/
RFC7252, June 2014,
<http://www.rfc-editor.org/info/rfc7252>.
Appendix A. Future Work
o More details on the definition of application state. Is server
involved and to what extent.
o Discuss design patterns, such as "Observing state (asynchronous
updates) of a resource", "Executing a Function", "Events as
State", "Conversion", "Collections", "robust communication in
network with high packet loss", "unreliable (best effort)
communication", "3-way commit", etc.
o Discuss directories, such as CoAP Resource Directory
Keranen, et al. Expires May 4, 2017 [Page 13]
Internet-Draft RESTful Design for IoT Systems October 2016
o More information on how to design resources; choosing what is
modeled as a resource, etc.
Authors' Addresses
Ari Keranen
Ericsson
Jorvas 02420
Finland
Email: ari.keranen@ericsson.com
Matthias Kovatsch
ETH Zurich
Universitaetstrasse 6
Zurich CH-8092
Switzerland
Email: kovatsch@inf.ethz.ch
Klaus Hartke
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Email: hartke@tzi.org
Keranen, et al. Expires May 4, 2017 [Page 14]