Group Communication for CoAP
draft-ietf-core-groupcomm-05

Abstract

CoAP is a RESTful transfer protocol for constrained devices and networks. It is anticipated that constrained devices will often naturally operate in groups (e.g. in a building automation scenario all lights in a given room may need to be switched on/off as a group). This document 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 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/.

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This Internet-Draft will expire on October 13, 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. CoAP Group Communication Based on IP Multicast
• 3.1. IP Multicast Routing Background
• 3.2. Group Definition and Naming
• 3.3. Port Configuration
• 3.4. Group Discovery and Member Discovery
• 3.5. Group Resource Manipulation
• 3.6. Multicast Request Acceptance and Response Suppression
• 3.7. Configuring Group Membership In Endpoints
• 3.8. Congestion Control
• 3.9. 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
• 5. Deployment Guidelines
• 5.1. Target Network Topologies
• 5.2. Advertising Membership of Multicast Groups
• 5.2.1. Using the Multicast Listener Discovery (MLD) 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
• 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].

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

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

Group Communication

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

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

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 preconfigured 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

This document describes how to use the CoAP protocol in a group communication context with IP Multicast running underneath CoAP. No changes to either CoAP or IP Multicast are required for this purpose. However, proper operation of group communication does require judicious use of these and a variety of other IETF protocols. The main contribution of this document lies in explaining how various IETF mechanisms may be used to fulfill CoAP group communication needs for specific use cases and deployments.

3. CoAP Group Communication Based on IP Multicast

3.1. IP Multicast Routing Background

IP Multicast routing protocols have been evolving for decades, resulting in proposed standards such as Protocol Independent Multicast - Sparse Mode (PIM-SM) [RFC4601]. Yet, due to various technical and marketing reasons, IP Multicast routing is not widely deployed on the general Internet. However, IP Multicast is very popular in specific deployments such as in enterprise networks (e.g. for video conferencing), smart home networks (e.g. UPnP) and carrier IPTV deployments. The packet economy and minimal host complexity of IP multicast make it attractive for group communication in constrained environments. 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 or forwarding protocol needs to be active on IP routers. An examples of a routing protocol specifically for LLNs is RPL (Section 12 of [RFC6550]) and an example of a forwarding protocol for LLNs is MPL [I-D.ietf-roll-trickle-mcast]. PIM-SM [RFC4601] is often used for multicast routing in un-constrained networks.

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

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

3.2. Group Definition and Naming

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

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

  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 multicast IP address via DNS resolution (if supported). Some examples of hierarchical Group FQDN naming (and scoping) for a building control application are shown below ([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 Configuration

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

Group communications 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 end-points. 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 preconfigured port number, if available. The pre-configuration mechanism MUST ensure that the same port number is preconfigured 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.

3.4. Group Discovery and 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.

3.5. Group Resource Manipulation

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

1. Preconfigured URI paths, if available. The pre-configuration mechanism MUST ensure that these URIs are preconfigured 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 service discovery, it uses URI paths which were retrieved from a "/.well-known/core" lookup on at least one group member endpoint; where the selected URI paths are known from application knowledge to be available in all 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.

3.6. Multicast Request Acceptance and Response Suppression

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

1. Multicast request acceptance; in what cases a request is accepted and executed, and when not.
2. Multicast response suppression; in what cases the response of an executed request is returned to the requesting endpoint, and when not.

This section aims to first summarize these normative requirements and then present guidelines, for a number of multicast example applications, in what way the request suppression and response suppression should be configured.

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. The case that an IP stack interface cannot provide this indication, is the exception case for the RECOMMENDED behaviours listed below. In that case, only response suppression (aspect 2.) can be supported for selected resources which are known (through application knowledge) and configured to be used for multicast requests.

For aspect 1 (request acceptance), the requirements are: Section 6.3 for examples of multicast boundary limiting methods.

• 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].
• 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.)

Regarding the first requirement see

For aspect 2 (response suppression), the requirements 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). This optional response suppression will be illustrated by some use cases in the remainder of this section.

The above response suppression requirements are complemented by the following guidelines in this document. CoAP servers should preferably implement configurable response suppression, enabling at least the following configuration items 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.

Below a number of group communication example applications are mentioned and in what way these could typically make use of response suppression as defined by the above four configuration items.

• CoAP resource discovery: suppression of 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).

3.7. Configuring Group Membership In Endpoints

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

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

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

To achieve smoother interoperability between nodes/endpoints from different manufacturers, an OPTIONAL RESTful method of configuring CoAP endpoints with relevant group information is specified here. This approach MUST use unicast methods (GET/PUT/POST) only as it is a method of configuring group information in individual endpoints. Using multicast operations in this situation may lead to unexpected (possibly circular) behavior in the network.

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

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

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

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

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.

The base CoAP draft [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 3.6 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 3.6 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. Exceptions

Group communication using IP multicast offers improved network efficiency and latency amongst other benefits. However, group communications may not always be possible to implement in a given network. The primary reason for this will be if IP multicast is not supported in the network. For example, in a LLN, if the RPL protocol is used and set to "Non-storing mode" [RFC6550] there will be no IP multicast routing in that network beyond link-local scope. This means that any CoAP group communications above link-local scope will not be supported in that 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: discovering the local CoRE RD which contains links (URIs) to resources stored on other 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) which is also a CoAP Resource Directory (RD).
• Light-2 and the Light-3 are connected to another router (Rtr-2) which is also a CoAP RD.
• The routers are connected to an IPv6 network backbone which is also multicast enabled. In the general case, this means the network backbone and Rtr-1/Rtr-2 support a PIM based multicast routing protocol, and MLD for forming groups. In a limited case, if the network backbone is one link, then the routers only have to support MLD-snooping (Appendix A) for the following use cases to work.
• The DNS server is optional. If the server is there 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 hardcoded into devices).

          

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

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

          
                                 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---->|
 |          |          |          |          |          |          |
 |          |          |          |          |          |          |