Internet DRAFT - draft-ietf-core-groupcomm-bis
draft-ietf-core-groupcomm-bis
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: 25 April 2024 RISE AB
23 October 2023
Group Communication for the Constrained Application Protocol (CoAP)
draft-ietf-core-groupcomm-bis-10
Abstract
This document specifies the use of the Constrained Application
Protocol (CoAP) for group communication, including the use of UDP/IP
multicast as the default 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 devices or networks that support CoAP. This
document replaces and obsoletes RFC 7390, while it updates RFC 7252
and RFC 7641.
Discussion Venues
This note is to be removed before publishing as an RFC.
Discussion of this document takes place on the CORE Working Group
mailing list (core@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/core/
(https://mailarchive.ietf.org/arch/browse/core/).
Source for this draft and an issue tracker can be found at
https://github.com/core-wg/groupcomm-bis (https://github.com/core-wg/
groupcomm-bis).
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Dijk, et al. Expires 25 April 2024 [Page 1]
Internet-Draft Group Communication for CoAP October 2023
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 25 April 2024.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
1.3. Changes to Other Documents . . . . . . . . . . . . . . . 6
2. Group Definition and Group Configuration . . . . . . . . . . 7
2.1. Group Definition . . . . . . . . . . . . . . . . . . . . 7
2.1.1. CoAP Group . . . . . . . . . . . . . . . . . . . . . 8
2.1.2. Application Group . . . . . . . . . . . . . . . . . . 8
2.1.3. Security Group . . . . . . . . . . . . . . . . . . . 9
2.1.4. Relations Between Group Types . . . . . . . . . . . . 9
2.2. Group Configuration . . . . . . . . . . . . . . . . . . . 12
2.2.1. Group Naming . . . . . . . . . . . . . . . . . . . . 12
2.2.2. Group Creation and Membership . . . . . . . . . . . . 17
2.2.3. Group Discovery . . . . . . . . . . . . . . . . . . . 18
2.2.4. Group Maintenance . . . . . . . . . . . . . . . . . . 22
3. CoAP Usage in Group Communication . . . . . . . . . . . . . . 23
3.1. Request/Response Model . . . . . . . . . . . . . . . . . 23
3.1.1. General . . . . . . . . . . . . . . . . . . . . . . . 23
3.1.2. Response Suppression . . . . . . . . . . . . . . . . 23
3.1.3. Repeating a Request . . . . . . . . . . . . . . . . . 24
3.1.4. Request/Response Matching and Distinguishing
Responses . . . . . . . . . . . . . . . . . . . . . . 24
3.1.5. Token Reuse . . . . . . . . . . . . . . . . . . . . . 25
3.1.6. Client Handling of Multiple Responses With Same
Token . . . . . . . . . . . . . . . . . . . . . . . . 26
Dijk, et al. Expires 25 April 2024 [Page 2]
Internet-Draft Group Communication for CoAP October 2023
3.2. Caching . . . . . . . . . . . . . . . . . . . . . . . . . 27
3.2.1. Freshness Model . . . . . . . . . . . . . . . . . . . 28
3.2.2. Validation Model . . . . . . . . . . . . . . . . . . 28
3.3. URI Path Selection . . . . . . . . . . . . . . . . . . . 29
3.4. Port Selection for UDP Transport . . . . . . . . . . . . 30
3.5. Proxy Operation . . . . . . . . . . . . . . . . . . . . . 30
3.5.1. Forward-Proxies . . . . . . . . . . . . . . . . . . . 30
3.5.2. Reverse-Proxies . . . . . . . . . . . . . . . . . . . 32
3.5.3. Single Group Request to Multiple Proxies . . . . . . 34
3.6. Congestion Control . . . . . . . . . . . . . . . . . . . 35
3.7. Observing Resources . . . . . . . . . . . . . . . . . . . 37
3.8. Block-Wise Transfer . . . . . . . . . . . . . . . . . . . 39
3.9. Transport Protocols . . . . . . . . . . . . . . . . . . . 39
3.9.1. UDP/IPv6 Multicast Transport . . . . . . . . . . . . 40
3.9.2. UDP/IPv6 Multicast Transport over 6LoWPAN . . . . . . 40
3.9.3. UDP/IPv4 Multicast Transport . . . . . . . . . . . . 41
3.9.4. TCP, TLS, and WebSockets . . . . . . . . . . . . . . 41
3.9.5. Other Transports . . . . . . . . . . . . . . . . . . 42
3.10. Interworking with Other Protocols . . . . . . . . . . . . 42
3.10.1. MLD/MLDv2/IGMP/IGMPv3 . . . . . . . . . . . . . . . 42
3.10.2. RPL . . . . . . . . . . . . . . . . . . . . . . . . 42
3.10.3. MPL . . . . . . . . . . . . . . . . . . . . . . . . 43
4. Unsecured Group Communication . . . . . . . . . . . . . . . . 44
5. Secured Group Communication using Group OSCORE . . . . . . . 45
5.1. Group OSCORE . . . . . . . . . . . . . . . . . . . . . . 45
5.2. Secure Group Maintenance . . . . . . . . . . . . . . . . 47
5.3. Proxy Security . . . . . . . . . . . . . . . . . . . . . 48
6. Security Considerations . . . . . . . . . . . . . . . . . . . 49
6.1. CoAP NoSec Mode . . . . . . . . . . . . . . . . . . . . . 49
6.2. Group OSCORE . . . . . . . . . . . . . . . . . . . . . . 50
6.2.1. Group Key Management . . . . . . . . . . . . . . . . 51
6.2.2. Source Authentication . . . . . . . . . . . . . . . . 52
6.2.3. Countering Attacks . . . . . . . . . . . . . . . . . 52
6.3. Risk of Amplification . . . . . . . . . . . . . . . . . . 55
6.4. Replay of Non-Confirmable Messages . . . . . . . . . . . 57
6.5. Use of CoAP No-Response Option . . . . . . . . . . . . . 57
6.6. 6LoWPAN and MPL . . . . . . . . . . . . . . . . . . . . . 58
6.7. Wi-Fi . . . . . . . . . . . . . . . . . . . . . . . . . . 59
6.8. Monitoring . . . . . . . . . . . . . . . . . . . . . . . 59
6.8.1. General Monitoring . . . . . . . . . . . . . . . . . 59
6.8.2. Pervasive Monitoring . . . . . . . . . . . . . . . . 59
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 60
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 60
8.1. Normative References . . . . . . . . . . . . . . . . . . 60
8.2. Informative References . . . . . . . . . . . . . . . . . 62
Appendix A. Use Cases . . . . . . . . . . . . . . . . . . . . . 66
A.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 66
A.1.1. Distributed Device Discovery . . . . . . . . . . . . 67
Dijk, et al. Expires 25 April 2024 [Page 3]
Internet-Draft Group Communication for CoAP October 2023
A.1.2. Distributed Service Discovery . . . . . . . . . . . . 67
A.1.3. Directory Discovery . . . . . . . . . . . . . . . . . 67
A.2. Operational Phase . . . . . . . . . . . . . . . . . . . . 68
A.2.1. Actuator Group Control . . . . . . . . . . . . . . . 68
A.2.2. Device Group Status Request . . . . . . . . . . . . . 68
A.2.3. Network-wide Query . . . . . . . . . . . . . . . . . 68
A.2.4. Network-wide / Group Notification . . . . . . . . . . 69
A.3. Software Update . . . . . . . . . . . . . . . . . . . . . 69
Appendix B. Examples of Group Naming for Application Groups . . 69
B.1. Group Naming using the URI Path Component . . . . . . . . 70
B.2. Group Naming using the URI Query Component . . . . . . . 71
B.3. Group Naming using the URI Authority Component . . . . . 71
B.4. Group Naming using the URI Host Subcomponent . . . . . . 72
B.5. Group Naming using the URI Port Subcomponent . . . . . . 72
B.6. Group Naming using a Custom CoAP Option . . . . . . . . . 73
Appendix C. Examples of Group Discovery from CoAP Servers . . . 73
C.1. Application Groups Associated with a CoAP Group . . . . . 73
C.2. Members of a Given Application Group . . . . . . . . . . 74
C.3. Members of any Application Group of a Given Type . . . . 75
C.4. Members of any Application Group in the Network . . . . . 76
Appendix D. Examples of Message Exchanges . . . . . . . . . . . 77
Appendix E. Document Updates . . . . . . . . . . . . . . . . . . 84
E.1. Version -09 to -10 . . . . . . . . . . . . . . . . . . . 84
E.2. Version -08 to -09 . . . . . . . . . . . . . . . . . . . 85
E.3. Version -07 to -08 . . . . . . . . . . . . . . . . . . . 85
E.4. Version -06 to -07 . . . . . . . . . . . . . . . . . . . 85
E.5. Version -05 to -06 . . . . . . . . . . . . . . . . . . . 86
E.6. Version -04 to -05 . . . . . . . . . . . . . . . . . . . 86
E.7. Version -03 to -04 . . . . . . . . . . . . . . . . . . . 87
E.8. Version -02 to -03 . . . . . . . . . . . . . . . . . . . 87
E.9. Version -01 to -02 . . . . . . . . . . . . . . . . . . . 87
E.10. Version -00 to -01 . . . . . . . . . . . . . . . . . . . 88
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 88
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 88
1. Introduction
This document specifies group communication using the Constrained
Application Protocol (CoAP) [RFC7252], together with UDP/IP multicast
as the default transport for CoAP group communication messages. 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
Dijk, et al. Expires 25 April 2024 [Page 4]
Internet-Draft Group Communication for CoAP October 2023
response message back to the client over UDP/IP unicast. Notable
CoAP implementations that support group communication include
"Eclipse Californium" [Californium], "Go-CoAP" [Go-CoAP] as well as
"libcoap" [libcoap].
Both unsecured and secured CoAP group communication 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)
[RFC9052][RFC9053].
This document replaces and obsoletes [RFC7390], while it updates both
[RFC7252] and [RFC7641]. A summary of the changes and additions to
these documents is provided in Section 1.3.
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 those solutions that use CoAP messages
over a "one-to-many" (i.e., non-unicast) transport protocol are in
the scope of this document. There are alternative methods to achieve
group communication using CoAP, using unicast only. One example is
Publish-Subscribe [I-D.ietf-core-coap-pubsub] which uses a central
broker server that CoAP clients access via unicast communication.
These alternative methods may be usable for the same or similar use
cases as the ones targeted in this document.
This document defines UDP/IP multicast as the default transport
protocol for CoAP group requests, as in [RFC7252]. Other transport
protocols (which may include broadcast, non-IP multicast, geocast,
etc.) are not described in detail and are not considered. Although
UDP/IP multicast transport is assumed in most of the text in this
document, we expect many of the considerations for UDP/IP multicast
can be re-used for alternative transport protocols.
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 left for
future work. General principles for secure group configuration are
in scope.
Dijk, et al. Expires 25 April 2024 [Page 5]
Internet-Draft Group Communication for CoAP October 2023
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.
This specification requires readers to be familiar with CoAP
terminology [RFC7252]. Terminology related to group communication is
defined in Section 2.1.
In addition, the following terms are extensively used.
* Group URI -- This is defined as a CoAP URI that has the "coap"
scheme and includes in the authority component 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 can contain a UDP port number
in the authority component. Group URIs follow the regular CoAP
URI syntax (see Section 6 of [RFC7252]).
* Security material -- This refers to any security keys, counters,
or parameters stored in a device that are required to participate
in secure group communication with other devices.
1.3. Changes to Other Documents
This document obsoletes and replaces [RFC7390] as follows.
* It provides separate definitions for CoAP groups, application
groups, and security groups, together with high-level guidelines
on their configuration (see Section 2).
* It defines the use of Group OSCORE
[I-D.ietf-core-oscore-groupcomm] as the security protocol to
protect group communication for CoAP, together with high-level
guidelines on secure group maintenance (see Section 5).
* It updates all the guidelines about using group communication for
CoAP (see Section 3).
* It strongly discourages unsecured group communication for CoAP
based on the CoAP NoSec (No Security) mode (see Section 4 and
Section 6.1), and highlights the risk of amplification attacks
(see Section 6.3).
Dijk, et al. Expires 25 April 2024 [Page 6]
Internet-Draft Group Communication for CoAP October 2023
* It updates all sections on transport protocols and interworking
with other protocols based on new IETF work done for these
protocols.
This document updates [RFC7252] as follows.
* It updates the request/response model for group communication, as
to response suppression (see Section 3.1.2) and token reuse time
(see Section 3.1.5).
* It updates the freshness model and validation model to use for
cached responses (see Section 3.2.1 and Section 3.2.2).
* It defines the measures against congestion risk specified in
[RFC7252] to be applicable also to alternative transports other
than IP multicast, and defines additional guidelines to reduce
congestion risks (see Section 3.6).
* It explicitly admits the use of the IPv6 multicast address scopes
realm-local (3), admin-local (4), and global (E). In particular,
it recommends that an IPv6 CoAP server supports at least link-
local (2), admin-local (4), and site-local (5) scopes with the
"All CoAP Nodes" multicast CoAP group (see Section 3.9.1). Also,
it recommends that the realm-local (3) scope is supported by an
IPv6 CoAP server on a 6LoWPAN node (see Section 3.9.1).
This document updates [RFC7641] as follows.
* It defines the use of the CoAP Observe Option in group requests,
for both the GET method and the FETCH method [RFC8132], together
with normative behavior for both CoAP clients and CoAP servers
(see Section 3.7).
2. Group Definition and Group Configuration
In the following, different group types are first defined in
Section 2.1. Then, Group configuration, including group creation and
maintenance by an application, user, or commissioning entity is
considered in Section 2.2.
2.1. Group Definition
Three types of groups and their mutual relations are defined in this
section: CoAP group, application group, and security group.
Dijk, et al. Expires 25 April 2024 [Page 7]
Internet-Draft Group Communication for CoAP October 2023
2.1.1. CoAP Group
A CoAP group is defined as a set of CoAP endpoints, where each
endpoint is configured to receive CoAP group messages that are sent
to the group's associated IP multicast address and UDP port. That
is, CoAP groups have relevance at the level of IP networks and CoAP
endpoints.
An endpoint may be a member of multiple CoAP groups, by subscribing
to multiple IP multicast addresses. A node may be a member of
multiple CoAP groups, by hosting multiple CoAP server endpoints on
different UDP ports. Membership(s) of an endpoint or node to a CoAP
group may dynamically change over time. A node or endpoint sending a
CoAP group message to a CoAP group is not necessarily itself a member
of that CoAP group: it is a member only if it also has a CoAP
endpoint listening on the associated IP multicast address and UDP
port associated with the CoAP group.
A CoAP group is identified by information encoded within a group URI.
Further details on identifying a CoAP group are provided in
Section 2.2.1.1.
2.1.2. Application Group
An application group is a set of CoAP server endpoints (hosted on
different nodes) that share a common set of CoAP resources. That is,
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.
An endpoint may be a member of multiple application groups. 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). Such relations are discussed in more
detail in Section 2.1.4.
An application group name may be explicitly encoded in the group URI
of a CoAP request, for example in the URI path component. If this is
not the case, the application group is implicitly derived by the
receiver, e.g., based on information in the CoAP request or other
contextual information. Further details on identifying an
application group are provided in Section 2.2.1.2.
Dijk, et al. Expires 25 April 2024 [Page 8]
Internet-Draft Group Communication for CoAP October 2023
2.1.3. Security Group
For secure group communication, a security group is required. A
security group comprises endpoints storing shared group security
material, such that they can use it to protect and verify mutually
exchanged messages.
That is, a client endpoint needs to be a member of a security group
in order to send a valid secured group communication message to that
group. A server endpoint needs to be a member of a security group in
order to receive and correctly verify a secured group communication
message sent to that group. An endpoint may be a member of multiple
security groups.
There can be a many-to-many relation between security groups and CoAP
groups, but often it is one-to-one. Also, there can be a many-to-
many relation between security groups and application groups, but
often it is one-to-one. Such relations are discussed in more detail
in Section 2.1.4.
Further details on identifying a security group are provided in
Section 2.2.1.3.
If the NoSec mode is used (see Section 4), group communication does
not rely on security at the transport layer nor at the CoAP layer,
hence the communicating endpoints do not refer to a security group.
2.1.4. Relations Between Group Types
Using the above group type definitions, a CoAP group communication
message sent by an endpoint can be associated with 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. This has the
benefit that typically less storage, configuration, and updating are
required for security material. In this case, a CoAP endpoint is
supposed to know the exact application group to refer to for each
message that is sent or received, based on, e.g., the server port
number used, the targeted resource, or the content and structure of
the message payload.
Dijk, et al. Expires 25 April 2024 [Page 9]
Internet-Draft Group Communication for CoAP October 2023
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 cryptographic 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 careful in
associating a single application group to multiple security groups.
In particular, it is NOT RECOMMENDED to use different security groups
to reflect different access policies for resources in the same
application group.
In fact, being a member of a security group actually grants access
only to exchange secured messages and enables authentication of group
members, while access control (authorization) to use resources in the
application group belongs to a separate security domain. Therefore,
access control to use resources in the application group should 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 Unified Modeling Language (UML) class
diagram notation. The class attributes in square brackets are
optionally defined.
Dijk, et al. Expires 25 April 2024 [Page 10]
Internet-Draft Group Communication for CoAP October 2023
+------------------------------+ +--------------------+
| Application group | | CoAP group |
|..............................| |....................|
| | | |
| [ - Application group name ] +--------------+ - IP mcast address |
| | 1...N 1 | - UDP port number |
| | | |
| | | |
+-------------+----------------+ +---------+----------+
| 1...N | 1...N
| |
| |
| | 1...N
| +------------+----------+
| | Security group |
| |.......................|
| | |
'----------------------------+ - Security group name |
1...N | - Security material |
| |
+-----------------------+
Figure 1: Relations 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). Although in real-life
deployments using group communication the number of servers and
resources would usually be higher, only limited numbers are shown
here for ease of representation.
There are three application groups (1, 2, 3) and two security groups
(1, 2). The Security Group 1 may, for example, include all lighting
devices on a floor of an office building, while Security Group 2
includes all Heating, Ventilation, and Air Conditioning (HVAC)
devices of that floor. Security Group 1 is used by both Application
Group 1 and 2. The Application Group 1 for example may consist of
all lights in a hallway, while Application Group 2 includes all
lights in a storage room. Three clients (Cli1, Cli2, Cli3) are
configured with security material for Security Group 1. These
clients may be motion sensors and a control panel (Cli3), that send
multicast messages to /resA to inform the lights of any motion or
user activity detected. The control panel Cli3 additionally sends a
multicast message to /resB to communicate the latest light preset
selected by a user. The latter action only influences the lighting
in the storage room (Application Group 2). Two clients (Cli2, Cli4)
are configured with security material for Security Group 2. These
clients may be temperature/humidity sensors that report measurements
Dijk, et al. Expires 25 April 2024 [Page 11]
Internet-Draft Group Communication for CoAP October 2023
periodically to all HVAC devices (Srv5, Srv6) in the Application
Group 3, using for example /resC to report temperature and /resD to
report humidity.
All the shown application groups may use the same CoAP group (not
shown in the figure), for example the CoAP group with site-local,
site-specific multicast IP address ff15::3456 and default UDP port
number 5683 on which all the shown resources are hosted for each
server. Other floors of the same building may replicate the shown
structure, but using different security groups and different CoAP
groups.
.------------------------------. .-----------------------------.
| | | |
| +---------------------+ | | +---------------------+ |
| | 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
The following defines how groups of different types are named,
created, discovered, and maintained.
2.2.1. Group Naming
Different types of groups are named as specified below, separately
for CoAP groups, application groups, and security groups.
2.2.1.1. CoAP Groups
A CoAP group is always defined by the two properties of IP multicast
address and UDP port number (see Section 2.1.1).
Dijk, et al. Expires 25 April 2024 [Page 12]
Internet-Draft Group Communication for CoAP October 2023
However, a CoAP group is for practical purposes identified and named
by the authority component in the group URI. This component includes
the host subcomponent and an optional UDP port number. The host
subcomponent directly defines the IP multicast address of the CoAP
group, in case the host consists of an IP literal. The host
subcomponent indirectly defines the IP multicast address of the CoAP
group, in case the host consists of a hostname: resolving the
hostname to an IP address in this case produces the IP multicast
address.
It follows that the same CoAP group might have multiple names, which
can be simultaneously and interchangeably used. For example, if the
two hostnames group1.com and group1.alias.com both resolve to the IP
multicast address [ff15::1234], then the following authority
components are all names for the same CoAP group.
* group1.example:7700
* group1.alias.example:7700
* [ff15::1234]:7700
Also note that, when using the "coap" scheme, the two authority
components <HOST> and <HOST>:5683 both identify the same CoAP group,
whose members listen to the CoAP default port number 5683.
Therefore, building on the above, the following authority components
are all names for the same CoAP group.
* group1.example
* group1.alias.example
* [ff15::1234]
* group1.example:5683
* group1.alias.example:5683
* [ff15::1234]:5683
When configuring a CoAP group membership, it is recommended to
configure an endpoint with an IP multicast address literal, instead
of a group hostname. 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.
Dijk, et al. Expires 25 April 2024 [Page 13]
Internet-Draft Group Communication for CoAP October 2023
Examples of hierarchical CoAP group FQDN naming (and scoping) for a
building control application were shown in Section 2.2 of [RFC7390].
2.2.1.2. Application Groups
An application group can be named in many ways through different
types of identifiers, such as name string, (integer) number, URI, or
other types of strings. The decision of whether and how exactly an
application group name is encoded and transported is application
specific.
The following discusses a number of possible methods to use, while
full examples for the different methods are provided in Appendix B.
An application group name can be explicitly encoded in a group URI.
In such a case, it can be encoded within one of the following URI
components.
* URI path component -- This is the most common and RECOMMENDED
method to encode the application group name. When using this
method in constrained networks, an application group name APPNAME
should be kept short.
A best practice for doing so is to use a URI path component such
that: i) it includes a path segment as delimiter with a designated
value, e.g., "gp", followed by ii) a path segment with value the
name of the application group, followed by iii) the path
segment(s) that identify the targeted resource within the
application group. For example, both /gp/APPNAME/res1 and
/base/gp/APPNAME/res1/res2 conform to this practice. Just like
application group names, the path segment used as delimiter should
be kept short in constrained networks.
Full examples are provided in Appendix B.1.
* URI query component -- This method can use the following formats.
In either case, when using this method in constrained networks, an
application group name APPNAME should be as short as possible.
- As a first alternative, the URI query component consists of
only one parameter, which has no value and has the name of the
application group as its own identifier. That is, the query
component ?APPNAME conforms to this format.
Dijk, et al. Expires 25 April 2024 [Page 14]
Internet-Draft Group Communication for CoAP October 2023
- As a second alternative, the URI query component includes a
query parameter as designated indicator, e.g., "gp", with value
the name of the application group. That is, assuming "gp" to
be used as designated indicator, both the query components
?gp=APPNAME and ?par1=v1&gp=APPNAME conform to this format.
Full examples are provided in Appendix B.2.
* URI authority component -- If this method is used, the application
group is identified by the authority component as a whole.
In particular, the application group has the same name of the CoAP
group expressed by the group URI (see Section 2.2.1.1). Thus,
this method can only be used if there is a one-to-one mapping
between CoAP groups and application groups (see Section 2.1.4).
While the host component of the Group URI can be a group hostname,
an implementation would likely rather use an IP address literal,
in order to reduce the size of the CoAP request. In particular,
the Uri-Host Option can be fully elided in this case.
A full example is provided in Appendix B.3.
* URI host subcomponent -- If this method is used, the application
group is identified solely by the host subcomponent of the
authority component.
Since an application group can be associated with only one CoAP
group (see Section 2.1.4), using this method implies that, given
any two CoAP groups, the port subcomponent of the URI authority
component MUST NOT be the only information distinguishing them.
Like for the previous case relying on the whole URI authority
component, an implementation would likely use an IP address
literal rather than the group hostname as host component of the
Group URI, in order to reduce the size of the CoAP request. In
particular, the Uri-Host Option can be fully elided in this case.
A full example is provided in Appendix B.4.
* URI port subcomponent -- By using this method, the application
group is uniquely identified by the destination port number
encoded in the port subcomponent of the authority component.
Since an application group can be associated with only one CoAP
group (see Section 2.1.4), using this method implies that any two
CoAP groups cannot differ only by their host subcomponent of the
URI authority component.
Dijk, et al. Expires 25 April 2024 [Page 15]
Internet-Draft Group Communication for CoAP October 2023
A full example is provided in Appendix B.5.
Alternatively, there are also methods to encode the application group
name within the CoAP request, even though it is not encoded within
the group URI. An example of such a method is summarized below.
* The application group name can be encoded in a new (e.g., custom,
application-specific) CoAP Option, which the client adds to the
CoAP request before sending it out.
Upon receiving the request as a member of the targeted CoAP group,
each CoAP server would, by design, understand this Option, decode
it, and treat the result as an application group name.
A full example is provided in Appendix B.6.
Furthermore, it is possible to encode the application group name
neither in the group URI nor within a CoAP request, thus yielding the
most compact representation on the wire. In this case, each CoAP
server needs to determine the right application group based on
contextual information, such as the client identity and/or the target
resource. For example, each application group on a server could
support a unique set of resources, such that it does not overlap with
the set of resources of any other application group.
Finally, Appendix A of [RFC9176] provides an example of an
application group registered to a Resource Directory (RD), along with
the CoAP group it uses and the resources it supports. In that
example, an application group name "lights" is encoded in the "ep"
(endpoint) attribute of the RD registration entry, while the CoAP
group ff35:30:2001:db8:f1::8000:1 is specified in the authority
component of the URI encoded in the "base" attribute.
2.2.1.3. Security Groups
A security group can be named in many ways through different types of
identifiers, such as name string, (integer) number, URI, or other
types of strings. Such a group name is generally not related to
other kinds of group identifiers that may be specific of the used
security solution.
The name of a security group is not expected to be used in messages
exchanged among its members, unless the application requires
otherwise. At the same time, it is useful to identify the security
group when performing a number of side tasks related to secure group
communication, such as the following ones.
Dijk, et al. Expires 25 April 2024 [Page 16]
Internet-Draft Group Communication for CoAP October 2023
* An administrator may have to request for an authorization to
configure security groups at an available Group Manager (see
Section 5). During the authorization process, as well as during
the interaction between the administrator and the Group Manager,
the group name identifies the specific security group that the
administrator wishes to configure and is authorized to.
* A CoAP endpoint may have to request for an authorization to join a
specific security group through the respective Group Manager, and
thus obtain the required group security material (see Section 5).
During the authorization process, as well as during the
interaction between the CoAP endpoint and the Group Manager, the
group name identifies the specific security group that the CoAP
endpoint wishes to join and is authorized to.
* A CoAP endpoint may first need to discover the specific security
groups to join through the respective Group Manager (see
Section 2.2.3.1). Results from the discovery process include the
name of the security groups to join, together with additional
information such as a pointer to the respective Group Manager.
It is discouraged to use "NoSec" and any of its lowercase/uppercase
combinations as name of a security group. Indications that endpoints
can use the NoSec mode MUST NOT rely on setting up and advertising a
pseudo security group with name "NoSec" or any of its lowercase/
uppercase combinations.
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 number 5683. Then, it
configures one or more devices as listeners to that IP multicast
address, with a CoAP endpoint listening on the CoAP 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 CoAP 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 group
URI.
The IETF does not define a mandatory protocol to accomplish CoAP
group creation. [RFC7390] defined an experimental protocol for
configuring memberships of CoAP groups for unsecured group
communication, based on JSON-formatted configuration resources, and
Dijk, et al. Expires 25 April 2024 [Page 17]
Internet-Draft Group Communication for CoAP October 2023
the experiment is concluded. For 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 a set of resources on CoAP endpoints, such that a
CoAP request sent to a group URI 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 members of the application group.
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 cryptographic algorithms and
parameters used in the security 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 5. After that, further security materials to
protect group communications have to be generated, compatible with
the configuration specified for the security group.
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 5.
For unsecure group communication using the NoSec mode (see
Section 4), there is no security material to be provided, hence there
is no security group for CoAP endpoints to participate in.
The configuration of groups and membership may be performed at
different moments in the life-cycle of a device. For example, it can
occur 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
The following describes how a CoAP endpoint can discover groups by
different means, i.e., by using a Resource Directory or directly from
the CoAP servers that are members of such groups.
Dijk, et al. Expires 25 April 2024 [Page 18]
Internet-Draft Group Communication for CoAP October 2023
2.2.3.1. Discovery through a Resource Directory
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 [RFC9176].
In particular, an application group can be registered to the RD,
specifying the reference IP multicast address of its associated CoAP
group. The registration of groups to the RD is typically performed
by a Commissioning Tool. Later on, CoAP endpoints can discover the
registered application groups and related CoAP group(s), by using the
lookup interface of the RD.
When secure communication is provided with Group OSCORE (see
Section 5), the approach described in
[I-D.tiloca-core-oscore-discovery] 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
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 names of 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.
2.2.3.2. Discovery from the CoAP Servers
It is possible for CoAP endpoints to discover application groups and
CoAP groups from the CoAP servers that are members of such groups, by
using a GET request targeting the /.well-known/core resource.
As discussed below, such a GET request may be sent to the IP
multicast address of an already known CoAP group associated with one
or more application groups; or to the "All CoAP Nodes" multicast
address, thus targeting all reachable CoAP servers in any CoAP group.
Also, the GET request may specify a query component, in order to
filter the application groups of interest.
These particular details concerning the GET request depend on the
specific discovery action intended by the client and on application-
specific means used to encode names of application groups and CoAP
groups, e.g., in group URIs and/or CoRE target attributes used with
resource links.
The following discusses a number of methods to discover application
groups and CoAP groups, building on the following assumptions.
First, application group names are encoded in the path component of
Dijk, et al. Expires 25 April 2024 [Page 19]
Internet-Draft Group Communication for CoAP October 2023
Group URIs (see Section 2.2.1.2), using the path segment "gp" as
designated delimiter. Second, the type of an application group is
encoded in the CoRE Link Format attribute "rt" of a group resource
with a value "g.<GROUPTYPE>".
Full examples for the different methods are provided in Appendix C.
* A CoAP client can discover all the application groups associated
with a specific CoAP group.
This is achieved by sending the GET request above to the IP
multicast address of the CoAP group, and specifying a wildcarded
group type "g.*" as resource type in the URI query parameter "rt".
For example, the request can use a Group URI with path and query
components "/.well-known/core?rt=g.*", so that the query matches
any application group resource type. Alternatively, the request
can use a Group URI with path and query components "/.well-known/
core?href=/gp/*", so that the query matches any application group
resources and also matches any sub-resources of those.
Through the corresponding responses, the query result is a list of
resources at CoAP servers that are members of the specified CoAP
group and have at least one application group associated with the
CoAP group. That is, the client gains knowledge of: i) the set of
servers that are members of the specified CoAP group and member of
any of the associated application groups; ii) for each of those
servers, the name of the application groups where the server is a
member and that are associated with the CoAP group.
A full example is provided in Appendix C.1.
* A CoAP client can discover the CoAP servers that are members of a
specific application group, the CoAP group associated with the
application group, and optionally the resources that those servers
host for each application group.
This is achieved by sending the GET request above to the "All CoAP
Nodes" IP multicast address (see Section 12.8 of [RFC7252]), with
a particular chosen scope (e.g., site-local or realm-local) if
IPv6 is used. Also, the request specifies the application group
name of interest in the URI query component, as defined in
Section 2.2.1.2. For example, the request can use a Group URI
with path and query components "/.well-known/core?href=/gp/gp1" to
specify the application group with name "gp1".
Through the corresponding responses, the query result is a list of
resources at CoAP servers that are members of the specified
application group and for each application group the associated
Dijk, et al. Expires 25 April 2024 [Page 20]
Internet-Draft Group Communication for CoAP October 2023
CoAP group. That is, the client gains knowledge of: i) the set of
servers that are members of the specified application group and of
the associated CoAP group; ii) for each of those servers,
optionally the resources it hosts within the application group.
If the client wishes to discover resources that a particular
server hosts within a particular application group, it may use
unicast discovery request(s) to this server.
A full example is provided in Appendix C.2.
* A CoAP client can discover the CoAP servers that are members of
any application group of a specific type, the CoAP group
associated with those application groups, and optionally the
resources that those servers host as members of those application
groups.
This is achieved by sending the GET request above to the "All CoAP
Nodes" IP multicast address (see Section 12.8 of [RFC7252]), with
a particular chosen scope (e.g., site-local or realm-local) if
IPv6 is used. Also, the request can specify the application group
type of interest in the URI query component as value of a query
parameter "rt". For example, the request can use a Group URI with
path and query components "/.well-known/core?rt=TypeA" to specify
the application group type "TypeA".
Through the corresponding responses, the query result is a list of
resources at CoAP servers that are members of any application
group of the specified type and of the CoAP group associated with
each of those application groups. That is, the client gains
knowledge of: i) the set of servers that are members of the
application groups of the specified type and of the associated
CoAP group; ii) optionally for each of those servers, the
resources it hosts within each of those application groups.
If the client wishes to discover resources that a particular
server hosts within a particular application group, it may use
unicast discovery request(s) to this server.
A full example is provided in Appendix C.3.
* A CoAP client can discover the CoAP servers that are members of
any application group configured in the 6LoWPAN wireless mesh
network of the client, the CoAP group associated with each
application group, and optionally the resources that those servers
host as members of the application group.
Dijk, et al. Expires 25 April 2024 [Page 21]
Internet-Draft Group Communication for CoAP October 2023
This is achieved by sending the GET request above with a query
specifying a wildcarded group type in the URI query parameter for
"rt". For example, the request can use a Group URI with path and
query components "/.well-known/core?rt=g.*", so that the query
matches any application group type. The request is sent to the
"All CoAP Nodes" IP multicast address (see Section 12.8 of
[RFC7252]), with a particular chosen scope if IPv6 is used.
Through the corresponding responses, the query result is a list of
group resources hosted by any server in the mesh network. Each
group resource denotes one application group membership of a
server. For each application group, the associated CoAP group is
obtained as the URI authority component of the corresponding
returned link.
If the client wishes to discover resources that a particular
server hosts within a particular application group, it may use
unicast discovery request(s) to this server.
Full examples are provided in Appendix C.4.
Note that the specific way of using the above methods, including the
ways shown by the examples in Appendix C.4, is application-specific.
That is, there is currently no standard way of encoding names of
application groups and CoAP groups in group URIs and/or CoRE target
attributes used with resource links. In particular, the discovery of
application groups and CoAP groups through the RD mentioned in
Section 2.2.3.1 is only defined for use with an RD, i.e., not
directly with CoAP servers as group members.
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 number 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 4), i.e., when the
NoSec mode is used, 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 5), the maintenance
operations of the protocol Group OSCORE
[I-D.ietf-core-oscore-groupcomm] MUST be implemented as well. When
using Group OSCORE, CoAP endpoints participating in group
Dijk, et al. Expires 25 April 2024 [Page 22]
Internet-Draft Group Communication for CoAP October 2023
communication are also members of a corresponding OSCORE security
group, and thus share common security material. Additional related
maintenance operations are discussed in Section 5.2.
3. CoAP Usage in Group Communication
This section specifies the usage of CoAP in group communication, both
unsecured and secured. This includes additional support for protocol
extensions, such as Observe (see Section 3.7) and block-wise transfer
(see Section 3.8).
How CoAP group messages are carried over various transport layers is
the subject of Section 3.9. Finally, Section 3.10 covers the
interworking of CoAP group communication with other protocols that
may operate in the same network.
3.1. Request/Response Model
3.1.1. General
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 number of the request.
All CoAP requests that are sent via IP multicast MUST be Non-
confirmable, see 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 a CoAP group request. The
unicast responses received by the CoAP client may carry a mixture of
success (e.g., 2.05 (Content)) and failure (e.g., 4.04 (Not Found))
response codes, depending on the individual server processing
results.
3.1.2. Response Suppression
A server MAY suppress its response for various reasons given in
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.
Dijk, et al. Expires 25 April 2024 [Page 23]
Internet-Draft Group Communication for CoAP October 2023
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 that a server supporting this option only takes it into
account when processing requests targeting selected resources, as
useful in the application context.
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.
3.1.3. Repeating a Request
A CoAP client MAY repeat a group request using the same Token value
and same Message ID 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 members of the CoAP group 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 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 repeat a group 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 members of the CoAP group, 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).
3.1.4. Request/Response Matching and Distinguishing Responses
A CoAP client can distinguish the origin of multiple server responses
by the source IP address of the message containing the CoAP response
and/or any other available application-specific source identifiers
contained in the CoAP response payload or CoAP response options, such
as an application-level unique ID associated with the server. If
secure communication is provided with Group OSCORE (see Section 5),
additional security-related identifiers in the CoAP response enable
Dijk, et al. Expires 25 April 2024 [Page 24]
Internet-Draft Group Communication for CoAP October 2023
the client to retrieve the right security material for decrypting
each response and authenticating its source.
While processing a response on the client, the source endpoint of the
response is not matched to the destination endpoint of the request,
since for a group request these will never match. This is specified
in Section 8.2 of [RFC7252], with reference to IP multicast.
Also, when UDP transport is used, this implies that a server MAY
respond from a UDP port number that differs from the destination UDP
port number of the request, although a CoAP server normally SHOULD
respond from the UDP port number that equals the destination port
number 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
number of the response MUST match to the client endpoint's UDP port
number, i.e., to the UDP source port number of the client's request.
3.1.5. Token Reuse
For CoAP group 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 [RFC9175]. Since for CoAP group
requests 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 reuse of Token values for different group
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
was done in [RFC7390], that is: the expected maximum response delay
over all servers that the client can send a CoAP group 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.
Dijk, et al. Expires 25 April 2024 [Page 25]
Internet-Draft Group Communication for CoAP October 2023
A preferred solution to meet this requirement is to generate a new
unique Token for every new group 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 MIN_TOKEN_REUSE_TIME = 500 seconds.
When securing CoAP group communication with Group OSCORE
[I-D.ietf-core-oscore-groupcomm], secure binding between requests and
responses is ensured (see Section 5). Thus, a client may reuse a
Token value after it has been freed up, as discussed above and
considering a reuse time greater than MIN_TOKEN_REUSE_TIME. If an
alternative security protocol for CoAP group communication is used
which does not ensure secure binding between requests and responses,
a client MUST follow the Token processing requirements as defined in
[RFC9175].
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.
3.1.6. Client Handling of Multiple Responses With Same Token
Since a client sending a group request with a Token T will accept
multiple responses with the same Token T, it is possible in
particular 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 acting out of specification as a malicious,
compromised or faulty server.
When this happens, the client normally processes at the CoAP layer
each of those responses to the same request coming from the same
server. If the processing of a response is successful, the client
delivers this response to the application as usual.
Then, the application is in a better position to decide what to do,
depending on the available context information. For instance, it
might accept and process all the responses from the same server, even
if they are not Observe notifications (i.e., they do not include an
Observe option). Alternatively, the application might accept and
process only one of those responses, such as the most recent one from
that server, e.g., when this can trigger a change of state within the
application.
Dijk, et al. Expires 25 April 2024 [Page 26]
Internet-Draft Group Communication for CoAP October 2023
As part of a long exchange between the client and any of the servers
in the CoAP group, the responses considered above are an example of
the more general concept elaborated in Section 2 of
[I-D.bormann-core-responses].
3.2. Caching
CoAP endpoints that are members of a CoAP group MAY cache responses
to a group request as defined in Section 5.6 of [RFC7252]. In
particular, these same rules apply to determine the set of request
options used as "Cache-Key".
Furthermore, building on what is defined in Section 8.2.1 of
[RFC7252]:
* A client sending a GET or FETCH group request MAY update a cache
with the responses from the servers in the CoAP group. Then, the
client uses both cached-still-fresh and new responses as the
result of the group request.
* A client sending a GET or FETCH group request MAY use a response
received from a server, to satisfy a subsequent sent request
intended to that server on the related unicast request URI. In
particular, the unicast request URI is obtained by replacing the
authority component of the request URI with the transport-layer
source address of the cached response message.
* A client MAY revalidate a cached response by making a GET or FETCH
request on the related unicast request URI.
Note that, in the presence of proxies, doing any of the above
(optional) unicast requests requires the client to distinguish the
different responses to a group request, as well as to distinguish the
different origin servers that responded. This in turn requires
additional means to provide the client with information about the
origin server of each response, e.g., using the forward-proxying
method defines in [I-D.tiloca-core-groupcomm-proxy].
The following subsections define the freshness model and validation
model to use for cached responses, which update the models defined in
Sections 5.6.1 and 5.6.2 of [RFC7252], respectively.
Dijk, et al. Expires 25 April 2024 [Page 27]
Internet-Draft Group Communication for CoAP October 2023
3.2.1. Freshness Model
For caching of group communication responses at client endpoints, the
same freshness model relying on the Max-Age Option as defined in
Section 5.6.1 of [RFC7252] applies, and the multicast caching rules
of Section 8.2.1 of [RFC7252] apply except for the one discussed
below.
In Section 8.2.1 of [RFC7252] it is stated that, regardless of the
presence of cached responses to the group request, the client
endpoint will always send out a new group request onto the network
because new members may have joined the CoAP group since the last
group request to the same CoAP group or resource. That is, a request
is never served from cached responses only. This document updates
[RFC7252] by adding the following exception case, where a client
endpoint MAY serve a request by using cached responses only, and not
send out a new group request onto the network:
* The client knows all current CoAP servers that are members of the
CoAP group; and, for each group member, the client's cache
currently stores a fresh response.
How the client in the case above determines the CoAP servers that are
currently members of the CoAP group is out of scope for this
document. It may be, for example, via a group manager server, or by
monitoring group joining protocol exchanges.
For caching at proxies, the freshness model defined in
[I-D.tiloca-core-groupcomm-proxy] can be used.
3.2.2. Validation Model
For validation of cached group communication responses at client
endpoints, the multicast validation rules in Section 8.2.1 of
[RFC7252] apply, except for the last paragraph which states "A GET
request to a multicast group MUST NOT contain an ETag option". This
document updates [RFC7252] by allowing a group request to contain
ETag Options as specified below.
For validation at proxies, the validation model defined in
[I-D.tiloca-core-groupcomm-proxy] can be used.
Dijk, et al. Expires 25 April 2024 [Page 28]
Internet-Draft Group Communication for CoAP October 2023
3.2.2.1. ETag Option in a Group Request/Response
A client endpoint MAY include one or more ETag Options in a GET or
FETCH group request, to validate one or more stored responses it has
cached. In case two or more servers in the CoAP group have responded
to a previous request to the same resource with an identical ETag
value, it is the responsibility of the client to handle this case.
In particular, if the client wishes to validate, using a group
request, a response from server 1 with an ETag value N, while it does
not wish to validate a response from server 2 with the same ETag
value N, there is no way to achieve this. In such cases where an
identical ETag value is returned by two or more servers, the client,
by default, SHOULD NOT include an ETag Option containing that ETag
value in a group request.
A server endpoint MUST process an ETag Option in a GET or FETCH group
request in the same way it processes an ETag Option for a unicast
request. A server endpoint that includes an ETag Option in a
response to a group request SHOULD construct the ETag Option value in
such a way that the value will be unique to this particular server
with a high probability. This practically prevents a collision of
the ETag values from different servers in the same CoAP group and
application group, which in turn allows the client to effectively
validate a particular response of an origin server. This can be
accomplished, for example, by embedding a compact ID of the server
within the ETag value, where the ID is unique (or unique with a high
probability) in the scope of the group.
Note: a legacy CoAP server might treat an ETag Option in a group
request as an unrecognized option per Sections 5.4 and 8.2.1 of
[RFC7252], causing it to ignore this (elective) ETag Option
regardless of its value, and process the request normally as if that
ETag Option was not included.
3.3. URI Path Selection
The URI Path used in a group request is preferably a path that is
known to be supported across all members of a CoAP group. However,
there are valid use cases where a group request is known to be
successful only for a subset of the CoAP group. For instance, the
subset may include only members of a specific application group,
while the members of the CoAP group for which the request is
unsuccessful (for example because they are outside the target
application group) either respond with an error status code or ignore
the group request (see also Section 3.1.2 on response suppression).
Dijk, et al. Expires 25 April 2024 [Page 29]
Internet-Draft Group Communication for CoAP October 2023
3.4. Port Selection for UDP Transport
A server that is a member of a CoAP group listens for CoAP request
messages on the group's IP multicast address, usually on the CoAP
default UDP port number 5683, or another non-default UDP port number
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 sending
group requests to that group.
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.
Also, a constrained network may be additionally burdened in this case
with multicast traffic that is eventually discarded at the UDP layer
by most nodes.
The port number 5684 is reserved for DTLS-secured unicast 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 number 5683 MUST be
supported (see Section 7.1 of [RFC7252]) for the "All CoAP Nodes"
CoAP group as detailed in Section 3.9.
3.5. Proxy Operation
This section defines how proxies operate in a group communication
scenario. In particular, Section 3.5.1 defines operations of
forward-proxies, while Section 3.5.2 defines operations of reverse-
proxies. Furthermore, Section 3.5.3 discusses the case where a
client sends a group request to multiple proxies at once. Security
operations for a proxy are discussed later in Section 5.3.
3.5.1. Forward-Proxies
CoAP enables a client to request a forward-proxy to process a CoAP
request on its behalf, as described in Sections 5.7.2 and 8.2.2 of
[RFC7252].
When intending to reach a CoAP group through a proxy, the client
sends a unicast CoAP group request to the proxy. The group URI where
the request has to be forwarded to is specified in the request,
Dijk, et al. Expires 25 April 2024 [Page 30]
Internet-Draft Group Communication for CoAP October 2023
either as a string in the Proxy-URI Option, or through the Proxy-
Scheme Option with the group URI constructed from the usual Uri-*
Options. Then, the forward-proxy resolves the group URI to a
destination CoAP group, i.e., it sends (e.g., multicasts) the CoAP
group request to the group URI, receives the responses and forwards
all the individual (unicast) responses back to the client.
However, there are certain issues and limitations with this approach:
* The CoAP client component that has sent the unicast CoAP group
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 forward the CoAP group request to
the group URI.
* 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.
* Unlike a CoAP client, the proxy is likely to lack "application
context". In particular, the proxy is not expected to know how
many members there are in the CoAP group (not even the order of
magnitude), how many members of that group will actually respond,
or the minimal amount/percentage of those that will respond.
Therefore, while still capable to forward the group request to the
CoAP group and the corresponding responses to the client, the
proxy does not know and cannot reliably determine for how long to
collect responses, before it stops forwarding them to the client.
In principle, a CoAP client that is not using a proxy might face
the same problems in collecting responses to a group request.
However, unlike a CoAP proxy, the client itself would typically
have application-specific rules or knowledge on how to handle this
situation. For example, a CoAP client could monitor incoming
responses and use this information to decide for how long to
continue collecting responses
A forward-proxying method using this approach and addressing the
issues raised above is defined in [I-D.tiloca-core-groupcomm-proxy].
Dijk, et al. Expires 25 April 2024 [Page 31]
Internet-Draft Group Communication for CoAP October 2023
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:
* Like for the approach discussed above, the proxy does not know for
how long to collect responses before sending back the aggregated
response to the client. Analogous considerations apply to this
approach too, both on the client and proxy side.
* 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
processes a request to be forwarded to a group URI only if it is
explicitly enabled to do so. If such functionality is not explicitly
enabled, the default response returned to the client is 5.01 Not
Implemented. Furthermore, a proxy SHOULD be explicitly configured
(e.g., by allow-listing and/or client authentication) to allow
proxied CoAP group requests only from specific client(s).
The operation of HTTP-to-CoAP proxies for multicast CoAP requests is
specified in Sections 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 may still be able to
identify the CoAP servers by other means such as application-specific
information in the response payload.
A forward-proxying method for HTTP-to-CoAP proxies addressing the
issues raised above is defined in [I-D.tiloca-core-groupcomm-proxy].
3.5.2. Reverse-Proxies
CoAP enables the use of a reverse-proxy, as an endpoint that stands
in for one or more other server(s), and satisfies requests on behalf
of these, doing any necessary translations (see Section 5.7.3 of
[RFC7252]).
Dijk, et al. Expires 25 April 2024 [Page 32]
Internet-Draft Group Communication for CoAP October 2023
In a group communication scenario, a reverse-proxy can rely on its
configuration and/or on information in a request from a client, in
order to determine that a group request has to be sent to servers in
a CoAP group, over a one-to-many transport such as IP/UDP multicast.
For example, specific resources on the reverse-proxy could be
allocated, each to a specific application group and/or CoAP group.
Or alternatively, the application group and/or CoAP group in question
could be encoded as URI path segments. The URI path encodings for a
reverse-proxy may also use a URI mapping template as described in
Section 5.4 of [RFC8075].
The reverse-proxy practically stands in for a CoAP group, thus
preventing the client from reaching the group as a whole with a
single group request directly addressed to that group (e.g., via
multicast). In addition to that, the reverse-proxy may also stand in
for each of the individual servers in the CoAP group (e.g., if acting
as firewall), thus also preventing the client from individually
reaching any server in the group with a unicast request directly
addressed to that server.
For a reverse-proxy that sends a request to servers in a CoAP group,
the considerations as defined in Section 5.7.3 of [RFC7252] hold,
with the following additions:
* The three issues and limitations defined in Section 3.5.1 for a
forward proxy apply to a reverse-proxy as well, and have to be
addressed, e.g., using the signaling method defined in
[I-D.tiloca-core-groupcomm-proxy] or other means.
* A reverse-proxy MAY have preconfigured time duration(s) that are
used for collecting server responses and forwarding these back to
the client. These duration(s) may be set as global configuration
or as resource-specific configurations. If there is such
preconfiguration, then an explicit signaling of the time period in
the client's request as defined in
[I-D.tiloca-core-groupcomm-proxy] is not necessarily needed. Note
that a reverse-proxy is in an explicit relation with the origin
servers it stands in for. Thus, compared to a forward-proxy, it
has a much better basis for determining and configuring such time
durations.
* A client that is configured to access a reverse-proxy resource
(i.e., one that triggers a CoAP group communication request)
SHOULD be configured also to handle potentially multiple responses
with the same Token value caused by a single request.
Dijk, et al. Expires 25 April 2024 [Page 33]
Internet-Draft Group Communication for CoAP October 2023
That is, the client needs to preserve the Token value used for the
request also after the reception of the first response forwarded
back by the proxy (see Section 3.1.6) and keep the request open to
potential further responses with this Token. This requirement can
be met by a combination of client implementation and proper
proxied group communication configuration on the client.
* A client might re-use a Token value in a valid new request to the
reverse-proxy, while the reverse-proxy still has an ongoing group
communication request for this client with the same Token value
(i.e., its time period for response collection has not ended yet).
If this happens, the reverse-proxy MUST stop the ongoing request
and associated response forwarding, it MUST NOT forward the new
request to the servers in the CoAP group, and it MUST send a 4.00
(Bad Request) error response to the client. The diagnostic
payload of the error response SHOULD indicate to the client that
the resource is a reverse-proxy resource, and that for this reason
immediate Token re-use is not possible.
If the reverse-proxy supports the signaling protocol of
[I-D.tiloca-core-groupcomm-proxy] it can include a Multicast-
Signaling Option in the error response to convey the reason for
the error in a machine-readable way.
For the operation of HTTP-to-CoAP reverse proxies, see the last two
paragraphs of Section 3.5.1 which applies also to the case of
reverse-proxies.
3.5.3. Single Group Request to Multiple Proxies
A client might send a group request to multiple proxies at once
(e.g., over IP multicast), so that each and every of those proxies
forwards it to the servers in the CoAP group. Assuming that no
message loss occurs and that N proxies receive and forward the group
request, this has the following implications.
* Each server receives N copies of the group request, i.e., one copy
from each proxy.
* If the NoSec mode is used (see Section 4), each server treats each
received copy of the group request as a different request from a
different client. Consistently:
Dijk, et al. Expires 25 April 2024 [Page 34]
Internet-Draft Group Communication for CoAP October 2023
- Each server can reply to each of the N received requests with
multiple responses over time (see Section 3.1.6). All the
responses to the same received request are sent to the same
proxy that has forwarded that request, which in turn relays
those responses to the client.
- From each proxy, the client receives all the responses to the
group request that each server has sent to that proxy. Even in
case the client is able to distinguish the different servers
originating the responses (e.g., by using the approach defined
in [I-D.tiloca-core-groupcomm-proxy]), the client would receive
the same response content originated by each server N times, as
relayed by the N proxies.
* If secure group communication with Group OSCORE is used (see
Section 5), each server is able to determine that each received
copy of the group request is in fact originated by the same
client. In particular, each server is able to determine that all
such received requests are copies of exactly the same group
request.
Consistently, each server S accepts only the first copy of the
group request received from one of the proxies, say P, while
discarding as replay any later copies received from any other
proxy.
After that, the server S can reply to the accepted request with
multiple responses over time (see Section 3.1.6). All those
responses are sent to the same proxy P that forwarded the only
accepted request, and that in turn relays those responses to the
client.
As a consequence, for each server, the client receives responses
originated by that server only from one proxy. That is, the
client receives a certain response content only once, like in the
case with only one proxy.
3.6. Congestion Control
CoAP group requests may result in a multitude of responses from
different nodes, potentially causing congestion. Therefore, both the
sending of CoAP group requests and the sending of the unicast CoAP
responses to these group requests should be conservatively
controlled.
CoAP [RFC7252] reduces IP multicast-specific congestion risks through
the following measures:
Dijk, et al. Expires 25 April 2024 [Page 35]
Internet-Draft Group Communication for CoAP October 2023
* 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]).
* A server should limit the support for IP multicast requests to
specific resources where multicast operation is required
(Section 11.3 of [RFC7252]).
* An IP multicast request MUST be Non-confirmable (Section 8.1 of
[RFC7252]).
* A response to an IP multicast request SHOULD be Non-confirmable
(Section 5.2.3 of [RFC7252]).
* 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]).
This document also defines these measures to be applicable to
alternative transports (other than IP multicast), if not defined
otherwise.
Independently of the used transport, additional guidelines to reduce
congestion risks defined in this document are as follows:
* A server in a constrained network SHOULD only support group
requests for resources that have a small representation (where the
representation may be retrieved via a GET, FETCH, or POST method
in the request). For example, "small" can be defined as a
response payload 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 3.9.2) is used on the constrained network.
* A server SHOULD minimize the payload size of a response to a group
GET or FETCH request 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].
* A server MAY minimize the payload size of a response to a group
GET or FETCH request (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 a CoAP group request to
"/.well-known/core" SHOULD support block-wise transfers. See also
Section 3.8.
Dijk, et al. Expires 25 April 2024 [Page 36]
Internet-Draft Group Communication for CoAP October 2023
* 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.
3.7. Observing Resources
The CoAP Observe Option [RFC7641] is a protocol extension of CoAP,
which 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
changed. [RFC7641] does not mention whether the Observe Option can
be combined with CoAP (multicast) group communication.
This section updates [RFC7641] with the use of the Observe Option in
a CoAP GET group request, and defines normative behavior for both
client and server. Consistent with Section 2.4 of [RFC8132], the
same rules apply when using the Observe Option in a CoAP FETCH group
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 3.1.2, depending on server-specific configuration.
A client that sends a group 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 members of the CoAP group did not
respond to the initial request. The client MAY additionally use the
same Message ID in the repeated request, to avoid that members of the
CoAP group 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 members of the CoAP group that receive this new message will
typically respond again, which increases the network load.
Dijk, et al. Expires 25 April 2024 [Page 37]
Internet-Draft Group Communication for CoAP October 2023
A client that has sent a group 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 member of the CoAP group
did not respond to the initial group request, although it was
expected to. In this case, the client MUST use a Message ID that
differs from that of the initial group request message.
Furthermore, consistent with Section 3.3.1 of [RFC7641] and following
its guidelines, a client MAY at any time send a new group/multicast
GET or FETCH request with the same Token value and same Observe
Option value as the original request. This allows the client 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. If a server adds a
client (as a new entry) to the list of observers for a resource due
to an Observe request, the server SHOULD respond to this request and
SHOULD NOT suppress the response. An exception to the above is the
overriding of response suppression according to a CoAP No-Response
Option [RFC7967] specified by the client in the GET or FETCH request
(see Section 3.1.2).
A server SHOULD have a mechanism to verify the aliveness of its
observing clients and the continued interest of these clients in
receiving the observe notifications. This can be implemented by
sending notifications occasionally 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 CoAP group (using serial
unicast to each known group member). In addition, a client MAY
Dijk, et al. Expires 25 April 2024 [Page 38]
Internet-Draft Group Communication for CoAP October 2023
explicitly deregister from all those servers at once, by sending a
group/multicast GET or FETCH request that includes the Token value of
the observation to be canceled 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 at a later point in time or the
group/multicast deregistration request MAY be repeated upon receiving
another observe response from a server.
For observing at servers that are members of a CoAP group through a
CoAP-to-CoAP proxy, the limitations stated in Section 3.5 apply. The
method defined in [I-D.tiloca-core-groupcomm-proxy] enables group
communication including resource observation through proxies and
addresses those limitations.
3.8. 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. Consistent with Section 2.5
of [RFC8132], the same can be done with a multicast FETCH request.
To retrieve any further blocks of the resource from a responding
server, the client then has to use unicast requests, separately
addressing each different server. Also, a server (member of a
targeted CoAP group) that needs to respond to a group 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 group/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 group 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.
[RFC9177] specifies a specialized alternative method for CoAP block-
wise transfer. It specifies that "servers MUST ignore multicast
requests that contain the Q-Block2 Option".
3.9. Transport Protocols
In this document UDP, both over IPv4 and IPv6, is considered as the
default transport protocol for CoAP group communication.
Dijk, et al. Expires 25 April 2024 [Page 39]
Internet-Draft Group Communication for CoAP October 2023
3.9.1. 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 3.10.2 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 number 5683 MUST be
supported as per Sections 7.1 and 12.8 of [RFC7252] for the "All CoAP
Nodes" multicast CoAP group. An IPv6 CoAP server SHOULD support the
"All CoAP Nodes" multicast CoAP group with at least link-local (2),
admin-local (4), and site-local (5) scopes. An IPv6 CoAP server on a
6LoWPAN node (see Section 3.9.2) SHOULD also support the realm-local
(3) scope.
Note that a client sending an IPv6 multicast CoAP message to a port
number 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].
3.9.2. UDP/IPv6 Multicast Transport over 6LoWPAN
In 6LoWPAN [RFC4944] [RFC6282] networks, an IPv6 packet (up to 1280
bytes) may be fragmented into multiple 6LoWPAN fragments, each
fragment small enough to be carried over an IEEE 802.15.4 MAC frame
(up to 127 bytes).
These 6LoWPAN fragments are exchanged between 6LoWPAN nodes,
potentially involving 6LoWPAN routers operating in a multi-hop
network topology. Although 6LoWPAN multicast routing protocols
usually define mechanisms to compensate for the loss of transmitted
fragments (e.g., using link-layer unicast acknowledgements, or
repeated link-layer broadcast transmissions as in MPL -- see
Section 3.10.3) a fragment may still be lost in transit. The loss of
a single fragment implies the loss of the entire IPv6 packet, because
the reassembly back into IPv6 packet will fail in that case. Also,
if this fragment loss causes the application-layer retransmission of
the entire multi-fragment IPv6 packet, it may happen that much of the
same data is transmitted yet again over the constrained network.
Dijk, et al. Expires 25 April 2024 [Page 40]
Internet-Draft Group Communication for CoAP October 2023
For this reason, the performance in terms of packet loss and
throughput of using larger, multi-fragment multicast IPv6 packets is
on average worse than the performance of smaller, single-fragment
IPv6 multicast packets. So it is recommended to design application
payloads for group communication sufficiently small: a CoAP request
sent over multicast over a 6LoWPAN network interface SHOULD fit in a
single IEEE 802.15.4 MAC frame, if possible.
On 6LoWPAN networks, multicast CoAP groups can be defined with realm-
local scope [RFC7346]. Such a realm-local CoAP group is restricted
to the local 6LoWPAN network/subnet. In other words, a multicast
request to that CoAP 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 CoAP group (see Section 3.9.1) in order to
discover only CoAP servers on the local 6LoWPAN network.
3.9.3. 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 number 5683 MUST be supported as per
Sections 7.1 and 12.8 of [RFC7252], for the "All CoAP Nodes" IPv4
multicast CoAP group.
Note that a client sending an IPv4 multicast CoAP message to a port
number 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].
3.9.4. TCP, TLS, and WebSockets
Because it supports unicast only, [RFC8323] (CoAP over TCP, TLS, and
WebSockets) has a restricted scope as a transport for CoAP group
communication. This is limited to the use of block-wise transfer
discussed in Section 3.8.
That is, after the first group request including the Block2 Option
and sent over UDP, the following unicast CoAP requests targeting
individual servers to retrieve further blocks may be sent over TCP or
WebSockets, possibly protected with TLS.
This requires the individually addressed servers to also support CoAP
over TCP/TLS/WebSockests for the targeted resource. A server can
indicate its support for multiple alternative transports, and
practically enable access to its resources through either of them, by
using the method defined in [I-D.ietf-core-transport-indication].
Dijk, et al. Expires 25 April 2024 [Page 41]
Internet-Draft Group Communication for CoAP October 2023
3.9.5. Other Transports
CoAP group communication may be used over transports other than UDP/
IP multicast. For example broadcast, non-UDP multicast, geocast,
serial unicast, etc. In such cases the particular considerations for
UDP/IP multicast in this document may need to be applied to that
particular transport.
3.10. Interworking with Other Protocols
3.10.1. MLD/MLDv2/IGMP/IGMPv3
A CoAP node that is an IP host (i.e., not an IP router) may be
unaware of the specific IP multicast routing/forwarding protocol
being used in its network. When such a node needs to join a specific
(CoAP) multicast group, the application process would typically
subscribe to the particular IP multicast group via an API method of
the IP stack on the node. Then the IP stack would execute a
particular (e.g., default) method to communicate its subscription to
on-link IP (multicast) routers.
The Multicast Listener Discovery Version 2 (MLDv2) protocol [RFC3810]
is the standard IPv6 method to communicate multicast subscriptions,
when other methods are not defined. The CoAP server nodes then act
in the role of MLD Multicast Address Listener. MLDv2 uses link-local
communication between Listeners and IP multicast routers.
Constrained IPv6 networks such as ones implementing either RPL (see
Section 3.10.2) or MPL (see Section 3.10.3) typically do not support
MLDv2 as they have their own mechanisms defined for subscribing to
multicast groups.
The Internet Group Management Protocol Version 3 (IGMPv3) protocol
[RFC3376] is the standard IPv4 method to signal subscriptions to
multicast group. This SHOULD be used by members of a CoAP group to
subscribe to its multicast IPv4 address on IPv4 networks unless
another method is defined for the network interface/technology used.
The guidelines from [RFC6636] on the tuning of MLD for mobile and
wireless networks may be useful when implementing MLD in constrained
networks.
3.10.2. RPL
IPv6 Routing Protocol for Low-Power and Lossy Networks (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.
Dijk, et al. Expires 25 April 2024 [Page 42]
Internet-Draft Group Communication for CoAP October 2023
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 set to a mode that
supports multicast, for example 3 (Storing mode with multicast
support) or 5 (Non-Storing Mode of Operation with ingress replication
multicast support) defined in [I-D.ietf-6lo-multicast-registration].
In mode 3, RPL DAO can be used by an RPL/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 members of
the CoAP group.
The same DAO mechanism can be used by an edge router (e.g., 6LBR) to
learn CoAP group membership information of the entire RPL network, 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 learns which IP multicast traffic it needs to selectively 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.
See [I-D.ietf-6lo-multicast-registration] for more details on RPL
Mode 5, and on subscribing to IPv6 multicast groups using 6LoWPAN
Neighbor Discovery (ND) and the Extended Address Registration Option
(EARO) in RPL networks.
3.10.3. 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 with 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
Dijk, et al. Expires 25 April 2024 [Page 43]
Internet-Draft Group Communication for CoAP October 2023
be used to propagate a CoAP multicast group request to all members of
the CoAP group.
However, a CoAP multicast request to a CoAP 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).
Methods to configure which multicast groups are to be propagated into
the MPL Domain could be:
* Manual configuration on each ingress MPL Forwarder.
* MLDv2 protocol [RFC3810], which works only in case all CoAP
servers joining a CoAP group are in link-local communication range
of an ingress MPL Forwarder. This is typically not the case on
mesh networks.
* Using 6LoWPAN Neighbor Discovery (ND) and Extended Address
Registration Option (EARO) as described in
[I-D.ietf-6lo-multicast-registration], in a network that supports
6LoWPAN-ND, RPL, and MPL.
* A new/custom protocol to register multicast groups at an ingress
MPL Forwarder. This could be for example a CoAP-based protocol
offering multicast group subscription features similar to MLDv2.
For security and performance reasons, also other filtering criteria
may be defined at an ingress MPL Forwarder. See Section 6.6 for more
details.
4. 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 NoSec mode does not require and does not make use of a security
group. Indications that endpoints can use the NoSec mode MUST NOT
rely on setting up and advertising a pseudo security group with name
"NoSec" or any of its lowercase/uppercase combinations.
A CoAP server in NoSec mode MUST NOT be accessible through the public
Internet. It is NOT RECOMMENDED to use CoAP group communication in
NoSec mode.
Dijk, et al. Expires 25 April 2024 [Page 44]
Internet-Draft Group Communication for CoAP October 2023
The possible, exceptional use of the NoSec mode ought to be limited
to specific, well-defined steps that are proven to not require
security or to not be able to attain it, e.g., early discovery of
devices and resources (see Section 6.1).
Before possibly and exceptionally using the NoSec mode in such
circumstances, the security implications in Section 6.1 must be very
well considered and understood, especially as to the risk and impact
of amplification attacks (see Section 6.3). Consistently with such
security implications, the use of the NoSec mode should still be
avoided whenever possible.
5. Secured Group Communication using Group OSCORE
This section discusses how CoAP group communication can be secured.
In particular, Section 5.1 describes how the Group OSCORE security
protocol [I-D.ietf-core-oscore-groupcomm] can be used to protect
messages exchanged in a CoAP group, while Section 5.2 provides
guidance on required maintenance operations for OSCORE groups used as
security groups.
5.1. 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 [RFC9052][RFC9053] to perform encryption operations
and protect a CoAP message carried 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 functionalities. 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.
Dijk, et al. Expires 25 April 2024 [Page 45]
Internet-Draft Group Communication for CoAP October 2023
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 group requests sent by a
CoAP client, e.g., over UDP/IP multicast, as well as multiple
corresponding CoAP responses sent as (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. That is, the recipient of a CoAP message
protected with Group OSCORE is able to securely verify whether the
CoAP endpoint that has generated and sent the message is indeed the
alleged one. This is achieved by means of two possible methods.
The first method, called 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.
The second method, called 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 security group, such as
all responses individually sent by servers, as well as requests
addressed to an individual server.
A Group Manager is responsible for managing one or multiple OSCORE
groups. In particular, the Group Manager acts as repository of the
security group members' authentication credentials including the
corresponding public keys; manages, renews, and provides security
material in the security group; and handles the join process of new
members in the security group.
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 configuration of the
security group 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
[RFC9200].
Dijk, et al. Expires 25 April 2024 [Page 46]
Internet-Draft Group Communication for CoAP October 2023
A CoAP endpoint can discover OSCORE groups and retrieve information
to join them through their respective Group Managers by using the
method described in [I-D.tiloca-core-oscore-discovery] and based on
the CoRE Resource Directory [RFC9176].
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 with an OSCORE security group, which includes:
* All members of the CoAP group, i.e., the CoAP endpoints that are
configured to receive CoAP group messages sent to the particular
CoAP group and -- in case of IP multicast transport -- that are
listening to the group's multicast IP address on the group's UDP
port.
* All further CoAP endpoints configured only as CoAP clients, that
may send CoAP group requests to the CoAP group.
5.2. Secure Group Maintenance
As part of group maintenance operations (see Section 2.2.4),
additional key management operations are required for an OSCORE
group, also depending on the security requirements of the application
(see Section 6.2.1). Specifically:
* 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. When
this happens, they are securely provided with the security
material to use in that OSCORE group.
Applications may need backward security. That is, they may
require that, after having joined an OSCORE group, a new member of
that group cannot read the cleartext of messages exchanged in the
group prior to its joining, even if it has recorded them.
In such a case, new security material to use in the OSCORE group
has first to be generated and distributed to the current members
of that group, before new endpoints are also provided with that
new security material upon their joining.
* 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 security material is generated and securely distributed
only to the remaining members of the OSCORE group, together with
the list of former members removed from that group.
Dijk, et al. Expires 25 April 2024 [Page 47]
Internet-Draft Group Communication for CoAP October 2023
This ensures forward security in the OSCORE group. That is, it
ensures that only the members intended to remain in the OSCORE
group are able to continue participating in the secure
communications within that group, while the evicted ones are not
able to participate after the distribution and installation of the
new security material.
Also, this ensures that the members intended to remain in the
OSCORE group are able to confidently assert the group membership
of other sender nodes, when receiving protected messages in the
OSCORE group after the distribution and installation of the new
security material (see Section 3.2 of
[I-D.ietf-core-oscore-groupcomm]).
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 as defined in [I-D.ietf-ace-key-groupcomm-oscore].
5.3. Proxy Security
Different solutions may be selected for secure group communication
via a proxy depending on proxy type, use case and deployment
requirements. In this section the options based on Group OSCORE are
listed.
For a client performing a group communication request via a forward-
proxy, end-to-end security should be implemented. The client then
creates a group request protected with Group OSCORE and unicasts this
to the proxy. The proxy adapts the request from a forward-proxy
request to a regular request and multicasts this adapted request to
the indicated CoAP group. During the adaptation, the security
provided by Group OSCORE persists, in either case of using the group
mode or using the pairwise mode. The first leg of communication from
client to proxy can optionally be further protected, e.g., by using
(D)TLS and/or OSCORE.
For a client performing a group communication request via a reverse-
proxy, either end-to-end-security or hop-by-hop security can be
implemented. The case of end-to-end security is the same as for the
forward-proxy case.
The case of hop-by-hop security is only possible if the proxy can be
completely trusted and it is configured as a member of the OSCORE
security group(s) that it needs to access, when sending a group
request on behalf of clients. The first leg of communication between
client and proxy is then protected with a security method for CoAP
unicast, such as (D)TLS, OSCORE, or a combination of such methods.
Dijk, et al. Expires 25 April 2024 [Page 48]
Internet-Draft Group Communication for CoAP October 2023
The second leg between proxy and servers is protected using Group
OSCORE. This can be useful in applications where for example the
origin client does not implement Group OSCORE, or the group
management operations are confined to a particular network domain and
the client is outside this domain.
For all the above cases, more details on using Group OSCORE are
defined in [I-D.tiloca-core-groupcomm-proxy].
6. Security Considerations
This section provides security considerations for CoAP group
communication, in general and for the particular transport of IP
multicast.
6.1. CoAP NoSec Mode
CoAP group communication, if not protected, is vulnerable to all the
attacks mentioned in Section 11 of [RFC7252] for IP multicast.
Moreover, as also discussed in
[I-D.irtf-t2trg-amplification-attacks], the NoSec mode is susceptible
to source IP address spoofing, hence amplification attacks are
especially feasible and greatly effective, since a single request can
result in multiple responses from multiple servers (see Section 6.3).
Therefore, it is NOT RECOMMENDED to use CoAP group communication in
NoSec mode, also in order to prevent proliferation of high-volume
amplification attacks as further discussed in Section 6.3. The
requirement in Section 4 on publically accessible CoAP servers also
aims to prevent amplification attacks.
Exceptionally, and only after the security implications have been
very well considered and understood, some applications may rely on a
limited use of the NoSec mode, when performing specific, well-defined
steps that are proven to not require security or to not be able to
attain it.
For example, early link-local discovery of devices and resources as
part of an onboarding protocol is a typical use case where the NoSec
mode or equivalent unsecured mode is used. In such a discovery step,
there may be a querying device that needs to discover nearby devices
capable of helping it with the network onboarding process. But there
are no mutual security relations configured on the querying device
and its neighbor devices at the time it performs the early discovery.
These relations are configured later in the process based on secure
device identities. Alternatively, a new device to be onboarded may
wait for advertisements of nearby devices able to help it do the
network onboarding process. Also in this case, these messages cannot
Dijk, et al. Expires 25 April 2024 [Page 49]
Internet-Draft Group Communication for CoAP October 2023
be secured initially because the new device does not yet have any
security relation configured with devices that are already a member
of the network. See [I-D.ietf-anima-constrained-voucher] for an
example of an onboarding protocol that can use CoAP multicast for
early link-local discovery.
As a further example, the NoSec mode may be useful in simple
applications that neither involve nor may have an impact on sensitive
data and personal information. These include, e.g., read-only
temperature sensors deployed in an environment where a client reads
temperature values but does not use this data to control actuators or
to perform other automated actions.
In the exception cases where NoSec mode is used, high-volume and
harmful amplifications need to be prevented through appropriate and
conservative device configurations: taking the early discovery query
as an example, only a few CoAP servers are expected to be configured
for responding to multicast group requests that are sent for
discovery. And the time window during which such unsecured requests
are accepted, can be limited as well. Furthermore the scope is also
limited: only link-local requests are accepted.
Except for the class of applications discussed above, and all the
more so in applications that obviously have hard security
requirements (e.g., health monitoring systems and alarm monitoring
systems), CoAP group communication MUST NOT be used in NoSec mode.
6.2. 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 specific dynamics of security group membership
(e.g., the problem of distributing security keys across large groups
with many members that come and go).
CoAP group communication MUST be protected by using Group OSCORE as
specified in [I-D.ietf-core-oscore-groupcomm], with the possible
exception of specific, well-defined steps that are proven to not
require security or to not be able to attain it (e.g., early
discovery).
The same security considerations from Section 13 of
[I-D.ietf-core-oscore-groupcomm] hold for this specification.
Dijk, et al. Expires 25 April 2024 [Page 50]
Internet-Draft Group Communication for CoAP October 2023
6.2.1. Group Key Management
A key management scheme for secure revocation and renewal of group
security material, namely group rekeying, is required to be adopted
in OSCORE groups. The key management scheme has to preserve forward
security in the OSCORE group, as well as backward security if this is
required by the application (see Section 5.2). In particular, the
key management scheme MUST comply with the functional steps defined
in Section 3.2 of [I-D.ietf-core-oscore-groupcomm].
Even though security group policies vary depending on the specific
applications and their security requirements, they SHOULD also take
into account:
* The expected amount of time that the key management scheme
requires to rekey the OSCORE group.
* The expected frequency of group membership changes in the OSCORE
group (i.e., nodes joining and leaving).
* The identity of the specific CoAP endpoints as they join and leave
the OSCORE group.
In particular, it can be desirable to not rekey the OSCORE group upon
every single membership change, in case group members frequently join
and leave, and at the same time a single group rekeying instance
takes a non-negligible time to complete.
In such a case, the Group Manager can cautiously consider to rekey
the OSCORE 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 time intervals of network
inactivity. This would prevent from paralyzing communications in the
OSCORE group, when a slow rekeying scheme is used and frequently
invoked.
At the same time, the security implications of delaying the rekeying
process have to be carefully considered and understood before
employing such security group policies.
Dijk, et al. Expires 25 April 2024 [Page 51]
Internet-Draft Group Communication for CoAP October 2023
In fact, this comes at the cost of not continuously preserving
backward and forward security, since group rekeying might not occur
upon every single change in the OSCORE group membership. That is,
most recently joined nodes would have access to the security material
used prior to their joining, and thus be able to access past group
communications protected with that security material. Similarly,
until the OSCORE group is rekeyed, most recently left nodes would
retain access to group communications protected with the existing
security material.
6.2.2. 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 signed 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 and sent by a specific and
identified CoAP endpoint as a member of the OSCORE group.
Note that Group OSCORE does not protect the addressing information
about the CoAP endpoint that has sent the message, e.g., the source
IP address and port number used when sending the message. Therefore,
Group OSCORE does not provide authentication of such source
addressing information.
6.2.3. 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.
Dijk, et al. Expires 25 April 2024 [Page 52]
Internet-Draft Group Communication for CoAP October 2023
* Since Group OSCORE provides end-to-end confidentiality and
integrity of request/response messages, proxies capable of group
communication 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 OSCORE group members. Also, with the notable
addition of signatures for the group mode, Group OSCORE protects
messages using the same procedure as OSCORE (see Sections 8 and 9
of [I-D.ietf-core-oscore-groupcomm]), and especially processes
CoAP options according to the same classification in U/I/E
classes.
* Group OSCORE limits the feasibility and impact of amplification
attacks (see Section 6.3 of this document and Section 11.3 of
[RFC7252]), thanks to the handling of protected group requests on
the server side. That is, upon receiving a group request
protected with Group OSCORE, a server verifies whether the request
is not a replay, and whether it has been originated and sent by
the alleged CoAP endpoint in the OSCORE group.
In order to perform the latter check of source authentication, the
server either: i) verifies the signature included in the request
by using the public key of the client, when the request is
protected using the group mode (see Section 8.2 of
[I-D.ietf-core-oscore-groupcomm]); or ii) decrypts and verifies
the request by means of an additionally derived pairwise key
associated with the client, when the request is protected using
the pairwise mode (see Section 9.4 of
[I-D.ietf-core-oscore-groupcomm]).
As also discussed in Section 8 of
[I-D.ietf-core-oscore-groupcomm], it is recommended that, when
failing to decrypt and verify an incoming group request protected
with the group mode, a server does not send back any error message
in case any of the following holds: the server determines that the
request was indeed sent to the whole CoAP group (e.g., over IP
multicast); or the server is not able to determine it altogether.
Such a message processing on the server limits an adversary to
leveraging an intercepted group request protected with Group
OSCORE, and then altering the source address to be the one of the
intended amplification victim.
Furthermore, the adversary needs to consider a group request that
specifically targets a resource for which the CoAP servers are
configured to respond. While this can be often correctly
inferable from the application context, it is not explicit from
Dijk, et al. Expires 25 April 2024 [Page 53]
Internet-Draft Group Communication for CoAP October 2023
the group request itself, since Group OSCORE protects the Uri-Path
and Uri-Query CoAP Options conveying the respective components of
the target URI.
As a further mitigation against amplification attacks, a server
can also rely on the Echo Option for CoAP defined in [RFC9175] and
include it in a response to a group request. By doing so, the
server can assert that the alleged sender of the group request
(i.e., the CoAP client associated with a certain authentication
credential including the corresponding public key) is indeed
reachable at the claimed source address of the group request,
especially if such an address differs from the one used in
previous group requests from the same (authenticated) device.
Although responses including the Echo Option do still result in
amplification, this is limited in volume compared to when all
servers reply with a full response.
Using the Echo Option does not provide authentication of source
addressing information about the sender of a CoAP message. Also,
using the Echo Option in itself does not provide source
authentication of exchanged messages, which is achieved by means
of Group OSCORE (see Section 6.2.2). Using the Echo Option
together with Group OSCORE allows a CoAP server in the OSCORE
group to assert the freshness of CoAP requests received from other
members in the group.
* Group OSCORE limits the impact of attacks based on IP spoofing
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 CoAP endpoints that are legitimate
members of the group.
Within an OSCORE group, the shared symmetric-key security material
strictly provides only group-level authentication. However,
source authentication of messages is also ensured, both in the
group mode by means of signatures (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 9.3 and 9.5
of [I-D.ietf-core-oscore-groupcomm]). Thus, recipient endpoints
can verify a message to have been originated and sent by the
alleged, identifiable CoAP endpoint in the OSCORE group.
As noted above, the server may additionally rely on the Echo
Option for CoAP defined in [RFC9175], in order to verify the
aliveness and reachability of the client sending a request from a
particular IP address.
Dijk, et al. Expires 25 April 2024 [Page 54]
Internet-Draft Group Communication for CoAP October 2023
* Group OSCORE does not require members of an OSCORE group 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 entropy-related 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 OSCORE group.
* Group OSCORE prevents making any single member of an OSCORE group
a target for subverting security in the whole 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 group. 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 signature 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).
6.3. Risk of Amplification
Section 11.3 of [RFC7252] highlights that CoAP group requests may be
used for accidentally or deliberately performing Denial of Service
attacks, especially in the form of a high-volume amplification
attack, by using all the servers in the CoAP group as attack vectors.
That is, following a group request sent to a CoAP group, each of the
servers in the group may reply with a response which is likely larger
in size than the group request. Thus, an attacker sending a single
group request may achieve a high amplification factor, i.e., a high
ratio between the size of the group request and the total size of the
corresponding responses intended to the attack victim.
Thus, consistently with Section 11.3 of [RFC7252], a server in a CoAP
group:
* SHOULD limit the support for CoAP group requests only to the group
resources of the application group(s) using that CoAP group;
* SHOULD NOT accept group requests that cannot be authenticated in
some way, i.e., for which it is not possible to securely assert
that they have been originated and sent by the alleged,
identifiable CoAP endpoint in the OSCORE group;
Dijk, et al. Expires 25 April 2024 [Page 55]
Internet-Draft Group Communication for CoAP October 2023
* SHOULD NOT provide large amplification factors through its
responses to a non-authenticated group request, possibly employing
CoAP block-wise transfers [RFC7959] to reduce the amount of
amplification.
Amplification attacks using CoAP are further discussed in
[I-D.irtf-t2trg-amplification-attacks], which also highlights how the
amplification factor would become even higher when CoAP group
communication is combined with resource observation [RFC7641]. That
is, a single group request may result in multiple notification
responses from each of the responding servers, throughout the
observation lifetime.
Thus, consistently with Section 7 of [RFC7641], a server in a CoAP
group MUST strictly limit the number of notifications it sends
between receiving acknowledgments that confirm the actual interest of
the client in continuing the observation.
Moreover, it is especially easy to perform an amplification attack
when the NoSec mode is used. Therefore, also in order to prevent an
easy proliferation of high-volume amplification attacks, it is
generally NOT RECOMMENDED to use CoAP group communication in NoSec
mode (see Section 6.1).
Besides building on very well understood security implications and
being limited to specific, well-defined steps that are proven to not
require security or to not be able to attain it, possible exceptions
should also be limited to use cases where accesses to a group
resource have a specific, narrow, and well understood scope, and
where only a few CoAP servers (or, ideally, only one) would possibly
respond to a group request.
A relevant exceptional example is a CoAP client performing the
discovery of hosts such as a group manager or a Resource Directory
[RFC9176], by probing for them through a group request sent to the
CoAP group. This early, unprotected step is relevant for a CoAP
client that does not know the address of such hosts in advance, and
that does not have yet configured a mutual security relation with
them. In this kind of deployments, such a discovery procedure does
not result in a considerable and harmful amplification, since only
the few CoAP servers that are the object of discovery are going to
respond to the group request targeting that specific resource. In
particular, those hosts can be the only CoAP servers in that specific
CoAP group (hence listening for group requests sent to that group),
and/or the only CoAP servers explicitly configured to respond to
group requests targeting specific group resources.
Dijk, et al. Expires 25 April 2024 [Page 56]
Internet-Draft Group Communication for CoAP October 2023
With the exception of such particular use cases, group communications
MUST be secured using Group OSCORE [I-D.ietf-core-oscore-groupcomm],
see Section 5. As discussed in Section 6.2.3, this limits the
feasibility and impact of amplification attacks.
6.4. 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 transmitted 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
typically not have a valid matching request active anymore, so this
attack would not accomplish anything in the client.
If Group OSCORE is used, such a replay attack on the servers is
prevented, since a client protects each 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 with 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 the
challenge-response synchronization method based on the Echo Option
for CoAP defined in [RFC9175] as described in Section 10 of
[I-D.ietf-core-oscore-groupcomm], in order to (re-)synchronize with
the client's sequence number.
6.5. Use of CoAP No-Response Option
When CoAP group communication is used in CoAP NoSec (No Security)
mode (see Section 4), the CoAP No-Response Option [RFC7967] could be
misused by a malicious client to evoke as many responses from servers
to a group request as possible, by using the value '0' -- Interested
in all responses. This might even override the default behavior of a
CoAP server to suppress the response in case there is nothing of
interest to respond with. Therefore, this option can be used to
perform an amplification attack (see Section 6.3).
Dijk, et al. Expires 25 April 2024 [Page 57]
Internet-Draft Group Communication for CoAP October 2023
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.
6.6. 6LoWPAN and MPL
In a 6LoWPAN network, the MPL [RFC7731] protocol may be used to
forward multicast packets throughout the network. A 6LoWPAN Router
that forwards a large IPv6 packet may have a relatively high impact
on the occupation of the wireless channel because sending a large
packet consists of the transmission of multiple link-layer IEEE
802.15.4 frames. Also, a constrained 6LoWPAN Router may experience a
high memory load due to buffering of the large packet -- MPL requires
an MPL Forwarder to store the packet for a longer duration, to allow
multiple forwarding transmissions to neighboring Forwarders. This
could allow an attacker on the 6LoWPAN network or outside the 6LoWPAN
network to execute a Denial of Service (DoS) attack by sending large
IPv6 multicast packets. This is also an amplification attack in
general, because each of potentially multiple MPL Forwarder(s)
repeats the transmission of the IPv6 packet potentially multiple
times, hence amplifying the original amount of data sent by the
attacker considerably.
The amplification factor may be even further increased by the loss of
link-layer frames. If one or more of the fragments are not received
correctly by an MPL Forwarder during its packet reassembly time
window, the Forwarder discards all received fragments and it will
likely at a future point in time trigger a neighboring MPL Forwarder
to send the IPv6 packet (fragments) again, because its internal state
marks this packet (that it failed to received previously) still as a
"new" IPv6 packet. Hence this leads to an MPL Forwarder signaling to
neighbors its "old" state, triggering additional transmission(s) of
all packet fragments.
For these reasons, a large IPv6 multicast packet is a possible attack
vector in a Denial of Service (DoS) amplification attack on a 6LoWPAN
network. See Section 6.3 of this document and 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 of the request packet. A 6LoWPAN Router or (MPL)
multicast forwarder SHOULD deprioritize forwarding for multi-fragment
6LoWPAN multicast packets. 6LoWPAN Border Routers are typical ingress
points where multicast traffic enters into a 6LoWPAN network.
Specific MPL Forwarders (whether located on a 6LBR or not) may also
Dijk, et al. Expires 25 April 2024 [Page 58]
Internet-Draft Group Communication for CoAP October 2023
be configured as ingress points. Any such ingress point 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 packets for which there is
no known multicast listener on the 6LoWPAN network. See Section 3.10
for protocols that allow multicast listeners to signal which groups
they would like to listen to. As part of multicast packet filtering,
the ingress point SHOULD implement a filtering criterion based on the
size of the multicast packet. Ingress multicast packets above a
defined size may then be dropped or deprioritized.
6.7. 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.
6.8. Monitoring
6.8.1. General Monitoring
CoAP group communication can be used to control a set of related
devices: for example, to simultaneously turn on all the lights in a
room. This intrinsically exposes the communicating devices to some
unique monitoring risks, which they are not as vulnerable to when not
using group communication.
For example, an attacker might be 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 CoAP group, some
network topology information may be deduced).
6.8.2. Pervasive Monitoring
CoAP traffic is typically used for the Internet of Things, and CoAP
(multicast) group communication may specifically be used for
conveniently controlling and monitoring critical infrastructure
(e.g., lights, alarms, HVAC, electrical grid, etc.).
Dijk, et al. Expires 25 April 2024 [Page 59]
Internet-Draft Group Communication for CoAP October 2023
However, this may be a prime target of pervasive monitoring attacks
[RFC7258], which have to be considered as a key additional threat for
group communication. 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 instance, the source node (controller) sends out CoAP
group messages, which easily identifies it as a controller.
CoAP group communication built on top of IP multicast is inherently
more vulnerable compared to communications solely relying on IP
unicast, since the same packet may be replicated over many multiple
links. In particular, this yields a higher probability of packet
capture by a pervasive monitoring system, which in turn results in
more information available to analyze within the same time interval.
Moreover, a single CoAP group request potentially results in multiple
CoAP responses, thus further contributing to the information
available to analyze.
This requires CoAP group communication solutions that are built on
top of IP multicast to pay particular attention to these dangers.
In order to limit the ease of interception of group communication
messages, one mitigation is to restrict the scope of IP multicast to
the minimal scope that fulfills the application need. See the
congestion control recommendations in the last bullet of Section 3.6
to minimize the scope. Thus, for example, realm-local IP multicast
scope is always preferred over site-local scope IP multicast, if it
fulfills the application needs.
Even if CoAP group communications are encrypted/protected (see
Section 5), an attacker may still attempt to capture this traffic and
perform an off-line attack in the future.
7. IANA Considerations
This document has no actions for IANA.
8. References
8.1. Normative References
[I-D.ietf-core-oscore-groupcomm]
Tiloca, M., Selander, G., Palombini, F., Mattsson, J. P.,
and J. Park, "Group Object Security for Constrained
RESTful Environments (Group OSCORE)", Work in Progress,
Internet-Draft, draft-ietf-core-oscore-groupcomm-20, 2
September 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-core-oscore-groupcomm-20>.
Dijk, et al. Expires 25 April 2024 [Page 60]
Internet-Draft Group Communication for CoAP October 2023
[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/rfc/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/rfc/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/rfc/rfc3376>.
[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/rfc/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/rfc/rfc4944>.
[RFC6282] Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
DOI 10.17487/RFC6282, September 2011,
<https://www.rfc-editor.org/rfc/rfc6282>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/rfc/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/rfc/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/rfc/rfc7641>.
Dijk, et al. Expires 25 April 2024 [Page 61]
Internet-Draft Group Communication for CoAP October 2023
[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/rfc/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/rfc/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/rfc/rfc8132>.
[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/rfc/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/rfc/rfc8613>.
[RFC9052] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Structures and Process", STD 96, RFC 9052,
DOI 10.17487/RFC9052, August 2022,
<https://www.rfc-editor.org/rfc/rfc9052>.
[RFC9053] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053,
August 2022, <https://www.rfc-editor.org/rfc/rfc9053>.
[RFC9175] Amsüss, C., Preuß Mattsson, J., and G. Selander,
"Constrained Application Protocol (CoAP): Echo, Request-
Tag, and Token Processing", RFC 9175,
DOI 10.17487/RFC9175, February 2022,
<https://www.rfc-editor.org/rfc/rfc9175>.
8.2. Informative References
[Californium]
Eclipse Foundation, "Eclipse Californium", June 2022,
<https://github.com/eclipse/californium>.
[Go-CoAP] Open Connectivity Foundation (OCF), "Go-CoAP", June 2022,
<https://github.com/go-ocf/go-coap>.
Dijk, et al. Expires 25 April 2024 [Page 62]
Internet-Draft Group Communication for CoAP October 2023
[I-D.bormann-core-responses]
Bormann, C. and C. Amsüss, "CoAP: Non-traditional response
forms", Work in Progress, Internet-Draft, draft-bormann-
core-responses-01, 3 February 2022,
<https://datatracker.ietf.org/doc/html/draft-bormann-core-
responses-01>.
[I-D.ietf-6lo-multicast-registration]
Thubert, P., "IPv6 Neighbor Discovery Multicast and
Anycast Address Listener Subscription", Work in Progress,
Internet-Draft, draft-ietf-6lo-multicast-registration-15,
30 May 2023, <https://datatracker.ietf.org/doc/html/draft-
ietf-6lo-multicast-registration-15>.
[I-D.ietf-ace-key-groupcomm-oscore]
Tiloca, M., Park, J., and F. Palombini, "Key Management
for OSCORE Groups in ACE", Work in Progress, Internet-
Draft, draft-ietf-ace-key-groupcomm-oscore-16, 6 March
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
ace-key-groupcomm-oscore-16>.
[I-D.ietf-ace-oscore-gm-admin]
Tiloca, M., Höglund, R., Van der Stok, P., and F.
Palombini, "Admin Interface for the OSCORE Group Manager",
Work in Progress, Internet-Draft, draft-ietf-ace-oscore-
gm-admin-09, 1 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-ace-
oscore-gm-admin-09>.
[I-D.ietf-anima-constrained-voucher]
Richardson, M., Van der Stok, P., Kampanakis, P., and E.
Dijk, "Constrained Bootstrapping Remote Secure Key
Infrastructure (BRSKI)", Work in Progress, Internet-Draft,
draft-ietf-anima-constrained-voucher-21, 7 July 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-anima-
constrained-voucher-21>.
[I-D.ietf-core-coap-pubsub]
Jimenez, J., Koster, M., and A. Keränen, "A publish-
subscribe architecture for the Constrained Application
Protocol (CoAP)", Work in Progress, Internet-Draft, draft-
ietf-core-coap-pubsub-13, 20 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
coap-pubsub-13>.
Dijk, et al. Expires 25 April 2024 [Page 63]
Internet-Draft Group Communication for CoAP October 2023
[I-D.ietf-core-transport-indication]
Amsüss, C., "CoAP Protocol Indication", Work in Progress,
Internet-Draft, draft-ietf-core-transport-indication-02,
13 March 2023, <https://datatracker.ietf.org/doc/html/
draft-ietf-core-transport-indication-02>.
[I-D.irtf-t2trg-amplification-attacks]
Mattsson, J. P., Selander, G., and C. Amsüss,
"Amplification Attacks Using the Constrained Application
Protocol (CoAP)", Work in Progress, Internet-Draft, draft-
irtf-t2trg-amplification-attacks-02, 12 April 2023,
<https://datatracker.ietf.org/doc/html/draft-irtf-t2trg-
amplification-attacks-02>.
[I-D.tiloca-core-groupcomm-proxy]
Tiloca, M. and E. Dijk, "Proxy Operations for CoAP Group
Communication", Work in Progress, Internet-Draft, draft-
tiloca-core-groupcomm-proxy-09, 31 August 2023,
<https://datatracker.ietf.org/doc/html/draft-tiloca-core-
groupcomm-proxy-09>.
[I-D.tiloca-core-oscore-discovery]
Tiloca, M., Amsüss, C., and P. Van der Stok, "Discovery of
OSCORE Groups with the CoRE Resource Directory", Work in
Progress, Internet-Draft, draft-tiloca-core-oscore-
discovery-14, 8 September 2023,
<https://datatracker.ietf.org/doc/html/draft-tiloca-core-
oscore-discovery-14>.
[libcoap] Bergmann, O., "libcoap", June 2022,
<https://github.com/obgm/libcoap>.
[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/rfc/rfc3810>.
[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/rfc/rfc6092>.
Dijk, et al. Expires 25 April 2024 [Page 64]
Internet-Draft Group Communication for CoAP October 2023
[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/rfc/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/rfc/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/rfc/rfc7258>.
[RFC7346] Droms, R., "IPv6 Multicast Address Scopes", RFC 7346,
DOI 10.17487/RFC7346, August 2014,
<https://www.rfc-editor.org/rfc/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/rfc/rfc7390>.
[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/rfc/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/rfc/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/rfc/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/rfc/rfc8710>.
Dijk, et al. Expires 25 April 2024 [Page 65]
Internet-Draft Group Communication for CoAP October 2023
[RFC9019] Moran, B., Tschofenig, H., Brown, D., and M. Meriac, "A
Firmware Update Architecture for Internet of Things",
RFC 9019, DOI 10.17487/RFC9019, April 2021,
<https://www.rfc-editor.org/rfc/rfc9019>.
[RFC9124] Moran, B., Tschofenig, H., and H. Birkholz, "A Manifest
Information Model for Firmware Updates in Internet of
Things (IoT) Devices", RFC 9124, DOI 10.17487/RFC9124,
January 2022, <https://www.rfc-editor.org/rfc/rfc9124>.
[RFC9176] Amsüss, C., Ed., Shelby, Z., Koster, M., Bormann, C., and
P. van der Stok, "Constrained RESTful Environments (CoRE)
Resource Directory", RFC 9176, DOI 10.17487/RFC9176, April
2022, <https://www.rfc-editor.org/rfc/rfc9176>.
[RFC9177] Boucadair, M. and J. Shallow, "Constrained Application
Protocol (CoAP) Block-Wise Transfer Options Supporting
Robust Transmission", RFC 9177, DOI 10.17487/RFC9177,
March 2022, <https://www.rfc-editor.org/rfc/rfc9177>.
[RFC9200] Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments Using the OAuth 2.0 Framework
(ACE-OAuth)", RFC 9200, DOI 10.17487/RFC9200, August 2022,
<https://www.rfc-editor.org/rfc/rfc9200>.
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
[RFC9176]; and there may be other types of directories that can be
Dijk, et al. Expires 25 April 2024 [Page 66]
Internet-Draft Group Communication for CoAP October 2023
used with CoAP.
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.
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 subcase 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] showed an example of discovering a CoRE
Resource Directory using CoAP group communication. As defined in
[RFC9176], a resource directory is a web entity that stores
information about web resources and implements REST interfaces for
Dijk, et al. Expires 25 April 2024 [Page 67]
Internet-Draft Group Communication for CoAP October 2023
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 cohesive collection, 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] showed examples of lighting control
of a set 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. A set of devices may be defined as a CoAP
group, e.g., as intended to cover "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 set of devices 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.
Dijk, et al. Expires 25 April 2024 [Page 68]
Internet-Draft Group Communication for CoAP October 2023
A.2.4. Network-wide / Group Notification
In some cases, a whole network, or subnet of multiple IP devices, or
a specific target set of devices 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
Group communication can be useful to efficiently distribute new
software (firmware, image, application, etc.) to a set of multiple
devices, e.g., by relying on the SUIT firmware update architecture
[RFC9019] and its manifest information model [RFC9124]. In this
case, a CoAP group can be defined in terms of the device type of its
members: all devices in the target CoAP 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 CoAP 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 a multicast software update, the CoAP 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. Examples of Group Naming for Application Groups
This section provides examples for the different methods that can be
used to name application groups, as defined in Section 2.2.1.2.
The shown examples consider a CoAP group identified by the group
hostname grp.example. Its members are CoAP servers listening to the
associated IP multicast address ff35:30:2001:db8:f1::8000:1 and port
number 5685.
Note that a group hostname is used here to have better-readable
examples. As discussed in Section 2.2.1.2 when considering the
authority component and its host subcomponent in the Group URI, in
Dijk, et al. Expires 25 April 2024 [Page 69]
Internet-Draft Group Communication for CoAP October 2023
practice an implementation would likely use an IP address literal as
the host component of the Group URI, in order to reduce the size of
the CoAP request. In particular, the Uri-Host Option can be fully
elided in this case.
Also note that the Uri-Port Option does not appear in the examples,
since the port number 5685 is already included in the CoAP request's
UDP header (which is not shown in the examples).
B.1. Group Naming using the URI Path Component
Figure 3 provides an example where the URI path component is used for
naming application groups.
Application group name: gp1
Group URI: coap://grp.example:5685/gp/gp1/light?foo=bar
CoAP group request
Header: GET (T=NON, Code=0.01, MID=0x7d41)
Uri-Host: grp.example
Uri-Path: gp
Uri-Path: gp1
Uri-Path: light
Uri-Query: foo=bar
Figure 3: Example of application group name in URI path (1/2)
Figure 4 provides a different example, where an IPv6 literal address
and the default CoAP port number 5683 are used in the authority
component, which yields a compact CoAP request. Also the resource
structure is different in this example.
Application group name: gp1
Group URI: coap://[ff35:30:2001:db8:f1::8000:1]/g/gp1/li
CoAP group request
Header: POST (T=NON, Code=0.02, MID=0x7d41)
Uri-Path: g
Uri-Path: gp1
Uri-Path: li
Figure 4: Example of application group name in URI path (2/2)
Dijk, et al. Expires 25 April 2024 [Page 70]
Internet-Draft Group Communication for CoAP October 2023
B.2. Group Naming using the URI Query Component
Figure 5 provides an example where the URI query component is used
for naming application groups. In particular, it considers the first
alternative discussed in Section 2.2.1.2, where the URI query
component consists of only one parameter, which has no value and has
the name of the application group as its own identifier.
Application group name: gp1
Group URI: coap://grp.example:5685/light?gp1
CoAP group request
Header: GET (T=NON, Code=0.01, MID=0x7d41)
Uri-Host: grp.example
Uri-Path: light
Uri-Query: gp1
Figure 5: Example of application group name in URI query (1/2)
Figure 6 provides another example, which considers the second
alternative discussed in Section 2.2.1.2. In particular, the URI
query component includes a query parameter "gp" as designated
indicator, with value the name of the application group.
Application group name: gp1
Group URI: coap://grp.example:5685/light?foo=bar&gp=gp1
CoAP group request
Header: GET (T=NON, Code=0.01, MID=0x7d41)
Uri-Host: grp.example
Uri-Path: light
Uri-Query: foo=bar
Uri-Query: gp=gp1
Figure 6: Example of application group name in URI query (2/2)
B.3. Group Naming using the URI Authority Component
Figure 7 provides an example where the URI authority component as a
whole is used for naming application groups.
Dijk, et al. Expires 25 April 2024 [Page 71]
Internet-Draft Group Communication for CoAP October 2023
Application group name: grp.example:5685
Group URI: coap://grp.example:5685/light?foo=bar
CoAP group request
Header: GET (T=NON, Code=0.01, MID=0x7d41)
Uri-Host: grp.example
Uri-Path: light
Uri-Query: foo=bar
Figure 7: Example of application group name in URI authority
B.4. Group Naming using the URI Host Subcomponent
Figure 8 provides an example where the URI host subcomponent of the
URI authority component is used for naming application groups.
Application group name: grp.example
Group URI: coap://grp.example:5685/light?foo=bar
CoAP group request
Header: GET (T=NON, Code=0.01, MID=0x7d41)
Uri-Host: grp.example
Uri-Path: light
Uri-Query: foo=bar
Figure 8: Example of application group name in URI host
B.5. Group Naming using the URI Port Subcomponent
Figure 9 provides an example where the URI port subcomponent of the
URI authority component is used for naming application groups.
Application group name: grp1, as inferable from port number 5685
Group URI: coap://grp.example:5685/light?foo=bar
CoAP group request
Header: GET(T=NON, Code=0.01, MID=0x7d41)
Uri-Host: grp.example
Uri-Path: light
Uri-Query: foo=bar
Figure 9: Example of application group name in URI port
Dijk, et al. Expires 25 April 2024 [Page 72]
Internet-Draft Group Communication for CoAP October 2023
B.6. Group Naming using a Custom CoAP Option
Figure 10 provides an example where a new, custom CoAP Option, namely
App-Group-Name, is used for naming application groups.
Application group name: grp1
Group URI: coap://grp.example:5685/light?foo=bar
CoAP group request
Header: GET (T=NON, Code=0.01, MID=0x7d41)
Uri-Host: grp.example
Uri-Path: light
Uri-Query: foo=bar
App-Group-Name: grp1 // new (e.g., custom) CoAP option
Figure 10: Example of application group name in a new CoAP Option
Appendix C. Examples of Group Discovery from CoAP Servers
This section provides examples for the different methods that a CoAP
client can use to discover application groups and CoAP groups by
interacting with CoAP servers, as defined in Section 2.2.3.2.
The examples build on the same assumptions considered in
Section 2.2.3.2. In addition, a CoAP group is used and is identified
by the URI authority grp.example:5685.
C.1. Application Groups Associated with a CoAP Group
Figure 11 provides an example where a CoAP client discovers all the
application groups associated with a specific CoAP group.
As a result, the client gains knowledge of: i) the set of servers
that are members of the specified CoAP group and member of any of the
associated application groups; ii) for each of those servers, the
name of the application groups where the server is a member and that
are associated with the CoAP group.
Each of the servers S1 and S2 is identified by the IP source address
of the CoAP response. If the client wishes to discover resources
that a particular server hosts within a particular application group,
it may use unicast discovery request(s) to this server, i.e., to its
respective unicast IP address. Alternatively the client may use the
discovered group resource type (e.g., rt=g.light) to infer which
resources are present below the group resource.
Dijk, et al. Expires 25 April 2024 [Page 73]
Internet-Draft Group Communication for CoAP October 2023
// Request to all members of the CoAP group
Req: GET coap://grp.example:5685/.well-known/core?rt=g.*
// Response from server S1, as member of:
// - The CoAP group "grp.example:5685"
// - The application group "gp1"
Res: 2.05 (Content)
Content-Format: 40
Payload:
</gp/gp1>;rt=g.light
// Response from server S2, as member of:
// - The CoAP group "grp.example:5685"
// - The application groups "gp1" and "gp2"
Res: 2.05 (Content)
Content-Format: 40
Payload:
</gp/gp1>;rt=g.light,
</gp/gp2>;rt=g.temp
Figure 11: Discovery of application groups associated with a CoAP
group
C.2. Members of a Given Application Group
Figure 12 provides an example where a CoAP client discovers the CoAP
servers that are members of a specific application group and the CoAP
group associated with the application group.
Note that, unlike in the example shown in Appendix C.1, now the
servers need to respond with an absolute URI and not a relative URI.
This is necessary because the responding CoAP endpoint serving the
Link Format document (on port 5683) is a different CoAP endpoint from
the one hosting the group resource "gp1" (on port 5685). Due to this
situation, the responding server includes the full (absolute) URI in
the Link Format response from which the client can conveniently gain
knowledge of the CoAP group.
Also note that a server could equally well respond with the literal
IPv6 multicast address within square brackets instead of the CoAP
group name "grp.example". In that case, the client would still gain
knowledge of the CoAP group, albeit in a different representation.
Dijk, et al. Expires 25 April 2024 [Page 74]
Internet-Draft Group Communication for CoAP October 2023
// Request to realm-local members of the application group "gp1"
Req: GET coap://[ff03::fd]/.well-known/core?href=/gp/gp1
// CoAP response from server S1, as member of:
// - The CoAP group "grp.example:5685"
// - The application group "gp1"
Res: 2.05 (Content)
Content-Format: 40
Payload:
<coap://grp.example:5685/gp/gp1>;rt=g.light
// CoAP response from server S2, as member of:
// - The CoAP group "grp.example:5685"
// - The application groups "gp1"
Res: 2.05 (Content)
Content-Format: 40
Payload:
<coap://grp.example:5685/gp/gp1>;rt=g.light
Figure 12: Discovery of members of an application group, together
with the associated CoAP group
C.3. Members of any Application Group of a Given Type
Figure 13 provides an example where a CoAP client discovers the CoAP
servers that are members of any application group of a specific type,
and the CoAP group associated with those application groups.
// Request to realm-local members of application groups
// with group type "g.temp"
Req: GET coap://[ff03::fd]/.well-known/core?rt=g.temp
// Response from server S1, as member of:
// - The CoAP group "grp.example:5685"
// - The application group "gp1" of type "g.temp"
Res: 2.05 (Content)
Content-Format: 40
Payload:
<coap://grp.example:5685/gp/gp1>;rt=g.temp
// Response from server S2, as member of:
// - The CoAP group "grp.example:5685"
// - The application groups "gp1" and "gp2" of type "g.temp"
Res: 2.05 (Content)
Content-Format: 40
Payload:
<coap://grp.example:5685/gp/gp1>;rt=g.temp,
<coap://grp.example:5685/gp/gp2>;rt=g.temp
Dijk, et al. Expires 25 April 2024 [Page 75]
Internet-Draft Group Communication for CoAP October 2023
Figure 13: Discovery of members of application groups of a
specified type, and of the associated CoAP group
C.4. Members of any Application Group in the Network
Figure 14 provides an example where a CoAP client discovers the CoAP
servers that are members of any application group configured in the
6LoWPAN wireless mesh network of the client, and the CoAP group
associated with each application group. In this example, the scope
is realm-local to address all servers in the current 6LoWPAN wireless
mesh network of the client.
// Request to realm-local members of any application group
Req: GET coap://[ff03::fd]/.well-known/core?rt=g.*
// Response from server S1, as member of:
// - The CoAP groups "grp.example:5685" and "grp2.example"
// - The application groups "gp1" and "gp5"
Res: 2.05 (Content)
Content-Format: 40
Payload:
<coap://grp.example:5685/gp/gp1>;rt=g.light,
<coap://grp2.example/gp/gp5>;rt=g.lock
// Response from server S2, as member of:
// - The CoAP group "grp.example:5685"
// - The application groups "gp1" and "gp2"
Res: 2.05 (Content)
Content-Format: 40
Payload:
<coap://grp.example:5685/gp/gp1>;rt=g.light,
<coap://grp.example:5685/gp/gp2>;rt=g.light
// Response from server S3, as member of:
// - The CoAP group "grp2.example"
// - The application group "gp5"
Res: 2.05 (Content)
Content-Format: 40
Payload:
<coap://grp2.example/gp/gp5>;rt=g.lock
Figure 14: Discovery of the resources and members of any
application group, and of the associated CoAP group
Alternatively, some applications may use the "rt" attribute on a
parent resource to denote support for a particular REST API to access
child resources.
Dijk, et al. Expires 25 April 2024 [Page 76]
Internet-Draft Group Communication for CoAP October 2023
For instance, Figure 15 provides a different example where a custom
Link Format attribute "gpt" is used to denote the group type within
the scope of the application/system. An alternative, shorter
encoding (not shown in the figure) is to use only the value "1" for
each "gpt" attribute, in order to denote that the resource is of type
application group. In that case, information about the semantics/API
of the group resource is disclosed only via the "rt" attribute as
shown in the figure.
// Request to realm-local members of any application group
Req: GET coap://[ff03::fd]/.well-known/core?gpt=*
// Response from server S1, as member of:
// - The CoAP groups "grp.example:5685" and "grp2.example"
// - The application groups "gp1" and "gp5"
Res: 2.05 (Content)
Content-Format: 40
Payload:
<coap://grp.example:5685/gp/gp1>;rt=oic.d.light;gpt=light,
<coap://grp2.example/gp/gp5>;rt=oic.d.smartlock;gpt=lock
// Response from server S2, as member of:
// - The CoAP group "grp.example:5685"
// - The application groups "gp1" and "gp2"
Res: 2.05 (Content)
Content-Format: 40
Payload:
<coap://grp.example:5685/gp/gp1>;rt=oic.d.light;gpt=light,
<coap://grp.example:5685/gp/gp2>;rt=oic.d.light;gpt=light
// Response from server S3, as member of:
// - The CoAP group "grp2.example"
// - The application group "gp5"
Res: 2.05 (Content)
Content-Format: 40
Payload:
<coap://grp2.example/gp/gp5>;rt=oic.d.smartlock;gpt=lock
Figure 15: Example of using a custom 'gpt' link attribute to
denote group type
Appendix D. Examples of Message Exchanges
This section provides examples of different message exchanges when
CoAP is used with group communication. The examples consider:
* A client with address ADDR_CLIENT and port number PORT_CLIENT.
Dijk, et al. Expires 25 April 2024 [Page 77]
Internet-Draft Group Communication for CoAP October 2023
* A CoAP group associated with the IP multicast address ADDR_GRP and
port number PORT_GRP.
* An application group "gp1" associated with the CoAP group above.
* Three servers A, B, and C, all of which are members of the CoAP
group above and of the application group "gp1". Each server X
(with X equal to A, B, or C): listens to its own address ADDR_X
and port number PORT_X; and listens to the address ADDR_GRP and
port number PORT_GRP. For each server its PORT_X may be different
from PORT_GRP or may be equal to it, in general.
In Figure 16, the client sends a Non-confirmable GET request to the
CoAP group, targeting the resource "temperature" in the application
group "gp1". All servers reply with a 2.05 (Content) response,
although the response from server B is lost. As source port number
of their response, servers A and B use the destination port number of
the request, i.e., PORT_GRP. Instead, server C uses its own port
number PORT_C.
Dijk, et al. Expires 25 April 2024 [Page 78]
Internet-Draft Group Communication for CoAP October 2023
Client A B C
| | | |
| GET | | |
+-------+------->| | | Source: ADDR_CLIENT:PORT_CLIENT
| \ | | | Destination: ADDR_GRP:PORT_GRP
| `.------->| | Header: GET (T=NON, Code=0.01, MID=0x7d41)
| `. | | | Token: 0x86
| `------->| Uri-Path: "gp"
| | | | Uri-Path: "gp1"
| | | | Uri-Path: "temperature"
| | | |
|<---------------+ | | Source: ADDR_A:PORT_GRP
| 2.05 | | | Destination: ADDR_CLIENT:PORT_CLIENT
| | | | Header: 2.05 (T=NON, Code=2.05, MID=0x60b1)
| | | | Token: 0x86
| | | | Payload: "22.3 C"
| | | |
| X---------------+ | Source: ADDR_B:PORT_GRP
| 2.05 | | | Destination: ADDR_CLIENT:PORT_CLIENT
| | | | Header: 2.05 (T=NON, Code=2.05, MID=0x01a0)
| | | | Token: 0x86
| | | | Payload: "20.9 C"
| | | |
| | | |
|<---------------------+ Source: ADDR_C:PORT_C
| 2.05 | | | Destination: ADDR_CLIENT:PORT_CLIENT
| | | | Header: 2.05 (T=NON, Code=2.05, MID=0x952a)
| | | | Token: 0x86
| | | | Payload: "21.0 C"
| | | |
Figure 16: Example of Non-confirmable group request, followed by
Non-confirmable Responses
In Figure 17, the client sends a Non-confirmable GET request to the
CoAP group, targeting and requesting to observe the resource
"temperature" in the application group "gp1". All servers reply with
a 2.05 (Content) notification response. As source port number of
their response, servers A and B use the destination port number of
the request, i.e., PORT_GRP. Instead, server C uses its own port
number PORT_C. Some time later, all servers send a 2.05 (Content)
notification response, with the new representation of the
"temperature" resource as payload.
Dijk, et al. Expires 25 April 2024 [Page 79]
Internet-Draft Group Communication for CoAP October 2023
Client A B C
| | | |
| GET | | |
+-------+------->| | | Source: ADDR_CLIENT:PORT_CLIENT
| \ | | | Destination: ADDR_GRP:PORT_GRP
| `.------->| | Header: GET (T=NON, Code=0.01, MID=0x7d41)
| `. | | | Token: 0x86
| `------->| Observe: 0 (register)
| | | | Uri-Path: "gp"
| | | | Uri-Path: "gp1"
| | | | Uri-Path: "temperature"
| | | |
|<---------------+ | | Source: ADDR_A:PORT_GRP
| 2.05 | | | Destination: ADDR_CLIENT:PORT_CLIENT
| | | | Header: 2.05 (T=NON, Code=2.05, MID=0x60b1)
| | | | Token: 0x86
| | | | Observe: 3
| | | | Payload: "22.3 C"
| | | |
|<------------------+ | Source: ADDR_B:PORT_GRP
| 2.05 | | | Destination: ADDR_CLIENT:PORT_CLIENT
| | | | Header: 2.05 (T=NON, Code=2.05, MID=0x01a0)
| | | | Token: 0x86
| | | | Observe: 13
| | | | Payload: "20.9 C"
| | | |
|<---------------------+ Source: ADDR_C:PORT_C
| 2.05 | | | Destination: ADDR_CLIENT:PORT_CLIENT
| | | | Header: 2.05 (T=NON, Code=2.05, MID=0x952a)
| | | | Token: 0x86
| | | | Observe: 23
| | | | Payload: "21.0 C"
| | | |
// The temperature changes ...
| | | |
|<---------------+ | | Source: ADDR_A:PORT_GRP
| 2.05 | | | Destination: ADDR_CLIENT:PORT_CLIENT
| | | | Header: 2.05 (T=NON, Code=2.05, MID=0x60b2)
| | | | Token: 0x86
| | | | Observe: 7
| | | | Payload: "32.3 C"
| | | |
|<------------------+ | Source: ADDR_B:PORT_GRP
| 2.05 | | | Destination: ADDR_CLIENT:PORT_CLIENT
| | | | Header: 2.05 (T=NON, Code=2.05, MID=0x01a1)
| | | | Token: 0x86
Dijk, et al. Expires 25 April 2024 [Page 80]
Internet-Draft Group Communication for CoAP October 2023
| | | | Observe: 18
| | | | Payload: "30.9 C"
| | | |
|<---------------------+ Source: ADDR_C:PORT_C
| 2.05 | | | Destination: ADDR_CLIENT:PORT_CLIENT
| | | | Header: 2.05 (T=NON, Code=2.05, MID=0x952b)
| | | | Token: 0x86
| | | | Observe: 29
| | | | Payload: "31.0 C"
| | | |
Figure 17: Example of Non-confirmable Observe group request,
followed by Non- confirmable Responses as Observe notifications
In Figure 18, the client sends a Non-confirmable GET request to the
CoAP group, targeting the resource "log" in the application group
"gp1", and requesting a blockwise transfer. All servers reply with a
2.05 (Content) response including the first block. As source port
number of its response, each server uses its own port number. After
obtaining the first block, the client requests the following blocks
separately from each server, by means of unicast exchanges.
Client A B C
| | | |
| GET | | |
+-------+------->| | | Source: ADDR_CLIENT:PORT_CLIENT
| \ | | | Destination: ADDR_GRP:PORT_GRP
| `.------->| | Header: GET (T=NON, Code=0.01, MID=0x7d41)
| `. | | | Token: 0x86
| `------->| Uri-Path: "gp"
| | | | Uri-Path: "gp1"
| | | | Uri-Path: "log"
| | | | Block2: 0/0/64
| | | |
|<---------------+ | | Source: ADDR_A:PORT_A
| 2.05 | | | Destination: ADDR_CLIENT:PORT_CLIENT
| | | | Header: 2.05 (T=NON, Code=2.05, MID=0x60b1)
| | | | Token: 0x86
| | | | Block2: 0/1/64
| | | | Payload: 0x0a00 ...
| | | |
|<------------------+ | Source: ADDR_B:PORT_B
| 2.05 | | | Destination: ADDR_CLIENT:PORT_CLIENT
| | | | Header: 2.05 (T=NON, Code=2.05, MID=0x01a0)
| | | | Token: 0x86
| | | | Block2: 0/1/64
| | | | Payload: 0x0b00 ...
| | | |
Dijk, et al. Expires 25 April 2024 [Page 81]
Internet-Draft Group Communication for CoAP October 2023
|<---------------------+ Source: ADDR_C:PORT_C
| 2.05 | | | Destination: ADDR_CLIENT:PORT_CLIENT
| | | | Header: 2.05 (T=NON, Code=4.04, MID=0x952a)
| | | | Token: 0x86
| | | | Block2: 0/1/64
| | | | Payload: 0x0c00 ...
| | | |
| GET | | |
+--------------->| | | Source: ADDR_CLIENT:PORT_CLIENT
| | | | Destination: ADDR_A:PORT_A
| | | | Header: GET (T=CON, Code=0.01, MID=0x7d42)
| | | | Token: 0xa6
| | | | Uri-Path: "gp"
| | | | Uri-Path: "gp1"
| | | | Uri-Path: "log"
| | | | Block2: 1/0/64
| | | |
|<---------------+ | | Source: ADDR_A:PORT_A
| 2.05 | | | Destination: ADDR_CLIENT:PORT_CLIENT
| | | | Header: 2.05 (T=ACK, Code=2.05, MID=0x7d42)
| | | | Token: 0xa6
| | | | Block2: 1/1/64
| | | | Payload: 0x0a01 ...
| | | |
| GET | | |
+--------------->| | | Source: ADDR_CLIENT:PORT_CLIENT
| | | | Destination: ADDR_A:PORT_A
| | | | Header: GET (T=CON, Code=0.01, MID=0x7d43)
| | | | Token: 0xa7
| | | | Uri-Path: "gp"
| | | | Uri-Path: "gp1"
| | | | Uri-Path: "log"
| | | | Block2: 2/0/64
| | | |
|<---------------+ | | Source: ADDR_A:PORT_A
| 2.05 | | | Destination: ADDR_CLIENT:PORT_CLIENT
| | | | Header: 2.05 (T=ACK, Code=2.05, MID=0x7d43)
| | | | Token: 0xa7
| | | | Block2: 2/0/64
| | | | Payload: 0x0a02 ...
| | | |
| GET | | |
+------------------>| | Source: ADDR_CLIENT:PORT_CLIENT
| | | | Destination: ADDR_B:PORT_B
| | | | Header: GET (T=CON, Code=0.01, MID=0x7d44)
| | | | Token: 0xb6
| | | | Uri-Path: "gp"
| | | | Uri-Path: "gp1"
Dijk, et al. Expires 25 April 2024 [Page 82]
Internet-Draft Group Communication for CoAP October 2023
| | | | Uri-Path: "log"
| | | | Block2: 1/0/64
| | | |
|<------------------+ | Source: ADDR_B:PORT_B
| 2.05 | | | Destination: ADDR_CLIENT:PORT_CLIENT
| | | | Header: 2.05 (T=ACK, Code=2.05, MID=0x7d44)
| | | | Token: 0xb6
| | | | Block2: 1/1/64
| | | | Payload: 0x0b01 ...
| | | |
| GET | | |
+------------------>| | Source: ADDR_CLIENT:PORT_CLIENT
| | | | Destination: ADDR_C:PORT_B
| | | | Header: GET (T=CON, Code=0.01, MID=0x7d45)
| | | | Token: 0xb7
| | | | Uri-Path: "gp"
| | | | Uri-Path: "gp1"
| | | | Uri-Path: "log"
| | | | Block2: 2/0/64
| | | |
|<------------------+ | Source: ADDR_B:PORT_B
| 2.05 | | | Destination: ADDR_CLIENT:PORT_CLIENT
| | | | Header: 2.05 (T=ACK, Code=2.05, MID=0x7d45)
| | | | Token: 0xb7
| | | | Block2: 2/0/64
| | | | Payload: 0x0b02 ...
| | | |
| GET | | |
+--------------------->| Source: ADDR_CLIENT:PORT_CLIENT
| | | | Destination: ADDR_C:PORT_C
| | | | Header: GET (T=CON, Code=0.01, MID=0x7d46)
| | | | Token: 0xc6
| | | | Uri-Path: "gp"
| | | | Uri-Path: "gp1"
| | | | Uri-Path: "log"
| | | | Block2: 1/0/64
| | | |
|<---------------------+ Source: ADDR_C:PORT_C
| 2.05 | | | Destination: ADDR_CLIENT:PORT_CLIENT
| | | | Header: 2.05 (T=ACK, Code=2.05, MID=0x7d46)
| | | | Token: 0xc6
| | | | Block2: 1/1/64
| | | | Payload: 0x0c01 ...
| | | |
| GET | | |
+--------------------->| Source: ADDR_CLIENT:PORT_CLIENT
| | | | Destination: ADDR_C:PORT_C
| | | | Header: GET (T=CON, Code=0.01, MID=0x7d47)
Dijk, et al. Expires 25 April 2024 [Page 83]
Internet-Draft Group Communication for CoAP October 2023
| | | | Token: 0xc7
| | | | Uri-Path: "gp"
| | | | Uri-Path: "gp1"
| | | | Uri-Path: "log"
| | | | Block2: 2/0/64
| | | |
|<---------------------+ Source: ADDR_C:PORT_C
| 2.05 | | | Destination: ADDR_CLIENT:PORT_CLIENT
| | | | Header: 2.05 (T=ACK, Code=2.05, MID=0x7d47)
| | | | Token: 0xc7
| | | | Block2: 2/0/64
| | | | Payload: 0x0c02 ...
| | | |
Figure 18: Example of Non-confirmable group request starting a
blockwise transfer, followed by Non-confirmable Responses with
the first block. The transfer continues over confirmable unicast
exchanges
Appendix E. Document Updates
This section is to be removed before publishing as an RFC.
RFC EDITOR: PLEASE REMOVE THIS SECTION.
E.1. Version -09 to -10
* Clarified use of NoSec for 'early discovery' and refer to cBRSKI.
* Clarified mandatory vs optional elements in CoAP group and CoAP
group URI.
* Replaced "GROUPNAME" with "APPNAME".
* Explicitly mentioning of the type of group (CoAP/application/
security).
* Use of .example for example hostnames.
* The name of a security group is not necessarily a string.
* Changed "has to" to "should" for enforcing access control based on
membership to security groups.
* Used normative language for policies about group rekeying.
* Further stressed that group communication ought to be secured.
Dijk, et al. Expires 25 April 2024 [Page 84]
Internet-Draft Group Communication for CoAP October 2023
* Avoid calling applications as "(non-)sensitive" and
"(non-)critical".
* Clarification on source authentication, source addresses, and Echo
Option.
* Editorial fixes and improvements.
E.2. Version -08 to -09
* Multiple responses in long exchanges are non-traditional
responses.
* Updated references.
E.3. Version -07 to -08
* Updated references.
E.4. Version -06 to -07
* Updated list of changes to other documents.
* Added real-life context and clarifications to examples.
* Clarified aliasing of CoAP group names.
* Clarified use of security group names.
* Clarified response suppression.
* Clarified response revalidation.
* Clarified limitations and peculiarities when using proxies.
* Discussed the case of group request sent to multiple proxies at
once.
* Discussed limited use of reliable transports with block-wise
transfer.
* Revised text on joining CoAP groups and multicast routing.
* Clarified use/avoidance of the CoAP NoSec mode.
* Moved examples of application group naming and group discovery to
appendix sections.
Dijk, et al. Expires 25 April 2024 [Page 85]
Internet-Draft Group Communication for CoAP October 2023
* Revised list of references.
* Updated list of implementations supporting group communication.
* Editorial improvements.
E.5. Version -05 to -06
* Harmonized use of "group URI".
* Clarifications about different group types.
* Revised methods to perform group naming.
* Revised methods to discover application groups and CoAP groups.
* Explicit difference between "authentication credential" and
"public key".
* Added examples of application group naming.
* Added examples of application/CoAP group discovery.
* Added examples of message exchanges.
* Reference to draft-mattsson-core-coap-attacks replaced with
reference to draft-mattsson-t2trg-amplification-attacks.
* Editorial improvements.
E.6. Version -04 to -05
* Clarified changes to other documents.
* Clarified relation between different group types.
* Clarified discovery of application groups.
* Discussed methods to express application group names in requests.
* Revised and extended text on the NoSec mode and amplification
attacks.
* Rephrased backward/forward security as properties.
* Removed appendix on Multi-ETag Option for response revalidation.
* Editorial improvements.
Dijk, et al. Expires 25 April 2024 [Page 86]
Internet-Draft Group Communication for CoAP October 2023
E.7. Version -03 to -04
* Multi-ETag Option for response revalidation moved to appendix.
* ETag Option usage added.
* Q-Block Options added in the block-wise transfer section.
* Caching at proxies moved to draft-tiloca-core-groupcomm-proxy.
* Client-Proxy response revalidation with the Group-ETag Option
moved to draft-tiloca-core-groupcomm-proxy.
* Security considerations on amplification attacks.
* Generalized transport protocols to include others than UDP/IP
multicast; and security protocols other than Group OSCORE.
* Overview of security cases with proxies.
* Editorial improvements.
E.8. Version -02 to -03
* Multiple responses from same server handled at the application.
* Clarifications about issues with forward-proxies.
* Operations for reverse-proxies.
* Caching of responses at proxies.
* Client-Server response revalidation, with Multi-ETag Option.
* Client-Proxy response revalidation, with the Group-ETag Option.
E.9. Version -01 to -02
* Clarified relation between security groups and application groups.
* Considered also FETCH for requests over IP multicast.
* More details on Observe re-registration.
* More details on Proxy intermediaries.
* More details on servers changing port number in the response.
Dijk, et al. Expires 25 April 2024 [Page 87]
Internet-Draft Group Communication for CoAP October 2023
* Usage of the Uri-Host Option to indicate an application group.
* Response suppression based on classes of error codes.
E.10. Version -00 to -01
* Clarifications on group memberships for the different group types.
* Simplified description of Token reusage, compared to the unicast
case.
* More details on the rationale for response suppression.
* Clarifications of creation and management of security groups.
* Clients more knowledgeable than proxies about stopping receiving
responses.
* Cancellation of group observations.
* Clarification on multicast scope to use.
* Both the group mode and pairwise mode of Group OSCORE are
considered.
* Updated security considerations.
* Editorial improvements.
Acknowledgments
The authors sincerely thank Christian Amsüss, Carsten Bormann, Thomas
Fossati, Rikard Höglund, Jaime Jiménez, John Preuß Mattsson, Jim
Schaad, and Jon Shallow 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 projects SIFIS-
Home (Grant agreement 952652) and ARCADIAN-IoT (Grant agreement
101020259).
Authors' Addresses
Esko Dijk
IoTconsultancy.nl
Utrecht
Netherlands
Email: esko.dijk@iotconsultancy.nl
Dijk, et al. Expires 25 April 2024 [Page 88]
Internet-Draft Group Communication for CoAP October 2023
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
SE-16440 Stockholm Kista
Sweden
Email: marco.tiloca@ri.se
Dijk, et al. Expires 25 April 2024 [Page 89]
