[Docs] [txt|pdf|xml|html] [Tracker] [Email] [Nits]

Versions: 00 draft-shelby-core-coap

6lowapp                                                        Z. Shelby
Internet-Draft                                                 Sensinode
Intended status: Informational                        M. Garrison Stuber
Expires: June 27, 2010                                             Itron
                                                               D. Sturek
                                                  Pacific Gas & Electric
                                                                B. Frank
                                                            Tridium, Inc
                                                               R. Kelsey
                                                       December 24, 2009

                         CoAP Feature Analysis


   This document considers the requirements and resulting features
   needed for the design of the Constrained Application Protocol (CoAP).
   Starting from requirements for energy and building automation
   applications, the basic features are identified along with an
   analysis of possible realizations.  The goal of the document is to
   provide a basis for protocol design and related discussion.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   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."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   This Internet-Draft will expire on June 27, 2010.

Shelby, et al.            Expires June 27, 2010                 [Page 1]

Internet-Draft            CoAP Feature Analysis            December 2009

Copyright Notice

   Copyright (c) 2009 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
   the Trust Legal Provisions and are provided without warranty as
   described in the BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Shelby, et al.            Expires June 27, 2010                 [Page 2]

Internet-Draft            CoAP Feature Analysis            December 2009

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  CoAP Requirements  . . . . . . . . . . . . . . . . . . . .  3
   2.  CoAP Feature Analysis  . . . . . . . . . . . . . . . . . . . .  5
     2.1.  Compact Header . . . . . . . . . . . . . . . . . . . . . .  5
     2.2.  Basic Messages . . . . . . . . . . . . . . . . . . . . . .  6
     2.3.  REST Methods . . . . . . . . . . . . . . . . . . . . . . .  6
     2.4.  Content-type encoding  . . . . . . . . . . . . . . . . . .  6
     2.5.  URLs . . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.6.  Caching  . . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.7.  Subscribe/Notify . . . . . . . . . . . . . . . . . . . . .  8
     2.8.  Transport Binding  . . . . . . . . . . . . . . . . . . . .  9
       2.8.1.  UDP  . . . . . . . . . . . . . . . . . . . . . . . . .  9
       2.8.2.  TCP  . . . . . . . . . . . . . . . . . . . . . . . . .  9
     2.9.  Resource Discovery . . . . . . . . . . . . . . . . . . . . 10
     2.10. HTTP Mapping . . . . . . . . . . . . . . . . . . . . . . . 10
   3.  Applicability  . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.1.  Energy Applications  . . . . . . . . . . . . . . . . . . . 10
     3.2.  Building Automation  . . . . . . . . . . . . . . . . . . . 11
     3.3.  General M2M Applications . . . . . . . . . . . . . . . . . 12
   4.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 12
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 13
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 13
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14

Shelby, et al.            Expires June 27, 2010                 [Page 3]

Internet-Draft            CoAP Feature Analysis            December 2009

1.  Introduction

   The use of web services on the Internet has become ubiquitous in most
   applications, and depends on the basic REST architecture of the web.
   The proposed Constrained RESTful Environments (CoRE) working group
   aims at extending the REST architecture to a suitable form for the
   most constrained nodes (e.g. 8-bit microcontrollers with limited RAM
   and ROM) and networks (e.g. 6LoWPAN).  One of the main goals of CoRE
   is to design a generic RESTful protocol for the special requirements
   of this constrained environment, especially considering energy and
   building automation applications.  The result of this work should be
   a Constrained Application Protocol (CoAP) which easily traslates to
   HTTP for integration with the web while meeting specialized
   requirements such as multicast support, very low overhead and

   This document first analyzes the requirements for CoAP from the
   proposed charter and related application requirement drafts in
   Section 1.1.  The key features needed for the CoAP protocol are then
   identified in Section 2.  Possible ways of realizing each feature are
   considered and recommendations made where possible.  Finally, the
   applicability of these features to energy, building automation and
   general M2M applications is considered in Section 3.

1.1.  CoAP Requirements

   Paragraph 1.  The following requirements for CoAP have been
   identified in the proposed charter of the working group, in the
   6lowapp problem statement [I-D.bormann-6lowpan-6lowapp-problem], or
   in the application specific requirement documents.  The requirements
   relevant to CoAP are summarizes as follows:

   REQ1:   Nodes have limited code size and limited RAM (typically on
           the order of 128K flash and 4K of RAM). [charter],

   REQ2:   A header size on the order of 10 bytes, and assumes an
           underlying network bandwidth of several dozen kbit/s.

   REQ3:   Ability to deal with sleeping nodes.  Devices may be powered
           off at any point in time but periodically "wake up" for brief
           periods of time. [charter], [I-D.sturek-6lowapp-smartenergy],

Shelby, et al.            Expires June 27, 2010                 [Page 4]

Internet-Draft            CoAP Feature Analysis            December 2009

   REQ4:   Protocol must support the caching of recent resource
           requests, along with caching subscriptions to sleeping nodes.

   REQ5:   Must support the manipulation of simple resources on
           constrained nodes and networks.  The architecture requires
           push, pull and a notify approach to manipulating resources.
           CoAP will be able to set and query a resource on a Device.
           It will allow a Device to publish a value or event to another
           Device. [charter], [I-D.sturek-6lowapp-smartenergy],

   REQ6:   Must define a mapping from CoAP to a HTTP REST API; this
           mapping will not depend on a specific application and must be
           as transparent as possible using standard protocol response
           and error codes where possible. [charter],
           [I-D.sturek-6lowapp-smartenergy], [I-D.gold-6lowapp-sensei]

   REQ7:   Each interface profile will define the Resources on the
           Device that applications can manipulate and what
           manipulations are possible.  CoAP must be able to discover
           Devices on the CNN and to interrogate them to find out which
           interface profiles they support. [charter]

   REQ8:   CoAP will support a non-reliable multicast message to be sent
           to a group of Devices to manipulate a resource on all the
           Devices simultaneously (roughly within 50 ms of each other)
           [charter].  The use of multicast for discovery and
           advertisement must be supported, along with the support of
           unicast responses [I-D.sturek-6lowapp-smartenergy].

   REQ9:   The protocol will operate by default over UDP, it may
           optionally be bound to TCP or other reliable transports.
           [charter], [I-D.sturek-6lowapp-smartenergy],

   REQ10:  Reliability must be provided for application layer messages
           [I-D.sturek-6lowapp-smartenergy].  Must achieve < 0.01%
           Application layer errors on all messages assuming a .1%
           Network layer error rate.

   REQ11:  Latency times should be mimimized of the Home Area Network
           (HAN), and ideally a typical exchange should consist of just
           a single request and a single response message.

Shelby, et al.            Expires June 27, 2010                 [Page 5]

Internet-Draft            CoAP Feature Analysis            December 2009

   REQ12:  Internet media type and transfer encoding type support.
           [I-D.sturek-6lowapp-smartenergy], [I-D.gold-6lowapp-sensei]

2.  CoAP Feature Analysis

   This section introduces the minimum set of features needed to realize
   CoAP, and looks at the possible options for realizing them.  These
   features are considered in light on the requirements listed in
   Section 1.1.  The goal is to consider the cross-dependencies,
   benefits and drawbacks of alternatives for realizing CoAP and to
   narrow down the options where obvious.

2.1.  Compact Header

   There is a requirement for a header overhead on the order of 10 bytes
   with limited complexity due to node limitations.  It should be noted
   that in wireless networks bits transmitted are much more expensive
   than processing cycles.  The following header design options are

   Fixed approach:  The simplest approach is to assume as fixed set of
         byte-aligned fields.  The use of variable length fields should
         be avoided if possible, one obvious exception being a string
         URL (see Section 2.5).  This results in a simple header that
         can be represented as a struct and easily parsed/created.  The
         disadvantages are difficult evolvability and the tendency to
         design missing tranport features on-top of CoAP.

   Extensible approach:  The approach of [I-D.frank-6lowapp-chopan] is
         to encode HTTP headers as binary tuples, assuming that a large
         number of optional headers will be needed.  A similar approach
         could be takenin CoAP, giving total header flexibility.  The
         disadvantage is header parsing complexity.

   Hybrid approach:  It is unclear how much extensibility is really
         required from the headers of CoAP.  Some of the fields in the
         protocol will obviously require a sufficient value space for
         future extensions, such as for indicating content type.  Other
         headers are clearly optional, such as those related to cache
         control (see Section 2.6) or even the URL (see Section 2.5).  A
         hybrid approach would be to design a small fixed header, with
         the ability to include extension headers, such as in ICMP

   Considering the features foreseen by this document, some kind of
   extensible hybrid approach is recommended.  Most features are fixed
   for messages, whereas only a some are expected to be optional.

Shelby, et al.            Expires June 27, 2010                 [Page 6]

Internet-Draft            CoAP Feature Analysis            December 2009

2.2.  Basic Messages

   It is assumed that basic Request and Response messages will be
   required by the CoAP protocol.  This also provides a natural mapping
   to HTTP (See Section 2.10) and the response may be useful as an
   acknowledgement in UDP reliability (See Section 2.8.1).  It can be
   considered that CoAP methods are different kinds of Request messages.

2.3.  REST Methods

   The core methods of REST must be supported within CoAP.  To minimize
   confusion with HTTP methods, having their own protocol semantics, in
   CoAP we call the basic REST methods READ, WRITE, CREATE, DESTROY.

   Additionally, CoAP must support a light-weight Subscribe/Notify
   mechanism (see Section 2.7).  This may require a new NOTIFY method.
   The discovery mechanism of CoAP may also require a new method called
   DISCOVER which has different semantics than a READ (see Section 2.9).
   In order to maintain compatibility with HTTP, these new messages must
   be mapped to a standard HTTP method.  See Section 2.10 for more about
   HTTP mapping.

2.4.  Content-type encoding

   In order to support hetergenous uses, it is important that CoAP is
   transparent to the use of different application payloads.  In order
   for the application process receiving a packet to properly parse a
   payload, its content-type and encoding should be explicitly known
   from the header (as e.g. with HTTP).  The use of typical binary
   encodings for XML is discussed in [I-D.shelby-6lowapp-encoding],
   which includes recommendations for header indication.  The draft
   recommends the indication of at least 10 Internet media types (MIME)
   [RFC2046] and 2 content transfer encodings.

   It is obvious that string names of Internet media types [RFC2046] are
   not appropriate for use in the CoAP header.  But then how to make
   this small yet extensible?  One possible solution is to simply assign
   codes to a small subset of common MIME and content transfer encoding
   types and have IANA maintain that.  A field of 16-bits should be
   sufficient for encoding both media and content transfer encoding

2.5.  URLs

   The Universal Resource Locator (URL) [RFC3986] is an important
   feature of the REST architecture, the relative part of the URL
   indicates which resource on the server is being manipulated.  It is
   surely useful for CoAP to support string URLs, which requires a

Shelby, et al.            Expires June 27, 2010                 [Page 7]

Internet-Draft            CoAP Feature Analysis            December 2009

   variable length-value field.  Although URLs can be designed for
   compactness, this still often results in 10s of bytes of overhead.
   The encoding of of a URL string needs to be considered, as this is
   becoming increasingly complex.  It is recommended that only US-ASCII
   is supported in URL strings for CoAP as defined in [RFC3986], or even
   a stricter subset as URL parsing is complex and may result in
   security problems.

   Constrained devices are not general purpose web servers, and thus
   often won't host but a small set of resources with fixed URLs.  Thus
   in addition to string URLs a feature for compressing fixed URLs would
   be useful.

   One way of achieving this would be to assign an integer identifier
   (7-8 bits should be sufficient) to each fixed URL in an off-line
   interface description (e.g.  Web Application Description Langauge
   (WADL)) or in its profile.  This identifier could be encoded in the
   URL length field instead of the string length.

2.6.  Caching

   The cachability of CoAP messages will be important, especially with
   the sleeping node configurations and power limitations typically
   found in constrained networks and nodes.  What features of
   cachability are really required and how much energy are we willing to
   spend on it?  Roughly 50% of the HTTP specifications are dedicated to
   sohpisticated caching.  With CoAP we should look at the bare minimum
   caching feature possible.

   Before talking about caching solutiongs, we should consider in what
   scenarios caching will actually be required.  The following two
   scenarios have been identified:

   o  An intermediate CoAP proxy may cache resources and answer READ
      requests using a cached version.  The resource may be cached from
      previous responses or notifications.  This requires at least Max-
      Age cache control information about each resource.

   o  An intermediate CoAP proxy may cache subscriptions to a sleeping
      node.  This requires at least Max-Age information about the

   Three possible approaches have been identified for caching support.

   In-band approach:  One approach is suggested in
         [I-D.frank-6lowapp-chopan], which analyses the subset of
         features from HTTP that could be used for simple sensor data
         purposes.  The proposal is that simply using the using the HTTP

Shelby, et al.            Expires June 27, 2010                 [Page 8]

Internet-Draft            CoAP Feature Analysis            December 2009

         Age header (for resource age) and Cache-Control header (for
         max-age).  Max-age may also be applied in requests.  Both
         headers make use of a 2-byte value in seconds.  The advantage
         of this approach is that cache control information is easily
         available from the header.  The disadvantage is the header

   Out-of-band approach:  Here the CoAP protocol would be agnostic to
         the cachability of the resources it is carrying, instead
         leaving the definition of cache control parameters to the body
         of the resources in an application specific way.  The
         disadvantage is that this makes proxies dependent on the

   Discovery approach:  In this approach the cache control information
         for resources is defined off-line in the list of a server's
         resources.  This approach is used e.g. in the SENSEI system
         [I-D.gold-6lowapp-sensei].  The disadvantage id that the
         caching is dependent on the profile, which may not be a problem
         if the cache information is in a universal format (see
         Section 2.9).

   Based on the current analysis either the In-band or Profile approach
   would be reasonable for CoAP considering the requirements.

2.7.  Subscribe/Notify

   CoAP is required to integrate a push model for interaction in
   addition to traditional request/response.  Meaning that interested
   clients could subscribe to a resource (a URL), and receive
   notifications to a call-back URL of their choice.  In its most basic
   form a notification would be sent each time the resource changes.
   There are many issues to consider including managing subscription
   leasing and timeouts, how to batch multiple changes and how to tune
   notify times.  Before considering the details, there are a few
   general general models possible for realizing the Subscribe/Notify

   Resource:  Subscribe is realized using CREATE on a well known
         resource (e.g. /subsribe) with the URL of the resource of
         interest and a URL call-back in the body).  Notifications would
         be made using a NOTIFY message to the call-back URL.  Likewise,
         de-subscribe is realized using DESTROY on the same well known
         resource with the URL in the body.  Notifications would cease
         after the DESTROY.

Shelby, et al.            Expires June 27, 2010                 [Page 9]

Internet-Draft            CoAP Feature Analysis            December 2009

   Watch:  This method would require a CREATE to a new URI to "create" a
         new watch resource.  WRITE is then used to add/remove a set of
         URIs being "watched" along with call-backs.

2.8.  Transport Binding

   The CoAP protocol will operate by default over UDP and it may
   optionally be bound to TCP or other reliable transports.  In this
   section we look at the issues regarding these bindings.

2.8.1.  UDP

   The goal of binding CoAP to UDP is to provide the bare minimum
   features for the protocol to operate over UDP, going nowhere near
   trying to re-create the full feature set of TCP.  The bare minimum
   features required would be:

   o  Stop-and-wait would be sufficient for reliability.  A simple
      response message itself would suffice as an acknowledgement with
      retransmission support.  Not all requests require reliability,
      thus this should be optional.  Performance is not the key here and
      for more sophisticated reliability and flow control TCP can be

   o  A sequence number (transaction ID) is needed to match responses to
      open requests and would be generated by the client.  A 12-16 bit
      unsigned interger would be sufficient.  [I-D.frank-6lowapp-chopan]
      also considered this solution.

   o  Multicast support.  Providing reliability with a multicast
      destination address would be very complex.  Therefore the goal is
      to provide non-reliable multicast service.  In many cases there
      may not be a response to a multi-cast message.  A multicast
      command might result in an action being taken at a device, but no
      response being sent.  Therefore a multicast request may be
      answered with a unicast response, however without reliability
      (retransmission e.g.).

2.8.2.  TCP

   The CoAP protocol also has the goal of defining a TCP binding.  As
   TCP provides a reliable stream this binding does not require anything
   special from the CoAP protcol design.  The same basic messages could
   be applied over TCP without stop-and-wait.  A transaction ID should
   still be used over TCP.

Shelby, et al.            Expires June 27, 2010                [Page 10]

Internet-Draft            CoAP Feature Analysis            December 2009

2.9.  Resource Discovery

   CoAP is required to support the discovery of resources offered by
   CoAP servers.  In order to achieve this, the protocol would need to
   suport multicast with optional responses for discovery (over UDP),
   along with unicast requests and posting of profiles (over UDP or
   TCP).  A well-known resource (/profile) could be used to enable
   profile discovery through a new DISCOVER method along with profile
   sending on an optional broker with a CREATE or modification with a
   WRITE to /profile.  The response to a DISCOVER message would include
   a list of resource URLs available, or those matched if the DISCOVER
   has a body with one or more URLs.  Such a resource list could include
   optional information such as the URL to a description of the

   Section 2.6 discusses different caching models.  If caching would be
   realized using the Profile model, then the resource list would need
   to indicate at least the Max-Age of each resource.

2.10.  HTTP Mapping

   It shall be possible to map from CoAP directly to HTTP, CoAP however
   only offers a subset of the HTTP protocol features.  As a result,
   programs implementing translation between HTTP and CoAP must either
   implement other HTTP 1.1 commands on behalf of the CoAP nodes (e.g.
   LINK, TRACE, OPTIONS), or must reject such request.  The primary
   responsibility of a program translating between HTTP and CoAP is to
   rewrite the headers, translating between the highly optimized CoAP
   headers and plain text HTTP headers.  It must also manage/maintain
   TCP sessions necessary for HTTP.  Depending on how some of the
   features of CoAP are realized, the mapping may also need to make
   further translations for subscription or caching.

3.  Applicability

   This sections looks at the applicability of the CoAP features for
   energy, building automation and other macine-to-machine (M2M)

3.1.  Energy Applications

   Rising energy prices, concerns about global warming and energy
   resource depletion, and societal interest in more ecologically
   friendly living have resulted in government mandates for Smart Energy
   solutions.  In a Smart Energy environment consumers of energy have
   direct, immediate access to information about their consumption, and
   are able to take action based on that information.  Smart Energy

Shelby, et al.            Expires June 27, 2010                [Page 11]

Internet-Draft            CoAP Feature Analysis            December 2009

   systems also allow device to device communication to optimize the
   transport, reliability, and safety of energy delivery systems.  While
   often Smart Energy solutions are electricity-centric, i.e.  Smart
   Grid, gas and water are also subject to the same pressures, and can
   benefit from the same technology.

   Smart Energy Transactions typically include the exchange of current
   consumption information, text messages from providers to consumers,
   and control signals requesting a reduction in consumption.  Advanced
   features such as billing information, energy prepayment transactions,
   management of distributed energy resources (e.g. generators and
   photo-voltaics), and management of electric vehicles are also being

   Smart Energy benefits from Metcalfe's Law. The more devices that are
   part of a smart energy network within the home or on the grid, the
   more valuable it becomes.  Showing a consumer how much energy they
   are using is useful.  Combining that with specific information about
   their major appliances, and enabling them to adjust their consumption
   based on current pricing and system demand is much much more
   powerful.  To do this however requires a system that is resillient,
   low cost, and easy to install.  In many areas this is being done with
   systems built around IEEE 802.15.4 radios.  In the United States,
   there are over 30 million electric meters that will be deployed with
   these radios.  These radios will be combined to form a mesh network,
   enabling Smart Energy communication within the home.  The maximum
   packet size for IEEE 802.15.4 is only 127 bytes.  Additionally, there
   is the well known issue of how TCP manages congestion working sub-
   optimally over wireless networks.  IEEE 802.15.4 is ideal for these
   applications because of its low cost and its support for battery
   powered devices; however, it is not as well suited for heavier
   protocols like HTTP.  These technical issues with IEEE 802.15.4
   networks combined with a desire to facilitate broader compatibility,
   makes a protocol like CoAP desireable.  Its REST architecture will
   allow seamless compatibility with the rest of the Internet, allowing
   it to be easily integrated with web browsers and web-based service
   providers, while at the same time being appropriately sized for the
   low-cost networks necessary for its success.

3.2.  Building Automation

   Building automation applications were analyzed in detail including
   use cases in [I-D.martocci-6lowapp-building-applications].  Although
   many of the embedded control solutions for building automation make
   use of industry-specific application protocols like BACnet over IP,
   there is a growing use of web services in building monitoring, remote
   control and IT integration.  The OASIS oBIX standard [ref] is one
   example of the use of web services for the monitoring and

Shelby, et al.            Expires June 27, 2010                [Page 12]

Internet-Draft            CoAP Feature Analysis            December 2009

   interconnection of heterogeneous building systems.  Several of the
   CoAP requirements have been taken from
   [I-D.martocci-6lowapp-building-applications].  The resulting features
   should allow for peer-to-peer interactions as well as node-server
   request/response and push interfactions for monitoring and some
   control purposes.  For building automation control with very strict
   timing requirements using e.g. multicast, further features may be
   required on top of CoAP.

3.3.  General M2M Applications

   CoAP provides a natural extension of the REST architecture into the
   domain of constrained nodes and networks, aiming at requirements from
   automation applications in energy and building automation.  A very
   wide range of machine-to-machine (M2M) applications have similar
   requirements to those considered in this document, and thus it is
   foreseen that CoAP may be widely applied in the industry.  One
   standardization group considering a general M2M architecture and API
   is the ETSI M2M TC [ref], which considers a wide range of
   applications including energy.  Another group developing solutions
   for general embedded device control is the OASIS Device Proile Web
   Services (DPWS) group.  The consideration of DPWS over 6LoWPAN is
   available in [I-D.moritz-6lowapp-dpws-enhancements].

4.  Conclusions

   This document analyzed the requirements associated with the design of
   the foreseen Constrained Application Protocol (CoAP).  Based on these
   requirements a list of minumum features was analyzed along with
   different options for realizing them.  If possible a recommendation
   was also made where obvious.  Finally, the identified features of
   CoAP are considered for energy, building automation and M2M
   applications.  This document is meant to serve as a basis for the
   design of the CoAP protocol and relevant discussion.

   CoAP is proposed as a transport agnostic extension of REST for
   deployment in confined computing environments.  The intent is to
   align CoAP with HTTP wherever possible to leverage the web services
   computing environment already in place.

   Whereas REST envisions just 4 primitives (READ, SET, CREATE and
   DESTROY), CoAP proposes to extend this paradigm with a NOTIFY
   primitive to enable publish/subscribe along with a DISCOVER primitive
   to support multicast discovery of services denoted by URL.  The main
   architectural difference between READ and the new discovery primitive
   is the requirement to not respond if the URL is not present on a
   local device.

Shelby, et al.            Expires June 27, 2010                [Page 13]

Internet-Draft            CoAP Feature Analysis            December 2009

   Finally, CoAP seeks to preserve the caching facilities of HTTP and
   extend that capability for power saving devices that are not always
   active on the network.

5.  Security Considerations

   Some of the features considered in this document will need further
   security considerations during a protocol design.  For example the
   use of string URLs may have entail security risks due to complex
   processing on limited microcontroller implementations.

   The CoAP protocol will be designed for use with (D)TLS or object
   security.  A protocol design should consider how integration with
   these security methods will be done, how to secure the CoAP header
   and other implications.

6.  IANA Considerations

   This draft requires no IANA consideration.

7.  Acknowledgments

   Thanks to Cullen Jennings for helpful comments and discussions.

8.  References

8.1.  Normative References

              Frank, B., "Chopan - Compressed HTTP Over PANs",
              draft-frank-6lowapp-chopan-00 (work in progress),
              September 2009.

              Gold, R., Krco, S., Gluhak, A., and Z. Shelby, "SENSEI
              6lowapp Requirements", draft-gold-6lowapp-sensei-00 (work
              in progress), October 2009.

              Martocci, J. and A. Schoofs, "Commercial Building
              Applications Requirements",
              draft-martocci-6lowapp-building-applications-00 (work in
              progress), October 2009.

Shelby, et al.            Expires June 27, 2010                [Page 14]

Internet-Draft            CoAP Feature Analysis            December 2009

              Shelby, Z., Luimula, M., and D. Peintner, "Efficient XML
              Encoding and 6LowApp", draft-shelby-6lowapp-encoding-00
              (work in progress), October 2009.

              Sturek, D., Shelby, Z., Lohman, D., Stuber, M., and S.
              Ashton, "Smart Energy Requiements for 6LowApp",
              draft-sturek-6lowapp-smartenergy-00 (work in progress),
              October 2009.

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              November 1996.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

8.2.  Informative References

              Bormann, C., Sturek, D., and Z. Shelby, "6LowApp: Problem
              Statement for 6LoWPAN and LLN Application Protocols",
              draft-bormann-6lowpan-6lowapp-problem-01 (work in
              progress), July 2009.

              Moritz, G., "DPWS for 6LoWPAN",
              draft-moritz-6lowapp-dpws-enhancements-00 (work in
              progress), December 2009.

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, September 1981.

Authors' Addresses

   Zach Shelby
   Kidekuja 2
   Vuokatti  88600

   Phone: +358407796297
   Email: zach@sensinode.com

Shelby, et al.            Expires June 27, 2010                [Page 15]

Internet-Draft            CoAP Feature Analysis            December 2009

   Michael Garrison Stuber
   2111 N. Molter Road
   Liberty Lake, WA  99025

   Phone: +1.509.891.3441
   Email: Michael.Stuber@itron.com

   Don Sturek
   Pacific Gas & Electric
   77 Beale Street
   San Francisco, CA

   Phone: +1-619-504-3615
   Email: d.sturek@att.net

   Brian Frank
   Tridium, Inc
   Richmond, VA

   Email: brian.tridium@gmail.com

   Richard Kelsey
   47 Farnsworth Street
   Boston, MA  02210

   Phone: +1.617.951.1201
   Email: richard.kelsey@ember.com

Shelby, et al.            Expires June 27, 2010                [Page 16]

Html markup produced by rfcmarkup 1.121, available from https://tools.ietf.org/tools/rfcmarkup/