RESTful Design for Internet of Things Systems
draft-keranen-t2trg-rest-iot-05

Network Working Group                                         A. Keranen
Internet-Draft                                                  Ericsson
Intended status: Informational                               M. Kovatsch
Expires: March 18, 2018                                       ETH Zurich
                                                               K. Hartke
                                                 Universitaet Bremen TZI
                                                      September 14, 2017


             RESTful Design for Internet of Things Systems
                    draft-keranen-t2trg-rest-iot-05

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
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   This Internet-Draft will expire on March 18, 2018.

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   document authors.  All rights reserved.

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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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Basics  . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Architecture  . . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  System design . . . . . . . . . . . . . . . . . . . . . .   8
     3.3.  Uniform Resource Identifiers (URIs) . . . . . . . . . . .   9
     3.4.  Representations . . . . . . . . . . . . . . . . . . . . .  10
     3.5.  HTTP/CoAP Methods . . . . . . . . . . . . . . . . . . . .  10
       3.5.1.  GET . . . . . . . . . . . . . . . . . . . . . . . . .  11
       3.5.2.  POST  . . . . . . . . . . . . . . . . . . . . . . . .  11
       3.5.3.  PUT . . . . . . . . . . . . . . . . . . . . . . . . .  12
       3.5.4.  DELETE  . . . . . . . . . . . . . . . . . . . . . . .  12
     3.6.  HTTP/CoAP Status/Response Codes . . . . . . . . . . . . .  12
   4.  REST Constraints  . . . . . . . . . . . . . . . . . . . . . .  13
     4.1.  Client-Server . . . . . . . . . . . . . . . . . . . . . .  13
     4.2.  Stateless . . . . . . . . . . . . . . . . . . . . . . . .  14
     4.3.  Cache . . . . . . . . . . . . . . . . . . . . . . . . . .  14
     4.4.  Uniform Interface . . . . . . . . . . . . . . . . . . . .  14
     4.5.  Layered System  . . . . . . . . . . . . . . . . . . . . .  15
     4.6.  Code-on-Demand  . . . . . . . . . . . . . . . . . . . . .  15
   5.  Hypermedia-driven Applications  . . . . . . . . . . . . . . .  16
     5.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .  16
     5.2.  Knowledge . . . . . . . . . . . . . . . . . . . . . . . .  17
     5.3.  Interaction . . . . . . . . . . . . . . . . . . . . . . .  18
   6.  Design Patterns . . . . . . . . . . . . . . . . . . . . . . .  18
     6.1.  Collections . . . . . . . . . . . . . . . . . . . . . . .  18
     6.2.  Calling a Procedure . . . . . . . . . . . . . . . . . . .  19
       6.2.1.  Instantly Returning Procedures  . . . . . . . . . . .  19
       6.2.2.  Long-running Procedures . . . . . . . . . . . . . . .  19
       6.2.3.  Conversion  . . . . . . . . . . . . . . . . . . . . .  20
       6.2.4.  Events as State . . . . . . . . . . . . . . . . . . .  20
     6.3.  Server Push . . . . . . . . . . . . . . . . . . . . . . .  21
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
   8.  Acknowledgement . . . . . . . . . . . . . . . . . . . . . . .  23
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  23
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  25
   Appendix A.  Future Work  . . . . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26







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1.  Introduction

   The Representational State Transfer (REST) architectural style [REST]
   is a set of guidelines and best practices for building distributed
   hypermedia systems.  At its core is a set of constraints, which when
   fulfilled enable desirable properties for distributed software
   systems such as scalability and modifiability.  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].  Since RESTful APIs are often simple and lightweight, they
   are a good fit for 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.
   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].

   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.  In RESTful IoT systems it's common for nodes to have
      more than one role (e.g., both server and client; see
      Section 3.1).

   Client State:  The state kept by a client between requests.  This
      typically includes the currently processed representation, the set
      of active requests, the history of requests, bookmarks (URIs
      stored for later retrieval), and application-specific state (e.g.,
      local variables).  (Note that this is called "Application State"
      in [REST], which has some ambiguity in modern (IoT) systems where



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      the overall state of the distributed application (i.e.,
      application state) is reflected in the union of all Client States
      and Resource States of all clients and servers involved.)

   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 or to construct a query locally.

   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:  A reverse proxy that provides an interface to a non-RESTful
      system such as legacy systems or alternative technologies such as
      Bluetooth ATT/GATT.  See also "Reverse Proxy".

   Hypermedia Control:  A component, such as a link or a form, embedded
      in a representation that identifies a resource for future
      hypermedia interactions.  If the client engages in an interaction
      with the identified resource, the result may be a change to
      resource state and/or client state.

   Idempotent Method:  A method where multiple identical requests with
      that method lead to the same visible resource state as a single
      such request.

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

   Link Relation Type:  An identifier that describes how the link target
      resource relates to the current resource (see [RFC5988]).

   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



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      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, an IoT application could
      send a request to a sensor 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:  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.

   Representation Format:  A set of rules for serializing resource
      state.  On the Web, the most prevalent representation format is
      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.

   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.



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   Resource Type:  An identifier that annotates the application-
      semantics of a resource (see Section 3.1 of [RFC6690]).

   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.

   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 3.3 for more details.

3.  Basics

3.1.  Architecture

   The components of a RESTful system are assigned one or both of two
   roles: client or server.  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.  Classic user agents (e.g.,
   Web browsers) are always in the client role and have the initiative
   to issue requests.  Origin servers always have the server role and
   govern over the resources they host.

                 ________                       _________
                |        |                     |         |
                | User  (C)-------------------(S) Origin |
                | Agent  |                     |  Server |
                |________|                     |_________|
                (Browser)                      (Web Server)

                   Figure 1: Client-Server Communication

   Intermediaries (such as forward proxies, reverse proxies, and
   gateways) 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.






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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.  In this case,
   reverse proxies are usually called gateways.  This property is
   enabled by the Layered System constraint of REST, which says that a
   client cannot see beyond the server it is connected to (i.e., it is
   left unaware of the protocol/paradigm change).

       ________                        __________       _________
      |        |                      |          |     |         |
      | User  (C)--------------------(S) Inter- (x)---(x) Origin |
      | Agent  |                      |  mediary |     |  Server |
      |________|                      |__________|     |_________|
      (Browser)                        (Gateway)     (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





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3.2.  System design

   When designing a RESTful system, the primary effort goes into
   modeling the state of the distributed application and assigning it to
   the different components (i.e., clients and servers).  How clients
   can navigate through the resources and modify state to achieve their
   goals is defined through hypermedia controls, that is, links and
   forms.  Hypermedia controls span a kind of a state machine where the
   nodes are resources and the transitions are links or forms.  Clients
   run this state machine (i.e., the application) by retrieving
   representations, processing the data, and following the included
   hypermedia controls.  In REST, remote state is changed by submitting
   forms.  This is usually done by retrieving the current state,
   modifying the state on the client side, and transferring the new
   state to the server in the form of new representations - rather than
   calling a service and modifying the state on the server side.

   Client state encompasses the current state of the described state
   machine and the possible next transitions derived from the hypermedia
   controls within the currently processed representation (see
   Section 2).  Furthermore, clients can have part of the state of the
   distributed application in local variables.

   Resource state includes the more persistent data of an application
   (i.e., independent of individual clients).  This can be static data
   such as device descriptions, persistent data such as system
   configurations, but also dynamic data such as the current value of a
   sensor on a thing.

   It is important to distinguish between "client state" and "resource
   state" and keep them separate.  Following the Stateless constraint,
   the client state must be kept only on clients.  That is, there is no
   establishment of shared information about past and future
   interactions between client and server (usually called a session).
   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 and scalable, 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 IoT systems, this constraint lowers the memory
   requirements for server implementations, which is particularly
   important for constrained servers (e.g., sensor nodes) and servers
   serving large amount of clients (e.g., Resource Directory).








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3.3.  Uniform Resource Identifiers (URIs)

   An 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.

   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
   scheme and naming authority.  The fragment allows to refer to some
   portion of the resource, such as a Record in a SenML Pack.  However,
   fragments are processed only at client side and not sent on the wire.
   [RFC7320] provides more details on URI design and ownership with best
   current practices for establishing URI structures, conventions, and
   formats.

   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



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   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.

3.4.  Representations

   Clients can retrieve the resource state from an origin server or
   manipulate resource state on the origin server by transferring
   resource representations.  Resource representations have a media type
   that tells how the representation should be interpreted by
   identifying the representation format used.

   Typical media types for IoT systems include:

   o  "text/plain" for simple UTF-8 text

   o  "application/octet-stream" for arbitrary binary data

   o  "application/json" for the JSON format [RFC7159]

   o  "application/senml+json" [I-D.ietf-core-senml] for Sensor Markup
      Language (SenML) formatted data

   o  "application/cbor" for CBOR [RFC7049]

   o  "application/exi" for EXI [W3C.REC-exi-20110310]

   A full list of registered Internet Media Types is available at the
   IANA registry [IANA-media-types] and numerical media types registered
   for use with CoAP are listed at CoAP Content-Formats IANA registry
   [IANA-CoAP-media].

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.  As part
   of the Uniform Interface constraint, each method can have certain
   properties that give guarantees to clients.

   Safe methods do not cause 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.



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   Idempotent methods can be applied multiple times to the same resource
   while causing 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
   idempotent 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.

   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, while the origin server must ensure that there are no side-
   effects on the resource state.  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.

   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
   (with HTTP) or Location-Path and/or Location-Query Options (with
   CoAP) that provide an identifier for the resource created.  The
   server also includes a representation that describes the status of
   the request while referring to the new resource(s).

   The POST method is not safe nor idempotent.








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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
   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 a successful request.  Codes starting
   with 3 indicate redirection; further action is needed to complete the



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   request.  Codes stating with 4 and 5 indicate errors.  The codes
   starting with 4 mean client error (e.g., bad syntax in the 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.  REST Constraints

   The REST architectural style defines a set of constraints for the
   system design.  When all constraints are applied correctly, REST
   enables architectural properties of key interest [REST]:

   o  Performance

   o  Scalability

   o  Reliability

   o  Simplicity

   o  Modifiability

   o  Visibility

   o  Portability

   The following sub-sections briefly summarize the REST constraints and
   explain how they enable the listed properties.

4.1.  Client-Server

   As explained in the Architecture section, RESTful system components
   have clear roles in every interaction.  Clients have the initiative
   to issue requests, intermediaries can only forward requests, and
   servers respond requests, while origin servers are the ultimate
   recipient of requests that intent to modify resource state.




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   This improves simplicity and visibility, as it is clear which
   component started an interaction.  Furthermore, it improves
   modifiability through a clear separation of concerns.

4.2.  Stateless

   The Stateless constraint requires messages to be self-contained.
   They must contain all the information to process it, independent from
   previous messages.  This allows to strictly separate the client state
   from the resource state.

   This improves scalability and reliability, since servers or worker
   threads can be replicated.  It also improves visibility because
   message traces contain all the information to understand the logged
   interactions.

   Furthermore, the Stateless constraint enables caching.

4.3.  Cache

   This constraint requires responses to have implicit or explicit
   cache-control metadata.  This enables clients and intermediary to
   store responses and re-use them to locally answer future requests.
   The cache-control metadata is necessary to decide whether the
   information in the cached response is still fresh or stale and needs
   to be discarded.

   Cache improves performance, as less data needs to be transferred and
   response times can be reduced significantly.  Less transfers also
   improves scalability, as origin servers can be protected from too
   many requests.  Local caches furthermore improve reliability, since
   requests can be answered even if the origin server is temporarily not
   available.

4.4.  Uniform Interface

   All RESTful APIs use the same, uniform interface independent of the
   application.  This simple interaction model is enabled by exchanging
   representations and modifying state locally, which simplifies the
   interface between clients and servers to a small set of methods to
   retrieve, update, and delete state - which applies to all
   applications.

   In contrast, in a service-oriented RPC approach, all required ways to
   modify state need to be modeled explicitly in the interface resulting
   in a large set of methods - which differs from application to
   application.  Moreover, it is also likely that different parties come
   up with different ways how to modify state, including the naming of



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   the procedures, while the state within an application is a bit easier
   to agree on.

   A REST interface is fully defined by:

   o  URIs to identify resources

   o  representation formats to represent (and retrieve and manipulate)
      resource state

   o  self-descriptive messages with a standard set of methods (e.g.,
      GET, POST, PUT, DELETE with their guaranteed properties)

   o  hypermedia controls within representations

   The concept of hypermedia controls is also known as HATEOAS:
   Hypermedia As The Engine Of Application State.  The origin server
   embeds controls for the interface into its representations and
   thereby informs the client about possible next requests.  The mostly
   used control for RESTful systems is Web Linking [RFC5590].
   Hypermedia forms are more powerful controls that describe how to
   construct more complex requests, including representations to modify
   resource state.

   While this is the most complex constraints (in particular the
   hypermedia controls), it improves many different key properties.  It
   improves simplicity, as uniform interfaces are easier to understand.
   The self-descriptive messages improve visibility.  The limitation to
   a known set of representation formats fosters portability.  Most of
   all, however, this constraint is the key to modifiability, as
   hypermedia-driven, uniform interfaces allow clients and servers to
   evolve independently, and hence enable a system to evolve.

4.5.  Layered System

   This constraint enforces that a client cannot see beyond the server
   with which it is interacting.

   A layered system is easier to modify, as topology changes become
   transparent.  Furthermore, this helps scalability, as intermediaries
   such as load balancers can be introduced without changing the client
   side.  The clean separation of concerns helps with simplicity.

4.6.  Code-on-Demand

   This principle enables origin servers to ship code to clients.





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   Code-on-Demand improves modifiability, since new features can be
   deployed during runtime (e.g., support for a new representation
   format).  It also improves performance, as the server can provide
   code for local pre-processing before transferring the data.

5.  Hypermedia-driven Applications

   Hypermedia-driven applications take advantage of hypermedia controls,
   i.e., links and forms, embedded in the resource representations.  A
   hypermedia client is a client that is capable of processing these
   hypermedia controls.  Hypermedia links can be used to give additional
   information about a resource representation (e.g., the source URI of
   the representation) or pointing to other resources.  The forms can be
   used to describe the structure of the data that can be sent (e.g.,
   with a POST or PUT method) to a server, or how a data retrieval
   (e.g., GET) request for a resource should be formed.  In a
   hypermedia-driven application the client interacts with the server
   using only the hypermedia controls, instead of selecting methods and/
   or constructing URIs based on out-of-band information, such as API
   documentation.

5.1.  Motivation

   The advantage of this approach is increased evolvability and
   extensibility.  This is important in scenarios where servers exhibit
   a range of feature variations, where it's expensive to keep evolving
   client knowledge and server knowledge in sync all the time, or where
   there are many different client and server implementations.
   Hypermedia controls serve as indicators in capability negotiation.
   In particular, they describe available resources and possible
   operations on these resources using links and forms, respectively.

   There are multiple reasons why a server might introduce new links or
   forms:

   o  The server implements a newer version of the application.  Older
      clients ignore the new links and forms, while newer clients are
      able to take advantage of the new features by following the new
      links and submitting the new forms.

   o  The server offers links and forms depending on the current state.
      The server can tell the client which operations are currently
      valid and thus help the client navigate the application state
      machine.  The client does not have to have knowledge which
      operations are allowed in the current state or make a request just
      to find out that the operation is not valid.





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   o  The server offers links and forms depending on the client's access
      control rights.  If the client is unauthorized to perform a
      certain operation, then the server can simply omit the links and
      forms for that operation.

5.2.  Knowledge

   A client needs to have knowledge of a couple of things for successful
   interaction with a server.  This includes what resources are
   available, what representations of resource states are available,
   what each representation describes, how to retrieve a representation,
   what state changing operations on a resource are possible, how to
   perform these operations, and so on.

   Some part of this knowledge, such as how to retrieve the
   representation of a resource state, is typically hard-coded in the
   client software.  For other parts, a choice can often be made between
   hard-coding the knowledge or acquiring it on-demand.  The key to
   success in either case is the use in-band information for identifying
   the knowledge that is required.  This enables the client to verify
   that is has all required knowledge and to acquire missing knowledge
   on-demand.

   A hypermedia-driven application typically uses the following
   identifiers:

   o  URI schemes that identify communication protocols,

   o  Internet Media Types that identify representation formats,

   o  link relation types or resource types that identify link
      semantics,

   o  form relation types that identify form semantics,

   o  variable names that identify the semantics of variables in
      templated links, and

   o  form field names that identify the semantics of form fields in
      forms.

   The knowledge about these identifiers as well as matching
   implementations have to be shared a priori in a RESTful system.








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5.3.  Interaction

   A client begins interacting with an application through a GET request
   on an entry point URI.  The entry point URI is the only URI a client
   is expected to know before interacting with an application.  From
   there, the client is expected to make all requests by following links
   and submitting forms that are provided in previous responses.  The
   entry point URI can be obtained, for example, by manual configuration
   or some discovery process (e.g., DNS-SD [RFC6763] or Resource
   Directory [I-D.ietf-core-resource-directory]).  For Constrained
   RESTful environments "/.well-known/core" relative URI is defined as a
   default entry point for requesting the links hosted by servers with
   known or discovered addresses [RFC6690].

6.  Design Patterns

   Certain kinds of design problems are often recurring in variety of
   domains, and often re-usable design patterns can be applied to them.
   Also some interactions with a RESTful IoT system are straightforward
   to design; a classic example of reading a temperature from a
   thermometer device is almost always implemented as a GET request to a
   resource that represents the current value of the thermometer.
   However, certain interactions, for example data conversions or event
   handling, do not have as straightforward and well established ways to
   represent the logic with resources and REST methods.

   The following sections describe how common design problems such as
   different interactions can be modeled with REST and what are the
   benefits of different approaches.

6.1.  Collections

   A common pattern in RESTful systems across different domains is the
   collection.  A collection can be used to combine multiple resources
   together by providing resources that consist of set of (often
   partial) representations of resources, called items, and links to
   resources.  The collection resource also defines hypermedia controls
   for managing and searching the items in the collection.

   Examples of the collection pattern in RESTful IoT systems are the
   CoRE Resource Directory [I-D.ietf-core-resource-directory], CoAP pub/
   sub broker [I-D.ietf-core-coap-pubsub], and resource discovery via
   ".well-known/core".  Collection+JSON [CollectionJSON] is an example
   of a generic collection Media Type.







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6.2.  Calling a Procedure

   To modify resource state, clients usually use GET to retrieve a
   representation from the server, modify that locally, and transfer the
   resulting state back to the server with a PUT (see Section 4.4).
   Sometimes, however, the state can only be modified on the server
   side, for instance, because representations would be too large to
   transfer or part of the required information shall not be accessible
   to clients.  In this case, resource state is modified by calling a
   procedure (or "function").  This is usually modeled with a POST
   request, as this method leaves the behavior semantics completely to
   the server.  Procedure calls can be divided into two different
   classes based on how long they are expected to execute: "instantly"
   returning and long-running.

6.2.1.  Instantly Returning Procedures

   When the procedure can return within the expected response time of
   the system, the result can be directly returned in the response.  The
   result can either be actual content or just a confirmation that the
   call was successful.  In either case, the response does not contain a
   representation of the resource, but a so-called action result.
   Action results can still have hypermedia controls to provide the
   possible transitions in the application state machine.

6.2.2.  Long-running Procedures

   When the procedure takes longer than the expected response time of
   the system, or even longer than the response timeout, it is a good
   pattern to create a new resource to track the "task" execution.  The
   server would respond instantly with a "Created" status (HTTP code 201
   or CoAP 2.01) and indicate the location of the task resource in the
   corresponding header field (or CoAP option) or as a link in the
   action result.  The created resource can be used to monitor the
   progress, to potentially modify queued tasks or cancel tasks, and to
   eventually retrieve the result.

   Monitoring information would be modeled as state of the task
   resource, and hence be retrievable as representation.  The result -
   when available - can be embedded in the representation or given as a
   link to another sub-resource.  Modifying tasks can be modeled with
   forms that either update sub-resources via PUT or do a partial write
   using PATCH or POST.  Canceling a task would be modeled with a form
   that uses DELETE to remove the task resource.







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6.2.3.  Conversion

   A conversion service is a good example where REST resources need to
   behave more like a procedure call.  The knowledge of converting from
   one representation to another is located only at the server to
   relieve clients from high processing or storing lots of data.  There
   are different approaches that all depend on the particular conversion
   problem.

   As mentioned in the previous sections, POST request are a good way to
   model functionality that does not necessarily affect resource state.
   When the input data for the conversion is small and the conversion
   result is deterministic, however, it can be better to use a GET
   request with the input data in the URI query part.  The query is
   parameterizing the conversion resource, so that it acts like a look-
   up table.  The benefit is that results can be cached also for HTTP
   (where responses to POST are not cacheable).  In CoAP, cacheability
   depends on the response code, so that also a response to a POST
   request can be made cacheable through a 2.05 Content code.

   When the input data is large or has a binary encoding, it is better
   to use POST requests with a proper Media Type for the input
   representation.  A POST request is also more suitable, when the
   result is time-dependent and the latest result is expected (e.g.,
   exchange rates).

6.2.4.  Events as State

   In event-centric paradigms such as pub/sub, events are usually
   represented by an incoming message that might even be identical for
   each occurrence.  Since the messages are queued, the receiver is
   aware of each occurrence of the event and can react accordingly.  For
   instance, in an event-centric system, ringing a door bell would
   result in a message being sent that represents the event that it was
   rung.

   In resource-oriented paradigms such as REST, messages usually carry
   the current state of the remote resource, independent from the
   changes (i.e., events) that have lead to that state.  In a naive yet
   natural design, a door bell could be modeled as a resource that can
   have the states unpressed and pressed.  There are, however, a few
   issues with this approach.  Polling is not an option, as it is highly
   unlikely to be able to observe the pressed state with any realistic
   polling interval.  When using CoAP Observe with Confirmable
   notifications, the server will usually send two notifications for the
   event that the door bell was pressed: notification for changing from
   unpressed to pressed and another one for changing back to unpressed.
   If the time between the state changes is very short, the server might



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   drop the first notification, as Observe only guarantees only eventual
   consistency (see Section 1.3 of [RFC7641]).

   The solution is to pick a state model that fits better to the
   application.  In the case of the door bell - and many other event-
   driven resources - the solution could be a counter that counts how
   often the bell was pressed.  The corresponding action is taken each
   time the client observes a change in the received representation.

   In the case of a network outage, this could lead to a ringing sound
   10 minutes after the bell was rung.  Also including a timestamp of
   the last counter increment in the state can help to suppress ringing
   a sound when the event has become obsolete.

6.3.  Server Push

   Overall, a universal mechanism for server push, that is, change-of-
   state notifications and stand-alone event notifications, is still an
   open issue that is being discussed in the Thing-to-Thing Research
   Group.  It is connected to the state-event duality problem and
   custody transfer, that is, the transfer of the responsibility that a
   message (e.g., event) is delivered successfully.

   A proficient mechanism for change-of-state notifications is currently
   only available for CoAP: Observing resources [RFC7641].  It offers
   enventual consistency, which guarantees "that if the resource does
   not undergo a new change in state, eventually all registered
   observers will have a current representation of the latest resource
   state".  It intrinsically deals with the challenges of lossy
   networks, where notifications might be lost, and constrained
   networks, where there might not be enough bandwidth to propagate all
   changes.

   For stand-alone event notifications, that is, where every single
   notification contains an identifiable event that must not be lost,
   observing resources is not a good fit.  A better strategy is to model
   each event as a new resource, whose existence is notified through
   change-of-state notifications of an index resource (cf.  Collection
   pattern).  Large numbers of events will cause the notification to
   grow large, as it needs to contain a large number of Web links.
   Blockwise transfers [RFC7959] can help here.  When the links are
   ordered by freshness of the events, the first block can already
   contain all links to new events.  Then, observers do not need to
   retrieve the remaining blocks from the server, but only the
   representations of the new event resources.

   An alternative pattern is to exploit the dual roles of IoT devices,
   in particular when using CoAP: they are usually client and server at



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   the same time.  A client observer would subscribe to events by
   registering a callback URI at the origin server, e.g., using a POST
   request and receiving the location of a temporary subscription
   resource as handle.  The origin server would then publish events by
   sending POST requests containing the event to the observer.  The
   cancellation can be modeled through deleting the subscription
   resource.  This pattern makes the origin server responsible for
   delivering the event notifications.  This goes beyond retransmissions
   of messages; the origin server is usually supposed to queue all
   undelivered events and to retry until successful delivery or explicit
   cancellation.  In HTTP, this pattern is known as REST Hooks.

   In HTTP, there exist a number of workarounds to enable server push,
   e.g., long polling and streaming [RFC6202] or server-sent events
   [W3C.REC-html5-20141028].  Long polling as an extension that both
   server and client need to be aware of.  In IoT systems, long polling
   can introduce a considerable overhead, as the request has to be
   repeated for each notification.  Streaming and server-sent events (in
   fact an evolved version of streaming) are more efficient, as only one
   request is sent.  However, there is only one response header and
   subsequent notifications can only have content.  There are no means
   for individual status and metadata, and hence no means for proficient
   error handling (e.g., when the resource is deleted).

7.  Security Considerations

   This document does not define new functionality and therefore does
   not introduce new security concerns.  We assume that system designers
   apply classic Web security on top of the basic RESTful guidance given
   in this document.  Thus, security protocols and considerations from
   related specifications apply to RESTful IoT design.  These include:

   o  Transport Layer Security (TLS): [RFC5246] and [RFC6347]

   o  Internet X.509 Public Key Infrastructure: [RFC5280]

   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]

   IoT-specific security is mainly work in progress at the time of
   writing.  First specifications include:

   o  (D)TLS Profiles for the Internet of Things: [RFC7925]




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   Further IoT security considerations are available in
   [I-D.irtf-t2trg-iot-seccons].

8.  Acknowledgement

   The authors would like to thank Mert Ocak, Heidi-Maria Back, Tero
   Kauppinen, Michael Koster, Robby Simpson, Ravi Subramaniam, Dave
   Thaler, Erik Wilde, and Niklas Widell for the reviews and feedback.

9.  References

9.1.  Normative References

   [I-D.ietf-core-object-security]
              Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security of CoAP (OSCOAP)", draft-ietf-core-
              object-security-04 (work in progress), July 2017.

   [I-D.ietf-core-resource-directory]
              Shelby, Z., Koster, M., Bormann, C., Stok, P., and C.
              Amsuess, "CoRE Resource Directory", draft-ietf-core-
              resource-directory-11 (work in progress), July 2017.

   [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,
              <https://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, <https://www.rfc-
              editor.org/info/rfc5246>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5590]  Harrington, D. and J. Schoenwaelder, "Transport Subsystem
              for the Simple Network Management Protocol (SNMP)",
              STD 78, RFC 5590, DOI 10.17487/RFC5590, June 2009,
              <https://www.rfc-editor.org/info/rfc5590>.




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   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
              DOI 10.17487/RFC5988, October 2010, <https://www.rfc-
              editor.org/info/rfc5988>.

   [RFC6202]  Loreto, S., Saint-Andre, P., Salsano, S., and G. Wilkins,
              "Known Issues and Best Practices for the Use of Long
              Polling and Streaming in Bidirectional HTTP", RFC 6202,
              DOI 10.17487/RFC6202, April 2011, <https://www.rfc-
              editor.org/info/rfc6202>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <https://www.rfc-editor.org/info/rfc6690>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://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,
              <https://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, <https://www.rfc-
              editor.org/info/rfc7231>.

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015, <https://www.rfc-
              editor.org/info/rfc7641>.

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016, <https://www.rfc-
              editor.org/info/rfc7959>.

   [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>.




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   [W3C.REC-html5-20141028]
              Hickson, I., Berjon, R., Faulkner, S., Leithead, T.,
              Navara, E., O&#039;Connor, T., and S. Pfeiffer, "HTML5",
              World Wide Web Consortium Recommendation REC-
              html5-20141028, October 2014,
              <http://www.w3.org/TR/2014/REC-html5-20141028>.

9.2.  Informative References

   [CollectionJSON]
              Amundsen, M., "Collection+JSON - Document Format",
              February 2013,
              <http://amundsen.com/media-types/collection/format/>.

   [I-D.ietf-core-coap-pubsub]
              Koster, M., Keranen, A., and J. Jimenez, "Publish-
              Subscribe Broker for the Constrained Application Protocol
              (CoAP)", draft-ietf-core-coap-pubsub-02 (work in
              progress), July 2017.

   [I-D.ietf-core-senml]
              Jennings, C., Shelby, Z., Arkko, J., Keranen, A., and C.
              Bormann, "Media Types for Sensor Measurement Lists
              (SenML)", draft-ietf-core-senml-10 (work in progress),
              July 2017.

   [I-D.irtf-t2trg-iot-seccons]
              Garcia-Morchon, O., Kumar, S., and M. Sethi, "State-of-
              the-Art and Challenges for the Internet of Things
              Security", draft-irtf-t2trg-iot-seccons-05 (work in
              progress), September 2017.

   [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>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.

   [RFC6943]  Thaler, D., Ed., "Issues in Identifier Comparison for
              Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May
              2013, <https://www.rfc-editor.org/info/rfc6943>.



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   [RFC7159]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
              2014, <https://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, <https://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, <https://www.rfc-
              editor.org/info/rfc7252>.

   [RFC7320]  Nottingham, M., "URI Design and Ownership", BCP 190,
              RFC 7320, DOI 10.17487/RFC7320, July 2014,
              <https://www.rfc-editor.org/info/rfc7320>.

   [RFC7925]  Tschofenig, H., Ed. and T. Fossati, "Transport Layer
              Security (TLS) / Datagram Transport Layer Security (DTLS)
              Profiles for the Internet of Things", RFC 7925,
              DOI 10.17487/RFC7925, July 2016, <https://www.rfc-
              editor.org/info/rfc7925>.

Appendix A.  Future Work

   o  Interface semantics: shared knowledge among system components (URI
      schemes, media types, relation types, well-known locations; see
      core-apps)

   o  Unreliable (best effort) communication, robust communication in
      network with high packet loss, 3-way commit

   o  Discuss directories, such as CoAP Resource Directory

   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





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   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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