CoRE Working Group A. Rahman, Ed.
Internet-Draft InterDigital Communications, LLC
Intended status: Informational E.O. Dijk, Ed.
Expires: April 20, 2013 Philips Research
October 19, 2012

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

Abstract

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

Status of This Memo

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

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http:/⁠/⁠datatracker.ietf.org/⁠drafts/⁠current/⁠.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on April 20, 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) or layer 3 (IP) mechanisms.
IP Multicast

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

Low power and Lossy Network (LLN): A type of constrained network where the devices are interconnected by a variety of low power, lossy links such as IEEE 802.15.4, Bluetooth, WiFi, wired or low power power-line communication links.

2. Introduction

2.1. Background

The Constrained Application Protocol (CoAP) is an application protocol (analogous to HTTP) for resource constrained devices operating in an IP network [I-D.ietf-core-coap]. Constrained devices can be large in number, but are often highly correlated to each other (e.g. by type or location). For example, all the light switches in a building may belong to one group and all the thermostats may belong to another group. Groups may be composed by function. For example, the group "all lights in building one" may consist of the groups "all lights on floor one of building one", "all lights on floor two of building one", etc. Groups may be preconfigured or dynamically formed. If information needs to be sent to or received from a group of devices, group communication mechanisms can improve efficiency and latency of communication and reduce bandwidth requirements for a given application. HTTP does not support any equivalent functionality to CoAP group communication.

2.2. Scope

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

2.3. Potential Solutions for Group Communication

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

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

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

3. CoAP Group Communication Based On IP Multicast

3.1. IP Multicast Background

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

To achieve IP multicast beyond a subnet, an IP multicast routing protocol needs to be active on routers. The RPL protocol [RFC6550] for example is able to route multicast traffic in constrained LLNs. While PIM-SM [RFC4601] is often used for multicast routing in un-constrained networks.

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

3.2. CoAP Group Definition and Naming

A group is defined as a set of CoAP endpoints, where each endpoint is configured to receive a multicast CoAP request that is sent to the group's associated IP multicast address. The group MAY have more than one associated IP multicast address. An endpoint MAY be a member of multiple groups. Group membership of an endpoint MAY dynamically change over time. The group MAY be identified by a Group Name ([I-D.vanderstok-core-dna]) which is defined as a prefix string of a Group FQDN. The Group FQDN can be uniquely mapped to a site-local or global multicast IP address via DNS resolution.

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

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

  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"
    

Examples of hierarchical group naming (and scoping) for a building control application are shown below.

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

3.3. Group Discovery and Member Discovery

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

3.3.1. DNS-SD

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

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

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

3.3.2. CoRE Resource Directory

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

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

3.4. Group Resource Manipulation

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

A unicast response per server MAY be sent back to answer the group request (e.g. response "2.05 Content" to a group GET request) taking into account the congestion control rules defined in [I-D.ietf-core-coap]. The unicast responses may be a mixture of success (e.g. 2.05 Content) and failure (e.g. 4.04 Not Found) codes depending on the individual server processing result.

Group communications SHALL NOT be used for non-idempotent methods (i.e. CoAP POST). This is because not all group members are guaranteed to receive the multicast request, and the sender can not readily find out which group members did not receive it.

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

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

3.5. Configuring Group Membership In Endpoints

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

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

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

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

	
   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   

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

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

3.6. Congestion Control

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

The base CoAP draft [I-D.ietf-core-coap] reduces multicast-specific congestion risks through the following measures:

Additional guidelines to reduce congestion risks are:

3.7. CoAP Multicast and HTTP Unicast Interworking

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

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

Figure 1: CoAP Multicast and HTTP Unicast Interworking

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

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

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

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

4. Use Cases and Corresponding Protocol Flows

4.1. Introduction

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

4.2. Network Configuration

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

          

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

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

4.3. Discovery of Resource Directory

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

  • The fixture for Light-2 is installed and powered on for the first time.
  • Light-2 will then search for the local RD (RD-2) by sending out a GET request (with the "/.well-known/core?rt=core.rd" request URI) to the site-local "All CoAP Nodes" address. In this case, the site is assumed to include all nodes in the subnet.
  • This multicast message will then go to each node in subnet-2. However, only Rtr-2 (RD-2) will respond because the GET is qualified by the query string "?rt=core.rd".
  • Note that the flow is shown only for Light-2 for clarity. Similar flows will happen for Light-1, Light-3 and the Light Switch when they are first powered on.

The RD may also be discovered by other means such as by assuming a default location (e.g. on a 6LBR), using DHCP, anycast address, etc. However, these approaches do not invoke CoAP group communication so are not further discussed here.

For other discovery use cases such as discovering local CoAP servers, services or resources group communication can be used in a similar fashion as in the above use case. Both Link-Local (LL) and site-local discovery are possible this way.

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

Figure 3: Resource Directory Discovery via Multicast Message

4.4. Lighting Control

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

  • 1) Startup phase: 6LoWPANs are formed. IPv6 addresses assigned to all devices. The CoAP network is formed.
  • 2) Commissioning phase (by applications): The IP multicast address of the group (Room-A-Lights) has been set in all the Lights. The URI of the group (Room-A-Lights) has been set in the Light Switch.
  • 3) The indicated MLD Report messages are link-local multicast. In each LoWPAN, it is assumed that a multicast routing/forwarding protocol in 6LRs will then propagate the Join information contained in the MLD Report over multiple hops to the 6LBR.

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

Figure 4: Joining Lighting Groups Using MLD

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

Figure 5: Sending Lighting Control Multicast Message

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

Figure 6: Sending Lighting Control Response to Multicast Message

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

5. Deployment Guidelines

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

5.1. Target Network Topologies

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

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

5.2. Multicast Routing

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

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

CoAP nodes that are IP hosts (not routers) are generally unaware of the specific multicast routing protocol being used. When such a host needs to join a specific (CoAP) multicast group, it usually requires a way to signal to the multicast routers which multicast traffic it wants to receive. For efficient multicast routing (i.e. avoid always flooding multicast IP packets), routers must know which hosts need to receive packets addressed to specific IP multicast destinations.

The Multicast Listener Discovery (MLD) protocol ([RFC3810], Appendix 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.

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 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

7. IANA Considerations

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

8. Conclusions

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

9. Acknowledgements

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

10. References

10.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P. and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
[RFC3986] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, January 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, February 2006.
[RFC4601] Fenner, B., Handley, M., Holbrook, H. and I. Kouvelas, "Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)", RFC 4601, August 2006.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J. and D. Culler, "Transmission of IPv6 Packets over IEEE 802.15.4 Networks", RFC 4944, September 2007.
[RFC5771] Cotton, M., Vegoda, L. and D. Meyer, "IANA Guidelines for IPv4 Multicast Address Assignments", BCP 51, RFC 5771, March 2010.
[RFC6550] Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur, JP. and R. Alexander, "RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks", RFC 6550, March 2012.
[RFC6636] Asaeda, H., Liu, H. and Q. Wu, "Tuning the Behavior of the Internet Group Management Protocol (IGMP) and Multicast Listener Discovery (MLD) for Routers in Mobile and Wireless Networks", RFC 6636, May 2012.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, August 2012.
[I-D.ietf-core-coap] Shelby, Z, Hartke, K, Bormann, C and B Frank, "Constrained Application Protocol (CoAP)", Internet-Draft draft-ietf-core-coap-08, October 2011.

10.2. Informative References

[I-D.cheshire-dnsext-dns-sd] Cheshire, S and M Krochmal, "DNS-Based Service Discovery", Internet-Draft draft-cheshire-dnsext-dns-sd-11, December 2011.
[I-D.ietf-core-observe] Hartke, K and Z Shelby, "Observing Resources in CoAP", Internet-Draft draft-ietf-core-observe-03, October 2011.
[I-D.shelby-core-resource-directory] Krco, S and Z Shelby, "CoRE Resource Directory", Internet-Draft draft-shelby-core-resource-directory-02, October 2011.
[I-D.vanderstok-core-bc] Stok, P and K Lynn, "CoAP Utilization for Building Control", Internet-Draft draft-vanderstok-core-bc-05, October 2011.
[I-D.lynn-core-discovery-mapping] Lynn, K and Z Shelby, "CoRE Link-Format to DNS-Based Service Discovery Mapping", Internet-Draft draft-lynn-core-discovery-mapping-01, July 2011.
[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.castellani-core-http-mapping] Castellani, A, Loreto, S, Rahman, A, Fossati, T and E Dijk, "Best practices for HTTP-CoAP mapping implementation", Internet-Draft draft-castellani-core-http-mapping-02, October 2011.
[I-D.castellani-core-advanced-http-mapping] Castellani, A, Loreto, S, Rahman, A, Fossati, T and E Dijk, "Best Practices for HTTP-CoAP Mapping Implementation", Internet-Draft draft-castellani-core-advanced-http-mapping-00, July 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.
[Lao05] Lao, L., Cui, J., Gerla, M. and D. Maggiorini, "A Comparative Study of Multicast Protocols: Top, Bottom, or In the Middle? ", 2005.
[Banerjee01] Banerjee, B. and B. Bhattacharjee, "A Comparative Study of Application Layer Multicast Protocols ", 2001.

Appendix A. Multicast Listener Discovery (MLD)

In order to extend the scope of IP multicast beyond link-local scope, an IP multicast routing protocol has to be active in routers on an LLN. To achieve efficient multicast routing (i.e. avoid always flooding multicast IP packets), routers have to learn which hosts need to receive packets addressed to specific IP multicast destinations.

The Multicast Listener Discovery (MLD) protocol [RFC3810] (or its IPv4 pendant IGMP) is today the method of choice used by an (IP multicast enabled) router to discover the presence of multicast listeners on directly attached links, and to discover which multicast addresses are of interest to those listening nodes. MLD was specifically designed to cope with fairly dynamic situations in which multicast listeners may join and leave at any time.

IGMP/MLD Snooping is a technique implemented in some corporate LAN routing/switching devices. An MLD snooping switch listens to MLD State Change Report messages from MLD listeners on attached links. Based on this, the switch learns on what LAN segments there is interest for what IP multicast traffic. If the switch receives at some point an IP multicast packet, it uses the stored information to decide onto which LAN segment(s) to send the packet. This improves network efficiency compared to the regular behavior of forwarding every incoming multicast packet onto all LAN segments. An MLD snooping switch may also send out MLD Query messages (which is normally done by a device in MLD Router role) if no MLD Router is present.

[RFC6636] discusses optimal tuning of the parameters of MLD for routers for mobile and wireless networks. These guidelines may be useful when implementing MLD in LLNs.

Appendix B. CoAP-Observe Alternative to Group Communication

The CoAP Observation extension [I-D.ietf-core-observe] can be used as a simple (but very limited) alternative for group communication. A group in this case consists of a CoAP server hosting a specific resource, plus all CoAP clients observing that resource. The server is the only group member that can send a group message. It does this by modifying the state of a resource under observation and subsequently notifying its observers of the change. Serial unicast is used for sending the notifications. This approach can be a simple alternative for networks where IP multicast is not available or too expensive.

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

Appendix C. Change Log

Changes from ietf-02 to ietf-03:

  • 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 [I-D.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