CoRE Working Group E. Dijk
Internet-Draft IoTconsultancy.nl
Obsoletes: 7390 (if approved) C. Wang
Updates: 7252, 7641 (if approved) InterDigital
Intended status: Standards Track M. Tiloca
Expires: May 6, 2021 RISE AB
November 02, 2020
Group Communication for the Constrained Application Protocol (CoAP)
draft-ietf-core-groupcomm-bis-02
Abstract
This document specifies the use of the Constrained Application
Protocol (CoAP) for group communication, using UDP/IP multicast as
the underlying data transport. Both unsecured and secured CoAP group
communication are specified. Security is achieved by use of the
Group Object Security for Constrained RESTful Environments (Group
OSCORE) protocol. The target application area of this specification
is any group communication use cases that involve resource-
constrained networks. The most common of such use cases are also
discussed. This document replaces [RFC7390] and updates [RFC7252]
and [RFC7641].
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
material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 6, 2021.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. General Group Communication Operation . . . . . . . . . . . . 5
2.1. Group Definition . . . . . . . . . . . . . . . . . . . . 5
2.1.1. CoAP Group . . . . . . . . . . . . . . . . . . . . . 5
2.1.2. Application Group . . . . . . . . . . . . . . . . . . 6
2.1.3. Security Group . . . . . . . . . . . . . . . . . . . 6
2.1.4. Relations Between Group Types . . . . . . . . . . . . 6
2.2. Group Configuration . . . . . . . . . . . . . . . . . . . 9
2.2.1. Group Naming . . . . . . . . . . . . . . . . . . . . 9
2.2.2. Group Creation and Membership . . . . . . . . . . . . 10
2.2.3. Group Discovery . . . . . . . . . . . . . . . . . . . 11
2.2.4. Group Maintenance . . . . . . . . . . . . . . . . . . 12
2.3. CoAP Usage . . . . . . . . . . . . . . . . . . . . . . . 12
2.3.1. Request/Response Model . . . . . . . . . . . . . . . 13
2.3.2. Port and URI Path Selection . . . . . . . . . . . . . 16
2.3.3. Proxy Operation . . . . . . . . . . . . . . . . . . . 17
2.3.4. Congestion Control . . . . . . . . . . . . . . . . . 18
2.3.5. Observing Resources . . . . . . . . . . . . . . . . . 19
2.3.6. Block-Wise Transfer . . . . . . . . . . . . . . . . . 22
2.4. Transport . . . . . . . . . . . . . . . . . . . . . . . . 22
2.4.1. UDP/IPv6 Multicast Transport . . . . . . . . . . . . 22
2.4.2. UDP/IPv4 Multicast Transport . . . . . . . . . . . . 23
2.4.3. 6LoWPAN . . . . . . . . . . . . . . . . . . . . . . . 23
2.5. Interworking with Other Protocols . . . . . . . . . . . . 23
2.5.1. MLD/MLDv2/IGMP/IGMPv3 . . . . . . . . . . . . . . . . 23
2.5.2. RPL . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.5.3. MPL . . . . . . . . . . . . . . . . . . . . . . . . . 25
3. Unsecured Group Communication . . . . . . . . . . . . . . . . 25
4. Secured Group Communication using Group OSCORE . . . . . . . 26
4.1. Secure Group Maintenance . . . . . . . . . . . . . . . . 27
5. Security Considerations . . . . . . . . . . . . . . . . . . . 28
5.1. CoAP NoSec Mode . . . . . . . . . . . . . . . . . . . . . 28
5.2. Group OSCORE . . . . . . . . . . . . . . . . . . . . . . 29
5.2.1. Group Key Management . . . . . . . . . . . . . . . . 29
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5.2.2. Source Authentication . . . . . . . . . . . . . . . . 30
5.2.3. Countering Attacks . . . . . . . . . . . . . . . . . 30
5.3. Replay of Non Confirmable Messages . . . . . . . . . . . 32
5.4. Use of CoAP No-Response Option . . . . . . . . . . . . . 32
5.5. 6LoWPAN . . . . . . . . . . . . . . . . . . . . . . . . . 33
5.6. Wi-Fi . . . . . . . . . . . . . . . . . . . . . . . . . . 33
5.7. Monitoring . . . . . . . . . . . . . . . . . . . . . . . 34
5.7.1. General Monitoring . . . . . . . . . . . . . . . . . 34
5.7.2. Pervasive Monitoring . . . . . . . . . . . . . . . . 34
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 35
7.1. Normative References . . . . . . . . . . . . . . . . . . 35
7.2. Informative References . . . . . . . . . . . . . . . . . 37
Appendix A. Use Cases . . . . . . . . . . . . . . . . . . . . . 39
A.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 39
A.1.1. Distributed Device Discovery . . . . . . . . . . . . 40
A.1.2. Distributed Service Discovery . . . . . . . . . . . . 40
A.1.3. Directory Discovery . . . . . . . . . . . . . . . . . 40
A.2. Operational Phase . . . . . . . . . . . . . . . . . . . . 41
A.2.1. Actuator Group Control . . . . . . . . . . . . . . . 41
A.2.2. Device Group Status Request . . . . . . . . . . . . . 41
A.2.3. Network-wide Query . . . . . . . . . . . . . . . . . 41
A.2.4. Network-wide / Group Notification . . . . . . . . . . 42
A.3. Software Update . . . . . . . . . . . . . . . . . . . . . 42
Appendix B. Document Updates . . . . . . . . . . . . . . . . . . 42
B.1. Version -01 to -02 . . . . . . . . . . . . . . . . . . . 42
B.2. Version -00 to -01 . . . . . . . . . . . . . . . . . . . 43
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 43
1. Introduction
This document specifies group communication using the Constrained
Application Protocol (CoAP) [RFC7252] together with UDP/IP multicast.
CoAP is a RESTful communication protocol that is used in resource-
constrained nodes, and in resource-constrained networks where packet
sizes should be small. This area of use is summarized as Constrained
RESTful Environments (CoRE).
One-to-many group communication can be achieved in CoAP, by a client
using UDP/IP multicast data transport to send multicast CoAP request
messages. In response, each server in the addressed group sends a
response message back to the client over UDP/IP unicast. Notable
CoAP implementations supporting group communication include the
framework "Eclipse Californium" 2.0.x [Californium] from the Eclipse
Foundation and the "Implementation of CoAP Server & Client in Go"
[Go-OCF] from the Open Connectivity Foundation (OCF).
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Both unsecured and secured CoAP group communication over UDP/IP
multicast are specified in this document. Security is achieved by
using Group Object Security for Constrained RESTful Environments
(Group OSCORE) [I-D.ietf-core-oscore-groupcomm], which in turn builds
on Object Security for Constrained Restful Environments (OSCORE)
[RFC8613]. This method provides end-to-end application-layer
security protection of CoAP messages, by using CBOR Object Signing
and Encryption (COSE) [I-D.ietf-cbor-7049bis][I-D.ietf-cose-rfc8152bi
s-struct][I-D.ietf-cose-rfc8152bis-algs].
All guidelines in [RFC7390] are updated by this document, which
replaces and obsoletes [RFC7390]. Furthermore, this document updates
[RFC7252], by specifying a group request/response model and by adding
security for CoAP group communication. Finally, this document also
updates [RFC7641], by adding the multicast usage of CoAP Observe for
both the GET and FETCH methods.
All sections in the body of this document are normative, while
appendices are informative. For additional background about use
cases for CoAP group communication in resource-constrained devices
and networks, see Appendix A.
1.1. Scope
For group communication, only solutions that use CoAP over UDP/IP
multicast are in the scope of this document. There are alternative
methods to achieve group communication using CoAP, for example
Publish-Subscribe [I-D.ietf-core-coap-pubsub] which uses a central
broker server that CoAP clients access via unicast communication.
These methods may be usable for the same or similar use cases as are
targeted in this document.
Furthermore, this document defines Group OSCORE
[I-D.ietf-core-oscore-groupcomm] as the default group communication
security solution for CoAP. Security solutions for group
communication and configuration other than Group OSCORE are not in
scope. General principles for secure group configuration are in
scope.
1.2. 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 BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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This specification requires readers to be familiar with CoAP
terminology [RFC7252]. Terminology related to group communication is
defined in Section 2.1.
Furthermore, "Security material" refers to any security keys,
counters or parameters stored in a device that are required to
participate in secure group communication with other devices.
2. General Group Communication Operation
The general operation of group communication, both unsecured and
secured, is specified in this section. First, different group types
are defined in Section 2.1. Group configuration, including group
creation and maintenance by an application, user or commissioning
entity is considered next in Section 2.2. Then the use of CoAP for
group communication including support for protocol extensions (block-
wise transfer, Observe) follows in Section 2.3. How CoAP group
messages are carried over various transport layers is the subject of
Section 2.4. Finally, Section 2.5 covers the interworking of CoAP
group communication with other protocols that may operate in the same
network.
2.1. Group Definition
Three types of groups and their mutual relations are defined in this
section: CoAP group, application group, and security group.
2.1.1. CoAP Group
A CoAP group is defined as a set of CoAP endpoints, where each
endpoint is configured to receive CoAP multicast messages that are
sent to the group's associated IP multicast address and UDP port. An
endpoint may be a member of multiple CoAP groups by subscribing to
multiple IP multicast groups and/or listening on multiple UDP ports.
Group membership(s) of an endpoint may dynamically change over time.
A device sending a CoAP multicast message to a CoAP group is not
necessarily itself a member of this CoAP group: it is a member only
if it also has a CoAP endpoint listening on the group's associated IP
multicast address and UDP port. A CoAP group can be encoded within a
Group URI. This is defined as a CoAP URI that has the "coap" scheme
and includes in the authority part either an IP multicast address or
a group hostname (e.g., a Group Fully Qualified Domain Name (FQDN))
that can be resolved to an IP multicast address. A Group URI also
contains an optional UDP port number in the authority part. Group
URIs follow the regular CoAP URI syntax (see Section 6 of [RFC7252]).
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2.1.2. Application Group
Besides CoAP groups, that have relevance at the level of IP networks
and CoAP endpoints, there are also application groups. An
application group is a set of CoAP server endpoints that share a
common set of CoAP resources. An endpoint may be a member of
multiple application groups. An application group has relevance at
the application level - for example an application group could denote
all lights in an office room or all sensors in a hallway. A client
endpoint that sends a group communication message to an application
group is not necessarily itself a member of this application group.
There can be a one-to-one or a one-to-many relation between a CoAP
group and application group(s). An application group identifier is
optionally encoded explicitly in the CoAP request. If not explicitly
encoded, the application group is implicitly derived by the receiver,
based on information in the CoAP request. See Section 2.2.1 for more
details on identifying the application group.
2.1.3. Security Group
For secure group communication, a security group is required. A
security group is a group of endpoints that each store group security
material, such that they can mutually exchange secured messages and
verify secured messages. So, a client endpoint needs to be a member
of a security group in order to send a valid secured group
communication message to this group. An endpoint may be a member of
multiple security groups. There can be a one-to-one or a one-to-many
relation between security groups and CoAP groups. Also, there can be
a one-to-one or a one-to-many relation between security groups and
application groups. A special security group named "NoSec"
identifies group communication without any security at the transport
layer and/or application layer.
2.1.4. Relations Between Group Types
Using the above group type definitions, a CoAP group communication
message sent by an endpoint can be represented as a tuple that
contains one instance of each group type:
(application group, CoAP group, security group)
A special note is appropriate about the possible relation between
security groups and application groups.
On one hand, multiple application groups may use the same security
group. Thus, the same group security material is used to protect the
messages targeting any of those application groups. In this case, a
CoAP endpoint is supposed to know the exact application group to
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refer to for each message, based on, e.g., the used server port
number, the targeted resource, or the content and structure of the
message payload.
On the other hand, a single application group may use multiple
security groups. Thus, different messages targeting the resources of
the application group can be protected with different security
material. This can be convenient, for example, if the security
groups differ with respect to the crytpographic algorithms and
related parameters they use. In this case, a CoAP client can join
just one of the security groups, based on what it supports and
prefers, while a CoAP server in the application group would rather
have to join all of them.
Beyond this particular case, applications should be greatly careful
in associating a same application group to multiple security groups.
In particular, it is NOT RECOMMENDED using different security groups
to reflect different access policies for resources in a same
application group. That is, being a member of a security group
actually grants access only to exchanged secure messages, while
access to resources in the application group belongs to a separate
security domain, and has to be separately enforced by leveraging the
resource properties or through dedicated access control credentials
assessed by separate means.
Figure 1 summarizes the relations between the different types of
groups described above in UML class diagram notation. The items in
square brackets are optionally defined.
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+------------------------+ +------------------+
| Application group | | CoAP group |
|........................| |..................|
| | | |
| [ group name/ +-----------------+ IP mcast address |
| identifier ] | 1...N 1 | UDP port |
| | | |
| | | |
+-------------+----------+ +---------+--------+
| 1...N | 1...N
| |
| |
| | 1...N
| +----------+----------+
| | Security group |
| |.....................|
| | |
\---------------------------+ Security group name |
1...N | Security material |
| |
+---------------------+
Figure 1: Relation Among Different Group Types
Figure 2 provides a deployment example of the relations between the
different types of groups. It shows six CoAP servers (Srv1-Srv6) and
their respective resources hosted (/resX). There are three
application groups (1, 2, 3) and two security groups (1, 2).
Security Group 1 is used by both Application Group 1 and 2. Three
clients (Cli1, Cli2, Cli3) are configured with security material for
Security Group 1. One cient (Cli4) is configured with security
material for Security Group 2. All the shown application groups use
the same CoAP group (not shown in the figure), i.e. one specific
multicast IP address and UDP port on which all the shown resources
are hosted for each server.
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_________________________________ _________________________________
/ \ / \
| +---------------------+ | | +---------------------+ |
| | Application Group 1 | | | | Application Group 3 | Cli2 |
| | | | | | | |
| Cli1 | Srv1 Srv2 Srv3 | | | | Srv5 Srv6 | Cli4 |
| | /resA /resA /resA | | | | /resC /resC | |
| Cli2 +---------------------+ | | | /resD /resD | |
| | | +---------------------+ |
| Cli3 Security Group 1 | | |
| | | Security Group 2 |
| +---------------------+ | \_________________________________/
| | Application Group 2 | |
| | | |
| | Srv1 Srv4 | |
| | /resB /resB | |
| +---------------------+ |
\_________________________________/
Figure 2: Deployment Example of Different Group Types
2.2. Group Configuration
2.2.1. Group Naming
A CoAP group is identified and named by the authority component in
the Group URI, which includes host (possibly an IP multicast address
literal) and an optional UDP port number. It is recommended to
configure an endpoint with an IP multicast address literal, instead
of a hostname, when configuring a CoAP group membership. This is
because DNS infrastructure may not be deployed in many constrained
networks. In case a group hostname is configured, it can be uniquely
mapped to an IP multicast address via DNS resolution - if DNS client
functionality is available in the endpoint being configured and the
DNS service is supported in the network. Some examples of
hierarchical CoAP group FQDN naming (and scoping) for a building
control application are shown in Section 2.2 of [RFC7390].
An application group can be named in many ways through different
types of identifiers, such as numbers, URIs or other strings. An
application group name or identifier, if explicitly encoded in a CoAP
request, is typically included in the path component or in the query
component of a Group URI. It may also be encoded using the Uri-Host
Option [RFC7252] in case application group members implement a
virtual CoAP server specific to that application group. The
application group can then be identified by the value of the Uri-Host
Option and each virtual server serves one specific application group.
However, encoding the application group in the Uri-Host Option is not
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the preferred method because in this case the application group
cannot be encoded in a Group URI, and also the Uri-Host Option is
being used for another purpose than encoding the host part of a URI
as intended by [RFC7252] - which is potentially confusing.
Appendix A of [I-D.ietf-core-resource-directory] shows an example
registration of an application group into a Resource Directory, along
with the CoAP group it uses and the resources supported by the
application group. In this example an application group identifier
is not explicitly encoded in the RD nor in CoAP requests made to the
group, but it implicitly follows from the CoAP group used for the
request. So there is a one-to-one binding between the CoAP group and
the application group. The "NoSec" security group is used.
A best practice for encoding application group into a Group URI is to
use one URI path component to identify the application group and use
the following URI paths component(s) to identify the resource within
this application group. For example, /<groupname>/res1 or
/base/<groupname>/res1/res2 conform to this practice. An application
group identifier (like <groupname>) should be as short as possible
when used in constrained networks.
A security group is identified by a stable and invariant string used
as group name, which is generally not related with other kinds of
group identifiers, specific to the chosen security solution. The
"NoSec" security group name MUST be only used to represent the case
of group communication without any security. It is typically
characterized by the absence of any security group name, identifier,
or security-related data structures in the CoAP message.
2.2.2. Group Creation and Membership
To create a CoAP group, a configuring entity defines an IP multicast
address (or hostname) for the group and optionally a UDP port number
in case it differs from the default CoAP port 5683. Then, it
configures one or more devices as listeners to that IP multicast
address, with a CoAP endpoint listening on the group's associated UDP
port. These endpoints/devices are the group members. The
configuring entity can be, for example, a local application with pre-
configuration, a user, a software developer, a cloud service, or a
local commissioning tool. Also, the devices sending CoAP requests to
the group in the role of CoAP client need to be configured with the
same information, even though they are not necessarily group members.
One way to configure a client is to supply it with a CoAP Group URI.
The IETF does not define a mandatory, standardized protocol to
accomplish CoAP group creation. [RFC7390] defines an experimental
protocol for configuration of group membership for unsecured group
communication, based on JSON-formatted configuration resources. For
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IPv6 CoAP groups, common multicast address ranges that are used to
configure group addresses from are ff1x::/16 and ff3x::/16.
To create an application group, a configuring entity may configure a
resource (name) or set of resources on CoAP endpoints, such that a
CoAP request with Group URI sent by a configured CoAP client will be
processed by one or more CoAP servers that have the matching URI path
configured. These servers are the application group members.
To create a security group, a configuring entity defines an initial
subset of the related security material. This comprises a set of
group properties including the crytpographic algorithms and
parameters used in the group, as well as additional information
relevant throughout the group life-cycle, such as the security group
name and description. This task MAY be entrusted to a dedicated
administrator, that interacts with a Group Manager as defined in
Section 4. After that, further security material to protect group
communications have to be generated, as compatible with the specified
group configuration.
To participate in a security group, CoAP endpoints have to be
configured with the group security material used to protect
communications in the associated application/CoAP groups. The part
of the process that involves secure distribution of group security
material MAY use standardized communication with a Group Manager as
defined in Section 4. For unsecure group communication using the
"NoSec" security group, any CoAP endpoint may become a group member
at any time: there is no (central) configuring entity that needs to
provide the security material for this group. This means that group
creation and membership cannot be tightly controlled for the "NoSec"
group.
The configuration of groups and membership may be performed at
different moments in the life-cycle of a device; for example during
product (software) creation, in the factory, at a reseller, on-site
during first deployment, or on-site during a system reconfiguration
operation.
2.2.3. Group Discovery
It is possible for CoAP endpoints to discover application groups as
well as CoAP groups, by using the RD-Groups usage pattern of the CoRE
Resource Directory (RD), as defined in Appendix A of
[I-D.ietf-core-resource-directory]. In particular, an application
group can be registered to the RD, specifying the reference IP
multicast address, hence its associated CoAP group. The registration
is typically performed by a Commissioning Tool. Later on, CoAP
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endpoints can discover the registered application groups and related
CoAP group, by using the lookup interface of the RD.
CoAP endpoints can also discover application groups by performing a
multicast discovery query using the /.well-known/core resource. Such
a request may be sent to a known CoAP group multicast address
associated to application group(s), or to the All CoAP Nodes
multicast address.
When secure communication is provided with Group OSCORE (see
Section 4), the approach described in
[I-D.tiloca-core-oscore-discovery] and also based on the RD can be
used, in order to discover the security group to join.
In particular, the responsible OSCORE Group Manager registers its own
security groups to the RD, as links to its own corresponding
resources for joining the security groups
[I-D.ietf-ace-key-groupcomm-oscore]. Later on, CoAP endpoints can
discover the registered security groups and related application
groups, by using the lookup interface of the RD, and then join the
security group through the respective Group Manager.
2.2.4. Group Maintenance
Maintenance of a group includes any necessary operations to cope with
changes in a system, such as: adding group members, removing group
members, changing group security material, reconfiguration of UDP
port and/or IP multicast address, reconfiguration of the Group URI,
renaming of application groups, splitting of groups, or merging of
groups.
For unsecured group communication (see Section 3), addition/removal
of CoAP group members is simply done by configuring these devices to
start/stop listening to the group IP multicast address on the group's
UDP port.
For secured group communication (see Section 4), the protocol Group
OSCORE [I-D.ietf-core-oscore-groupcomm] is mandatory to implement.
When using Group OSCORE, CoAP endpoints participating in group
communication are also members of a corresponding OSCORE security
group, and thus share common security material. Additional related
maintenance operations are discussed in Section 4.1.
2.3. CoAP Usage
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2.3.1. Request/Response Model
A CoAP client is an endpoint able to transmit CoAP requests and
receive CoAP responses. Since the underlying UDP transport supports
multiplexing by means of UDP port number, there can be multiple
independent CoAP clients operational on a single host. On each UDP
port, an independent CoAP client can be hosted. Each independent
CoAP client sends requests that use the associated endpoint's UDP
port number as the UDP source port of the request.
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 deduplication by
both clients and servers, as detailed in Section 4.5 of [RFC7252].
A server sends back a unicast response to the CoAP group request -
but the server MAY suppress the response for various reasons
(Section 8.2 of [RFC7252]). This document adds the requirement that
a server SHOULD suppress the response in case of error or in case
there is nothing useful to respond, unless the application related to
a particular resource requires such a response to be made for that
resource. 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.
The CoAP No-Response Option [RFC7967] can be used by a client to
influence the default response suppression on the server side. It is
RECOMMENDED for a server to support this option only on selected
resources where it is useful in the application context. If the
Option is supported on a resource, it MUST override the default
response suppression of that resource.
Any default response suppression by a server SHOULD be performed
consistently, as follows: if a request on a resource produces a
particular Response Code and this response is not suppressed, then
another request on the same resource that produces a response of the
same Response Code class is also not suppressed. For example, if a
4.05 Method Not Allowed error response code is suppressed by default
on a resource, then a 4.15 Unsupported Content-Format error response
code is also suppressed by default for that resource.
A CoAP client MAY repeat a multicast request using the same Token
value and same Message ID value, in order to ensure that enough (or
all) group members have been reached with the request. This is
useful in case a number of group members did not respond to the
initial request and the client suspects that the request did not
reach these group members. However, in case one or more servers did
receive the initial request but the response to that request was
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lost, this repeat does not help to retrieve the lost response(s) if
the server(s) implement the optional Message ID based deduplication
(Section 4.5 of [RFC7252]).
A CoAP client MAY also repeat a multicast request using the same
Token value and a different Message ID, in which case all servers
that received the initial request will again process the repeated
request since it appears within a new CoAP message. This is useful
in case a client suspects that one or more response(s) to its
original request were lost and the client needs to collect more, or
even all, responses from group members, even if this comes at the
cost of the overhead of certain group members responding twice (once
to the original request, and once to the repeated request with
different Message ID).
The CoAP client can distinguish the origin of multiple server
responses by the source IP address of the UDP message containing the
CoAP response and/or any other available application-specific source
identifiers contained in the CoAP response, such as an application-
level unique ID associated to the server. If secure communication is
provided with Group OSCORE (see Section 4), additional security-
related identifiers enable the client to retrieve the right security
material for decrypting each response and authenticating its source.
While processing a response, the source endpoint of the response is
not matched to the destination endpoint of the request, since for a
multicast request these will never match. This is specified in
Section 8.2 of [RFC7252]. It implies also that a server MAY respond
from a UDP port number that differs from the destination UDP port
number of the request, yet a CoAP server normally SHOULD respond from
the UDP port number that equals the destination port of the request -
following the convention for UDP-based protocols. In case a single
client has sent multiple group requests and concurrent CoAP
transactions are ongoing, the responses received by that client are
matched to an active request using only the Token value. Due to UDP
level multiplexing, the UDP destination port of the response MUST
match to the client endpoint's UDP port value, i.e. to the UDP source
port of the client's request.
For multicast CoAP requests, there are additional constraints on the
reuse of Token values at the client, compared to the unicast case
defined in [RFC7252] and updated by [I-D.ietf-core-echo-request-tag].
Since for multicast CoAP the number of responses is not bound a
priori, the client cannot use the reception of a response as a
trigger to "free up" a Token value for reuse. Reusing a Token value
too early could lead to incorrect response/request matching on the
client, and would be a protocol error. Therefore, the time between
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reuse of Token values used in multicast requests MUST be greater
than:
MIN_TOKEN_REUSE_TIME = (NON_LIFETIME + MAX_LATENCY +
MAX_SERVER_RESPONSE_DELAY)
where NON_LIFETIME and MAX_LATENCY are defined in Section 4.8 of
[RFC7252]. This specification defines MAX_SERVER_RESPONSE_DELAY as
in [RFC7390], that is: 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]. However, CoAP does not define a time limit for the server
response delay. Using the default CoAP parameters, the Token reuse
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 new multicast request, such that a Token value
is never reused. If a client has to reuse 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 reuses is in that case set to a value greater
than 500 seconds.
When securing Group CoAP communications with Group OSCORE
[I-D.ietf-core-oscore-groupcomm], secure binding between requests and
responses is ensured (see Section 4). Thus, a client may reuse a
Token value after it has been freed up, as discussed above for the
multicast case and considering a reuse time greater than
MIN_TOKEN_REUSE_TIME. If an alternative security protocol for Group
CoAP is defined in the future and it does not ensure secure binding
between requests and responses, a client MUST follow the Token
processing requirements for the unicast case discussed above, as
defined in [I-D.ietf-core-echo-request-tag].
Another method to more easily meet the above constraint is to
instantiate multiple CoAP clients at multiple UDP ports on the same
host. The Token values only have to be unique within the context of
a single CoAP client, so using multiple clients can make it easier to
meet the constraint.
Since a client sending a multicast request with a Token T will accept
multiple responses with the same Token T, there is a risk that the
same server sends multiple responses with the same Token T back to
the client. For example, this server might not implement the
optional CoAP message deduplication based on Message ID, or it might
be a malicious/compromised server acting out of specification. To
mitigate issues with multiple responses from one server bound to a
same multicast request, the client has to ensure that, as long as the
the CoAP Token used for a multicast request is retained, at most one
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response to that request per server is accepted, with the exception
of Observe notifications [RFC7641] (see Section 2.3.5).
To this end, upon receiving a response corresponding to a multicast
request, the client MUST perform the following actions. First, the
client checks whether it previously received a valid response to this
request from the same originating server of the just-received
response. If the check yields a positive match and the response is
not an Observe notification (i.e., it does not include an Observe
option), the client SHALL stop processing the response. Upon
eventually freeing up the Token value of a multicast request for
possible reuse, the client MUST also delete the list of responding
servers associated to that request.
2.3.2. Port and URI Path Selection
A server that is a member of a CoAP group listens for CoAP messages
on the group's IP multicast address, usually on the CoAP default UDP
port 5683, or another non-default UDP port if configured. Regardless
of the method for selecting the port number, the same port number
MUST be used across all CoAP servers that are members of a CoAP group
and across all CoAP clients performing the requests to that group.
The URI Path used in the request is preferably a path that is known
to be supported across all group members. However there are valid
use cases where a request is known to be successful for a subset of
the CoAP group, for example only members of a specific application
group, while those group members for which the request is
unsuccessful (for example because they are outside the application
group) either ignore the multicast request or respond with an error
status code.
One way to create multiple CoAP groups is using different UDP ports
with the same IP multicast address, in case the devices' network
stack only supports a limited number of multicast address
subscriptions. However, it must be taken into account that this
incurs additional processing overhead on each CoAP server
participating in at least one of these groups: messages to groups
that are not of interest to the node are only discarded at the higher
transport (UDP) layer instead of directly at the network (IP) layer.
Port 5684 is reserved for DTLS-secured CoAP and MUST NOT be used for
any CoAP group communication.
For a CoAP server node that supports resource discovery as defined in
Section 2.4 of [RFC7252], the default port 5683 MUST be supported
(see Section 7.1 of [RFC7252]) for the "All CoAP Nodes" multicast
group as detailed in Section 2.4.
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2.3.3. Proxy Operation
CoAP enables a client to request a forward-proxy to process a CoAP
request on its behalf, as described in Section 5.7.2 and 8.2.2 of
[RFC7252]. For this purpose, the client specifies either the request
group URI as a string in the Proxy-URI option or it uses the Proxy-
Scheme option with the group URI constructed from the usual Uri-*
options. The forward-proxy then resolves the group URI to a
destination CoAP group, multicasts the CoAP request, receives the
responses and forwards all the individual (unicast) responses back to
the client.
However, there are certain issues and limitations with this approach:
o The CoAP client component that sent a unicast CoAP request to the
proxy may be expecting only one (unicast) response, as usual for a
CoAP unicast request. Instead, it receives multiple (unicast)
responses, potentially leading to fault conditions in the
component or to discarding any received responses following the
first one. This issue may occur even if the application calling
the CoAP client component is aware that the forward-proxy is going
to execute a CoAP group URI request.
o Each individual CoAP response received by the client will appear
to originate (based on its 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, unless the server identity is contained as a part
of the response payload or inside a CoAP Option in the response.
A method based on this approach and addressing the issues raised
above is defined in [I-D.tiloca-core-groupcomm-proxy].
An alternative solution is for the proxy to collect all the
individual (unicast) responses to a CoAP group request and then send
back only a single (aggregated) response to the client. However,
this solution brings up new issues:
o The proxy does not know how many members there are in the CoAP
group or how many group members will actually respond. Also, the
proxy does not know for how long to collect responses before
sending back the aggregated response to the client. A CoAP client
that is not using a Proxy might face the same problems in
collecting responses to a multicast request. However, the client
itself would typically have application-specific rules or
knowledge on how to handle this situation, while an application-
agnostic CoAP Proxy would typically not have this knowledge. For
example, a CoAP client could monitor incoming responses and use
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this information to decide how long to continue collecting
responses - which is something a proxy cannot do.
o There is no default format defined in CoAP for aggregation of
multiple responses into a single response. Such a format could be
standardized based on, for example, the multipart content-format
[RFC8710].
Due to the above issues, it is RECOMMENDED that a CoAP Proxy only
processes a group URI request if it is explicitly enabled to do so.
The default response (if the function is not explicitly enabled) to a
group URI request is 5.01 (Not Implemented). Furthermore, a proxy
SHOULD be explicitly configured (e.g. by white-listing and/or client
authentication) to allow proxied CoAP multicast requests only from
specific client(s).
The operation of HTTP-to-CoAP proxies for multicast CoAP requests is
specified in Section 8.4 and 10.1 of [RFC8075]. In this case, the
"application/http" media type is used to let the proxy return
multiple CoAP responses - each translated to a HTTP response - back
to the HTTP client. Of course, in this case the HTTP client sending
a group URI to the proxy needs to be aware that it is going to
receive this format, and needs to be able to decode it into the
responses of multiple CoAP servers. Also, the IP source address of
each CoAP response cannot be determined anymore from the
"application/http" response. The HTTP client still identify the CoAP
servers by other means such as application-specific information in
the response payload.
2.3.4. Congestion Control
CoAP group 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 is nothing useful to respond to, e.g., error or empty
response (see Section 8.2 of [RFC7252]).
o A server should limit the support for IP multicast requests to
specific resources where multicast operation is required
(Section 11.3 of [RFC7252]).
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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 of
[RFC7252]).
Additional guidelines to reduce congestion risks defined in this
document are as follows:
o A server in a constrained network should only support group
communication with GET and FETCH for resources that are small.
This can consist, for example, in having the payload of the
response as limited to approximately 5% of the IP Maximum Transmit
Unit (MTU) size, so that it fits into a single link-layer frame in
case IPv6 over Low-Power Wireless Personal Area Networks (6LoWPAN)
(see Section 4 of [RFC4944]) is used.
o A server SHOULD minimize the payload size of a response to a
multicast GET or FETCH on "/.well-known/core" by using hierarchy
in arranging link descriptions for the response. An example of
this is given in Section 5 of [RFC6690].
o A server MAY minimize the payload size of a response to a
multicast GET or FETCH (e.g., on "/.well-known/core") by using
CoAP block-wise transfers [RFC7959] in case the payload is long,
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 block-wise
transfers. See also Section 2.3.6.
o A client SHOULD be configured to use CoAP groups 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.
Similarly, realm-local scope is always preferred over site-local
scope if this fulfills the application needs.
2.3.5. Observing Resources
The CoAP Observe Option [RFC7641] is a protocol extension of CoAP,
that allows a CoAP client to retrieve a representation of a resource
and automatically keep this representation up-to-date over a longer
period of time. The client gets notified when the representation has
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changed. [RFC7641] does not mention whether the Observe Option can
be combined with CoAP multicast.
This section updates [RFC7641] with the use of the Observe Option in
a CoAP multicast GET request, and defines normative behavior for both
client and server. Consistently with Section 2.4 of [RFC8132], it is
also possible to use the Observe Option in a CoAP multicast FETCH
request.
Multicast Observe is a useful way to start observing a particular
resource on all members of a CoAP group at the same time. Group
members that do not have this particular resource or do not allow the
GET or FETCH method on it will either respond with an error status -
4.04 Not Found or 4.05 Method Not Allowed, respectively - or will
silently suppress the response following the rules of Section 2.3.1,
depending on server-specific configuration.
A client that sends a multicast GET or FETCH request with the Observe
Option MAY repeat this request using the same Token value and the
same Observe Option value, in order to ensure that enough (or all)
members of the CoAP group have been reached with the request. This
is useful in case a number of group members did not respond to the
initial request. The client MAY additionally use the same Message ID
in the repeated request to avoid that group members that had already
received the initial request would respond again. Note that using
the same Message ID in a repeated request will not be helpful in case
of loss of a response message, since the server that responded
already will consider the repeated request as a duplicate message.
On the other hand, if the client uses a different, fresh Message ID
in the repeated request, then all the group members that receive this
new message will typically respond again, which increases the network
load.
A client that has sent a multicast GET or FETCH request with the
Observe Option MAY follow up by sending a new unicast CON request
with the same Token value and same Observe Option value to a
particular server, in order to ensure that the particular server
receives the request. This is useful in case a specific group
member, that was expected to respond to the initial group request,
did not respond to the initial request. In this case, the client
always uses a Message ID that differs from the initial multicast
message.
Furthermore, consistently with Section 3.3.1 of [RFC7641] and
following its guidelines, a client MAY at any time send a new
multicast GET or FETCH request with the same Token value and same
Observe Option value as the original request. This allows the client
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to verify that it has an up-to-date representation of an observed
resource and/or to re-register its interest to observe a resource.
In the above client behaviors, the Token value is kept identical to
the initial request to avoid that a client is included in more than
one entry in the list of observers (Section 4.1 of [RFC7641]).
Before repeating a request as specified above, the client SHOULD wait
for at least the expected round-trip time plus the Leisure time
period defined in Section 8.2 of [RFC7252], to give the server time
to respond.
A server that receives a GET or FETCH request with the Observe
Option, for which request processing is successful, SHOULD respond to
this request and not suppress the response. A server that adds a
client to the list of observers for a resource due to an Observe
request MUST respond to this request and not suppress it.
A server SHOULD have a mechanism to verify liveness of its observing
clients and the continued interest of these clients in receiving the
observe notifications. This can be implemented by sending
notifications occassionally using a Confirmable message. See
Section 4.5 of [RFC7641] for details. This requirement overrides the
regular behavior of sending Non-Confirmable notifications in response
to a Non-Confirmable request.
A client can use the unicast cancellation methods of Section 3.6 of
[RFC7641] and stop the ongoing observation of a particular resource
on members of a CoAP group. This can be used to remove specific
observed servers, or even all servers in the group. In addition, a
client MAY explicitly deregister from all those servers at once, by
sending a multicast GET or FETCH request that includes the Token
value of the observation to be cancelled and includes an Observe
Option with the value set to 1 (deregister). In case not all the
servers in the CoAP group received this deregistration request,
either the unicast cancellation methods can be used or the multicast
deregistration request MAY be repeated upon receiving another observe
response from a server.
For observing a group of servers through a CoAP-to-CoAP proxy, the
limitations stated in Section 2.3.3 apply. The method defined in
[I-D.tiloca-core-groupcomm-proxy] enables group communication through
proxies and addresses those limitations.
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2.3.6. Block-Wise Transfer
Section 2.8 of [RFC7959] specifies how a client can use block-wise
transfer (Block2 Option) in a multicast GET request to limit the size
of the initial response of each server. Consistently with
Section 2.5 of [RFC8132], the same can be done with a multicast FETCH
request.
The client has to use unicast for any further request, separately
addressing each different server, in order to retrieve more blocks of
the resource from that server, if any. Also, a server (member of a
targeted CoAP group) that needs to respond to a multicast request
with a particularly large resource can use block-wise transfer
(Block2 Option) at its own initiative, to limit the size of the
initial response. Again, a client would have to use unicast for any
further requests to retrieve more blocks of the resource.
A solution for multicast block-wise transfer using the Block1 Option
is not specified in [RFC7959] nor in the present document. Such a
solution would be useful for multicast FETCH/PUT/POST/PATCH/iPATCH
requests, to efficiently distribute a large request payload as
multiple blocks to all members of a CoAP group. Multicast usage of
Block1 is non-trivial due to potential message loss (leading to
missing blocks or missing confirmations), and potential diverging
block size preferences of different members of the CoAP group.
2.4. Transport
In this document only UDP is considered as a transport protocol, both
over IPv4 and IPv6. Therefore, [RFC8323] (CoAP over TCP, TLS, and
WebSockets) is not in scope as a transport for group communication.
2.4.1. UDP/IPv6 Multicast Transport
CoAP group communication can use UDP over IPv6 as a transport
protocol, provided that IPv6 multicast is enabled. IPv6 multicast
MAY be supported in a network only for a limited scope. For example,
Section 2.5.2 describes the potential limited support of RPL for
multicast, depending on how the protocol is configured.
For a CoAP server node that supports resource discovery as defined in
Section 2.4 of [RFC7252], the default port 5683 MUST be supported as
per Section 7.1 and 12.8 of [RFC7252] for the "All CoAP Nodes"
multicast group. An IPv6 CoAP server SHOULD support the "All CoAP
Nodes" groups with at least link-local (2), admin-local (4) and site-
local (5) scopes. An IPv6 CoAP server on a 6LoWPAN node (see
Section 2.4.3) SHOULD also support the realm-local (3) scope.
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Note that a client sending an IPv6 multicast CoAP message to a port
that is not supported by the server will not receive an ICMPv6 Port
Unreachable error message from that server, because the server does
not send it in this case, per Section 2.4 of [RFC4443].
2.4.2. UDP/IPv4 Multicast Transport
CoAP group communication can use UDP over IPv4 as a transport
protocol, provided that IPv4 multicast is enabled. For a CoAP server
node that supports resource discovery as defined in Section 2.4 of
[RFC7252], the default port 5683 MUST be supported as per Section 7.1
and 12.8 of [RFC7252], for the "All CoAP Nodes" IPv4 multicast group.
Note that a client sending an IPv4 multicast CoAP message to a port
that is not supported by the server will not receive an ICMP Port
Unreachable error message from that server, because the server does
not send it in this case, per Section 3.2.2 of [RFC1122].
2.4.3. 6LoWPAN
In 6LoWPAN [RFC4944] networks, IPv6 packets (up to 1280 bytes) may be
fragmented into smaller IEEE 802.15.4 MAC frames (up to 127 bytes),
if the packet size requires this. Every 6LoWPAN IPv6 router that
receives a multi-fragment packet reassembles the packet and
refragments it upon transmission. Since the loss of a single
fragment implies the loss of the entire IPv6 packet, the performance
in terms of packet loss and throughput of multi-fragment multicast
IPv6 packets is typically far worse than the performance of single-
fragment IPv6 multicast packets. For this reason, a CoAP request
sent over multicast in 6LoWPAN networks SHOULD be sized in such a way
that it fits in a single IEEE 802.15.4 MAC frame, if possible.
On 6LoWPAN networks, multicast groups can be defined with realm-local
scope [RFC7346]. Such a realm-local group is restricted to the local
6LoWPAN network/subnet. In other words, a multicast request to that
group does not propagate beyond the 6LoWPAN network segment where the
request originated. For example, a multicast discovery request can
be sent to the realm-local "All CoAP Nodes" IPv6 multicast group (see
Section 2.4.1) in order to discover only CoAP servers on the local
6LoWPAN network.
2.5. Interworking with Other Protocols
2.5.1. MLD/MLDv2/IGMP/IGMPv3
CoAP nodes that are IP hosts (i.e., not IP routers) are generally
unaware of the specific IP multicast routing/forwarding protocol
being used in their network. When such a host needs to join a
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specific (CoAP) multicast group, it requires a way to signal to IP
multicast routers which IP multicast address(es) it needs to listen
to.
The MLDv2 protocol [RFC3810] is the standard IPv6 method to achieve
this; therefore, this method SHOULD be used by members of a CoAP
group to subscribe to its multicast IPv6 address, on IPv6 networks
that support it. CoAP server nodes then act in the role of MLD
Multicast Address Listener. Constrained IPv6 networks that implement
either RPL (see Section 2.5.2) or MPL (see Section 2.5.3) typically
do not support MLD as they have their own mechanisms defined.
The IGMPv3 protocol [RFC3376] is the standard IPv4 method to signal
multicast group subscriptions. This SHOULD be used by members of a
CoAP group to subscribe to its multicast IPv4 address on IPv4
networks.
The guidelines from [RFC6636] on the tuning of MLD for mobile and
wireless networks may be useful when implementing MLD in constrained
networks.
2.5.2. RPL
RPL [RFC6550] is an IPv6 based routing protocol suitable for low-
power, lossy networks (LLNs). In such a context, CoAP is often used
as an application protocol.
If only RPL is used in a network for routing and its optional
multicast support is disabled, there will be no IP multicast routing
available. Any IPv6 multicast packets in this case will not
propagate beyond a single hop (to direct neighbors in the LLN). This
implies that any CoAP group request will be delivered to link-local
nodes only, for any scope value >= 2 used in the IPv6 destination
address.
RPL supports (see Section 12 of [RFC6550]) advertisement of IP
multicast destinations using Destination Advertisement Object (DAO)
messages and subsequent routing of multicast IPv6 packets based on
this. It requires the RPL mode of operation to be 3 (Storing mode
with multicast support).
In this mode, RPL DAO can be used by a CoAP node that is either an
RPL router or RPL Leaf Node, to advertise its CoAP group membership
to parent RPL routers. Then, RPL will route any IP multicast CoAP
requests over multiple hops to those CoAP servers that are group
members.
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The same DAO mechanism can be used to convey CoAP group membership
information to an edge router (e.g., 6LBR), in case the edge router
is also the root of the RPL Destination-Oriented Directed Acyclic
Graph (DODAG). This is useful because the edge router then learns
which IP multicast traffic it needs to pass through from the backbone
network into the LLN subnet. In LLNs, such ingress filtering helps
to avoid congestion of the resource-constrained network segment, due
to IP multicast traffic from the high-speed backbone IP network.
2.5.3. MPL
The Multicast Protocol for Low-Power and Lossy Networks (MPL)
[RFC7731] can be used for propagation of IPv6 multicast packets
throughout a defined network domain, over multiple hops. MPL is
designed to work in LLNs and can operate alone or in combination with
RPL. The protocol involves a predefined group of MPL Forwarders to
collectively distribute IPv6 multicast packets throughout their MPL
Domain. An MPL Forwarder may be associated to multiple MPL Domains
at the same time. Non-Forwarders will receive IPv6 multicast packets
from one or more of their neighboring Forwarders. Therefore, MPL can
be used to propagate a CoAP multicast request to all group members.
However, a CoAP multicast request to a group that originated outside
of the MPL Domain will not be propagated by MPL - unless an MPL
Forwarder is explicitly configured as an ingress point that
introduces external multicast packets into the MPL Domain. Such an
ingress point could be located on an edge router (e.g., 6LBR). The
method to configure which multicast groups are to be propagated into
the MPL Domain could be:
o Manual configuration on the ingress MPL Forwarder.
o A protocol to register multicast groups at an ingress MPL
Forwarder. This could be a protocol offering features similar to
MLDv2.
3. Unsecured Group Communication
CoAP group communication can operate in CoAP NoSec (No Security)
mode, without using application-layer and transport-layer security
mechanisms. The NoSec mode uses the "coap" scheme, and is defined in
Section 9 of [RFC7252]. The conceptual "NoSec" security group as
defined in Section 2.1 is used for unsecured group communication.
Before using this mode of operation, the security implications
(Section 5.1) must be well understood.
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4. Secured Group Communication using Group OSCORE
The application-layer protocol Object Security for Constrained
RESTful Environments (OSCORE) [RFC8613] provides end-to-end
encryption, integrity and replay protection of CoAP messages
exchanged between two CoAP endpoints. These can act both as CoAP
Client as well as CoAP Server, and share an OSCORE Security Context
used to protect and verify exchanged messages. The use of OSCORE
does not affect the URI scheme and OSCORE can therefore be used with
any URI scheme defined for CoAP.
OSCORE uses COSE
[I-D.ietf-cose-rfc8152bis-struct][I-D.ietf-cose-rfc8152bis-algs] to
perform encryption operations and protect a CoAP message in a COSE
object, by using an Authenticated Encryption with Associated Data
(AEAD) algorithm. In particular, OSCORE takes as input an
unprotected CoAP message and transforms it into a protected CoAP
message transporting the COSE object.
OSCORE makes it possible to selectively protect different parts of a
CoAP message in different ways, while still allowing intermediaries
(e.g., CoAP proxies) to perform their intended funtionalities. That
is, some message parts are encrypted and integrity protected; other
parts are only integrity protected to be accessible to, but not
modifiable by, proxies; and some parts are kept as plain content to
be both accessible to and modifiable by proxies. Such differences
especially concern the CoAP options included in the unprotected
message.
Group OSCORE [I-D.ietf-core-oscore-groupcomm] builds on OSCORE, and
provides end-to-end security of CoAP messages exchanged between
members of an OSCORE group, while fulfilling the same security
requirements.
In particular, Group OSCORE protects CoAP requests sent over IP
multicast by a CoAP client, as well as multiple corresponding CoAP
responses sent over IP unicast by different CoAP servers. However,
the same security material can also be used to protect CoAP requests
sent over IP unicast to a single CoAP server in the OSCORE group, as
well as the corresponding responses.
Group OSCORE ensures source authentication of all messages exchanged
within the OSCORE group, by means of two possible methods.
The first method, namely group mode, relies on digital signatures.
That is, sender devices sign their outgoing messages using their own
private key, and embed the signature in the protected CoAP message.
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The second method, namely pairwise mode, relies on a symmetric key,
which is derived from a pairwise shared secret computed from the
asymmetric keys of the message sender and recipient. This method is
intended for one-to-one messages sent in the group, such as all
responses as individually sent by servers, as well as requests
addressed to an individual server.
A Group Manager is responsible for one or multiple OSCORE groups. In
particular, the Group Manager acts as repository of public keys of
group members; manages, renews and provides security material in the
group; and handles the join process of new group members.
As defined in [I-D.ietf-ace-oscore-gm-admin], an administrator entity
can interact with the Group Manager to create OSCORE groups and
specify their configuration (see Section 2.2.2). During the lifetime
of the OSCORE group, the administrator can further interact with the
Group Manager, in order to possibly update the group configuration
and eventually delete the group.
As recommended in [I-D.ietf-core-oscore-groupcomm], a CoAP endpoint
can join an OSCORE group by using the method described in
[I-D.ietf-ace-key-groupcomm-oscore] and based on the ACE framework
for Authentication and Authorization in constrained environments
[I-D.ietf-ace-oauth-authz].
A CoAP endpoint can discover OSCORE groups and retrieve information
to join them through their Group Managers by using the method
described in [I-D.tiloca-core-oscore-discovery] and based on the CoRE
Resource Directory [I-D.ietf-core-resource-directory].
If security is required, CoAP group communication as described in
this specification MUST use Group OSCORE. In particular, a CoAP
group as defined in Section 2.1 and using secure group communication
is associated to an OSCORE security group, which includes:
o All members of the CoAP group, i.e. the CoAP endpoints configured
(also) as CoAP servers and listening to the group's multicast IP
address.
o All further CoAP endpoints configured only as CoAP clients, that
send (multicast) CoAP requests to the CoAP group.
4.1. Secure Group Maintenance
Additional key management operations on the OSCORE group are
required, depending also on the security requirements of the
application (see Section 5.2). That is:
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o Adding new members to a CoAP group or enabling new client-only
endpoints to interact with that group require also that each of
such members/endpoints join the corresponding OSCORE group. By
doing so, they are securely provided with the necessary
cryptographic material. In case backward security is needed, this
also requires to first renew such material and distribute it to
the current members/endpoints, before new ones are added and join
the OSCORE group.
o In case forward security is needed, removing members from a CoAP
group or stopping client-only endpoints from interacting with that
group requires removing such members/endpoints from the
corresponding OSCORE group. To this end, new cryptographic
material is generated and securely distributed only to the
remaining members/endpoints. This ensures that only the members/
endpoints intended to remain are able to continue participating in
secure group communication, while the evicted ones are not able
to.
The key management operations mentioned above are entrusted to the
Group Manager responsible for the OSCORE group
[I-D.ietf-core-oscore-groupcomm], and it is RECOMMENDED to perform
them according to the approach described in
[I-D.ietf-ace-key-groupcomm-oscore].
5. Security Considerations
This section provides security considerations for CoAP group
communication using IP multicast.
5.1. CoAP NoSec Mode
CoAP group communication, if not protected, is vulnerable to all the
attacks mentioned in Section 11 of [RFC7252] for IP multicast.
Thus, for sensitive and mission-critical applications (e.g., health
monitoring systems and alarm monitoring systems), it is NOT
RECOMMENDED to deploy CoAP group communication in NoSec mode.
Without application-layer security, CoAP group communication SHOULD
only be deployed in applications that are non-critical, and that do
not involve or may have an impact on sensitive data and personal
sphere. These include, e.g., read-only temperature sensors deployed
in non-sensitive environments, where the client reads out the values
but does not use the data to control actuators or to base an
important decision on.
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Discovery of devices and resources is a typical use case where NoSec
mode is applied, since the devices involved do not have yet
configured any mutual security relations at the time the discovery
takes place.
5.2. Group OSCORE
Group OSCORE provides end-to-end application-level security. This
has many desirable properties, including maintaining security
assurances 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).
For sensitive and mission-critical applications, CoAP group
communication MUST be protected by using Group OSCORE as specified in
[I-D.ietf-core-oscore-groupcomm]. The same security considerations
from Section 10 of [I-D.ietf-core-oscore-groupcomm] hold for this
specification.
5.2.1. Group Key Management
A key management scheme for secure revocation and renewal of group
security material, namely group rekeying, should be adopted in OSCORE
groups. In particular, the key management scheme should preserve
backward and forward security in the OSCORE group, if the application
requires so (see Section 3.1 of [I-D.ietf-core-oscore-groupcomm]).
Group policies should also take into account the time that the key
management scheme requires to rekey the group, on one hand, and the
expected frequency of group membership changes, i.e. nodes' joining
and leaving, on the other hand.
In fact, it may be desirable to not rekey the group upon every single
membership change, in case members' joining and leaving are frequent,
and at the same time a single group rekeying instance takes a non
negligible time to complete.
In such a case, the Group Manager may consider to rekey the group,
e.g., after a minimum number of nodes has joined or left the group
within a pre-defined time interval, or according to communication
patterns with predictable intervals of network inactivity. This
would prevent paralyzing communications in the group, when a slow
rekeying scheme is used and frequently invoked.
This comes at the cost of not continuously preserving backward and
forward security, since group rekeying might not occur upon every
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single group membership change. That is, most recently joined nodes
would have access to the security material used prior to their join,
and thus be able to access past group communications protected with
that security material. Similarly, until the group is rekeyed, most
recently left nodes would preserve access to group communications
protected with the retained security material.
5.2.2. Source Authentication
Both the group mode and the pairwise mode of Group OSCORE ensure
source authentication of messages exchanged by CoAP endpoints through
CoAP group communication.
To this end, outgoing messages are either countersigned by the
message sender endpoint with its own private key (group mode), or
protected with a symmetric key, which is in turn derived using the
asymmetric keys of the message sender and recipient (pairwise mode).
Thus, both modes allow a recipient CoAP endpoint to verify that a
message has actually been originated by a specific and identified
member of the OSCORE group.
Appendix F of [I-D.ietf-core-oscore-groupcomm] discusses a number of
cases where a recipient CoAP endpoint may skip the verification of
countersignatures in messages protected with the group mode, possibly
on a per-message basis. However, this is NOT RECOMMENDED. That is,
a CoAP endpoint receiving a message secured with the group mode of
Group OSCORE SHOULD always verify the countersignature.
5.2.3. Countering Attacks
As discussed below, Group OSCORE addresses a number of security
attacks mentioned in Section 11 of [RFC7252], with particular
reference to their execution over IP multicast.
o Since Group OSCORE provides end-to-end confidentiality and
integrity of request/response messages, proxies in multicast
settings cannot break message protection, and thus cannot act as
man-in-the-middle beyond their legitimate duties (see Section 11.2
of [RFC7252]). In fact, intermediaries such as proxies are not
assumed to have access to the OSCORE Security Context used by
group members. Also, with the notable addition of
countersignatures for the group mode, Group OSCORE protects
messages using the same constructions of OSCORE (see Sections 8.1
and 8.3 of [I-D.ietf-core-oscore-groupcomm]), and especially
processes CoAP options according to the same classification in
U/I/E classes.
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o Group OSCORE protects against amplification attacks (see
Section 11.3 of [RFC7252]), which are made e.g. by injecting
(small) requests over IP multicast from the (spoofed) IP address
of a victim client, and thus triggering the transmission of
several (much bigger) responses back to that client. In fact,
upon receiving a request over IP multicast as protected with Group
OSCORE in group mode, a server is able to verify whether the
request is fresh and originates from the alleged sender in the
OSCORE group, by verifying the countersignature included in the
request using the public key of that sender (see Section 8.2 of
[I-D.ietf-core-oscore-groupcomm]). Furthermore, as also discussed
in Section 8 of [I-D.ietf-core-oscore-groupcomm], it is
recommended that servers failing to decrypt and verify an incoming
message do not send back any error message.
o Group OSCORE limits the impact of attacks based on IP spoofing
also over IP multicast (see Section 11.4 of [RFC7252]). In fact,
requests and corresponding responses sent in the OSCORE group can
be correctly generated only by legitimate group members.
Within an OSCORE group, the shared symmetric-key security material
strictly provides only group-level authentication (see
Section 10.1 of [I-D.ietf-core-oscore-groupcomm]). However,
source authentication of messages is also ensured, both in the
group mode by means of countersignatures (see Sections 8.1 and 8.3
of [I-D.ietf-core-oscore-groupcomm]), and in the pairwise mode by
using additionally derived pairwise keys (see Sections 9.1 and 9.3
of [I-D.ietf-core-oscore-groupcomm]). Thus, recipient endpoints
can verify a message to be originated by the alleged, identifiable
sender in the OSCORE group.
Note that the server may additionally rely on the Echo option for
CoAP described in [I-D.ietf-core-echo-request-tag], in order to
verify the aliveness and reachability of the client sending a
request from a particular IP address.
o Group OSCORE does not require group members to be equipped with a
good source of entropy for generating security material (see
Section 11.6 of [RFC7252]), and thus does not contribute to create
an attack vector against such (constrained) CoAP endpoints. In
particular, the symmetric keys used for message encryption and
decryption are derived through the same HMAC-based HKDF scheme
used for OSCORE (see Section 3.2 of [RFC8613]). Besides, the
OSCORE Master Secret used in such derivation is securely generated
by the Group Manager responsible for the OSCORE group, and
securely provided to the CoAP endpoints when they join the group.
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o Group OSCORE prevents to make any single group member a target for
subverting security in the whole OSCORE group (see Section 11.6 of
[RFC7252]), even though all group members share (and can derive)
the same symmetric-key security material used in the OSCORE group
(see Section 10.1 of [I-D.ietf-core-oscore-groupcomm]). In fact,
source authentication is always ensured for exchanged CoAP
messages, as verifiable to be originated by the alleged,
identifiable sender in the OSCORE group. This relies on including
a countersignature computed with a node's individual private key
(in the group mode), or on protecting messages with a pairwise
symmetric key, which is in turn derived from the asymmetric keys
of the sender and recipient CoAP endpoints (in the pairwise mode).
5.3. Replay of Non Confirmable Messages
Since all requests sent over IP multicast are Non-confirmable, a
client might not be able to know if an adversary has actually
captured one of its trasmitted requests and later re-injected it in
the group as a replay to the server nodes. In fact, even if the
servers sent back responses to the replayed request, the client would
not have a valid matching request anymore to suspect of the attack.
If Group OSCORE is used, such a replay attack on the servers is
prevented, since a client protects every different request with a
different Sequence Number value, which is in turn included as Partial
IV in the protected message and takes part in the construction of the
AEAD cipher nonce. Thus, a server would be able to detect the
replayed request, by checking the conveyed Partial IV against its own
replay window in the OSCORE Recipient Context associated to the
client.
This requires a server to have a synchronized, up to date view of the
sequence number used by the client. If such synchronization is lost,
e.g. due to a reboot, or suspected so, the server should use one of
the methods described in Appendix E of
[I-D.ietf-core-oscore-groupcomm], such as the one based on the Echo
option for CoAP described in [I-D.ietf-core-echo-request-tag], in
order to (re-)synchronize with the client's sequence number.
5.4. Use of CoAP No-Response Option
When CoAP group communication is used in CoAP NoSec (No Security)
mode (see Section 3), the CoAP No-Response Option [RFC7967] could be
misused by a malicious client to evoke as much responses from servers
to a multicast request as possible, by using the value '0' -
Interested in all responses. This even overrides the default
behaviour of a CoAP server to suppress the response in case there is
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nothing of interest to respond with. Therefore, this option can be
used to perform an amplification attack.
A proposed mitigation is to only allow this Option to relax the
standard suppression rules for a resource in case the Option is sent
by an authenticated client. If sent by an unauthenticated client,
the Option can be used to expand the classes of responses suppressed
compared to the default rules but not to reduce the classes of
responses suppressed.
5.5. 6LoWPAN
In a 6LoWPAN network, a multicast IPv6 packet may be fragmented prior
to transmission. A 6LoWPAN Router that forwards a fragmented packet
can have a relatively high impact on the occupation of the wireless
channel and on the memory load of the local node due to packet buffer
occupation. For example, the MPL [RFC7731] protocol requires an MPL
Forwarder to store the packet for a longer duration, to allow
multiple forwarding transmissions to neighboring Forwarders. If only
one of the fragments is not received correctly by an MPL Forwarder,
the receiver needs to discard all received fragments and it needs to
receive all the packet fragments again on a future occasion.
For these reasons, a fragmented IPv6 multicast packet is a possible
attack vector in a Denial of Service (DoS) amplification attack. See
Section 11.3 of [RFC7252] for more details on amplification. To
mitigate the risk, applications sending multicast IPv6 requests to
6LoWPAN hosted CoAP servers SHOULD limit the size of the request to
avoid 6LoWPAN fragmentation. A 6LoWPAN Router or multicast forwarder
SHOULD deprioritize forwarding for multi-fragment 6LoWPAN multicast
packets. Also, a 6LoWPAN Border Router SHOULD implement multicast
packet filtering to prevent unwanted multicast traffic from entering
a 6LoWPAN network from the outside. For example, it could filter out
all multicast packet for which there is no known multicast listener
on the 6LoWPAN network.
5.6. Wi-Fi
In a home automation scenario using Wi-Fi, Wi-Fi security should be
enabled to prevent rogue nodes from joining. The Customer Premises
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 IP
multicast transmissions and listeners should be site-local (5) or
smaller. For site-local scope, the CPE will be an appropriate
multicast scope boundary point.
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5.7. Monitoring
5.7.1. General Monitoring
CoAP group communication can be used to control a set of related
devices: for example, simultaneously turn on all the lights in a
room. This intrinsically exposes the group to some unique monitoring
risks that 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 an observed CoAP
group communication message to the observed coordinated group action
- even if the CoAP message is (partly) encrypted.
This will give the attacker side-channel information to plan further
attacks (e.g., by determining the members of the group some network
topology information may be deduced).
5.7.2. Pervasive Monitoring
A key additional threat consideration for group communication is
pervasive monitoring [RFC7258]. CoAP group communication solutions
that are built on top of IP multicast need to pay particular heed to
these dangers. This is because IP multicast is easier to intercept
(and to secretly record) compared to IP unicast. Also, CoAP traffic
is meant for the Internet of Things. This means that CoAP multicast
may be used for the control and monitoring of critical infrastructure
(e.g., lights, alarms, etc.) that may be prime targets for attack.
For example, an attacker may attempt to record all the CoAP traffic
going over a smart grid (i.e., networked electrical utility) and try
to determine critical nodes for further attacks. For example, the
source node (controller) sends out CoAP group communication messages
which easily identifies it as a controller.
CoAP multicast traffic is inherently more vulnerable (compared to
unicast) as the same packet may be replicated over many links,
leading to a higher probability of packet capture by a pervasive
monitoring system.
One mitigation is to restrict the scope of 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.
Even if all CoAP multicast traffic is encrypted/protected, an
attacker may still attempt to capture this traffic and perform an
off-line attack in the future.
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6. IANA Considerations
This document has no actions for IANA.
7. References
7.1. Normative References
[I-D.ietf-cbor-7049bis]
Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", draft-ietf-cbor-7049bis-16 (work
in progress), September 2020.
[I-D.ietf-core-echo-request-tag]
Amsuess, C., Mattsson, J., and G. Selander, "CoAP: Echo,
Request-Tag, and Token Processing", draft-ietf-core-echo-
request-tag-10 (work in progress), July 2020.
[I-D.ietf-core-oscore-groupcomm]
Tiloca, M., Selander, G., Palombini, F., and J. Park,
"Group OSCORE - Secure Group Communication for CoAP",
draft-ietf-core-oscore-groupcomm-10 (work in progress),
November 2020.
[I-D.ietf-cose-rfc8152bis-algs]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", draft-ietf-cose-rfc8152bis-algs-12
(work in progress), September 2020.
[I-D.ietf-cose-rfc8152bis-struct]
Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", draft-ietf-cose-rfc8152bis-
struct-14 (work in progress), September 2020.
[RFC1122] Braden, R., Ed., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122,
DOI 10.17487/RFC1122, October 1989,
<https://www.rfc-editor.org/info/rfc1122>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, DOI 10.17487/RFC3376, October 2002,
<https://www.rfc-editor.org/info/rfc3376>.
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[RFC3810] Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
DOI 10.17487/RFC3810, June 2004,
<https://www.rfc-editor.org/info/rfc3810>.
[RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
Control Message Protocol (ICMPv6) for the Internet
Protocol Version 6 (IPv6) Specification", STD 89,
RFC 4443, DOI 10.17487/RFC4443, March 2006,
<https://www.rfc-editor.org/info/rfc4443>.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<https://www.rfc-editor.org/info/rfc4944>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>.
[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<https://www.rfc-editor.org/info/rfc7959>.
[RFC8075] Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
E. Dijk, "Guidelines for Mapping Implementations: HTTP to
the Constrained Application Protocol (CoAP)", RFC 8075,
DOI 10.17487/RFC8075, February 2017,
<https://www.rfc-editor.org/info/rfc8075>.
[RFC8132] van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
FETCH Methods for the Constrained Application Protocol
(CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
<https://www.rfc-editor.org/info/rfc8132>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/info/rfc8613>.
7.2. Informative References
[Californium]
Eclipse Foundation, "Eclipse Californium", March 2019,
<https://github.com/eclipse/californium/tree/2.0.x/
californium-core/src/main/java/org/eclipse/californium/
core>.
[Go-OCF] Open Connectivity Foundation (OCF), "Implementation of
CoAP Server & Client in Go", March 2019,
<https://github.com/go-ocf/go-coap>.
[I-D.ietf-ace-key-groupcomm-oscore]
Tiloca, M., Park, J., and F. Palombini, "Key Management
for OSCORE Groups in ACE", draft-ietf-ace-key-groupcomm-
oscore-09 (work in progress), November 2020.
[I-D.ietf-ace-oauth-authz]
Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments (ACE) using the OAuth 2.0
Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-35
(work in progress), June 2020.
[I-D.ietf-ace-oscore-gm-admin]
Tiloca, M., Hoeglund, R., Stok, P., Palombini, F., and K.
Hartke, "Admin Interface for the OSCORE Group Manager",
draft-ietf-ace-oscore-gm-admin-01 (work in progress),
November 2020.
[I-D.ietf-core-coap-pubsub]
Koster, M., Keranen, A., and J. Jimenez, "Publish-
Subscribe Broker for the Constrained Application Protocol
(CoAP)", draft-ietf-core-coap-pubsub-09 (work in
progress), September 2019.
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[I-D.ietf-core-resource-directory]
Shelby, Z., Koster, M., Bormann, C., Stok, P., and C.
Amsuess, "CoRE Resource Directory", draft-ietf-core-
resource-directory-25 (work in progress), July 2020.
[I-D.tiloca-core-groupcomm-proxy]
Tiloca, M. and E. Dijk, "Proxy Operations for CoAP Group
Communication", draft-tiloca-core-groupcomm-proxy-02 (work
in progress), November 2020.
[I-D.tiloca-core-oscore-discovery]
Tiloca, M., Amsuess, C., and P. Stok, "Discovery of OSCORE
Groups with the CoRE Resource Directory", draft-tiloca-
core-oscore-discovery-07 (work in progress), November
2020.
[RFC6092] Woodyatt, J., Ed., "Recommended Simple Security
Capabilities in Customer Premises Equipment (CPE) for
Providing Residential IPv6 Internet Service", RFC 6092,
DOI 10.17487/RFC6092, January 2011,
<https://www.rfc-editor.org/info/rfc6092>.
[RFC6550] Winter, T., Ed., Thubert, P., Ed., 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,
DOI 10.17487/RFC6550, March 2012,
<https://www.rfc-editor.org/info/rfc6550>.
[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, DOI 10.17487/RFC6636,
May 2012, <https://www.rfc-editor.org/info/rfc6636>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <https://www.rfc-editor.org/info/rfc7258>.
[RFC7346] Droms, R., "IPv6 Multicast Address Scopes", RFC 7346,
DOI 10.17487/RFC7346, August 2014,
<https://www.rfc-editor.org/info/rfc7346>.
[RFC7390] Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
the Constrained Application Protocol (CoAP)", RFC 7390,
DOI 10.17487/RFC7390, October 2014,
<https://www.rfc-editor.org/info/rfc7390>.
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[RFC7731] Hui, J. and R. Kelsey, "Multicast Protocol for Low-Power
and Lossy Networks (MPL)", RFC 7731, DOI 10.17487/RFC7731,
February 2016, <https://www.rfc-editor.org/info/rfc7731>.
[RFC7967] Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
Bose, "Constrained Application Protocol (CoAP) Option for
No Server Response", RFC 7967, DOI 10.17487/RFC7967,
August 2016, <https://www.rfc-editor.org/info/rfc7967>.
[RFC8323] Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
Application Protocol) over TCP, TLS, and WebSockets",
RFC 8323, DOI 10.17487/RFC8323, February 2018,
<https://www.rfc-editor.org/info/rfc8323>.
[RFC8710] Fossati, T., Hartke, K., and C. Bormann, "Multipart
Content-Format for the Constrained Application Protocol
(CoAP)", RFC 8710, DOI 10.17487/RFC8710, February 2020,
<https://www.rfc-editor.org/info/rfc8710>.
Appendix A. Use Cases
To illustrate where and how CoAP-based group communication can be
used, this section summarizes the most common use cases. These use
cases include both secured and non-secured CoAP usage. Each
subsection below covers one particular category of use cases for
CoRE. Within each category, a use case may cover multiple
application areas such as home IoT, commercial building IoT (sensing
and control), industrial IoT/control, or environmental sensing.
A.1. Discovery
Discovery of physical devices in a network, or discovery of
information entities hosted on network devices, are operations that
are usually required in a system during the phases of setup or
(re)configuration. When a discovery use case involves devices that
need to interact without having been configured previously with a
common security context, unsecured CoAP communication is typically
used. Discovery may involve a request to a directory server, which
provides services to aid clients in the discovery process. One
particular type of directory server is the CoRE Resource Directory
[I-D.ietf-core-resource-directory]; and there may be other types of
directories that can be used with CoAP.
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A.1.1. Distributed Device Discovery
Device discovery is the discovery and identification of networked
devices - optionally only devices of a particular class, type, model,
or brand. Group communication is used for distributed device
discovery, if a central directory server is not used. Typically in
distributed device discovery, a multicast request is sent to a
particular address (or address range) and multicast scope of
interest, and any devices configured to be discoverable will respond
back. For the alternative solution of centralized device discovery a
central directory server is accessed through unicast, in which case
group communication is not needed. This requires that the address of
the central directory is either preconfigured in each device or
configured during operation using a protocol.
In CoAP, device discovery can be implemented by CoAP resource
discovery requesting (GET) a particular resource that the sought
device class, type, model or brand is known to respond to. It can
also be implemented using CoAP resource discovery (Section 7 of
[RFC7252]) and the CoAP query interface defined in Section 4 of
[RFC6690] to find these particular resources. Also, a multicast GET
request to /.well-known/core can be used to discover all CoAP
devices.
A.1.2. Distributed Service Discovery
Service discovery is the discovery and identification of particular
services hosted on network devices. Services can be identified by
one or more parameters such as ID, name, protocol, version and/or
type. Distributed service discovery involves group communication to
reach individual devices hosting a particular service; with a central
directory server not being used.
In CoAP, services are represented as resources and service discovery
is implemented using resource discovery (Section 7 of [RFC7252]) and
the CoAP query interface defined in Section 4 of [RFC6690].
A.1.3. Directory Discovery
This use case is a specific sub-case of Distributed Service Discovery
(Appendix A.1.2), in which a device needs to identify the location of
a Directory on the network to which it can e.g. register its own
offered services, or to which it can perform queries to identify and
locate other devices/services it needs to access on the network.
Section 3.3 of [RFC7390] shows an example of discovering a CoRE
Resource Directory using CoAP group communication. As defined in
[I-D.ietf-core-resource-directory], a resource directory is a web
entity that stores information about web resources and implements
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REST interfaces for registration and lookup of those resources. For
example, a device can register itself to a resource directory to let
it be found by other devices and/or applications.
A.2. Operational Phase
Operational phase use cases describe those operations that occur most
frequently in a networked system, during its operational lifetime and
regular operation. Regular usage is when the applications on
networked devices perform the tasks they were designed for and
exchange of application-related data using group communication
occurs. Processes like system reconfiguration, group changes,
system/device setup, extra group security changes, etc. are not part
of regular operation.
A.2.1. Actuator Group Control
Group communication can be beneficial to control actuators that need
to act in synchrony, as a group, with strict timing (latency)
requirements. Examples are office lighting, stage lighting, street
lighting, or audio alert/Public Address systems. Sections 3.4 and
3.5 of [RFC7390] show examples of lighting control of a group of
6LoWPAN-connected lights.
A.2.2. Device Group Status Request
To properly monitor the status of systems, there may be a need for
ad-hoc, unplanned status updates. Group communication can be used to
quickly send out a request to a (potentially large) number of devices
for specific information. Each device then responds back with the
requested data. Those devices that did not respond to the request
can optionally be polled again via reliable unicast communication to
complete the dataset. The device group may be defined e.g. as "all
temperature sensors on floor 3", or "all lights in wing B". For
example, it could be a status request for device temperature, most
recent sensor event detected, firmware version, network load, and/or
battery level.
A.2.3. Network-wide Query
In some cases a whole network or subnet of multiple IP devices needs
to be queried for status or other information. This is similar to
the previous use case except that the device group is not defined in
terms of its function/type but in terms of its network location.
Technically this is also similar to distributed service discovery
(Appendix A.1.2) where a query is processed by all devices on a
network - except that the query is not about services offered by the
device, but rather specific operational data is requested.
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A.2.4. Network-wide / Group Notification
In some cases a whole network, or subnet of multiple IP devices, or a
specific target group needs to be notified of a status change or
other information. This is similar to the previous two use cases
except that the recipients are not expected to respond with some
information. Unreliable notification can be acceptable in some use
cases, in which a recipient does not respond with a confirmation of
having received the notification. In such a case, the receiving CoAP
server does not have to create a CoAP response. If the sender needs
confirmation of reception, the CoAP servers can be configured for
that resource to respond with a 2.xx success status after processing
a notification request successfully.
A.3. Software Update
Multicast can be useful to efficiently distribute new software
(firmware, image, application, etc.) to a group of multiple devices.
In this case, the group is defined in terms of device type: all
devices in the target group are known to be capable of installing and
running the new software. The software is distributed as a series of
smaller blocks that are collected by all devices and stored in
memory. All devices in the target group are usually responsible for
integrity verification of the received software; which can be done
per-block or for the entire software image once all blocks have been
received. Due to the inherent unreliability of CoAP multicast, there
needs to be a backup mechanism (e.g. implemented using CoAP unicast)
by which a device can individually request missing blocks of a whole
software image/entity. Prior to multicast software update, the group
of recipients can be separately notified that there is new software
available and coming, using the above network-wide or group
notification.
Appendix B. Document Updates
RFC EDITOR: PLEASE REMOVE THIS SECTION.
B.1. Version -01 to -02
o Clarified relation between security groups and application groups.
o Considered also FETCH for requests over IP multicast.
o More details on Observe re-registration.
o More details on Proxy intermediaries.
o More details on servers changing port number in the response.
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o Usage of the Uri-Host Option to indicate an application group.
o Response suppression based on classes of error codes.
B.2. Version -00 to -01
o Clarifications on group memberships for the different group types.
o Simplified description of Token reusage, compared to the unicast
case.
o More details on the rationale for response suppression.
o Clarifications of creation and management of security groups.
o Clients more knowledgeable than proxies about stopping receiving
responses.
o Cancellation of group observations.
o Clarification on multicast scope to use.
o Both the group mode and pairwise mode of Group OSCORE are
considered.
o Updated security considerations.
o Editorial improvements.
Acknowledgments
The authors sincerely thank Thomas Fossati and Jim Schaad for their
comments and feedback.
The work on this document has been partly supported by VINNOVA and
the Celtic-Next project CRITISEC; and by the H2020 project SIFIS-Home
(Grant agreement 952652).
Authors' Addresses
Esko Dijk
IoTconsultancy.nl
\________________\
Utrecht
Netherlands
Email: esko.dijk@iotconsultancy.nl
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Chonggang Wang
InterDigital
1001 E Hector St, Suite 300
Conshohocken PA 19428
United States
Email: Chonggang.Wang@InterDigital.com
Marco Tiloca
RISE AB
Isafjordsgatan 22
Kista SE-16440 Stockholm
Sweden
Email: marco.tiloca@ri.se
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