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CoRE Working Group                                            C. Bormann
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Standards Track                        October 22, 2012
Expires: April 25, 2013


                 CoRE Roadmap and Implementation Guide
                     draft-bormann-core-roadmap-02

Abstract

   The CoRE set of protocols, in particular the CoAP protocol, is
   defined in draft-ietf-core-coap in conjunction with a number of
   specifications that are currently nearing completion.  There are also
   several dozen more individual Internet-Drafts in various states of
   development, with various levels of WG review and interest.

   Today, this is simply a bewildering array of documents.  Beyond the
   main four documents, it is hard to find relevant information and
   assess the status of proposals.  At the level of Internet-Drafts, the
   IETF has only adoption as a WG document to assign status - too crude
   an instrument to assess the level of development and standing for
   anyone who does not follow the daily proceedings of the WG.

   With a more long-term perspective, as additional drafts mature and
   existing specifications enter various levels of spec maintenance, the
   entirety of these specifications may become harder to understand,
   pose specific implementation problems, or be simply inconsistent.

   The present guide aims to provide a roadmap to these documents as
   well as provide specific advice how to use these specifications in
   combination.  In certain cases, it may provide clarifications or even
   corrections to the specifications referenced.

   This guide is intended as a continued work-in-progress, i.e. a long-
   lived Internet-Draft, to be updated whenever new information becomes
   available and new consensus on how to handle issues is formed.
   Similar to the ROHC implementation guide, RFC 4815, it might be
   published as an RFC at some future time later in the acceptance curve
   of the specifications.

   This document does not describe a new protocol or attempt to set a
   new standard of any kind - it mostly describes good practice in using
   the existing specifications, but it may also document emerging
   consensus where a correction needs to be made.

Status of this Memo




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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 25, 2013.

Copyright Notice

   Copyright (c) 2012 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 Simplified BSD License.






















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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  The Main Four  . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  The CoAP protocol  . . . . . . . . . . . . . . . . . . . .  5
     2.2.  Discovery  . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.3.  Further reading  . . . . . . . . . . . . . . . . . . . . .  7
   3.  Informational Drafts . . . . . . . . . . . . . . . . . . . . .  8
     3.1.  Implementation . . . . . . . . . . . . . . . . . . . . . .  8
     3.2.  Multicast and Group Communication  . . . . . . . . . . . .  8
     3.3.  Security . . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.4.  Intermediaries . . . . . . . . . . . . . . . . . . . . . .  9
     3.5.  Congestion Control . . . . . . . . . . . . . . . . . . . . 10
   4.  CoAP over X  . . . . . . . . . . . . . . . . . . . . . . . . . 11
   5.  Optional components of CoRE  . . . . . . . . . . . . . . . . . 12
     5.1.  CoAP-misc  . . . . . . . . . . . . . . . . . . . . . . . . 12
     5.2.  Generalizing Media Types . . . . . . . . . . . . . . . . . 12
     5.3.  Patience, Leisure, Pledge, or: Timing extensions . . . . . 13
     5.4.  Extending Observe  . . . . . . . . . . . . . . . . . . . . 13
     5.5.  Service discovery  . . . . . . . . . . . . . . . . . . . . 13
     5.6.  Server discovery, Naming, etc. . . . . . . . . . . . . . . 14
     5.7.  More support for sleepy nodes  . . . . . . . . . . . . . . 14
   6.  Replaced drafts  . . . . . . . . . . . . . . . . . . . . . . . 16
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 20
     10.2. Informative References . . . . . . . . . . . . . . . . . . 20
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 25




















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1.  Introduction

   (To be written - for now please see the Abstract.)

1.1.  Terminology

   This document is a guide.  However, it might evolve to make specific
   recommendations on how to use standards-track specifications.
   Therefore: The key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
   "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in RFC
   2119.  They indicate requirement levels for compliant CoRE
   implementations [RFC2119].  Note that these keywords are not only
   used where a correction or clarification is intended; the latter are
   explicitly identified as such.

   The term "byte" is used in its now customary sense as a synonym for
   "octet".

































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2.  The Main Four

   The main component of the CoRE architecture is the Constrained
   Application Protocol (CoAP).  It aims to provide a RESTful transfer
   service, not unlike HTTP, but radically simplified for the use on
   constrained devices on constrained networks.  REST is the
   architectural style that informed the design of HTTP [REST].  The
   terms "constrained device" and "constrained network" refer to
   limited-capability devices such as sensors operating on networks such
   as the IEEE 802.15.4 based 6LoWPAN [RFC4919].
   [I-D.ietf-lwig-guidance] provides a more detailed discussion of what
   we mean by these terms.

2.1.  The CoAP protocol

   The CoAP protocol is defined in three specifications:

   o  [I-D.ietf-core-coap]

   o  [I-D.ietf-core-block]

   o  [I-D.ietf-core-observe]

   The first specification, [I-D.ietf-core-coap], provides the core
   transfer protocol, including the means to provide communication
   security using the DTLS protocol [RFC6347] (compare this to the way
   [RFC2616] and [RFC2818] define HTTP and HTTPS).  The protocol is
   structured into a message layer, which provides duplicate detection
   and optional message reliability on top of UDP, and a request/
   response layer, which provides the usual REST operations GET, PUT,
   POST, and DELETE.  A highly efficient protocol encoding carries the
   4-byte base header, a sequence of _Options_, and the payload (body)
   of a message.  The main extension points of CoAP are its Options,
   similar to the way new header fields are used to extend HTTP.

   Since CoAP is a very simple protocol based on UDP, it is limited in
   its transfer size by the datagram sizes provided by UDP.  As a
   further constraint, many constrained networks do not provide good
   reliability of delivery once their small frame sizes are exceeded and
   the adaptation layer is forced to fragment [WEI].  This may lead to a
   practical limitation to payload sizes as small as 64 bytes.
   [I-D.ietf-core-block] extends the base CoAP protocol with three
   options that enable _blockwise_ transfer, i.e., splitting up a larger
   transfer into a sequence of smaller transactions, as well as the
   early determination of the overall size of the resource
   representation.

   In HTTP, transactions are always client initiated, and it is the



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   responsibility of the client to perform GET operations again and
   again (polling) if it wants to stay up to date about the status of a
   resource.  This "pull model" becomes expensive in an environment with
   limited power, limited network resources, and nodes that sleep most
   of the time.  Some more or less savory workarounds have been
   developed for HTTP [RFC6202], but, as a new protocol, CoAP can do
   better.  [I-D.ietf-core-observe] extends the base CoAP protocol with
   an option that a client can use to indicate its interest in further
   updates from a resource.  If the server accepts this option, the
   client becomes an _Observer_ of this resource and receives an
   asynchronous notification message each time it changes.  Each such
   notification message is identical in structure to the response to the
   initial GET request.

   While the "Block" and "Observe" specifications are optional additions
   to the CoAP protocol (just as the core specification already defines
   14 options most of which will not need to be used in every message),
   they together form what is now generally considered to be the CoAP
   protocol.  All three CoAP specifications are, at the time of this
   writing, in the process of being updated based on the comments to the
   first Working-Group Last-Call [RFC2418], a prerequisite to submitting
   them to the IESG for publication as a Standards-Track RFC.  The
   specifications, together with link-format (below), have been widely
   implemented in highly interoperable implementations: an ETSI
   "plugtest" event in March 2012 was attended by 15 organizations with
   20 implementations; in over 3000 tests performed only about 6 %
   failed.

2.2.  Discovery

   The fourth specification in the main set now nearing completion does
   not extend the CoAP protocol but addresses a different problem.

   In the Web, a number of methods for discovery of resources are
   common.  Initially, Web discovery was just performed by humans based
   on an entry resource to a server (e.g., "/index.html").  This
   resource then includes links that directly or indirectly allow a
   human to reach the other Web resources that make up the Web site.

   Web discovery can be performed by machines if standardized interfaces
   and resource descriptions are available.  Among the component
   mechanisms for Web discovery that are standardized in the IETF are
   the well-known resource path "/.well-known/..."  [RFC5785] and the
   HTTP link header [RFC5988].  Several related techniques are in common
   use today.

   Clearly, in the machine-to-machine environments that will be typical
   of CoAP applications, it is important to enable devices to discover



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   each other and their resources.  Autonomous devices and embedded
   systems necessitate uniform, interoperable resource discovery.

   A basic component for this is provided by a standardized description
   format for the resources a server provides, the _link-format_.
   Unless other methods of discovery are available, CoAP servers should
   provide such a description via the well-known URI
   "/.well-known/core", available for access via a GET request on that
   URI.  (More advanced resource discovery schemes might make the same
   description available by other means, e.g. by posting it to a
   resource directory.)

   The description format has been adapted from the format used in the
   HTTP link header [RFC5988], which is simple and easy to parse.  In
   contrast to the HTTP specification, link-format is specified as an
   Internet media type (what used to be called "MIME type") and intended
   to be carried around in the payload [RFC6690].

   [RFC6690] is the first RFC of the CoRE working group.

2.3.  Further reading

   A recent article provides a more detailed overview over the CoRE
   documents nearing completion [SB].

   While the specification documents themselves have to go into
   meticulous details on every aspect of their protocols, they are the
   ultimate reference source and are the recommended reading if this
   basic overview is not sufficient.






















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3.  Informational Drafts

3.1.  Implementation

   In the IETF, a separate working group is working on informational
   documents concerning guidance in lightweight implementation of
   protocols, the LWIG working group.  LWIG has several drafts pertinent
   here:

   [I-D.ietf-lwig-guidance] is the main working document of the WG.  It
   contains some discussion about CoAP implementation in its section
   3.4.2, including the efficient representation of managing duplicate
   detection state.  [I-D.kovatsch-lwig-class1-coap] contains additional
   considerations that, over time, might move into
   [I-D.ietf-lwig-guidance].

   [I-D.castellani-lwig-coap-separate-responses] contains some examples
   for message exchanges, focusing on elaborating exchanges involving
   separate responses.

   Two drafts show how to build minimal implementations of security
   protocols relevant for CoAP: [I-D.tschofenig-lwig-tls-minimal] for
   TLS, which is relevant for CoAP's use of DTLS; and
   [I-D.kivinen-ipsecme-ikev2-minimal] for IKEv2, the protocol for
   setting up IPsec security associations.  See also the discussion of
   [I-D.hartke-core-codtls] in Section 3.3.

3.2.  Multicast and Group Communication

   As it is based on UDP, CoAP easily supports the use of IP multicast
   to confer messages.  However, there are difficult issues around
   making the desirable multicast applications actually work well.

   This led to an additional milestone on the CoRE charter:

   Nov 2012:  Using CoAP for group communications to IESG as
      Informational

   The informational WG draft [I-D.ietf-core-groupcomm] discusses
   fundamentals and use cases for group communication with CoAP.  In
   parts, it is still in an explorative mode and will require additional
   investigation before conclusive results become available.

   [I-D.dijk-core-groupcomm-misc] gives some additional considerations,
   listing requirements, providing some taxonomy, proposing deployment
   guidelines, and discussing approaches that are not (yet?) in the
   focus of the WG.  Its section 5 can serve as an overview over the
   status of multicast in constrained node/networks.



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3.3.  Security

   Several individual drafts analyze the issues around the security of
   constrained devices in constrained networks.

  | draft-garcia-core-security                      |     | 2012-03-26 |
  | draft-sarikaya-core-sbootstrapping              | -05 | 2012-07-10 |
  | draft-jennings-core-transitive-trust-enrollment | -01 | 2012-10-13 |

                                 Figure 1

   [I-D.garcia-core-security] in particular describes the "Thing
   Lifecycle" and discusses resulting architectural considerations.
   [I-D.jennings-core-transitive-trust-enrollment] demonstrates a
   specific approach to securing the Thing Lifecycle based on defined
   roles of security players, including a Manufacturer, an Introducer,
   and a Transfer Agent.

   Further work around Thing Lifecycles is also expected to occur in the
   SOLACE initiative (Smart Object Lifecycle Architecture for
   Constrained Environments), with its early mailing list at
   solace@ietf.org -- developed after the model of the COMAN initiative
   (Management for Constrained Management Networks and Devices,
   coman@ietf.org, [I-D.ersue-constrained-mgmt]).

   [I-D.hartke-core-codtls] looks specifically into the use of DTLS in
   constrained networks.  It raises issues that pertain both to the LWIG
   and CoRE working groups of the IETF.

   Other drafts in this space:

   | draft-wang-core-profile-secflag-options | -02 | 2012-10-12 |

                                 Figure 2

3.4.  Intermediaries

   [I-D.castellani-core-http-mapping] discusses some ideas about what
   HTTP/CoAP intermediaries could do beyond the basic mapping defined in
   [I-D.ietf-core-coap].  In the IETF83 WG meeting, it was discussed to
   evolve this draft into one document describing best practices for
   mapping between HTTP and CoAP (beyond what is already described in
   [I-D.ietf-core-coap]), and one document that describes usages that
   serve as additional useful examples for more advanced forms of
   mapping, a first draft is available in
   [I-D.castellani-core-advanced-http-mapping].





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3.5.  Congestion Control

   [I-D.ietf-core-coap] only defines a very basic congestion control
   scheme that is focused on being safe in a wide variety of
   applications.  Additional documents will define more advanced
   congestion control schemes that can provide more optimized
   performance in exchange for more implementation complexity and/or a
   narrower field of application.

   Several drafts are contributing to this active subject of discussion
   in the WG:

 | draft-bormann-core-congestion-control            | -02 | 2012-08-01 |
 | draft-bormann-core-cocoa                         | -00 | 2012-08-13 |

                                 Figure 3

   [I-D.greevenbosch-core-minimum-request-interval] proposes adding an
   option that allows a server to indicate its desire for some pacing of
   the requests sent to it by one client; enabling a form of server load
   control.






























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4.  CoAP over X

   [I-D.becker-core-coap-sms-gprs] shows how to run CoAP over cellular
   SMS and in mixed SMS/GPRS environments.  This draft optionally makes
   use of an SMS-oriented encoding for CoAP that is described in
   [I-D.bormann-coap-misc].

   [I-D.li-core-coap-payload-length-option] defines a way to indicate
   the length of the payload in case the underlying transport does not
   provide a suitable definite length indication.









































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5.  Optional components of CoRE

   Additional sub-protocols are being discussed in the IETF that may
   become optional protocols in CoREs.

   The present document will track these sub-protocols and be amended
   once the sub-protocols reach formal status in the IETF.

   Since the WG is cautious in adopting additional work while the main
   specifications near completion, none of the additional protocols
   proposed have become WG documents yet.

5.1.  CoAP-misc

   One draft is a little different from the other drafts in this
   category: [I-D.bormann-coap-misc] is a running document capturing
   CoAP extensions that are in various states of being cooked.

   Some of these extensions may finally be adopted for the WG documents
   and then vanish from CoAP-misc.  For other extensions, we may decide
   that they are not very good ideas.  Instead of deleting them from
   CoAP-misc, they are moved to an appendix.  This documents the
   approach, the best implementation of that approach that was reached,
   and the reasons why it was not adopted.  This documentation should
   spare the WG and its contributors from the continuous reinvention of
   bad ideas.

   As of the time of writing, the main body of CoAP-misc is almost
   empty, as most urgent developments have found their way into the WG
   documents, and many other ideas wait in the "nursery" section of the
   document.

5.2.  Generalizing Media Types

   CoAP defines a registry for combinations of an Internet Media Type
   ("MIME type") and a Content Encoding (e.g. some form of compression),
   enabling its compact encoding of this information in one or two
   bytes.  Each entry in the registry defines a single, fixed set of
   media type parameters (as in ";charset=utf-8"), if any.  This does
   not work well with media types that rely on more complex combinations
   of parameter settings.  [I-D.doi-core-parameter-option] proposes to
   add an option to carry parameters for media types.

   [I-D.fossati-core-multipart-ct] defines a new media type that can
   carry multiple embedded representations employing different media
   types using a binary type-length-value format.





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5.3.  Patience, Leisure, Pledge, or: Timing extensions

   Several proposals intend to extend the amount of information
   available during an exchange about the timing requirements of the
   participants.

   | draft-li-core-coap-patience-option | -01 | 2012-10-22 |

                                 Figure 4

   Another discussion is in Appendix B.4 of [I-D.bormann-coap-misc].

   The question of whether some of this functionality should be
   introduced into the main WG documents now is currently also the
   subject of an active issue tracker ticket [CoRE204].

5.4.  Extending Observe

5.5.  Service discovery

   Basic service discovery is defined in [RFC6690].  A JSON
   representation of the same information is defined in
   [I-D.bormann-core-links-json].  The intention is to make this
   information available in an equivalent format that is more accessible
   to classic Web servers, both as a file format (Internet media type)
   and as a format that can be used in e.g. a JavaScript API.

   [I-D.arkko-core-dev-urn] defines a new Uniform Resource Name (URN)
   namespace that can be used to provide hardware device identifiers in
   resource descriptions.

   [I-D.shelby-core-interfaces] provides additional semantics that can
   be used to make resource descriptions more directly machine-
   interpretable.  This ties in to a more general discussion about CoRE
   profiles that has only just begun.

   [I-D.greevenbosch-core-profile-description] ties into this and
   defines a basic JSON format for indicating what CoAP Options and what
   Content-Formats (still called media-types there) are available for a
   resource.

   [I-D.fossati-core-fp-link-format-attribute] defines a link-format
   attribute that indicates a certain resource is best reached via a
   specific proxy.

   Other drafts in this space:





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 | draft-wang-core-profile-secflag-options          | -01 | 2012-07-16 |

                                 Figure 5

5.6.  Server discovery, Naming, etc.

   On the boundary between service and server discovery, resource
   directory servers provide a way to collect resource descriptions from
   multiple servers into one accessible location.

   [I-D.bormann-core-simple-server-discovery] provided a basic way to
   discover such servers in a constrained node/network without
   necessarily having to resort to multicast.  It has been merged into
   [I-D.shelby-core-resource-directory], which defines protocol elements
   that can be used for setting up such a resource directory.

   Additional drafts include:

 | draft-he-core-energy-aware-pd                    | -01 | 2012-07-16 |
 | draft-cao-core-pd                                | -02 | 2012-07-16 |

                                 Figure 6

   An attempt to merge mDNS/DNS-SD-based discovery (colloquially known
   as zeroconf or Bonjour), including recent approaches to extend these
   for constrained networks, into the picture is documented in
   [I-D.vanderstok-core-dna].

5.7.  More support for sleepy nodes

   The basic communication model of CoAP was imported from the Web. This
   applies well to some communication requirements in constrained node/
   networks, but leaves some other requirements open.

   The assumption underlying the current set of WG documents is that the
   communication layers below the application provide support functions
   for sleeping nodes.  Adding support at the application layer might be
   able to further reduce the power requirements of nodes that can sleep
   most of the time.  A number of drafts aim to extend the CoAP
   communication model towards more support for sleepy nodes.











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   | draft-vial-core-mirror-proxy               | -01 | 2012-07-13 |
   | draft-fossati-core-publish-option          | -00 | 2012-07-09 |
   | draft-fossati-core-monitor-option          | -00 | 2012-07-09 |
   | draft-rahman-core-sleepy                   | -01 | 2012-10-16 |
   | draft-rahman-core-sleepy-problem-statement | -01 | 2012-10-21 |
   | draft-vial-core-mirror-server              | -00 | 2012-10-08 |
   | draft-castellani-core-alive                |     | 2012-03-29 |
   | draft-giacomin-core-sleepy-option          |     | 2012-02-29 |

                                 Figure 7

   This is currently a very active subject of discussion in the WG; it
   is clear that there is a high level of interest in the WG in
   addressing application-level support for sleepy nodes in future
   specifications.

   [I-D.arkko-core-cellular] provides a well-founded discussion of
   methods for power conservation in CoAP nodes connected via cellular
   networks.
































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6.  Replaced drafts

   Internet-Drafts often get replaced by merged drafts or get promoted
   to WG drafts.  As the relationships between drafts are not always
   accurately captured by the secretariat tools, this table provides a
   mapping from current drafts to any previous drafts they are
   replacing:

   +-----------------------------------+-------------------------------+
   | current draft                     | replaced draft                |
   +-----------------------------------+-------------------------------+
   | [I-D.ietf-core-coap]              | draft-shelby-core-coap        |
   |                                   |                               |
   | [I-D.ietf-core-block]             | draft-bormann-core-coap-block |
   |                                   |                               |
   |                                   | draft-li-core-coap-size-optio |
   |                                   | n                             |
   |                                   |                               |
   | [I-D.ietf-core-observe]           | draft-hartke-coap-observe     |
   |                                   |                               |
   | [RFC6690]                         | draft-shelby-core-link-format |
   |                                   |                               |
   | [I-D.ietf-core-groupcomm]         | draft-rahman-core-groupcomm   |
   |                                   |                               |
   |                                   | draft-dijk-core-groupcomm-mis |
   |                                   | c                             |
   |                                   |                               |
   | [I-D.becker-core-coap-sms-gprs]   | draft-li-core-coap-over-sms   |
   |                                   |                               |
   | [I-D.vanderstok-core-dna]         | draft-vanderstok-core-bc      |
   |                                   |                               |
   | [I-D.shelby-core-resource-directo | draft-bormann-core-simple-ser |
   | ry]                               | ver-discovery                 |
   |                                   |                               |
   | [I-D.greevenbosch-core-minimum-re | draft-greevenbosch-core-block |
   | quest-interval]                   | -minimum-time                 |
   +-----------------------------------+-------------------------------+

   Note that draft-scim-core-schema is just named against the naming
   conventions and actually unrelated to the CoRE working group.











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7.  IANA Considerations

   This document has no actions for IANA.
















































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8.  Security Considerations

   (None so far; this section will certainly grow as additional security
   considerations beyond those listed in the base specifications become
   known.)














































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9.  Acknowledgements

   (The concept for this document is borrowed from [RFC4815], which was
   invented by Lars-Erik Jonsson.  Thanks!)















































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10.  References

10.1.  Normative References

   [I-D.ietf-core-block]
              Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
              draft-ietf-core-block-10 (work in progress), October 2012.

   [I-D.ietf-core-coap]
              Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
              "Constrained Application Protocol (CoAP)",
              draft-ietf-core-coap-12 (work in progress), October 2012.

   [I-D.ietf-core-observe]
              Hartke, K., "Observing Resources in CoAP",
              draft-ietf-core-observe-06 (work in progress),
              September 2012.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              April 2010.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, August 2012.

10.2.  Informative References

   [CoRE204]  "Introduce a minimal version of Pledge", CoRE ticket #204,
              <http://trac.tools.ietf.org/wg/core/trac/ticket/204>.

   [I-D.arkko-core-cellular]
              Arkko, J., Eriksson, A., and A. Keranen, "Building Power-
              Efficient CoAP Devices for Cellular Networks",
              draft-arkko-core-cellular-00 (work in progress),
              July 2012.

   [I-D.arkko-core-dev-urn]
              Arkko, J., Jennings, C., and Z. Shelby, "Uniform Resource
              Names for Device Identifiers", draft-arkko-core-dev-urn-03
              (work in progress), July 2012.

   [I-D.becker-core-coap-sms-gprs]



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              Becker, M., Li, K., Kuladinithi, K., and T. Poetsch,
              "Transport of CoAP over SMS, USSD and GPRS",
              draft-becker-core-coap-sms-gprs-02 (work in progress),
              July 2012.

   [I-D.bormann-coap-misc]
              Bormann, C. and K. Hartke, "Miscellaneous additions to
              CoAP", draft-bormann-coap-misc-21 (work in progress),
              October 2012.

   [I-D.bormann-core-links-json]
              Bormann, C., "Representing CoRE Link Collections in JSON",
              draft-bormann-core-links-json-01 (work in progress),
              July 2012.

   [I-D.bormann-core-simple-server-discovery]
              Bormann, C., "CoRE Simple Server Discovery",
              draft-bormann-core-simple-server-discovery-01 (work in
              progress), March 2012.

   [I-D.castellani-core-advanced-http-mapping]
              Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
              E. Dijk, "Best Practices for HTTP-CoAP Mapping
              Implementation",
              draft-castellani-core-advanced-http-mapping-00 (work in
              progress), July 2012.

   [I-D.castellani-core-http-mapping]
              Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
              E. Dijk, "Best Practices for HTTP-CoAP Mapping
              Implementation", draft-castellani-core-http-mapping-05
              (work in progress), July 2012.

   [I-D.castellani-lwig-coap-separate-responses]
              Castellani, A., "Learning CoAP separate responses by
              examples",
              draft-castellani-lwig-coap-separate-responses-00 (work in
              progress), March 2012.

   [I-D.dijk-core-groupcomm-misc]
              Dijk, E. and A. Rahman, "Miscellaneous CoAP Group
              Communication Topics", draft-dijk-core-groupcomm-misc-02
              (work in progress), October 2012.

   [I-D.doi-core-parameter-option]
              Doi, Y. and K. Lynn, "CoAP Content-Type Parameter Option",
              draft-doi-core-parameter-option-01 (work in progress),
              October 2012.



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   [I-D.ersue-constrained-mgmt]
              Ersue, M., Romascanu, D., and J. Schoenwaelder,
              "Management of Networks with Constrained Devices: Use
              Cases and Requirements", draft-ersue-constrained-mgmt-02
              (work in progress), October 2012.

   [I-D.fossati-core-fp-link-format-attribute]
              Fossati, T. and S. Loreto, "Resource Discovery through
              Proxies", draft-fossati-core-fp-link-format-attribute-00
              (work in progress), July 2012.

   [I-D.fossati-core-multipart-ct]
              Fossati, T., "Multipart Content Format Encoding for CoAP",
              draft-fossati-core-multipart-ct-01 (work in progress),
              October 2012.

   [I-D.garcia-core-security]
              Garcia-Morchon, O., Keoh, S., Kumar, S., Hummen, R., and
              R. Struik, "Security Considerations in the IP-based
              Internet of Things", draft-garcia-core-security-04 (work
              in progress), March 2012.

   [I-D.greevenbosch-core-minimum-request-interval]
              Greevenbosch, B., "CoAP Minimum Request Interval",
              draft-greevenbosch-core-minimum-request-interval-00 (work
              in progress), September 2012.

   [I-D.greevenbosch-core-profile-description]
              Greevenbosch, B., Hoebeke, J., and I. Ishaq, "CoAP Profile
              Description Format",
              draft-greevenbosch-core-profile-description-01 (work in
              progress), October 2012.

   [I-D.hartke-core-codtls]
              Hartke, K. and O. Bergmann, "Datagram Transport Layer
              Security in Constrained Environments",
              draft-hartke-core-codtls-02 (work in progress), July 2012.

   [I-D.ietf-core-groupcomm]
              Rahman, A. and E. Dijk, "Group Communication for CoAP",
              draft-ietf-core-groupcomm-03 (work in progress),
              October 2012.

   [I-D.ietf-lwig-guidance]
              Bormann, C., "Guidance for Light-Weight Implementations of
              the Internet Protocol Suite", draft-ietf-lwig-guidance-02
              (work in progress), August 2012.




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   [I-D.jennings-core-transitive-trust-enrollment]
              Jennings, C., "Transitive Trust Enrollment for Constrained
              Devices",
              draft-jennings-core-transitive-trust-enrollment-01 (work
              in progress), October 2012.

   [I-D.kivinen-ipsecme-ikev2-minimal]
              Kivinen, T., "Minimal IKEv2",
              draft-kivinen-ipsecme-ikev2-minimal-01 (work in progress),
              October 2012.

   [I-D.kovatsch-lwig-class1-coap]
              Kovatsch, M., "Implementing CoAP for Class 1 Devices",
              draft-kovatsch-lwig-class1-coap-00 (work in progress),
              October 2012.

   [I-D.li-core-coap-payload-length-option]
              Li, K. and X. Sun, "CoAP Payload-Length Option Extension",
              draft-li-core-coap-payload-length-option-00 (work in
              progress), May 2012.

   [I-D.shelby-core-interfaces]
              Shelby, Z. and M. Vial, "CoRE Interfaces",
              draft-shelby-core-interfaces-03 (work in progress),
              July 2012.

   [I-D.shelby-core-resource-directory]
              Shelby, Z., Krco, S., and C. Bormann, "CoRE Resource
              Directory", draft-shelby-core-resource-directory-04 (work
              in progress), July 2012.

   [I-D.tschofenig-lwig-tls-minimal]
              Tschofenig, H. and J. Gilger, "A Minimal (Datagram)
              Transport Layer Security Implementation",
              draft-tschofenig-lwig-tls-minimal-00 (work in progress),
              October 2012.

   [I-D.vanderstok-core-dna]
              Stok, P., Lynn, K., and A. Brandt, "CoRE Discovery,
              Naming, and Addressing", draft-vanderstok-core-dna-02
              (work in progress), July 2012.

   [REST]     Fielding, R., "Architectural Styles and the Design of
              Network-based Software Architectures", Ph.D. Dissertation,
              University of California, Irvine, 2000, <http://
              www.ics.uci.edu/~fielding/pubs/dissertation/
              fielding_dissertation.pdf>.




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   [RFC2418]  Bradner, S., "IETF Working Group Guidelines and
              Procedures", BCP 25, RFC 2418, September 1998.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC4815]  Jonsson, L-E., Sandlund, K., Pelletier, G., and P. Kremer,
              "RObust Header Compression (ROHC): Corrections and
              Clarifications to RFC 3095", RFC 4815, February 2007.

   [RFC4919]  Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
              over Low-Power Wireless Personal Area Networks (6LoWPANs):
              Overview, Assumptions, Problem Statement, and Goals",
              RFC 4919, August 2007.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988, October 2010.

   [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,
              April 2011.

   [SB]       Bormann, C., Castellani, A., and Z. Shelby, "CoAP: An
              Application Protocol for Billions of Tiny Internet Nodes",
              DOI 10.1109/MIC.2012.29, 2012.

   [WEI]      Shelby, Z. and C. Bormann, "6LoWPAN: the Wireless Embedded
              Internet", ISBN 9780470747995, 2009.




















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Author's Address

   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63921
   Email: cabo@tzi.org









































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