CoRE Working Group A. Rahman, Ed.
Internet-Draft InterDigital Communications, LLC
Intended status: Informational E. Dijk, Ed.
Expires: June 1, 2014 Philips Research
November 28, 2013
Group Communication for CoAP
draft-ietf-core-groupcomm-17
Abstract
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.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on June 1, 2014.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Background . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Conventions and Terminology . . . . . . . . . . . . . . . 4
2. Protocol Considerations . . . . . . . . . . . . . . . . . . . 5
2.1. IP Multicast Background . . . . . . . . . . . . . . . . . 5
2.2. Group Definition and Naming . . . . . . . . . . . . . . . 6
2.3. Port and URI Configuration . . . . . . . . . . . . . . . 7
2.4. Group REST Methods . . . . . . . . . . . . . . . . . . . 8
2.5. Messaging and Responses . . . . . . . . . . . . . . . . . 8
2.6. Group Member Discovery . . . . . . . . . . . . . . . . . 9
2.7. Configuring Group Memberships in Endpoints . . . . . . . 9
2.7.1. Background . . . . . . . . . . . . . . . . . . . . . 9
2.7.2. RESTful Interface . . . . . . . . . . . . . . . . . . 9
2.8. Multicast Request Acceptance and Response Suppression . . 14
2.9. Congestion Control . . . . . . . . . . . . . . . . . . . 16
2.10. Proxy Operation . . . . . . . . . . . . . . . . . . . . . 17
2.11. Exceptions . . . . . . . . . . . . . . . . . . . . . . . 18
3. Use Cases and Corresponding Protocol Flows . . . . . . . . . 18
3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 19
3.2. Network Configuration . . . . . . . . . . . . . . . . . . 19
3.3. Discovery of Resource Directory . . . . . . . . . . . . . 21
3.4. Lighting Control . . . . . . . . . . . . . . . . . . . . 22
3.5. Lighting Control in MLD Enabled Network . . . . . . . . . 25
3.6. Commissioning the Network Based On Resource Directory . . 26
4. Deployment Guidelines . . . . . . . . . . . . . . . . . . . . 27
4.1. Target Network Topologies . . . . . . . . . . . . . . . . 27
4.2. Networks Using the MLD Protocol . . . . . . . . . . . . . 27
4.3. Networks Using RPL Multicast Without MLD . . . . . . . . 28
4.4. Networks Using MPL Forwarding Without MLD . . . . . . . . 28
4.5. 6LoWPAN Specific Guidelines for the 6LBR . . . . . . . . 30
5. Security Considerations . . . . . . . . . . . . . . . . . . . 30
5.1. Security Configuration . . . . . . . . . . . . . . . . . 30
5.2. Threats . . . . . . . . . . . . . . . . . . . . . . . . . 30
5.3. Threat Mitigation . . . . . . . . . . . . . . . . . . . . 31
5.3.1. WiFi Scenario . . . . . . . . . . . . . . . . . . . . 31
5.3.2. 6LoWPAN Scenario . . . . . . . . . . . . . . . . . . 31
5.3.3. Future Evolution . . . . . . . . . . . . . . . . . . 31
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
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6.1. New 'core.gp' Resource Type . . . . . . . . . . . . . . . 32
6.2. New 'coap-group+json' Internet Media Type . . . . . . . . 32
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 33
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 33
8.1. Normative References . . . . . . . . . . . . . . . . . . 33
8.2. Informative References . . . . . . . . . . . . . . . . . 35
Appendix A. Multicast Listener Discovery (MLD) . . . . . . . . . 36
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 36
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 45
1. Introduction
1.1. Background
Constrained Application Protocol (CoAP) is a Representational State
Transfer (REST) based web transfer protocol for resource constrained
devices operating in an IP network [I-D.ietf-core-coap]. CoAP has
many similarities to HTTP [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.
1.2. Scope
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.
The normative protocol aspects of running CoAP on top of IP Multicast
and processing the responses are given in [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.
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1.3. Conventions and Terminology
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
[RFC2119].
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.
This document assumes readers are familiar with the terms and
concepts that are used in [I-D.ietf-core-coap]. In addition, this
document defines the following terminology:
Group Communication
A source node sends a single message which is delivered to
multiple destination nodes, where all destinations are identified
to belong to a specific group. The source node 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.
Reliable Group Communication
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.
Multicast
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.
IP Multicast
A specific multicast solution based on the use of IP multicast
addresses as defined in "IANA Guidelines for IPv4 Multicast
Address Assignments" [RFC5771] and "IP Version 6 Addressing
Architecture" [RFC4291].
Low power and Lossy Network (LLN)
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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.
2. Protocol Considerations
2.1. IP Multicast Background
IP Multicast protocols have been evolving for decades, resulting in
standards such as Protocol Independent Multicast - Sparse Mode (PIM-
SM) [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.
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 [RFC6550]) and an example of a forwarding protocol for LLNs is
Multicast Protocol for Low power and Lossy Networks (MPL)
[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 [RFC4601] is often used
for multicast routing in traditional IP networks (i.e. networks that
are not constrained).
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.
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 Section 4.2).
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 Section 1.3.
However, the level of reliability can be increased by employing a
multicast protocol that performs periodic retransmissions as is done
for example in MPL.
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2.2. Group Definition and Naming
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.
Any CoAP server node SHOULD join the "All CoAP Nodes" multicast group
([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.
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
[I-D.ietf-core-coap].
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
[I-D.ietf-core-coap] should be used in such way that a CoAP multicast
request is generated.
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:
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"
Similarly, if supported, reverse mapping (from IP multicast address
to Group FQDN) is possible using the reverse DNS resolution technique
([RFC1033]).
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2.3. Port and URI Configuration
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.
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:
1. 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.
2. 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.
3. Use the default CoAP UDP port (5683).
For a CoAP server node that supports multicast resource discovery,
the default port 5683 MUST be supported (Section 7.1 of
[I-D.ietf-core-coap]) for the "All CoAP Nodes" group.
All CoAP multicast requests SHOULD operate on URI paths in one of the
following ways:
1. 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
[I-D.ietf-appsawg-uri-get-off-my-lawn] prescribes that any
specification must not constrain, define structure or semantics
for any path component.
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2. 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.
3. 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.
4. 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.
2.4. Group REST Methods
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.
2.5. Messaging and Responses
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
[I-D.ietf-core-coap].
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 Section 2.9.
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 [I-D.ietf-core-coap]).
The CoAP client can distinguish the origin of multiple server
responses by source IP address of the UDP message containing the CoAP
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response. In case a CoAP client sent multiple group requests, the
responses are as usual matched to a request using the CoAP Token.
2.6. Group Member Discovery
CoAP Groups, and the membership of these groups, can be discovered
via the lookup interfaces defined in
[I-D.ietf-core-resource-directory]. An example of doing some of
these lookups is given in Section 3.6.
2.7. Configuring Group Memberships in Endpoints
2.7.1. Background
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).
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).
For the second case, a CoAP endpoint may look up its group membership
using techniques such as DNS-SD and Resource Directory
[I-D.ietf-core-resource-directory]. The latter case is detailed more
in Section 3.6.
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.
2.7.2. RESTful Interface
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.
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Also, the (unicast) methods to configure group membership SHOULD use
DTLS-secured CoAP [I-D.ietf-core-coap]. Thus only authorized
controllers should be allowed by an endpoint to configure its group
membership.
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 [RFC3433]).
2.7.2.1. CoAP-Group Resource Type and Media Type
CoAP endpoints implementing the RESTful interface MUST support the
CoAP group configuration Internet Media Type "application/coap-
group+json" (Section 6.2).
A resource offering this representation can be annotated for direct
discovery [RFC6690] using the resource type (rt) "core.gp" where "gp"
is shorthand for "group" (Section 6.1). An authorized client uses
this media type to query/manage group membership of a CoAP endpoint
as defined in the following subsections.
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 Section 2.7.2.5. 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.
Examples of four different group membership objects are:
{ "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]" }
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
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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 [I-D.ietf-core-coap] and [RFC3986].
group-address = IPv4address [ ":" port ]
/ "[" IPv6address "]" [":" port ]
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.
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.
2.7.2.2. Creating a new multicast group membership (POST)
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
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.
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
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in "a" a multicast address to which the endpoint is already
subscribed, the endpoint MUST re-register to that multicast address.
2.7.2.3. Deleting a group membership (DELETE)
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
2.7.2.4. Reading a group membership (GET)
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"}
2.7.2.5. Reading all group memberships (GET)
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.
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
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{ "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" }
}
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.
2.7.2.6. Updating a group memberships (PUT)
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
2.7.2.7. Creating/updating all group memberships at once (PUT)
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.
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
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Example: (clearing all group memberships at once)
Req: PUT /coap-group
Content-Format: application/coap-group+json
{}
Res: 2.04 Changed
2.8. Multicast Request Acceptance and Response Suppression
CoAP [I-D.ietf-core-coap] and CoRE Link Format [RFC6690] define
normative behaviors for:
1. Multicast request acceptance - in which cases a CoAP request is
accepted and executed, and when not.
2. Multicast response suppression - in which cases the CoAP response
to an already-executed request is returned to the requesting
endpoint, and when not.
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.
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 [RFC3542].
For multicast request acceptance, the REQUIRED behaviors are:
o 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)
[I-D.ietf-core-coap]. See Section 5.3 for examples of multicast
boundary limiting methods.
o A server SHOULD NOT accept a multicast discovery request with a
query string (as defined in CoRE Link Format [RFC6690]) if
filtering ([RFC6690]) is not supported by the server.
o 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 [RFC6690]). Implementers are
advised to disable multicast support by default on any other
resource, until explicitly enabled by an application or by
configuration.)
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o Otherwise accept the multicast request.
For multicast response suppression, the REQUIRED behaviors are:
o A server SHOULD NOT respond to a multicast discovery request if
the filter specified by the request's query string does not match.
o A server MAY choose not to respond to a multicast request, if
there's nothing useful to respond (e.g., error or empty response).
o Otherwise respond to the multicast request.
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:
o Suppression of all 2.xx success responses;
o Suppression of all 4.xx client errors;
o Suppression of all 5.xx server errors;
o Suppression of all 2.05 responses with empty payload.
A number of group communication example applications are given below
to illustrate how to make use of response suppression:
o CoAP resource discovery: Suppress 2.05 responses with empty
payload and all 4.xx and 5.xx errors.
o Lighting control: Suppress all 2.xx responses after a lighting
change command.
o 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.
o 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.
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o 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).
2.9. Congestion Control
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.
CoAP [I-D.ietf-core-coap] reduces multicast-specific congestion risks
through the following measures:
o A server MAY choose not to respond to a multicast request if
there's nothing useful to respond (e.g., error or empty response).
See Section 2.8 for more detailed guidelines on response
suppression.
o A server SHOULD limit the support for multicast requests to
specific resources where multicast operation is required.
o A multicast request MUST be Non-confirmable.
o A response to a multicast request SHOULD be Non-confirmable
(Section 5.2.3 of [I-D.ietf-core-coap]).
o A server does not respond immediately to a multicast request, but
SHOULD first wait for a time that is randomly picked within a
predetermined time interval called the Leisure.
Additional guidelines to reduce congestion risks defined in this
document are:
o 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 [RFC4944] is
used.
o A server can minimize the payload length in response to a
multicast GET on "/.well-known/core" by using hierarchy in
arranging link descriptions for the response. An example of this
is given in Section 5 of [RFC6690].
o A server can also minimize the payload length of a response to a
multicast GET (e.g., on "/.well-known/core") using CoAP blockwise
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transfers [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.
o 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.
More guidelines specific to use of CoAP in 6LoWPAN networks [RFC4944]
are given in Section 4.5.
2.10. Proxy Operation
CoAP [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.
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:
o 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.
o There is no default format defined in CoAP for aggregation of
multiple responses into a single response.
Alternatively, if a proxy follows directly the specification for a
CoAP Proxy [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:
o 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
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multiple (unicast) responses potentially leading to fault
conditions in the application.
o 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.
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:
o The CoAP Proxy MUST be explicitly configured (whitelist) to allow
proxied multicast requests by specific client(s).
o 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.
o 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.
2.11. Exceptions
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.
For example, if only the RPL protocol [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.
3. Use Cases and Corresponding Protocol Flows
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3.1. Introduction
The use of CoAP group communication is shown in the context of the
following two use cases and corresponding protocol flows:
o Discovery of Resource Directory (RD,
[I-D.ietf-core-resource-directory]): discovering the local CoAP RD
which contains links to resources stored on other CoAP servers
[RFC6690].
o Lighting Control: synchronous operation of a group of
IPv6-connected lights (e.g., 6LoWPAN [RFC4944] lights).
3.2. Network Configuration
To illustrate the use cases we define two IPv6 network
configurations. Both are based on the topology as shown in Figure 1.
The two configurations using this topology are:
1. Subnets are 6LoWPAN networks; the routers Rtr-1 and Rtr-2 are
6LoWPAN Border Routers (6LBRs, [RFC6775]).
2. Subnets are Ethernet links; the routers Rtr-1 and Rtr-2 are
multicast-capable Ethernet routers.
Both configurations are further specified by the following:
o A large room (Room-A) with three lights (Light-1, Light-2,
Light-3) controlled by a Light Switch. The devices are organized
into two subnets. In reality, there could be more lights (up to
several hundreds) but these are not shown for clarity.
o Light-1 and the Light Switch are connected to a router (Rtr-1).
o Light-2 and the Light-3 are connected to another router (Rtr-2).
o The routers are connected to an IPv6 network backbone which is
also multicast enabled. In the general case, this means the
network backbone and Rtr-1/Rtr-2 support a PIM based multicast
routing protocol, and Multicast Listener Discovery (MLD) for
forming groups.
o A CoAP RD is connected to the network backbone.
o 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
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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).
o A Controller (CoAP client) is connected to the backbone, which is
able to control various building functions including lighting.
################################################
# ********************** 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 |
+------------+
Figure 1: Network Topology of a Large Room (Room-A)
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3.3. Discovery of Resource Directory
The protocol flow for discovery of the CoAP RD for the given network
(of Figure 1) is shown in Figure 2:
o Light-2 is installed and powered on for the first time.
o 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).
o 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).
o 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.
o Note that the flow is shown only for Light-2 for clarity. Similar
flows will happen for Light-1, Light-3 and the Light Switch when
they are first installed.
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
[I-D.ietf-core-resource-directory] for more details).
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.
Light CoAP
Light-1 Light-2 Light-3 Switch Rtr-1 Rtr-2 RD
| | | | | | |
| | | | | | |
********************************** | | |
* Light-2 is installed * | | |
* and powers on for first time * | | |
********************************** | | |
| | | | | | |
| | | | | | |
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| | COAP NON Mcast(GET | |
| | /.well-known/core?rt=core.rd) | |
| |--------->-------------------------------->| |
| | | | | |--------->|
| | | | | | |
| | | | | | |
| | COAP NON (2.05 Content | |
| | </rd>;rt="core.rd";ins="Primary") |<---------|
| |<------------------------------------------| |
| | | | | | |
Figure 2: Resource Directory Discovery via Multicast Request
3.4. Lighting Control
The protocol flow for a building automation lighting control scenario
for the network (Figure 1) is shown in Figure 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 Section 2.8 for more details on response suppression
by a server.)
In addition, Figure 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.
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 [I-D.ietf-roll-trickle-mcast].
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We assume the following steps have already occurred before the
illustrated flows:
1. Startup phase: 6LoWPANs are formed. IPv6 addresses assigned to
all devices. The CoAP network is formed.
2. Network configuration (application-independent): 6LBRs are
configured with multicast addresses, or address blocks, to filter
out or to pass through to/from the 6LoWPAN.
3. 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.
4. 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.
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.
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 * | | | |
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* simultaneously) * | | | |
*********************** | | | |
| | | | | | |
Figure 3: Light Switch Sends Multicast Control Message
Light Network
Light-1 Light-2 Light-3 Switch Rtr-1 Rtr-2 Backbone
| | | | | | |
| COAP NON (2.04 Changed) | | | |
|------------------------------->| | | |
| | | | | | |
| | | | | | |
| COAP NON (2.04 Changed) | | |
| |------------------------------------------>| |
| | | | | |--------->|
| | | | |<--------------------|
| | | |<---------| | |
| | | | | | |
| | COAP NON (5.00 Internal Server Error) |
| | |------------------------------->| |
| | | | | |--------->|
| | | | |<--------------------|
| | | |<---------| | |
| | | | | | |
Figure 4: Lights (Optionally) Respond to Multicast CoAP Request
Another, but similar, lighting control use case is shown in Figure 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.
Network
Light-1 Light-2 Light-3 Rtr-1 Rtr-2 Backbone Controller
| | | | | | |
| | | | | COAP NON Mcast(PUT,
| | | | | Payload=lights ON)
| | | | | |<-------|
| | | |<----------<---------| |
|<--------------------------------| | | |
ON | | | | | |
| |<----------<---------------------| | |
| ON ON | | | |
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^ ^ ^ | | | |
*********************** | | | |
* Lights in Room-A * | | | |
* turn on (nearly * | | | |
* simultaneously) * | | | |
*********************** | | | |
| | | | | | |
| | | | | | |
| COAP NON (2.04 Changed) | | | |
|-------------------------------->| | | |
| | | |-------------------->| |
| | COAP NON (2.04 Changed) | |------->|
| |-------------------------------->| | |
| | | | |--------->| |
| | | COAP NON (2.04 Changed) |------->|
| | |--------------------->| | |
| | | | |--------->| |
| | | | | |------->|
| | | | | | |
Figure 5: Controller On Backbone Sends Multicast Control Message
3.5. Lighting Control in MLD Enabled Network
The use case of previous section can also apply in networks where
nodes support the MLD protocol [RFC3810]. The Lights then take on
the role of MLDv2 listener and the routers (Rtr-1, Rtr-2) are MLDv2
Routers. In the Ethernet based network configuration, MLD may be
available on all involved network interfaces. Use of MLD in the
6LoWPAN based configuration is also possible, but requires MLD
support in all nodes in the 6LoWPAN. In current 6LoWPAN
implementations, MLD is however not supported.
The resulting protocol flow is shown in Figure 6. 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.
After the shown sequence of MLD Report messages has been executed,
both Rtr-1 and Rtr-2 are automatically configured to forward
multicast traffic destined to Room-A-Lights onto their connected
subnet. Hence, no manual Network Configuration of routers, as
previously indicated in Section 3.4, is needed anymore.
Light Network
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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---->|
| | | | | | |
| | | | | | |
Figure 6: Joining Lighting Groups Using MLD
3.6. Commissioning the Network Based On Resource Directory
This section outlines how devices in the lighting use case (both
Switches and Lights) can be commissioned, making use of Resource
Directory [I-D.ietf-core-resource-directory] and its group
configuration feature.
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.
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
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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 [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 Section 2.7.2.2.
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.
4. Deployment Guidelines
This section provides guidelines how IP Multicast based CoAP group
communication can be deployed in various network configurations.
4.1. Target Network Topologies
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.
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:
1. Routing/forwarding of IP multicast packets over multiple hops
2. Routing/forwarding of IP multicast packets over subnet boundaries
between traditional and constrained (e.g. LLN) networks.
The remainder of this section discusses solutions to enable both
features.
4.2. Networks Using the MLD Protocol
CoAP nodes that are IP hosts (i.e., not IP routers) are generally
unaware of the specific multicast routing/forwarding protocol being
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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.
The Multicast Listener Discovery (MLD) protocol [RFC3810] (see
Appendix A) 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.
The guidelines from [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.
4.3. Networks Using RPL Multicast Without MLD
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 [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).
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.
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.
4.4. Networks Using MPL Forwarding Without MLD
The MPL forwarding protocol [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.
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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.
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 Figure 1 where the Subnets are
6LoWPAN subnets and per 6LoWPAN subnet one Realm-Local
([I-D.droms-6man-multicast-scopes]) MPL Domain is defined. The
backbone network in this case is not part of any MPL Domain.
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.
To solve the above issue, the following solutions could be applied:
1. 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.
2. 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).
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3. 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
[I-D.ietf-roll-trickle-mcast].
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.
4.5. 6LoWPAN Specific Guidelines for the 6LBR
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 Appendix A) on the backbone link.
5. Security Considerations
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.
5.1. Security Configuration
As defined in [I-D.ietf-core-coap], CoAP group communication based on
IP multicast:
o Will operate in CoAP NoSec (No Security) mode, until a future
group security solution is developed (see also Section 5.3.3).
o MUST NOT use "coaps" scheme. That is, all group communication
MUST use only "coap" scheme.
5.2. Threats
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 [I-D.ietf-core-coap] for IP multicast.
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5.3. Threat Mitigation
The [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.
5.3.1. WiFi Scenario
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 [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.
5.3.2. 6LoWPAN Scenario
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.
5.3.3. Future Evolution
In the future, to further mitigate the threats, the developing
approach for DTLS-based IP multicast security for CoAP networks (see
[I-D.keoh-tls-multicast-security]) or similar approaches should be
considered once they mature.
6. IANA Considerations
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6.1. New 'core.gp' Resource Type
This memo registers a new resource type (rt) from the CoRE Parameters
Registry called 'core.gp'.
(Note to IANA/RFC Editor: This registration follows the process
described in section 7.4 of [RFC6690]).
Attribute Value: core.gp
Description: Group Configuration resource. This resource is used to
query/manage the group membership of a CoAP server.
Reference: See Section 2.7.2.
6.2. New 'coap-group+json' Internet Media Type
This memo registers a new Internet Media Type for CoAP group
configuration resource called 'application/coap-group+json'.
(Note to IANA/RFC Editor: This registration follows the guidance from
[RFC6839], and (last paragraph) of section 12.3 of
[I-D.ietf-core-coap].
Type name: application
Subtype name: coap-group+json
Required parameters: None
Optional parameters: None
Encoding considerations: 8bit UTF-8.
JSON to be represented using UTF-8 which is 8bit compatible (and most
efficient for resource constrained implementations).
Security considerations:
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.
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Interoperability considerations: None
Published specification: (This I-D when it becomes an RFC)
Applications that use this media type:
CoAP client and server implementations that wish to set/read the
group configuration resource via 'application/coap-group+json'
payload as described in Section 2.7.2.
Additional Information:
Magic number(s): None
File extension(s): *.json
Macintosh file type code(s): TEXT
Intended usage: COMMON
Restrictions on usage: None
Author: CoRE WG
Change controller: IETF
7. Acknowledgements
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.
8. References
8.1. Normative References
[RFC1033] Lottor, M., "Domain administrators operations guide", RFC
1033, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
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[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, October 2002.
[RFC3433] Bierman, A., Romascanu, D., and K. Norseth, "Entity Sensor
Management Information Base", RFC 3433, December 2002.
[RFC3542] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
"Advanced Sockets Application Program Interface (API) for
IPv6", RFC 3542, May 2003.
[RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, September 2007.
[RFC5771] Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for
IPv4 Multicast Address Assignments", BCP 51, RFC 5771,
March 2010.
[RFC6092] Woodyatt, J., "Recommended Simple Security Capabilities in
Customer Premises Equipment (CPE) for Providing
Residential IPv6 Internet Service", RFC 6092, January
2011.
[RFC6550] Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R.,
Levis, P., Pister, K., Struik, R., Vasseur, JP., and R.
Alexander, "RPL: IPv6 Routing Protocol for Low-Power and
Lossy Networks", RFC 6550, March 2012.
[RFC6636] Asaeda, H., Liu, H., and Q. Wu, "Tuning the Behavior of
the Internet Group Management Protocol (IGMP) and
Multicast Listener Discovery (MLD) for Routers in Mobile
and Wireless Networks", RFC 6636, May 2012.
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[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, August 2012.
[RFC6775] Shelby, Z., Chakrabarti, S., Nordmark, E., and C. Bormann,
"Neighbor Discovery Optimization for IPv6 over Low-Power
Wireless Personal Area Networks (6LoWPANs)", RFC 6775,
November 2012.
[RFC6839] Hansen, T. and A. Melnikov, "Additional Media Type
Structured Syntax Suffixes", RFC 6839, January 2013.
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., and C. Bormann, "Constrained
Application Protocol (CoAP)", draft-ietf-core-coap-18
(work in progress), June 2013.
8.2. Informative References
[I-D.ietf-core-block]
Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
draft-ietf-core-block-14 (work in progress), October 2013.
[I-D.ietf-roll-trickle-mcast]
Hui, J. and R. Kelsey, "Multicast Protocol for Low power
and Lossy Networks (MPL)", draft-ietf-roll-trickle-
mcast-05 (work in progress), August 2013.
[I-D.keoh-tls-multicast-security]
Keoh, S., Kumar, S., and E. Dijk, "DTLS-based Multicast
Security for Low-Power and Lossy Networks (LLNs)", draft-
keoh-tls-multicast-security-00 (work in progress), October
2012.
[I-D.ietf-core-resource-directory]
Shelby, Z., Krco, S., and C. Bormann, "CoRE Resource
Directory", draft-ietf-core-resource-directory-00 (work in
progress), June 2013.
[I-D.ietf-appsawg-uri-get-off-my-lawn]
Nottingham, M., "Standardising Structure in URIs", draft-
ietf-appsawg-uri-get-off-my-lawn-00 (work in progress),
September 2013.
[I-D.droms-6man-multicast-scopes]
Droms, R., "IPv6 Multicast Address Scopes", draft-droms-
6man-multicast-scopes-02 (work in progress), July 2013.
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Appendix A. Multicast Listener Discovery (MLD)
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.
The Multicast Listener Discovery (MLD) protocol [RFC3810] (or its
IPv4 equivalent IGMP [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.
[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.
Appendix B. Change Log
[Note to RFC Editor: Please remove this section before publication.]
Changes from ietf-16 to ietf-17:
o Added guidelines on joining of IPv6/IPv4 "All CoAP Nodes"
multicast addresses (#356).
o Added MUST support default port in case multicast discovery is
available.
o In section 2.1 (IP Multicast Background), clarified that IP
multicast is not guaranteed and referenced a definition of
Reliable Group Communication (#355).
o Added section 2.5 (Messages and Responses) to clarify how
responses are identified and how Token/MID are used in multicast
CoAP.
o 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.
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o Updated section 2.6.2 (RESTful Interface for Configuring Group
Memberships) to allow deletion of individual group memberships
(#354).
o Various editorial updates based on comments by Peter van der Stok.
Removed reference to expired draft-vanderstok-core-dna at request
of its author.
o Various editorial updates for improved readability.
Changes from ietf-15 to ietf-16:
o In section 2.6.2, changed DELETE in group management interface to
a PUT with empty JSON array to clear the list (#345).
o 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).
o Addressed some more editorial comments provided by Carsten Bormann
in preparation for WGLC.
o Various editorial updates for improved readability.
Changes from ietf-14 to ietf-15:
o In section 2.2, provided guidance on how implementers should parse
URIs for group communication (#339).
o 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).
o 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).
o In section 2.6.2.1, specified that notation of IP address in group
membership configuration interface should follow standard notation
(#342).
o In section 6.2, "coap-group+json" Media Type encoding simplified
to just support UTF-8 (and not UTF-16 and UTF-32) (#344).
o Various editorial updates for improved readability.
Changes from ietf-13 to ietf-14:
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o 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.
o Changed "ip" field to "a" in Section 2.6 (#337)
Changes from ietf-12 to ietf-13:
o 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.
o 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.
Changes from ietf-11 to ietf-12:
o 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.
o Various editorial updates for improved readability.
Changes from ietf-10 to ietf-11:
o 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).
o Various editorial updates for improved readability.
Changes from ietf-09 to ietf-10:
o Various editorial updates including:
o Added a fourth option in section 3.3 on ways to obtain the URI
path for a group request.
o Clarified use of content format in GET/PUT requests for
Configuring Group Membership in Endpoints (in section 3.6).
o Changed reference "draft-shelby-core-resource-directory" to
"draft-ietf-core-resource-directory".
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o Clarified (in section 3.7) that ACKs are never used for a
multicast request (from #296).
o Clarified (in section 5.2/5.2.3) that MPL does not support group
membership advertisement.
o Adding introductory paragraph to Scope (section 2.2).
o Wrote out fully the URIs in table section 3.2.
o Reworded security text in section 7.2 (New Internet Media Type) to
make it consistent with section 3.6 (Configuring Group
Membership).
o Fixed formatting of hyperlinks in sections 6.3 and 7.2.
Changes from ietf-08 to ietf-09:
o 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).
o 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]".
o Changed text in section 3.4 (Group Methods) to allow multicast
POST under specific conditions and highlighting the risks with
using it (#328).
o Various editorial updates for improved readability.
Changes from ietf-07 to ietf-08:
o 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).
o Addressed various comments from Peter van der Stok (#296).
o Various editorial updates for improved readability including
defining all acronyms.
Changes from ietf-06 to ietf-07:
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o 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).
o Clarified semantics (in section 3.6) of group management JSON
format (#300).
o Added details of IANA request (in section 7.1) for a new CORE
Resource Type called 'core.gp'.
o Clarified that DELETE method (in section 3.6) is also a valid
group management operation.
o Various editorial updates for improved readability.
Changes from ietf-05 to ietf-06:
o Added a new section on commissioning flow when using discovery
services when end devices discover in which multicast group they
are allocated (#295).
o 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).
o Added use case of multicasting controller on the backbone (#279).
o 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).
o 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.
o Clarified relationship of this document to [I-D.ietf-core-coap] in
section 2.2 (Scope).
o Removed IPSec related requirement, as IPSec is not part of
[I-D.ietf-core-coap] anymore.
o 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) .
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o Editorial rewording of section 3.7 (Multicast Request Acceptance
and Response Suppression) to make the logic easier to comprehend
(parse).
o Various editorial updates for improved readability.
Changes from ietf-04 to ietf-05:
o Added a new section 3.9 (Exceptions) that highlights that IP
multicast (and hence group communication) is not always available
(#187).
o Updated text on the use of [RFC2119] language (#271) in Section 1.
o Included guidelines on when (not) to use CoAP responses to
multicast requests and when (not) to accept multicast requests
(#273).
o Added guideline on use of core-block for minimizing response size
(#275).
o Restructured section 6 (Security Considerations) to more fully
describe threats and threat mitigation (#277).
o Clearly indicated that DNS resolution and reverse DNS lookup are
optional.
o 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.
o Removed repetitive text about the fact that group communication is
not guaranteed.
o 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).
o 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).
o Extended group resource manipulation guidelines with use of pre-
configured ports/paths for the multicast group.
o Consolidated all text relating to ports in a new section 3.3 (Port
Configuration).
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o 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).
o Various editorial updates for improved readability, including
editorial comments by Peter van der Stok to WG list of December
18th, 2012.
Changes from ietf-03 to ietf-04:
o 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).
o Added reference to draft-keoh-tls-multicast-security to section 6
(Security Considerations).
o Removed Appendix B (CoAP-Observe Alternative to Group
Communications) as it is as an alternative to IP Multicast that
the WG has not adopted and moved section to draft-dijk-core-
groupcomm-misc (#267).
o Deleted section 8 (Conclusions) as it is redundant (#268).
o Simplified light switch use case (#269) by splitting into basic
operations and additional functions (#269).
o Moved section 3.7 (CoAP Multicast and HTTP Unicast Interworking)
to draft-dijk-core-groupcomm-misc (#270).
o Moved section 3.3.1 (DNS-SD) and 3.3.2 (CoRE Resource Directory)
to draft-dijk-core-groupcomm-misc as these sections essentially
just repeated text from other drafts regarding DNS based features.
Clarified remaining text in this draft relating to DNS based
features to clearly indicate that these features are optional
(#272).
o Focus section 3.5 (Configuring Group Membership) on a single
proposed solution.
o Scope of section 5.3 (Use of MLD) widened to multicast destination
advertisement methods in general.
o Rewrote section 2.2 (Scope) for improved readability.
o Moved use cases that are not addressed to draft-dijk-core-
groupcomm-misc.
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o Various editorial updates for improved readability.
Changes from ietf-02 to ietf-03:
o Clarified that a group resource manipulation may return back a
mixture of successful and unsuccessful responses (section 3.4 and
Figure 6) (#251).
o Clarified that security option for group communication must be
NoSec mode (section 6) (#250).
o Added mechanism for group membership configuration (#249).
o Removed IANA request for multicast addresses (section 7) and
replaced with a note indicating that the request is being made in
[I-D.ietf-core-coap] (#248).
o Made the definition of 'group' more specific to group of CoAP
endpoints and included text on UDP port selection (#186).
o Added explanatory text in section 3.4 regarding why not to use
group communication for non-idempotent messages (i.e. CoAP POST)
(#186).
o Changed link-local RD discovery to site-local in RD discovery use
case to make it more realistic.
o Fixed lighting control use case CoAP proxying; now returns
individual CoAP responses as in coap-12.
o Replaced link format I-D with RFC6690 reference.
o Various editorial updates for improved readability
Changes from ietf-01 to ietf-02:
o Rewrote congestion control section based on latest CoAP text
including Leisure concept (#188)
o Updated the CoAP/HTTP interworking section and example use case
with more details and use of MLD for multicast group joining
o Key use cases added (#185)
o References to draft-vanderstok-core-dna and draft-castellani-core-
advanced-http-mapping added
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o Moved background sections on "MLD" and "CoAP-Observe" to
Appendices
o Removed requirements section (and moved it to draft-dijk-core-
groupcomm-misc)
o Added details for IANA request for group communication multicast
addresses
o Clarified text to distinguish between "link local" and general
multicast cases
o Moved lengthy background section 5 to draft-dijk-core-groupcomm-
misc and replaced with a summary
o Various editorial updates for improved readability
o Change log added
Changes from ietf-00 to ietf-01:
o Moved CoAP-observe solution section to section 2
o Editorial changes
o Moved security requirements into requirements section
o Changed multicast POST to PUT in example use case
o Added CoAP responses in example use case
Changes from rahman-07 to ietf-00:
o Editorial changes
o Use cases section added
o CoRE Resource Directory section added
o Removed section 3.3.5. IP Multicast Transmission Methods
o Removed section 3.4 Overlay Multicast
o Removed section 3.5 CoAP Application Layer Group Management
o Clarified section 4.3.1.3 RPL Routers with Non-RPL Hosts case
o References added and some normative/informative status changes
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Authors' Addresses
Akbar Rahman (editor)
InterDigital Communications, LLC
Email: Akbar.Rahman@InterDigital.com
Esko Dijk (editor)
Philips Research
Email: esko.dijk@philips.com
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