RESTful Design for Internet of Things Systems
draft-keranen-t2trg-rest-iot-00

Network Working Group                                         A. Keranen
Internet-Draft                                                  Ericsson
Intended status: Informational                               M. Kovatsch
Expires: April 21, 2016                                       ETH Zurich
                                                               K. Hartke
                                                 Universitaet Bremen TZI
                                                        October 19, 2015

             RESTful Design for Internet of Things Systems
                    draft-keranen-t2trg-rest-iot-00

Abstract

   This document gives guidance for designing Internet of Things (IoT)
   systems that follow the principles of the Representational State
   Transfer (REST) architectural style.

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   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  . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Architecture  . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  System design . . . . . . . . . . . . . . . . . . . . . .   7
     3.3.  Resource modeling . . . . . . . . . . . . . . . . . . . .   7
     3.4.  Uniform Resource Identifiers (URIs) . . . . . . . . . . .   7
     3.5.  HTTP/CoAP Methods . . . . . . . . . . . . . . . . . . . .   8
       3.5.1.  GET . . . . . . . . . . . . . . . . . . . . . . . . .   9
       3.5.2.  POST  . . . . . . . . . . . . . . . . . . . . . . . .   9
       3.5.3.  PUT . . . . . . . . . . . . . . . . . . . . . . . . .   9
       3.5.4.  DELETE  . . . . . . . . . . . . . . . . . . . . . . .  10
     3.6.  HTTP/CoAP Status/Response Codes . . . . . . . . . . . . .  10
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   5.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  11
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Future Work  . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

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 a 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 [RFC7231] 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.

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2.  Terminology

   This section explains some of the common terminology that is used in
   the context of RESTful design for IoT systems.

   Application State: The set of pending requests, history of requests,
   bookmarks (URIs stored for later retrieval), and application-specific
   state that the client keeps between requests.

   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.

   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
   and network bandwidth or energy consumption.

   Gateway: See "Reverse Proxy".

   Hypermedia Control: A component embedded in a representation that
   describes a request.  By performing the request, the client can
   change resource state and/or move the application state forward.

   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 result.

   Link: A hypermedia control that enables a client to navigate between
   resources and thereby change the application state.

   Media Type: A sequence of characters 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.

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   Method: A procedure 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 client's content
   negotiation preferences.  For example, in an IoT application, the
   preferences of a client could be media types "application/senml+json"
   and "text/plain".

   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 included in
   the list.

   Representation Format: A set of rules for encoding information in a
   sequence of bytes.  In the Web, the most prevalent representation
   format is HTML.  Other common formats include plain text (in UTF-8 or
   any other encoding), JSON or XML.  With IoT systems, often compact
   formats such as JSON, CBOR, and EXI are used.

   Representation: A sequence of bytes, plus representation metadata,
   that captures the current or intended state of a resource and that
   can be transferred between clients and servers (possibly via one or
   more intermediaries).

   Representational State Transfer (REST): An architectural style for
   Internet-scale distributed hypermedia systems.

   Resource State: A mapping of a resource to a set of values that may
   change over time.

   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.

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   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.

   Server: A node that listens for requests, applies the requested
   actions to resources, and sends responses back to the clients.

   Uniform Resource Identifier (URI): A global identifier for resources.
   See Section 3.4 for more details.

3.  Basics

3.1.  Architecture

   The components of a REST 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, 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

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    ________       __________                        _________
   |        |     |          |                      |         |
   | 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.

    ________                        __________       _________
   |        |                      |          |     |         |
   | 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 CoAP Resource Directory)
   and act as origin server at the same time (e.g., to serve sensor
   values).

    ________                                         _________
   |        |                                       |         |
   | Thing (C)-------------------------------------(S) Origin |
   |       (S)                                      |  Server |
   |________| \                                     |_________|
    (Sensor)   \   ________                     (Resource Directory)
                \ |        |
                 (C) Thing |
                  |________|
                 (Controller)

                Figure 4: Constrained RESTful environments

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3.2.  System design

   When designing a REST 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.

   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.jennings-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 interact with a
   resource, to reference a resource from another resource, to advertise
   or bookmark a resource, or to index a resource by search engines.

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     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
   scheme and naming authority.  The path is hierarchical; the query is
   non-hierarchical.  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.

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.

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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.

   A response to a successful GET request is cacheable; a cache may use
   it to satisfy future, equivalent GET requests.  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.

   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.

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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
   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" or "4.04")
   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.

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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 for the review comments.

6.  References

6.1.  Normative References

   [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>.

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   [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>.

6.2.  Informative References

   [I-D.jennings-core-senml]
              Jennings, C., Shelby, Z., Arkko, J., and A. Keranen,
              "Media Types for Sensor Markup Language (SENML)", draft-
              jennings-core-senml-01 (work in progress), July 2015.

   [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>.

   [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>.

   [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

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   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

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