One document matched: draft-ietf-core-groupcomm-17.xml
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<rfc category="info" ipr="trust200902" docName="draft-ietf-core-groupcomm-17">
<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 specialized web transfer protocol for constrained devices and constrained 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. 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="Introduction" -->
<section title="Introduction">
<!-- section anchor="sec-1.1" title="Background" -->
<section title="Background">
<t>
Constrained Application Protocol (CoAP) is a Representational State Transfer (REST)
based web transfer protocol for resource constrained devices operating in an IP
network <xref target="I-D.ietf-core-coap"/>. CoAP has many similarities to HTTP <xref target="RFC2616" />
but also has some key differences. Constrained devices can be large in numbers, but are often related to
each other in function 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-1.2" title="Scope" -->
<section title="Scope">
<t>Group communication involves a one-to-many relationship between CoAP endpoints.
Specifically, a single CoAP client will simultaneously get (or set) resource representations
from multiple CoAP servers using CoAP over IP multicast. An example would be a CoAP light
switch turning on/off multiple lights in a room with a single CoAP group communication PUT
request, and handling the potential multitude of (unicast) responses.</t>
<t>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 several important group communication features. Among the topics covered are
group definition, group resource manipulation, and group configuration. Also, proxy operation and
minimizing network congestion for group communication is discussed. Finally, specific use cases
and deployment guidelines for CoAP group communication are outlined.
</t>
</section>
<!-- section anchor="sec-1.3" title="Conventions and Terminology" -->
<section title="Conventions and Terminology" anchor="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>
The above key words are used to establish a set of guidelines
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 of guidelines.
</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="Reliable Group Communication"><vspace />
Group Communication where for each destination node it is guaranteed
that the node either 1) eventually receives the message sent by the source node,
or 2) does not receive the message and the source node
is notified of the non-reception event.
</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
low-power and lossy links. The links may be may composed of one or more
technologies such as IEEE 802.15.4, Bluetooth Low Energy (BLE), Digital
Enhanced Cordless Telecommunication (DECT), and IEEE P1901.2 power-line
communication.</t>
</list>
</t>
</section>
</section>
<!-- section anchor="sec-2" title="Protocol Considerations" -->
<section title="Protocol Considerations" anchor="ProtocolConsiderations">
<!-- section anchor="sec-2.1" title="IP Multicast Background" -->
<section title="IP Multicast Background">
<t>
IP Multicast protocols have been evolving for decades, resulting in
standards such as Protocol Independent Multicast - Sparse
Mode (PIM-SM) <xref target="RFC4601" />. IP Multicast is very popular in specific
deployments such as in enterprise networks
(e.g., for video conferencing), smart home networks (e.g., Universal Plug and Play (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 link-local scope, an IP multicast routing or forwarding protocol needs to be
active on IP routers. An example of a routing protocol specifically for LLNs is
the IPv6 Routing Protocol for Low-Power and Lossy Networks (RPL)
(Section 12 of <xref target="RFC6550"/>) and an example of a forwarding protocol for LLNs
is Multicast Protocol for Low power and Lossy Networks (MPL)
<xref target="I-D.ietf-roll-trickle-mcast" />. RPL and MPL do not depend on each other;
each can be used in isolation and both can be used in combination in a network.
Finally, PIM-SM <xref target="RFC4601" /> is
often used for multicast routing in traditional IP networks (i.e. networks that are not constrained).
</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
"listener" protocol may be needed for the devices to subscribe to groups
(see <xref target="deploy_mld"/>).
</t>
<t>IP multicast is generally classified as an unreliable service in that packets are not guaranteed
to be delivered to each and every member of the group. In other words, it cannot be directly used
as a basis for "reliable group communication" as defined in <xref target="terminology"/>.
However, the level of reliability can be
increased by employing a multicast protocol that performs periodic retransmissions as is done
for example in MPL.
</t>
</section>
<!-- section anchor="sec-2.2" 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
multicast CoAP requests that are 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>Any CoAP server node SHOULD join the "All CoAP Nodes" multicast group (<xref target="I-D.ietf-core-coap"/>,
Section 12.8) by default to enable discovery
via CoAP multicast. For IPv4, the address is 224.0.1.187 and for IPv6 a server node joins at least both the
link-local scoped address FF02::FD and the site-local scoped address FF05::FD. IPv6 addresses of other scopes
may also be enabled.
</t>
<t>A Group URI has the scheme 'coap' and includes in the authority part either a group IP multicast address or a hostname
(e.g., Group Fully Qualified Domain Name (FQDN)) that can be resolved to the group IP multicast address.
A Group URI also contains an optional CoAP port number in the authority part.
Group URIs follow the CoAP URI syntax <xref target="I-D.ietf-core-coap"/>.</t>
<t>Note: A Group URI is needed to initiate CoAP group communications. If a CoAP implementation
accepts a CoAP URI as input in the group communications request API, then
the parsing method of Section 6.4 of <xref target="I-D.ietf-core-coap"/>
should be used in such way that a CoAP multicast request is generated.</t>
<t> It is recommended, for sending nodes, to use the IP multicast address literal in a Group URI.
In case a Group hostname is used, it can be uniquely mapped to an IP multicast address via DNS
resolution (if supported). Some examples of hierarchical Group FQDN naming (and scoping) for
a building control application are shown below:
<figure><artwork>
URI authority Targeted group of nodes
--------------------------------------- --------------------------
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.example.com "all nodes in floor 1,
west wing, building 6"
all.bu036.floor1.west.bldg6.example.com "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="RFC1033" />).
</t>
</section>
<!-- section anchor="sec-2.3" title="Port and URI Configuration" -->
<section title="Port and URI Configuration">
<t>
A CoAP server that is a member of a group listens for CoAP messages on the group's IP multicast address,
on a specified UDP port. 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 is diversity in the authority
port (e.g., different dynamic port addresses across the group) or if other parts of the
URI such as the path, or the query, differ 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 a port number according to one of below options:
<list style="numbers">
<t> A pre-configured port number. 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
for the targeted CoAP multicast group.</t>
<t> Use the default CoAP UDP port (5683).</t>
</list>
For a CoAP server node that supports multicast resource discovery, the default port 5683 MUST be supported
(Section 7.1 of <xref target="I-D.ietf-core-coap"/>) for the "All CoAP Nodes" group.
</t>
<t>All CoAP multicast requests SHOULD operate on URI paths in one of the following ways:
<list style="numbers">
<t> Pre-configured URI paths, if available. The pre-configuration mechanism SHOULD ensure that these paths
are pre-configured across all CoAP servers in a group and all CoAP clients performing
the group requests. Note that <xref target="I-D.ietf-appsawg-uri-get-off-my-lawn"/> prescribes that
any specification must not constrain, define structure or semantics for any path component.
</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>
<t> If the client has received a Group URI through a previous RESTful interaction with a trusted server,
for the purpose of the client using this URI in a request, it can use this URI in a multicast request.
For example, a commissioning tool may instruct a sensor device in this way to which target (multicast
URI) it should report sensor events.</t>
</list>
</t>
</section>
<!-- section anchor="sec-2.4" title="Group Methods" -->
<section title="Group REST Methods">
<t>Idempotent methods (i.e., CoAP GET, PUT, and DELETE) SHOULD be used for group communication,
with one exception as follows. A non-idempotent method (i.e., CoAP POST) MAY be used
for group communication if the resource being POSTed to has been designed to cope with
the lossy nature of multicast. Note that 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 title="Messaging and Responses">
<t>All CoAP messages that are sent via multicast MUST be Non-confirmable. The Message ID
in a multicast CoAP message is used for optional message deduplication as detailed in
Section 4.5 of <xref target="I-D.ietf-core-coap"/>.
</t>
<t>
A server MAY send back a unicast response to the group communication 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 by the
CoAP client 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 results (see section 8 of
<xref target="I-D.ietf-core-coap"/>).
</t>
<t>The CoAP client can distinguish the origin of multiple server responses by source IP address
of the UDP message containing the CoAP response. In case a CoAP client sent multiple group
requests, the responses are as usual matched to a request using the CoAP Token.
</t>
</section>
<!-- section anchor="sec-2.5" title="Group Discovery" -->
<section title="Group Member Discovery" anchor="MemberDiscovery">
<t>CoAP Groups, and the membership of these groups, can be discovered via the lookup interfaces
defined in <xref target="I-D.ietf-core-resource-directory"/>. An example of doing some
of these lookups is given in <xref target="CommissioningWithRD"/>.
</t>
</section>
<!-- section anchor="sec-2.6" title="Configuring Group Memberships in Endpoints" -->
<section title="Configuring Group Memberships in Endpoints" anchor="ConfiguringMembers">
<!-- section anchor="sec-2.6.1" title="Background" -->
<section title="Background" anchor="ConfiguringMembersBackground">
<t>The group membership of a CoAP endpoint may be configured in one of the following ways.
First, the group membership may be pre-configured before node deployment. Second, a node may
be programmed to discover (query) its group membership using a specific service
discovery means. Third, it may be configured by another node
(e.g., a commissioning device).
</t>
<t>
In the first case, the pre-configured group information may be either an IP multicast
address or a hostname (FQDN) which is resolved later (during operation) to a IP multicast address
by the endpoint using DNS (if supported).
</t>
<t>
For the second case, a CoAP endpoint may look up its group membership using techniques such as
DNS-SD and Resource Directory <xref target="I-D.ietf-core-resource-directory"/>. The latter
case is detailed more in <xref target="CommissioningWithRD"/>.
</t>
<t>
In the third case, typical in scenarios such as building control, a dynamic commissioning tool
determines to which group 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 an IP multicast address or a hostname.
</t>
</section>
<!-- End Background Section -->
<!-- section anchor="sec-2.6.2" title="RESTful Interface" -->
<section title="RESTful Interface" anchor="ConfiguringMembersRestful">
<t>
To achieve better interoperability between endpoints from different manufacturers,
an OPTIONAL RESTful CoAP interface for configuring endpoints with relevant group
information is described here. This interface provides a solution for the third
case mentioned above. To access this interface a client MUST use unicast CoAP methods
(GET/PUT/POST/DELETE) only as it is a method of configuring group information in
individual endpoints.</t>
<t>
Also, the (unicast) methods to configure group membership SHOULD use DTLS-secured CoAP
<xref target="I-D.ietf-core-coap"/>. Thus only authorized controllers should be allowed by
an endpoint to configure its group membership.
</t>
<t>
It is important to note that other approaches may be used to configure CoAP endpoints with relevant group information.
These alternate approaches may support a subset or superset of the RESTful CoAP interface described in this document.
For example, a simple interface to just read the endpoint group information may be implemented via a classical
Management Information Base (MIB) approach (e.g. following approach of <xref target="RFC3433" />).
</t>
<section title="CoAP-Group Resource Type and Media Type">
<t>
CoAP endpoints implementing the RESTful interface MUST
support the CoAP group configuration Internet Media Type "application/coap-group+json"
(<xref target="InternetMediaType"/>).
</t>
<t>
A resource offering this representation can be annotated for direct discovery
<xref target="RFC6690"/> using the resource type (rt) "core.gp" where "gp" is shorthand for
"group" (<xref target="ResourceType"/>). An authorized client uses this media type to query/manage group membership
of a CoAP endpoint as defined in the following subsections.
</t>
<t>
The group configuration resource and its sub-resources have a JSON-based content format (as indicated by the
"application/coap-group+json" media type). The resource includes zero or more group membership JSON objects
in a format as defined in <xref target="ConfiguringMembersRestfulGetAll"/>. A group membership JSON object
contains one or more key/value pairs as defined below. It represents a single IP multicast group
membership for the CoAP endpoint.
</t>
<t>
Examples of four different group membership objects are:
<figure><artwork>
{ "n": "All-Devices.floor1.west.bldg6.example.com",
"a": "[ff15::4200:f7fe:ed37:abcd]:4567" }
{ "n": "sensors.floor2.east.bldg6.example.com" }
{ "n": "coap-test",
"a": "224.0.1.187:56789" }
{ "a": "[ff15::c0a7:15:c00l]" }
</artwork></figure>
</t>
<t>
The OPTIONAL "n" key/value pair stands for "name" and identifies the group
with a hostname, for example a FQDN.
The OPTIONAL "a" key/value pair specifies the IP multicast address (and optionally the port number)
of the group. It contains an IPv4 address (in dotted decimal notation) or an IPv6 address. The following ABNF
rule can be used for parsing the address, referring to the definitions in
Section 6 of <xref target="I-D.ietf-core-coap"/> and <xref target="RFC3986"/>.
<figure><artwork>
group-address = IPv4address [ ":" port ]
/ "[" IPv6address "]" [":" port ]
</artwork></figure>
</t>
<t>
If the port number is not provided then it is assumed to be the default CoAP port (5683).
In a response, the "a" key/value pair MUST be included if the IP address is known at the time of generating
the response, and SHOULD NOT be included if unknown.
If the "a" value is not provided in a request, the "n" value in the same group membership object
SHOULD be a valid hostname that can
be translated to an IP multicast address via DNS resolution.
At least one of the "n"/"a" pairs MUST be given per group object.
</t>
<t>
After any change on a Group configuration resource, the endpoint MUST effect
registration/de-registration from the corresponding IP multicast group(s) as soon as possible.
</t>
</section>
<section title="Creating a new multicast group membership (POST)" anchor="ConfiguringMembersPOST">
<t>
<figure><artwork>
Method: POST
URI Template: /{+gp}
Location-URI Template: /{+gp}/{index}
URI Template Variables:
gp - Group Configuration Function Set path (mandatory).
index - Group index, SHOULD be a string of 1 or 2 alphanumerical
characters.
Example:
Req: POST /coap-group
Content-Format: application/coap-group+json
{ "n": "All-Devices.floor1.west.bldg6.example.com",
"a": "[ff15::4200:f7fe:ed37:abcd]:4567" }
Res: 2.01 Created
Location-Path: /coap-group/12
</artwork></figure>
For the 'gp' variable we recommend use of the path "coap-group" by default.
If the "a" key/value pair is given, this takes priority and the "n"
pair becomes informational. If only the "n" pair is given, the CoAP
endpoint may perform DNS resolution (if supported) to obtain the IP
multicast address from the hostname.
</t><t>
After any change on a Group configuration resource, the endpoint MUST
effect registration/de-registration from the corresponding IP
multicast group(s) as soon as possible. When a POST payload contains
in "a" a multicast address to which the endpoint is already
subscribed, the endpoint MUST re-register to that multicast address.
</t>
</section>
<section title="Deleting a group membership (DELETE)">
<t>
<figure><artwork>
Method: DELETE
URI Template: /{+location}
URI Template Variables:
location - The Location-Path returned by the CoAP server as a result
of a successful group creation.
Example:
Req: DELETE /coap-group/12
Res: 2.02 Deleted
</artwork></figure>
</t>
</section>
<section title="Reading a group membership (GET)">
<t>
<figure><artwork>
Method: GET
URI Template 1: /{+location}
URI Template 2: /{+gp}/{index}
URI Template Variables:
location, gp, index - see earlier definitions
Example:
Req: GET /coap-group/12
Res: 2.05 Content
Content-Format: application/coap-group+json
{"n": "All-Devices.floor1.west.bldg6.example.com",
"a": "[ff15::4200:f7fe:ed37:abcd]:4567"}
</artwork></figure>
</t>
</section>
<section title="Reading all group memberships (GET)" anchor="ConfiguringMembersRestfulGetAll">
<t>
A (unicast)
GET on the CoAP-group resource returns a JSON object containing multiple keys and values, the keys being group indices
and the values the corresponding group objects. Each group object is a group membership JSON object that indicates one
multicast group membership.
So, the group index is used as a JSON key to point to the group membership object, as shown below.
<figure><artwork>
Method: GET
URI Template: /{+gp}
URI Template Variables:
gp - see earlier definition
Example:
Req: GET /coap-group
Res: 2.05 Content
Content-Format: application/coap-group+json
{ "8" :{ "a": "[ff15::4200:f7fe:ed37:14ca]" },
"11":{ "n": "sensors.floor1.west.bldg6.example.com",
"a": "[ff15::4200:f7fe:ed37:25cb]" },
"12":{ "n": "All-Devices.floor1.west.bldg6.example.com",
"a": "[ff15::4200:f7fe:ed37:abcd]:4567" }
}
</artwork></figure>
Note: the returned IPv6 address may be a different string from the one originally submitted in group membership creation, due to different choices in IPv6 string representation formatting that may be allowed for the same address.
</t>
</section>
<section title="Updating a group memberships (PUT)">
<t>
<figure><artwork>
Method: PUT
URI Template 1: /{+location}
URI Template 2: /{+gp}/{index}
URI Template Variables:
location, gp, index - see earlier definitions
Example: (group name and multicast port change)
Req: PUT /coap-group/12
Content-Format: application/coap-group+json
{"n": "All-My-Devices.floor1.west.bldg6.example.com",
"a": "[ff15::4200:f7fe:ed37:abcd]"}
Res: 2.04 Changed
</artwork></figure>
</t>
</section>
<section title="Creating/updating all group memberships at once (PUT)">
<t>
A (unicast) PUT with a group configuration media type as payload will replace all current group memberships
in the endpoint with the new ones defined in the PUT request.
This operation SHOULD only be used to delete or update group membership objects for which the CoAP client, invoking this operation, is responsible.
</t>
<t>
<figure><artwork>
Method: PUT
URI Template: /{+gp}
URI Template Variables:
gp - see earlier definition
Example: (replacing all existing group memberships with two new groups)
Req: PUT /coap-group
Content-Format: application/coap-group+json
{ "1":{ "a": "[ff15::4200:f7fe:ed37:1234]" },
"2":{ "a": "[ff15::4200:f7fe:ed37:5678]" }
}
Res: 2.04 Changed
Example: (clearing all group memberships at once)
Req: PUT /coap-group
Content-Format: application/coap-group+json
{}
Res: 2.04 Changed
</artwork></figure>
</t>
</section>
</section>
<!-- End RESTful Interface Section -->
</section>
<!-- End Configuring Group Memberships Section -->
<!-- section anchor="sec-2.7" 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 CoAP request is accepted and executed, and when not.</t>
<t>Multicast response suppression - in which cases the CoAP response to an already-executed request is returned to
the requesting endpoint, and when not.</t>
</list>
A CoAP response differs from a CoAP ACK; ACKs are never sent by servers in response to a multicast CoAP
request. 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, a CoAP server must be aware that an incoming request
arrived via multicast by making use of APIs such as IPV6_RECVPKTINFO <xref target="RFC3542"/>.
</t>
<t>
For multicast request acceptance, the REQUIRED 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 the resource "/.well-known/core", multicast support is required
if "multicast resource discovery" is supported as specified in section 1.2.1 of <xref target="RFC6690"/>).
Implementers are advised to disable multicast support by default on any other resource, until explicitly
enabled by an application or by configuration.)</t>
<t>Otherwise accept the multicast request.</t>
</list>
</t>
<t>
For multicast response suppression, the REQUIRED 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>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 options per resource that supports multicast requests:
<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 given below to illustrate
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 configuration data in a group of devices using multicast PUT: No suppression at all. The client uses collected responses
to identify which group members did not receive the new configuration; then attempts using CoAP CON unicast to
update those specific group members. Note that in this case the client implements a Reliable CoAP Group Communication
function using additional, non-standardized functions above the CoAP layer.</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 data 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-2.8" title="Congestion Control" -->
<section title="Congestion Control" anchor="Congestion_Control">
<t>
Multicast CoAP requests may result in a multitude of responses from
different nodes, potentially causing congestion. Therefore both the sending of multicast requests,
and the sending of the unicast CoAP responses to these 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 of <xref target="I-D.ietf-core-coap"/>).</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>
</list>
</t>
<t>
Additional guidelines to reduce congestion risks defined in this document are:
<list style="symbols">
<t>A server in an LLN should only support multicast GET for resources that are small. For example, the
payload of the response is limited to 5% of the IP Maximum Transmit Unit (MTU) size so it fits
into a single link-layer frame in case 6LoWPAN <xref target="RFC4944"/> is used.</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>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 CoRE Link Format description. For this reason, a CoAP client
sending a multicast CoAP request to "/.well-known/core" SHOULD support core-block.</t>
<t>A client should use CoAP multicast with the smallest possible multicast scope
that fulfills the application needs. As an example, site-local scope is always preferred
over global scope IP multicast if this fulfills the application needs.</t>
</list>
</t>
<t>More guidelines specific to use of CoAP in 6LoWPAN networks <xref target="RFC4944"/>
are given in <xref target="sixlowpanSpecific"/>.</t>
</section>
<!-- section anchor="sec-2.9" 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.</t>
<t>A proxy may buffer all the individual (unicast) responses to a group communication request and then
send back only a single (aggregated) response to the client. However there are some issues with
this aggregation approach:
<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. Also the proxy does not know how long to wait before deciding to send back
the aggregated response to the client.</t>
<t>There is no default format defined in CoAP for aggregation of multiple responses into a single
response.</t>
</list>
Alternatively, if a proxy follows directly 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 with this approach:
<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.</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).
The exception case here occurs when a (future) standardized aggregation format is being used.</t>
<t>It MUST be known to the person/entity doing the configuration of the proxy, or otherwise 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-2.10" 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 implementable in
a given network. The primary reason for this will be that IP multicast is not (fully) supported in
the network.</t>
<t>For example, if only the RPL protocol <xref target="RFC6550"/> is used in a network with its optional
multicast support disabled, there will be no IP multicast routing at all. The only multicast that works
in this case is link-local IPv6 multicast. This implies that any CoAP multicast request will
be delivered to nodes on the local link only, regardless of the scope value used in the IPv6
destination address.
</t>
</section>
</section>
<!-- section anchor="sec-3" title="Use Cases and Corresponding Protocol Flows" -->
<section title="Use Cases and Corresponding Protocol Flows" anchor="Use_Cases">
<!-- section anchor="sec-3.1" 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.ietf-core-resource-directory"/>):
discovering the local CoAP RD which
contains links 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-3.2" title="Network Configuration" -->
<section title="Network Configuration">
<t>To illustrate the use cases we define two IPv6 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 Multicast Listener Discovery (MLD)
for forming groups.</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 hostname to IP multicast
address). If the DNS server is not there, then different provisioning of the network
is required (e.g., IP multicast addresses are hard-coded into devices, or manually configured,
or obtained via a service discovery method).</t>
<t>A Controller (CoAP 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[ ################################################
# ********************** Room-A #
# ** Subnet-1 ** # Network
# * ** # Backbone
# * +----------+ * # |
# * | Light |-------+ * # |
# * | Switch | | * # |
# * +----------+ +---------+ * # |
# * | Rtr-1 |-----------------------------+
# * +---------+ * # |
# * +----------+ | * # |
# * | Light-1 |--------+ * # |
# * +----------+ * # |
# ** ** # |
# ************************** # |
# # |
# ********************** # +------------+ |
# ** Subnet-2 ** # | DNS Server | |
# * ** # | (Optional) |--+
# * +----------+ * # +------------+ |
# * | Light-2 |-------+ * # |
# * | | | * # |
# * +----------+ +---------+ * # |
# * | Rtr-2 |-----------------------------+
# * +---------+ * # |
# * +----------+ | * # |
# * | Light-3 |--------+ * # |
# * +----------+ * # +------------+ |
# ** ** # | Controller |--+
# ************************** # | Client | |
################################################ +------------+ |
+------------+ |
| CoAP | |
| Resource |-----------------+
| Directory |
+------------+
]]>
</artwork>
</figure>
</section>
<!-- section anchor="sec-3.3" 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>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 (FF05:::FD).</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 contains a resource of type core.rd).</t>
<t>The CoAP RD will then send back a unicast response containing the requested content,
which is a CoRE Link Format representation of a resource of type 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
installed.</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. (See
<xref target="I-D.ietf-core-resource-directory"/> for more details).</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. For example, Link-Local (LL),
admin-local or site-local scoped discovery can be done this way.
</t>
<figure anchor="Example_Protocol_Flow_1" title="Resource Directory Discovery via Multicast Request" 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-3.4" 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 <xref target="Example_Protocol_Flow_3" />.
The network is assumed to be in a 6LoWPAN configuration.
Also, it is assumed that the CoAP servers
in each Light are configured to suppress CoAP responses for any multicast CoAP requests
related to lighting control. (See <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 case, the Light Switch does not check that all Lights
in the group received the multicast request by examining the responses.
This is because the Light Switch is not configured with an exhaustive
list of the 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. In case a CoAP message is delivered multiple times to a Light,
the subsequent CoAP messages can be filtered out as duplicates, based on the CoAP Message ID.
</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 using randomized retransmission 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 addresses, or 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 6LoWPAN/CoAP software stack supports sending
unicast, multicast or proxied unicast 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-3.5" 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. In current 6LoWPAN implementations, MLD is however not supported.
</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 anchor="sec-3.6" title="Commissioning the Network Based on Resource Directory" -->
<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.ietf-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 Light/Switch 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 tool 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 three POST commands to store the endpoints associated with the three lights
as prescribed by the RD specification <xref target="I-D.ietf-core-resource-directory"/>. Finally the commissioning
tool defines the group in the RD to contain these three endpoints. Also the commissioning tool writes the multicast
address in the Light endpoints with, for example, the POST command discussed in <xref target="ConfiguringMembersPOST"/>.
</t><t>
The light switch can discover the group in RD and thus learn the multicast address of the group. The light
switch will use this address to send multicast commands to the members of the group. When the message
arrives the Lights should recognize the multicast address and accept the message.
</t>
</section>
</section>
<!-- section anchor="sec-4" title="Deployment Guidelines" -->
<section title="Deployment Guidelines">
<t>
This section provides guidelines how IP Multicast based
CoAP group communication can be deployed in various network configurations.
</t>
<!-- section anchor="sec-4.1" 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 traditional IP network, or a LLN such as a 6LoWPAN network, or
consist of mixed traditional/constrained 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 its
nodes reachable in a single IP hop (fully connected), 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. To enable all the above target
network topologies, an implementation of CoAP group communication needs to allow:
<list style="numbers">
<t>Routing/forwarding of IP multicast packets over multiple hops</t>
<t>Routing/forwarding of IP multicast packets over subnet boundaries between traditional
and constrained (e.g. LLN) networks.</t>
</list>
The remainder of this section discusses solutions to enable both features.
</t>
</section>
<!-- section anchor="sec-4.2.1" title="Networks Using the MLD Protocol" -->
<section title="Networks Using the MLD Protocol" anchor="deploy_mld">
<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 requires a way to signal to multicast routers which multicast traffic it wants to receive.
</t>
<t>
The Multicast Listener Discovery (MLD) protocol <xref target="RFC3810" /> (see <xref target="mld"/>)
is the standard IPv6 method to achieve this; therefore this approach should be used on traditional
IP networks. CoAP server nodes would then act in the role of MLD Multicast Address Listener.
</t>
<t>
The guidelines from <xref target="RFC6636"/> on tuning of MLD
for mobile and wireless networks may be useful when implementing MLD in LLNs.
However, on LLNs and 6LoWPAN networks the use of MLD may not be feasible at all due to constraints
on code size, memory, or network capacity.
</t>
</section>
<!-- section anchor="sec-4.2.2" title="Networks Using RPL Multicast Without MLD" -->
<section title="Networks Using RPL Multicast Without MLD" anchor="deploy_rpl">
<t>
It is assumed in this section that the MLD protocol is not implemented in a network,
for example due to resource constraints. The RPL routing protocol (see Section 12
of <xref target="RFC6550"/>) defines the advertisement of IP multicast
destinations using DAO messages and routing of multicast
IPv6 packets based on this. It requires the RPL Mode of Operation (MOP)
to be 3 (Storing Mode with multicast support).
</t><t>
Hence, RPL DAO can be used by CoAP nodes that are RPL Routers, or are RPL Leaf Nodes,
to advertise IP multicast group membership to parent routers. Then, the RPL protocol is
used to route multicast CoAP requests over multiple hops to the correct CoAP servers.
</t><t>
The same DAO mechanism can be used 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 is useful because
the edge router then learns which IP multicast traffic it needs to pass through from
the backbone network into the LLN subnet.
In 6LoWPAN networks, such selective "filtering" helps to avoid congestion of a 6LoWPAN subnet
by IP multicast traffic from the traditional backbone IP network.
</t>
</section>
<!-- section anchor="sec-4.2.3" title="Networks Using MPL Forwarding Protocol Without MLD" -->
<section title="Networks Using MPL Forwarding Without MLD" anchor="deploy_mpl">
<t>
The MPL forwarding protocol <xref target="I-D.ietf-roll-trickle-mcast" /> can be used
for propagation of IPv6 multicast packets to all MPL Forwarders within a predefined network domain, over
multiple hops. MPL is designed to work in LLNs. In this section it is again assumed
that Multicast Listener Discovery (MLD) is not implemented in the network, for example due to resource
limitations in an LLN.
</t>
<t>The purpose of MPL is
to let a predefined group of Forwarders collectively work towards the goal of distributing an IPv6 multicast
packet throughout an MPL Domain. (A Forwarder node may be associated to multiple MPL Domains at the same
time.)
So it would appear there is no need for CoAP servers to advertise their multicast group membership, since any IP
multicast packet that enters the MPL Domain is distributed to all MPL Forwarders without regard
to what multicast addresses the individual nodes are listening to.
</t>
<t>However, if an IP multicast request originates just outside the MPL Domain, the request will
not be propagated by MPL. An example of such a case is the network
topology of <xref target="Example_Topology" /> where the Subnets are 6LoWPAN subnets and
per 6LoWPAN subnet one Realm-Local (<xref target="I-D.droms-6man-multicast-scopes"/>) MPL Domain is defined.
The backbone network in this case is not part of any MPL Domain.
</t>
<t>This situation can become a problem in building control use cases. For example, when the Controller
Client needs to send a single CoAP multicast request to the group Room-A-Lights.
By default, the request would be blocked
by Rtr-1 and by Rtr-2, and not enter the Realm-Local MPL Domains associated to Subnet-1 and Subnet-2. The
reason is that Rtr-1 and Rtr-2 do not have the knowledge that devices in Subnet-1/2 want to
listen for IP packets destined to multicast group Room-A-Lights.
</t>
<t>
To solve the above issue, the following solutions could be applied:
<list style="numbers">
<t>Extend the MPL Domain. E.g. in above example, include the Network Backbone to be part
of each of the two MPL Domains. Or in above example, create just a single MPL Domain
that includes both 6LoWPAN subnets plus the backbone link, which is possible since
MPL is not tied to a single link-layer technology.</t>
<t>Manual configuration of edge router(s) as MPL Seed(s) for specific multicast traffic. E.g. in above example,
first configure Rtr-1 and Rtr-2 to act as MLD Address Listeners for the Room-A-Lights multicast group.
This step allows any (other) routers on the backbone to learn that at least one node on the backbone link is interested to
receive any multicast traffic to Room-A-Lights.
Second, configure both routers to "inject" any IP multicast packets destined to group Room-A-Lights into
the (Realm-Local) MPL Domain that is associated to that router. Third, configure both routers to propagate
any IPv6 multicast packets originating from within their associated MPL Domain to the backbone, if at least
one node on the backbone has indicated interest to receive such IPv6 packets (for which MLD is used on
the backbone).
</t>
<t>Use an additional protocol/mechanism for injection of multicast traffic from outside an MPL Domain into
that MPL Domain, based on multicast group subscriptions of Forwarders within the MPL Domain.
Such protocol is currently not defined in <xref target="I-D.ietf-roll-trickle-mcast" />.</t>
</list>
</t>
<t>
Concluding, MPL can be used directly in case all sources of multicast CoAP requests (CoAP clients) and also
all the destinations
(CoAP servers) are inside a single MPL Domain. Then, each source node acts as an MPL Seed.
In all other cases, MPL can only be used with additional protocols and/or configuration on
how IP multicast packets can be injected from outside into an MPL Domain.
</t>
</section>
<!-- section anchor="sec-4.3" title="6LoWPAN Specific Guidelines for the 6LBR" -->
<section title="6LoWPAN Specific Guidelines for the 6LBR" anchor="sixlowpanSpecific">
<t>
To support multi-subnet 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 (see <xref target="mld"/>) on the backbone link.
</t>
</section>
</section>
<!-- section anchor="sec-5" 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-5.1" 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:
<list style="symbols">
<t>Will operate in CoAP NoSec (No Security) mode, until a future group security
solution is developed (see also <xref target="FutureEvolution" />).</t>
<t>MUST NOT use "coaps" scheme. That is, all group
communication MUST use only "coap" scheme.</t>
</list>
</t>
</section>
<!-- section anchor="sec-5.2" 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-5.3" title="Threat Mitigation" -->
<section title="Threat Mitigation" anchor="Security_Mitigation">
<t> The <xref target="I-D.ietf-core-coap" /> identifies various threat mitigation techniques for CoAP
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-5.3.1" 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. The Customer Premise Equipment (CPE) that enables access to the
Internet should also have its multicast filters set so that it enforces multicast scope
boundaries to isolate local multicast groups from the rest of the Internet (e.g., as per
<xref target="RFC6092"/>). In addition, the scope of the IP multicast should be set
to be site-local or smaller scope. For site-local scope, the CPE will
be an appropriate multicast scope boundary point.
</t>
</section>
<!-- section anchor="sec-5.3.2" title="6LoWPAN Scenario" -->
<section title="6LoWPAN Scenario">
<t>In a building automation scenario, a particular room may have a single 6LoWPAN network
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 (6LoWPAN-bound) 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 network segment can also be configured to
block incoming IP multicast traffic.
</t>
</section>
<!-- section anchor="sec-5.3.3" title="Future Evolution" -->
<section title="Future Evolution" anchor="FutureEvolution">
<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-6" title="IANA Considerations" -->
<section title="IANA Considerations">
<!-- section anchor="sec-6.1" title="New 'core.gp' Resource Type" -->
<section title="New 'core.gp' Resource Type" anchor="ResourceType">
<t> This memo registers a new resource type (rt) from the CoRE Parameters Registry called 'core.gp'.
</t>
<t>(Note to IANA/RFC Editor: This registration follows the process described in section
7.4 of <xref target="RFC6690"/>).
</t>
<t>Attribute Value: core.gp
</t>
<t>Description: Group Configuration resource. This resource is used to
query/manage the group membership of a CoAP server.
</t>
<t>Reference: See <xref target="ConfiguringMembersRestful"/>.
</t>
</section>
<!-- section anchor="sec-6.2" title="New 'coap-group+json' Internet Media Type" -->
<section title="New 'coap-group+json' Internet Media Type" anchor="InternetMediaType">
<t>This memo registers a new Internet Media Type for CoAP group configuration resource called
'application/coap-group+json'.
</t>
<t>(Note to IANA/RFC Editor: This registration follows the guidance from <xref target="RFC6839" />, and
(last paragraph) of section 12.3 of <xref target="I-D.ietf-core-coap"/>.
</t>
<t>Type name: application
</t>
<t>Subtype name: coap-group+json
</t>
<t>Required parameters: None
</t>
<t>Optional parameters: None
</t>
<t>Encoding considerations: 8bit UTF-8.
</t>
<t>JSON to be represented using UTF-8 which is 8bit compatible (and most efficient for resource constrained implementations).
</t>
<t>Security considerations:
</t>
<t>Denial of Service attacks could be performed by constantly setting
the group configuration resource of a CoAP endpoint to different values. This will cause
the endpoint to register (or de-register) from the related IP multicast group. To prevent this it is
recommended that DTLS-secured (unicast) CoAP communication be used for setting the group
configuration resource. Thus only authorized clients will be allowed by a server
to configure its group membership.
</t>
<t>Interoperability considerations: None
</t>
<t>Published specification: (This I-D when it becomes an RFC)
</t>
<t>Applications that use this media type:
</t>
<t>CoAP client and server implementations that wish to
set/read the group configuration resource via 'application/coap-group+json' payload
as described in <xref target="ConfiguringMembersRestful"/>.
</t>
<t>Additional Information:
</t>
<t>Magic number(s): None
</t>
<t>File extension(s): *.json
</t>
<t>Macintosh file type code(s): TEXT
</t>
<t>Intended usage: COMMON
</t>
<t>Restrictions on usage: None
</t>
<t>Author: CoRE WG
</t>
<t>Change controller: IETF
</t>
</section>
</section>
<!-- section anchor="sec-7" title="Acknowledgements" -->
<section title="Acknowledgements">
<t>
Thanks to Peter Bigot, Carsten Bormann, Anders Brandt, Angelo Castellani, Bjoern Hoehrmann,
Matthias Kovatsch, Guang Lu, Salvatore Loreto, Kerry Lynn, Andrew McGregor, 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-8" title="References" -->
<references title="Normative References">
&RFC1033;
&RFC2119;
&RFC2616;
&RFC3376;
&RFC3433;
&RFC3542;
&RFC3810;
&RFC3986;
&RFC4291;
&RFC4601;
&RFC4944;
&RFC5771;
&RFC6092;
&RFC6550;
&RFC6636;
&RFC6690;
&RFC6775;
&RFC6839;
&I-D.ietf-core-coap;
</references>
<references title="Informative References">
&I-D.ietf-core-block;
&I-D.ietf-roll-trickle-mcast;
&I-D.keoh-tls-multicast-security;
&I-D.ietf-core-resource-directory;
&I-D.ietf-appsawg-uri-get-off-my-lawn;
&I-D.droms-6man-multicast-scopes;
</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 IP multicast 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 equivalent IGMP <xref target="RFC3376" />) 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>
<xref target="RFC6636"/> discusses optimal tuning of the parameters of MLD/IGMP 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>[Note to RFC Editor: Please remove this section before publication.]</t>
<t>Changes from ietf-16 to ietf-17:
<list style="symbols">
<t>Added guidelines on joining of IPv6/IPv4 "All CoAP Nodes" multicast addresses (#356).</t>
<t>Added MUST support default port in case multicast discovery is available.</t>
<t>In section 2.1 (IP Multicast Background), clarified that IP multicast is not guaranteed and referenced
a definition of Reliable Group Communication (#355).</t>
<t>Added section 2.5 (Messages and Responses) to clarify how responses are identified and
how Token/MID are used in multicast CoAP.</t>
<t>In section 2.6.2 (RESTful Interface for Configuring Group Memberships),
clarified that group management interface is an optional approach for dynamic commissioning and that other
approaches can also be used if desired.</t>
<t>Updated section 2.6.2 (RESTful Interface for Configuring Group Memberships) to allow
deletion of individual group memberships (#354).</t>
<t>Various editorial updates based on comments by Peter van der Stok. Removed reference to expired
draft-vanderstok-core-dna at request of its author.</t>
<t>Various editorial updates for improved readability.</t>
</list>
</t>
<t>Changes from ietf-15 to ietf-16:
<list style="symbols">
<t>In section 2.6.2, changed DELETE in group management interface to a PUT with empty JSON array
to clear the list (#345).</t>
<t>In section 2.6.2, aligned the syntax for IP addresses to follow RFC 3986 URI syntax, which
is also used by coap-18. This allows re-use of the parsing code for CoAP URIs for this purpose (#342).</t>
<t>Addressed some more editorial comments provided by Carsten Bormann in preparation for WGLC.</t>
<t>Various editorial updates for improved readability.</t>
</list>
</t>
<t>Changes from ietf-14 to ietf-15:
<list style="symbols">
<t>In section 2.2, provided guidance on how implementers should parse URIs
for group communication (#339).</t>
<t>In section 2.6.2.1, specified that for group membership configuration interface
the "ip" (i.e. "a" parameter) key/value is not required when it is unknown (#338).</t>
<t>In section 2.6.2.1, specified that for group membership configuration interface
the port configuration be defaulted to standard CoAP port 5683, and if
not default then should follow standard notation (#340).</t>
<t>In section 2.6.2.1, specified that notation of IP address in group
membership configuration interface should follow standard notation (#342).</t>
<t>In section 6.2, "coap-group+json" Media Type encoding simplified
to just support UTF-8 (and not UTF-16 and UTF-32) (#344).</t>
<t>Various editorial updates for improved readability.</t>
</list>
</t>
<t>Changes from ietf-13 to ietf-14:
<list style="symbols">
<t>Update to address final editorial comments from the Chair's review (by Carsten Bormann) of the draft.
This included restructuring of Section 2.6 (Configuring Group Memberships) and Section 4
(Deployment Guidelines) to make it easier to read. Also various other editorial changes.</t>
<t>Changed "ip" field to "a" in Section 2.6 (#337)</t>
</list>
</t>
<t>Changes from ietf-12 to ietf-13:
<list style="symbols">
<t>Extensive editorial updates due to comments from the Chair's review (by Carsten Bormann) of the draft.
The best way to see the changes will be to do a -Diff with Rev. 12.</t>
<t>The technical comments from the Chair's review will be addressed in a future revision after tickets
are generated and the solutions are agreed to on the WG E-mail list.</t>
</list>
</t>
<t>Changes from ietf-11 to ietf-12:
<list style="symbols">
<t>Removed reference to "CoAP Ping" in Section 3.5 (Group Member Discovery) and replaced it with the more efficient
support of discovery of groups and group members via the CORE RD as suggested by Zach Shelby.</t>
<t>Various editorial updates for improved readability.</t>
</list>
</t>
<t>Changes from ietf-10 to ietf-11:
<list style="symbols">
<t>Added text to section 3.8 (Congestion Control) to clarify that a
"CoAP client sending a multicast CoAP request to /.well-known/core SHOULD support core-block" (#332).</t>
<t>Various editorial updates for improved readability.</t>
</list>
</t>
<t>Changes from ietf-09 to ietf-10:
<list style="symbols">
<t>Various editorial updates including:</t>
<t>Added a fourth option in section 3.3 on ways to obtain the URI path for a group request.</t>
<t>Clarified use of content format in GET/PUT requests for Configuring Group Membership in Endpoints (in section 3.6).</t>
<t>Changed reference "draft-shelby-core-resource-directory" to "draft-ietf-core-resource-directory".</t>
<t>Clarified (in section 3.7) that ACKs are never used for a multicast request (from #296).</t>
<t>Clarified (in section 5.2/5.2.3) that MPL does not support group membership advertisement.</t>
<t>Adding introductory paragraph to Scope (section 2.2).</t>
<t>Wrote out fully the URIs in table section 3.2.</t>
<t>Reworded security text in section 7.2 (New Internet Media Type) to make it consistent with section
3.6 (Configuring Group Membership).</t>
<t>Fixed formatting of hyperlinks in sections 6.3 and 7.2.</t>
</list>
</t>
<t>Changes from ietf-08 to ietf-09:
<list style="symbols">
<t>Cleaned up requirements language in general. Also, requirements language are now only used in section 3
(Protocol Considerations) and section 6 (Security Considerations).
Requirements language has been removed from other sections to keep them to a minimum (#271).</t>
<t>Addressed final comment from Peter van der Stok to define what "IP stack" meant (#296).
Following the lead of CoAP-17, we know refer instead to "APIs such as IPV6_RECVPKTINFO [RFC3542]".</t>
<t>Changed text in section 3.4 (Group Methods) to allow multicast POST under specific conditions
and highlighting the risks with using it (#328).</t>
<t>Various editorial updates for improved readability.</t>
</list>
</t>
<t>Changes from ietf-07 to ietf-08:
<list style="symbols">
<t>Updated text in section 3.6 (Configuring Group Membership in Endpoints) to make it more
explicit that the Internet Media Type is used in the processing rules (#299).</t>
<t>Addressed various comments from Peter van der Stok (#296).</t>
<t>Various editorial updates for improved readability including defining all acronyms.</t>
</list>
</t>
<t>Changes from ietf-06 to ietf-07:
<list style="symbols">
<t>Added an IANA request (in section 7.2) for a dedicated content-format (Internet Media type) for the group management
JSON format called 'application/coap-group+json' (#299).</t>
<t>Clarified semantics (in section 3.6) of group management JSON format (#300).</t>
<t>Added details of IANA request (in section 7.1) for a new CORE Resource Type called 'core.gp'.</t>
<t>Clarified that DELETE method (in section 3.6) is also a valid group management operation.</t>
<t>Various editorial updates for improved readability.</t>
</list>
</t>
<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 draft-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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