Internet DRAFT - draft-ietf-core-groupcomm
draft-ietf-core-groupcomm
CoRE Working Group A. Rahman, Ed.
Internet-Draft InterDigital Communications, LLC
Intended status: Experimental E. Dijk, Ed.
Expires: March 16, 2015 Philips Research
September 12, 2014
Group Communication for CoAP
draft-ietf-core-groupcomm-25
Abstract
The Constrained Application Protocol (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
specification defines how the CoAP protocol should be used in a group
communication context. An approach for using CoAP on top of IP
multicast is detailed based on both existing CoAP functionality as
well as new features introduced in this specification. 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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on March 16, 2015.
Copyright Notice
Copyright (c) 2014 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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. RESTful Methods . . . . . . . . . . . . . . . . . . . . . 9
2.5. Request and Response Model . . . . . . . . . . . . . . . 9
2.6. Membership Configuration . . . . . . . . . . . . . . . . 10
2.6.1. Background . . . . . . . . . . . . . . . . . . . . . 10
2.6.2. Membership Configuration RESTful Interface . . . . . 11
2.7. Request Acceptance and Response Suppression Rules . . . . 17
2.8. Congestion Control . . . . . . . . . . . . . . . . . . . 19
2.9. Proxy Operation . . . . . . . . . . . . . . . . . . . . . 20
2.10. Exceptions . . . . . . . . . . . . . . . . . . . . . . . 21
3. Use Cases and Corresponding Protocol Flows . . . . . . . . . 22
3.1. Introduction . . . . . . . . . . . . . . . . . . . . . . 22
3.2. Network Configuration . . . . . . . . . . . . . . . . . . 22
3.3. Discovery of Resource Directory . . . . . . . . . . . . . 24
3.4. Lighting Control . . . . . . . . . . . . . . . . . . . . 26
3.5. Lighting Control in MLD Enabled Network . . . . . . . . . 30
3.6. Commissioning the Network Based On Resource Directory . . 31
4. Deployment Guidelines . . . . . . . . . . . . . . . . . . . . 32
4.1. Target Network Topologies . . . . . . . . . . . . . . . . 32
4.2. Networks Using the MLD Protocol . . . . . . . . . . . . . 33
4.3. Networks Using RPL Multicast Without MLD . . . . . . . . 33
4.4. Networks Using MPL Forwarding Without MLD . . . . . . . . 34
4.5. 6LoWPAN Specific Guidelines for the 6LBR . . . . . . . . 35
5. Security Considerations . . . . . . . . . . . . . . . . . . . 35
5.1. Security Configuration . . . . . . . . . . . . . . . . . 35
5.2. Threats . . . . . . . . . . . . . . . . . . . . . . . . . 36
5.3. Threat Mitigation . . . . . . . . . . . . . . . . . . . . 36
5.3.1. WiFi Scenario . . . . . . . . . . . . . . . . . . . . 37
5.3.2. 6LoWPAN Scenario . . . . . . . . . . . . . . . . . . 37
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5.3.3. Future Evolution . . . . . . . . . . . . . . . . . . 37
5.4. Monitoring Considerations . . . . . . . . . . . . . . . . 38
5.4.1. General Monitoring . . . . . . . . . . . . . . . . . 38
5.4.2. Pervasive Monitoring . . . . . . . . . . . . . . . . 38
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 39
6.1. New 'core.gp' Resource Type . . . . . . . . . . . . . . . 39
6.2. New 'coap-group+json' Internet Media Type . . . . . . . . 39
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 41
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 41
8.1. Normative References . . . . . . . . . . . . . . . . . . 41
8.2. Informative References . . . . . . . . . . . . . . . . . 43
Appendix A. Multicast Listener Discovery (MLD) . . . . . . . . . 44
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 44
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 57
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 [RFC7252]. CoAP has many
similarities to HTTP [RFC7230] but also has some key differences.
Constrained devices can be large in numbers, but are often related to
each other in function or by 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 can simultaneously get
(or set) resources 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 base protocol aspects of sending CoAP requests on top of IP
multicast, and processing the (unicast IP) responses are given in
Section 8 of [RFC7252]. To provide a more complete CoAP group
communication functionality, this specification introduces new CoAP
protocol processing functionality (e.g., new rules for re-use of
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Token values, request suppression, and proxy operation) and a new
management interface for RESTful group membership configuration.
CoAP group communication will run in Any Source Multicast (ASM) mode
[RFC5110] of IP multicast operation. This means that there is no
restriction on the source node which sends (originates) the CoAP
messages to the IP multicast group. For example, the source node may
be part of the IP multicast group or not. Also, there is no
restriction on the number of source nodes.
While Section 9.1 of [RFC7252] supports various modes of DTLS-based
security for CoAP over unicast IP, it does not specify any security
modes for CoAP over IP multicast. That is, [RFC7252] assumes that
CoAP over IP multicast is not encrypted, nor authenticated, nor
access controlled. This document assumes the same security model
(see Section 5.1). However, there are several promising security
approaches being developed that should be considered in the future
for protecting CoAP group communications (see Section 5.3.3).
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] when they appear in ALL CAPS. When these words are not in
ALL CAPS (such as "should" or "Should"), they have their usual
English meanings, and are not to be interpreted as [RFC2119] key
words.
Note that this document refers back to other RFCs, and especially
[RFC7252], to help explain overall CoAP group communication features.
However use of [RFC2119] key words is reserved for new CoAP
functionality introduced by this specification.
This document assumes readers are familiar with the terms and
concepts that are used in [RFC7252]. In addition, this document
defines the following terminology:
Group Communication
A source node sends a single application layer (e.g., CoAP)
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
mechanisms for CoAP group communication are UDP/IP multicast for
the requests, and unicast UDP/IP for the responses. The network
involved may be a constrained network such as a low-power, lossy
network.
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Reliable Group Communication
A special case of 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. An example of a reliable group communication protocol is
[RFC5740].
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 approach 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]. A complete IP multicast solution may
include support for managing group memberships, and IP multicast
routing/forwarding (see Section 2.1).
Low power and Lossy Network (LLN)
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)
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[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). Also, a multicast
forwarding proxy node [RFC4605] may be required.
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.
2.2. Group Definition and Naming
A CoAP group is defined as a set of CoAP endpoints, where each
endpoint is configured to receive CoAP group communication requests
that are sent to the group's associated IP multicast address. The
individual response by each endpoint receiver to a CoAP group
communication request is always sent back as unicast. An endpoint
may be a member of multiple groups. Group membership of an endpoint
may dynamically change over time.
All CoAP server nodes SHOULD join the "All CoAP Nodes" multicast
group (Section 12.8 of [RFC7252]) by default to enable CoAP
discovery. 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 be enabled.
A CoAP 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 regular
CoAP URI syntax (Section 6 of [RFC7252].
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Note: A group URI is needed to initiate CoAP group communications.
For CoAP client implementations it is recommended to use the URI
decomposition method of Section 6.4 of [RFC7252] in such way that,
from a group URI, a CoAP group communication request is generated.
For sending nodes, it is recommended to use the IP multicast address
literal in a group URI. (This is because DNS infrastructure may not
be deployed in many constrained network deployments). However, 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]). Reverse mapping is important, for example, in trouble
shooting to translate IP multicast addresses back to human readable
hostnames to show in a diagnostics user interface.
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, usually on the CoAP default UDP
port, 5683. If the group uses a specified non-default UDP port, be
careful to ensure that all group members are configured to use that
same port.
Different ports for the same IP multicast address are preferably not
used to specify different CoAP groups. If disjoint groups share the
same IP multicast address, then all the devices interested in one
group will accept IP traffic also for the other disjoint groups, only
to ultimately discard the traffic higher in their IP stack (based on
UDP port discrimination).
CoAP 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 group URI such as the path, or the
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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 group communication requests MUST
be sent using a port number according to one of below options:
1. A pre-configured port number.
2. If the client is configured to use service discovery including
URI and port discovery, it uses the port number obtained via a
service discovery lookup operation for the targeted CoAP group.
3. Use the default CoAP UDP port (5683).
For a CoAP server node that supports resource discovery, the default
port 5683 must be supported (Section 7.1 of [RFC7252]) for the "All
CoAP Nodes" group. Regardless of the method of selecting the port
number, the same port MUST be used across all CoAP servers in a group
and across all CoAP clients performing the group requests.
All CoAP group communication requests SHOULD operate on group URI
paths in one of the following ways:
1. Pre-configured group URI paths, if available. Implementers are
free to define the paths as they see fit. However, note that
[RFC7320] prescribes that a specification must not constrain,
define structure or semantics for any path component. So for
this reason, a pre-defined URI path is not specified in this
document and also must not be provided in other specifications.
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 group 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 it can use this URI in a CoAP
group communication request. For example, a commissioning tool
may instruct a sensor device in this way to which target group
(group URI) it should report sensor events.
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However the URI path is selected, the same path MUST be used across
all CoAP servers in a group and all CoAP clients performing the group
requests.
2.4. RESTful Methods
Group communication most often uses the idempotent CoAP methods GET
and PUT. The idempotent method DELETE can also be used. The non-
idempotent CoAP method POST may only be used for group communication
if the resource being POSTed to has been designed to cope with the
unreliable and lossy nature of IP multicast. For example, a client
may re-send a multicast POST request for additional reliability.
Some servers will receive the request two times while others may
receive it only once. For idempotent methods all these servers will
be in the same state, while for POST this is not guaranteed; so the
resource POST operation must be specifically designed to take message
loss into account.
2.5. Request and Response Model
All CoAP requests that are sent via IP multicast must be Non-
confirmable (Section 8.1 of [RFC7252]). The Message ID in an IP
multicast CoAP message is used for optional message de-duplication as
detailed in Section 4.5 of [RFC7252].
A server optionally sends back a unicast response to the CoAP group
communication request (e.g., response "2.05 Content" to a group GET
request). 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.
Detailed processing rules for IP multicast request acceptance and
unicast response suppression are given in Section 2.7.
A CoAP request sent over IP multicast and any unicast response it
causes must take into account the congestion control rules defined in
Section 2.8.
The CoAP client can distinguish the origin of multiple server
responses by source IP address of the UDP message containing the CoAP
response, or any other available unique identifier (e.g., contained
in the CoAP payload). In case a CoAP client sent multiple group
requests, the responses are as usual matched to a request using the
CoAP Token.
For multicast CoAP requests there are additional constraints on the
re-use of Token values, compared to the unicast case. In the unicast
case, receiving a response effectively frees up its Token value for
re-use since no more responses will follow. However, for multicast
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CoAP the number of responses is not bounded a-priori. Therefore the
reception of a response cannot be used as a trigger to "free up" a
Token value for re-use. Re-using a Token value too early could lead
to incorrect response/request matching in the client, which is a
protocol error. Therefore the time between re-use of Token values
used in multicast requests MUST be greater than:
NON_LIFETIME + MAX_LATENCY + MAX_SERVER_RESPONSE_DELAY
where NON_LIFETIME and MAX_LATENCY are defined in Section 4.8 of
[RFC7252]. MAX_SERVER_RESPONSE_DELAY is defined here as the expected
maximum response delay over all servers that the client can send a
multicast request to. This delay includes the maximum Leisure time
period as defined in Section 8.2 of [RFC7252]. The CoAP protocol
does not define a time limit for the server response delay. Using
the default CoAP protocol parameters, the Token re-use time MUST be
greater than 250 seconds plus MAX_SERVER_RESPONSE_DELAY. A preferred
solution to meet this requirement is to generate a new unique Token
for every multicast request, such that a Token value is never re-
used. If a client has to re-use Token values for some reason, and
also MAX_SERVER_RESPONSE_DELAY is unknown, then using
MAX_SERVER_RESPONSE_DELAY = 250 seconds is a reasonable guideline.
The time between Token re-uses is in that case set to a value greater
than 500 seconds.
2.6. Membership Configuration
2.6.1. Background
2.6.1.1. Member Discovery
CoAP Groups, and the membership of these groups, can be discovered
via the lookup interfaces in the Resource Directory (RD) defined in
[I-D.ietf-core-resource-directory]. This discovery interface is not
required to invoke CoAP group communications. However, it is a
potential complementary interface useful for overall management of
CoAP groups. Other methods to discover groups (e.g., proprietary
management systems) can also be used. An example of doing some of
the RD based lookups is given in Section 3.6.
2.6.1.2. Configuring Members
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).
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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 an 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].
In the third case, typical in scenarios such as building control, a
dynamic commissioning tool determines to which group(s) a sensor or
actuator node belongs, and writes this information to the node, which
can subsequently join the correct IP multicast group(s) on its
network interface. The information written per group may again be an
IP multicast address or a hostname.
2.6.2. Membership Configuration RESTful Interface
To achieve better interoperability between endpoints from different
manufacturers, an OPTIONAL CoAP membership configuration RESTful
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 will use
unicast CoAP methods (GET/PUT/POST/DELETE). This interface is a
method of configuring group information in individual endpoints.
Also, a form of authorization (preferably making use of unicast DTLS-
secured CoAP of Section 9.1 of [RFC7252]) should be used such that
only authorized controllers are 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
alternative approaches may support a subset or super-set of the
membership configuration RESTful 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]).
2.6.2.1. CoAP-Group Resource Type and Media Type
CoAP endpoints implementing the membership configuration 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"
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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
JavaScript Object Notation (JSON) based content format (as indicated
by the "application/coap-group+json" media type). The resource
includes zero or more group membership JSON objects [RFC7159] in a
format as defined in Section 2.6.2.4. A group membership JSON object
contains one or more key/value pairs as defined below, and represents
a single IP multicast group membership for the CoAP endpoint. Each
key/value pair is encoded as a member of the JSON object, where the
key is the member name and the value is the member's value.
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:c001]" }
The OPTIONAL "n" key/value pair stands for "name" and identifies the
group with a hostname (and optionally the port number), 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 3.2.2 of [RFC3986] which are also used in
the base CoAP protocol (Section 6 of [RFC7252].
group-address = IPv4address [ ":" port ]
/ "[" IPv6address "]" [":" port ]
In any group membership object, if the IP address is known when the
object is created, it is included in the "a" key/value pair. If the
"a" value cannot be provided, the "n" value MUST be included,
containing a valid hostname with optional port number that can be
translated to an IP multicast address via DNS.
group-name = host [ ":" port ]
If the port number is not provided, then the endpoint will attempt to
look up the port number from DNS if it supports a method to do this.
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The possible DNS methods include DNS-SRV [RFC2782] or DNS-SD
[RFC6763]. If port lookup is not supported or not provided by DNS,
the default CoAP port (5683) is assumed.
After any change on a Group configuration resource, the endpoint MUST
effect registration/de-registration from the corresponding IP
multicast group(s) by making use of APIs such as IPV6_RECVPKTINFO
[RFC3542].
2.6.2.2. 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. Index MUST be a string of maximum two (2) alphanumeric ASCII
characters (case insensitive). It MUST be locally unique to the endpoint server.
It indexes the particular endpoint's list of group memberships.
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 it is recommended to use the path "coap-group"
by default. The "a" key/value pair is always used if it is given.
The "n" pair is only used when there is no "a" pair. If only the "n"
pair is given, the CoAP endpoint performs DNS resolution to obtain
the IP multicast address from the hostname in the "n" pair. If DNS
resolution is not successful, then the endpoint does not attempt
joining or listening to any multicast group for this case since the
IP multicast address is unknown.
After any change on a Group configuration resource, the endpoint MUST
effect registration/de-registration from the corresponding IP
multicast group(s) by making use of APIs such as IPV6_RECVPKTINFO
[RFC3542]. When a POST payload contains in "a" an IP multicast
address to which the endpoint is already subscribed, no change to
that subscription is needed.
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2.6.2.3. Deleting a single 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
2.6.2.4. Reading all group memberships at once (GET)
A (unicast) GET on the CoAP-group resource returns a JSON object
containing multiple keys and values. The keys (member names) are
group indices and the values (member values) are the corresponding
group membership objects. Each group membership object describes one
IP multicast group membership. If no group memberships are
configured then an empty JSON object is returned.
Method: GET
URI Template: /{+gp}
URI Template Variables:
gp - see Section 2.6.2.2
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" }
}
Note: the returned IPv6 address string will represent the same IPv6
address that was originally submitted in group membership creation,
though it might be a different string because of different choices in
IPv6 string representation formatting that may be allowed for the
same address (see [RFC5952]).
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2.6.2.5. Reading a single group membership (GET)
Similar to Section 2.6.2.4 but only a single group membership is
read. If the requested group index does not exist then a 4.04 Not
Found response is returned.
Method: GET
URI Template 1: {+location}
URI Template 2: /{+gp}/{index}
URI Template Variables:
location - see Section 2.6.2.3
gp, index - see Section 2.6.2.2
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"}
2.6.2.6. 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 MUST only be used to
delete or update group membership objects for which the CoAP client,
invoking this operation, is responsible. The responsibility is based
on application level knowledge. For example, a commissioning tool
will be responsible for any group membership objects that it created.
Method: PUT
URI Template: /{+gp}
URI Template Variables:
gp - see Section 2.6.2.2
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Example: (replacing all existing group memberships with two new group memberships)
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
After a successful PUT on the Group configuration resource, the
endpoint MUST effect registration to any new IP multicast group(s)
and de-registration from any previous IP multicast group(s), i.e.,
not any more present in the new memberships. An API such as
IPV6_RECVPKTINFO [RFC3542] should be used for this purpose. Also it
MUST take into account the group indices present in the new resource
during the generation of any new unique group indices in the future.
2.6.2.7. Updating a single group membership (PUT)
A (unicast) PUT with a group membership JSON object will replace an
existing group membership in the endpoint with the new one defined in
the PUT request. This can be used to update the group membership.
Method: PUT
URI Template 1: {+location}
URI Template 2: /{+gp}/{index}
URI Template Variables:
location - see Section 2.6.2.3
gp, index - see Section 2.6.2.2
Example: (group name and IP 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
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After a successful PUT on the Group configuration resource, the
endpoint MUST effect registration to any new IP multicast group(s)
and de-registration from any previous IP multicast group(s), i.e.,
not any more present in the new membership. An API such as
IPV6_RECVPKTINFO [RFC3542] should be used for this purpose.
2.7. Request Acceptance and Response Suppression Rules
CoRE Link Format [RFC6690], and Section 8 of CoAP [RFC7252] define
behaviors for:
1. IP multicast request acceptance - in which cases a CoAP request
is accepted and executed, and when not.
2. IP 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 an IP multicast CoAP request. This section
first summarizes these behaviors and then presents additional
guidelines for response suppression. Also a number of IP 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 IP
multicast by making use of APIs such as IPV6_RECVPKTINFO [RFC3542].
For IP multicast request acceptance, the behaviors are:
o A server should not accept an IP multicast request that cannot be
"authenticated" in some way (i.e, cryptographically or by some
multicast boundary limiting the potential sources) (Section 11.3
of [RFC7252]. See Section 5.3 for examples of multicast boundary
limiting methods.
o A server should not accept an IP 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 an IP multicast request that acts on a
specific resource for which IP multicast support is not required.
(Note that for the resource "/.well-known/core", IP multicast
support is required if "multicast resource discovery" is supported
as specified in Section 1.2.1 of [RFC6690]). Implementers are
advised to disable IP 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 IP multicast request.
For IP multicast response suppression, the behaviors are:
o A server should not respond to an IP 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 an IP multicast request, if
there's nothing useful to respond (e.g., error or empty response).
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 IP 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 CoAP 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 group
communication 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 group communication" (as
defined in Section 1.3) function using additional, non-
standardized functions above the CoAP layer.
o IP multicast firmware update by sending blocks of data: Suppress
all 2.xx and 5.xx responses. After having sent all IP 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 group
URI query: Suppress all 2.05 responses with empty payload (i.e.,
if a query produces no matching results).
2.8. Congestion Control
CoAP group communication requests may result in a multitude of
responses from different nodes, potentially causing congestion.
Therefore both the sending of IP multicast requests, and the sending
of the unicast CoAP responses to these multicast requests should be
conservatively controlled.
CoAP [RFC7252] reduces IP multicast-specific congestion risks through
the following measures:
o A server may choose not to respond to an IP multicast request if
there's nothing useful to respond (e.g., error or empty
response)(Section 8.2 [RFC7252]). See Section 2.7 for more
detailed guidelines on response suppression.
o A server should limit the support for IP multicast requests to
specific resources where multicast operation is required
(Section 11.3 of [RFC7252]).
o An IP multicast request must be Non-confirmable (Section 8.1 of
[RFC7252]).
o A response to an IP multicast request should be Non-confirmable
(Section 5.2.3 of [RFC7252]).
o A server does not respond immediately to an IP multicast request,
and should first wait for a time that is randomly picked within a
predetermined time interval called the Leisure (Section 8.2
[RFC7252]).
Additional guidelines to reduce congestion risks defined in this
document are:
o A server in an LLN should only support group communication GET for
resources that are small. For example, the payload of the
response is limited to approximately 5% of the IP Maximum Transmit
Unit (MTU) size so it fits into a single link-layer frame in case
6LoWPAN (see Section 4 of [RFC4944]) is used.
o A server can minimize the payload length in response to a group
communication 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].
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o A server can also minimize the payload length of a response to a
group communication GET (e.g., on "/.well-known/core") using CoAP
blockwise transfers [I-D.ietf-core-block], returning only a first
block of the CoRE Link Format description. For this reason, a
CoAP client sending an IP multicast CoAP request to "/.well-known/
core" should support core-block.
o A client should use CoAP group communication with the smallest
possible IP 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 [RFC4919]
are given in Section 4.5.
2.9. Proxy Operation
CoAP (Section 5.7.2 of [RFC7252]) allows a client to request a
forward-proxy to process its CoAP request. For this purpose the
client either specifies the request group URI as a string in the
Proxy-URI option, or it specifies the Proxy-Scheme option with the
group 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 CoAP group
communication request made in this manner through a proxy.
A proxy may buffer all the individual (unicast) responses to a CoAP
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 CoAP 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 (Section 5.7.2 of [RFC7252]), the proxy would simply
forward all the individual (unicast) responses to a CoAP group
communication request to the client (i.e., no aggregation). There
are also issues with this approach:
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o The client may be confused as it may not have known that the
Proxy-URI contained a group URI 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.
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 CoAP Proxy SHOULD NOT support processing an IP
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 if all the following conditions are met:
o The CoAP Proxy MUST be explicitly configured (whitelist) to allow
proxied IP 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.
2.10. Exceptions
CoAP group communication using IP multicast offers improved network
efficiency and latency among 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 group
communication request will be delivered to nodes on the local link
only, regardless of the scope value used in the IPv6 destination
address.
CoAP Observe [I-D.ietf-core-observe] is a feature for a client to
"observe" resources (i.e. to retrieve a representation of a resource
and keep this representation updated by the server over a period of
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time). CoAP Observe does not support a group communication mode.
CoAP Observe only supports a unicast mode of operation.
3. Use Cases and Corresponding Protocol Flows
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 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, for clarity, only three are shown.
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.
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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
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.
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################################################
# ********************** 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)
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.
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o Light-2 will then search for the local CoAP RD by sending out a
group communication 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 it into to the Network Backbone where it
will be received by the CoAP RD. All other nodes in subnet-2 will
ignore the group communication GET request 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, CoAP 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.
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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") |<---------|
| |<------------------------------------------| |
| | | | | | |
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 IP multicast CoAP requests related to lighting
control. (See Section 2.7 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 IP 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 IP 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.
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Reliability of IP 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 IP multicast forwarding/routing will be of sufficient quality to
provide on average a very high probability of packet delivery to all
CoAP endpoints in an IP multicast group. An example protocol to
accomplish this using randomized retransmission is the MPL forwarding
protocol for LLNs [I-D.ietf-roll-trickle-mcast].
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 IP multicast addresses, or address blocks, to
filter out or to pass through to/from the 6LoWPAN.
3a) 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.
3b) 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 an IP multicast CoAP request.
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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) * | | | |
*********************** | | | |
| | | | | | |
Figure 3: Light Switch Sends Multicast Control Message
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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 group communication request to turn on all lights in Room-A.
Every Light sends back a CoAP response to the Controller after being
turned on.
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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) |------->|
| | |--------------------->| | |
| | | | |--------->| |
| | | | | |------->|
| | | | | | |
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
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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 IP
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
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.
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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 unicast 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 IP 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 IP multicast address. For each light in the list the tool
learns the IP address of the light and instructs the RD with three
(unicast) 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 IP multicast address in the Light
endpoints with, for example, the (unicast) POST command discussed in
Section 2.6.2.2.
The light switch can discover the group in RD and thus learn the IP
multicast address of the group. The light switch will use this
address to send CoAP group communication requests to the members of
the group. When the message arrives the Lights should recognize the
IP 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
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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 IP 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 IP multicast routers
which IP 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 Destination
Advertisement Object (DAO) messages and routing of multicast IPv6
packets based on this. It requires the RPL Mode of Operation 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 IP 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
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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.
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 IP
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 IP 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.
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2. Manual configuration of edge router(s) as MPL Seed(s) for
specific IP multicast traffic. In the above example, this could
be done through the following three steps: First, configure Rtr-1
and Rtr-2 to act as MLD Address Listeners for the Room-A-Lights
IP 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 IP 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).
3. Use an additional protocol/mechanism for injection of IP
multicast traffic from outside an MPL Domain into that MPL
Domain, based on IP 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 IP
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 Sections 8.1 and 9.1 of [RFC7252], CoAP group
communication based on IP multicast:
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o Will operate in CoAP NoSec (No Security) mode, until a future
group security solution is developed (see also Section 5.3.3).
o Will use the "coap" scheme. The "coaps" scheme should only be
used when a future group security solution is developed (see also
Section 5.3.3).
Essentially the above configuration means that there is currently no
security at the CoAP layer for group communication. Therefore, for
sensitive and mission critical applications (e.g., health monitoring
systems, alarm monitoring systems) it is currently recommended to
deploy CoAP group communication with an application-layer security
mechanism (e.g, data object security) for improved security.
Application level security has many desirable properties including
maintaining security properties while forwarding traffic through
intermediaries (proxies). Application level security also tends to
more cleanly separate security from the dynamics of group membership
(e.g., the problem of distributing security keys across large groups
with many members that come and go).
Without application-layer security, CoAP group communication should
only be currently deployed in non-critical applications (e.g., read-
only temperature sensors). Only when security solutions at the CoAP
layer are mature enough (see Section 5.3.3) should CoAP group
communication without application-layer security be considered for
sensitive and mission-critical applications.
5.2. Threats
As noted above, there is currently 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 Section 11 of
[RFC7252] for IP multicast.
5.3. Threat Mitigation
Section 11 of [RFC7252] identifies various threat mitigation
techniques for CoAP group communication. 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.
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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 IP 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, security enhancements
need to be developed at IETF for group communications. This will
allow introduction of a secure mode of CoAP group communication, and
use of the "coaps" scheme for that purpose.
At the time of writing of this specification, there are various
approaches being considered for security enhancements for group
communications. Specifically, a lot of the current effort at IETF is
geared towards developing a DTLS-based group communication. This is
primarily motivated by the fact that the unicast CoAP security is
DTLS-based (Section 9.1 of [RFC7252]. For example,
[I-D.keoh-dice-multicast-security] proposes a DTLS-based IP multicast
security. However, it is too early to conclude if this is the best
approach. Alternatively,
[I-D.mglt-dice-ipsec-for-application-payload] proposes an IPSec-based
IP multicast security. This approach also needs further
investigation and validation.
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5.4. Monitoring Considerations
5.4.1. General Monitoring
CoAP group communication is meant to be used to control a set of
related devices (e.g., simultaneously turn on all the lights in a
room). This intrinsically exposes the group to some unique
monitoring risks that solitary devices (i.e., devices not in a group)
are not as vulnerable to. For example, assume an attacker is able to
physically see a set of lights turn on in a room. Then the attacker
can correlate a CoAP group communication message to that easily
observable coordinated group action even if the contents of the
message are encrypted by a future security solution (see
Section 5.3.3). This will give the attacker side channel information
to plan further attacks (e.g. by determining the members of the group
then some network topology information may be deduced).
One mitigation to group communication monitoring risks that should be
explored in the future is methods to de-correlate coordinated group
actions. For example, if a CoAP group communication GET is sent to
all the alarm sensors in a house, then their (unicast) responses
should be as de-correlated as possible. This will introduce greater
entropy into the system and will make it harder for an attacker to
monitor and gather side channel information.
5.4.2. Pervasive Monitoring
A key additional threat consideration for group communication is
pointed to by [RFC7258] which warns of the dangers of pervasive
monitoring. CoAP group communication solutions which are built on
top of IP multicast need to pay particular heed to these dangers.
This is because IP multicast is easier to intercept (e.g., and to
secretly record) compared to unicast traffic. Also, CoAP traffic is
meant for the Internet of Things. This means that CoAP traffic (once
future security solutions are developed as in Section 5.3.3) may be
used for the control and monitoring of critical infrastructure (e.g.,
lights, alarms, etc.) which may be prime targets for attack.
For example, an attacker may attempt to record all the CoAP traffic
going over the smart grid (i.e., networked electrical utility) of a
country and try to determine critical nodes for further attacks. For
example, the source node (controller) sending out the CoAP group
communication messages. CoAP multicast traffic is inherently more
vulnerable (compared to a unicast packet) as the same packet may be
replicated over many links so there is a much higher probability of
it getting captured by a pervasive monitoring system.
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One useful mitigation to pervasive monitoring is to restrict the
scope of the IP multicast to the minimal scope that fulfills the
application need. Thus, for example, site-local IP multicast scope
is always preferred over global scope IP multicast if this fulfills
the application needs. This approach has the added advantage that it
coincides with the guidelines for minimizing congestion control (see
Section 2.8.
In the future, even if all the CoAP multicast traffic is encrypted,
an attacker may still attempt to capture the traffic and perform an
off-line attack. Though of course having the multicast traffic
protected is always desirable as it significantly raises the cost to
an attacker (e.g., to break the encryption) versus unprotected
multicast traffic.
6. IANA Considerations
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 2.6.2.
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
[RFC6838], and (last paragraph) of Section 12.3 of [RFC7252].
Type name: application
Subtype name: coap-group+json
Required parameters: None
Optional parameters: None
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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
(re-)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 a form of authorization (making use of unicast DTLS-
secured CoAP) be used such that only authorized controllers are
allowed by an endpoint to configure its group membership.
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 2.6.2.
Fragment identifier considerations: N/A
Additional Information:
Deprecated alias names for this type: None
Magic number(s): None
File extension(s): *.json
Macintosh file type code(s): TEXT
Person and email address to contact for further information: Esko
Dijk ("Esko.Dijk@Philips.com")
Intended usage: COMMON
Restrictions on usage: None
Author: CoRE WG
Change controller: IETF
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Provisional registration? (standards tree only): N/A
7. Acknowledgements
Thanks to Jari Arkko, Peter Bigot, Anders Brandt, Ben Campbell,
Angelo Castellani, Alissa Cooper, Spencer Dawkins, Badis Djamaa,
Adrian Farrel, Stephen Farrell, Thomas Fossati, Brian Haberman,
Bjoern Hoehrmann, Matthias Kovatsch, Guang Lu, Salvatore Loreto,
Kerry Lynn, Andrew McGregor, Kathleen Moriarty, Pete Resnick, Dale
Seed, Martin Stiemerling, Zach Shelby, Peter van der Stok, Gengyu
Wei, and Juan Carlos Zuniga for their helpful comments and
discussions that have helped shape this document.
Special thanks to Carsten Bormann and Barry Leiba for their extensive
and thoughtful Chair and AD reviews of the document. Their reviews
helped to immeasurably improve the document quality.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[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.
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[RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas,
"Protocol Independent Multicast - Sparse Mode (PIM-SM):
Protocol Specification (Revised)", RFC 4601, August 2006.
[RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
over Low-Power Wireless Personal Area Networks (6LoWPANs):
Overview, Assumptions, Problem Statement, and Goals", RFC
4919, August 2007.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, September 2007.
[RFC5110] Savola, P., "Overview of the Internet Multicast Routing
Architecture", RFC 5110, January 2008.
[RFC5771] Cotton, M., Vegoda, L., and D. Meyer, "IANA Guidelines for
IPv4 Multicast Address Assignments", BCP 51, RFC 5771,
March 2010.
[RFC5952] Kawamura, S. and M. Kawashima, "A Recommendation for IPv6
Address Text Representation", RFC 5952, August 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.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, August 2012.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, February 2013.
[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.
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[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13, RFC
6838, January 2013.
[RFC7159] Bray, T., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, March 2014.
[RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Message Syntax and Routing", RFC 7230, June
2014.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252, June 2014.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, May 2014.
[RFC7320] Nottingham, M., "URI Design and Ownership", BCP 190, RFC
7320, July 2014.
8.2. Informative References
[RFC1033] Lottor, M., "Domain administrators operations guide", RFC
1033, November 1987.
[RFC4605] Fenner, B., He, H., Haberman, B., and H. Sandick,
"Internet Group Management Protocol (IGMP) / Multicast
Listener Discovery (MLD)-Based Multicast Forwarding
("IGMP/MLD Proxying")", RFC 4605, August 2006.
[RFC5740] Adamson, B., Bormann, C., Handley, M., and J. Macker,
"NACK-Oriented Reliable Multicast (NORM) Transport
Protocol", RFC 5740, November 2009.
[I-D.ietf-core-block]
Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
draft-ietf-core-block-15 (work in progress), July 2014.
[I-D.ietf-core-resource-directory]
Shelby, Z., Bormann, C., and S. Krco, "CoRE Resource
Directory", draft-ietf-core-resource-directory-01 (work in
progress), December 2013.
[I-D.ietf-core-observe]
Hartke, K., "Observing Resources in CoAP", draft-ietf-
core-observe-14 (work in progress), June 2014.
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[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-09 (work in progress), April 2014.
[I-D.keoh-dice-multicast-security]
Keoh, S., Kumar, S., Garcia-Morchon, O., Dijk, E., and A.
Rahman, "DTLS-based Multicast Security in Constrained
Environments", draft-keoh-dice-multicast-security-08 (work
in progress), July 2014.
[I-D.droms-6man-multicast-scopes]
Droms, R., "IPv6 Multicast Address Scopes", draft-droms-
6man-multicast-scopes-02 (work in progress), July 2013.
[I-D.mglt-dice-ipsec-for-application-payload]
Migault, D. and C. Bormann, "IPsec/ESP for Application
Payload", draft-mglt-dice-ipsec-for-application-payload-00
(work in progress), July 2014.
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 IP 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 IP
multicast listeners on directly attached links, and to discover which
IP multicast addresses are of interest to those listening nodes. MLD
was specifically designed to cope with fairly dynamic situations in
which IP 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-24 to ietf-25:
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o Updated with the remaining agreed minor comments from Ben
Campbell's GEN-ART review. Specifically, addressed the two
comments on section 2.6.2.1 (which was section 2.7.2.1 in rev-21)
as called out in "http://www.ietf.org/mail-
archive/web/core/current/msg05604.html".
o Updated with the clarification comment from Badis Djamaa in
Section 2.3 as called out in "http://www.ietf.org/mail-
archive/web/core/current/msg05612.html".
o Minor editorial updates.
Changes from ietf-23 to ietf-24:
o Clarified in section 2.6.1.2 (Configuring Members) that ABNF rules
from Section 3.2.2 of [RFC 3986] should be used for the IP address
parsing.
o Minor editorial updates.
Changes from ietf-22 to ietf-23:
o Updated requirements language (RFC 2119) to follow Barry Leiba's
suggestions #1, #2b, and #2.1 as per "http://www.ietf.org/mail-
archive/web/core/current/msg05593.html".
o Clarified that [RFC 7320] implies that also other specifications
cannot pre-define URI structure.
o Added MUST to Token re-use time as it is additional specification
to CoAP [RFC 7252].
o Clarified use of multicast POSTing in Section 2.4, in response to
Jari Arkko's COMMENTs in "http://www.ietf.org/mail-
archive/web/core/current/msg05572.html".
o Added to Section 5.1 (Security Configuration) the possibility to
use application-layer (data object) security, which enables to use
CoAP group communication also for critical applications, pointed
out by Jari Arkko's COMMENTs in "http://www.ietf.org/mail-
archive/web/core/current/msg05572.html".
o Fixed subtle error in hex string "c00l" to "c001".
o Clarified in Section 2.11 (Exceptions) that CoAP Observe feature
does not support group communication as per Jari's Arkko's comment
in "http://www.ietf.org/mail-archive/web/core/current/
msg05592.html".
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o Moved section 2.6 (Member Discovery) into a new subsection as part
of 2.7.1 (Membership Configuration - Background).
o Minor editorial updates.
Changes from ietf-21 to ietf-22:
o Updated with comments from IESG review as follows:
1. Changed Status from Informational to Experimental.
2. Addressed Brian Haberman's DISCUSS (to put in reference to
ASM) in section 1.2 as called out in "http://www.ietf.org/
mail-archive/web/core/current/msg05547.html".
3. Addressed Brian Haberman's DISCUSS (to put in reference to
multicast forwarding proxies) in section 2.1 as called out in
"http://www.ietf.org/mail-archive/web/core/current/
msg05547.html".
4. Addressed Brian Haberman's DISCUSS (to put in reference to
getting port numbers from URIs) in section 2.3 as called out
in "http://www.ietf.org/mail-archive/web/core/current/
msg05563.html".
5. Addressed Brian Haberman's DISCUSS (to put in reference to
IGMP/MLD API) in section 2.7.2.1, 2.7.2.2, 2.7.2.6 and
2.7.2.7 as called out in "http://www.ietf.org/mail-
archive/web/core/current/msg05547.html".
6. Addressed Brian Haberman's COMMENT (to put in reference to
reliable multicast RFC) in section 1.3 as called out in
"http://www.ietf.org/mail-archive/web/core/current/
msg05545.html".
7. Addressed Kathleen Moriarty's DISCUSS (to broaden to cover
general monitoring) in section 5.4 as called out in
"http://www.ietf.org/mail-archive/web/core/current/
msg05566.html".
8. Addressed Martin Stiemerling's DISCUSS (to clearly indicate
that the draft introduces new CoAP protocol functionality) in
the Abstract and section 1.2 as called out in
"http://www.ietf.org/mail-archive/web/core/current/
msg05542.html".
9. Addressed Martin Stiemerling's DISCUSS (to clarify selected
requirements language) in section 2.7.2 as called out in
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"http://www.ietf.org/mail-archive/web/core/current/
msg05542.html". (Note that the other sections are not
impacted as they truly are new requirements and not
repetition of the CoAP RFC 7252)
10. Addressed Spencer Dawkins' COMMENT as called out in
"http://www.ietf.org/mail-archive/web/core/current/
msg05557.html".
11. Addressed Alissa Cooper's COMMENT as called out in
"http://www.ietf.org/mail-archive/web/core/current/
msg05567.html".
12. Addressed selected Stephen Farrell's COMMENTs as called out
in "http://www.ietf.org/mail-archive/web/core/current/
msg05576.html".
13. Addressed selected Pete Resnick's COMMENTs as called out in
"http://www.ietf.org/mail-archive/web/core/current/
msg05568.html".
14. Addressed selected Adrian Farrel's COMMENTs as called out in
"http://www.ietf.org/mail-archive/web/core/current/
msg05565.html".
15. Addressed selected Jari Arkko's COMMENTs as called out in
"http://www.ietf.org/mail-archive/web/core/current/
msg05572.html".
o Updated with comments from GEN-ART review as follows:
1. Addressed major issue #1 from Ben Campbell's GEN-ART review
(about introducing new functionality beyond CoAP RFC 7252) by
changing the status of document to Experimental, and updating
Abstract and section 2.1 as called out in
"http://www.ietf.org/mail-archive/web/core/current/
msg05551.html".
2. Addressed major issue #2 from Ben Campbell's GEN-ART review
(about giving a stronger recommendation not to use CoAP group
communication in many scenarios until stronger security
solutions are available) in section 5.1 and section 5.4 as
called out in "http://www.ietf.org/mail-
archive/web/core/current/msg05551.html".
3. Addressed selected minor issues and nits from Ben Campbell's
GEN-ART review comments from "http://www.ietf.org/mail-
archive/web/core/current/msg05535.html".
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o Various minor editorial updates.
Changes from ietf-20 to ietf-21:
o Updated with comments from AD review by Barry Leiba. The details
of the updates can be seen by looking at the WG mailing list from
July 26-31, 2014.
o Various minor editorial updates.
Changes from ietf-19 to ietf-20:
o Replaced obsolete reference [RFC 2616] by [RFC 7230].
o Replaced outdated reference draft-ietf-appsawg-uri-get-off-my-lawn
by [RFC 7320] and moved to Normative reference.
o Replaced outdated reference draft-ietf-core-coap by [RFC 7252].
o Moved [RFC 1033] to Informative reference.
o Updated to latest revision numbers for informative draft
references by regenerating file through xml2rfc tool.
o Re-ran IETF spell check tool and corrected some minor spelling
errors.
o Various minor editorial updates.
Changes from ietf-18 to ietf-19:
o Added guideline on Token value re-use in section 2.5.
o Updated section 5.1 (Security Configuration) and 5.3.3 (Future
Security Evolution) to point to latest security developments
happening in DICE WG for support of group security.
o Added Pervasive Monitoring considerations in section 5.4.
o Various editorial updates for improved readability.
Changes from ietf-17 to ietf-18:
o Extensive editorial updates based on WGLC comments by Thomas
Fossati and Gengyu Wei.
o Addressed ticket #361: Added text for single membership PUT
section 2.7.2.7 (Updating a single group membership (PUT)).
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o Addressed ticket #360: Added text for server duties upon all-at-
once PUT section 2.7.2.6 (Creating/updating all group memberships
at once (PUT)).
o Addressed ticket #359: Fixed requirements text for Section 2.7.2.2
(Creating a new multicast group membership (POST)).
o Addressed ticket #358: Fixed requirements text for Section 2.7.2.1
(CoAP-Group Resource Type and Media Type).
o Addressed ticket #357: Added that "IPv6 addresses of other scopes
MAY be enabled" in section 2.2 (Group Definition and Naming).
o Various editorial updates for improved readability.
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.
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:
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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:
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:
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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".
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.
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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 [RFC 3542]".
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:
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'.
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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 draft-ietf-core-coap in
section 2.2 (Scope).
o Removed IPSec related requirement, as IPSec is not part of draft-
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) .
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:
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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).
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.
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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.
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).
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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
draft-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
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
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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
Authors' Addresses
Akbar Rahman (editor)
InterDigital Communications, LLC
Email: Akbar.Rahman@InterDigital.com
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Esko Dijk (editor)
Philips Research
Email: esko.dijk@philips.com
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