CoRE Working Group                                        A. Rahman, Ed.
Internet-Draft                          InterDigital Communications, LLC
Intended status: Informational                              E. Dijk, Ed.
Expires: January 31, 2014                               Philips Research
                                                           July 30, 2013


                      Group Communication for CoAP
                      draft-ietf-core-groupcomm-12

Abstract

   CoAP is a RESTful transfer protocol for constrained devices and
   constrained networks.  It is anticipated that constrained devices
   will often naturally operate in groups (e.g., in a building
   automation scenario all lights in a given room may need to be
   switched on/off as a group).  This document provides guidance for how
   the CoAP protocol should be used in a group communication context.
   An approach for using CoAP on top of IP multicast is detailed.  Also,
   various use cases and corresponding protocol flows are provided to
   illustrate important concepts.  Finally, guidance is provided for
   deployment in various network topologies.

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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   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
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   This Internet-Draft will expire on January 31, 2014.

Copyright Notice

   Copyright (c) 2013 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



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   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.

Table of Contents

   1.  Conventions and Terminology . . . . . . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Background  . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Protocol Considerations . . . . . . . . . . . . . . . . . . .   5
     3.1.  IP Multicast Background . . . . . . . . . . . . . . . . .   5
     3.2.  Group Definition and Naming . . . . . . . . . . . . . . .   5
     3.3.  Port and URI Configuration  . . . . . . . . . . . . . . .   6
     3.4.  Group Methods . . . . . . . . . . . . . . . . . . . . . .   7
     3.5.  Group Member Discovery  . . . . . . . . . . . . . . . . .   8
     3.6.  Configuring Group Membership in Endpoints . . . . . . . .   8
     3.7.  Multicast Request Acceptance and Response Suppression . .  10
     3.8.  Congestion Control  . . . . . . . . . . . . . . . . . . .  12
     3.9.  Proxy Operation . . . . . . . . . . . . . . . . . . . . .  13
     3.10. Exceptions  . . . . . . . . . . . . . . . . . . . . . . .  15
   4.  Use Cases and Corresponding Protocol Flows  . . . . . . . . .  15
     4.1.  Introduction  . . . . . . . . . . . . . . . . . . . . . .  15
     4.2.  Network Configuration . . . . . . . . . . . . . . . . . .  15
     4.3.  Discovery of Resource Directory . . . . . . . . . . . . .  17
     4.4.  Lighting Control  . . . . . . . . . . . . . . . . . . . .  18
     4.5.  Lighting Control in MLD Enabled Network . . . . . . . . .  21
     4.6.  Commissioning the Network Based On Resource Directory . .  22
   5.  Deployment Guidelines . . . . . . . . . . . . . . . . . . . .  23
     5.1.  Target Network Topologies . . . . . . . . . . . . . . . .  23
     5.2.  Advertising Membership of Multicast Groups  . . . . . . .  24
       5.2.1.  Using the MLD Listener Protocol . . . . . . . . . . .  24
       5.2.2.  Using the RPL Routing Protocol  . . . . . . . . . . .  24
       5.2.3.  Using the MPL Forwarding Protocol . . . . . . . . . .  25
     5.3.  6LoWPAN Specific Guidelines . . . . . . . . . . . . . . .  25
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  26
     6.1.  Security Configuration  . . . . . . . . . . . . . . . . .  26
     6.2.  Threats . . . . . . . . . . . . . . . . . . . . . . . . .  26
     6.3.  Threat Mitigation . . . . . . . . . . . . . . . . . . . .  26
       6.3.1.  WiFi Scenario . . . . . . . . . . . . . . . . . . . .  26
       6.3.2.  6LoWPAN Scenario  . . . . . . . . . . . . . . . . . .  27
       6.3.3.  Future Evolution  . . . . . . . . . . . . . . . . . .  27
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
     7.1.  New 'core.gp' Resource Type . . . . . . . . . . . . . . .  27
     7.2.  New 'coap-group+json' Internet Media Type . . . . . . . .  28



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   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  29
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  29
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  29
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  30
   Appendix A.  Multicast Listener Discovery (MLD) . . . . . . . . .  31
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  38

1.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

   The above key words are used to establish a set of guidelines for
   CoAP group communication.  An implementation of CoAP group
   communication MAY implement these guidelines; an implementation
   claiming compliance to this document MUST implement the set of
   guidelines.

   This document assumes readers are familiar with the terms and
   concepts that are used in [I-D.ietf-core-coap].  In addition, this
   document defines the following terminology:

   Group Communication
      A source node sends a single message which is delivered to
      multiple destination nodes, where all destinations are identified
      to belong to a specific group.  The source node itself may be part
      of the group.  The underlying mechanism for group communication is
      assumed to be multicast based.  The network involved may be a
      constrained network such as a low-power, lossy network.

   Multicast
      Sending a message to multiple destination nodes with one network
      invocation.  There are various options to implement multicast
      including layer 2 (Media Access Control) and layer 3 (IP)
      mechanisms.

   IP Multicast
      A specific multicast solution based on the use of IP multicast
      addresses as defined in "IANA Guidelines for IPv4 Multicast
      Address Assignments" [RFC5771] and "IP Version 6 Addressing
      Architecture" [RFC4291].

   Low power and Lossy Network (LLN)
      A type of constrained IP network where devices are interconnected
      by a variety of low-power and lossy links (such as IEEE 802.15.4,



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      Bluetooth Low Energy (BLE), Digital Enhanced Cordless
      Telecommunication (DECT)) or lossy links (such as IEEE P1901.2
      power-line communication).

2.  Introduction

2.1.  Background

   Constrained Application Protocol (CoAP) is a Representational State
   Transfer (REST) based approach for resource constrained devices
   operating in an IP network [I-D.ietf-core-coap].  CoAP has many
   similarities to HTTP [RFC2616] but also has some key differences.
   Constrained devices can be large in number, but are often highly
   correlated to each other (e.g., by type or location).  For example,
   all the light switches in a building may belong to one group and all
   the thermostats may belong to another group.  Groups may be pre-
   configured before deployment or dynamically formed during operation.
   If information needs to be sent to or received from a group of
   devices, group communication mechanisms can improve efficiency and
   latency of communication and reduce bandwidth requirements for a
   given application.  HTTP does not support any equivalent
   functionality to CoAP group communication.

2.2.  Scope

   Group communication involves a one-to-many relationship between CoAP
   endpoints.  Specifically, a single CoAP client will simultaneously
   get (or set) resource representations from multiple CoAP servers
   using CoAP over IP multicast.  An example would be a CoAP light
   switch turning on/off multiple lights in a room with a single CoAP
   group communication PUT request, and handling the potential multitude
   of (unicast) responses.

   The normative protocol aspects of running CoAP on top of IP Multicast
   and processing the responses are given in [I-D.ietf-core-coap].  The
   main contribution of this document lies in providing additional
   guidance for several important group communication features.  Among
   the topics covered are group definition, group resource manipulation,
   and group configuration.  Also, proxy operation and minimizing
   network congestion for group communication is discussed.  Finally,
   specific use cases and deployment guidelines are for CoAP group
   communication outlined.









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3.  Protocol Considerations

3.1.  IP Multicast Background

   IP Multicast protocols have been evolving for decades, resulting in
   proposed standards such as Protocol Independent Multicast - Sparse
   Mode (PIM-SM) [RFC4601].  Yet, due to various technical and business
   reasons, IP Multicast is not widely deployed on the general Internet.
   However, IP Multicast is very popular in specific deployments such as
   in enterprise networks (e.g., for video conferencing), smart home
   networks (e.g., Universal Plug and Play (UPnP)) and carrier IPTV
   deployments.  The packet economy and minimal host complexity of IP
   multicast make it attractive for group communication in constrained
   environments.

   To achieve IP multicast beyond a subnet, an IP multicast routing or
   forwarding protocol needs to be active on IP routers.  An example of
   a routing protocol specifically for LLNs is the IPv6 Routing Protocol
   for Low-Power and Lossy Networks (RPL) (Section 12 of [RFC6550]) and
   an example of a forwarding protocol for LLNs is Multicast Protocol
   for Low power and Lossy Networks (MPL) [I-D.ietf-roll-trickle-mcast].
   Finally, PIM-SM [RFC4601] is often used for multicast routing in un-
   constrained networks.

   IP multicast can also be run in a Link-Local (LL) scope.  This means
   that there is no routing involved and an IP multicast message is only
   received over the link on which it was sent.

   For a complete IP multicast solution, in addition to a routing/
   forwarding protocol, a so-called "listener" protocol is needed for
   the devices to subscribe to groups (see Section 5.2).

3.2.  Group Definition and Naming

   A group is defined as a set of CoAP endpoints, where each endpoint is
   configured to receive multicast CoAP requests that are sent to the
   group's associated IP multicast address.  An endpoint MAY be a member
   of multiple groups.  Group membership of an endpoint MAY dynamically
   change over time.

   To initiate CoAP group communication, a Group URI is used as the
   request URI in a CoAP request.  A Group URI has the scheme 'coap' and
   includes in the authority part either a group IP multicast address or
   a hostname (e.g., Group Fully Qualified Domain Name (FQDN)) that can
   be resolved to the group IP multicast address.  A Group URI also
   contains an optional CoAP port number in the authority part.  Group
   URIs follow the CoAP URI syntax [I-D.ietf-core-coap].




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   It is recommended, for sending nodes, to use the IP multicast address
   literal in a Group URI.  In case a Group hostname is used, it can be
   uniquely mapped to a site-local or global IP multicast address via
   DNS resolution (if supported).  Some examples of hierarchical Group
   FQDN naming (and scoping) for a building control application are
   shown below ([I-D.vanderstok-core-dna]):

   URI authority                           Targeted group of nodes
   --------------------------------------- --------------------------
   all.bldg6.example.com                   "all nodes in building 6"
   all.west.bldg6.example.com              "all nodes in west wing,
                                            building 6"
   all.floor1.west.bldg6.example.com       "all nodes in floor 1,
                                            west wing, building 6"
   all.bu036.floor1.west.bldg6.example.com "all nodes in office bu036,
                                            floor1, west wing,
                                            building 6"


   Similarly, if supported, reverse mapping (from IP multicast address
   to Group FQDN) is possible using the reverse DNS resolution technique
   ([I-D.vanderstok-core-dna]).

3.3.  Port and URI Configuration

   A CoAP server that is a member of a group listens for CoAP messages
   on the group's IP multicast address, on a specified UDP port.  The
   default UDP port is the CoAP default port 5683 but a non-default UDP
   port MAY be specified for the group; in which case implementers MUST
   ensure that all group members are configured to use this same port.

   Multicast based group communication will not work if there is
   diversity in the authority port (e.g., different dynamic port
   addresses across the group) or if the resources are located at
   different paths on different endpoints.  Therefore, some measures
   must be present to ensure uniformity in port number and resource
   names/locations within a group.  All CoAP multicast requests MUST be
   sent using a port number according to one of below options:

   1.  A pre-configured port number.  The pre-configuration mechanism
       MUST ensure that the same port number is pre-configured across
       all endpoints in a group and across all CoAP clients performing
       the group requests.

   2.  If the client is configured to use service discovery including
       port discovery, it uses a port number obtained via a service
       discovery lookup operation for the targeted CoAP multicast group.




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   3.  Use the default CoAP UDP port (5683).

   All CoAP multicast requests SHOULD operate on URI paths in one of the
   following ways:

   1.  Pre-configured URI paths, if available.  The pre-configuration
       mechanism SHOULD ensure that these paths are pre-configured
       across all CoAP servers in a group and all CoAP clients
       performing the group requests.

   2.  If the client is configured to use default CoRE resource
       discovery, it uses URI paths retrieved from a "/.well-known/core"
       lookup on a group member.  The URI paths the client will use MUST
       be known to be available also in all other endpoints in the
       group.  The URI path configuration mechanism on servers MUST
       ensure that these URIs (identified as being supported by the
       group) are configured on all group endpoints.

   3.  If the client is configured to use another form of service
       discovery, it uses URI paths from an equivalent service discovery
       lookup which returns the resources supported by all group
       members.

   4.  If the client has received a Group URI through a previous RESTful
       interaction with a trusted server, for the purpose of the client
       using this URI in a request, it can use this URI in a multicast
       request.  For example, a commissioning tool may instruct a sensor
       device in this way to which target (multicast URI) it should
       report sensor events.

3.4.  Group Methods

   Idempotent methods (i.e., CoAP GET, PUT, and DELETE) SHOULD be used
   for group communication, with one exception as follows.  A non-
   idempotent method (i.e., CoAP POST) MAY be used for group
   communication if the resource being POSTed to has been designed to
   cope with the lossy nature of multicast.  Note that not all group
   members are guaranteed to receive the multicast request, and the
   sender cannot readily find out which group members did not receive
   it.

   All CoAP messages that are sent via multicast MUST be Non-
   Confirmable.  A unicast response per server MAY be sent back to
   answer the group communication request (e.g., response "2.05 Content"
   to a group GET request) taking into account the congestion control
   rules defined in Section 3.8.  The unicast responses received may be
   a mixture of success (e.g., 2.05 Content) and failure (e.g., 4.04 Not
   Found) codes depending on the individual server processing results.



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3.5.  Group Member Discovery

   CoAP Groups, and the membership of these groups, can be discovered
   via the lookup interfaces defined in
   [I-D.ietf-core-resource-directory].  An example of doing some of
   these lookups is given in Section 4.6.

3.6.  Configuring Group Membership in Endpoints

   The group membership of a CoAP endpoint may be configured in one of
   the following ways.  First, the group membership may be pre-
   configured before node deployment.  Second, a node may be programmed
   to discover (query) its group membership during operation using a
   specific service discovery means.  Third, it may be configured during
   operation by another node (e.g., a commissioning device).

   In the first case, the pre-configured group information may be either
   directly a IP multicast address, or a hostname (FQDN) which is during
   operation resolved to a IP multicast address by the endpoint using
   DNS (if supported).

   For the second case, a CoAP endpoint may look up its group membership
   using techniques such as DNS-SD and Resource Directory
   [I-D.ietf-core-resource-directory].  The latter is detailed more in
   Section 4.6.

   In the third case, typical in scenarios such as building control, a
   commissioning tool determines to which group a sensor or actuator
   node belongs, and writes this information to the node, which can
   subsequently join the correct IP multicast group on its network
   interface.  The information written may again be an IP multicast
   address or a hostname.

   To achieve better interoperability between endpoints from different
   manufacturers, an OPTIONAL RESTful interface for configuring CoAP
   endpoints with relevant group information is described here.  This
   interface provides a solution for the third case mentioned above.  To
   access this interface a client MUST use unicast methods (GET/PUT/POST
   /DELETE) only as it is a method of configuring group information in
   individual endpoints.  Using multicast operations in this situation
   may lead to unexpected (possibly circular) behavior in the network.










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   CoAP endpoints implementing this optional mechanism MUST support the
   group configuration Internet Media Type "application/coap-group+json"
   (Section 7.2).  A resource offering this representation can be
   annotated for direct discovery [RFC6690] using the resource type (rt)
   "core.gp" where "gp" is shorthand for "group" (Section 7.1).  An
   authorized controller uses this media type to query/manage group
   membership of a CoAP endpoint as defined below.

   The group configuration resource has a JSON-based content format (as
   indicated by the media type).  A (unicast) GET on a CoAP endpoint
   with a resource with this format returns a JSON array of group
   objects, each group object being a JSON object.  Below example shows
   a client requesting group membership to a CoAP server, where the
   response is in the "application/coap-group+json" content format
   containing a single group object:

   Req: GET /gp
   Res: 2.05 Content (Content-Format: application/coap-group+json)
   [ { "n": "Room-A-Lights.floor1.west.bldg6.example.com",
       "ip": "ff15::4200:f7fe:ed37:14ca" }
   ]


   In a response, the OPTIONAL "n" key/value pair stands for "name" and
   identifies the group with a hostname, for example a FQDN.  The
   REQUIRED "ip" key/value pair specifies the IP multicast address of
   the group.  Its value can be empty if unknown at the time of
   generating the response.

   Note that each group object in the JSON array represents a single IP
   multicast group for the endpoint.  If there are multiple elements in
   the array then the endpoint is a member of multiple IP multicast
   groups.

   When the content format is used in a request, the "ip" key/value are
   OPTIONAL to define the group's associated IP multicast address.  The
   "n" key/value are also OPTIONAL then.  If the "ip" key and its value
   are given, this takes priority.  The "n" key/value are just
   informational in this case.  If only the "n" key/value are given, the
   CoAP endpoint has to do DNS resolution (if supported) to obtain the
   IP multicast address from the hostname.  At least one of the "n" or
   "ip" key/value MUST be given in a group object in a request.

   A (unicast) POST with a group configuration media type as payload
   instructs the CoAP endpoint to join the defined group(s).  The
   endpoint adds the specified IP multicast address(es) to its network
   interface configuration.  The endpoint also updates the resource by
   adding the specified group object(s) to the existing ones:



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   Req: POST /gp (Content-Format: application/coap-group+json)
   [ { "n": "floor1.west.bldg6.example.com",
     "ip": "ff15::4200:f7fe:ed37:14cb" } ]
   Res: 2.04 Changed


   A (unicast) PUT with a group configuration media type as payload will
   replace all current group memberships in the endpoint with the new
   ones defined in the PUT request.  A (unicast) DELETE with a group
   configuration media type will delete all group memberships from the
   endpoint.

   After any change on a Group configuration resource, the endpoint MUST
   effect registration/de-registration from the corresponding IP
   multicast group(s) as soon as possible.  Finally, any (unicast)
   operation to change a CoAP endpoint group membership configuration
   (i.e., PUT/POST/DELETE) SHOULD use DTLS-secured CoAP
   [I-D.ietf-core-coap].  Thus only authorized controllers should be
   allowed by an endpoint to configure its group membership.

3.7.  Multicast Request Acceptance and Response Suppression

   CoAP [I-D.ietf-core-coap] and CoRE Link Format [RFC6690] define
   normative behaviors for:

   1.  Multicast request acceptance - in which cases a coAP request is
       accepted and executed, and when not.

   2.  Multicast response suppression - in which cases the CoAP response
       to an already-executed request is returned to the requesting
       endpoint, and when not.

   Note that a CoAP response differs from a CoAP ACK; ACKs are never
   sent by servers in response to a multicast CoAP request.  This
   section first summarizes these normative behaviors and then presents
   additional guidelines for response suppression.  Also a number of
   multicast example applications are given to illustrate the overall
   approach.

   To apply any rules for request and/or response suppression, a CoAP
   server must be aware that an incoming request arrived via multicast
   by making use of APIs such as IPV6_RECVPKTINFO [RFC3542].

   For multicast request acceptance, the REQUIRED behaviors are:

   o  A server SHOULD NOT accept a multicast request that cannot be
      "authenticated" in some way (cryptographically or by some
      multicast boundary limiting the potential sources)



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      [I-D.ietf-core-coap].  See Section 6.3 for examples of multicast
      boundary limiting methods.

   o  A server SHOULD NOT accept a multicast discovery request with a
      query string (as defined in CoRE Link Format [RFC6690]) if
      filtering ([RFC6690]) is not supported by the server.

   o  A server SHOULD NOT accept a multicast request that acts on a
      specific resource for which multicast support is not required.
      (Note that for the discovery resource "/.well-known/core"
      multicast support is always required.  Implementers are advised to
      disable multicast support by default on any other resource, until
      explicitly enabled by an application or by configuration.)

   o  Otherwise accept the multicast request.

   For multicast response suppression, the REQUIRED behaviors are:

   o  A server SHOULD NOT respond to a multicast discovery request if
      the filter specified by the request's query string does not match.

   o  A server MAY choose not to respond to a multicast request, if
      there's nothing useful to respond (e.g., error or empty response).

   o  If the server API cannot indicate that an incoming message was
      multicast, then the server SHOULD NOT respond for incoming
      messages for selected resources which are known (through
      application knowledge) to be used for multicast requests.

   o  Otherwise respond to the multicast request.

   The above response suppression behaviors are complemented by the
   following guidelines.  CoAP servers SHOULD implement configurable
   response suppression, enabling at least the following options per
   resource that supports multicast requests:

   o  Suppression of all 2.xx success responses;

   o  Suppression of all 4.xx client errors;

   o  Suppression of all 5.xx server errors;

   o  Suppression of all 2.05 responses with empty payload.

   A number of group communication example applications are given below
   to illustrate how to make use of response suppression:





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   o  CoAP resource discovery: Suppress 2.05 responses with empty
      payload and all 4.xx and 5.xx errors.

   o  Lighting control: Suppress all 2.xx responses after a lighting
      change command.

   o  Update configuration data in a group of devices using multicast
      PUT: No suppression at all.  The client uses collected responses
      to identify which group members did not receive the new
      configuration; then attempts using CoAP CON unicast to update
      those specific group members.

   o  Multicast firmware update by sending blocks of data: Suppress all
      2.xx and 5.xx responses.  After having sent all multicast blocks,
      the client checks each endpoint by unicast to identify which data
      blocks are still missing in each endpoint.

   o  Conditional reporting for a group (e.g., sensors) based on a URI
      query: Suppress all 2.05 responses with empty payload (i.e., if a
      query produces no matching results).

3.8.  Congestion Control

   Multicast CoAP requests may result in a multitude of responses from
   different nodes, potentially causing congestion.  Therefore both the
   sending of multicast requests, and the sending of the unicast CoAP
   responses to these multicast requests should be conservatively
   controlled.

   CoAP [I-D.ietf-core-coap] reduces multicast-specific congestion risks
   through the following measures:

   o  A server MAY choose not to respond to a multicast request if
      there's nothing useful to respond (e.g., error or empty response).
      See Section 3.7 for more detailed guidelines on response
      suppression.

   o  A server SHOULD limit the support for multicast requests to
      specific resources where multicast operation is required.

   o  A multicast request MUST be Non-Confirmable.

   o  A response to a multicast request SHOULD be Non-Confirmable
      (Section 5.2.3 of [I-D.ietf-core-coap]).

   o  A server does not respond immediately to a multicast request, but
      SHOULD first wait for a time that is randomly picked within a
      predetermined time interval called the Leisure.



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   o  A server SHOULD NOT accept multicast requests that can not be
      authenticated in some way.  See Section 3.7 for more details on
      request suppression and multicast source authentication.

   Additional guidelines to reduce congestion risks defined in this
   document are:

   o  A server in an LLN should only support multicast GET for resources
      that are small.  For example, the payload of the response is 5% of
      the IP Maximum Transmit Unit (MTU) size (e.g. so it fits into a
      single link-layer frame).

   o  A server can minimize the payload length in response to a
      multicast GET on "/.well-known/core" by using hierarchy in
      arranging link descriptions for the response.  An example of this
      is given in Section 5 of [RFC6690].

   o  Alternatively a server can also minimize the payload length of a
      response to a multicast GET (e.g., on "/.well-known/core") using
      CoAP blockwise transfers [I-D.ietf-core-block], returning only a
      first block of the CoRE Link Format description.  For this reason,
      a CoAP client sending a multicast CoAP request to "/.well-known/
      core" SHOULD support core-block.

   o  A client should always aim to use IP multicast with link-local
      scope if possible.  If this is not possible, then site-local scope
      IP multicast should be considered.  If this is not possible, then
      global scope IP multicast should be considered as a last resort
      only.

   More guidelines specific to use of CoAP in 6LoWPAN networks are given
   in Section 5.3.

3.9.  Proxy Operation

   CoAP [I-D.ietf-core-coap] allows a client to request a forward-proxy
   to process its CoAP request.  For this purpose the client either
   specifies the request URI as a string in the Proxy-URI option, or it
   specifies the Proxy-Scheme option with the URI constructed from the
   usual Uri-* options.  This approach works well for unicast requests.
   However, there are certain issues and limitations of processing the
   (unicast) responses to a group communication request made in this
   manner through a proxy.  Specifically, if a proxy would apply
   aggregation of responses in such a case:







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   o  Aggregation of (unicast) responses to a group communication
      request in a proxy is difficult.  This is because the proxy does
      not know how many members there are in the group or how many group
      members will actually respond.

   o  There is no default format defined in CoAP for aggregation of
      multiple responses into a single response.

   Alternatively, if a proxy follows directly the specification for a
   CoAP Proxy [I-D.ietf-core-coap], the proxy would simply forward all
   the individual (unicast) responses to a group communication request
   to the client (i.e., no aggregation).  There are also issues with
   this approach:

   o  The client may be confused as it may not have known that the
      Proxy-URI contained a multicast target.  That is, the client may
      be expecting only one (unicast) response but instead receives
      multiple (unicast) responses potentially leading to fault
      conditions in the application.

   o  Each individual CoAP response will appear to originate (IP Source
      address) from the CoAP Proxy, and not from the server that
      produced the response.  This makes it impossible for the client to
      identify the server that produced each response.

   Due to above issues, a guideline is defined here that a CoAP Proxy
   SHOULD NOT support processing a multicast CoAP request but rather
   return a 501 (Not Implemented) response in such case.  The exception
   case here (i.e., to process it) is allowed under following
   conditions:

   o  The CoAP Proxy MUST be explicitly configured (whitelist) to allow
      proxied multicast requests by specific client(s).

   o  The proxy SHOULD return individual (unicast) CoAP responses to the
      client (i.e., not aggregated).  The exception case here occurs
      when a (future) standardized aggregation format is being used.

   o  It MUST be known to the person/entity doing the configuration of
      the proxy, or otherwise verified in some way, that the client
      configured in the whitelist supports receiving multiple responses
      to a proxied unicast CoAP request.









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3.10.  Exceptions

   Group communication using IP multicast offers improved network
   efficiency and latency amongst other benefits.  However, group
   communication may not always be possible to implement in a given
   network.  The primary reason for this will be if IP multicast is not
   (fully) supported in the network.  For example, in an LLN where the
   RPL protocol is used for routing in "Non-storing mode" [RFC6550] and
   no other routing/forwarding protocol is defined, there will be no IP
   multicast routing beyond link-local scope.  This means that any CoAP
   group communication above link-local scope will not be supported in
   this network.

4.  Use Cases and Corresponding Protocol Flows

4.1.  Introduction

   The use of CoAP group communication is shown in the context of the
   following two use cases and corresponding protocol flows:

   o  Discovery of Resource Directory (RD,
      [I-D.ietf-core-resource-directory]): discovering the local CoAP RD
      which contains links to resources stored on other CoAP servers
      [RFC6690].

   o  Lighting Control: synchronous operation of a group of
      IPv6-connected lights (e.g., 6LoWPAN [RFC4944] lights).

4.2.  Network Configuration

   To illustrate the use cases we define two network configurations.
   Both are based on the topology as shown in Figure 1.  The two
   configurations using this topology are:

   1.  Subnets are 6LoWPAN networks; the routers Rtr-1 and Rtr-2 are
       6LoWPAN Border Routers (6LBRs, [RFC6775]).

   2.  Subnets are Ethernet links; the routers Rtr-1 and Rtr-2 are
       multicast-capable Ethernet routers.

   Both configurations are further specified by the following:

   o  A large room (Room-A) with three lights (Light-1, Light-2,
      Light-3) controlled by a Light Switch.  The devices are organized
      into two subnets.  In reality, there could be more lights (up to
      several hundreds) but these are not shown for clarity.

   o  Light-1 and the Light Switch are connected to a router (Rtr-1).



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   o  Light-2 and the Light-3 are connected to another router (Rtr-2).

   o  The routers are connected to an IPv6 network backbone which is
      also multicast enabled.  In the general case, this means the
      network backbone and Rtr-1/Rtr-2 support a PIM based multicast
      routing protocol, and Multicast Listener Discovery (MLD) for
      forming groups.  In a limited case where the network backbone is
      one link, then the routers only have to support MLD-snooping
      (Appendix A) for the following use cases to work.

   o  A CoAP RD is connected to the network backbone.

   o  The DNS server is optional.  If the server is there (connected to
      the network backbone) then certain DNS based features are
      available (e.g., DNS resolution of hostname to IP multicast
      address).  If the DNS server is not there, then different
      provisioning of the network is required (e.g., IP multicast
      addresses are hard-coded into devices, or manually configured, or
      obtained via a service discovery method).

   o  A Controller (CoAP client) is connected to the backbone, which is
      able to control various building functions including lighting.


     ################################################
     #         **********************        Room-A #
     #       **  Subnet-1            **             #           Network
     #     *                           **           #          Backbone
     #    *     +----------+             *          #                 |
     #   *      |  Light   |-------+      *         #                 |
     #  *       |  Switch  |       |       *        #                 |
     #  *       +----------+  +---------+  *        #                 |
     #  *                     |  Rtr-1  |-----------------------------+
     #  *                     +---------+  *        #                 |
     #  *       +----------+        |      *        #                 |
     #   *      |  Light-1 |--------+     *         #                 |
     #    *     +----------+             *          #                 |
     #     **                          **           #                 |
     #       **************************             #                 |
     #                                              #                 |
     #         **********************               # +------------+  |
     #       **  Subnet-2            **             # | DNS Server |  |
     #     *                           **           # | (Optional) |--+
     #    *     +----------+             *          # +------------+  |
     #   *      |  Light-2 |-------+      *         #                 |
     #  *       |          |       |       *        #                 |
     #  *       +----------+  +---------+  *        #                 |
     #  *                     |  Rtr-2  |-----------------------------+



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     #  *                     +---------+  *        #                 |
     #  *       +----------+        |      *        #                 |
     #   *      |  Light-3 |--------+     *         #                 |
     #    *     +----------+             *          # +------------+  |
     #     **                          **           # | Controller |--+
     #       **************************             # | Client     |  |
     ################################################ +------------+  |
                                       +------------+                 |
                                       |   CoAP     |                 |
                                       |  Resource  |-----------------+
                                       |  Directory |
                                       +------------+


            Figure 1: Network Topology of a Large Room (Room-A)

4.3.  Discovery of Resource Directory

   The protocol flow for discovery of the CoAP RD for the given network
   (of Figure 1) is shown in Figure 2:

   o  Light-2 is installed and powered on for the first time.

   o  Light-2 will then search for the local CoAP RD by sending out a
      GET request (with the "/.well-known/core?rt=core.rd" request URI)
      to the site-local "All CoAP Nodes" multicast address.

   o  This multicast message will then go to each node in subnet-2.
      Rtr-2 will then forward into to the Network Backbone where it will
      be received by the CoAP RD.  All other nodes in subnet-2 will
      ignore the multicast GET because it is qualified by the query
      string "?rt=core.rd" (which indicates it should only be processed
      by the endpoint if it contains a resource of type core.rd).

   o  The CoAP RD will then send back a unicast response containing the
      requested content, which is a CoRE Link Format representation of a
      resource of type core.rd.

   o  Note that the flow is shown only for Light-2 for clarity.  Similar
      flows will happen for Light-1, Light-3 and the Light Switch when
      they are first powered on.

   The CoAP RD may also be discovered by other means such as by assuming
   a default location (e.g., on a 6LBR), using DHCP, anycast address,
   etc.  However, these approaches do not invoke CoAP group
   communication so are not further discussed here.





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   For other discovery use cases such as discovering local CoAP servers,
   services or resources group communication can be used in a similar
   fashion as in the above use case.  Both Link-Local (LL) and site-
   local discovery are possible this way.


                                    Light                           CoAP
   Light-1   Light-2    Light-3     Switch     Rtr-1     Rtr-2       RD
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    **********************************          |          |          |
    *   Light-2 is installed         *          |          |          |
    *   and powers on for first time *          |          |          |
    **********************************          |          |          |
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    |          | COAP NON Mcast(GET                        |          |
    |          |           /.well-known/core?rt=core.rd)   |          |
    |          |--------->-------------------------------->|          |
    |          |          |          |          |          |--------->|
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    |          | COAP NON (2.05 Content                    |          |
    |          |         </rd>;rt="core.rd";ins="Primary") |<---------|
    |          |<------------------------------------------|          |
    |          |          |          |          |          |          |



       Figure 2: Resource Directory Discovery via Multicast Request

4.4.  Lighting Control

   The protocol flow for a building automation lighting control scenario
   for the network (Figure 1) is shown in Figure 3.  The network is
   assumed to be in a 6LoWPAN configuration.  Also, it is assumed that
   the CoAP servers in each Light are configured to suppress CoAP
   responses for any multicast CoAP requests related to lighting
   control.  (See Section 3.7 for more details on response suppression
   by a server.)

   In addition, Figure 4 shows a protocol flow example for the case that
   servers do respond to a lighting control multicast request with
   (unicast) CoAP NON responses.  There are two success responses and
   one 5.00 error response.  In this particular case, the Light Switch
   does not check that all Lights in the group received the multicast
   request by examining the responses.  This is because the Light Switch
   is not configured with an exhaustive list of the IP addresses of all



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   Lights belonging to the group.  However, based on received error
   responses it could take additional action such as logging a fault or
   alerting the user via its LCD display.

   Reliability of CoAP multicast is not guaranteed.  Therefore, one or
   more lights in the group may not have received the CoAP control
   request due to packet loss.  In this use case there is no detection
   nor correction of such situations: the application layer expects that
   the multicast forwarding/routing will be of sufficient quality to
   provide on average a very high probability of packet delivery to all
   CoAP endpoints in a multicast group.  An example protocol to
   accomplish this using randomized retransmission is the MPL forwarding
   protocol for LLNs [I-D.ietf-roll-trickle-mcast].

   We assume the following steps have already occurred before the
   illustrated flows:

   1.  Startup phase: 6LoWPANs are formed.  IPv6 addresses assigned to
       all devices.  The CoAP network is formed.

   2.  Network configuration (application-independent): 6LBRs are
       configured with multicast addresses, or address blocks, to filter
       out or to pass through to/from the 6LoWPAN.

   3.  Commissioning phase (application-related): The IP multicast
       address of the group (Room-A-Lights) has been configured in all
       the Lights and in the Light Switch.

   4.  As an alternative to the previous step, when a DNS server is
       available, the Light Switch and/or the Lights have been
       configured with a group hostname which each nodes resolves to the
       above IP multicast address of the group.

   Note for the Commissioning phase: the switch's 6LoWPAN/CoAP software
   stack supports sending unicast, multicast or proxied unicast CoAP
   requests, including processing of the multiple responses that may be
   generated by a multicast CoAP request.


                                    Light                        Network
   Light-1   Light-2    Light-3     Switch    Rtr-1      Rtr-2  Backbone
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    |          |          ***********************          |          |
    |          |          *   User flips on     *          |          |
    |          |          *   light switch to   *          |          |
    |          |          *   turn on all the   *          |          |
    |          |          *   lights in Room A  *          |          |



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    |          |          ***********************          |          |
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    |          |          |    COAP NON Mcast(PUT,         |          |
    |          |          |    Payload=lights ON)          |          |
    |<-------------------------------+--------->|          |          |
    ON         |          |          |          |-------------------->|
    |          |          |          |          |          |<---------|
    |          |<---------|<-------------------------------|          |
    |          ON         ON         |          |          |          |
    ^          ^          ^          |          |          |          |
    ***********************          |          |          |          |
    *   Lights in Room-A  *          |          |          |          |
    *   turn on (nearly   *          |          |          |          |
    *   simultaneously)   *          |          |          |          |
    ***********************          |          |          |          |
    |          |          |          |          |          |          |


          Figure 3: Light Switch Sends Multicast Control Message


                                    Light                        Network
   Light-1   Light-2    Light-3     Switch    Rtr-1      Rtr-2  Backbone
    |          |          |          |          |          |          |
    |     COAP NON (2.04 Changed)    |          |          |          |
    |------------------------------->|          |          |          |
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    |          COAP NON (2.04 Changed)          |          |          |
    |          |------------------------------------------>|          |
    |          |          |          |          |          |--------->|
    |          |          |          |          |<--------------------|
    |          |          |          |<---------|          |          |
    |          |          |          |          |          |          |
    |          |        COAP NON (5.00 Internal Server Error)         |
    |          |          |------------------------------->|          |
    |          |          |          |          |          |--------->|
    |          |          |          |          |<--------------------|
    |          |          |          |<---------|          |          |
    |          |          |          |          |          |          |


      Figure 4: Lights (Optionally) Respond to Multicast CoAP Request







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   Another, but similar, lighting control use case is shown in Figure 5.
   In this case a controller connected to the Network Backbone sends a
   CoAP multicast request to turn on all lights in Room-A. Every Light
   sends back a CoAP response to the Controller after being turned on.


                                                       Network
   Light-1   Light-2    Light-3     Rtr-1      Rtr-2  Backbone Controller
    |          |          |           |          |          |        |
    |          |          |           |          |        COAP NON Mcast(PUT,
    |          |          |           |          |        Payload=lights ON)
    |          |          |           |          |          |<-------|
    |          |          |           |<----------<---------|        |
    |<--------------------------------|          |          |        |
    ON         |          |           |          |          |        |
    |          |<----------<---------------------|          |        |
    |          ON         ON          |          |          |        |
    ^          ^          ^           |          |          |        |
    ***********************           |          |          |        |
    *   Lights in Room-A  *           |          |          |        |
    *   turn on (nearly   *           |          |          |        |
    *   simultaneously)   *           |          |          |        |
    ***********************           |          |          |        |
    |          |          |           |          |          |        |
    |          |          |           |          |          |        |
    |    COAP NON (2.04 Changed)      |          |          |        |
    |-------------------------------->|          |          |        |
    |          |          |           |-------------------->|        |
    |          |  COAP NON (2.04 Changed)        |          |------->|
    |          |-------------------------------->|          |        |
    |          |          |           |          |--------->|        |
    |          |          | COAP NON (2.04 Changed)         |------->|
    |          |          |--------------------->|          |        |
    |          |          |           |          |--------->|        |
    |          |          |           |          |          |------->|
    |          |          |           |          |          |        |


     Figure 5: Controller On Backbone Sends Multicast Control Message

4.5.  Lighting Control in MLD Enabled Network

   The use case of previous section can also apply in networks where
   nodes support the MLD protocol [RFC3810].  The Lights then take on
   the role of MLDv2 listener and the routers (Rtr-1, Rtr-2) are MLDv2
   Routers.  In the Ethernet based network configuration, MLD may be
   available on all involved network interfaces.  Use of MLD in the
   6LoWPAN based configuration is also possible, but requires MLD



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   support in all nodes in the 6LoWPAN which is usually not implemented
   in many deployments.

   The resulting protocol flow is shown in Figure 6.  This flow is
   executed after the commissioning phase, as soon as Lights are
   configured with a group address to listen to.  The (unicast) MLD
   Reports may require periodic refresh activity as specified by the MLD
   protocol.  In the figure, LL denotes Link Local communication.

   After the shown sequence of MLD Report messages has been executed,
   both Rtr-1 and Rtr-2 are automatically configured to forward
   multicast traffic destined to Room-A-Lights onto their connected
   subnet.  Hence, no manual Network Configuration of routers, as
   previously indicated in Section 4.4, is needed anymore.


                                    Light                        Network
   Light-1   Light-2    Light-3     Switch    Rtr-1      Rtr-2  Backbone
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    | MLD Report: Join    |          |          |          |          |
    | Group (Room-A-Lights)          |          |          |          |
    |---LL------------------------------------->|          |          |
    |          |          |          |          |MLD Report: Join     |
    |          |          |          |          |Group (Room-A-Lights)|
    |          |          |          |          |---LL---->----LL---->|
    |          |          |          |          |          |          |
    |          | MLD Report: Join    |          |          |          |
    |          | Group (Room-A-Lights)          |          |          |
    |          |---LL------------------------------------->|          |
    |          |          |          |          |          |          |
    |          |          | MLD Report: Join    |          |          |
    |          |          | Group (Room-A-Lights)          |          |
    |          |          |---LL-------------------------->|          |
    |          |          |          |          |          |          |
    |          |          |          |          |MLD Report: Join     |
    |          |          |          |          |Group (Room-A-Lights)|
    |          |          |          |          |<--LL-----+---LL---->|
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |


                Figure 6: Joining Lighting Groups Using MLD

4.6.  Commissioning the Network Based On Resource Directory





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   This section outlines how devices in the lighting use case (both
   Switches and Lights) can be commissioned, making use of Resource
   Directory [I-D.ietf-core-resource-directory] and its group
   configuration feature.

   Once the Resource Directory (RD) is discovered, the Switches and
   Lights need to be discovered and their groups need to be defined.
   For the commissioning of these devices, a commissioning tool can be
   used that defines the entries in the RD.  The commissioning tool has
   the authority to change the contents of the RD and the Light/Switch
   nodes.  DTLS based security is used by the commissioning tool to
   modify operational data in RD, Switches and Lights.

   In our particular use case, a group of three lights is defined with
   one multicast address and hostname
   "Room-A-Lights.floor1.west.bldg6.example.com".  The commissioning
   tool has a list of the three lights and the associated multicast
   address.  For each light in the list the tool learns the IP address
   of the light and instructs the RD with three POST commands to store
   the endpoints associated with the three lights as prescribed by the
   RD specification [I-D.ietf-core-resource-directory].  Finally the
   commissioning tool defines the group in the RD to contain these three
   endpoints.  Also the commissioning tool writes the multicast address
   in the Light endpoints with, for example, the POST /gp command
   discussed in Section 3.6.

   The light switch can discover the group in RD and thus learn the
   multicast address of the group.  The light switch will use this
   address to send multicast commands to the members of the group.  When
   the message arrives the Lights should recognize the multicast address
   and accept the message.

5.  Deployment Guidelines

   This section provides guidelines how an IP Multicast based solution
   for CoAP group communication can be deployed in various network
   configurations.

5.1.  Target Network Topologies

   CoAP group communication can be deployed in various network
   topologies.  First, the target network may be a regular IP network,
   or a LLN such as a 6LoWPAN network, or consist of mixed constrained/
   unconstrained network segments.  Second, it may be a single subnet
   only or multi-subnet; e.g., multiple 6LoWPAN networks joined by a
   single backbone LAN.  Third, a wireless network segment may have all
   its nodes reachable in a single IP hop (fully connected), or it may
   require multiple IP hops for some pairs of nodes to reach each other.



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   Each topology may pose different requirements on the configuration of
   routers and protocol(s), in order to enable efficient CoAP group
   communication.

5.2.  Advertising Membership of Multicast Groups

   If a multicast routing/forwarding protocol is used in a network,
   server nodes that intend to receive CoAP multicast requests generally
   require a method to advertise the specific IP multicast address(es)
   they want to receive (i.e., a method to join specific IP multicast
   groups).  This section identifies two ways in which group joining is
   accomplished (with MLD, with RPL) and one situation (with MPL) where
   group joining is not required.

5.2.1.  Using the MLD Listener Protocol

   CoAP nodes that are IP hosts (i.e., not IP routers) are generally
   unaware of the specific multicast routing/forwarding protocol being
   used.  When such a host needs to join a specific (CoAP) multicast
   group, it requires a way to signal to multicast routers which
   multicast traffic it wants to receive.  For efficient multicast
   routing (i.e., avoid always flooding IP multicast packets), routers
   must know which hosts need to receive packets addressed to specific
   IP multicast destinations.

   The Multicast Listener Discovery (MLD) protocol [RFC3810]
   (Appendix A) is the standard IPv6 method to achieve this.  [RFC6636]
   discusses tuning of MLD for mobile and wireless networks.  These
   guidelines may be useful when implementing MLD in LLNs.

   Alternatively, to avoid the use of MLD in LLN deployments, either all
   nodes can be configured as multicast routers in an LLN, or a
   multicast forwarding/flooding protocol can be used that forwards any
   IP multicast packet to all forwarders (routers) in the LLN.

5.2.2.  Using the RPL Routing Protocol

   The RPL routing protocol [RFC6550] defines in Section 12 the
   advertisement of IP multicast destinations using DAO messages.  This
   mechanism can be used by CoAP nodes (which are also RPL routers) to
   advertise IP multicast group membership to other RPL routers.  Then,
   the RPL protocol can route multicast CoAP requests over multiple hops
   to the correct CoAP servers.

   This mechanism can be used as a means to convey IP multicast group
   membership information to an edge router (e.g., 6LBR), in case the
   edge router is also the root of the RPL DODAG.  This could be useful
   in LLN segments where MLD is not available and the edge router needs



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   to know what IP multicast traffic to pass through from the backbone
   network into the LLN subnet.

5.2.3.  Using the MPL Forwarding Protocol

   The MPL forwarding protocol [I-D.ietf-roll-trickle-mcast] can be used
   in a predefined network domain for propagation of IP multicast
   packets to all MPL routers, over multiple hops.  MPL is designed to
   work in LLN deployments.  There is one specific case in which there
   is no need for CoAP server nodes to advertise IP multicast group
   membership.  This case occurs when any IP multicast source is inside
   the MPL domain and if all nodes that listen to IP multicast CoAP
   requests are also MPL routers.

5.3.  6LoWPAN Specific Guidelines

   To support multi-LoWPAN scenarios for CoAP group communication, it is
   recommended that a 6LoWPAN Border Router (6LBR) will act in an MLD
   Router role on the backbone link.  If this is not possible then the
   6LBR should be configured to act as an MLD Multicast Address Listener
   and/or MLD Snooper (Appendix A) on the backbone link.

   To avoid that backbone IP multicast traffic needlessly congests
   6LoWPAN network segments, it is recommended that a filtering means is
   implemented to block IP multicast traffic from 6LoWPAN segments where
   none of the 6LoWPAN nodes listen to this traffic.  Possible means
   are:

   o  Filtering in 6LBRs based on information from the routing/
      forwarding protocol.  This allows a 6LBR to only forward multicast
      traffic onto the LoWPAN, for which it is known that there exists
      at least one listener on the LoWPAN.  This does not work for all
      protocols, for example in MPL this is not defined.

   o  Filtering in 6LBRs based on MLD reports.  Similar as previous but
      based directly on MLD reports from 6LoWPAN nodes.  This only works
      in a single-IP-hop 6LoWPAN network, such as a mesh-under routing
      network or a star topology network, because MLD relies on link-
      local communication.

   o  Filtering in 6LBRs based on settings.  Filtering tables with
      blacklists/whitelists can be configured in the 6LBR by system
      administration for all 6LBRs or configured on a per-6LBR basis.








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   o  Filtering in router(s) or firewalls that provide access to
      constrained network segments.  For example, in an access router/
      bridge that connects a regular intranet LAN to a building control
      IPv6 segment.  This building control segment connects multiple
      6LoWPAN subnets, each subnet connected via one 6LBR.

6.  Security Considerations

   This section describes the relevant security configuration for CoAP
   group communication using IP multicast.  The threats to CoAP group
   communication are also identified and various approaches to mitigate
   these threats are summarized.

6.1.  Security Configuration

   As defined in [I-D.ietf-core-coap], CoAP group communication based on
   IP multicast:

   o  MUST operate in CoAP NoSec (No Security) mode.

   o  MUST NOT use "coaps" scheme.  That is, all group communication
      MUST use only "coap" scheme.

6.2.  Threats

   Essentially the above configuration means that there is no security
   at the CoAP layer for group communication.  This is due to the fact
   that the current DTLS based approach for CoAP is exclusively unicast
   oriented and does not support group security features such as group
   key exchange and group authentication.  As a direct consequence of
   this, CoAP group communication is vulnerable to all attacks mentioned
   in [I-D.ietf-core-coap] for IP multicast.

6.3.  Threat Mitigation

   The [I-D.ietf-core-coap] identifies various threat mitigation
   techniques for CoAP multicast.  In addition to those guidelines, it
   is recommended that for sensitive data or safety-critical control, a
   combination of appropriate link-layer security and administrative
   control of IP multicast boundaries should be used.  Some examples are
   given below.

6.3.1.  WiFi Scenario

   In a home automation scenario (using WiFi), the WiFi encryption
   should be enabled to prevent rogue nodes from joining.  Also, if MAC
   address filtering at the WiFi Access Point is supported that should
   also be enabled.  The IP router should have the firewall enabled to



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   isolate the home network from the rest of the Internet.  In addition,
   the domain of the IP multicast should be set to be either link-local
   scope or site-local scope.  Finally, if possible, devices should be
   configured to accept only Source Specific Multicast (SSM) packets
   (see [RFC4607]) from within the trusted home network.  For example,
   all lights in a particular room should only accept IP multicast
   traffic originating from the master light switch in that room.  In
   this case, the Spoofed Source Address considerations of Section 7.4
   of [RFC4607] should be heeded.

6.3.2.  6LoWPAN Scenario

   In a building automation scenario, a particular room may have a
   single 6LoWPAN network with a single Edge Router (6LBR).  Nodes on
   the subnet can use link-layer encryption to prevent rogue nodes from
   joining.  The 6LBR can be configured so that it blocks any incoming
   (6LoWPAN-bound) IP multicast traffic.  Another example topology could
   be a multi-subnet 6LoWPAN in a large conference room.  In this case,
   the backbone can implement port authentication (IEEE 802.1X) to
   ensure only authorized devices can join the Ethernet backbone.  The
   access router to this secured network segment can also be configured
   to block incoming IP multicast traffic.

6.3.3.  Future Evolution

   In the future, to further mitigate the threats, the developing
   approach for DTLS-based IP multicast security for CoAP networks (see
   [I-D.keoh-tls-multicast-security]) or similar approaches should be
   considered once they mature.

7.  IANA Considerations

7.1.  New 'core.gp' Resource Type

   This memo registers a new resource type (rt) from the CoRE Parameters
   Registry called 'core.gp'.

   (Note to IANA/RFC Editor: This registration follows the process
   described in section 7.4 of [RFC6690]).

   Attribute Value: core.gp

   Description: Group Configuration resource.  This resource is used to
   query/manage the group membership of a CoAP server.

   Reference: See Section 3.6.





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7.2.  New 'coap-group+json' Internet Media Type

   This memo registers a new Internet Media Type for CoAP group
   configuration resource called 'application/coap-group+json'.

   (Note to IANA/RFC Editor: This registration follows the guidance from
   [RFC6839], and (last paragraph) of section 12.3 of
   [I-D.ietf-core-coap].

   Type name: application

   Subtype name: coap-group+json

   Required parameters: None

   Optional parameters: None

   Encoding considerations: 8bit if UTF-8; binary if UTF-16 or UTF-32.

   JSON may be represented using UTF-8, UTF-16, or UTF-32.  When JSON is
   written in UTF-8, JSON is 8bit compatible.  When JSON is written in
   UTF-16 or UTF-32, the binary content-transfer-encoding must be used.

   If the client is aware that the server group configuration resource
   is 8bit encoded (which is most efficient for a constrained device),
   that encoding should be respected by the client (i.e., it should not
   try to replace it by a binary encoded group configuration resource).

   Security considerations:

   Denial of Service attacks could be performed by constantly setting
   the group configuration resource of a CoAP endpoint to different
   values.  This will cause the endpoint to register (or de-register)
   from the related IP multicast group.  To prevent this it is
   recommended that DTLS-secured CoAP communication be used for setting
   the group configuration resource.  Thus only authorized clients will
   be allowed by a server to configure its group membership.

   Interoperability considerations: None

   Published specification: (This I-D when it becomes an RFC)

   Applications that use this media type:

   CoAP client and server implementations that wish to set/read the
   group configuration resource via 'application/coap-group+json'
   payload as described in Section 3.6.




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   Additional Information:

   Magic number(s): None

   File extension(s): *.json

   Macintosh file type code(s): TEXT

   Intended usage: COMMON

   Restrictions on usage: None

   Author: CoRE WG

   Change controller: IETF

8.  Acknowledgements

   Thanks to Peter Bigot, Carsten Bormann, Anders Brandt, Angelo
   Castellani, Bjoern Hoehrmann, Matthias Kovatsch, Guang Lu, Salvatore
   Loreto, Kerry Lynn, Dale Seed, Zach Shelby, Peter van der Stok, and
   Juan Carlos Zuniga for their helpful comments and discussions that
   have helped shape this document.

9.  References

9.1.  Normative References

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

   [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.

   [RFC3542]  Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
              "Advanced Sockets Application Program Interface (API) for
              IPv6", RFC 3542, May 2003.

   [RFC3810]  Vida, R. and L. Costa, "Multicast Listener Discovery
              Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC4601]  Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
              "Protocol Independent Multicast - Sparse Mode (PIM-SM):
              Protocol Specification (Revised)", RFC 4601, August 2006.



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   [RFC4607]  Holbrook, H. and B. Cain, "Source-Specific Multicast for
              IP", RFC 4607, August 2006.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, September 2007.

   [RFC5771]  Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for
              IPv4 Multicast Address Assignments", BCP 51, RFC 5771,
              March 2010.

   [RFC6550]  Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R.,
              Levis, P., Pister, K., Struik, R., Vasseur, JP., and R.
              Alexander, "RPL: IPv6 Routing Protocol for Low-Power and
              Lossy Networks", RFC 6550, March 2012.

   [RFC6636]  Asaeda, H., Liu, H., and Q. Wu, "Tuning the Behavior of
              the Internet Group Management Protocol (IGMP) and
              Multicast Listener Discovery (MLD) for Routers in Mobile
              and Wireless Networks", RFC 6636, May 2012.

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

   [RFC6775]  Shelby, Z., Chakrabarti, S., Nordmark, E., and C. Bormann,
              "Neighbor Discovery Optimization for IPv6 over Low-Power
              Wireless Personal Area Networks (6LoWPANs)", RFC 6775,
              November 2012.

   [RFC6839]  Hansen, T. and A. Melnikov, "Additional Media Type
              Structured Syntax Suffixes", RFC 6839, January 2013.

   [I-D.ietf-core-coap]
              Shelby, Z., Hartke, K., and C. Bormann, "Constrained
              Application Protocol (CoAP)", draft-ietf-core-coap-18
              (work in progress), June 2013.

9.2.  Informative References

   [I-D.ietf-core-block]
              Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
              draft-ietf-core-block-12 (work in progress), June 2013.

   [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.




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   [I-D.ietf-roll-trickle-mcast]
              Hui, J. and R. Kelsey, "Multicast Protocol for Low power
              and Lossy Networks (MPL)", draft-ietf-roll-trickle-
              mcast-04 (work in progress), February 2013.

   [I-D.keoh-tls-multicast-security]
              Keoh, S., Kumar, S., and E. Dijk, "DTLS-based Multicast
              Security for Low-Power and Lossy Networks (LLNs)", draft-
              keoh-tls-multicast-security-00 (work in progress), October
              2012.

   [I-D.ietf-core-resource-directory]
              Shelby, Z., Krco, S., and C. Bormann, "CoRE Resource
              Directory", draft-ietf-core-resource-directory-00 (work in
              progress), June 2013.

Appendix A.  Multicast Listener Discovery (MLD)

   In order to extend the scope of IP multicast beyond link-local scope,
   an IP multicast routing or forwarding protocol has to be active in
   routers on an LLN.  To achieve efficient multicast routing (i.e.,
   avoid always flooding IP multicast packets), routers have to learn
   which hosts need to receive packets addressed to specific IP
   multicast destinations.

   The Multicast Listener Discovery (MLD) protocol [RFC3810] (or its
   IPv4 pendant IGMP) is today the method of choice used by an (IP
   multicast enabled) router to discover the presence of multicast
   listeners on directly attached links, and to discover which multicast
   addresses are of interest to those listening nodes.  MLD was
   specifically designed to cope with fairly dynamic situations in which
   multicast listeners may join and leave at any time.

   IGMP/MLD Snooping is a technique implemented in some corporate LAN
   routing/switching devices.  An MLD snooping switch listens to MLD
   State Change Report messages from MLD listeners on attached links.
   Based on this, the switch learns on what LAN segments there is
   interest for what IP multicast traffic.  If the switch receives at
   some point an IP multicast packet, it uses the stored information to
   decide onto which LAN segment(s) to send the packet.  This improves
   network efficiency compared to the regular behavior of forwarding
   every incoming multicast packet onto all LAN segments.  An MLD
   snooping switch may also send out MLD Query messages (which is
   normally done by a device in MLD Router role) if no MLD Router is
   present.






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   [RFC6636] discusses optimal tuning of the parameters of MLD for
   routers for mobile and wireless networks.  These guidelines may be
   useful when implementing MLD in LLNs.

Appendix B.  Change Log

   Changes from ietf-11 to ietf-12:

   o  Removed reference to "CoAP Ping" in Section 3.5 (Group Member
      Discovery) and replaced it with the more efficient support of
      discovery of groups and group members via the CORE RD as suggested
      by Zach Shelby.

   o  Various editorial updates for improved readability.

   Changes from ietf-10 to ietf-11:

   o  Added text to section 3.8 (Congestion Control) to clarify that a
      "CoAP client sending a multicast CoAP request to /.well-known/core
      SHOULD support core-block" (#332).

   o  Various editorial updates for improved readability.

   Changes from ietf-09 to ietf-10:

   o  Various editorial updates including:

   o  Added a fourth option in section 3.3 on ways to obtain the URI
      path for a group request.

   o  Clarified use of content format in GET/PUT requests for
      Configuring Group Membership in Endpoints (in section 3.6).

   o  Changed reference "draft-shelby-core-resource-directory" to
      "draft-ietf-core-resource-directory".

   o  Clarified (in section 3.7) that ACKs are never used for a
      multicast request (from #296).

   o  Clarified (in section 5.2/5.2.3) that MPL does not support group
      membership advertisement.

   o  Adding introductory paragraph to Scope (section 2.2).

   o  Wrote out fully the URIs in table section 3.2.






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   o  Reworded security text in section 7.2 (New Internet Media Type) to
      make it consistent with section 3.6 (Configuring Group
      Membership).

   o  Fixed formatting of hyperlinks in sections 6.3 and 7.2.

   Changes from ietf-08 to ietf-09:

   o  Cleaned up requirements language in general.  Also, requirements
      language are now only used in section 3 (Protocol Considerations)
      and section 6 (Security Considerations).  Requirements language
      has been removed from other sections to keep them to a minimum
      (#271).

   o  Addressed final comment from Peter van der Stok to define what "IP
      stack" meant (#296).  Following the lead of CoAP-17, we know refer
      instead to "APIs such as IPV6_RECVPKTINFO [RFC3542]".

   o  Changed text in section 3.4 (Group Methods) to allow multicast
      POST under specific conditions and highlighting the risks with
      using it (#328).

   o  Various editorial updates for improved readability.

   Changes from ietf-07 to ietf-08:

   o  Updated text in section 3.6 (Configuring Group Membership in
      Endpoints) to make it more explicit that the Internet Media Type
      is used in the processing rules (#299).

   o  Addressed various comments from Peter van der Stok (#296).

   o  Various editorial updates for improved readability including
      defining all acronyms.

   Changes from ietf-06 to ietf-07:

   o  Added an IANA request (in section 7.2) for a dedicated content-
      format (Internet Media type) for the group management JSON format
      called 'application/coap-group+json' (#299).

   o  Clarified semantics (in section 3.6) of group management JSON
      format (#300).

   o  Added details of IANA request (in section 7.1) for a new CORE
      Resource Type called 'core.gp'.





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   o  Clarified that DELETE method (in section 3.6) is also a valid
      group management operation.

   o  Various editorial updates for improved readability.

   Changes from ietf-05 to ietf-06:

   o  Added a new section on commissioning flow when using discovery
      services when end devices discover in which multicast group they
      are allocated (#295).

   o  Added a new section on CoAP Proxy Operation (section 3.9) that
      outlines the potential issues and limitations of doing CoAP
      multicast requests via a CoAP Proxy (#274).

   o  Added use case of multicasting controller on the backbone (#279).

   o  Use cases were updated to show only a single CoAP RD (to replace
      the previous multiple RDs with one in each subnet).  This is a
      more efficient deployment and also avoids RD specific issues such
      as synchronization of RD information between serves (#280).

   o  Added text to section 3.6 (Configuring Group Membership in
      Endpoints) that clarified that any (unicast) operation to change
      an endpoint's group membership must use DTLS-secured CoAP.

   o  Clarified relationship of this document to [I-D.ietf-core-coap] in
      section 2.2 (Scope).

   o  Removed IPSec related requirement, as IPSec is not part of
      [I-D.ietf-core-coap] anymore.

   o  Editorial reordering of subsections in section 3 to have a better
      flow of topics.  Also renamed some of the (sub)sections to better
      reflect their content.  Finally, moved the URI Configuration text
      to the same section as the Port Configuration section as it was a
      more natural grouping (now in section 3.3) .

   o  Editorial rewording of section 3.7 (Multicast Request Acceptance
      and Response Suppression) to make the logic easier to comprehend
      (parse).

   o  Various editorial updates for improved readability.

   Changes from ietf-04 to ietf-05:






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   o  Added a new section 3.9 (Exceptions) that highlights that IP
      multicast (and hence group communication) is not always available
      (#187).

   o  Updated text on the use of [RFC2119] language (#271) in Section 1.

   o  Included guidelines on when (not) to use CoAP responses to
      multicast requests and when (not) to accept multicast requests
      (#273).

   o  Added guideline on use of core-block for minimizing response size
      (#275).

   o  Restructured section 6 (Security Considerations) to more fully
      describe threats and threat mitigation (#277).

   o  Clearly indicated that DNS resolution and reverse DNS lookup are
      optional.

   o  Removed confusing text about a single group having multiple IP
      addresses.  If multiple IP addresses are required then multiple
      groups (with the same members) should be created.

   o  Removed repetitive text about the fact that group communication is
      not guaranteed.

   o  Merged previous section 5.2 (Multicast Routing) into 3.1 (IP
      Multicast Routing Background) and added link to section 5.2
      (Advertising Membership of Multicast Groups).

   o  Clarified text in section 3.8 (Congestion Control) regarding
      precedence of use of IP multicast domains (i.e. first try to use
      link-local scope, then site-local scope, and only use global IP
      multicast as a last resort).

   o  Extended group resource manipulation guidelines with use of pre-
      configured ports/paths for the multicast group.

   o  Consolidated all text relating to ports in a new section 3.3 (Port
      Configuration).

   o  Clarified that all methods (GET/PUT/POST) for configuring group
      membership in endpoints should be unicast (and not multicast) in
      section 3.7 (Configuring Group Membership In Endpoints).

   o  Various editorial updates for improved readability, including
      editorial comments by Peter van der Stok to WG list of December
      18th, 2012.



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   Changes from ietf-03 to ietf-04:

   o  Removed section 2.3 (Potential Solutions for Group Communication)
      as it is purely background information and moved section to draft-
      dijk-core-groupcomm-misc (#266).

   o  Added reference to draft-keoh-tls-multicast-security to section 6
      (Security Considerations).

   o  Removed Appendix B (CoAP-Observe Alternative to Group
      Communications) as it is as an alternative to IP Multicast that
      the WG has not adopted and moved section to draft-dijk-core-
      groupcomm-misc (#267).

   o  Deleted section 8 (Conclusions) as it is redundant (#268).

   o  Simplified light switch use case (#269) by splitting into basic
      operations and additional functions (#269).

   o  Moved section 3.7 (CoAP Multicast and HTTP Unicast Interworking)
      to draft-dijk-core-groupcomm-misc (#270).

   o  Moved section 3.3.1 (DNS-SD) and 3.3.2 (CoRE Resource Directory)
      to draft-dijk-core-groupcomm-misc as these sections essentially
      just repeated text from other drafts regarding DNS based features.
      Clarified remaining text in this draft relating to DNS based
      features to clearly indicate that these features are optional
      (#272).

   o  Focus section 3.5 (Configuring Group Membership) on a single
      proposed solution.

   o  Scope of section 5.3 (Use of MLD) widened to multicast destination
      advertisement methods in general.

   o  Rewrote section 2.2 (Scope) for improved readability.

   o  Moved use cases that are not addressed to draft-dijk-core-
      groupcomm-misc.

   o  Various editorial updates for improved readability.

   Changes from ietf-02 to ietf-03:

   o  Clarified that a group resource manipulation may return back a
      mixture of successful and unsuccessful responses (section 3.4 and
      Figure 6) (#251).




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   o  Clarified that security option for group communication must be
      NoSec mode (section 6) (#250).

   o  Added mechanism for group membership configuration (#249).

   o  Removed IANA request for multicast addresses (section 7) and
      replaced with a note indicating that the request is being made in
      [I-D.ietf-core-coap] (#248).

   o  Made the definition of 'group' more specific to group of CoAP
      endpoints and included text on UDP port selection (#186).

   o  Added explanatory text in section 3.4 regarding why not to use
      group communication for non-idempotent messages (i.e. CoAP POST)
      (#186).

   o  Changed link-local RD discovery to site-local in RD discovery use
      case to make it more realistic.

   o  Fixed lighting control use case CoAP proxying; now returns
      individual CoAP responses as in coap-12.

   o  Replaced link format I-D with RFC6690 reference.

   o  Various editorial updates for improved readability

   Changes from ietf-01 to ietf-02:

   o  Rewrote congestion control section based on latest CoAP text
      including Leisure concept (#188)

   o  Updated the CoAP/HTTP interworking section and example use case
      with more details and use of MLD for multicast group joining

   o  Key use cases added (#185)

   o  References to [I-D.vanderstok-core-dna] and draft-castellani-core-
      advanced-http-mapping added

   o  Moved background sections on "MLD" and "CoAP-Observe" to
      Appendices

   o  Removed requirements section (and moved it to draft-dijk-core-
      groupcomm-misc)

   o  Added details for IANA request for group communication multicast
      addresses




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   o  Clarified text to distinguish between "link local" and general
      multicast cases

   o  Moved lengthy background section 5 to draft-dijk-core-groupcomm-
      misc and replaced with a summary

   o  Various editorial updates for improved readability

   o  Change log added

   Changes from ietf-00 to ietf-01:

   o  Moved CoAP-observe solution section to section 2

   o  Editorial changes

   o  Moved security requirements into requirements section

   o  Changed multicast POST to PUT in example use case

   o  Added CoAP responses in example use case

   Changes from rahman-07 to ietf-00:

   o  Editorial changes

   o  Use cases section added

   o  CoRE Resource Directory section added

   o  Removed section 3.3.5.  IP Multicast Transmission Methods

   o  Removed section 3.4 Overlay Multicast

   o  Removed section 3.5 CoAP Application Layer Group Management

   o  Clarified section 4.3.1.3 RPL Routers with Non-RPL Hosts case

   o  References added and some normative/informative status changes

Authors' Addresses

   Akbar Rahman (editor)
   InterDigital Communications, LLC

   Email: Akbar.Rahman@InterDigital.com





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Internet-Draft        Group Communication for CoAP             July 2013


   Esko Dijk (editor)
   Philips Research

   Email: esko.dijk@philips.com















































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