One document matched: draft-ietf-core-groupcomm-03.xml
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<rfc category="info" ipr="trust200902" docName="draft-ietf-core-groupcomm-03">
<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="2012"/>
<area>Applications</area>
<workgroup>CoRE Working Group</workgroup>
<abstract>
<t>
CoAP is a RESTful transfer protocol for constrained devices. It is
anticipated that constrained devices will often naturally operate in groups
(e.g. in a building automation scenario all lights in a given room may
need to be switched on/off as a group). This document defines how the CoAP
protocol should be used in a group communication context. An approach for
using CoAP on top of IP multicast is detailed for both constrained and
un-constrained networks. Also, various use causes and corresponding protocol
flows are provided to illustrate important concepts. Finally, guidance is
provided for deployment in various network topologies.
</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", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in <xref target="RFC2119" />.
</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 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</t>
<t hangText="Multicast"><vspace />
Sending a message to multiple destination nodes simultaneously.
There are various options to implement multicast including
layer 2 (Media Access Control) or layer 3 (IP) mechanisms.</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 />
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.</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 composed by function. For example, the group
"all lights in building one" may consist of the groups "all lights
on floor one of building one", "all lights on floor two of building
one", etc. Groups may be preconfigured or dynamically formed. If information
needs to be sent to or received from a group of devices, group communication
mechanisms can improve efficiency and latency of communication and reduce
bandwidth requirements for a given application. HTTP does not
support any equivalent functionality to CoAP group communication.</t>
</section>
<!-- section anchor="sec-2.2" title="Scope" -->
<section title="Scope">
<t>
In this draft, we address the issues related to CoAP group communication in
detail, with use cases, recommended approaches and analysis of the impact
to the CoAP protocol and to implementations. The guiding principle is to apply
wherever possible existing IETF protocols to achieve group communication
functionality. In many cases the contribution of this document lies in
explaining how existing mechanisms may be used to together fulfill CoAP
group communication needs for specific use cases.
</t>
</section>
<!-- section anchor="sec-2.3" title="Potential Solutions for Group Coummunication" -->
<section title="Potential Solutions for Group Communication">
<t>
The classic concept of group communications is that of a single source
distributing content to multiple destination recipients that are all part of a group.
Before content can be distributed, there is a separate process to form the group.
The source may be either a member or non-member of the group.</t>
<t>Group communication solutions have evolved from "bottom" to "top", i.e., from layer 2
(Media Access Control broadcast/multicast) and layer 3 (IP multicast)
to application layer group communication, also referred to as application layer
multicast. A study published in 2005 <xref target="Lao05" /> identified new solutions in the "middle"
(referred to as overlay multicast) that utilize an infrastructure based on proxies.
</t>
<t>
Each of these classes of solutions may be compared <xref target="Lao05" /> using
metrics such as link stress and level of host complexity <xref target="Banerjee01" />. The
results show for a realistic internet topology that IP Multicast is the most resource-efficient,
with the downside being that it requires the most effort to deploy in the infrastructure. IP
Multicast is the solution adopted by this draft for CoAP group communication.
</t>
</section>
</section>
<!-- section anchor="sec-3" title="IP Multicast Based Group Communication" -->
<section title="CoAP Group Communication Based On IP Multicast" anchor="IPMulticast">
<!-- section anchor="sec-3.1" title="Introduction" -->
<section title="IP Multicast 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/mDNS) and carrier IPTV deployments. The
packet economy and minimal host complexity of IP multicast make it attractive for
group communication in constrained environments. Therefore IP multicast is the recommended
underlying mechanism for CoAP group communication, and the approach assumed in this document.
</t>
<t>To achieve IP multicast beyond a subnet, an IP multicast routing protocol needs to be
active on routers. The RPL protocol <xref target="RFC6550"/> for example is able to
route multicast traffic in constrained LLNs. While 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 on the network link on which it was sent.
</t>
</section>
<!-- section anchor="sec-3.2" title="Group URIs and Multicast Addresses" -->
<section title="CoAP 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. The group
MAY have more than one associated IP multicast address. An endpoint MAY be a member of
multiple groups. Group membership of an endpoint MAY dynamically change over time. The group
MAY be identified by a Group Name (<xref target="I-D.vanderstok-core-dna" />) which is defined
as a prefix string of a Group FQDN.
The Group FQDN can be uniquely mapped to a site-local or global multicast IP address via DNS
resolution.
</t><t>
A CoAP multicast request that addresses a group includes a Group URI as the request URI. A
Group URI has the scheme 'coap' and includes in the authority part either a group IP address or
a hostname that can be resolved to the group IP address (e.g., a Group Name or Group FQDN).
Group URIs MUST follow the URI syntax <xref target="RFC3986" />.
</t><t>
A CoAP node becomes a group member by listening for CoAP messages on the group's IP multicast
address, assuming the default CoAP UDP port. Note that a non-default UDP port MAY be specified
for the group; in which case implementers MUST ensure that all group members are configured to
use this same port.
</t><t>
Examples of hierarchical group naming (and scoping) for a building control application are shown below.
<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>Reverse mapping (from IP multicast address to group FQDN) is supported
using the reverse DNS resolution technique (<xref target="I-D.vanderstok-core-dna" />).
</t>
</section>
<!-- section anchor="sec-3.3" title="Group Discovery" -->
<section title="Group Discovery and Member Discovery" anchor="GroupDiscovery">
<t>
CoAP defines a resource discovery capability, but does not yet specify how
to discover groups (e.g. find a group to join or send a multicast message to)
or to discover members of a group (e.g. to address selected group members by unicast).
These topics are elaborated in
more detail in <xref target="I-D.vanderstok-core-dna" /> including examples for using
DNS-SD and CoRE Resource Directory.
</t>
<!-- section anchor="sec-3.3.1" title="DNS-SD" -->
<section title="DNS-SD" anchor="DNSSD">
<t>
DNS-based Service Discovery <xref target="I-D.cheshire-dnsext-dns-sd" /> defines a
conventional way to configure DNS PTR, SRV, and TXT records to enable
enumeration of services, such as services offered by CoAP nodes,
or enumeration of all CoAP nodes, within specified subdomains.
A service is specified by a name of the form <Instance>.<ServiceType>.<Domain>,
where the service type for CoAP nodes is _coap._udp and the domain is a DNS
domain name that identifies a group as in the examples above.
For each CoAP end-point in a group, a PTR record with the name
_coap._udp and/or a PTR record with the name _coap._udp.<Domain> is
defined and it points to an SRV record having the
<Instance>.<ServiceType>.<Domain> name.
</t><t>
All CoAP nodes in a given subdomain may be enumerated by sending a
query for PTR records named _coap._udp to the authoritative DNS server
for that zone. A list of SRV records is returned. Each SRV record
contains the port and host name (AAAA record) of a CoAP node. The IP
address of the node is obtained by resolving the host name. DNS-SD
also specifies an optional TXT record, having the same name as the
SRV record, which can contain "key=value" attributes. This can be
used to store information about the device, e.g. schema=DALI,
type=switch, group=lighting.bldg6, etc.
</t><t>
Another feature of DNS-SD is the ability to specify service subtypes
using PTR records. For example, one could represent all the CoAP
groups in a subdomain by PTR records with the name
_group._sub._coap._udp or alternatively _group._sub._coap._udp.<Domain>.
</t>
</section>
<!-- section anchor="sec-3.3.2" title="CoRE Resource Directory" -->
<section title="CoRE Resource Directory">
<t>
CoRE Resource Directory <xref target="I-D.shelby-core-resource-directory"/>
defines the concept of a Resource Directory (RD) server where CoAP servers
can register their resources offered and CoAP clients can discover these
resources by querying the RD server. RD syntax can be mapped to DNS-SD
syntax and vice versa <xref target="I-D.lynn-core-discovery-mapping"/>, such
that the above approach can be reused for group discovery and group member
discovery.
</t><t>
Specifically, the Domain (d) parameter can be set to the group URI by an
end-point registering to the RD. If an end-point wants to join multiple
groups, it has to repeat the registration process for each group it wants
to join.
</t>
</section>
</section>
<!-- section anchor="sec-3.4" title="Group Resource Manipulation" -->
<section title="Group Resource Manipulation">
<t>Group communications SHALL only be used for idempotent methods
(i.e. CoAP GET, PUT, DELETE). The CoAP messages that are sent
via multicast SHALL be Non-Confirmable. </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="I-D.ietf-core-coap"/>. The unicast responses may be a mixture
of success (e.g. 2.05 Content) and failure (e.g. 4.04 Not Found)
codes depending on the individual server processing result.
</t>
<t>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.
</t>
<t>
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.
</t>
<t>
Therefore, some measures must be present to ensure uniformity in port number
and resource names/locations within a group. This document proposes the following measures:
<list style="symbols">
<t> All CoAP multicast requests MUST be sent either to the default CoAP UDP port
(i.e. default Uri-Port as defined in <xref target="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.</t>
<t> All CoAP multicast requests SHOULD operate only on URIs (links) which were retreived
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.</t>
</list>
</t>
</section>
<!-- section anchor="sec-3.5" title="Configuring Group Membeship In Endpoints" -->
<section title="Configuring Group Membership In Endpoints">
<t>
In some use cases, the group membership of endpoints needs to be configurable after the network
has been deployed. Example use cases can be found in building control: a commissioning tool
determines to which groups a light or sensor node belongs, and writes this information to all
nodes, which can subsequently join the correct group.
</t><t>
To achieve smoother interoperability between nodes/endpoints from different manufacturers, it is
proposed here to define an OPTIONAL standardized RESTful means of configuring CoAP endpoints
with relevant group information.
</t><t>
CoAP endpoints implementing this mechanism MUST support a discoverable "Group Configuration" resource
of resource type (rt) <xref target="RFC6690"/> "core.gp". This resource (and perhaps its
sub-resources, TBD) are used to manage group membership. Three design options for this mechanism
are presented here as a placeholder (TBD).
</t><t>
Design 1: use CoRE link format payloads to communicate group membership to endpoints.
(TBD Not clear how this should be realized.)
</t><t>
Design 2: use a JSON type resource. For example, a GET on the "core.gp" resource returns a
JSON array of group objects.
<figure><artwork>
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" }
]
</artwork></figure>
where "n" defines the Group FQDN and "ip" defines the associated multicast IP address.
As a next example, the same endpoint can be added to another group by a POST on the resource
with a JSON group object:
<figure><artwork>
Req: POST /gp (Content-Format: application/json)
{ "n": "floor1.west.bldg6.example.com",
"ip": "ff05::4200:f7fe:ed37:14cb" }
Res: 2.04 Changed
</artwork></figure>
</t><t>
Design 3: define named sub-resources, each sub-resource representing a group membership. The payload of
the sub-resource may be JSON or a simple pre-defined format. Or alternatively, information is provided
via POST with query parameters.
</t>
</section>
<!-- section anchor="sec-3.6" title="Congestion Control" -->
<section title="Congestion Control">
<t>
Multicast CoAP requests may result in a multitude of replies from
different nodes, potentially causing congestion. Therefore sending multicast requests
should be conservatively controlled.
</t><t>
The base CoAP draft <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).</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 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.</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. for an LLN
that could mean 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>Preferably IP multicast with link-local scope should be used, rather than global or site-local.</t>
<t>The Hop Limit field in the IPv6 packet should be chosen as low as possible (if the CoAP/IP stack
allows setting of this value. TBD - discuss whether this guideline is relevant/realistic in CoAP context)</t>
</list>
</t>
</section>
<!-- section anchor="sec-3.7" title="CoAP Multicast and HTTP Unicast Interworking" -->
<section title="CoAP Multicast and HTTP Unicast Interworking">
<t>
CoAP supports operation over UDP multicast, while HTTP does not. For use cases where it is required
that CoAP group communication is initiated from an HTTP end-point, it would be advantageous if
the HTTP-CoAP Proxy supports mapping of HTTP unicast to CoAP group communication based on IP multicast.
One possible way of operation of
such HTTP-CoAP Proxy is illustrated in <xref target="fig-http-coap"/>. Note that this topic is covered in
more detail in <xref target="I-D.castellani-core-advanced-http-mapping"/>.
</t>
<figure anchor="fig-http-coap" title="CoAP Multicast and HTTP Unicast Interworking" align="center">
<artwork>
<![CDATA[
CoAP Mcast CoAP Mcast HTTP-CoAP HTTP
Node 1 Rtr1 Node 2 Rtr2 Proxy Node 3
| | | | | |
|MLD REQUEST | | | |
|(Join Group X) | | | |
|--LL-->| | | | |
| | |MLD REQUEST | |
| | |(Join Group X) | |
| | |--LL-->| | |
| | | | | HTTP REQUEST |
| | | | | (URI to |
| | | | | unicast addr) |
| | | | |< -----------------|
| | | | | |
| | | Resolve HTTP Request-Line URI |
| | | to Group X multicast address |
| | | | | |
| CoAP REQUEST (to multicast addr)| |
|< ------<---------<-------<------| |
| | | | | |
| | | |
| (optional) CoAP RESPONSE(s) | |
| |-------------->| |
|-----------------|-------------->| Aggregated |
| | | HTTP RESPONSE |
| | |------------------>|
| | | |
]]>
</artwork>
</figure>
<t>
Note that <xref target="fig-http-coap"/> illustrates the case of IP multicast as the
underlying group communications mechanism. MLD denotes the Multicast Listener Discovery
protocol (<xref target="RFC3810" />, <xref target="mld"/>) and LL denotes a Link-Local multicast.
</t>
<t>
A key point in <xref target="fig-http-coap"/> is that the incoming HTTP Request (from
node 3) will carry a Host request-header field that resolves in the general
Internet to the proxy node. At the proxy node, this hostname and/or the Request-Line URI will
then possibly be mapped
(as detailed in <xref target="I-D.castellani-core-http-mapping"/>) and
again resolved (with the CoAP scheme) to an IP multicast address.
This may be accomplished, for example, by using DNS or DNS-SD
(<xref target="GroupDiscovery" />). The proxy node will then IP multicast
the CoAP Request (corresponding to the received HTTP Request) via multicast routers
to the appropriate nodes (i.e. nodes 1 and 2).
</t>
<t>
In terms of the HTTP Response, <xref target="fig-http-coap"/> illustrates that it will be
generated by the proxy node based on aggregated responses of the CoAP
nodes and sent back to the client in the
general Internet that sent the HTTP Request (i.e. node 1).
In <xref target="I-D.castellani-core-advanced-http-mapping" /> the HTTP Response
that the Proxy may use to aggregate multiple CoAP responses is described
in more detail. So in terms of overall operation, the CoAP proxy can be considered to
be a "non-transparent" proxy according to <xref target="RFC2616" />.
Specifically, <xref target="RFC2616" /> states that a "non-transparent
proxy is a proxy that modifies the request or response in order to
provide some added service to the user agent, such as group annotation
services, media type transformation, protocol reduction or anonymity
filtering."
</t><t>
An alternative to the above is using a Forward Proxy. In this case, the
CoAP request URI is carried in the HTTP Request-Line (as defined in
<xref target="I-D.ietf-core-coap" /> Section 10.2) in a HTTP request sent to the
IP address of the Proxy.
</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 use
cases and corresponding protocol flows:
<list style="symbols">
<t>Discovery of Resource Directory: discovering the local CoRE RD which
contains links (URIs) to resources stored on other servers
<xref target="RFC6690" />.
</t>
<t>Lighting Control: synchronous operation of a group of
6LoWPAN <xref target="RFC4944"/> IPv6-connected lights
</t>
<t>Parameter Update: updating parameters/settings simultaneously in a large group
of devices in a building/campus control
(<xref target="I-D.vanderstok-core-bc"/>) application --- TBD
</t>
<t>Firmware Update: efficiently updating firmware simultaneously in a large group
of devices in a building/campus control
(<xref target="I-D.vanderstok-core-bc"/>) application --- TBD suggests a
multicast extension of core-block.
</t>
<t>Group Status Report: requesting status information or event reports from a group
of devices in a building/campus control application --- TBD, may require reliable
group communication to be feasible.
</t>
</list>
</t>
</section>
<!-- section anchor="sec-4.2" title="Network Configuration" -->
<section title="Network Configuration">
<t> We assume the following network configuration for all the use cases
as shown in <xref target="Example_Topology" />:
<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 6LoWPAN subnets.</t>
<t>Light-1 and the Light Switch are connected to a router (Rtr-1) which is also a CoAP
Proxy, a CoAP Resource Directory (RD) and a 6LoWPAN Border Router (6LBR).</t>
<t>Light-2 and the Light-3 are connected to another router (Rtr-2) which is also a CoAP
Proxy, a CoAP RD and a 6LBR.</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 6LBRs 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>
</list>
</t>
<figure anchor="Example_Topology" title="Network Topology of a Large Room (Room-A)" align="center">
<artwork>
<![CDATA[
Network
Backbone
|
################################################ |
# Room-A # |
# ********************** # |
# ** LoWPAN-1 (subnet-1) ** # |
# * * # |
# * +----------+ * # |
# * | Light |-------+ * # |
# * | Switch | | * # |
# * +----------+ +---------+ * # |
# * | Rtr-1 |-----------------------------|
# * +---------+ * # |
# * +----------+ | * # |
# * | Light-1 |--------+ * # |
# * +----------+ * # |
# * * # |
# ** ** # |
# ********************** # |
# # |
# # |
# ********************** # |
# ** LoWPAN-2 (subnet-2) ** # |
# * * # |
# * +----------+ * # |
# * | Light-2 |-------+ * # |
# * | | | * # |
# * +----------+ +---------+ * # |
# * | Rtr-2 |-----------------------------|
# * +---------+ * # |
# * +----------+ | * # |
# * | Light-3 |--------+ * # |
# * +----------+ * # |
# * * # |
# ** ** # |
# ********************** # |
# # |
################################################# |
|
+--------+ |
| DNS |------------------|
| Server |
+--------+
]]>
</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 a 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 RD (RD-2) by sending
out a GET request (with the "/.well-known/core?rt=core.rd" request URI)
to the site-local "All CoAP Nodes" address. In this case, the site is
assumed to include all nodes in the subnet.</t>
<t>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".</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 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 Rtr-1 Rtr-2 Network
Light-1 Light-2 Light-3 Switch (RD-1) (RD-2) Backbone
| | | | | | |
| | | | | | |
********************************** | | |
* Light-2 is installed * | | |
* and powers on for first time * | | |
********************************** | | |
| | | | | | |
| | | | | | |
| | COAP NON (GET | |
| | /.well-known/core?rt=core.rd) | |
| |--------->-------------------------------->| |
| | | | | | |
| | | | | | |
| | | | | | |
| | COAP NON (2.05 Content | |
| | </rd>;rt="core.rd";ins="Primary") | |
| |<------------------------------------------| |
| | | | | | |
]]>
</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" />) is shown in sequence in
<xref target="Example_Protocol_Flow_2" />, <xref target="Example_Protocol_Flow_3" />,
and <xref target="Example_Protocol_Flow_4" />. We assume the following steps occur
before the illustrated flow:
<list style="symbols">
<t>1) Startup phase: 6LoWPANs are formed. IPv6 addresses assigned to all devices.
The CoAP network is formed.</t>
<t>2) Commissioning phase (by applications): The IP multicast address of the group
(Room-A-Lights) has been set in all the Lights. The URI of the group
(Room-A-Lights) has been set in the Light Switch.</t>
<t>3) The indicated MLD Report messages are link-local multicast. In each LoWPAN,
it is assumed that a multicast routing/forwarding protocol in 6LRs
will then propagate the Join information contained in the MLD Report
over multiple hops to the 6LBR. </t>
</list>
</t>
<figure anchor="Example_Protocol_Flow_2" title="Joining Lighting Groups Using MLD" align="center">
<artwork>
<![CDATA[
Light Rtr-1 Rtr-2 Network
Light-1 Light-2 Light-3 Switch (CoAP (CoAP Backbone
| | | | Proxy) Proxy) |
| | | | | | |
| | | | | | |
| MLD Report: Join | | | | |
| Group (Room-A-Lights) | | | |
|---LL------------------------------------->| | |
| | | | |MLD Report: Join |
| | | | |Group (Room-A-Lights)|
| | | | |---LL--------------->|
| | | | | | |
| | MLD Report: Join | | | |
| | Group (Room-A-Lights) | | |
| |---LL------------------------------------->| |
| | | | | | |
| | | MLD Report: Join | | |
| | | Group (Room-A-Lights) | |
| | |---LL-------------------------->| |
| | | | | | |
| | | | |MLD Report: Join |
| | | | |Group (Room-A-Lights)|
| | | | | |---LL---->|
| | | | | | |
| | | | | | |
]]>
</artwork>
</figure>
<figure anchor="Example_Protocol_Flow_3" title="Sending Lighting Control Multicast Message" align="center">
<artwork>
<![CDATA[
Light Rtr-1 Rtr-2 Network
Light-1 Light-2 Light-3 Switch (CoAP (CoAP Backbone
| | | | Proxy) Proxy) |
| | | | | | |
| | *********************** | |
| | * User flips on * | |
| | * light switch to * | |
| | * turn on all the * | |
| | * lights in Room A * | |
| | *********************** | |
| | | | | | |
| | | | | | |
| | | COAP NON (PUT | | |
| | | Proxy-URI | | |
| | | URI for Room-A-Lights |
| | | Payload=turn on lights) |
| | | |--------->| | |
| | | | | | |
| | | | | | |
| | | | Request DNS resolution of |
| | | | URI for Room-A-Lights |
| | | | |-------------------->|
| | | | | | |
| | | | | | |
| | | | DNS returns: AAAA |
| | | | Group (Room-A-Lights) |
| | | | IPv6 multicast address |
| | | | |<--------------------|
| | | | | | |
| | | | | | |
| | | COAP NON (Put |
| | | | URI Path |
| | | | Payload=turn on lights)|
| | | | Destination IP Address = |
| | | | IP multicast address |
| | | | for Group (Room-A-Lights)|
| | | | Originating IP Address = |
| | | | RTR-1 |
| | | | |-------------------->|
|<------------------------------------------| | |
| | | | | | |
| | | | | |<---------|
| |<---------|<-------------------------------| |
| | | | | | |
| | | | | | |
]]>
</artwork>
</figure>
<figure anchor="Example_Protocol_Flow_4" title="Sending Lighting Control Response to Multicast Message" align="center">
<artwork>
<![CDATA[
Light Rtr-1 Rtr-2 Network
Light-1 Light-2 Light-3 Switch (CoAP (CoAP Backbone
| | | | Proxy) Proxy) |
| | | | | | |
*********************** | | | |
* Lights in Room-A * | | | |
* turn on (nearly * | | | |
* simultaneously) * | | | |
*********************** | | | |
| | | | | | |
| | | | | | |
| COAP NON (2.04 Changed) | | | |
|------------------------------------------>| | |
| | | | | | |
| | | | | | |
| COAP NON (2.04 Changed) | | |
| |------------------------------->| | |
| | | | | | |
| | | | | | |
| | COAP NON (5.00 Internal Server Error) |
| | |-------------------->| | |
| | | | | | |
| | | ****************************** |
| | | * Rtr-1 as CoAP Proxy * |
| | | * | sends the individual * |
| | | * | responses back to * |
| | | * v originator * |
| | | ****************************** |
| | | | | | |
| | | COAP NON (2.04 Changed) | |
| | | |<---------| | |
| | | | | | |
| | | COAP NON (2.04 Changed) | |
| | | |<---------| | |
| | | | | | |
| | | COAP NON (5.00 Internal Server Error) |
| | | |<---------| | |
| | | | | | |
]]>
</artwork>
</figure>
<t>
NOTE: In the last step of <xref target="Example_Protocol_Flow_4"/>, Rtr-1 acting as
CoAP proxy, returns multiple individual CoAP responses to the client. Each response
echoes the Token of the client's request. The client can identify the original source
of each response by ...TBD.
</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-5.2" title="Implementation in 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 eachother.
</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 title="Multicast Routing">
<t>
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 <xref target="RFC6550"/>) and Trickle Multicast
Forwarding <xref target="I-D.ietf-roll-trickle-mcast" />.
</t>
</section>
<section title="Use of the Multicast Listener Discovery (MLD) protocol">
<t>
CoAP nodes that are IP hosts (not routers) are generally unaware of the specific multicast routing protocol
being used. When such a host needs to join a specific (CoAP) multicast group, it usually requires a way to
signal to the multicast routers which multicast traffic it wants to receive.
For efficient multicast routing (i.e. avoid always flooding multicast IP packets), routers
must know which hosts need to receive packets addressed to specific IP multicast
destinations.
</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 addition of MLD in LLN deployments, all nodes can be configured
as multicast routers.
</t>
</section>
<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>As defined in <xref target="I-D.ietf-core-coap" />, CoAP group communications based on
IP multicast must use the following security approach:
<list style="symbols">
<t>Group communications MUST operate in CoAP NoSec (No Security) mode.
</t>
<t>Group communications MUST NOT use "coaps" scheme. That is, all group
communications MUST use only "coap" scheme.
</t>
<t>Group communications MUST NOT use IPSec.
</t>
</list>
A consequence is that CoAP group communications is vulnerable to all attacks mentioned
in <xref target="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.
</t>
</section>
<!-- section anchor="sec-7" title="IANA Considerations" -->
<section title="IANA Considerations">
<t>No request is made to IANA. (Note: The required multicast address
request to IANA is made in <xref target="I-D.ietf-core-coap"/>).
</t>
</section>
<!-- section anchor="sec-8" title="Conclusions" -->
<section title="Conclusions">
<t>
IP multicast as outlined in <xref target="IPMulticast" /> is recommended to
be adopted as the base solution for CoAP Group Communication for situations
where the use case and network characteristics allow use of IP multicast. This approach
requires no standards changes to the IP multicast suite of protocols and it provides
interoperability with IP multicast group communication on un-constrained backbone networks.
</t>
</section>
<!-- section anchor="sec-9" 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>
<references title="Normative References">
&RFC2119;
&RFC2616;
&RFC3810;
&RFC3986;
&RFC4291;
&RFC4601;
&RFC4944;
&RFC5771;
&RFC6550;
&RFC6636;
&RFC6690;
&I-D.ietf-core-coap;
</references>
<references title="Informative References">
&I-D.cheshire-dnsext-dns-sd;
&I-D.ietf-core-observe;
&I-D.shelby-core-resource-directory;
&I-D.vanderstok-core-bc;
&I-D.lynn-core-discovery-mapping;
&I-D.vanderstok-core-dna;
&I-D.castellani-core-http-mapping;
&I-D.castellani-core-advanced-http-mapping;
&I-D.ietf-roll-trickle-mcast;
<reference anchor="Lao05" target="http://www.cs.ucla.edu/NRL/hpi/AggMC/papers/comparison_gi_2005.pdf">
<front>
<title>
A Comparative Study of Multicast Protocols: Top, Bottom, or In the Middle?
</title>
<author initials="L." surname="Lao" fullname="Li Lao"/>
<author initials="J." surname="Cui" fullname="Jun-Hong Cui"/>
<author initials="M." surname="Gerla" fullname="Mario Gerla,"/>
<author initials="D." surname="Maggiorini" fullname="Dario Maggiorini"/>
<date year="2005"/>
</front>
</reference>
<reference anchor="Banerjee01" target="http://wmedia.grnet.gr/P2PBackground/a-comparative-study-ofALM.pdf">
<front>
<title>
A Comparative Study of Application Layer Multicast Protocols
</title>
<author initials="B." surname="Banerjee" fullname="Suman Banerjee"/>
<author initials="B." surname="Bhattacharjee" fullname="Bobby Bhattacharjee"/>
<date year="2001"/>
</front>
</reference>
</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 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="CoAP-Observe Alternative to Group Communication" -->
<section title="CoAP-Observe Alternative to Group Communication">
<t>
The CoAP Observation extension <xref target="I-D.ietf-core-observe" /> can be
used as a simple (but very limited) alternative for group communication.
A group in this case consists of a CoAP server hosting a specific resource, plus
all CoAP clients observing that resource. The server is the only
group member that can send a group message. It does this by modifying the state
of a resource under observation and subsequently notifying its observers of the
change. Serial unicast is used for sending the notifications. This approach can
be a simple alternative for networks where IP multicast is not available or too expensive.
</t><t>
The CoAP-Observe approach is unreliable in the sense that, even though Confirmable
CoAP messages may be used, there are no guarantees that an update will be
received. For example, a client may believe it is observing a resource
while in reality the server rebooted and lost its listener state.
</t>
</section>
<!-- section anchor="Appendix C" title="Change Log" -->
<section title="Change Log">
<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 readibility</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
<xref target="I-D.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 readibility</t>
<t>Changelog 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>
| PAFTECH AB 2003-2026 | 2026-04-23 04:36:36 |