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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 22, 2013                                 Philips Research
                                                        October 19, 2012


                      Group Communication for CoAP
                      draft-ietf-core-groupcomm-03

Abstract

   CoAP is a RESTful transfer protocol for constrained devices.  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
   defines 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 causes 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 22, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of



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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.


Table of Contents

   1.  Conventions and Terminology  . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Background . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.2.  Scope  . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.3.  Potential Solutions for Group Communication  . . . . . . .  4
   3.  CoAP Group Communication Based On IP Multicast . . . . . . . .  4
     3.1.  IP Multicast Background  . . . . . . . . . . . . . . . . .  5
     3.2.  CoAP Group Definition and Naming . . . . . . . . . . . . .  5
     3.3.  Group Discovery and Member Discovery . . . . . . . . . . .  6
       3.3.1.  DNS-SD . . . . . . . . . . . . . . . . . . . . . . . .  6
       3.3.2.  CoRE Resource Directory  . . . . . . . . . . . . . . .  7
     3.4.  Group Resource Manipulation  . . . . . . . . . . . . . . .  7
     3.5.  Configuring Group Membership In Endpoints  . . . . . . . .  8
     3.6.  Congestion Control . . . . . . . . . . . . . . . . . . . .  9
     3.7.  CoAP Multicast and HTTP Unicast Interworking . . . . . . . 10
   4.  Use Cases and Corresponding Protocol Flows . . . . . . . . . . 11
     4.1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . 11
     4.2.  Network Configuration  . . . . . . . . . . . . . . . . . . 12
     4.3.  Discovery of Resource Directory  . . . . . . . . . . . . . 14
     4.4.  Lighting Control . . . . . . . . . . . . . . . . . . . . . 15
   5.  Deployment Guidelines  . . . . . . . . . . . . . . . . . . . . 19
     5.1.  Target Network Topologies  . . . . . . . . . . . . . . . . 19
     5.2.  Multicast Routing  . . . . . . . . . . . . . . . . . . . . 19
     5.3.  Use of the Multicast Listener Discovery (MLD) protocol . . 19
     5.4.  6LoWPAN-Specific Guidelines  . . . . . . . . . . . . . . . 20
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   8.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 21
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 21
     10.2. Informative References . . . . . . . . . . . . . . . . . . 22
   Appendix A.  Multicast Listener Discovery (MLD)  . . . . . . . . . 24
   Appendix B.  CoAP-Observe Alternative to Group Communication . . . 24
   Appendix C.  Change Log  . . . . . . . . . . . . . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 27





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1.  Conventions and Terminology

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

   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 may or may not be
      part of the group.  The underlying mechanism for group
      communication is assumed to be multicast based.  The network where
      the group communication takes place can be either a constrained or
      a regular (un-constrained) network

   Multicast
      Sending a message to multiple destination nodes simultaneously.
      There are various options to implement multicast including layer 2
      (Media Access Control) or 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)
      Low power and Lossy Network (LLN): A type of constrained network
      where the devices are interconnected by a variety of low power,
      lossy links such as IEEE 802.15.4, Bluetooth, WiFi, wired or low
      power power-line communication links.


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 composed by function.  For example,



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   the group "all lights in building one" may consist of the groups "all
   lights on floor one of building one", "all lights on floor two of
   building one", etc.  Groups may be preconfigured or dynamically
   formed.  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

   In this draft, we address the issues related to CoAP group
   communication in detail, with use cases, recommended approaches and
   analysis of the impact to the CoAP protocol and to implementations.
   The guiding principle is to apply wherever possible existing IETF
   protocols to achieve group communication functionality.  In many
   cases the contribution of this document lies in explaining how
   existing mechanisms may be used to together fulfill CoAP group
   communication needs for specific use cases.

2.3.  Potential Solutions for Group Communication

   The classic concept of group communications is that of a single
   source distributing content to multiple destination recipients that
   are all part of a group.  Before content can be distributed, there is
   a separate process to form the group.  The source may be either a
   member or non-member of the group.

   Group communication solutions have evolved from "bottom" to "top",
   i.e., from layer 2 (Media Access Control broadcast/multicast) and
   layer 3 (IP multicast) to application layer group communication, also
   referred to as application layer multicast.  A study published in
   2005 [Lao05] identified new solutions in the "middle" (referred to as
   overlay multicast) that utilize an infrastructure based on proxies.

   Each of these classes of solutions may be compared [Lao05] using
   metrics such as link stress and level of host complexity
   [Banerjee01].  The results show for a realistic internet topology
   that IP Multicast is the most resource-efficient, with the downside
   being that it requires the most effort to deploy in the
   infrastructure.  IP Multicast is the solution adopted by this draft
   for CoAP group communication.


3.  CoAP Group Communication Based On IP Multicast






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3.1.  IP Multicast 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/mDNS) and
   carrier IPTV deployments.  The packet economy and minimal host
   complexity of IP multicast make it attractive for group communication
   in constrained environments.  Therefore IP multicast is the
   recommended underlying mechanism for CoAP group communication, and
   the approach assumed in this document.

   To achieve IP multicast beyond a subnet, an IP multicast routing
   protocol needs to be active on routers.  The RPL protocol [RFC6550]
   for example is able to route multicast traffic in constrained LLNs.
   While 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 on the network link on which it was sent.

3.2.  CoAP 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.  The group MAY have more
   than one associated IP multicast address.  An endpoint MAY be a
   member of multiple groups.  Group membership of an endpoint MAY
   dynamically change over time.  The group MAY be identified by a Group
   Name ([I-D.vanderstok-core-dna]) which is defined as a prefix string
   of a Group FQDN.  The Group FQDN can be uniquely mapped to a site-
   local or global multicast IP address via DNS resolution.

   A CoAP multicast request that addresses a group includes a Group URI
   as the request URI.  A Group URI has the scheme 'coap' and includes
   in the authority part either a group IP address or a hostname that
   can be resolved to the group IP address (e.g., a Group Name or Group
   FQDN).  Group URIs MUST follow the URI syntax [RFC3986].

   A CoAP node becomes a group member by listening for CoAP messages on
   the group's IP multicast address, assuming the default CoAP UDP port.
   Note that 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.



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   Examples of hierarchical group 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"

   Reverse mapping (from IP multicast address to group FQDN) is
   supported using the reverse DNS resolution technique
   ([I-D.vanderstok-core-dna]).

3.3.  Group Discovery and Member Discovery

   CoAP defines a resource discovery capability, but does not yet
   specify how to discover groups (e.g. find a group to join or send a
   multicast message to) or to discover members of a group (e.g. to
   address selected group members by unicast).  These topics are
   elaborated in more detail in [I-D.vanderstok-core-dna] including
   examples for using DNS-SD and CoRE Resource Directory.

3.3.1.  DNS-SD

   DNS-based Service Discovery [I-D.cheshire-dnsext-dns-sd] defines a
   conventional way to configure DNS PTR, SRV, and TXT records to enable
   enumeration of services, such as services offered by CoAP nodes, or
   enumeration of all CoAP nodes, within specified subdomains.  A
   service is specified by a name of the form
   <Instance>.<ServiceType>.<Domain>, where the service type for CoAP
   nodes is _coap._udp and the domain is a DNS domain name that
   identifies a group as in the examples above.  For each CoAP end-point
   in a group, a PTR record with the name _coap._udp and/or a PTR record
   with the name _coap._udp.<Domain> is defined and it points to an SRV
   record having the <Instance>.<ServiceType>.<Domain> name.

   All CoAP nodes in a given subdomain may be enumerated by sending a
   query for PTR records named _coap._udp to the authoritative DNS
   server for that zone.  A list of SRV records is returned.  Each SRV
   record contains the port and host name (AAAA record) of a CoAP node.
   The IP address of the node is obtained by resolving the host name.
   DNS-SD also specifies an optional TXT record, having the same name as
   the SRV record, which can contain "key=value" attributes.  This can
   be used to store information about the device, e.g. schema=DALI,
   type=switch, group=lighting.bldg6, etc.




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   Another feature of DNS-SD is the ability to specify service subtypes
   using PTR records.  For example, one could represent all the CoAP
   groups in a subdomain by PTR records with the name
   _group._sub._coap._udp or alternatively
   _group._sub._coap._udp.<Domain>.

3.3.2.  CoRE Resource Directory

   CoRE Resource Directory [I-D.shelby-core-resource-directory] defines
   the concept of a Resource Directory (RD) server where CoAP servers
   can register their resources offered and CoAP clients can discover
   these resources by querying the RD server.  RD syntax can be mapped
   to DNS-SD syntax and vice versa [I-D.lynn-core-discovery-mapping],
   such that the above approach can be reused for group discovery and
   group member discovery.

   Specifically, the Domain (d) parameter can be set to the group URI by
   an end-point registering to the RD.  If an end-point wants to join
   multiple groups, it has to repeat the registration process for each
   group it wants to join.

3.4.  Group Resource Manipulation

   Group communications SHALL only be used for idempotent methods (i.e.
   CoAP GET, PUT, 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
   [I-D.ietf-core-coap].  The unicast responses 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 communications 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 can not
   readily find out which group members did not receive it.

   All nodes in a given group SHOULD receive the same request with high
   probability.  This will not be the case if there is diversity in the
   authority port (i.e. a diversity of dynamic port addresses across the
   group) or if the targeted resource is located at different paths on
   different nodes.

   Therefore, some measures must be present to ensure uniformity in port
   number and resource names/locations within a group.  This document
   proposes the following measures:



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   o  All CoAP multicast requests MUST be sent either to the default
      CoAP UDP port (i.e. default Uri-Port as defined in
      [I-D.ietf-core-coap]), or to a port number obtained via a service
      discovery lookup operation as a valid CoAP port for the targeted
      multicast group.

   o  All CoAP multicast requests SHOULD operate only on URIs (links)
      which were retreived either from a "/.well-known/core" lookup on
      at least one group member node, or from an equivalent service
      discovery lookup which returns either the resources supported by
      all group members or resources supported by one particular group
      member.

3.5.  Configuring Group Membership In Endpoints

   In some use cases, the group membership of endpoints needs to be
   configurable after the network has been deployed.  Example use cases
   can be found in building control: a commissioning tool determines to
   which groups a light or sensor node belongs, and writes this
   information to all nodes, which can subsequently join the correct
   group.

   To achieve smoother interoperability between nodes/endpoints from
   different manufacturers, it is proposed here to define an OPTIONAL
   standardized RESTful means of configuring CoAP endpoints with
   relevant group information.

   CoAP endpoints implementing this mechanism MUST support a
   discoverable "Group Configuration" resource of resource type (rt)
   [RFC6690] "core.gp".  This resource (and perhaps its sub-resources,
   TBD) are used to manage group membership.  Three design options for
   this mechanism are presented here as a placeholder (TBD).

   Design 1: use CoRE link format payloads to communicate group
   membership to endpoints.  (TBD Not clear how this should be
   realized.)

   Design 2: use a JSON type resource.  For example, a GET on the
   "core.gp" resource returns a JSON array of group objects.

      Req: GET /gp
      Res: 2.05 Content (Content-Format: application/json)
      [ { "n": "Room-A-Lights.floor1.west.bldg6.example.com",
          "ip": "ff05::4200:f7fe:ed37:14ca" }
      ]

   where "n" defines the Group FQDN and "ip" defines the associated
   multicast IP address.  As a next example, the same endpoint can be



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   added to another group by a POST on the resource with a JSON group
   object:

      Req: POST /gp (Content-Format: application/json)
      { "n": "floor1.west.bldg6.example.com",
        "ip": "ff05::4200:f7fe:ed37:14cb" }
      Res: 2.04 Changed

   Design 3: define named sub-resources, each sub-resource representing
   a group membership.  The payload of the sub-resource may be JSON or a
   simple pre-defined format.  Or alternatively, information is provided
   via POST with query parameters.

3.6.  Congestion Control

   Multicast CoAP requests may result in a multitude of replies from
   different nodes, potentially causing congestion.  Therefore sending
   multicast requests should be conservatively controlled.

   The base CoAP draft [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).

   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 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.

   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. for an LLN that could mean 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].




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   o  Preferably IP multicast with link-local scope should be used,
      rather than global or site-local.

   o  The Hop Limit field in the IPv6 packet should be chosen as low as
      possible (if the CoAP/IP stack allows setting of this value.  TBD
      - discuss whether this guideline is relevant/realistic in CoAP
      context)

3.7.  CoAP Multicast and HTTP Unicast Interworking

   CoAP supports operation over UDP multicast, while HTTP does not.  For
   use cases where it is required that CoAP group communication is
   initiated from an HTTP end-point, it would be advantageous if the
   HTTP-CoAP Proxy supports mapping of HTTP unicast to CoAP group
   communication based on IP multicast.  One possible way of operation
   of such HTTP-CoAP Proxy is illustrated in Figure 1.  Note that this
   topic is covered in more detail in
   [I-D.castellani-core-advanced-http-mapping].


           CoAP    Mcast    CoAP    Mcast   HTTP-CoAP           HTTP
          Node 1   Rtr1    Node 2    Rtr2    Proxy             Node 3
            |       |         |       |       |                   |
            |MLD REQUEST      |       |       |                   |
            |(Join Group X)   |       |       |                   |
            |--LL-->|         |       |       |                   |
            |       |         |MLD REQUEST    |                   |
            |       |         |(Join Group X) |                   |
            |       |         |--LL-->|       |                   |
            |       |         |       |       |  HTTP REQUEST     |
            |       |         |       |       |    (URI to        |
            |       |         |       |       |   unicast addr)   |
            |       |         |       |       |< -----------------|
            |       |         |       |       |                   |
            |       |         |   Resolve HTTP Request-Line URI   |
            |       |         |   to Group X multicast address    |
            |       |         |       |       |                   |
            | CoAP REQUEST (to multicast addr)|                   |
            |< ------<---------<-------<------|                   |
            |       |         |       |       |                   |
            |                 |               |                   |
            |     (optional) CoAP RESPONSE(s) |                   |
            |                 |-------------->|                   |
            |-----------------|-------------->|   Aggregated      |
            |                 |               |  HTTP RESPONSE    |
            |                 |               |------------------>|
            |                 |               |                   |




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          Figure 1: CoAP Multicast and HTTP Unicast Interworking

   Note that Figure 1 illustrates the case of IP multicast as the
   underlying group communications mechanism.  MLD denotes the Multicast
   Listener Discovery protocol ([RFC3810], Appendix A) and LL denotes a
   Link-Local multicast.

   A key point in Figure 1 is that the incoming HTTP Request (from node
   3) will carry a Host request-header field that resolves in the
   general Internet to the proxy node.  At the proxy node, this hostname
   and/or the Request-Line URI will then possibly be mapped (as detailed
   in [I-D.castellani-core-http-mapping]) and again resolved (with the
   CoAP scheme) to an IP multicast address.  This may be accomplished,
   for example, by using DNS or DNS-SD (Section 3.3).  The proxy node
   will then IP multicast the CoAP Request (corresponding to the
   received HTTP Request) via multicast routers to the appropriate nodes
   (i.e. nodes 1 and 2).

   In terms of the HTTP Response, Figure 1 illustrates that it will be
   generated by the proxy node based on aggregated responses of the CoAP
   nodes and sent back to the client in the general Internet that sent
   the HTTP Request (i.e. node 1).  In
   [I-D.castellani-core-advanced-http-mapping] the HTTP Response that
   the Proxy may use to aggregate multiple CoAP responses is described
   in more detail.  So in terms of overall operation, the CoAP proxy can
   be considered to be a "non-transparent" proxy according to [RFC2616].
   Specifically, [RFC2616] states that a "non-transparent proxy is a
   proxy that modifies the request or response in order to provide some
   added service to the user agent, such as group annotation services,
   media type transformation, protocol reduction or anonymity
   filtering."

   An alternative to the above is using a Forward Proxy.  In this case,
   the CoAP request URI is carried in the HTTP Request-Line (as defined
   in [I-D.ietf-core-coap] Section 10.2) in a HTTP request sent to the
   IP address of the Proxy.


4.  Use Cases and Corresponding Protocol Flows

4.1.  Introduction

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

   o  Discovery of Resource Directory: discovering the local CoRE RD
      which contains links (URIs) to resources stored on other servers
      [RFC6690].



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   o  Lighting Control: synchronous operation of a group of 6LoWPAN
      [RFC4944] IPv6-connected lights

   o  Parameter Update: updating parameters/settings simultaneously in a
      large group of devices in a building/campus control
      ([I-D.vanderstok-core-bc]) application --- TBD

   o  Firmware Update: efficiently updating firmware simultaneously in a
      large group of devices in a building/campus control
      ([I-D.vanderstok-core-bc]) application --- TBD suggests a
      multicast extension of core-block.

   o  Group Status Report: requesting status information or event
      reports from a group of devices in a building/campus control
      application --- TBD, may require reliable group communication to
      be feasible.

4.2.  Network Configuration

   We assume the following network configuration for all the use cases
   as shown in Figure 2:

   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 6LoWPAN subnets.

   o  Light-1 and the Light Switch are connected to a router (Rtr-1)
      which is also a CoAP Proxy, a CoAP Resource Directory (RD) and a
      6LoWPAN Border Router (6LBR).

   o  Light-2 and the Light-3 are connected to another router (Rtr-2)
      which is also a CoAP Proxy, a CoAP RD and a 6LBR.

   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 6LBRs 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
      work.











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                                                                 Network
                                                                Backbone
                                                                       |
      ################################################                 |
      #                                       Room-A #                 |
      #         **********************               #                 |
      #       **  LoWPAN-1 (subnet-1) **             #                 |
      #     *                            *           #                 |
      #    *     +----------+             *          #                 |
      #   *      |  Light   |-------+      *         #                 |
      #  *       |  Switch  |       |       *        #                 |
      #  *       +----------+  +---------+  *        #                 |
      #  *                     |  Rtr-1  |-----------------------------|
      #  *                     +---------+  *        #                 |
      #  *       +----------+        |      *        #                 |
      #   *      |  Light-1 |--------+     *         #                 |
      #    *     +----------+             *          #                 |
      #     *                            *           #                 |
      #       **                      **             #                 |
      #         **********************               #                 |
      #                                              #                 |
      #                                              #                 |
      #        **********************                #                 |
      #       **  LoWPAN-2 (subnet-2) **             #                 |
      #     *                            *           #                 |
      #    *     +----------+             *          #                 |
      #   *      |  Light-2 |-------+      *         #                 |
      #  *       |          |       |       *        #                 |
      #  *       +----------+  +---------+  *        #                 |
      #  *                     |  Rtr-2  |-----------------------------|
      #  *                     +---------+  *        #                 |
      #  *       +----------+        |      *        #                 |
      #   *      |  Light-3 |--------+     *         #                 |
      #    *     +----------+             *          #                 |
      #     *                            *           #                 |
      #       **                      **             #                 |
      #         **********************               #                 |
      #                                              #                 |
     #################################################                 |
                                                                       |
                                           +--------+                  |
                                           |  DNS   |------------------|
                                           | Server |
                                           +--------+


            Figure 2: Network Topology of a Large Room (Room-A)




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4.3.  Discovery of Resource Directory

   The protocol flow for discovery of a RD for the given network (of
   Figure 2) is shown in Figure 3:

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

   o  Light-2 will then search for the local RD (RD-2) by sending out a
      GET request (with the "/.well-known/core?rt=core.rd" request URI)
      to the site-local "All CoAP Nodes" address.  In this case, the
      site is assumed to include all nodes in the subnet.

   o  This multicast message will then go to each node in subnet-2.
      However, only Rtr-2 (RD-2) will respond because the GET is
      qualified by the query string "?rt=core.rd".

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

   The 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.





















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



       Figure 3: Resource Directory Discovery via Multicast Message

4.4.  Lighting Control

   The protocol flow for a building automation lighting control scenario
   for the network (Figure 2) is shown in sequence in Figure 4,
   Figure 5, and Figure 6.  We assume the following steps occur before
   the illustrated flow:

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

   o  2) Commissioning phase (by applications): The IP multicast address
      of the group (Room-A-Lights) has been set in all the Lights.  The
      URI of the group (Room-A-Lights) has been set in the Light Switch.

   o  3) The indicated MLD Report messages are link-local multicast.  In
      each LoWPAN, it is assumed that a multicast routing/forwarding
      protocol in 6LRs will then propagate the Join information
      contained in the MLD Report over multiple hops to the 6LBR.








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


                Figure 4: Joining Lighting Groups Using MLD























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                                    Light      Rtr-1     Rtr-2   Network
   Light-1   Light-2    Light-3     Switch    (CoAP     (CoAP   Backbone
    |          |          |          |         Proxy)    Proxy)       |
    |          |          |          |          |          |          |
    |          |          ***********************          |          |
    |          |          *   User flips on     *          |          |
    |          |          *   light switch to   *          |          |
    |          |          *   turn on all the   *          |          |
    |          |          *   lights in Room A  *          |          |
    |          |          ***********************          |          |
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    |          |          | COAP NON (PUT       |          |          |
    |          |          |           Proxy-URI |          |          |
    |          |          |           URI for Room-A-Lights           |
    |          |          |           Payload=turn on lights)         |
    |          |          |          |--------->|          |          |
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    |          |          |          |     Request DNS resolution of  |
    |          |          |          |     URI for Room-A-Lights      |
    |          |          |          |          |-------------------->|
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    |          |          |          |     DNS returns: AAAA          |
    |          |          |          |     Group (Room-A-Lights)      |
    |          |          |          |     IPv6 multicast address     |
    |          |          |          |          |<--------------------|
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    |          |          |          COAP NON (Put                    |
    |          |          |          |         URI Path               |
    |          |          |          |         Payload=turn on lights)|
    |          |          |          |    Destination IP Address =    |
    |          |          |          |       IP multicast address     |
    |          |          |          |       for Group (Room-A-Lights)|
    |          |          |          |    Originating IP Address =    |
    |          |          |          |        RTR-1                   |
    |          |          |          |          |-------------------->|
    |<------------------------------------------|          |          |
    |          |          |          |          |          |          |
    |          |          |          |          |          |<---------|
    |          |<---------|<-------------------------------|          |
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |


           Figure 5: Sending Lighting Control Multicast Message



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                                    Light      Rtr-1     Rtr-2   Network
   Light-1   Light-2    Light-3     Switch    (CoAP     (CoAP   Backbone
    |          |          |          |         Proxy)    Proxy)       |
    |          |          |          |          |          |          |
    ***********************          |          |          |          |
    *   Lights in Room-A  *          |          |          |          |
    *   turn on (nearly   *          |          |          |          |
    *   simultaneously)   *          |          |          |          |
    ***********************          |          |          |          |
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    |     COAP NON (2.04 Changed)    |          |          |          |
    |------------------------------------------>|          |          |
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    |          COAP NON (2.04 Changed)          |          |          |
    |          |------------------------------->|          |          |
    |          |          |          |          |          |          |
    |          |          |          |          |          |          |
    |          |        COAP NON (5.00 Internal Server Error)         |
    |          |          |-------------------->|          |          |
    |          |          |          |          |          |          |
    |          |          |          ******************************   |
    |          |          |          *      Rtr-1 as CoAP Proxy   *   |
    |          |          |          *  |   sends the individual  *   |
    |          |          |          *  |   responses back to     *   |
    |          |          |          *  v   originator            *   |
    |          |          |          ******************************   |
    |          |          |          |          |          |          |
    |          |          |    COAP NON (2.04 Changed)     |          |
    |          |          |          |<---------|          |          |
    |          |          |          |          |          |          |
    |          |          |    COAP NON (2.04 Changed)     |          |
    |          |          |          |<---------|          |          |
    |          |          |          |          |          |          |
    |          |          |    COAP NON (5.00 Internal Server Error)  |
    |          |          |          |<---------|          |          |
    |          |          |          |          |          |          |


     Figure 6: Sending Lighting Control Response to Multicast Message

   NOTE: In the last step of Figure 6, Rtr-1 acting as CoAP proxy,
   returns multiple individual CoAP responses to the client.  Each
   response echoes the Token of the client's request.  The client can
   identify the original source of each response by ...TBD.





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5.  Deployment Guidelines

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

5.1.  Target Network Topologies

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

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

5.2.  Multicast Routing

   If a network (segment) requires multiple IP hops to reach certain
   nodes, a multicast routing protocol is required to propagate
   multicast UDP packets to these nodes.  Examples of routing/forwarding
   protocols specifically for LLNs, able to route multicast, are RPL
   (Section 12 of [RFC6550]) and Trickle Multicast Forwarding
   [I-D.ietf-roll-trickle-mcast].

5.3.  Use of the Multicast Listener Discovery (MLD) protocol

   CoAP nodes that are IP hosts (not routers) are generally unaware of
   the specific multicast routing protocol being used.  When such a host
   needs to join a specific (CoAP) multicast group, it usually requires
   a way to signal to the 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 addition of MLD in LLN deployments, all
   nodes can be configured as multicast routers.




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5.4.  6LoWPAN-Specific Guidelines

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

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

   o  Filtering in 6LBRs based on information from the routing 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

   As defined in [I-D.ietf-core-coap], CoAP group communications based
   on IP multicast must use the following security approach:

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

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





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   o  Group communications MUST NOT use IPSec.

   A consequence is that CoAP group communications is vulnerable to all
   attacks mentioned in [I-D.ietf-core-coap] for the NoSec mode.  For
   sensitive data or safety-critical control, appropriate link-layer
   security or application-level object security SHOULD be used instead
   of DTLS security.


7.  IANA Considerations

   No request is made to IANA.  (Note: The required multicast address
   request to IANA is made in [I-D.ietf-core-coap]).


8.  Conclusions

   IP multicast as outlined in Section 3 is recommended to be adopted as
   the base solution for CoAP Group Communication for situations where
   the use case and network characteristics allow use of IP multicast.
   This approach requires no standards changes to the IP multicast suite
   of protocols and it provides interoperability with IP multicast group
   communication on un-constrained backbone networks.


9.  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.


10.  References

10.1.  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.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform



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              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.

   [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.

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

10.2.  Informative References

   [I-D.cheshire-dnsext-dns-sd]
              Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", draft-cheshire-dnsext-dns-sd-11 (work in
              progress), December 2011.

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




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

   [I-D.vanderstok-core-bc]
              Stok, P. and K. Lynn, "CoAP Utilization for Building
              Control", draft-vanderstok-core-bc-05 (work in progress),
              October 2011.

   [I-D.lynn-core-discovery-mapping]
              Lynn, K. and Z. Shelby, "CoRE Link-Format to DNS-Based
              Service Discovery Mapping",
              draft-lynn-core-discovery-mapping-01 (work in progress),
              July 2011.

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

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

   [I-D.ietf-roll-trickle-mcast]
              Hui, J. and R. Kelsey, "Multicast Forwarding Using
              Trickle", draft-ietf-roll-trickle-mcast-01 (work in
              progress), July 2012.

   [Lao05]    Lao, L., Cui, J., Gerla, M., and D. Maggiorini, "A
              Comparative Study of Multicast Protocols: Top, Bottom, or
              In the Middle?", 2005, <http://www.cs.ucla.edu/NRL/hpi/
              AggMC/papers/comparison_gi_2005.pdf>.

   [Banerjee01]
              Banerjee, B. and B. Bhattacharjee, "A Comparative Study of
              Application Layer Multicast Protocols", 2001, <http://
              wmedia.grnet.gr/P2PBackground/



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              a-comparative-study-ofALM.pdf>.


Appendix A.  Multicast Listener Discovery (MLD)

   In order to extend the scope of IP multicast beyond link-local scope,
   an IP multicast routing 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
   present.

   [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.  CoAP-Observe Alternative to Group Communication

   The CoAP Observation extension [I-D.ietf-core-observe] can be used as
   a simple (but very limited) alternative for group communication.  A
   group in this case consists of a CoAP server hosting a specific
   resource, plus all CoAP clients observing that resource.  The server
   is the only group member that can send a group message.  It does this
   by modifying the state of a resource under observation and
   subsequently notifying its observers of the change.  Serial unicast
   is used for sending the notifications.  This approach can be a simple



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   alternative for networks where IP multicast is not available or too
   expensive.

   The CoAP-Observe approach is unreliable in the sense that, even
   though Confirmable CoAP messages may be used, there are no guarantees
   that an update will be received.  For example, a client may believe
   it is observing a resource while in reality the server rebooted and
   lost its listener state.


Appendix C.  Change Log

   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).

   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 readibility

   Changes from ietf-01 to ietf-02:

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



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   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
      [I-D.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 readibility

   o  Changelog added

   Changes from ietf-00 to ietf-01:

   o  Moved CoAP-observe solution section to section 2

   o  Editorial changes

   o  Moved security requirements into requirements section

   o  Changed multicast POST to PUT in example use case

   o  Added CoAP responses in example use case

   Changes from rahman-07 to ietf-00:

   o  Editorial changes

   o  Use cases section added

   o  CoRE Resource Directory section added





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   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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