Group Communication for the Constrained Application Protocol (CoAP)
draft-ietf-core-groupcomm-bis-05

Document Type Active Internet-Draft (core WG)
Authors Esko Dijk  , Chonggang Wang  , Marco Tiloca 
Last updated 2021-10-25
Replaces draft-dijk-core-groupcomm-bis
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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: 28 April 2022                                           RISE AB
                                                         25 October 2021

  Group Communication for the Constrained Application Protocol (CoAP)
                    draft-ietf-core-groupcomm-bis-05

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 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/.

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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 28 April 2022.

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   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Changes to Other Documents  . . . . . . . . . . . . . . .   6
   2.  Group Definition and Group Configuration  . . . . . . . . . .   6
     2.1.  Group Definition  . . . . . . . . . . . . . . . . . . . .   6
       2.1.1.  CoAP Group  . . . . . . . . . . . . . . . . . . . . .   7
       2.1.2.  Application Group . . . . . . . . . . . . . . . . . .   7
       2.1.3.  Security Group  . . . . . . . . . . . . . . . . . . .   7
       2.1.4.  Relations Between Group Types . . . . . . . . . . . .   8
     2.2.  Group Configuration . . . . . . . . . . . . . . . . . . .  10
       2.2.1.  Group Naming  . . . . . . . . . . . . . . . . . . . .  10
       2.2.2.  Group Creation and Membership . . . . . . . . . . . .  12
       2.2.3.  Group Discovery . . . . . . . . . . . . . . . . . . .  13
       2.2.4.  Group Maintenance . . . . . . . . . . . . . . . . . .  15
   3.  CoAP Usage in Group Communication . . . . . . . . . . . . . .  16
     3.1.  Request/Response Model  . . . . . . . . . . . . . . . . .  16
       3.1.1.  General . . . . . . . . . . . . . . . . . . . . . . .  16
       3.1.2.  Response Suppression  . . . . . . . . . . . . . . . .  17
       3.1.3.  Repeating a Request . . . . . . . . . . . . . . . . .  17
       3.1.4.  Request/Response Matching and Distinguishing
               Responses . . . . . . . . . . . . . . . . . . . . . .  18
       3.1.5.  Token Reuse . . . . . . . . . . . . . . . . . . . . .  18
       3.1.6.  Client Handling of Multiple Responses With Same
               Token . . . . . . . . . . . . . . . . . . . . . . . .  19

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     3.2.  Caching . . . . . . . . . . . . . . . . . . . . . . . . .  20
       3.2.1.  Freshness Model . . . . . . . . . . . . . . . . . . .  21
       3.2.2.  Validation Model  . . . . . . . . . . . . . . . . . .  21
     3.3.  URI Path Selection  . . . . . . . . . . . . . . . . . . .  22
     3.4.  Port Selection for UDP Transport  . . . . . . . . . . . .  23
     3.5.  Proxy Operation . . . . . . . . . . . . . . . . . . . . .  23
       3.5.1.  Forward-Proxies . . . . . . . . . . . . . . . . . . .  24
       3.5.2.  Reverse-Proxies . . . . . . . . . . . . . . . . . . .  25
     3.6.  Congestion Control  . . . . . . . . . . . . . . . . . . .  27
     3.7.  Observing Resources . . . . . . . . . . . . . . . . . . .  28
     3.8.  Block-Wise Transfer . . . . . . . . . . . . . . . . . . .  30
     3.9.  Transport Protocols . . . . . . . . . . . . . . . . . . .  31
       3.9.1.  UDP/IPv6 Multicast Transport  . . . . . . . . . . . .  31
       3.9.2.  UDP/IPv4 Multicast Transport  . . . . . . . . . . . .  32
       3.9.3.  6LoWPAN . . . . . . . . . . . . . . . . . . . . . . .  32
       3.9.4.  Other Transports  . . . . . . . . . . . . . . . . . .  32
     3.10. Interworking with Other Protocols . . . . . . . . . . . .  33
       3.10.1.  MLD/MLDv2/IGMP/IGMPv3  . . . . . . . . . . . . . . .  33
       3.10.2.  RPL  . . . . . . . . . . . . . . . . . . . . . . . .  33
       3.10.3.  MPL  . . . . . . . . . . . . . . . . . . . . . . . .  34
   4.  Unsecured Group Communication . . . . . . . . . . . . . . . .  35
   5.  Secured Group Communication using Group OSCORE  . . . . . . .  35
     5.1.  Group OSCORE  . . . . . . . . . . . . . . . . . . . . . .  35
     5.2.  Secure Group Maintenance  . . . . . . . . . . . . . . . .  37
     5.3.  Proxy Security  . . . . . . . . . . . . . . . . . . . . .  38
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  39
     6.1.  CoAP NoSec Mode . . . . . . . . . . . . . . . . . . . . .  39
     6.2.  Group OSCORE  . . . . . . . . . . . . . . . . . . . . . .  40
       6.2.1.  Group Key Management  . . . . . . . . . . . . . . . .  41
       6.2.2.  Source Authentication . . . . . . . . . . . . . . . .  41
       6.2.3.  Countering Attacks  . . . . . . . . . . . . . . . . .  42
     6.3.  Risk of Amplification . . . . . . . . . . . . . . . . . .  44
     6.4.  Replay of Non-Confirmable Messages  . . . . . . . . . . .  45
     6.5.  Use of CoAP No-Response Option  . . . . . . . . . . . . .  46
     6.6.  6LoWPAN and MPL . . . . . . . . . . . . . . . . . . . . .  47
     6.7.  Wi-Fi . . . . . . . . . . . . . . . . . . . . . . . . . .  47
     6.8.  Monitoring  . . . . . . . . . . . . . . . . . . . . . . .  48
       6.8.1.  General Monitoring  . . . . . . . . . . . . . . . . .  48
       6.8.2.  Pervasive Monitoring  . . . . . . . . . . . . . . . .  48
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  49
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  49
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  49
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  51
   Appendix A.  Use Cases  . . . . . . . . . . . . . . . . . . . . .  54
     A.1.  Discovery . . . . . . . . . . . . . . . . . . . . . . . .  54
       A.1.1.  Distributed Device Discovery  . . . . . . . . . . . .  55
       A.1.2.  Distributed Service Discovery . . . . . . . . . . . .  55
       A.1.3.  Directory Discovery . . . . . . . . . . . . . . . . .  55

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     A.2.  Operational Phase . . . . . . . . . . . . . . . . . . . .  56
       A.2.1.  Actuator Group Control  . . . . . . . . . . . . . . .  56
       A.2.2.  Device Group Status Request . . . . . . . . . . . . .  56
       A.2.3.  Network-wide Query  . . . . . . . . . . . . . . . . .  56
       A.2.4.  Network-wide / Group Notification . . . . . . . . . .  57
     A.3.  Software Update . . . . . . . . . . . . . . . . . . . . .  57
   Appendix B.  Document Updates . . . . . . . . . . . . . . . . . .  57
     B.1.  Version -04 to -05  . . . . . . . . . . . . . . . . . . .  57
     B.2.  Version -03 to -04  . . . . . . . . . . . . . . . . . . .  58
     B.3.  Version -02 to -03  . . . . . . . . . . . . . . . . . . .  58
     B.4.  Version -01 to -02  . . . . . . . . . . . . . . . . . . .  59
     B.5.  Version -00 to -01  . . . . . . . . . . . . . . . . . . .  59
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  59
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  60

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
   response message back to the client over UDP/IP unicast.  Notable
   CoAP implementations supporting group communication include the
   framework "Eclipse Californium" 2.0.x [Californium] from the Eclipse
   Foundation and the "Implementation of CoAP Server & Client in Go"
   [Go-OCF] from the Open Connectivity Foundation (OCF).

   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)
   [I-D.ietf-cose-rfc8152bis-struct][I-D.ietf-cose-rfc8152bis-algs].

   This documents 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.

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   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 left for future work.
   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.

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.

   Furthermore, "Security material" refers to any security keys,
   counters or parameters stored in a device that are required to
   participate in secure group communication with other devices.

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1.3.  Changes to Other Documents

   This document obsoletes and replaces [RFC7390] as follows.

   *  It defines 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 "NoSec" mode (see Section 4 and Section 6.1) and
      highlights the risk of amplification attacks (see Section 6.3).

   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).

   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.

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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.  An
   endpoint may be a member of multiple CoAP groups by subscribing to
   multiple IP multicast groups and/or listening on multiple UDP ports.
   Group membership(s) of an endpoint may dynamically change over time.
   A device sending a CoAP group message to a CoAP group is not
   necessarily itself a member of this CoAP group: it is a member only
   if it also has a CoAP endpoint listening on the group's associated IP
   multicast address and UDP port.  A CoAP group can be encoded within a
   Group URI.  This is defined as a CoAP URI that has the "coap" scheme
   and includes in the authority part either an IP multicast address or
   a group hostname (e.g., a Group Fully Qualified Domain Name (FQDN))
   that can be resolved to an IP multicast address.  A Group URI also
   contains an optional UDP port number in the authority part.  Group
   URIs follow the regular CoAP URI syntax (see Section 6 of [RFC7252]).

2.1.2.  Application Group

   Besides CoAP groups, that have relevance at the level of IP networks
   and CoAP endpoints, there are also application groups.  An
   application group is a set of CoAP server endpoints that share a
   common set of CoAP resources.  An endpoint may be a member of
   multiple application groups.  An application group has relevance at
   the application level -- for example an application group could
   denote all lights in an office room or all sensors in a hallway.  A
   client endpoint that sends a group communication message to an
   application group is not necessarily itself a member of this
   application group.  There can be a one-to-one or a one-to-many
   relation between a CoAP group and application group(s).  An
   application group name is optionally encoded explicitly in the CoAP
   request, for example in the URI path.  If not explicitly encoded, the
   application group is implicitly derived by the receiver, based on
   information in the CoAP request.  See Section 2.2.1 for more details
   on identifying the application group.

2.1.3.  Security Group

   For secure group communication, a security group is required.  A
   security group is a group of endpoints that each store group security
   material, such that they can mutually exchange secured messages and
   verify secured messages.  So, a client endpoint needs to be a member
   of a security group in order to send a valid secured group
   communication message to this group.  An endpoint may be a member of
   multiple security groups.  There can be a many-to-many relation
   between security groups and CoAP groups, but often it is one-to-one.

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   Also, there can be a many-to-many relation between security groups
   and application groups, but often it is one-to-one.  A special
   security group named "NoSec" identifies group communication without
   any security at the transport layer nor at the CoAP layer.

2.1.4.  Relations Between Group Types

   Using the above group type definitions, a CoAP group communication
   message sent by an endpoint can be represented as a tuple that
   contains one instance of each group type:

   (application group, CoAP group, security group)

   A special note is appropriate about the possible relation between
   security groups and application groups.

   On one hand, multiple application groups may use the same security
   group.  Thus, the same group security material is used to protect the
   messages targeting any of those application groups.  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 used server
   port number, the targeted resource, or the content and structure of
   the message payload.

   On the other hand, a single application group may use multiple
   security groups.  Thus, different messages targeting the resources of
   the application group can be protected with different security
   material.  This can be convenient, for example, if the security
   groups differ with respect to the 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 same 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 a same application
   group.  That is, 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.  It has to be separately enforced by leveraging the resource
   properties or through dedicated access control credentials assessed
   by separate means.

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   Figure 1 summarizes the relations between the different types of
   groups described above in UML class diagram notation.  The class
   attributes in square brackets are optionally defined.

    +------------------------+                 +--------------------+
    |   Application group    |                 |    CoAP group      |
    |........................|                 |....................|
    |                        |                 |                    |
    | [ - group name ]       +-----------------+ - IP mcast address |
    |                        |  1...N       1  | - UDP port         |
    |                        |                 |                    |
    |                        |                 |                    |
    +-------------+----------+                 +---------+----------+
                  |  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).  There are three
   application groups (1, 2, 3) and two security groups (1, 2).
   Security Group 1 is used by both Application Group 1 and 2.  Three
   clients (Cli1, Cli2, Cli3) are configured with security material for
   Security Group 1.  Two clients (Cli2, Cli4) are configured with
   security material for Security Group 2.  All the shown application
   groups use the same CoAP group (not shown in the figure), i.e., one
   specific multicast IP address and UDP port on which all the shown
   resources are hosted for each server.

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  ________________________________    _________________________________
 /                                \  /                                 \
 |       +---------------------+  |  |  +---------------------+        |
 |       | Application Group 1 |  |  |  | Application Group 3 |  Cli2  |
 |       |                     |  |  |  |                     |        |
 | Cli1  | Srv1   Srv2   Srv3  |  |  |  | Srv5   Srv6         |  Cli4  |
 |       | /resA  /resA  /resA |  |  |  | /resC  /resC        |        |
 | Cli2  +---------------------+  |  |  | /resD  /resD        |        |
 |                                |  |  +---------------------+        |
 | Cli3     Security Group 1      |  |                                 |
 |                                |  |        Security Group 2         |
 |       +---------------------+  |  \_________________________________/
 |       | Application Group 2 |  |
 |       |                     |  |
 |       | Srv1   Srv4         |  |
 |       | /resB  /resB        |  |
 |       +---------------------+  |
 \________________________________/

         Figure 2: Deployment Example of Different Group Types

2.2.  Group Configuration

   The following defines how groups of different types are named,
   created, discovered and maintained.

2.2.1.  Group Naming

   A CoAP group is identified and named by the authority component in
   the Group URI, which includes host (possibly an IP multicast address
   literal) and an optional UDP port number.  It is recommended to
   configure an endpoint with an IP multicast address literal, instead
   of a hostname, when configuring a CoAP group membership.  This is
   because DNS infrastructure may not be deployed in many constrained
   networks.  In case a group hostname is configured, it can be uniquely
   mapped to an IP multicast address via DNS resolution - if DNS client
   functionality is available in the endpoint being configured and the
   DNS service is supported in the network.  Some examples of
   hierarchical CoAP group FQDN naming (and scoping) for a building
   control application were shown in Section 2.2 of [RFC7390].

   An application group can be named in many ways through different
   types of identifiers, such as name string, (integer) number, URI or
   other type of string.  An application group name may be explicitly
   encoded in a CoAP Group URI, or it may be not included in the Group
   URI.  This is an implementation-specific decision.  If the
   application group name is explicitly encoded in a CoAP Group URI, it
   can be encoded within one of the

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   *  URI path component: this is the most common and RECOMMENDED method
      to encode the application group name.  A best practice for
      encoding application group into a Group URI is to use one URI path
      component to identify the application group and use the following
      URI paths component(s) to identify the resource within this
      application group.  For example, /<groupname>/res1 or
      /base/<groupname>/res1/res2 conform to this practice.  An
      application group name (like <groupname>) should be as short as
      possible when used in constrained networks.

   *  URI query component: using this method, the query may consist of
      the group name (?<groupname>) or it may be one parameter of the
      query (?g=<groupname> or ?param1=value1&gn=<groupname>).

   *  URI host subcomponent: using this method, the application group
      becomes equal to the CoAP group.  This can only be used if there
      is a one-to-one mapping between CoAP groups and application
      groups.

   *  URI port subcomponent: using this method, the application group is
      identified by a number that is encoded in some way in the
      destination port.

   There are also methods to encode the application group name within
   the CoAP request even though it is not encoded within the Group URI.
   Examples of such methods are:

   *  encode in a Uri-Host Option [RFC7252] which is added to the CoAP
      request by the client before sending it out.  Each CoAP server
      that is part of the CoAP group, receiving this request, decodes
      the Uri-Host Option and treats it as an application group name.
      (It can also treat the application group name in this Option as a
      "virtual CoAP server" specific to that application group, exactly
      in the same way that the Uri-Host Option was intended to allow
      support for multiple virtual servers hosted on the same port.  The
      net effect of both treatments is the same.)

   *  encode in a new (custom/application-specific) CoAP Option which is
      added to the CoAP request by the client before sending it out.
      Each CoAP server that is part of the CoAP group, receiving this
      request, would by design understand this Option, would decode it,
      and treat it as an application group name.

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   Finally, it is possible to not encode the application group name at
   all within the CoAP request.  This yields the most compact
   representation on the wire.  In this case, each CoAP server needs to
   determine the application group based on contextual information, such
   as client identity and/or target resource.  For example, each
   application group on a server could have a unique set of resources
   that does not overlap with any resources of other application groups.

   Appendix A of [I-D.ietf-core-resource-directory] shows an example
   registration of an application group into a Resource Directory (RD),
   along with the CoAP group it uses and the resources supported by the
   application group.  In this example an application group name
   "lights" is encoded in the "ep" (endpoint) attribute of the RD
   registration entry.  The CoAP group is ff35:30:2001:db8:f1::8000:1
   and the "NoSec" security group is used.

   A security group is identified by a stable and invariant string used
   as group name, which is generally not related with other kinds of
   group identifiers, specific to the chosen security solution.  The
   "NoSec" security group name MUST be only used to represent the case
   of group communication without any security.  It is typically
   characterized by the absence of any security group name, identifier,
   or security-related data structures in the CoAP message.

2.2.2.  Group Creation and Membership

   To create a CoAP group, a configuring entity defines an IP multicast
   address (or hostname) for the group and optionally a UDP port number
   in case it differs from the default CoAP port 5683.  Then, it
   configures one or more devices as listeners to that IP multicast
   address, with a CoAP endpoint listening on the group's associated UDP
   port.  These endpoints/devices are the group members.  The
   configuring entity can be, for example, a local application with pre-
   configuration, a user, a software developer, a cloud service, or a
   local commissioning tool.  Also, the devices sending CoAP requests to
   the group in the role of CoAP client need to be configured with the
   same information, even though they are not necessarily group members.
   One way to configure a client is to supply it with a CoAP Group URI.
   The IETF does not define a mandatory protocol to accomplish CoAP
   group creation.  [RFC7390] defined an experimental protocol for
   configuration of group membership for unsecured group communication,
   based on JSON-formatted configuration resources.  For IPv6 CoAP
   groups, common multicast address ranges that are used to configure
   group addresses from are ff1x::/16 and ff3x::/16.

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   To create an application group, a configuring entity may configure a
   resource (name) or set of resources on CoAP endpoints, such that a
   CoAP request with Group URI sent by a configured CoAP client will be
   processed by one or more CoAP servers that have the matching URI path
   configured.  These servers are the application group members.

   To create a security group, a configuring entity defines an initial
   subset of the related security material.  This comprises a set of
   group properties including the cryptographic algorithms and
   parameters used in the group, as well as additional information
   relevant throughout the group life-cycle, such as the security group
   name and description.  This task MAY be entrusted to a dedicated
   administrator, that interacts with a Group Manager as defined in
   Section 5.  After that, further security materials to protect group
   communications have to be generated, compatible with the specified
   group configuration.

   To participate in a security group, CoAP endpoints have to be
   configured with the group security material used to protect
   communications in the associated application/CoAP groups.  The part
   of the process that involves secure distribution of group security
   material MAY use standardized communication with a Group Manager as
   defined in Section 5.  For unsecure group communication using the
   "NoSec" security group, any CoAP endpoint may become a group member
   at any time: there is no configuring entity that needs to provide
   security material for this group, as there is no security material
   for it.  This means that group creation and membership cannot be
   tightly controlled for the "NoSec" group.

   The configuration of groups and membership may be performed at
   different moments in the life-cycle of a device; for example during
   product (software) creation, in the factory, at a reseller, on-site
   during first deployment, or on-site during a system reconfiguration
   operation.

2.2.3.  Group Discovery

   It is possible for CoAP endpoints to discover application groups as
   well as CoAP groups, by using the RD-Groups usage pattern of the CoRE
   Resource Directory (RD), as defined in Appendix A of
   [I-D.ietf-core-resource-directory].  In particular, an application
   group can be registered to the RD, specifying the reference IP
   multicast address of its associated CoAP group.  The registration of
   groups 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.

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   CoAP endpoints can also discover application groups and/or CoAP
   groups using a GET request on the /.well-known/core resource.  Such a
   request may be sent to a known CoAP group multicast address that is
   associated to one or more application group(s), or to the All CoAP
   Nodes multicast address; and the request may be sent with or without
   a query component.  These particular details of the request are
   selected depending on the intented discovery action of the client and
   on the particular encoding of group names in names and/or attributes
   of resources which is application-specific.  The following types of
   request may typically be used:

   *  Discovery of all application groups that are part of a CoAP group.
      For example, in case the application group name is encoded in the
      URI path as "/grp/<groupname>" then the discovery query may use
      URI "/.well-known/core?href=/grp/*" and it is sent to the IP
      multicast address of the CoAP group.  The query matches any
      application group name.  The result of this query is a list of
      resources at CoAP servers that are a member of at least one
      application group associated to the CoAP group.  Each resource
      represents an application group membership at one CoAP server.

   *  Discovery of a particular application group with a known name.
      For example, in case the known application group name is
      "mygroup1" and is encoded in the URI path as "/grp/<groupname>"
      then the discovery query may use URI "/.well-known/core?href=/grp/
      mygroup1" and it is sent to the IPv6 All CoAP Nodes multicast
      address of a particular chosen scope (e.g. site-local, or realm-
      local).  The result of this query is a list of resources at CoAP
      servers that are a member of "mygroup1".  Each resource represents
      the membership of the responding server to the application group
      "mygroup1".

   *  Discovery of all application groups of a particular type.  For
      example, in case the application group type is "mytype1" and is
      encoded as a resource type (rt) attribute of the application group
      resource, and the application group resource is "/grp/<groupname>"
      then the discovery query may use URI "/.well-known/
      core?rt=mytype1" and it is sent to the All CoAP Nodes multicast
      address.  The query result is a list of resources at CoAP servers
      that have at least one application group of type "mytype1"; each
      server response identifies the membership to one or more
      application group(s) of type "mytype1" for that server.  Each
      resource in the list represents one membership of a server to one
      application group of type "mytype1".

   *  Discovery of all application groups that are configured on the
      client's 6LoWPAN wireless mesh network.  In the following example
      a request without query is used.  So, the URI "/.well-known/core"

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      is used and the request is sent to the IPv6 All CoAP Nodes
      multicast address of realm-local scope.  Each CoAP server
      (including any application group members) will return a list of
      its resources, which in this example includes the resources used
      to encode the application group name.  If each application group
      resource is known to be "/g/<groupname>" then the client marks
      each returned resource under the root resource "/g" as a
      discovered application group.  Not using a query has the
      disadvantage that potentially a lot of response traffic will be
      generated on the mesh network, including responses from servers
      that are not a member of any application group.  But it may be
      needed in particular use cases, e.g. if some of the CoAP servers
      do not support the optional link format query functionality (as
      mentioned in Section 4.1 of [RFC6690]).

   Note that all the above examples are application-specific; there is
   currently no standard way of encoding application group names or CoAP
   group names in CoAP resources and/or CoRE link attributes at the CoAP
   server.  (The aforementioned registration and discovery of groups in
   the RD is only defined for use with an RD, not for discovery on CoAP
   servers without using an 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 own
   security groups to the RD, as links to its own corresponding
   resources for joining the security groups
   [I-D.ietf-ace-key-groupcomm-oscore].  Later on, CoAP endpoints can
   discover the registered security groups and related application
   groups, by using the lookup interface of the RD, and then join the
   security group through the respective Group Manager.

2.2.4.  Group Maintenance

   Maintenance of a group includes any necessary operations to cope with
   changes in a system, such as: adding group members, removing group
   members, changing group security material, reconfiguration of UDP
   port and/or IP multicast address, reconfiguration of the Group URI,
   renaming of application groups, splitting of groups, or merging of
   groups.

   For unsecured group communication (see Section 4) i.e., the "NoSec"
   security group, 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.

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   For secured group communication (see Section 5), the maintenance
   operations of the protocol Group OSCORE
   [I-D.ietf-core-oscore-groupcomm] MUST be implemented.  When using
   Group OSCORE, CoAP endpoints participating in group communication are
   also members of a corresponding OSCORE security group, and thus share
   common security material.  Additional related maintenance operations
   are discussed in Section 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 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 be a mixture of
   success (e.g., 2.05 Content) and failure (e.g., 4.04 Not Found)
   codes, depending on the individual server processing results.

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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.

   The CoAP No-Response Option [RFC7967] can be used by a client to
   influence the default response suppression on the server side.  It is
   RECOMMENDED for a server to support this option only on selected
   resources where it is useful in the application context.  If the
   option is supported on a resource, it MUST override the default
   response suppression of that resource.

   Any default response suppression by a server SHOULD be performed
   consistently, as follows: if a request on a resource produces a
   particular Response Code and this response is not suppressed, then
   another request on the same resource that produces a response of the
   same Response Code class is also not suppressed.  For example, if a
   4.05 Method Not Allowed error response code is suppressed by default
   on a resource, then a 4.15 Unsupported Content-Format error response
   code is also suppressed by default for that resource.

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)
   group members have been reached with the request.  This is useful in
   case a number of group members did not respond to the initial request
   and the client suspects that the request did not reach these group
   members.  However, in case one or more servers did receive the
   initial request but the response to that request was 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 group members, even if this comes at the cost of the overhead of
   certain group members responding twice (once to the original request,
   and once to the repeated request with different Message ID).

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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 to the server.  If
   secure communication is provided with Group OSCORE (see Section 5),
   additional security-related identifiers in the CoAP response enable
   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 of the
   request -- following the convention for UDP-based protocols.  In case
   a single client has sent multiple group requests and concurrent CoAP
   transactions are ongoing, the responses received by that client are
   matched to an active request using only the Token value.  Due to UDP
   level multiplexing, the UDP destination port of the response MUST
   match to the client endpoint's UDP port value, i.e., to the UDP
   source port of the client's request.

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 [I-D.ietf-core-echo-request-tag].
   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.

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   This delay includes the maximum Leisure time period as defined in
   Section 8.2 of [RFC7252].  However, CoAP does not define a time limit
   for the server response delay.  Using the default CoAP parameters,
   the Token reuse time MUST be greater than 250 seconds plus
   MAX_SERVER_RESPONSE_DELAY.  A preferred solution to meet this
   requirement is to generate a new unique Token for every new 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
   [I-D.ietf-core-echo-request-tag].

   Another method to more easily meet the above constraint is to
   instantiate multiple CoAP clients at multiple UDP ports on the same
   host.  The Token values only have to be unique within the context of
   a single CoAP client, so using multiple clients can make it easier to
   meet the constraint.

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

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   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.

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 part 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.

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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 group members may have joined the group since the last
   group request to the same group/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 server group members; and, for
      each group member, the client's cache currently stores a fresh
      response.

   How the client in the case above determines the current CoAP server
   group members is out of scope for this document.  It may be, for
   example, via a group manager server, or by observing group join
   requests, or observing IGMP/MLD multicast group join messages, etc.

   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.

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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 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 of identical ETag
   values returned by two or more servers, the client, by default,
   SHOULD NOT include an ETag Option in a group request containing that
   ETag value.

   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 can be done, 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 group members.  However there are
   valid use cases where a group request is known to be successful only
   for a subset of the CoAP group, for example only members of a
   specific application group, while those group members for which the
   request is unsuccessful (for example because they are outside the
   application group) either ignore the group request or respond with an
   error status code.

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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 5683, or another non-default UDP port if configured.
   Regardless of the method for selecting the port number, the same port
   number MUST be used across all CoAP servers that are members of a
   CoAP group and across all CoAP clients 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.

   Port 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 5683 MUST be supported
   (see Section 7.1 of [RFC7252]) for the "All CoAP Nodes" multicast
   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.  Security operations for a proxy are discussed later in
   Section 5.3.

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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].  For this purpose, the client specifies either the request
   group URI as a string in the Proxy-URI Option or it uses the Proxy-
   Scheme Option with the group URI constructed from the usual Uri-*
   Options.  The forward-proxy then resolves the group URI to a
   destination CoAP group, sends (e.g., multicasts) the CoAP group
   request, receives the responses and forwards all the individual
   (unicast) responses back to the client.

   However, there are certain issues and limitations with this approach:

   *  The CoAP client component that sent a unicast CoAP request to the
      proxy may be expecting only one (unicast) response, as usual for a
      CoAP unicast request.  Instead, it receives multiple (unicast)
      responses, potentially leading to fault conditions in the
      component or to discarding any received responses following the
      first one.  This issue may occur even if the application calling
      the CoAP client component is aware that the forward-proxy is going
      to execute a CoAP group URI request.

   *  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.

   *  The proxy does not necessarily know how many members there are in
      the CoAP group or how many group members will actually respond.
      Also, the proxy does not know for how long to collect responses
      before it stops forwarding them to the client.  A CoAP client that
      is not using a Proxy might face the same problems in collecting
      responses to a group request.  However, the client itself would
      typically have application-specific rules or knowledge on how to
      handle this situation, while an application-agnostic CoAP Proxy
      would typically not have this knowledge.  For example, a CoAP
      client could monitor incoming responses and use this information
      to decide how long to continue collecting responses - which is
      something a proxy cannot do.

   A forward-proxying method using this approach and addressing the
   issues raised above is defined in [I-D.tiloca-core-groupcomm-proxy].

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   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 only
   processes a group URI request if it is explicitly enabled to do so.
   The default response (if the function is not explicitly enabled) to a
   group URI request is 5.01 Not Implemented.  Furthermore, a proxy
   SHOULD be explicitly configured (e.g., by 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 still identify the CoAP
   servers by other means such as application-specific information in
   the response payload.

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]).

   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 a group of
   servers over a one-to-many transport such as IP/UDP multicast.

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   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].

   Furthermore, the reverse-proxy can actually stand in for (and thus
   prevent to directly reach) only the whole set of servers in the
   group, or also for each of those individual servers (e.g., if acting
   as firewall).

   For a reverse-proxy that sends a request to a group of servers, 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 the collecting of server responses and forwarding these
      back to the client.  These duration(s) may be set as global
      configuration or 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.

   *  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.

      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).

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      If this happens, the reverse-proxy MUST stop the ongoing request
      and associated response forwarding, it MUST NOT forward the new
      request to the group of servers, 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 signalling 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
   paragraph of Section 3.5.1 which applies also to the case of reverse-
   proxies.

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:

   *  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]).

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   This document also defines these measures to be applicable to
   alternative transports (other than IP multicast), if not defined
   otherwise.  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 3.9.3) 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.

   *  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,
   that allows a CoAP client to retrieve a representation of a resource
   and automatically keep this representation up-to-date over a longer
   period of time.  The client gets notified when the representation has
   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], it is
   also possible to use the Observe Option in a CoAP FETCH group
   request.

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   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 group members did not respond to the
   initial request.  The client MAY additionally use the same Message ID
   in the repeated request to avoid that group members that had already
   received the initial request would respond again.  Note that using
   the same Message ID in a repeated request will not be helpful in case
   of loss of a response message, since the server that responded
   already will consider the repeated request as a duplicate message.
   On the other hand, if the client uses a different, fresh Message ID
   in the repeated request, then all the group members that receive this
   new message will typically respond again, which increases the network
   load.

   A client that has sent a 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 group member, that was
   expected to respond to the initial group request, did not respond to
   the initial request.  In this case, the client MUST use a Message ID
   that differs from 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.

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   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 liveness 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 group (using serial
   unicast to each known group member).  In addition, a client MAY
   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 cancelled and includes an Observe Option with
   the value set to 1 (deregister).  In case not all the servers in the
   CoAP group received this deregistration request, either the unicast
   cancellation methods can be used 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 a group of servers 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.

   The client has to use unicast for any further request, separately
   addressing each different server, in order to retrieve more blocks of
   the resource from that server, if any.  Also, a server (member of a

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   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.

   [I-D.ietf-core-new-block] specifies an 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.

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 5683 MUST be supported as
   per Sections 7.1 and 12.8 of [RFC7252] for the "All CoAP Nodes"
   multicast group.  An IPv6 CoAP server SHOULD support the "All CoAP
   Nodes" groups with at least link-local (2), admin-local (4) and site-
   local (5) scopes.  An IPv6 CoAP server on a 6LoWPAN node (see
   Section 3.9.3) SHOULD also support the realm-local (3) scope.

   Note that a client sending an IPv6 multicast CoAP message to a port
   that is not supported by the server will not receive an ICMPv6 Port
   Unreachable error message from that server, because the server does
   not send it in this case, per Section 2.4 of [RFC4443].

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3.9.2.  UDP/IPv4 Multicast Transport

   CoAP group communication can use UDP over IPv4 as a transport
   protocol, provided that IPv4 multicast is enabled.  For a CoAP server
   node that supports resource discovery as defined in Section 2.4 of
   [RFC7252], the default port 5683 MUST be supported as per Sections
   7.1 and 12.8 of [RFC7252], for the "All CoAP Nodes" IPv4 multicast
   group.

   Note that a client sending an IPv4 multicast CoAP message to a port
   that is not supported by the server will not receive an ICMP Port
   Unreachable error message from that server, because the server does
   not send it in this case, per Section 3.2.2 of [RFC1122].

3.9.3.  6LoWPAN

   In 6LoWPAN [RFC4944] [RFC6282] networks, IPv6 packets (up to 1280
   bytes) may be fragmented into smaller IEEE 802.15.4 MAC frames (up to
   127 bytes), if the packet size requires this.  Every 6LoWPAN IPv6
   router that receives a multi-fragment packet reassembles the packet
   and refragments it upon transmission.  Since the loss of a single
   fragment implies the loss of the entire IPv6 packet, the performance
   in terms of packet loss and throughput of multi-fragment multicast
   IPv6 packets is typically far worse than the performance of single-
   fragment IPv6 multicast packets.  For this reason, a CoAP request
   sent over multicast in 6LoWPAN networks SHOULD be sized in such a way
   that it fits in a single IEEE 802.15.4 MAC frame, if possible.

   On 6LoWPAN networks, multicast groups can be defined with realm-local
   scope [RFC7346].  Such a realm-local group is restricted to the local
   6LoWPAN network/subnet.  In other words, a multicast request to that
   group does not propagate beyond the 6LoWPAN network segment where the
   request originated.  For example, a multicast discovery request can
   be sent to the realm-local "All CoAP Nodes" IPv6 multicast group (see
   Section 3.9.1) in order to discover only CoAP servers on the local
   6LoWPAN network.

3.9.4.  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.

   Because it supports unicast only, [RFC8323] (CoAP over TCP, TLS, and
   WebSockets) is not in scope as a transport for CoAP group
   communication.

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3.10.  Interworking with Other Protocols

3.10.1.  MLD/MLDv2/IGMP/IGMPv3

   CoAP nodes that are IP hosts (i.e., not IP routers) are generally
   unaware of the specific IP multicast routing/forwarding protocol
   being used in their network.  When such a host needs to join a
   specific (CoAP) multicast group, it requires a way to signal to IP
   multicast routers which IP multicast address(es) it needs to listen
   to.

   The MLDv2 protocol [RFC3810] is the standard IPv6 method to achieve
   this; therefore, this method SHOULD be used by members of a CoAP
   group to subscribe to its multicast IPv6 address, on IPv6 networks
   that support it.  CoAP server nodes then act in the role of MLD
   Multicast Address Listener.  MLDv2 uses link-local communication
   between Listeners and IP multicast routers.  Constrained IPv6
   networks that implement 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 IGMPv3 protocol [RFC3376] is the standard IPv4 method to signal
   multicast group subscriptions.  This SHOULD be used by members of a
   CoAP group to subscribe to its multicast IPv4 address on IPv4
   networks.

   The guidelines from [RFC6636] on the tuning of MLD for mobile and
   wireless networks may be useful when implementing MLD in constrained
   networks.

3.10.2.  RPL

   RPL [RFC6550] is an IPv6 based routing protocol suitable for low-
   power, lossy networks (LLNs).  In such a context, CoAP is often used
   as an application protocol.

   If only RPL is used in a network for routing and its optional
   multicast support is disabled, there will be no IP multicast routing
   available.  Any IPv6 multicast packets in this case will not
   propagate beyond a single hop (to direct neighbors in the LLN).  This
   implies that any CoAP group request will be delivered to link-local
   nodes only, for any scope value >= 2 used in the IPv6 destination
   address.

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   RPL supports (see Section 12 of [RFC6550]) advertisement of IP
   multicast destinations using Destination Advertisement Object (DAO)
   messages and subsequent routing of multicast IPv6 packets based on
   this.  It requires the RPL mode of operation to be 3 (Storing mode
   with multicast support).

   In this mode, RPL DAO can be used by a CoAP node that is either an
   RPL router or RPL Leaf Node, to advertise its CoAP group membership
   to parent RPL routers.  Then, RPL will route any IP multicast CoAP
   requests over multiple hops to those CoAP servers that are group
   members.

   The same DAO mechanism can be used to convey CoAP group membership
   information to an edge router (e.g., 6LBR), in case the edge router
   is also the root of the RPL Destination-Oriented Directed Acyclic
   Graph (DODAG).  This is useful because the edge router then learns
   which IP multicast traffic it needs to pass through from the backbone
   network into the LLN subnet, and which traffic not.  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.

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 to multiple MPL Domains
   at the same time.  Non-Forwarders will receive IPv6 multicast packets
   from one or more of their neighboring Forwarders.  Therefore, MPL can
   be used to propagate a CoAP multicast group request to all group
   members.

   However, a CoAP multicast request to a group that originated outside
   of the MPL Domain will not be propagated by MPL - unless an MPL
   Forwarder is explicitly configured as an ingress point that
   introduces external multicast packets into the MPL Domain.  Such an
   ingress point could be located on an edge router (e.g., 6LBR).
   Methods to configure which multicast groups are to be propagated into
   the MPL Domain could be:

   *  Manual configuration on each ingress MPL Forwarder.

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   *  MLDv2 protocol, which works only in case all CoAP servers joining
      a group are in link-local communication range of an ingress MPL
      Forwarder.  This is typically not the case on mesh networks.

   *  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 conceptual "NoSec" security group as
   defined in Section 2.1 is used for unsecured group communication.

   It is NOT RECOMMENDED to use CoAP group communication in NoSec mode,
   even in case of non-sensitive and non-critical applications.

   Before possibly and exceptionally using the NoSec mode, 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).

5.  Secured Group Communication using Group OSCORE

   This section defines 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.

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   OSCORE uses COSE [I-D.ietf-cose-rfc8152bis-struct]
   [I-D.ietf-cose-rfc8152bis-algs] 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 funtionalities.  That
   is, some message parts are encrypted and integrity protected; other
   parts are only integrity protected to be accessible to, but not
   modifiable by, proxies; and some parts are kept as plain content to
   be both accessible to and modifiable by proxies.  Such differences
   especially concern the CoAP options included in the unprotected
   message.

   Group OSCORE [I-D.ietf-core-oscore-groupcomm] builds on OSCORE, and
   provides end-to-end security of CoAP messages exchanged between
   members of an OSCORE group, while fulfilling the same security
   requirements.

   In particular, Group OSCORE protects CoAP 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, 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 group, such as all
   responses individually sent by servers, as well as requests addressed
   to an individual server.

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   A Group Manager is responsible for managing one or multiple OSCORE
   groups.  In particular, the Group Manager acts as repository of
   public keys of group members; manages, renews and provides security
   material in the group; and handles the join process of new group
   members.

   As defined in [I-D.ietf-ace-oscore-gm-admin], an administrator entity
   can interact with the Group Manager to create OSCORE groups and
   specify their configuration (see Section 2.2.2).  During the lifetime
   of the OSCORE group, the administrator can further interact with the
   Group Manager, in order to possibly update the group configuration
   and eventually delete the group.

   As recommended in [I-D.ietf-core-oscore-groupcomm], a CoAP endpoint
   can join an OSCORE group by using the method described in
   [I-D.ietf-ace-key-groupcomm-oscore] and based on the ACE framework
   for Authentication and Authorization in constrained environments
   [I-D.ietf-ace-oauth-authz].

   A CoAP endpoint can discover OSCORE groups and retrieve information
   to join them through their respective Group Managers by using the
   method described in [I-D.tiloca-core-oscore-discovery] and based on
   the CoRE Resource Directory [I-D.ietf-core-resource-directory].

   If security is required, CoAP group communication as described in
   this specification MUST use Group OSCORE.  In particular, a CoAP
   group as defined in Section 2.1 and using secure group communication
   is associated to an OSCORE security group, which includes:

   *  All members of the CoAP group, i.e., the CoAP endpoints configured
      to receive CoAP group messages sent to the particular group and --
      in case of IP multicast transport -- 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:

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   *  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 group
      member cannot access 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.

      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 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.

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   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, 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.  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, if the NoSec mode is used, amplification attacks are
   especially feasible to perform and greatly effective in their impact
   [I-D.mattsson-core-coap-attacks].

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   Therefore, even in case of non-sensitive and non-critical
   applications, it is generally NOT RECOMMENDED to use CoAP group
   communication in NoSec mode, also in order to prevent an easy
   proliferation of high-volume amplification attacks as further
   discussed in Section 6.3.

   Exceptionally, early discovery of devices and resources is a typical
   use case where NoSec mode is applied.  In such a situation, the
   querying devices do not have yet configured any mutual security
   relations at the time they perform the discovery.  Also, high-volume
   and harmful amplifications can be prevented through appropriate and
   conservative configurations, since only a few CoAP servers are
   expected to be configured for responding to the group requests sent
   for discovery (see Section 6.3).

   CoAP group communication in NoSec mode SHOULD NOT be deployed for
   sensitive and mission-critical applications (e.g., health monitoring
   systems and alarm monitoring systems).

   CoAP group communication without application-layer security SHOULD be
   deployed only in applications that are non-critical and that do not
   involve or may have an impact on sensitive data and personal sphere.
   These include, e.g., read-only temperature sensors deployed in non-
   sensitive environments, where the client reads out the values but
   does not use the data to control actuators or to base an important
   decision on.

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 dynamics of group membership (e.g., the problem of
   distributing security keys across large groups with many members that
   come and go).

   For sensitive and mission-critical applications, CoAP group
   communication MUST be protected by using Group OSCORE as specified in
   [I-D.ietf-core-oscore-groupcomm].  The same security considerations
   from Section 11 of [I-D.ietf-core-oscore-groupcomm] hold for this
   specification.

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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].

   Group policies should also take into account the time that the key
   management scheme requires to rekey the group, on one hand, and the
   expected frequency of group membership changes, i.e., nodes' joining
   and leaving, on the other hand.

   That is, it may be desirable to not rekey the group upon every single
   membership change, in case members' joining and leaving are frequent,
   and at the same time a single group rekeying instance takes a non-
   negligible time to complete.

   In such a case, the Group Manager may cautiously consider to rekey
   the group, e.g., after a minimum number of nodes has joined or left
   the group within a pre-defined time interval, or according to
   communication patterns with predictable time intervals of network
   inactivity.  This would prevent paralyzing communications in the
   group, when a slow rekeying scheme is used and frequently invoked.

   At the same, the security implications of delaying the rekeying
   process have to be carefully considered and understood, before
   enforcing such group policies.

   In fact, this comes at the cost of not continuously preserving
   backward and forward security, since group rekeying might not occur
   upon every single group membership change.  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 group is
   rekeyed, most recently left nodes would preserve access to group
   communications protected with the retained 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.

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   To this end, outgoing messages are either countersigned by the
   message sender endpoint with its own private key (group mode), or
   protected with a symmetric key, which is in turn derived using the
   asymmetric keys of the message sender and recipient (pairwise mode).

   Thus, both modes allow a recipient CoAP endpoint to verify that a
   message has actually been originated by a specific and identified
   member of the OSCORE group.

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.

   *  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 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].

      In fact, upon receiving a group request protected with Group
      OSCORE in group mode, a server is able to verify whether the
      request is not a replay and originates from the alleged sender in
      the OSCORE group, by verifying the signature included in the
      request using the public key of that sender (see Section 8.2 of
      [I-D.ietf-core-oscore-groupcomm]).  Furthermore, as also discussed
      in Section 8 of [I-D.ietf-core-oscore-groupcomm], it is
      recommended that servers failing to decrypt and verify an incoming
      message do not send back any error message.

      This 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 assumed

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      or inferrable from the application context, it is not explicit
      from 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
      [I-D.ietf-core-echo-request-tag] 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 to a certain public key) is indeed reachable at the
      claimed source address, especially if this differs from the one
      used in previous group requests from the same CoAP client.
      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-fledged response.

   *  Group OSCORE limits the impact of attacks based on IP spoofing
      also over IP multicast (see Section 11.4 of [RFC7252]).  In fact,
      requests and corresponding responses sent in the OSCORE group can
      be correctly generated only by legitimate group members.

      Within an OSCORE group, the shared symmetric-key security material
      strictly provides only group-level authentication.  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 be originated by the alleged, identifiable
      sender in the OSCORE group.

      As noted above, the server may additionally rely on the Echo
      Option for CoAP defined in [I-D.ietf-core-echo-request-tag], in
      order to verify the aliveness and reachability of the client
      sending a request from a particular IP address.

   *  Group OSCORE does not require group members to be equipped with a
      good source of entropy for generating security material (see
      Section 11.6 of [RFC7252]), and thus does not contribute to create
      an 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 group.

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   *  Group OSCORE prevents to make any single group member a target for
      subverting security in the whole OSCORE group (see Section 11.6 of
      [RFC7252]), even though all group members share (and can derive)
      the same symmetric-key security material used in the OSCORE group.
      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 can not be authenticated in
      some way;

   *  SHOULD NOT provide large amplification factors through its
      responses to a non-authenticated group request, and can possibly
      rely on CoAP block-wise transfers [RFC7959] to reduce the amount
      of amplification.

   Section 3.1 of [I-D.mattsson-core-coap-attacks] further discusses
   amplification attacks, and also notes 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.

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   Thus, consistently with Section 7 of [RFC7641], a server in a CoAP
   group MUST strictly limit the number of notifications it sends
   between receiving acknowledgements 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, even in case of non-
   sensitive and non-critical applications, it is generally NOT
   RECOMMENDED to use CoAP group communication in NoSec mode, in order
   to prevent an easy proliferation of high-volume amplification
   attacks.

   Exceptions should be carefully limited to use cases and accesses to a
   group resource that 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
   [I-D.ietf-core-resource-directory], 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 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.

   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.

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   If Group OSCORE is used, such a replay attack on the servers is
   prevented, since a client protects every different request with a
   different Sequence Number value, which is in turn included as Partial
   IV in the protected message and takes part in the construction of the
   AEAD cipher nonce.  Thus, a server would be able to detect the
   replayed request, by checking the conveyed Partial IV against its own
   replay window in the OSCORE Recipient Context associated to the
   client.

   This requires a server to have a synchronized, up to date view of the
   sequence number used by the client.  If such synchronization is lost,
   e.g., due to a reboot, or suspected so, the server should use the
   challenge-response synchronization method described in Appendix E of
   [I-D.ietf-core-oscore-groupcomm] and based on the Echo Option for
   CoAP defined in [I-D.ietf-core-echo-request-tag], 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 much responses from servers
   to a group request as possible, by using the value '0' - Interested
   in all responses.  This even overrides the default behaviour of a
   CoAP server to suppress the response in case there is nothing of
   interest to respond with.  Therefore, this option can be used to
   perform an amplification attack (see Section 6.3).

   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.

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6.6.  6LoWPAN and MPL

   In a 6LoWPAN network, a multicast IPv6 packet may be fragmented prior
   to transmission.  A 6LoWPAN Router that forwards a fragmented packet
   may have a relatively high impact on the occupation of the wireless
   channel and may locally experience high memory load due to packet
   buffering.  For example, the MPL [RFC7731] protocol requires an MPL
   Forwarder to store the packet for a longer duration, to allow
   multiple forwarding transmissions to neighboring Forwarders.  If 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 at a future point in time it
   needs to receive again all the packet fragments (this time, possibly
   from another neighboring MPL Forwarder).

   For these reasons, a fragmented IPv6 multicast packet is a possible
   attack vector in a Denial of Service (DoS) amplification attack.  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 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 criterium 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.

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6.8.  Monitoring

6.8.1.  General Monitoring

   CoAP group communication can be used to control a set of related
   devices: for example, simultaneously turn on all the lights in a
   room.  This intrinsically exposes the group to some unique monitoring
   risks that devices not in a group are not as vulnerable to.  For
   example, assume an attacker is able to physically see a set of lights
   turn on in a room.  Then the attacker can correlate an observed CoAP
   group communication message to the observed coordinated group action
   -- even if the CoAP message is (partly) encrypted.
   This will give the attacker side-channel information to plan further
   attacks (e.g., by determining the members of the group some network
   topology information may be deduced).

6.8.2.  Pervasive Monitoring

   A key additional threat consideration for group communication is
   pervasive monitoring [RFC7258].  CoAP group communication solutions
   that are built on top of IP multicast need to pay particular heed to
   these dangers.  This is because IP multicast is easier to intercept
   compared to IP unicast.  Also, CoAP traffic is typically used for the
   Internet of Things.  This means that CoAP (multicast) group
   communication may be used for the control and monitoring of critical
   infrastructure (e.g., lights, alarms, HVAC, electrical grid, etc.)
   that may be prime targets for attack.

   For example, an attacker may attempt to record all the CoAP traffic
   going over a smart grid (i.e., networked electrical utility) and try
   to determine critical nodes for further attacks.  For example, the
   source node (controller) sends out CoAP group communication messages
   which easily identifies it as a controller.  CoAP multicast traffic
   is inherently more vulnerable compared to unicast, as the same packet
   may be replicated over many more links, leading to a higher
   probability of packet capture by a pervasive monitoring system.

   One mitigation is to restrict the scope of IP multicast to the
   minimal scope that fulfills the application need.  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 this fulfills the application needs.

   Even if all CoAP multicast traffic is encrypted/protected, an
   attacker may still attempt to capture this traffic and perform an
   off-line attack in the future.

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7.  IANA Considerations

   This document has no actions for IANA.

8.  References

8.1.  Normative References

   [I-D.ietf-core-echo-request-tag]
              Amsüss, C., Mattsson, J. P., and G. Selander, "CoAP: Echo,
              Request-Tag, and Token Processing", Work in Progress,
              Internet-Draft, draft-ietf-core-echo-request-tag-14, 4
              October 2021, <https://www.ietf.org/archive/id/draft-ietf-
              core-echo-request-tag-14.txt>.

   [I-D.ietf-core-oscore-groupcomm]
              Tiloca, M., Selander, G., Palombini, F., Mattsson, J. P.,
              and J. Park, "Group OSCORE - Secure Group Communication
              for CoAP", Work in Progress, Internet-Draft, draft-ietf-
              core-oscore-groupcomm-13, 25 October 2021,
              <https://www.ietf.org/archive/id/draft-ietf-core-oscore-
              groupcomm-13.txt>.

   [I-D.ietf-cose-rfc8152bis-algs]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Initial Algorithms", Work in Progress, Internet-Draft,
              draft-ietf-cose-rfc8152bis-algs-12, 24 September 2020,
              <https://www.ietf.org/archive/id/draft-ietf-cose-
              rfc8152bis-algs-12.txt>.

   [I-D.ietf-cose-rfc8152bis-struct]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", Work in Progress, Internet-Draft,
              draft-ietf-cose-rfc8152bis-struct-15, 1 February 2021,
              <https://www.ietf.org/archive/id/draft-ietf-cose-
              rfc8152bis-struct-15.txt>.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <https://www.rfc-editor.org/info/rfc1122>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

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   [RFC3376]  Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
              Thyagarajan, "Internet Group Management Protocol, Version
              3", RFC 3376, DOI 10.17487/RFC3376, October 2002,
              <https://www.rfc-editor.org/info/rfc3376>.

   [RFC3810]  Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
              Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
              DOI 10.17487/RFC3810, June 2004,
              <https://www.rfc-editor.org/info/rfc3810>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/info/rfc4443>.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
              <https://www.rfc-editor.org/info/rfc4944>.

   [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/info/rfc6282>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <https://www.rfc-editor.org/info/rfc6690>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <https://www.rfc-editor.org/info/rfc7641>.

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,
              <https://www.rfc-editor.org/info/rfc7959>.

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   [RFC8075]  Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
              E. Dijk, "Guidelines for Mapping Implementations: HTTP to
              the Constrained Application Protocol (CoAP)", RFC 8075,
              DOI 10.17487/RFC8075, February 2017,
              <https://www.rfc-editor.org/info/rfc8075>.

   [RFC8132]  van der Stok, P., Bormann, C., and A. Sehgal, "PATCH and
              FETCH Methods for the Constrained Application Protocol
              (CoAP)", RFC 8132, DOI 10.17487/RFC8132, April 2017,
              <https://www.rfc-editor.org/info/rfc8132>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/info/rfc8613>.

8.2.  Informative References

   [Californium]
              Eclipse Foundation, "Eclipse Californium", March 2019,
              <https://github.com/eclipse/californium/tree/2.0.x/
              californium-core/src/main/java/org/eclipse/californium/
              core>.

   [Go-OCF]   Open Connectivity Foundation (OCF), "Implementation of
              CoAP Server & Client in Go", March 2019,
              <https://github.com/go-ocf/go-coap>.

   [I-D.ietf-ace-key-groupcomm-oscore]
              Tiloca, M., Park, J., and F. Palombini, "Key Management
              for OSCORE Groups in ACE", Work in Progress, Internet-
              Draft, draft-ietf-ace-key-groupcomm-oscore-12, 25 October
              2021, <https://www.ietf.org/archive/id/draft-ietf-ace-key-
              groupcomm-oscore-12.txt>.

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) using the OAuth 2.0
              Framework (ACE-OAuth)", Work in Progress, Internet-Draft,
              draft-ietf-ace-oauth-authz-45, 29 August 2021,
              <https://www.ietf.org/archive/id/draft-ietf-ace-oauth-
              authz-45.txt>.

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   [I-D.ietf-ace-oscore-gm-admin]
              Tiloca, M., Höglund, R., Stok, P. V. D., and F. Palombini,
              "Admin Interface for the OSCORE Group Manager", Work in
              Progress, Internet-Draft, draft-ietf-ace-oscore-gm-admin-
              04, 25 October 2021, <https://www.ietf.org/archive/id/
              draft-ietf-ace-oscore-gm-admin-04.txt>.

   [I-D.ietf-core-coap-pubsub]
              Koster, M., Keranen, A., and J. Jimenez, "Publish-
              Subscribe Broker for the Constrained Application Protocol
              (CoAP)", Work in Progress, Internet-Draft, draft-ietf-
              core-coap-pubsub-09, 30 September 2019,
              <https://www.ietf.org/archive/id/draft-ietf-core-coap-
              pubsub-09.txt>.

   [I-D.ietf-core-new-block]
              Boucadair, M. and J. Shallow, "Constrained Application
              Protocol (CoAP) Block-Wise Transfer Options Supporting
              Robust Transmission", Work in Progress, Internet-Draft,
              draft-ietf-core-new-block-14, 26 May 2021,
              <https://www.ietf.org/archive/id/draft-ietf-core-new-
              block-14.txt>.

   [I-D.ietf-core-resource-directory]
              Amsüss, C., Shelby, Z., Koster, M., Bormann, C., and P. V.
              D. Stok, "CoRE Resource Directory", Work in Progress,
              Internet-Draft, draft-ietf-core-resource-directory-28, 7
              March 2021, <https://www.ietf.org/archive/id/draft-ietf-
              core-resource-directory-28.txt>.

   [I-D.mattsson-core-coap-attacks]
              Mattsson, J. P., Fornehed, J., Selander, G., Palombini,
              F., and C. Amsüss, "CoAP Attacks", Work in Progress,
              Internet-Draft, draft-mattsson-core-coap-attacks-01, 27
              July 2021, <https://www.ietf.org/archive/id/draft-
              mattsson-core-coap-attacks-01.txt>.

   [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-05, 25 October 2021,
              <https://www.ietf.org/archive/id/draft-tiloca-core-
              groupcomm-proxy-05.txt>.

   [I-D.tiloca-core-oscore-discovery]
              Tiloca, M., Amsuess, C., and P. V. D. Stok, "Discovery of
              OSCORE Groups with the CoRE Resource Directory", Work in
              Progress, Internet-Draft, draft-tiloca-core-oscore-

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              discovery-10, 25 October 2021,
              <https://www.ietf.org/archive/id/draft-tiloca-core-oscore-
              discovery-10.txt>.

   [RFC6092]  Woodyatt, J., Ed., "Recommended Simple Security
              Capabilities in Customer Premises Equipment (CPE) for
              Providing Residential IPv6 Internet Service", RFC 6092,
              DOI 10.17487/RFC6092, January 2011,
              <https://www.rfc-editor.org/info/rfc6092>.

   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
              Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
              JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
              Low-Power and Lossy Networks", RFC 6550,
              DOI 10.17487/RFC6550, March 2012,
              <https://www.rfc-editor.org/info/rfc6550>.

   [RFC6636]  Asaeda, H., Liu, H., and Q. Wu, "Tuning the Behavior of
              the Internet Group Management Protocol (IGMP) and
              Multicast Listener Discovery (MLD) for Routers in Mobile
              and Wireless Networks", RFC 6636, DOI 10.17487/RFC6636,
              May 2012, <https://www.rfc-editor.org/info/rfc6636>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <https://www.rfc-editor.org/info/rfc7258>.

   [RFC7346]  Droms, R., "IPv6 Multicast Address Scopes", RFC 7346,
              DOI 10.17487/RFC7346, August 2014,
              <https://www.rfc-editor.org/info/rfc7346>.

   [RFC7390]  Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
              the Constrained Application Protocol (CoAP)", RFC 7390,
              DOI 10.17487/RFC7390, October 2014,
              <https://www.rfc-editor.org/info/rfc7390>.

   [RFC7731]  Hui, J. and R. Kelsey, "Multicast Protocol for Low-Power
              and Lossy Networks (MPL)", RFC 7731, DOI 10.17487/RFC7731,
              February 2016, <https://www.rfc-editor.org/info/rfc7731>.

   [RFC7967]  Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
              Bose, "Constrained Application Protocol (CoAP) Option for
              No Server Response", RFC 7967, DOI 10.17487/RFC7967,
              August 2016, <https://www.rfc-editor.org/info/rfc7967>.

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   [RFC8323]  Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
              Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
              Application Protocol) over TCP, TLS, and WebSockets",
              RFC 8323, DOI 10.17487/RFC8323, February 2018,
              <https://www.rfc-editor.org/info/rfc8323>.

   [RFC8710]  Fossati, T., Hartke, K., and C. Bormann, "Multipart
              Content-Format for the Constrained Application Protocol
              (CoAP)", RFC 8710, DOI 10.17487/RFC8710, February 2020,
              <https://www.rfc-editor.org/info/rfc8710>.

Appendix A.  Use Cases

   To illustrate where and how CoAP-based group communication can be
   used, this section summarizes the most common use cases.  These use
   cases include both secured and non-secured CoAP usage.  Each
   subsection below covers one particular category of use cases for
   CoRE.  Within each category, a use case may cover multiple
   application areas such as home IoT, commercial building IoT (sensing
   and control), industrial IoT/control, or environmental sensing.

A.1.  Discovery

   Discovery of physical devices in a network, or discovery of
   information entities hosted on network devices, are operations that
   are usually required in a system during the phases of setup or
   (re)configuration.  When a discovery use case involves devices that
   need to interact without having been configured previously with a
   common security context, unsecured CoAP communication is typically
   used.  Discovery may involve a request to a directory server, which
   provides services to aid clients in the discovery process.  One
   particular type of directory server is the CoRE Resource Directory
   [I-D.ietf-core-resource-directory]; and there may be other types of
   directories that can be used with CoAP.

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A.1.1.  Distributed Device Discovery

   Device discovery is the discovery and identification of networked
   devices -- optionally only devices of a particular class, type,
   model, or brand.  Group communication is used for distributed device
   discovery, if a central directory server is not used.  Typically in
   distributed device discovery, a multicast request is sent to a
   particular address (or address range) and multicast scope of
   interest, and any devices configured to be discoverable will respond
   back.  For the alternative solution of centralized device discovery a
   central directory server is accessed through unicast, in which case
   group communication is not needed.  This requires that the address of
   the central directory is either preconfigured in each device or
   configured during operation using a protocol.

   In CoAP, device discovery can be implemented by CoAP resource
   discovery requesting (GET) a particular resource that the sought
   device class, type, model or brand is known to respond to.  It can
   also be implemented using CoAP resource discovery (Section 7 of
   [RFC7252]) and the CoAP query interface defined in Section 4 of
   [RFC6690] to find these particular resources.  Also, a multicast GET
   request to /.well-known/core can be used to discover all CoAP
   devices.

A.1.2.  Distributed Service Discovery

   Service discovery is the discovery and identification of particular
   services hosted on network devices.  Services can be identified by
   one or more parameters such as ID, name, protocol, version and/or
   type.  Distributed service discovery involves group communication to
   reach individual devices hosting a particular service; with a central
   directory server not being used.

   In CoAP, services are represented as resources and service discovery
   is implemented using resource discovery (Section 7 of [RFC7252]) and
   the CoAP query interface defined in Section 4 of [RFC6690].

A.1.3.  Directory Discovery

   This use case is a specific sub-case of Distributed Service Discovery
   (Appendix A.1.2), in which a device needs to identify the location of
   a Directory on the network to which it can e.g., register its own
   offered services, or to which it can perform queries to identify and
   locate other devices/services it needs to access on the network.
   Section 3.3 of [RFC7390] showed an example of discovering a CoRE
   Resource Directory using CoAP group communication.  As defined in
   [I-D.ietf-core-resource-directory], a resource directory is a web
   entity that stores information about web resources and implements

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   REST interfaces for registration and lookup of those resources.  For
   example, a device can register itself to a resource directory to let
   it be found by other devices and/or applications.

A.2.  Operational Phase

   Operational phase use cases describe those operations that occur most
   frequently in a networked system, during its operational lifetime and
   regular operation.  Regular usage is when the applications on
   networked devices perform the tasks they were designed for and
   exchange of application-related data using group communication
   occurs.  Processes like system reconfiguration, group changes,
   system/device setup, extra group security changes, etc. are not part
   of regular operation.

A.2.1.  Actuator Group Control

   Group communication can be beneficial to control actuators that need
   to act in synchrony, as a group, with strict timing (latency)
   requirements.  Examples are office lighting, stage lighting, street
   lighting, or audio alert/Public Address systems.  Sections 3.4 and
   3.5 of [RFC7390] showed examples of lighting control of a group of
   6LoWPAN-connected lights.

A.2.2.  Device Group Status Request

   To properly monitor the status of systems, there may be a need for
   ad-hoc, unplanned status updates.  Group communication can be used to
   quickly send out a request to a (potentially large) number of devices
   for specific information.  Each device then responds back with the
   requested data.  Those devices that did not respond to the request
   can optionally be polled again via reliable unicast communication to
   complete the dataset.  The device group may be defined e.g., as "all
   temperature sensors on floor 3", or "all lights in wing B".  For
   example, it could be a status request for device temperature, most
   recent sensor event detected, firmware version, network load, and/or
   battery level.

A.2.3.  Network-wide Query

   In some cases a whole network or subnet of multiple IP devices needs
   to be queried for status or other information.  This is similar to
   the previous use case except that the device group is not defined in
   terms of its function/type but in terms of its network location.
   Technically this is also similar to distributed service discovery
   (Appendix A.1.2) where a query is processed by all devices on a
   network - except that the query is not about services offered by the
   device, but rather specific operational data is requested.

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A.2.4.  Network-wide / Group Notification

   In some cases a whole network, or subnet of multiple IP devices, or a
   specific target group needs to be notified of a status change or
   other information.  This is similar to the previous two use cases
   except that the recipients are not expected to respond with some
   information.  Unreliable notification can be acceptable in some use
   cases, in which a recipient does not respond with a confirmation of
   having received the notification.  In such a case, the receiving CoAP
   server does not have to create a CoAP response.  If the sender needs
   confirmation of reception, the CoAP servers can be configured for
   that resource to respond with a 2.xx success status after processing
   a notification request successfully.

A.3.  Software Update

   Group communication can be useful to efficiently distribute new
   software (firmware, image, application, etc.) to a group of multiple
   devices.  In this case, the group is defined in terms of device type:
   all devices in the target group are known to be capable of installing
   and running the new software.  The software is distributed as a
   series of smaller blocks that are collected by all devices and stored
   in memory.  All devices in the target group are usually responsible
   for integrity verification of the received software; which can be
   done per-block or for the entire software image once all blocks have
   been received.  Due to the inherent unreliability of CoAP multicast,
   there needs to be a backup mechanism (e.g., implemented using CoAP
   unicast) by which a device can individually request missing blocks of
   a whole software image/entity.  Prior to a multicast software update,
   the group of recipients can be separately notified that there is new
   software available and coming, using the above network-wide or group
   notification.

Appendix B.  Document Updates

   RFC EDITOR: PLEASE REMOVE THIS SECTION.

B.1.  Version -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.

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   *  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.

B.2.  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.

B.3.  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.

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B.4.  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.

   *  Usage of the Uri-Host Option to indicate an application group.

   *  Response suppression based on classes of error codes.

B.5.  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 Amsuess, Carsten Bormann,
   Thomas Fossati, Rikard Hoeglund, John Mattsson and Jim Schaad for
   their comments and feedback.

Dijk, et al.              Expires 28 April 2022                [Page 59]
Internet-Draft        Group Communication for CoAP          October 2021

   The work on this document has been partly supported by VINNOVA and
   the Celtic-Next project CRITISEC; and by the H2020 project SIFIS-Home
   (Grant agreement 952652).

Authors' Addresses

   Esko Dijk
   IoTconsultancy.nl
   \________________\
   Utrecht
   Netherlands

   Email: esko.dijk@iotconsultancy.nl

   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

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