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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

CoRE Working Group                                        A. Rahman, Ed.
Internet-Draft                          InterDigital Communications, LLC
Intended status: Informational                            E.O. Dijk, Ed.
Expires: October 14, 2013                               Philips Research
                                                          April 12, 2013

                      Group Communication for CoAP


   CoAP is a RESTful transfer protocol for constrained devices and
   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 for both constrained
   and un-constrained networks.  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 October 14, 2013.

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

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   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Conventions and Terminology . . . . . . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Background  . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Protocol Considerations . . . . . . . . . . . . . . . . . . .   4
     3.1.  IP Multicast Routing Background . . . . . . . . . . . . .   4
     3.2.  Group Definition and Naming . . . . . . . . . . . . . . .   5
     3.3.  Port and URI Configuration  . . . . . . . . . . . . . . .   6
     3.4.  Group Methods . . . . . . . . . . . . . . . . . . . . . .   7
     3.5.  Group Member Discovery  . . . . . . . . . . . . . . . . .   7
     3.6.  Configuring Group Membership In Endpoints . . . . . . . .   7
     3.7.  Multicast Request Acceptance and Response Suppression . .   9
     3.8.  Congestion Control  . . . . . . . . . . . . . . . . . . .  11
     3.9.  Proxy Operation . . . . . . . . . . . . . . . . . . . . .  12
     3.10. Exceptions  . . . . . . . . . . . . . . . . . . . . . . .  13
   4.  Use Cases and Corresponding Protocol Flows  . . . . . . . . .  13
     4.1.  Introduction  . . . . . . . . . . . . . . . . . . . . . .  13
     4.2.  Network Configuration . . . . . . . . . . . . . . . . . .  14
     4.3.  Discovery of Resource Directory . . . . . . . . . . . . .  16
     4.4.  Lighting Control  . . . . . . . . . . . . . . . . . . . .  17
     4.5.  Lighting Control in MLD Enabled Network . . . . . . . . .  20
     4.6.  Commissioning the Network Based On Resource Directory . .  21
   5.  Deployment Guidelines . . . . . . . . . . . . . . . . . . . .  22
     5.1.  Target Network Topologies . . . . . . . . . . . . . . . .  22
     5.2.  Advertising Membership of Multicast Groups  . . . . . . .  22
       5.2.1.  Using the Multicast Listener Discovery (MLD) Protocol  23
       5.2.2.  Using the RPL Routing Protocol  . . . . . . . . . . .  23
       5.2.3.  Using the MPL Forwarding Protocol . . . . . . . . . .  23
     5.3.  6LoWPAN-Specific Guidelines . . . . . . . . . . . . . . .  24
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  24
     6.1.  Security Configuration  . . . . . . . . . . . . . . . . .  24
     6.2.  Threats . . . . . . . . . . . . . . . . . . . . . . . . .  25
     6.3.  Threat Mitigation . . . . . . . . . . . . . . . . . . . .  25
       6.3.1.  WiFi Scenario . . . . . . . . . . . . . . . . . . . .  25
       6.3.2.  6LoWPAN Scenario  . . . . . . . . . . . . . . . . . .  25
       6.3.3.  Future Evolution  . . . . . . . . . . . . . . . . . .  26
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  26

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   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  26
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  27
   Appendix A.  Multicast Listener Discovery (MLD) . . . . . . . . .  28
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

1.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

   These key words are used to establish a set of best practices 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.

   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.

      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)

   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,
      Bluetooth LE, DECT, DECT ULE) or lossy links (such as IEEE P1901.2
      power-line communication).

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2.  Introduction

2.1.  Background

   The Constrained Application Protocol (CoAP) is an application
   protocol (analogous to HTTP) for resource constrained devices
   operating in an IP network [I-D.ietf-core-coap].  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 sending a CoAP Request as an IP
   Multicast message and handling the potential multitude of (unicast)
   CoAP 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 key group communication features.
   Among the topics covered are group definition, group resource
   manipulation, and group configuration.  Also, proxy operation and
   minimizing congestion scenarios for group communication is discussed.
   Finally, specific use case behavior and deployment guidelines are
   outlined for CoAP group communication.

3.  Protocol Considerations

3.1.  IP Multicast Routing Background

   IP Multicast routing 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 marketing reasons, IP Multicast routing 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.  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 examples of

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   a routing protocol specifically for LLNs is RPL (Section 12 of
   [RFC6550]) and an example of a forwarding protocol for LLNs is MPL
   [I-D.ietf-roll-trickle-mcast].  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 a multicast CoAP request that is 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.

   For group communication, the Group URI will be the CoAP request URI.
   A Group URI has the scheme 'coap' and includes in the authority part
   either a group IP multicast address plus optional port number or a
   hostname plus optional port number that can be resolved to the group
   IP multicast address (e.g., a Group Name or Group FQDN).  Group URIs
   follow the CoAP URI syntax [I-D.ietf-core-coap].  It is recommended
   for sending nodes to use the IP multicast address literal in the
   authority for the Group URI as the default.

   If a Group FQDN is used, it can be uniquely mapped to a site-local or
   global multicast IP address via DNS resolution (if supported).  Some
   examples of hierarchical Group FQDN naming (and scoping) for a
   building control application are shown below

   URI authority                  Targeted group
   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.examp... "all nodes in floor 1, west wing,
                                   building 6"
   all.bu036.floor1.west.bldg6... "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

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3.3.  Port and URI Configuration

   A CoAP group member listens for CoAP messages on the group's IP
   multicast address, on a specified UDP port.  Note that 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 diversity
   in the authority port (i.e.  a diversity of 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 the port
   number as follows:

   1.  A pre-configured port number, if available.  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 as a valid CoAP port for the targeted
       CoAP multicast group.

   3.  Otherwise use the default CoAP UDP port.

   All CoAP multicast requests SHOULD operate on URI paths ("links") as

   1.  Pre-configured URI paths, if available.  The pre-configuration
       mechanism MUST ensure that these URIs 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

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3.4.  Group Methods

   Group communication SHALL only be used for idempotent methods (i.e.
   CoAP GET, PUT, and DELETE).  The CoAP messages that are sent via
   multicast SHALL be Non-Confirmable.

   A unicast response per server MAY be sent back to answer the group
   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 result.

   Group communication SHALL NOT be used for non-idempotent methods
   (i.e.  CoAP POST).  This is because 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.

3.5.  Group Member Discovery

   CoAP defines a resource discovery capability [RFC6690], but does not
   specify how to discover groups (e.g.  find a group to join or send a
   multicast message to) or how to discover members of a group (e.g.  to
   address selected group members by unicast).  A simple ad-hoc method
   to discover members of a CoAP group would be to send a multicast
   "CoAP ping" [I-D.ietf-core-coap].  The collected responses to the
   ping would then give an indication of the group members.

3.6.  Configuring Group Membership In Endpoints

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

   In the first case, the pre-configured group may be a multicast IP
   address or a hostname which is during operation resolved to a
   multicast IP address by the endpoint using DNS.

   In the second case, typical in e.g.  building control, a
   commissioning tool determines to which groups 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 a multicast IP
   address or a hostname.

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   For the third case, specific methods to use for a CoAP server to look
   up its group membership(s) may be DNS-SD and Resource Directory
   [I-D.shelby-core-resource-directory].  The latter is detailed more in
   section Section 4.6.

   To achieve better interoperability between nodes/endpoints from
   different manufacturers, an OPTIONAL default RESTful interface for
   configuring CoAP endpoints with relevant group information is
   specified here.  This interface thus provides a solution for the
   second 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.

   CoAP endpoints implementing this optional mechanism MUST support at
   least one discoverable "Group Configuration" resource of resource
   type (rt) [RFC6690] "core.gp" where "gp" is shorthand for "group".
   This resource is used by an authorized endpoint to manage group
   membership of the CoAP endpoint.

   The resource of type "core.gp" has a JSON content format.  A
   (unicast) GET on this resource returns a JSON array of group objects,
   each group object formatted as shown below:

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

   where the OPTIONAL "n" key/value pair defines the Group name as FQDN
   and the OPTIONAL "ip" key/value pair defines the associated multicast
   IP address.  A CoAP endpoint can be added to another group by a
   (unicast) POST on the resource with a single JSON group object, which
   updates the existing resource by adding the group object to the
   existing ones:

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

   A (unicast) PUT with as payload an array of JSON group objects will
   replace all current group memberships with the new ones as defined in
   the payload.  After a change effected on the "core.gp" type resource,

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   the endpoint MUST effect registration/deregistration from
   corresponding IP multicast groups as soon as possible.

   Any (unicast) operation (i.e.  PUT/POST) to change a CoAP endpoint
   group membership configuration MUST use DTLS-secured CoAP
   [I-D.ietf-core-coap].  Thus only authorized clients will be allowed
   by a server to configure the server's (endpoint) 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 request is
       accepted and executed, and when not.

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

   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, the IP
   stack interface of a CoAP server must be able to indicate for an
   incoming request that the destination address of the request was

   For multicast request acceptance, the 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 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 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.)

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   o  Otherwise accept the multicast request.

   For multicast response suppression, the 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 IP stack interface 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 per resource:

   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 described
   below illustrating 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 group configuration data using multicast PUT: No
      suppression at all.  Use collected responses to identify which
      group members did not receive the new configuration; then attempt
      using CoAP CON unicast to update those 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
      blocks are still missing in each endpoint.

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   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 replies from
   different nodes, potentially causing congestion.  Therefore both the
   sending of multicast requests and sending unicast CoAP responses to
   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

   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).

   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.

   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 are:

   o  A server in an LLN should only support multicast GET for resources
      that are small, e.g.  the payload of the response 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

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      CoAP blockwise transfers [I-D.ietf-core-block], returning only a
      first block of the link format description.

   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

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:

   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.

   But if a proxy would follow 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:

   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 or CoAP stack.

   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

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   case here (i.e.  to process it) is allowed under following

   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.  (This condition MAY be removed once
      an aggregation format is standardized.)

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

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
   supported in the network.  For example, in a LLN, if the RPL protocol
   is used for routing multicast packets and RPL routers operate in
   "Non-storing mode" [RFC6550] there will be no IP multicast routing in
   that network beyond link-local scope.  This means that any CoAP group
   communication above link-local scope will not be supported in that

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.shelby-core-resource-directory]): discovering the local CoAP
      RD which contains links (URIs) 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).

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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).

   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 MLD for forming groups.  In a limited case,
      if the network backbone is one link, then the routers only have to
      support MLD-snooping (Appendix A) for the following use cases to

   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 URI to IP multicast address).
      If the DNS server is not there, then different manual provisioning
      of the network is required (e.g.  IP multicast addresses are hard-
      coded into devices).

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

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

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

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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  The fixture for Light-2 is installed and powered on for the first

   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 is a RD).

   o  The CoAP RD will then send back a unicast response containing the
      requested content.

   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.

   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                        |          |

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    |          |           /.well-known/core?rt=core.rd)   |          |
    |          |--------->-------------------------------->|          |
    |          |          |          |          |          |--------->|
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    |          | COAP NON (2.05 Content                    |          |
    |          |         </rd>;rt="core.rd";ins="Primary") |<---------|
    |          |<------------------------------------------|          |
    |          |          |          |          |          |          |

       Figure 2: Resource Directory Discovery via Multicast Message

4.4.  Lighting Control

   The protocol flow for a building automation lighting control scenario
   for the network (Figure 1) in 6LoWPAN configuration is shown in
   sequence in Figure 3 for the case that the CoAP servers in each Light
   are configured to not generate a CoAP response to lighting control
   CoAP multicast requests.  (See section 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 use case the Light
   Switch does not check, based on the responses, that all Lights in the
   group actually received the multicast request, because it is not
   configured with an exhaustive list of 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.

   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 is the MPL forwarding protocol for LLNs

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

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   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 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 software supports
   sending unicast, multicast or proxied unicast/multicast 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)   *          |          |          |          |
    ***********************          |          |          |          |
    |          |          |          |          |          |          |

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          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

   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.

   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

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
   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    |          |          |          |          |

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    | 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

   This section outlines how devices in the lighting use case (both
   Switches and Lights) can be commissioned, making use of Resource
   Directory [I-D.shelby-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 nodes.  DTLS
   based security is used by the commissioning tool to modify
   operational data in RD, Switches and Lights.

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   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
   device 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 3 POST commands to store the
   end-points associated with the three lights as prescribed by RD.
   Finally the commissioning device defines the group in the RD to
   contain these three end-points.  Also the commissioning tool writes
   the MC address in the Lights with e.g.  the POST /gp command
   discussed in Section 3.6.

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

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
   nodes reachable in a single IP hop, 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

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 the ways in which this can be

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5.2.1.  Using the Multicast Listener Discovery (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 usually requires a way to signal to multicast routers which
   multicast traffic it wants to receive.  For efficient multicast
   routing (i.e.  avoid always flooding multicast IP 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 subnet (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
   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.  Due to its property of propagating all
   (non-link-local) IP multicast packets to all MPL routers, there is in
   principle no need for CoAP server nodes to advertise IP multicast
   group membership assuming that any IP multicast source is also part
   of the MPL domain.

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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

   o  Filtering in 6LBRs based on information from the routing 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.

   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.

   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 backbone.  This backbone connects multiple 6LoWPAN segments,
      each segment connected via a 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 must use the following security modes:

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

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   o  Group communication 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

   [I-D.ietf-core-coap] identifies various threat mitigation techniques
   for CoAP IP 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 fire wall enabled to
   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 topology 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
   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

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   router to this secured 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

   tbd: allocation of "core.gp" resource type in relevant registry.

   (Note to RFC Editor: The required multicast address request to IANA
   is made in [I-D.ietf-core-coap]).

8.  Acknowledgements

   Thanks to Peter Bigot, Carsten Bormann, Anders Brandt, Angelo
   Castellani, 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.

   [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.

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

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   [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.

              Shelby, Z., Hartke, K., and C. Bormann, "Constrained
              Application Protocol (CoAP)", draft-ietf-core-coap-14
              (work in progress), March 2013.

9.2.  Informative References

              Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
              draft-ietf-core-block-11 (work in progress), March 2013.

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

              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.


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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

              Shelby, Z., Krco, S., and C. Bormann, "CoRE Resource
              Directory", draft-shelby-core-resource-directory-05 (work
              in progress), February 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 multicast IP 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

   [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-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

   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

   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

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   o  Added guideline on use of core-block for minimizing response size

   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

   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

   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

   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-

   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)

   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

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

   o  Added details for IANA request for group communication multicast

   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 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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