Group Communication for CoAP
draft-ietf-core-groupcomm-08

Abstract

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

Status of This Memo

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

Internet-Drafts are working documents of the Internet Engineering 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 November 28, 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 (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
• 2. Introduction
• 2.1. Background
• 2.2. Scope
• 3. Protocol Considerations
• 3.1. IP Multicast Background
• 3.2. Group Definition and Naming
• 3.3. Port and URI Configuration
• 3.4. Group Methods
• 3.5. Group Member Discovery
• 3.6. Configuring Group Membership in Endpoints
• 3.7. Multicast Request Acceptance and Response Suppression
• 3.8. Congestion Control
• 3.9. Proxy Operation
• 3.10. Exceptions
• 4. Use Cases and Corresponding Protocol Flows
• 4.1. Introduction
• 4.2. Network Configuration
• 4.3. Discovery of Resource Directory
• 4.4. Lighting Control
• 4.5. Lighting Control in MLD Enabled Network
• 4.6. Commissioning the Network Based On Resource Directory
• 5. Deployment Guidelines
• 5.1. Target Network Topologies
• 5.2. Advertising Membership of Multicast Groups
• 5.2.1. Using the MLD Listener Protocol
• 5.2.2. Using the RPL Routing Protocol
• 5.2.3. Using the MPL Forwarding Protocol
• 5.3. 6LoWPAN-Specific Guidelines
• 6. Security Considerations
• 6.1. Security Configuration
• 6.2. Threats
• 6.3. Threat Mitigation
• 6.3.1. WiFi Scenario
• 6.3.2. 6LoWPAN Scenario
• 6.3.3. Future Evolution
• 7. IANA Considerations
• 7.1. New 'core.gp' Resource Type
• 7.2. New 'coap-group+json' Internet Media Type
• 8. Acknowledgements
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Appendix A. Multicast Listener Discovery (MLD)
• Appendix B. Change Log
• Authors' Addresses

1. Conventions and Terminology

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

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

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

Group Communication

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

Multicast

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

IP Multicast

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

Low power and Lossy Network (LLN)

A type of constrained IP network where devices are interconnected by a variety of low-power and lossy links (such as IEEE 802.15.4, Bluetooth Low Energy (BLE), Digital Enhanced Cordless Telecommunication (DECT)) or lossy links (such as IEEE P1901.2 power-line communication).

2. Introduction

2.1. Background

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

2.2. Scope

Group communication involves 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 Background

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

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

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

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

3.2. Group Definition and Naming

A group is defined as a set of CoAP endpoints, where each endpoint is configured to receive 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 (e.g., Group Fully Qualified Domain Name (FQDN)) plus optional port number that can be resolved to the group IP multicast address. 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 as the default for the Group URI.

  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"
    

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

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

3.3. Port and URI Configuration

A CoAP group member listens for CoAP messages on the group's IP multicast address, on a specified UDP port. The default UDP port is the CoAP default port of 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 (5683).

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

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 members.

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

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

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

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

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

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

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

The group configuration resource has a JSON-based content format (as indicated by the media type). A (unicast) GET on a resource accepting this format returns a JSON array of group objects, each group object formatted as shown below:

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

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

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

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

3.7. Multicast Request Acceptance and Response Suppression

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

1. Multicast request acceptance - in which cases a 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 multicast.

For multicast request acceptance, the behaviors are:

• 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.
• 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.
• 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.)
• Otherwise accept the multicast request.

For multicast response suppression, the behaviors are:

• A server SHOULD NOT respond to a multicast discovery request if the filter specified by the request's query string does not match.
• A server MAY choose not to respond to a multicast request, if there's nothing useful to respond (e.g., error or empty response).
• 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.
• 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:

• Suppression of all 2.xx success responses;
• Suppression of all 4.xx client errors;
• Suppression of all 5.xx server errors;
• Suppression of all 2.05 responses with empty payload.

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

• CoAP resource discovery: Suppress 2.05 responses with empty payload and all 4.xx and 5.xx errors.
• Lighting control: Suppress all 2.xx responses after a lighting change command.
• 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.
• 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.
• 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:

• A server MAY choose not to respond to a multicast request if there's nothing useful to respond (e.g., error or empty response). See Section 3.7 for more detailed guidelines on response suppression.
• A server SHOULD limit the support for multicast requests to specific resources where multicast operation is required.
• A multicast request MUST be Non-Confirmable.
• A response to a multicast request SHOULD be Non-Confirmable (Section 5.2.3).
• 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.
• 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:

• 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.
• 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].
• Alternatively a server can also minimize the payload length of a response to a multicast GET (e.g., on "/.well-known/core") using CoAP blockwise transfers [I-D.ietf-core-block], returning only a first block of the link format description.
• A client should always aim to use IP multicast with link-local scope if possible. If this is not possible, then site-local scope IP multicast should be considered. If this is not possible, then global scope IP multicast should be considered as a last resort only.

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

• 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.
• 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

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

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

• The CoAP Proxy MUST be explicitly configured (whitelist) to allow proxied multicast requests by specific client(s).
• 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.)
• 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 (fully) supported in the network. For example, assume an LLN where the MPL protocol is not supported, but the RPL protocol is used used for routing multicast packets. Also assume the RPL routers operate in "Non-storing mode" [RFC6550]. Then there will be no IP multicast routing in this network beyond link-local scope. This means that any CoAP group communication above link-local scope will not be supported in this network.

4. Use Cases and Corresponding Protocol Flows

4.1. Introduction

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

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

4.2. Network Configuration

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

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

Both configurations are further specified by the following:

• 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.
• Light-1 and the Light Switch are connected to a router (Rtr-1).
• Light-2 and the Light-3 are connected to another router (Rtr-2).
• The routers are connected to an IPv6 network backbone which is also multicast enabled. In the general case, this means the network backbone and Rtr-1/Rtr-2 support a PIM based multicast routing protocol, and Multicast Listener Discovery (MLD) for forming groups. In a limited case, 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.
• A CoAP RD is connected to the network backbone.
• 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).
• A Controller (client) is connected to the backbone, which is able to control various building functions including lighting.

          
          
          
          
          
          
          
          
          
          
          
          
          

                                                             Network
                                                            Backbone
  ################################################                 |
  #         **********************        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) |-----------------+
                                    +------------+
          
        

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

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

          
                                 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)         |
 |          |          |------------------------------->|          |
 |          |          |          |          |          |--------->|
 |          |          |          |          |<--------------------|
 |          |          |          |<---------|          |          |
 |          |          |          |          |          |          |
          
        

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

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