CoRE Working Group A. Rahman, Ed.
Internet-Draft InterDigital Communications, LLC
Intended status: Informational E.O. Dijk, Ed.
Expires: June 23, 2013 Philips Research
December 20, 2012

Group Communication for CoAP
draft-ietf-core-groupcomm-04

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 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

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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 June 23, 2013.

Copyright Notice

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

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Table of Contents

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) 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 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, 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

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 together to fulfill CoAP group communication needs for specific use cases and deployments.

3. CoAP Group Communication Based On IP Multicast

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) 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.

A CoAP group member listens 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.

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 or a hostname 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"
    

The Group FQDN 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]):

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 [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.

3.4. 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 3.6. 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. The following are recommended measures:

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 IP multicast group.

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.

CoAP endpoints implementing this mechanism MUST support at least one discoverable "Group Configuration" resource of resource type (rt) [RFC6690] "core.gp". 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": "ff05::4200:f7fe:ed37:14ca" }
   ]   
	
   Req: POST /gp (Content-Format: application/json)
   { "n": "floor1.west.bldg6.example.com",
     "ip": "ff05::4200:f7fe:ed37:14cb" }
   Res: 2.04 Changed   

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

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:

Additional guidelines to reduce congestion risks are:

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:

4.2. Network Configuration

To illustrate all 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:

          

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

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

4.3. Discovery of Resource Directory

The protocol flow for discovery of a RD for the given network (of Figure 1) is shown in Figure 2:

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.

          
                                 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") |          |
 |          |<------------------------------------------|          |
 |          |          |          |          |          |          |
 
          
        

Figure 2: Resource Directory Discovery via Multicast Message

4.4. Lighting Control

The protocol flow for a building automation lighting control scenario for the network (Figure 1) in 6LoWPAN configuration is shown in sequence in Figure 3 for the case that the CoAP servers in each Light are configured to not generate a CoAP response to lighting control CoAP multicast requests. (Following section 8.2 of [I-D.ietf-core-coap], a server MAY choose not to generate a response to a multicast request.)

In addition, Figure 4 shows an additional protocol flow example for the case that servers do respond to a lighting control multicast request. There are two success responses and one 5.00 error response. In this particular use case the Light Switch does not check, based on the responses, that all Lights in the group actually received the multicast request, because it is not configured with an exhaustive list of IP addresses of all Lights belonging to the group. However, based on received error responses it could take additional action such as logging a fault in its log or alerting the user via its LCD display.

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

  1. Startup phase: 6LoWPANs are formed. IPv6 addresses assigned to all devices. The CoAP network is formed.
  2. Network configuration (application-independent): 6LBRs are configured with multicast address blocks to filter out or to pass through to/from the 6LoWPAN.
  3. Commissioning phase (application): 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.

Note for the Commissioning phase: the switch's software supports sending unicast, multicast or proxied unicast/multicast CoAP requests, including processing of the multiple responses that may be generated by a multicast CoAP request.

          
                                 Light                        Network
Light-1   Light-2    Light-3     Switch    Rtr-1      Rtr-2  Backbone
 |          |          |          |          |          |          |
 |          |          |          |          |          |          |
 |          |          ***********************          |          |
 |          |          *   User flips on     *          |          |
 |          |          *   light switch to   *          |          |
 |          |          *   turn on all the   *          |          |
 |          |          *   lights in Room A  *          |          |
 |          |          ***********************          |          |
 |          |          |          |          |          |          |
 |          |          |    ______|______    |          |          |
 |          |          |    COAP NON (PUT    |          |          |
 |          |          |    Destination IP Address =    |          |
 |          |          |    IP multicast address        |          |
 |          |          |    for Group (Room-A-Lights)   |          |
 |          |          |    Payload=lights on)          |          |
 |<-------------------------------+--------->|          |          |
 ON         |          |          |          |-------------------->|
 |          |          |          |          |          |<---------|
 |          |<---------|<-------------------------------|          |
 |          ON         ON         |          |          |          |
 ^          ^          ^          |          |          |          |
 ***********************          |          |          |          |
 *   Lights in Room-A  *          |          |          |          |
 *   turn on (nearly   *          |          |          |          |
 *   simultaneously)   *          |          |          |          |
 ***********************          |          |          |          |
 |          |          |          |          |          |          |
          
        

Figure 3: Light Switch Sends Multicast Control Message

          
                                 Light                        Network
Light-1   Light-2    Light-3     Switch    Rtr-1      Rtr-2  Backbone
 |          |          |          |          |          |          |
 |     COAP NON (2.04 Changed)    |          |          |          |
 |------------------------------->|          |          |          |
 |          |          |          |          |          |          |
 |          |          |          |          |          |          |
 |          COAP NON (2.04 Changed)          |          |          |
 |          |------------------------------------------>|          |
 |          |          |          |          |          |--------->|
 |          |          |          |          |<--------------------|
 |          |          |          |<---------|          |          |
 |          |          |          |          |          |          |
 |          |        COAP NON (5.00 Internal Server Error)         |
 |          |          |------------------------------->|          |
 |          |          |          |          |          |--------->|
 |          |          |          |          |<--------------------|
 |          |          |          |<---------|          |          |
 |          |          |          |          |          |          |
          
        

Figure 4: Lights (Optionally) Respond to Multicast CoAP Request

4.5. Lighting Control in MLD Enabled Network

The use case of previous section can also apply in networks where nodes support the MLD protocol [RFC3810]. The Lights then take on the role of MLDv2 listener and the routers (Rtr-1, Rtr-2) are MLDv2 Routers. In the Ethernet based network configuration, MLD may be available on all involved network interfaces. Use of MLD in the 6LoWPAN based configuration is also possible, but requires MLD support in all nodes in the 6LoWPAN which is usually not implemented in many deployments.

The resulting protocol flow is shown in Figure 5. This flow is executed after the commissioning phase, as soon as Lights are configured with a group address to listen to. The MLD Reports may require periodic refresh activity as specified by the MLD protocol.

After the shown sequence of MLD Report messages has been executed, both Rtr-1 and Rtr-2 are automatically configured to forward multicast traffic destined to Room-A-Lights onto their connected subnet. Hence, no manual Network Configuration of routers, as previously indicated in Section 4.4, is needed anymore.

          
                                 Light                        Network
Light-1   Light-2    Light-3     Switch    Rtr-1      Rtr-2  Backbone
 |          |          |          |          |          |          |
 |          |          |          |          |          |          |
 |          |          |          |          |          |          |
 | MLD Report: Join    |          |          |          |          |
 | 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 5: Joining Lighting Groups Using MLD

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 each other.

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 MPL [I-D.ietf-roll-trickle-mcast].

5.3. Advertising Membership of Multicast Groups

If a multicast routing/forwarding protocol is used in a network, server nodes that intend to receive CoAP multicast requests generally require a method to advertise the specific IP multicast address(es) they want to receive, i.e. a method to join specific IP multicast groups. This section identifies the ways in which this can be accomplished.

5.3.1. Using the Multicast Listener Discovery (MLD) Protocol

CoAP nodes that are IP hosts (i.e. 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 in an LLN.

5.3.2. Using the RPL Routing Protocol

The RPL routing protocol [RFC6550] defines in Section 12 the advertisement of IP multicast destinations using DAO messages. This mechanism can be used by CoAP nodes (which are also RPL routers) to advertise IP multicast group membership to other RPL nodes. Then, the RPL protocol can route multicast CoAP requests over multiple hops to the correct CoAP servers.

This mechanism could also be used as a means to convey IP multicast group membership information to an edge router (e.g. 6LBR), in case the edge router is also the root of the RPL DODAG. This could be useful in LLN segments where MLD is not available.

5.3.3. Using the MPL Forwarding Protocol

The MPL forwarding protocol [I-D.ietf-roll-trickle-mcast] can be used in a predefined network domain for propagation of IP multicast packets to all MPL routers, over multiple hops. MPL is designed to work in LLN deployments. Due to its property of propagating all (non-link-local) IP multicast packets to all MPL routers, there is in principle no need for CoAP server nodes to advertise IP multicast group membership assuming that any IP multicast source is also part of the MPL domain.

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:

  • 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.
  • 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.
  • 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.
  • 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: [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.

  • Group communications MUST operate in CoAP NoSec (No Security) mode.
  • Group communications MUST NOT use "coaps" scheme. That is, all group communications MUST use only "coap" scheme.
  • Group communications MUST NOT use IPSec.

A consequence is that CoAP group communications is vulnerable to all attacks mentioned in

Also, there is an approach for DTLS-based IP multicast security for CoAP networks (see [I-D.keoh-tls-multicast-security]) that should be considered once it matures.

7. IANA Considerations

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

8. Acknowledgements

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

9. References

9.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, February 2006.
[RFC4601] Fenner, B., Handley, M., Holbrook, H. and I. Kouvelas, "Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)", RFC 4601, August 2006.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J. and D. Culler, "Transmission of IPv6 Packets over IEEE 802.15.4 Networks", RFC 4944, September 2007.
[RFC5771] Cotton, M., Vegoda, L. and D. Meyer, "IANA Guidelines for IPv4 Multicast Address Assignments", BCP 51, RFC 5771, March 2010.
[RFC6550] Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur, JP. and R. Alexander, "RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks", RFC 6550, March 2012.
[RFC6636] Asaeda, H., Liu, H. and Q. Wu, "Tuning the Behavior of the Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) for Routers in Mobile and Wireless Networks", RFC 6636, May 2012.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, August 2012.
[RFC6775] Shelby, Z., Chakrabarti, S., Nordmark, E. and C. Bormann, "Neighbor Discovery Optimization for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)", RFC 6775, November 2012.
[I-D.ietf-core-coap] Shelby, Z, Hartke, K, Bormann, C and B Frank, "Constrained Application Protocol (CoAP)", Internet-Draft draft-ietf-core-coap-08, October 2011.

9.2. Informative References

[I-D.vanderstok-core-dna] Stok, P, Lynn, K and A Brandt, "CoRE Discovery, Naming, and Addressing", Internet-Draft draft-vanderstok-core-dna-01, March 2012.
[I-D.ietf-roll-trickle-mcast] Hui, J and R Kelsey, "Multicast Forwarding Using Trickle", Internet-Draft draft-ietf-roll-trickle-mcast-00, April 2011.
[I-D.keoh-tls-multicast-security] Keoh, S, Kumar, S and E Dijk, "DTLS-based Multicast Security for Low-Power and Lossy Networks (LLNs)", Internet-Draft draft-keoh-tls-multicast-security-00, October 2012.

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. Change Log

Changes from ietf-03 to ietf-04:

  • Removed section 2.3 (Potential Solutions for Group Communications) as it is purely background information and moved section to draft-dijk-core-groupcomm-misc (#266).
  • Added reference to draft-keoh-tls-multicast-security to section 6 (Security Considerations).
  • Removed Appendix B (CoAP-Observe Alternative to Group Communications) as it is as an alternative to IP Multicast that the WG has not adopted and moved section to draft-dijk-core-groupcomm-misc (#267).
  • Deleted section 8 (Conclusions) as it is redundant (#268).
  • Simplified light switch use case (#269) by splitting into basic operations and additional functions (#269).
  • Moved section 3.7 (CoAP Multicast and HTTP Unicast Interworking) to draft-dijk-core-groupcomm-misc (#270).
  • Moved section 3.3.1 (DNS-SD) and 3.3.2 (CoRE Resource Directory) to draft-dijk-core-groupcomm-misc as these sections essentially just repeated text from other drafts regarding DNS based features. Clarified remaining text in this draft relating to DNS based features to clearly indicate that these features are optional (#272).
  • Focus section 3.5 (Configuring Group Membership) on a single proposed solution.
  • Scope of section 5.3 (Use of MLD) widened to multicast destination advertisement methods in general.
  • Rewrote section 2.2 (Scope) for improved readibility.
  • Moved use cases that are not adressed to draft-dijk-core-groupcomm-misc.
  • Various editorial updates for improved readibility.

Changes from ietf-02 to ietf-03:

  • Clarified that a group resource manipulation may return back a mixture of successful and unsuccessful responses (section 3.4 and Figure 6) (#251).
  • Clarified that security option for group communication must be NoSec mode (section 6) (#250).
  • Added mechanism for group membership configuration (#249).
  • 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).
  • Made the definition of 'group' more specific to group of CoAP endpoints and included text on UDP port selection (#186).
  • Added explanatory text in section 3.4 regarding why not to use group communication for non-idempotent messages (i.e. CoAP POST) (#186).
  • Changed link-local RD discovery to site-local in RD discovery use case to make it more realistic.
  • Fixed lighting control use case CoAP proxying; now returns individual CoAP responses as in coap-12.
  • Replaced link format I-D with RFC6690 reference.
  • Various editorial updates for improved readibility

Changes from ietf-01 to ietf-02:

  • Rewrote congestion control section based on latest CoAP text including Leisure concept (#188)
  • Updated the CoAP/HTTP interworking section and example use case with more details and use of MLD for multicast group joining
  • Key use cases added (#185)
  • References to [I-D.vanderstok-core-dna] and draft-castellani-core-advanced-http-mapping added
  • Moved background sections on "MLD" and "CoAP-Observe" to Appendices
  • Removed requirements section (and moved it to draft-dijk-core-groupcomm-misc)
  • Added details for IANA request for group communication multicast addresses
  • Clarified text to distinguish between "link local" and general multicast cases
  • Moved lengthy background section 5 to draft-dijk-core-groupcomm-misc and replaced with a summary
  • Various editorial updates for improved readibility
  • Changelog added

Changes from ietf-00 to ietf-01:

  • Moved CoAP-observe solution section to section 2
  • Editorial changes
  • Moved security requirements into requirements section
  • Changed multicast POST to PUT in example use case
  • Added CoAP responses in example use case

Changes from rahman-07 to ietf-00:

  • Editorial changes
  • Use cases section added
  • CoRE Resource Directory section added
  • Removed section 3.3.5. IP Multicast Transmission Methods
  • Removed section 3.4 Overlay Multicast
  • Removed section 3.5 CoAP Application Layer Group Management
  • Clarified section 4.3.1.3 RPL Routers with Non-RPL Hosts case
  • 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