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Versions: (draft-rahman-core-groupcomm) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 RFC 7390

CoRE Working Group                                        A. Rahman, Ed.
Internet-Draft                          InterDigital Communications, LLC
Intended status: Informational                              E. Dijk, Ed.
Expires: April 05, 2014                                 Philips Research
                                                        October 02, 2013


                      Group Communication for CoAP
                      draft-ietf-core-groupcomm-16

Abstract

   CoAP is a specialized web 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
   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 05, 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.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Background  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.3.  Conventions and Terminology . . . . . . . . . . . . . . .   4
   2.  Protocol Considerations . . . . . . . . . . . . . . . . . . .   4
     2.1.  IP Multicast Background . . . . . . . . . . . . . . . . .   4
     2.2.  Group Definition and Naming . . . . . . . . . . . . . . .   5
     2.3.  Port and URI Configuration  . . . . . . . . . . . . . . .   6
     2.4.  Group Methods . . . . . . . . . . . . . . . . . . . . . .   7
     2.5.  Group Member Discovery  . . . . . . . . . . . . . . . . .   8
     2.6.  Configuring Group Membership in Endpoints . . . . . . . .   8
       2.6.1.  Background  . . . . . . . . . . . . . . . . . . . . .   8
       2.6.2.  RESTful Interface . . . . . . . . . . . . . . . . . .   9
     2.7.  Multicast Request Acceptance and Response Suppression . .  11
     2.8.  Congestion Control  . . . . . . . . . . . . . . . . . . .  13
     2.9.  Proxy Operation . . . . . . . . . . . . . . . . . . . . .  14
     2.10. Exceptions  . . . . . . . . . . . . . . . . . . . . . . .  15
   3.  Use Cases and Corresponding Protocol Flows  . . . . . . . . .  16
     3.1.  Introduction  . . . . . . . . . . . . . . . . . . . . . .  16
     3.2.  Network Configuration . . . . . . . . . . . . . . . . . .  16
     3.3.  Discovery of Resource Directory . . . . . . . . . . . . .  18
     3.4.  Lighting Control  . . . . . . . . . . . . . . . . . . . .  20
     3.5.  Lighting Control in MLD Enabled Network . . . . . . . . .  23
     3.6.  Commissioning the Network Based On Resource Directory . .  24
   4.  Deployment Guidelines . . . . . . . . . . . . . . . . . . . .  25
     4.1.  Target Network Topologies . . . . . . . . . . . . . . . .  25
     4.2.  Networks Using the MLD Protocol . . . . . . . . . . . . .  25
     4.3.  Networks Using RPL Multicast Without MLD  . . . . . . . .  26
     4.4.  Networks Using MPL Forwarding Without MLD . . . . . . . .  26
     4.5.  6LoWPAN Specific Guidelines for the 6LBR  . . . . . . . .  27
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  28
     5.1.  Security Configuration  . . . . . . . . . . . . . . . . .  28
     5.2.  Threats . . . . . . . . . . . . . . . . . . . . . . . . .  28
     5.3.  Threat Mitigation . . . . . . . . . . . . . . . . . . . .  28
       5.3.1.  WiFi Scenario . . . . . . . . . . . . . . . . . . . .  28
       5.3.2.  6LoWPAN Scenario  . . . . . . . . . . . . . . . . . .  29
       5.3.3.  Future Evolution  . . . . . . . . . . . . . . . . . .  29
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
     6.1.  New 'core.gp' Resource Type . . . . . . . . . . . . . . .  29



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     6.2.  New 'coap-group+json' Internet Media Type . . . . . . . .  29
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  31
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  31
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  31
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  32
   Appendix A.  Multicast Listener Discovery (MLD) . . . . . . . . .  33
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . .  33
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  41

1.  Introduction

1.1.  Background

   Constrained Application Protocol (CoAP) is a Representational State
   Transfer (REST) based web transfer protocol 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 numbers, but are
   often related to each other in function 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.

1.2.  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 for CoAP group
   communication are outlined.




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1.3.  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 low-power and lossy links.  The links may be may composed of
      one or more technologies such as IEEE 802.15.4, Bluetooth Low
      Energy (BLE), Digital Enhanced Cordless Telecommunication (DECT),
      and IEEE P1901.2 power-line communication.

2.  Protocol Considerations

2.1.  IP Multicast Background




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   IP Multicast protocols have been evolving for decades, resulting in
   standards such as Protocol Independent Multicast - Sparse Mode (PIM-
   SM) [RFC4601].  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 link-local scope, 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].  RPL and MPL do not depend on each
   other; each can be used in isolation and both can be used in
   combination in a network.  Finally, PIM-SM [RFC4601] is often used
   for multicast routing in traditional IP networks (i.e. networks that
   are not constrained).

   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 "listener" protocol may be needed for the
   devices to subscribe to groups (see Section 4.2).

2.2.  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.

   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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   Note: A Group URI is needed to initiate CoAP group communications.
   If a CoAP implementation accepts a CoAP URI as input in the group
   communications request API, then the parsing method of Section 6.4 of
   [I-D.ietf-core-coap] should be used in such way that a CoAP multicast
   request is generated.

   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 (and are derived from [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]).

2.3.  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 other parts of the URI such as the
   path, or the query, differ 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:







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   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.

   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.  Note that
       [I-D.ietf-appsawg-uri-get-off-my-lawn] prescribes that any
       specification must not constrain, define structure or semantics
       for any path component.

   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.

2.4.  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



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   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 2.8.  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
   (see section 8 of [I-D.ietf-core-coap]).

2.5.  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 3.6.

2.6.  Configuring Group Membership in Endpoints

2.6.1.  Background

   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 3.6.

   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.



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2.6.2.  RESTful Interface

   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) 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.
   Also, the (unicast) methods to configure group membership 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.

   CoAP endpoints implementing this optional mechanism MUST support the
   group configuration Internet Media Type "application/coap-group+json"
   (Section 6.2).  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 6.1).  An
   authorized controller uses this media type to query/manage group
   membership of a CoAP endpoint as defined below.

2.6.2.1.  GET Interface

   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 that indicates one
   multicast group membership.  The example below illustrates a request
   to an endpoint querying its group membership information, 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",
       "a": "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.








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   The "a" key/value pair specifies the IP multicast address (and
   optionally the port number) of the group.  It contains an IPv4
   address or an IPv6 address in dotted decimal notation.  The following
   ABNF rule can be used for parsing the address, referring to the
   definitions in Section 6 of [I-D.ietf-core-coap] and [RFC3986].

   group-address = IPv4address [ ":" port ]
                   / "[" IPv6address "]" [":" port ]


   If the port number is not provided then it is assumed to be the
   default CoAP port (5683).  The "a" key/value pair MUST be included if
   the IP address is known at the time of generating the response, and
   SHOULD NOT be included if unknown.

   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.

2.6.2.2.  POST Interface

   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:

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


   In a request, the "a" key/value pair is OPTIONAL to define the
   group's associated IP multicast address.  The "n" pair is also
   OPTIONAL.  If the "a" pair is given, this takes priority and the "n"
   pair becomes informational.  If only the "n" pair is given, the CoAP
   endpoint may perform DNS resolution (if supported) to obtain the IP
   multicast address from the hostname.  At least one of the "n"/"a"
   pairs MUST be given per group object.

   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.  When a POST payload contains
   in "a" a multicast address to which the endpoint is already
   subscribed, the endpoint MUST re-register to that multicast address.




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2.6.2.3.  PUT Interface

   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.  The format of the group
   configuration payload, and the requirements for it, are the same as
   in Section 2.6.2.2.

   A (unicast) PUT with an empty array [] will delete all group
   memberships from the endpoint.  Note that it is not possible to
   delete individual group memberships on an endpoint.  (A DELETE
   interface is not defined as the underlying resource should not be
   removed even if it is empty).

2.7.  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.

   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)
      [I-D.ietf-core-coap].  See Section 5.3 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.




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   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 resource "/.well-known/core", multicast support
      is required if "multicast resource discovery" is supported as
      specified in section 1.2.1 of [RFC6690]).  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  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:

   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.



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   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).

2.8.  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 2.7 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.

   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).







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   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  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 use CoAP multicast with the smallest possible
      multicast scope that fulfills the application needs.  As an
      example, site-local scope is always preferred over global scope IP
      multicast if this fulfills the application needs.

   More guidelines specific to use of CoAP in 6LoWPAN networks [RFC4944]
   are given in Section 4.5.

2.9.  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.

   A proxy may buffer all the individual (unicast) responses to a group
   communication request and then send back only a single (aggregated)
   response to the client.  However there are some issues with this
   aggregation approach:

   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.  Also the proxy does not know
      how long to wait before deciding to send back the aggregated
      response to the client.

   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



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   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.

2.10.  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" (Mode Of Operation=1) or "Storing mode with no
   multicast support" (Mode Of Operation=2) [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.



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3.  Use Cases and Corresponding Protocol Flows

3.1.  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).

3.2.  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).

   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.

   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



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      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.











































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     ################################################
     #         **********************        Room-A #
     #       **  Subnet-1            **             #           Network
     #     *                           **           #          Backbone
     #    *     +----------+             *          #                 |
     #   *      |  Light   |-------+      *         #                 |
     #  *       |  Switch  |       |       *        #                 |
     #  *       +----------+  +---------+  *        #                 |
     #  *                     |  Rtr-1  |-----------------------------+
     #  *                     +---------+  *        #                 |
     #  *       +----------+        |      *        #                 |
     #   *      |  Light-1 |--------+     *         #                 |
     #    *     +----------+             *          #                 |
     #     **                          **           #                 |
     #       **************************             #                 |
     #                                              #                 |
     #         **********************               # +------------+  |
     #       **  Subnet-2            **             # | DNS Server |  |
     #     *                           **           # | (Optional) |--+
     #    *     +----------+             *          # +------------+  |
     #   *      |  Light-2 |-------+      *         #                 |
     #  *       |          |       |       *        #                 |
     #  *       +----------+  +---------+  *        #                 |
     #  *                     |  Rtr-2  |-----------------------------+
     #  *                     +---------+  *        #                 |
     #  *       +----------+        |      *        #                 |
     #   *      |  Light-3 |--------+     *         #                 |
     #    *     +----------+             *          # +------------+  |
     #     **                          **           # | Controller |--+
     #       **************************             # | Client     |  |
     ################################################ +------------+  |
                                       +------------+                 |
                                       |   CoAP     |                 |
                                       |  Resource  |-----------------+
                                       |  Directory |
                                       +------------+


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

3.3.  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.





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   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 installed.

   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.  (See
   [I-D.ietf-core-resource-directory] for more details).

   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                    |          |



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    |          |         </rd>;rt="core.rd";ins="Primary") |<---------|
    |          |<------------------------------------------|          |
    |          |          |          |          |          |          |



       Figure 2: Resource Directory Discovery via Multicast Request

3.4.  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 2.7 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
   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.  In case a CoAP message is
   delivered multiple times to a Light, the subsequent CoAP messages can
   be filtered out as duplicates, based on the CoAP Message ID.

   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.





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   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  *          |          |
    |          |          ***********************          |          |
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    |          |          |    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



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                                    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

   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)   *           |          |          |        |
    ***********************           |          |          |        |
    |          |          |           |          |          |        |
    |          |          |           |          |          |        |



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    |    COAP NON (2.04 Changed)      |          |          |        |
    |-------------------------------->|          |          |        |
    |          |          |           |-------------------->|        |
    |          |  COAP NON (2.04 Changed)        |          |------->|
    |          |-------------------------------->|          |        |
    |          |          |           |          |--------->|        |
    |          |          | COAP NON (2.04 Changed)         |------->|
    |          |          |--------------------->|          |        |
    |          |          |           |          |--------->|        |
    |          |          |           |          |          |------->|
    |          |          |           |          |          |        |


     Figure 5: Controller On Backbone Sends Multicast Control Message

3.5.  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
   support in all nodes in the 6LoWPAN.  In current 6LoWPAN
   implementations, MLD is however not supported.

   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 3.4, 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     |



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    |          |          |          |          |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

3.6.  Commissioning the Network Based On Resource Directory

   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 2.6.2.2.




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   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.

4.  Deployment Guidelines

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

4.1.  Target Network Topologies

   CoAP group communication can be deployed in various network
   topologies.  First, the target network may be a traditional IP
   network, or a LLN such as a 6LoWPAN network, or consist of mixed
   traditional/constrained 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.

   Each topology may pose different requirements on the configuration of
   routers and protocol(s), in order to enable efficient CoAP group
   communication.  To enable all the above target network topologies, an
   implementation of CoAP group communication needs to allow:

   1.  Routing/forwarding of IP multicast packets over multiple hops

   2.  Routing/forwarding of IP multicast packets over subnet boundaries
       between traditional and constrained (e.g. LLN) networks.

   The remainder of this section discusses solutions to enable this.

4.2.  Networks Using the MLD 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.

   The Multicast Listener Discovery (MLD) protocol [RFC3810] (see
   Appendix A) is the standard IPv6 method to achieve this; therefore
   this approach should be used on traditional IP networks.  CoAP server
   nodes would then act in the role of MLD Multicast Address Listener.




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   The guidelines from [RFC6636] on tuning of MLD for mobile and
   wireless networks may be useful when implementing MLD in LLNs.
   However, on LLNs and 6LoWPAN networks the use of MLD may not be
   feasible at all due to constraints on code size, memory, or network
   capacity.

4.3.  Networks Using RPL Multicast Without MLD

   It is assumed in this section that the MLD protocol is not
   implemented in a network, for example due to resource constraints.
   The RPL routing protocol (see Section 12 of [RFC6550]) defines the
   advertisement of IP multicast destinations using DAO messages and
   routing of multicast IP packets based on this.  It requires the RPL
   Mode of Operation (MOP) to be 3 (Storing Mode with multicast
   support).

   Hence, RPL DAO can be used by CoAP nodes (that are also RPL routers)
   to advertise IP multicast group membership to parent routers.  Then,
   the RPL protocol is used to route multicast CoAP requests over
   multiple hops to the correct CoAP servers.

   The same mechanism can be used 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 is useful
   because the edge router then learns which IP multicast traffic it
   needs to pass through from the backbone network into the LLN subnet.
   In 6LoWPAN networks, such selective "filtering" may avoid congestion
   of a 6LoWPAN subnet by IP multicast traffic from the traditional
   backbone network.

4.4.  Networks Using MPL Forwarding Without MLD

   The MPL forwarding protocol [I-D.ietf-roll-trickle-mcast] can be used
   for propagation of IP multicast packets to all MPL Forwarders within
   a predefined network domain, over multiple hops.  MPL is designed to
   work in LLNs.  In this section it is again assumed that the MLD
   protocol is not implemented in the network, for example due to
   resource limitations in an LLN.

   In a typical use of MPL, all nodes in an LLN are MPL Forwarders.  So
   it would appear there is no need for CoAP servers to advertise their
   multicast group membership, since any IP multicast packet that enters
   the MPL Domain is distributed to all MPL Forwarders without regard to
   what multicast addresses the individual nodes are listening to.

   However, if an IP multicast request originates outside the MPL
   Domain, this request will by default not be propagated by MPL to the
   CoAP server(s) within the MPL Domain that need to receive it.  This



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   situation can become a problem in building control use cases.  For
   example, in a network topology of Figure 1 where the Subnets are
   6LoWPAN subnets and per 6LoWPAN subnet one Realm-Local MPL Domain is
   defined.  Suppose that the Controller Client needs to send a single
   CoAP multicast request to group Room-A-Lights.  By default, the
   request would be blocked by Rtr-1 and by Rtr-2, and not enter the
   Realm-Local MPL Domains associated to Subnet-1 and Subnet-2.  The
   reason is that Rtr-1 and Rtr-2 do not have the knowledge that devices
   in Subnet-1/2 want to listen for IP packets destined to multicast
   group Room-A-Lights.

   To solve the above issue, the following solutions could be applied:

   1.  Extend the MPL Domain.  E.g. in above example, include the
       Network Backbone to be part of the MPL Domain.

   2.  Manual configuration of edge router(s) as MPL Seed(s) for
       specific multicast traffic.  E.g. in above example, configure
       Rtr-1 and Rtr-2 to act as MLD Address Listeners for the
       Room-A-Lights multicast group.  Then any received traffic from
       the backbone, destined to that group, would be injected into the
       MPL Domain that is associated to that router.  Also a whitelist
       filtering should be enabled in these routers, to only pass
       through multicast traffic to configured groups such as
       Room-A-Lights.  This would prevent unnecessarily flooding of the
       MPL Domain with multicast packets, which is especially important
       if the subnets are LLNs.

   3.  Use an additional protocol for injection of multicast traffic
       from outside an MPL Domain into the MPL Domain, based on
       multicast group subscriptions of Forwarders within the MPL
       Domain.  Such protocol is currently not defined in IETF and
       outside scope of [I-D.ietf-roll-trickle-mcast].

   Concluding, MPL can be used directly in case all sources of multicast
   CoAP requests (CoAP clients) and also all the destinations (CoAP
   servers) are inside a single MPL Domain.  Then, each source node acts
   as an MPL Seed.  In all other cases, MPL can only be used with
   additional protocols and/or configuration.

4.5.  6LoWPAN Specific Guidelines for the 6LBR

   To support multi-subnet 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
   (see Appendix A) on the backbone link.




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5.  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.

5.1.  Security Configuration

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

   o  Will operate in CoAP NoSec (No Security) mode, until a future
      group security solution is developed (see also Section 5.3.3).

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

5.2.  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.

5.3.  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.

5.3.1.  WiFi Scenario

   In a home automation scenario (using WiFi), the WiFi encryption
   should be enabled to prevent rogue nodes from joining.  The Customer
   Premise Equipment (CPE) that enables access to the Internet should
   also have its multicast filters set so that it enforces multicast
   scope boundaries to isolate local multicast groups from the rest of
   the Internet (e.g., as per [RFC6092]).  In addition, the domain of
   the IP multicast should be set to be either link-local scope or site-
   local scope.  For site-local scope, the CPE will be an appropriate
   multicast scope boundary point.



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5.3.2.  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.

5.3.3.  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.

6.  IANA Considerations

6.1.  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 2.6.2.

6.2.  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




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   Subtype name: coap-group+json

   Required parameters: None

   Optional parameters: None

   Encoding considerations: 8bit UTF-8.

   JSON to be represented using UTF-8 which is 8bit compatible (and most
   efficient for resource constrained implementations).

   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 (unicast) 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 2.6.2.

   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




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7.  Acknowledgements

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

8.  References

8.1.  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.

   [RFC3376]  Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
              Thyagarajan, "Internet Group Management Protocol, Version
              3", RFC 3376, October 2002.

   [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.

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

   [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.

   [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.



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   [RFC6092]  Woodyatt, J., "Recommended Simple Security Capabilities in
              Customer Premises Equipment (CPE) for Providing
              Residential IPv6 Internet Service", RFC 6092, January
              2011.

   [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.

8.2.  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.

   [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-05 (work in progress), August 2013.

   [I-D.keoh-tls-multicast-security]



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              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.

   [I-D.ietf-appsawg-uri-get-off-my-lawn]
              Nottingham, M., "Standardising Structure in URIs", draft-
              ietf-appsawg-uri-get-off-my-lawn-00 (work in progress),
              September 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 equivalent IGMP [RFC3376]) 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.

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

Appendix B.  Change Log

   [Note to RFC Editor: Please remove this section before publication.]

   Changes from ietf-15 to ietf-16:

   o  In section 2.6.2, changed DELETE in group management interface to
      a PUT with empty JSON array to clear the list (#345).

   o  In section 2.6.2, aligned the syntax for IP addresses to follow
      RFC 3986 URI syntax, which is also used by coap-18.  This allows
      re-use of the parsing code for CoAP URIs for this purpose (#342).



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   o  Addressed some more editorial comments provided by Carsten Bormann
      in preparation for WGLC.

   o  Various editorial updates for improved readability.

   Changes from ietf-14 to ietf-15:

   o  In section 2.2, provided guidance on how implementers should parse
      URIs for group communication (#339).

   o  In section 2.6.2.1, specified that for group membership
      configuration interface the "ip" (i.e. "a" parameter) key/value is
      not required when it is unknown (#338).

   o  In section 2.6.2.1, specified that for group membership
      configuration interface the port configuration be defaulted to
      standard CoAP port 5683, and if not default then should follow
      standard notation (#340).

   o  In section 2.6.2.1, specified that notation of IP address in group
      membership configuration interface should follow standard notation
      (#342).

   o  In section 6.2, "coap-group+json" Media Type encoding simplified
      to just support UTF-8 (and not UTF-16 and UTF-32) (#344).

   o  Various editorial updates for improved readability.

   Changes from ietf-13 to ietf-14:

   o  Update to address final editorial comments from the Chair's review
      (by Carsten Bormann) of the draft.  This included restructuring of
      Section 2.6 (Configuring Group Memberships) and Section 4
      (Deployment Guidelines) to make it easier to read.  Also various
      other editorial changes.

   o  Changed "ip" field to "a" in Section 2.6 (#337)

   Changes from ietf-12 to ietf-13:

   o  Extensive editorial updates due to comments from the Chair's
      review (by Carsten Bormann) of the draft.  The best way to see the
      changes will be to do a -Diff with Rev. 12.

   o  The technical comments from the Chair's review will be addressed
      in a future revision after tickets are generated and the solutions
      are agreed to on the WG E-mail list.




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   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.

   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:




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   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'.

   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:






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   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:

   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).



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   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.

   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).




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   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).

   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).




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   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

   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



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   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


   Esko Dijk (editor)
   Philips Research

   Email: esko.dijk@philips.com









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