CoRE Working Group A. Rahman, Ed.
Internet-Draft InterDigital Communications, LLC
Intended status: Informational E. 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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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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.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these documents
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Table of Contents
1. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 3
2.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. CoAP Group Communication Based On IP Multicast . . . . . . . . 4
3.1. IP Multicast Background . . . . . . . . . . . . . . . . . 4
3.2. CoAP Group Definition and Naming . . . . . . . . . . . . . 5
3.3. Group Discovery and Member Discovery . . . . . . . . . . . 5
3.4. Group Resource Manipulation . . . . . . . . . . . . . . . 6
3.5. Configuring Group Membership In Endpoints . . . . . . . . 7
3.6. Congestion Control . . . . . . . . . . . . . . . . . . . . 8
4. Use Cases and Corresponding Protocol Flows . . . . . . . . . . 8
4.1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 8
4.2. Network Configuration . . . . . . . . . . . . . . . . . . 9
4.3. Discovery of Resource Directory . . . . . . . . . . . . . 11
4.4. Lighting Control . . . . . . . . . . . . . . . . . . . . . 12
4.5. Lighting Control in MLD Enabled Network . . . . . . . . . 14
5. Deployment Guidelines . . . . . . . . . . . . . . . . . . . . 15
5.1. Target Network Topologies . . . . . . . . . . . . . . . . 15
5.2. Multicast Routing . . . . . . . . . . . . . . . . . . . . 16
5.3. Advertising Membership of Multicast Groups . . . . . . . . 16
5.3.1. Using the Multicast Listener Discovery (MLD)
Protocol . . . . . . . . . . . . . . . . . . . . . . . 16
5.3.2. Using the RPL Routing Protocol . . . . . . . . . . . . 16
5.3.3. Using the MPL Forwarding Protocol . . . . . . . . . . 17
5.4. 6LoWPAN-Specific Guidelines . . . . . . . . . . . . . . . 17
6. Security Considerations . . . . . . . . . . . . . . . . . . . 18
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Normative References . . . . . . . . . . . . . . . . . . . 18
9.2. Informative References . . . . . . . . . . . . . . . . . . 20
Appendix A. Multicast Listener Discovery (MLD) . . . . . . . . . 20
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
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1. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This document assumes readers are familiar with the terms and
concepts that are used in [I-D.ietf-core-coap]. In addition, this
document defines the following terminology:
Group Communication
A source node sends a single message which is delivered to
multiple destination nodes, where all destinations are identified
to belong to a specific group. The source node may or may not be
part of the group. The underlying mechanism for group
communication is assumed to be multicast based. The network where
the group communication takes place can be either a constrained or
a regular (un-constrained) network.
Multicast
Sending a message to multiple destination nodes simultaneously.
There are various options to implement multicast including layer 2
(Media Access Control) 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,
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the group "all lights in building one" may consist of the groups "all
lights on floor one of building one", "all lights on floor two of
building one", etc. Groups may be preconfigured or dynamically
formed. If information needs to be sent to or received from a group
of devices, group communication mechanisms can improve efficiency and
latency of communication and reduce bandwidth requirements for a
given application. HTTP does not support any equivalent
functionality to CoAP group communication.
2.2. Scope
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.
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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.
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]):
URI authority Targeted group
all.bldg6.example.com "all nodes in building 6"
all.west.bldg6.example.com "all nodes in west wing, building 6"
all.floor1.west.bldg6.examp... "all nodes in floor 1, west wing,
building 6"
all.bu036.floor1.west.bldg6... "all nodes in office bu036, floor1,
west wing, building 6"
Reverse mapping (from IP multicast address to Group FQDN) is
supported using the reverse DNS resolution technique
([I-D.vanderstok-core-dna]).
3.3. Group Discovery and Member Discovery
CoAP defines a resource discovery capability [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
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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:
o All CoAP multicast requests MUST be sent either to the default
CoAP UDP port (i.e. default Uri-Port as defined in
[I-D.ietf-core-coap]), or to a port number obtained via a service
discovery lookup operation as a valid CoAP port for the targeted
multicast group.
o All CoAP multicast requests SHOULD operate only on URIs (links)
which were retrieved either from a "/.well-known/core" lookup on
at least one group member node, or from an equivalent service
discovery lookup which returns either the resources supported by
all group members or resources supported by one particular group
member.
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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.
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:
Req: GET /gp
Res: 2.05 Content (Content-Format: application/json)
[ { "n": "Room-A-Lights.floor1.west.bldg6.example.com",
"ip": "ff05::4200:f7fe:ed37:14ca" }
]
where the OPTIONAL "n" key/value pair defines the Group name as FQDN
and "ip" defines the associated multicast IP address. A CoAP
endpoint can be added to another group by a POST on the resource with
a single JSON group object, which updates the existing resource by
adding the group object to the existing ones:
Req: POST /gp (Content-Format: application/json)
{ "n": "floor1.west.bldg6.example.com",
"ip": "ff05::4200:f7fe:ed37:14cb" }
Res: 2.04 Changed
A PUT with as payload an array of JSON group objects will replace all
current group memberships with the new ones as defined in the
payload. After a change effected on the "core.gp" type resource, the
endpoint MUST effect registration/deregistration from corresponding
IP multicast groups.
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3.6. Congestion Control
Multicast CoAP requests may result in a multitude of replies from
different nodes, potentially causing congestion. Therefore sending
multicast requests should be conservatively controlled.
The base CoAP draft [I-D.ietf-core-coap] reduces multicast-specific
congestion risks through the following measures:
o A server MAY choose not to respond to a multicast request if
there's nothing useful to respond (e.g. error or empty response).
o A server SHOULD limit the support for multicast requests to
specific resources where multicast operation is required.
o A multicast request MUST be Non-Confirmable.
o A server does not respond immediately to a multicast request, but
SHOULD first wait for a time that is randomly picked within a
predetermined time interval called the Leisure.
o A server SHOULD NOT accept multicast requests that can not be
authenticated.
Additional guidelines to reduce congestion risks are:
o A server in an LLN should only support multicast GET for resources
that are small, e.g. for an LLN that could mean the payload of the
response fits into a single link-layer frame.
o A server can minimize the payload length in response to a
multicast GET on "/.well-known/core" by using hierarchy in
arranging link descriptions for the response. An example of this
is given in Section 5 of [RFC6690].
o Preferably IP multicast with link-local scope should be used,
rather than global or site-local.
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:
o Discovery of Resource Directory: discovering the local CoRE RD
which contains links (URIs) to resources stored on other servers
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[RFC6690].
o Lighting Control: synchronous operation of a group of IPv6-
connected lights (e.g., 6LoWPAN [RFC4944] lights).
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:
o A large room (Room-A) with three lights (Light-1, Light-2,
Light-3) controlled by a Light Switch. The devices are organized
into two subnets. In reality, there could be more lights (up to
several hundreds) but these are not shown for clarity.
o Light-1 and the Light Switch are connected to a router (Rtr-1)
which is also a CoAP Resource Directory (RD).
o Light-2 and the Light-3 are connected to another router (Rtr-2)
which is also a CoAP RD.
o The routers are connected to an IPv6 network backbone which is
also multicast enabled. In the general case, this means the
network backbone and 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.
o 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
greater manual provisioning of the network is requried (e.g. IP
multicast addresses are hardcoded into devices).
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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)
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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:
o The fixture for Light-2 is installed and powered on for the first
time.
o Light-2 will then search for the local RD (RD-2) by sending out a
GET request (with the "/.well-known/core?rt=core.rd" request URI)
to the site-local "All CoAP Nodes" address. In this case, the
site is configured to include at least all nodes in the subnet.
o This multicast message will then go to each node in subnet-2.
However, only Rtr-2 (RD-2) will respond because the GET is
qualified by the query string "?rt=core.rd". Note that the router
Rtr-2 is configured not to forward this multicast request further
onto the backbone.
o Note that the flow is shown only for Light-2 for clarity. Similar
flows will happen for Light-1, Light-3 and the Light Switch when
they are first powered on.
The RD may also be discovered by other means such as by assuming a
default location (e.g. on a 6LBR), using DHCP, anycast address, etc.
However, these approaches do not invoke CoAP group communication so
are not further discussed here.
For other discovery use cases such as discovering local CoAP servers,
services or resources group communication can be used in a similar
fashion as in the above use case. Both Link-Local (LL) and site-
local discovery are possible this way.
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Light Rtr-1 Rtr-2 Network
Light-1 Light-2 Light-3 Switch (RD-1) (RD-2) Backbone
| | | | | | |
| | | | | | |
********************************** | | |
* Light-2 is installed * | | |
* and powers on for first time * | | |
********************************** | | |
| | | | | | |
| | | | | | |
| | COAP NON 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:
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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) * | | | |
*********************** | | | |
| | | | | | |
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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.
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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
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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.
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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:
o Filtering in 6LBRs based on information from the routing protocol.
This allows a 6LBR to only forward multicast traffic onto the
LoWPAN, for which it is known that there exists at least one
listener on the LoWPAN.
o Filtering in 6LBRs based on MLD reports. Similar as previous but
based directly on MLD reports from 6LoWPAN nodes. This only works
in a single-IP-hop 6LoWPAN network, such as a mesh-under routing
network or a star topology network, because MLD relies on link-
local communication.
o Filtering in 6LBRs based on settings. Filtering tables with
blacklists/whitelists can be configured in the 6LBR by system
administration for all 6LBRs or configured on a per-6LBR basis.
o Filtering in router(s) or firewalls that provide access to
constrained network segments. For example, in an access router/
bridge that connects a regular intranet LAN to a building control
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IPv6 backbone. This backbone connects multiple 6LoWPAN segments,
each segment connected via a 6LBR.
6. Security Considerations
As defined in [I-D.ietf-core-coap], CoAP group communications based
on IP multicast must use the following security approach:
o Group communications MUST operate in CoAP NoSec (No Security)
mode.
o Group communications MUST NOT use "coaps" scheme. That is, all
group communications MUST use only "coap" scheme.
o Group communications MUST NOT use IPSec.
A consequence is that CoAP group communications is vulnerable to all
attacks mentioned in [I-D.ietf-core-coap] for the NoSec mode. For
sensitive data or safety-critical control, appropriate link-layer
security or application-level object security SHOULD be used instead
of DTLS security.
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.
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[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)",
draft-ietf-core-coap-13 (work in progress), December 2012.
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9.2. Informative References
[I-D.vanderstok-core-dna]
Stok, P., Lynn, K., and A. Brandt, "CoRE Discovery,
Naming, and Addressing", draft-vanderstok-core-dna-02
(work in progress), July 2012.
[I-D.ietf-roll-trickle-mcast]
Hui, J. and R. Kelsey, "Multicast Protocol for Low power
and Lossy Networks (MPL)",
draft-ietf-roll-trickle-mcast-02 (work in progress),
October 2012.
[I-D.keoh-tls-multicast-security]
Keoh, S., Kumar, S., and E. Dijk, "DTLS-based Multicast
Security for Low-Power and Lossy Networks (LLNs)",
draft-keoh-tls-multicast-security-00 (work in progress),
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
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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:
o 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).
o Added reference to draft-keoh-tls-multicast-security to section 6
(Security Considerations).
o 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).
o Deleted section 8 (Conclusions) as it is redundant (#268).
o Simplified light switch use case (#269) by splitting into basic
operations and additional functions (#269).
o Moved section 3.7 (CoAP Multicast and HTTP Unicast Interworking)
to draft-dijk-core-groupcomm-misc (#270).
o 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).
o Focus section 3.5 (Configuring Group Membership) on a single
proposed solution.
o Scope of section 5.3 (Use of MLD) widened to multicast destination
advertisement methods in general.
o Rewrote section 2.2 (Scope) for improved readibility.
o Moved use cases that are not adressed to
draft-dijk-core-groupcomm-misc.
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o Various editorial updates for improved readibility.
Changes from ietf-02 to ietf-03:
o Clarified that a group resource manipulation may return back a
mixture of successful and unsuccessful responses (section 3.4 and
Figure 6) (#251).
o Clarified that security option for group communication must be
NoSec mode (section 6) (#250).
o Added mechanism for group membership configuration (#249).
o Removed IANA request for multicast addresses (section 7) and
replaced with a note indicating that the request is being made in
[I-D.ietf-core-coap] (#248).
o Made the definition of 'group' more specific to group of CoAP
endpoints and included text on UDP port selection (#186).
o Added explanatory text in section 3.4 regarding why not to use
group communication for non-idempotent messages (i.e. CoAP POST)
(#186).
o Changed link-local RD discovery to site-local in RD discovery use
case to make it more realistic.
o Fixed lighting control use case CoAP proxying; now returns
individual CoAP responses as in coap-12.
o Replaced link format I-D with RFC6690 reference.
o Various editorial updates for improved readibility
Changes from ietf-01 to ietf-02:
o Rewrote congestion control section based on latest CoAP text
including Leisure concept (#188)
o Updated the CoAP/HTTP interworking section and example use case
with more details and use of MLD for multicast group joining
o Key use cases added (#185)
o References to [I-D.vanderstok-core-dna] and
draft-castellani-core-advanced-http-mapping added
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o Moved background sections on "MLD" and "CoAP-Observe" to
Appendices
o Removed requirements section (and moved it to
draft-dijk-core-groupcomm-misc)
o Added details for IANA request for group communication multicast
addresses
o Clarified text to distinguish between "link local" and general
multicast cases
o Moved lengthy background section 5 to
draft-dijk-core-groupcomm-misc and replaced with a summary
o Various editorial updates for improved readibility
o Changelog added
Changes from ietf-00 to ietf-01:
o Moved CoAP-observe solution section to section 2
o Editorial changes
o Moved security requirements into requirements section
o Changed multicast POST to PUT in example use case
o Added CoAP responses in example use case
Changes from rahman-07 to ietf-00:
o Editorial changes
o Use cases section added
o CoRE Resource Directory section added
o Removed section 3.3.5. IP Multicast Transmission Methods
o Removed section 3.4 Overlay Multicast
o Removed section 3.5 CoAP Application Layer Group Management
o Clarified section 4.3.1.3 RPL Routers with Non-RPL Hosts case
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o References added and some normative/informative status changes
Authors' Addresses
Akbar Rahman (editor)
InterDigital Communications, LLC
Email: Akbar.Rahman@InterDigital.com
Esko Dijk (editor)
Philips Research
Email: esko.dijk@philips.com
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