One document matched: draft-ietf-core-groupcomm-06.xml
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<rfc category="info" ipr="trust200902" docName="draft-ietf-core-groupcomm-06">
<front>
<title abbrev="Group Communication for CoAP">Group Communication for CoAP</title>
<author fullname="Akbar Rahman" initials="A." surname="Rahman" role="editor">
<organization>InterDigital Communications, LLC</organization>
<address>
<email>Akbar.Rahman@InterDigital.com</email>
</address>
</author>
<author fullname="Esko Dijk" initials="E.O." surname="Dijk" role="editor">
<organization>Philips Research</organization>
<address>
<email>esko.dijk@philips.com</email>
</address>
</author>
<date year="2013"/>
<area>Applications</area>
<workgroup>CoRE Working Group</workgroup>
<abstract>
<t>
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 provides guidance for
how the CoAP protocol should be used in a group communication context. An approach for
using CoAP on top of IP multicast is detailed for both constrained and
un-constrained networks. Also, various use cases and corresponding protocol
flows are provided to illustrate important concepts. Finally, guidance is
provided for deployment in various network topologies.
</t>
</abstract>
<!--
<note title="Requirements Language">
<t>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 <xref target="RFC2119">RFC 2119</xref>.
</t>
</note>
-->
</front>
<middle>
<!-- section anchor="sec-1" title="Conventions and Terminology" -->
<section title="Conventions and Terminology">
<t>
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in <xref target="RFC2119" />.
</t>
<t>
These key words are used to establish a set of best practices
for CoAP group communication. An implementation of CoAP group communication
MAY implement these guidelines; an implementation claiming compliance to
this document MUST implement the set.
</t>
<t>This document assumes readers are familiar with the terms and
concepts that are used in <xref target="I-D.ietf-core-coap"/>. In
addition, this document defines the following terminology:
<list style="hanging">
<t hangText="Group Communication"><vspace />
A source node sends a single message which is delivered
to multiple destination nodes, where all destinations are
identified to belong to a specific group. The
source node itself may be part of the group. The underlying
mechanism for group communication is assumed to be multicast
based. The network involved may be a constrained network
such as a low-power, lossy network.</t>
<t hangText="Multicast"><vspace />
Sending a message to multiple destination nodes with one network
invocation. There are various options to implement multicast including
layer 2 (Media Access Control) and layer 3 (IP) mechanisms.</t>
<t hangText="IP Multicast"><vspace />
A specific multicast solution based on the use of IP multicast
addresses as defined in "IANA Guidelines for IPv4 Multicast Address
Assignments" <xref target="RFC5771" /> and "IP Version 6 Addressing
Architecture" <xref target="RFC4291" />.</t>
<t hangText="Low power and Lossy Network (LLN)"><vspace />
A type of constrained IP network where devices are interconnected by
a variety of low-power and lossy links (such as IEEE 802.15.4, Bluetooth LE,
DECT, DECT ULE) or lossy links (such as IEEE P1901.2 power-line communication).</t>
</list>
</t>
</section>
<!-- section anchor="sec-2" title="Introduction" -->
<section title="Introduction">
<!-- section anchor="sec-2.1" title="Background" -->
<section title="Background">
<t>
The Constrained Application Protocol (CoAP) is an application protocol (analogous to HTTP)
for resource constrained devices operating in an IP network <xref target="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 pre-configured before deployment or dynamically formed during operation. If information
needs to be sent to or received from a group of devices, group communication
mechanisms can improve efficiency and latency of communication and reduce
bandwidth requirements for a given application. HTTP does not
support any equivalent functionality to CoAP group communication.</t>
</section>
<!-- section anchor="sec-2.2" title="Scope" -->
<section title="Scope">
<t>
Group communication involves sending a CoAP Request as an IP Multicast message and
handling the potential multitude of (unicast) CoAP Responses. The normative protocol aspects of
running CoAP on top of IP Multicast and processing the responses are given
in <xref target="I-D.ietf-core-coap"/>. The main contribution of this document lies in
providing additional guidance for key group communication features. Among the topics covered are
group definition, group resource manipulation, and group configuration. Also, proxy operation and
minimizing congestion scenarios for group communication is discussed. Finally, specific use case
behavior and deployment guidelines are outlined for CoAP group communication.
</t>
</section>
</section>
<!-- section anchor="sec-3" title="Protocol Considerations" -->
<section title="Protocol Considerations" anchor="ProtocolConsiderations">
<!-- section anchor="sec-3.1" title="IP Multicast Routing Background" -->
<section title="IP Multicast Routing Background">
<t>
IP Multicast routing protocols have been evolving for decades, resulting in
proposed standards such as Protocol Independent Multicast - Sparse
Mode (PIM-SM) <xref target="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.
</t>
<t>To achieve IP multicast beyond a subnet, an IP multicast routing or forwarding protocol needs to be
active on IP routers. An examples of a routing protocol specifically for LLNs is
RPL (Section 12 of <xref target="RFC6550"/>) and an example of a forwarding protocol for LLNs is MPL
<xref target="I-D.ietf-roll-trickle-mcast" />. PIM-SM <xref target="RFC4601" /> is
often used for multicast routing in un-constrained networks.
</t>
<t>IP multicast can also be run in a Link-Local (LL) scope. This means that there is no routing
involved and an IP multicast message is only received over the link on which it was sent.
</t>
<t>For a complete IP multicast solution, in addition to a routing/forwarding protocol, a so-called
"listener" protocol is needed for the devices to subscribe to groups
(see <xref target="Advertising_Memership"/>).
</t>
</section>
<!-- section anchor="sec-3.2" title="Group Definition and Naming" -->
<section title="Group Definition and Naming">
<t>
A group is defined as a set of CoAP endpoints, where each endpoint is configured to receive a
multicast CoAP request that is sent to the group's associated IP multicast address. An endpoint
MAY be a member of multiple groups. Group membership of an endpoint MAY dynamically change over time.
</t>
<t>For group communication, the Group URI will be the CoAP request URI. A Group URI has the
scheme 'coap' and includes in the authority part either a group IP multicast address plus
optional port number or a hostname plus optional port number that can be resolved to
the group IP multicast address (e.g., a Group Name or Group FQDN).
Group URIs follow the CoAP URI syntax <xref target="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. </t>
<t>If a Group FQDN is used, it can be uniquely mapped to a site-local or global multicast IP address via DNS
resolution (if supported). Some examples of hierarchical Group FQDN naming (and scoping) for
a building control application are shown below (<xref target="I-D.vanderstok-core-dna" />):
<figure><artwork>
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"
</artwork></figure>
</t>
<t>Similarly, if supported, reverse mapping (from IP multicast address to Group FQDN) is possible
using the reverse DNS resolution technique (<xref target="I-D.vanderstok-core-dna" />).
</t>
</section>
<!-- section anchor="sec-3.3" title="Port and URI Configuration" -->
<section title="Port and URI Configuration">
<t>
A CoAP group member listens for CoAP messages on the group's IP multicast address,
on a specified UDP port. Note that the default UDP port is the CoAP default port 5683 but
a non-default UDP port MAY be specified for the group; in which case implementers
MUST ensure that all group members are configured to use this same port.
</t>
<t>
Multicast based group communication will not work if there diversity in the authority
port (i.e. a diversity of dynamic port addresses across the group)
or if the resources are located at different paths on different endpoints.
Therefore, some measures must be present to ensure uniformity in port number
and resource names/locations within a group.
All CoAP multicast requests MUST be sent using the port number as follows:
<list style="numbers">
<t> A pre-configured port number, if available. The pre-configuration mechanism MUST ensure
that the same port number is pre-configured across all endpoints in a group and across
all CoAP clients performing the group requests.</t>
<t> If the client is configured to use service discovery including port discovery,
it uses a port number obtained via a service discovery lookup operation as a valid
CoAP port for the targeted CoAP multicast group.</t>
<t> Otherwise use the default CoAP UDP port.</t>
</list>
</t>
<t>All CoAP multicast requests SHOULD operate on URI paths ("links") as follows:
<list style="numbers">
<t> Pre-configured URI paths, if available. The pre-configuration mechanism MUST ensure that these URIs
are pre-configured across all CoAP servers in a group and all CoAP clients performing
the group requests.</t>
<t> If the client is configured to use default CoRE resource discovery, it uses URI paths
retrieved from a "/.well-known/core" lookup on a
group member. The URI paths the client will use MUST be known
to be available also in all other endpoints in the group. The URI path configuration mechanism on servers
MUST ensure that these URIs (identified as being supported by the group) are configured on
all group endpoints.</t>
<t> If the client is configured to use another form of service discovery, it uses URI paths
from an equivalent service discovery lookup which returns the
resources supported by all group members.</t>
</list>
</t>
</section>
<!-- section anchor="sec-3.4" title="Group Methods" -->
<section title="Group Methods">
<t>Group communication SHALL only be used for idempotent methods
(i.e. CoAP GET, PUT, and DELETE). The CoAP messages that are sent
via multicast SHALL be Non-Confirmable. </t>
<t>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
<xref target="Congestion_Control"/>. The unicast responses received may be a mixture
of success (e.g. 2.05 Content) and failure (e.g. 4.04 Not Found)
codes depending on the individual server processing result.
</t>
<t>Group communication SHALL NOT be used for non-idempotent
methods (i.e. CoAP POST). This is because not all group members
are guaranteed to receive the multicast request, and the sender
cannot readily find out which group members did not receive it.
</t>
</section>
<!-- section anchor="sec-3.5" title="Group Member Discovery" -->
<section title="Group Member Discovery" anchor="MemberDiscovery">
<t>
CoAP defines a resource discovery capability <xref target="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"
<xref target="I-D.ietf-core-coap"/>. The collected responses to the ping would then give
an indication of the group members.
</t>
</section>
<!-- section anchor="sec-3.6" title="Configuring Group Membeship In Endpoints" -->
<section title="Configuring Group Membership In Endpoints" anchor="ConfiguringMembers">
<t>The group membership of a CoAP server may be determined in one or more of the following ways.
First, the group membership may be pre-configured before node deployment. Second, it may be
configured during operation by another node e.g. a commissioning device. Third, a node may
be programmed to discover (query) its group membership during operation using a specific service
discovery means.
</t>
<t>
In the first case, the pre-configured group may be a multicast IP address or a hostname which is
during operation resolved to a multicast IP address by the endpoint using DNS.
</t>
<t>
In the second case, typical in e.g. building control, a commissioning tool
determines to which groups a sensor or actuator node belongs, and writes this information to the
node, which can subsequently join the correct IP multicast group on its network interface. The
information written may again be a multicast IP address or a hostname.
</t><t>
For the third case, specific methods to use for a CoAP server to look up its group membership(s)
may be DNS-SD and Resource Directory <xref target="I-D.shelby-core-resource-directory"/>. The latter
is detailed more in section <xref target="CommissioningWithRD"/>.
</t><t>
To achieve better interoperability between nodes/endpoints from different manufacturers,
an OPTIONAL default RESTful interface for configuring CoAP endpoints with relevant group
information is specified here. This interface thus provides a solution for the second
case mentioned above. To access this interface a client MUST use unicast methods (GET/PUT/POST)
only as it is a method of configuring group information in individual
endpoints. Using multicast operations in this situation may lead to unexpected (possibly circular)
behavior in the network.
</t><t>
CoAP endpoints implementing this optional mechanism MUST support at least one discoverable "Group Configuration" resource
of resource type (rt) <xref target="RFC6690"/> "core.gp" where "gp" is shorthand for "group". This resource is
used by an authorized endpoint to manage group membership of the CoAP endpoint.
</t><t>
The resource of type "core.gp" has a JSON content format. A (unicast) GET on this resource returns a
JSON array of group objects, each group object formatted as shown below:
<figure><artwork>
Req: GET /gp
Res: 2.05 Content (Content-Format: application/json)
[ { "n": "Room-A-Lights.floor1.west.bldg6.example.com",
"ip": "ff15::4200:f7fe:ed37:14ca" }
]
</artwork></figure>
where the OPTIONAL "n" key/value pair defines the Group name as FQDN and the OPTIONAL
"ip" key/value pair defines the associated multicast IP address.
A CoAP endpoint can be added to another group by a (unicast) POST on the resource
with a single JSON group object, which updates the existing resource by adding the group object to the existing ones:
<figure><artwork>
Req: POST /gp (Content-Format: application/json)
{ "n": "floor1.west.bldg6.example.com",
"ip": "ff15::4200:f7fe:ed37:14cb" }
Res: 2.04 Changed
</artwork></figure>
A (unicast) 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 as soon as possible.
</t>
<t> Any (unicast) operation (i.e. PUT/POST) to change a CoAP endpoint group membership configuration MUST
use DTLS-secured CoAP <xref target="I-D.ietf-core-coap"/>. Thus only authorized clients
will be allowed by a server to configure the server's (endpoint) group membership.
</t>
</section>
<!-- section anchor="sec-3.7" title="Multicast Request Acceptance and Response Suppression" -->
<section title="Multicast Request Acceptance and Response Suppression" anchor="ResponseSuppression">
<t>
CoAP <xref target="I-D.ietf-core-coap"/> and CoRE Link Format <xref target="RFC6690"/> define
normative behaviors for:
<list style="numbers">
<t>Multicast request acceptance - in which cases a request is accepted and executed, and when not.</t>
<t>Multicast response suppression - in which cases the response of an executed request is returned to
the requesting endpoint, and when not.</t>
</list>
This section first summarizes these normative behaviors and then presents additional guidelines
for response suppression. Also a number of multicast example applications are given to illustrate the overall approach.
</t>
<t>
To apply any rules for request and/or response suppression, the IP stack interface of a CoAP server
must be able to indicate for an incoming request that the destination address of the request was multicast.
</t>
<t>
For multicast request acceptance, the behaviors are:
<list style="symbols">
<t>A server SHOULD NOT accept a multicast request that cannot be "authenticated" in some way (cryptographically
or by some multicast boundary limiting the potential sources) <xref target="I-D.ietf-core-coap"/>. See
<xref target="Security_Mitigation"/> for examples of multicast boundary limiting methods.
</t>
<t>A server SHOULD NOT accept a multicast discovery request with a query string (as defined in CoRE Link Format
<xref target="RFC6690"/>) if filtering (<xref target="RFC6690"/>) is not supported by the server.</t>
<t>A server SHOULD NOT accept a multicast request that acts on a specific resource for which multicast support
is not required. (Note that for discovery resource "/.well-known/core" multicast support is always required. Implementers are advised
to disable multicast support by default on any other resource, until explicitly enabled by an application.)</t>
<t>Otherwise accept the multicast request.</t>
</list>
</t>
<t>
For multicast response suppression, the behaviors are:
<list style="symbols">
<t>A server SHOULD NOT respond to a multicast discovery request if the filter specified by the request's
query string does not match.</t>
<t>A server MAY choose not to respond to a multicast request, if there's nothing useful to respond (e.g. error
or empty response).</t>
<t>If the IP stack interface cannot indicate that an incoming message was multicast, then the server SHOULD NOT
respond for incoming messages for selected resources which are known (through application knowledge)
to be used for multicast requests.</t>
<t>Otherwise respond to the multicast request.</t>
</list>
</t>
<t>
The above response suppression behaviors are complemented by the following guidelines.
CoAP servers should implement configurable response suppression, enabling at least
the following per resource:
<list style="symbols">
<t>Suppression of all 2.xx success responses;</t>
<t>Suppression of all 4.xx client errors;</t>
<t>Suppression of all 5.xx server errors;</t>
<t>Suppression of all 2.05 responses with empty payload.</t>
</list>
</t>
<t>
A number of group communication example applications are described below illustrating how to make use of response suppression:
<list style="symbols">
<t>CoAP resource discovery: Suppress 2.05 responses with empty payload and all 4.xx and 5.xx errors.</t>
<t>Lighting control: Suppress all 2.xx responses after a lighting change command.</t>
<t>Update group configuration data using multicast PUT: No suppression at all. Use collected responses
to identify which group members did not receive the new configuration; then attempt using CoAP CON unicast to
update those group members.</t>
<t>Multicast firmware update by sending blocks of data: Suppress all 2.xx and 5.xx responses. After having sent
all multicast blocks, the client checks each endpoint by unicast to identify which blocks are still missing
in each endpoint.</t>
<t>Conditional reporting for a group (e.g. sensors) based on a URI query: Suppress all 2.05 responses with empty payload (i.e. if
a query produces no matching results).</t>
</list>
</t>
</section>
<!-- section anchor="sec-3.8" title="Congestion Control" -->
<section title="Congestion Control" anchor="Congestion_Control">
<t>
Multicast CoAP requests may result in a multitude of replies from
different nodes, potentially causing congestion. Therefore both the sending of multicast requests
and sending unicast CoAP responses to multicast requests should be conservatively controlled.
</t><t>
CoAP <xref target="I-D.ietf-core-coap"/> reduces multicast-specific congestion
risks through the following measures:
<list style="symbols">
<t>A server MAY choose not to respond to a multicast request if there's nothing useful
to respond (e.g. error or empty response). See <xref target="ResponseSuppression"/> for
more detailed guidelines on response suppression.</t>
<t>A server SHOULD limit the support for multicast requests to specific resources
where multicast operation is required.</t>
<t>A multicast request MUST be Non-Confirmable.</t>
<t>A response to a multicast request SHOULD be Non-Confirmable (Section 5.2.3).</t>
<t>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.</t>
<t>A server SHOULD NOT accept multicast requests that can not be authenticated in some way.
See <xref target="ResponseSuppression"/> for more details on request suppression
and multicast source authentication.</t>
</list>
</t>
<t>
Additional guidelines to reduce congestion risks are:
<list style="symbols">
<t>A server in an LLN should only support multicast GET for resources that are small, e.g. the
payload of the response fits into a single link-layer frame.</t>
<t>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 <xref target="RFC6690"/>.</t>
<t>Alternatively a server can also minimize the payload length of a response to a multicast GET (e.g.
on "/.well-known/core") using CoAP blockwise transfers <xref target="I-D.ietf-core-block"/>,
returning only a first block of the link format description.</t>
<t>A client should always aim to use IP multicast with link-local scope if possible. If this is not
possible, then site-local scope IP multicast should be considered. If this is not possible, then
global scope IP multicast should be considered as a last resort only.</t>
</list>
</t>
</section>
<!-- section anchor="sec-3.9" title="Proxy Operation" -->
<section title="Proxy Operation">
<t> CoAP <xref target="I-D.ietf-core-coap"/> allows a client to request a forward-proxy to process
its CoAP request. For this purpose the client either specifies the request URI as a string in the
Proxy-URI option, or it specifies the Proxy-Scheme option with the URI constructed from the
usual Uri-* options. This approach
works well for unicast requests. However, there are certain issues and limitations of processing
the (unicast) responses to a group communication request made in this manner through a proxy.
Specifically, if a proxy would apply aggregation of responses in such a case:
<list style="symbols">
<t>Aggregation of (unicast) responses to a group communication request in a proxy is difficult.
This is because the proxy does not know how many members there are in the group or how many
group members will actually respond.</t>
<t>There is no default format defined in CoAP for aggregation of multiple responses into a single
response.</t>
</list>
But if a proxy would follow the specification for a CoAP Proxy <xref target="I-D.ietf-core-coap"/>,
the proxy would simply forward all the individual (unicast) responses to a group communication request
to the client (i.e. no aggregation), there are also issues:
<list style="symbols">
<t>The client may be confused as it may not have known that the Proxy-URI contained a multicast target.
That is, the client may be expecting only one (unicast) response but instead receives multiple (unicast)
responses potentially leading to fault conditions in the application or CoAP stack.</t>
<t>
Each individual CoAP response will appear to originate (IP Source address) from the CoAP Proxy, and
not from the server that produced the response. This makes it impossible for the client to identify
the server that produced each response.
</t>
</list>
</t>
<t>
Due to above issues, a guideline is defined here that a CoAP Proxy SHOULD NOT support processing
a multicast CoAP request but rather return a 501 (Not Implemented) response in such case. The
exception case here (i.e. to process it) is allowed under following conditions:
<list style="symbols">
<t>The CoAP Proxy MUST be explicitly configured (whitelist) to allow proxied multicast requests by
specific client(s).</t>
<t>The proxy SHOULD return individual (unicast) CoAP responses to the client, i.e. not aggregated.
(This condition MAY be removed once an aggregation format is standardized.)</t>
<t>It MUST be known to the person/entity doing the configuration of the proxy, or verified in
some way, that the client
configured in the whitelist supports receiving multiple responses to a proxied unicast CoAP
request.</t>
</list>
</t>
</section>
<!-- section anchor="sec-3.10" title="Exceptions" -->
<section title="Exceptions">
<t>Group communication using IP multicast offers improved network efficiency and latency amongst
other benefits. However, group communication may not always be possible to implement in
a given network. The primary reason for this will be if IP multicast is not supported in
the network. For example, in a LLN, if the RPL protocol is used for routing multicast packets
and RPL routers operate in "Non-storing mode"
<xref target="RFC6550"/> there will be no IP multicast routing in that network beyond link-local
scope. This means that any CoAP group communication above link-local scope will not be supported in that network.
</t>
</section>
</section>
<!-- section anchor="sec-4" title="Use Cases and Corresponding Protocol Flows" -->
<section title="Use Cases and Corresponding Protocol Flows" anchor="Use_Cases">
<!-- section anchor="sec-4.1" title="Introduction" -->
<section title="Introduction">
<t>The use of CoAP group communication is shown in the context of the following two use
cases and corresponding protocol flows:
<list style="symbols">
<t>Discovery of Resource Directory (RD, <xref target="I-D.shelby-core-resource-directory"/>):
discovering the local CoAP RD which
contains links (URIs) to resources stored on other CoAP servers
<xref target="RFC6690" />.
</t>
<t>Lighting Control: synchronous operation of a group of
IPv6-connected lights (e.g., 6LoWPAN <xref target="RFC4944"/> lights).
</t>
</list>
</t>
</section>
<!-- section anchor="sec-4.2" title="Network Configuration" -->
<section title="Network Configuration">
<t>To illustrate the use cases we define two network configurations. Both are based on
the topology as shown in <xref target="Example_Topology" />. The two configurations
using this topology are:
<list style="numbers">
<t>Subnets are 6LoWPAN networks; the routers Rtr-1 and Rtr-2 are 6LoWPAN Border Routers
(6LBRs, <xref target="RFC6775"/>).</t>
<t>Subnets are Ethernet links; the routers Rtr-1 and Rtr-2 are multicast-capable Ethernet routers.</t>
</list>
Both configurations are further specified by the following:
<list style="symbols">
<t>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.</t>
<t>Light-1 and the Light Switch are connected to a router (Rtr-1).</t>
<t>Light-2 and the Light-3 are connected to another router (Rtr-2).</t>
<t>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
(<xref target="mld"/>) for the following use cases to work.</t>
<t>A CoAP RD is connected to the network backbone.</t>
<t>The DNS server is optional. If the server is there (connected to the network backbone)
then certain DNS based features are available (e.g. DNS resolution of URI to IP multicast
address). If the DNS server is not there, then different manual provisioning of the network
is required (e.g. IP multicast addresses are hard-coded into devices).</t>
<t>A Controller (client) is connected to the backbone, which is able to control
various building functions including lighting.</t>
</list>
</t>
<figure anchor="Example_Topology" title="Network Topology of a Large Room (Room-A)" align="center">
<artwork>
<![CDATA[
Network
Backbone
################################################ |
# ********************** Room-A # |
# ** Subnet-1 ** # |
# * ** # |
# * +----------+ * # |
# * | Light |-------+ * # |
# * | Switch | | * # |
# * +----------+ +---------+ * # |
# * | Rtr-1 |-----------------------------+
# * +---------+ * # |
# * +----------+ | * # |
# * | Light-1 |--------+ * # |
# * +----------+ * # |
# * * # |
# ** ** # |
# ********************** # |
# # |
# ********************** # |
# ** Subnet-2 ** # |
# * ** # |
# * +----------+ * # |
# * | Light-2 |-------+ * # |
# * | | | * # |
# * +----------+ +---------+ * # |
# * | Rtr-2 |-----------------------------+
# * +---------+ * # |
# * +----------+ | * # |
# * | Light-3 |--------+ * # |
# * +----------+ * # +------------+ |
# * * # | Controller |--+
# ** ** # | Client | |
# ********************** # +------------+ |
################################################ |
|
+------------+ |
| CoAP | |
| Resource |-----------------+
| Directory | |
+------------+ |
+------------+ |
| DNS Server | |
| (Optional) |-----------------+
+------------+
]]>
</artwork>
</figure>
</section>
<!-- section anchor="sec-4.3" title="Discovery of Resource Directory" -->
<section title="Discovery of Resource Directory" anchor="Discovery_Use_Case">
<t>
The protocol flow for discovery of the CoAP RD for the given network
(of <xref target="Example_Topology" />) is shown in <xref target="Example_Protocol_Flow_1" />:
<list style="symbols">
<t>The fixture for Light-2 is installed and powered on for the first time.</t>
<t>Light-2 will then search for the local CoAP RD by sending
out a GET request (with the "/.well-known/core?rt=core.rd" request URI)
to the site-local "All CoAP Nodes" multicast address.</t>
<t>This multicast message will then go to each node in subnet-2. Rtr-2 will
then forward into to the Network Backbone where it will be received
by the CoAP RD. All other nodes in subnet-2 will ignore the multicast
GET because it is qualified by the query string "?rt=core.rd"
(which indicates it should only be processed by the endpoint if it is a RD).</t>
<t>The CoAP RD will then send back a unicast response containing the requested content.</t>
<t>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.</t>
</list>
The CoAP 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.
</t>
<t>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.
</t>
<figure anchor="Example_Protocol_Flow_1" title="Resource Directory Discovery via Multicast Message" align="center">
<artwork>
<![CDATA[
Light CoAP
Light-1 Light-2 Light-3 Switch Rtr-1 Rtr-2 RD
| | | | | | |
| | | | | | |
********************************** | | |
* Light-2 is installed * | | |
* and powers on for first time * | | |
********************************** | | |
| | | | | | |
| | | | | | |
| | COAP NON Mcast(GET | |
| | /.well-known/core?rt=core.rd) | |
| |--------->-------------------------------->| |
| | | | | |--------->|
| | | | | | |
| | | | | | |
| | COAP NON (2.05 Content | |
| | </rd>;rt="core.rd";ins="Primary") |<---------|
| |<------------------------------------------| |
| | | | | | |
]]>
</artwork>
</figure>
</section>
<!-- section anchor="sec-4.4" title="Lighting Control" -->
<section title="Lighting Control" anchor="Lighting_Control_Use_Case">
<t>
The protocol flow for a building automation lighting control scenario
for the network (<xref target="Example_Topology" />) in 6LoWPAN configuration
is shown in sequence in <xref target="Example_Protocol_Flow_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. (See section <xref target="ResponseSuppression"/> for more details on
response suppression by a server.)
</t>
<t>In addition, <xref target="Example_Protocol_Flow_4" />
shows a protocol flow example for the case that servers do respond to a
lighting control multicast request with (unicast) CoAP NON responses.
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 or alerting the user via its
LCD display.
</t>
<t>Reliability of CoAP multicast is not guaranteed. Therefore, one or more lights in the
group may not have received the CoAP control request due to packet loss. In this use case
there is no detection nor correction of such situations: the application layer expects
that the multicast forwarding/routing will be of sufficient quality to provide on average
a very high probability of packet delivery to all CoAP endpoints in a multicast group.
An example protocol to accomplish this is the MPL forwarding protocol for LLNs
<xref target="I-D.ietf-roll-trickle-mcast"/>.
</t>
<t>
We assume the following steps have already occurred before the illustrated flows:
<list style="numbers">
<t>Startup phase: 6LoWPANs are formed. IPv6 addresses assigned to all devices.
The CoAP network is formed.</t>
<t>Network configuration (application-independent): 6LBRs are configured with
multicast address blocks to filter out or to pass through to/from the 6LoWPAN.</t>
<t>Commissioning phase (application-related): The IP multicast address of the group
(Room-A-Lights) has been configured in all the Lights and in the Light Switch.</t>
<t>As an alternative to the previous step, when a DNS server is available, the
Light Switch and/or the Lights
have been configured with a group hostname which each nodes resolves to the above
IP multicast address of the group. </t>
</list>
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.
</t>
<figure anchor="Example_Protocol_Flow_3" title="Light Switch Sends Multicast Control Message" align="center">
<artwork>
<![CDATA[
Light Network
Light-1 Light-2 Light-3 Switch Rtr-1 Rtr-2 Backbone
| | | | | | |
| | | | | | |
| | *********************** | |
| | * User flips on * | |
| | * light switch to * | |
| | * turn on all the * | |
| | * lights in Room A * | |
| | *********************** | |
| | | | | | |
| | | | | | |
| | | COAP NON Mcast(PUT, | |
| | | Payload=lights ON) | |
|<-------------------------------+--------->| | |
ON | | | |-------------------->|
| | | | | |<---------|
| |<---------|<-------------------------------| |
| ON ON | | | |
^ ^ ^ | | | |
*********************** | | | |
* Lights in Room-A * | | | |
* turn on (nearly * | | | |
* simultaneously) * | | | |
*********************** | | | |
| | | | | | |
]]>
</artwork>
</figure>
<figure anchor="Example_Protocol_Flow_4" title="Lights (Optionally) Respond to Multicast CoAP Request" align="center">
<artwork>
<![CDATA[
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) |
| | |------------------------------->| |
| | | | | |--------->|
| | | | |<--------------------|
| | | |<---------| | |
| | | | | | |
]]>
</artwork>
</figure>
<t>
Another, but similar, lighting control use case is shown in <xref target="Example_Protocol_Flow_5"/>.
In this case a controller connected to the Network Backbone sends a CoAP multicast request
to turn on all lights in Room-A. Every Light sends back a CoAP response to the Controller after
being turned on.
</t>
<figure anchor="Example_Protocol_Flow_5" title="Controller On Backbone Sends Multicast Control Message" align="center">
<artwork>
<![CDATA[
Network
Light-1 Light-2 Light-3 Rtr-1 Rtr-2 Backbone Controller
| | | | | | |
| | | | | COAP NON Mcast(PUT,
| | | | | Payload=lights ON)
| | | | | |<-------|
| | | |<----------<---------| |
|<--------------------------------| | | |
ON | | | | | |
| |<----------<---------------------| | |
| ON ON | | | |
^ ^ ^ | | | |
*********************** | | | |
* Lights in Room-A * | | | |
* turn on (nearly * | | | |
* simultaneously) * | | | |
*********************** | | | |
| | | | | | |
| | | | | | |
| COAP NON (2.04 Changed) | | | |
|-------------------------------->| | | |
| | | |-------------------->| |
| | COAP NON (2.04 Changed) | |------->|
| |-------------------------------->| | |
| | | | |--------->| |
| | | COAP NON (2.04 Changed) |------->|
| | |--------------------->| | |
| | | | |--------->| |
| | | | | |------->|
| | | | | | |
]]>
</artwork>
</figure>
</section>
<!-- section anchor="sec-4.5" title="Lighting Control in MLD Enabled Network" -->
<section title="Lighting Control in MLD Enabled Network" anchor="Lighting_Control_Use_Case_MLD">
<t>
The use case of previous section can also apply in networks where nodes support the
MLD protocol <xref target="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.
</t><t>
The resulting protocol flow is shown in <xref target="Example_Protocol_Flow_2"/>. This flow
is executed after the commissioning phase, as soon as Lights are configured with a group
address to listen to. The (unicast) MLD Reports may require periodic refresh activity as specified
by the MLD protocol. In the figure, LL denotes Link Local communication.
</t><t>
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 <xref target="Lighting_Control_Use_Case"/>, is needed anymore.
</t><t>
<figure anchor="Example_Protocol_Flow_2" title="Joining Lighting Groups Using MLD" align="center">
<artwork>
<![CDATA[
Light Network
Light-1 Light-2 Light-3 Switch Rtr-1 Rtr-2 Backbone
| | | | | | |
| | | | | | |
| | | | | | |
| MLD Report: Join | | | | |
| Group (Room-A-Lights) | | | |
|---LL------------------------------------->| | |
| | | | |MLD Report: Join |
| | | | |Group (Room-A-Lights)|
| | | | |---LL---->----LL---->|
| | | | | | |
| | MLD Report: Join | | | |
| | Group (Room-A-Lights) | | |
| |---LL------------------------------------->| |
| | | | | | |
| | | MLD Report: Join | | |
| | | Group (Room-A-Lights) | |
| | |---LL-------------------------->| |
| | | | | | |
| | | | |MLD Report: Join |
| | | | |Group (Room-A-Lights)|
| | | | |<--LL-----+---LL---->|
| | | | | | |
| | | | | | |
]]>
</artwork>
</figure>
</t>
</section>
<section title="Commissioning the Network Based On Resource Directory" anchor="CommissioningWithRD">
<t>
This section outlines how devices in the lighting use case (both Switches and Lights) can
be commissioned, making use of Resource Directory <xref target="I-D.shelby-core-resource-directory"/>
and its group configuration feature.
</t><t>
Once the Resource Directory (RD) is discovered, the Switches and Lights need to be discovered and their groups need to be defined. For the commissioning of these devices, a commissioning tool can be used that defines the entries in the RD. The commissioning tool has the authority to change the contents of the RD and the nodes. DTLS based security is used by the commissioning tool to modify operational data in RD, Switches and Lights.
</t><t>
In our particular use case, a group of three lights is defined with one multicast address and hostname Room-A-Lights.floor1.west.bldg6.example.com. The commissioning device has a list of the three lights and the associated multicast address. For each light in the list the tool learns the IP address of the light and instructs the RD with 3 POST commands to store the end-points associated with the three lights as prescribed by RD. Finally the commissioning device defines the group in the RD to contain these three end-points. Also the commissioning tool writes the MC address in the Lights with e.g. the POST /gp command discussed in <xref target="ConfiguringMembers"/>.
</t><t>
The light switch can discover the group in RD and learn the MC address of the group. The light switch will use this address to send MC commands to the members of the group. When the message arrives the Lights should recognize the MC address and accept the message.
</t>
</section>
</section>
<!-- section anchor="sec-5" title="Deployment Guidelines" -->
<section title="Deployment Guidelines">
<t>
This section provides guidelines how an IP Multicast based solution
for CoAP group communication can be deployed in various network configurations.
</t>
<!-- section anchor="sec-5.1" title="Target Network Topologies" -->
<section title="Target Network Topologies">
<t>
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.
</t><t>
Each topology may pose different requirements on the configuration of routers and protocol(s),
in order to enable efficient CoAP group communication.
</t>
</section>
<!-- section anchor="sec-5.2" title="Advertising Membership of Multicast Groups" -->
<section title="Advertising Membership of Multicast Groups" anchor="Advertising_Memership">
<t>
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.
</t>
<section title="Using the Multicast Listener Discovery (MLD) Protocol">
<t>
CoAP nodes that are IP hosts (i.e. not IP routers) are generally unaware of the specific multicast
routing/forwarding protocol being used. When such a host needs to join a specific (CoAP) multicast
group, it usually requires a way to
signal to 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.
</t>
<t>
The Multicast Listener Discovery (MLD) protocol (<xref target="RFC3810" />, <xref target="mld"/>)
is the standard IPv6 method to achieve this.
<xref target="RFC6636"/> discusses tuning of MLD for mobile and wireless networks. These guidelines
may be useful when implementing MLD in LLNs.
</t><t>
Alternatively, to avoid the use of MLD in LLN deployments, either all
nodes can be configured as multicast routers in an LLN, or a multicast forwarding/flooding protocol
can be used that forwards any IP multicast packet to all forwarders (routers) in the subnet (LLN).
</t>
</section>
<section title="Using the RPL Routing Protocol">
<t>
The RPL routing protocol <xref target="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 routers. Then, the RPL protocol can route multicast CoAP
requests over multiple hops to the correct CoAP servers.
</t><t>
This mechanism can 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
and the edge router needs to know what IP multicast traffic to pass through from
the backbone network into the LLN subnet.
</t>
</section>
<section title="Using the MPL Forwarding Protocol">
<t>
The MPL forwarding protocol <xref target="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.
</t>
</section>
</section>
<!-- section anchor="sec-5.3" title="6LoWPAN-Specific Guidelines" -->
<section title="6LoWPAN-Specific Guidelines">
<t>
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 (<xref target="mld"/>) on the backbone link.
</t>
<t>
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:
<list style="symbols">
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
</list>
</t>
</section>
</section>
<!-- section anchor="sec-6" title="Security Considerations" -->
<section title="Security Considerations" anchor="security">
<t>This section describes the relevant security configuration for CoAP group communication using
IP multicast. The threats to CoAP group communication are also identified and various approaches to
mitigate these threats are summarized.</t>
<!-- section anchor="sec-6.1" title="Security Configuration" -->
<section title="Security Configuration">
<t>As defined in <xref target="I-D.ietf-core-coap" />, CoAP group communication based on
IP multicast must use the following security modes:
<list style="symbols">
<t>Group communication MUST operate in CoAP NoSec (No Security) mode.</t>
<t>Group communication MUST NOT use "coaps" scheme. That is, all group
communication MUST use only "coap" scheme.</t>
</list>
</t>
</section>
<!-- section anchor="sec-6.2" title="Threats" -->
<section title="Threats">
<t>Essentially the above configuration means that there is no security at the CoAP layer for
group communication. This is due to the fact that the current
DTLS based approach for CoAP is exclusively unicast oriented and does not support
group security features such as group key exchange and group authentication. As a
direct consequence of this, CoAP group communication is vulnerable to all attacks mentioned in
<xref target="I-D.ietf-core-coap" /> for IP multicast.
</t>
</section>
<!-- section anchor="sec-6.3" title="Threat Mitigation" -->
<section title="Threat Mitigation" anchor="Security_Mitigation">
<t><xref target="I-D.ietf-core-coap" /> identifies various threat mitigation techniques for CoAP IP
multicast. In addition to those guidelines, it is recommended that for sensitive data or
safety-critical control, a combination of appropriate link-layer security and administrative
control of IP multicast boundaries should be used. Some examples are given below.</t>
<!-- section anchor="sec-6.3.1" title="WiFi Scenario" -->
<section title="WiFi Scenario">
<t> In a home automation scenario (using WiFi), the WiFi encryption should be enabled to prevent
rogue nodes from joining. Also, if MAC address filtering at the WiFi Access Point is
supported that should also be enabled.
The IP router should have the fire wall enabled to isolate the home network from the
rest of the Internet. In addition, the domain of the IP multicast should be set to be
either link-local scope or site-local scope. Finally, if possible, devices should
be configured to accept only Source Specific Multicast (SSM) packets
(see <xref target="RFC4607"/>) from within the trusted home network. For example, all
lights in a particular room should only accept IP multicast traffic originating from the
master light switch in that room. In this case, the Spoofed Source Address considerations
of Section 7.4 of <xref target="RFC4607"/> should be heeded.
</t>
</section>
<!-- section anchor="sec-6.3.2" title="6LoWPAN Scenario" -->
<section title="6LoWPAN Scenario">
<t>In a building automation scenario, a particular room may have a single 6LoWPAN topology
with a single Edge Router (6LBR). Nodes on the subnet can use link-layer encryption
to prevent rogue nodes from joining. The 6LBR can be configured so that it blocks any
incoming IP multicast traffic. Another example topology could be a multi-subnet
6LoWPAN in a large conference room. In this case, the backbone can implement port
authentication (IEEE 802.1X) to ensure only authorized devices can join the Ethernet
backbone. The access router to this secured segment can also be configured to
block incoming IP multicast traffic.
</t>
</section>
<!-- section anchor="sec-6.3.3" title="Future Evolution" -->
<section title="Future Evolution">
<t>In the future, to further mitigate the threats, the developing approach for DTLS-based IP multicast
security for CoAP networks (see <xref target="I-D.keoh-tls-multicast-security" />) or similar
approaches should be considered once they mature.
</t>
</section>
</section>
</section>
<!-- section anchor="sec-7" title="IANA Considerations" -->
<section title="IANA Considerations">
<t>
tbd: allocation of "core.gp" resource type in relevant registry.
</t>
<t>(Note to RFC Editor: The required multicast address
request to IANA is made in <xref target="I-D.ietf-core-coap"/>).
</t>
</section>
<!-- section anchor="sec-8" title="Acknowledgements" -->
<section title="Acknowledgements">
<t>
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.
</t>
</section>
</middle>
<back>
<!-- section anchor="sec-9" title="References" -->
<references title="Normative References">
&RFC2119;
&RFC2616;
&RFC3810;
&RFC4291;
&RFC4601;
&RFC4607;
&RFC4944;
&RFC5771;
&RFC6550;
&RFC6636;
&RFC6690;
&RFC6775;
&I-D.ietf-core-coap;
</references>
<references title="Informative References">
&I-D.ietf-core-block;
&I-D.vanderstok-core-dna;
&I-D.ietf-roll-trickle-mcast;
&I-D.keoh-tls-multicast-security;
&I-D.shelby-core-resource-directory;
</references>
<!-- section anchor="Appendix A" title="Multicast Listener Discovery" -->
<section anchor="mld" title="Multicast Listener Discovery (MLD)">
<t>
In order to extend the scope of IP multicast beyond link-local scope, an IP multicast
routing or forwarding 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.
</t><t>
The Multicast Listener Discovery (MLD) protocol <xref target="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.
</t><t>
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.
</t><t>
<xref target="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.
</t>
</section>
<!-- section anchor="Appendix B" title="Change Log" -->
<section title="Change Log">
<t>Changes from ietf-05 to ietf-06:
<list style="symbols">
<t>Added a new section on commissioning flow when using discovery services when end devices discover
in which multicast group they are allocated (#295).</t>
<t>Added a new section on CoAP Proxy Operation (section 3.9) that outlines the potential issues and limitations of
doing CoAP multicast requests via a CoAP Proxy (#274).</t>
<t>Added use case of multicasting controller on the backbone (#279).</t>
<t>Use cases were updated to show only a single CoAP RD (to replace the previous multiple RDs with one in each subnet).
This is a more efficient deployment and also avoids RD specific issues such as synchronization
of RD information between serves (#280).</t>
<t>Added text to section 3.6 (Configuring Group Membership in Endpoints) that clarified that any (unicast) operation to
change an endpoint's group membership must use DTLS-secured CoAP.</t>
<t>Clarified relationship of this document to <xref target="I-D.ietf-core-coap"/> in section 2.2 (Scope).</t>
<t>Removed IPSec related requirement, as IPSec is not part of <xref target="I-D.ietf-core-coap"/> anymore.</t>
<t>Editorial reordering of subsections in section 3 to have a better flow of topics. Also renamed some of the
(sub)sections to better reflect their content. Finally, moved the URI Configuration text to the same
section as the Port Configuration section as it was a more natural grouping (now in section 3.3) .</t>
<t>Editorial rewording of section 3.7 (Multicast Request Acceptance and Response Suppression) to make
the logic easier to comprehend (parse).</t>
<t>Various editorial updates for improved readability.</t>
</list>
</t>
<t>Changes from ietf-04 to ietf-05:
<list style="symbols">
<t>Added a new section 3.9 (Exceptions) that highlights that IP multicast (and hence group communication)
is not always available (#187).</t>
<t>Updated text on the use of <xref target="RFC2119"/> language (#271) in Section 1.</t>
<t>Included guidelines on when (not) to use CoAP responses to multicast requests and when
(not) to accept multicast requests (#273).</t>
<t>Added guideline on use of core-block for minimizing response size (#275).</t>
<t>Restructured section 6 (Security Considerations) to more fully describe threats and threat mitigation (#277).</t>
<t>Clearly indicated that DNS resolution and reverse DNS lookup are optional.</t>
<t>Removed confusing text about a single group having multiple IP addresses. If multiple IP addresses
are required then multiple groups (with the same members) should be created.</t>
<t>Removed repetitive text about the fact that group communication is not guaranteed.</t>
<t>Merged previous section 5.2 (Multicast Routing) into 3.1 (IP Multicast Routing Background) and added
link to section 5.2 (Advertising Membership of Multicast Groups).</t>
<t>Clarified text in section 3.8 (Congestion Control) regarding precedence of use of IP
multicast domains (i.e. first try to use link-local scope, then site-local scope,
and only use global IP multicast as a last resort).</t>
<t>Extended group resource manipulation guidelines with use of pre-configured ports/paths
for the multicast group.</t>
<t>Consolidated all text relating to ports in a new section 3.3 (Port Configuration).</t>
<t>Clarified that all methods (GET/PUT/POST) for configuring group membership in endpoints
should be unicast (and not multicast) in section 3.7 (Configuring Group Membership
In Endpoints).</t>
<t>Various editorial updates for improved readability, including editorial comments by Peter
van der Stok to WG list of December 18th, 2012.</t>
</list>
</t>
<t>Changes from ietf-03 to ietf-04:
<list style="symbols">
<t>Removed section 2.3 (Potential Solutions for Group Communication) as it is
purely background information and moved section to draft-dijk-core-groupcomm-misc (#266).</t>
<t>Added reference to draft-keoh-tls-multicast-security to section 6 (Security Considerations).</t>
<t>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).</t>
<t>Deleted section 8 (Conclusions) as it is redundant (#268).</t>
<t>Simplified light switch use case (#269) by splitting into basic operations and additional
functions (#269).</t>
<t>Moved section 3.7 (CoAP Multicast and HTTP Unicast Interworking) to draft-dijk-core-groupcomm-misc
(#270).</t>
<t>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).</t>
<t>Focus section 3.5 (Configuring Group Membership) on a single proposed solution.</t>
<t>Scope of section 5.3 (Use of MLD) widened to multicast destination advertisement methods
in general.</t>
<t>Rewrote section 2.2 (Scope) for improved readability.</t>
<t>Moved use cases that are not addressed to draft-dijk-core-groupcomm-misc.</t>
<t>Various editorial updates for improved readability.</t>
</list>
</t>
<t>Changes from ietf-02 to ietf-03:
<list style="symbols">
<t>Clarified that a group resource manipulation may return back a mixture of successful
and unsuccessful responses (section 3.4 and Figure 6) (#251).</t>
<t>Clarified that security option for group communication must be NoSec mode (section 6) (#250).</t>
<t>Added mechanism for group membership configuration (#249).</t>
<t>Removed IANA request for multicast addresses (section 7) and replaced with a note
indicating that the request is being made in <xref target="I-D.ietf-core-coap"/> (#248).</t>
<t>Made the definition of 'group' more specific to group of CoAP endpoints and included
text on UDP port selection (#186).</t>
<t>Added explanatory text in section 3.4 regarding why not to use group communication
for non-idempotent messages (i.e. CoAP POST) (#186).</t>
<t>Changed link-local RD discovery to site-local in RD discovery use case to make it
more realistic.</t>
<t>Fixed lighting control use case CoAP proxying; now returns individual CoAP responses
as in coap-12.</t>
<t>Replaced link format I-D with RFC6690 reference.</t>
<t>Various editorial updates for improved readability</t>
</list>
</t>
<t>Changes from ietf-01 to ietf-02:
<list style="symbols">
<t>Rewrote congestion control section based on latest CoAP text including Leisure concept (#188)</t>
<t>Updated the CoAP/HTTP interworking section and example use case with more details and use
of MLD for multicast group joining</t>
<t>Key use cases added (#185)</t>
<t>References to <xref target="I-D.vanderstok-core-dna" /> and
draft-castellani-core-advanced-http-mapping added</t>
<t>Moved background sections on "MLD" and "CoAP-Observe" to Appendices</t>
<t>Removed requirements section (and moved it to draft-dijk-core-groupcomm-misc)</t>
<t>Added details for IANA request for group communication multicast addresses</t>
<t>Clarified text to distinguish between "link local" and general multicast cases</t>
<t>Moved lengthy background section 5 to draft-dijk-core-groupcomm-misc and replaced with a summary</t>
<t>Various editorial updates for improved readability</t>
<t>Change log added</t>
</list>
</t>
<t>Changes from ietf-00 to ietf-01:
<list style="symbols">
<t>Moved CoAP-observe solution section to section 2</t>
<t>Editorial changes</t>
<t>Moved security requirements into requirements section</t>
<t>Changed multicast POST to PUT in example use case</t>
<t>Added CoAP responses in example use case</t>
</list>
</t>
<t>Changes from rahman-07 to ietf-00:
<list style="symbols">
<t>Editorial changes</t>
<t>Use cases section added</t>
<t>CoRE Resource Directory section added</t>
<t>Removed section 3.3.5. IP Multicast Transmission Methods</t>
<t>Removed section 3.4 Overlay Multicast</t>
<t>Removed section 3.5 CoAP Application Layer Group Management</t>
<t>Clarified section 4.3.1.3 RPL Routers with Non-RPL Hosts case</t>
<t>References added and some normative/informative status changes</t>
</list>
</t>
</section>
</back>
</rfc>
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