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Versions: 00 01 02

CoRE Working Group                                               E. Dijk
Internet-Draft                                         IoTconsultancy.nl
Obsoletes: 7390 (if approved)                                    C. Wang
Updates: 7252, 7641, 7959 (if approved)                     InterDigital
Intended status: Standards Track                               M. Tiloca
Expires: May 7, 2020                                             RISE AB
                                                       November 04, 2019


  Group Communication for the Constrained Application Protocol (CoAP)
                    draft-dijk-core-groupcomm-bis-02

Abstract

   This document specifies the use of the Constrained Application
   Protocol (CoAP) for group communication, using UDP/IP multicast as
   the underlying data transport.  Both unsecured and secured CoAP group
   communication are specified.  Security is achieved by use of the
   Group Object Security for Constrained RESTful Environments (Group
   OSCORE) protocol.  The target application area of this specification
   is any group communication use cases that involve resource-
   constrained network nodes.  The most common of such use cases are
   listed in this document.

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/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 7, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents



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   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  General Group Communication Operation . . . . . . . . . . . .   4
     2.1.  Group Configuration . . . . . . . . . . . . . . . . . . .   5
       2.1.1.  Group Definition  . . . . . . . . . . . . . . . . . .   5
       2.1.2.  Group Naming  . . . . . . . . . . . . . . . . . . . .   5
       2.1.3.  Group Creation and Membership . . . . . . . . . . . .   6
       2.1.4.  Group Maintenance . . . . . . . . . . . . . . . . . .   7
     2.2.  CoAP Usage  . . . . . . . . . . . . . . . . . . . . . . .   7
       2.2.1.  Request/Response Model  . . . . . . . . . . . . . . .   7
       2.2.2.  Port and URI Path Selection . . . . . . . . . . . . .  10
       2.2.3.  Proxy Operation . . . . . . . . . . . . . . . . . . .  10
       2.2.4.  Congestion Control  . . . . . . . . . . . . . . . . .  12
       2.2.5.  Observing Resources . . . . . . . . . . . . . . . . .  13
       2.2.6.  Block-Wise Transfer . . . . . . . . . . . . . . . . .  15
     2.3.  Transport . . . . . . . . . . . . . . . . . . . . . . . .  15
       2.3.1.  UDP/IPv6 Multicast Transport  . . . . . . . . . . . .  15
       2.3.2.  UDP/IPv4 Multicast Transport  . . . . . . . . . . . .  16
       2.3.3.  6LoWPAN . . . . . . . . . . . . . . . . . . . . . . .  16
     2.4.  Interworking with Other Protocols . . . . . . . . . . . .  16
       2.4.1.  MLD/MLDv2/IGMP/IGMPv3 . . . . . . . . . . . . . . . .  16
       2.4.2.  RPL . . . . . . . . . . . . . . . . . . . . . . . . .  17
       2.4.3.  MPL . . . . . . . . . . . . . . . . . . . . . . . . .  18
   3.  Unsecured Group Communication . . . . . . . . . . . . . . . .  18
   4.  Secured Group Communication using Group OSCORE  . . . . . . .  18
     4.1.  Secure Group Maintenance  . . . . . . . . . . . . . . . .  20
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  21
     5.1.  CoAP NoSec Mode . . . . . . . . . . . . . . . . . . . . .  21
     5.2.  Group OSCORE  . . . . . . . . . . . . . . . . . . . . . .  21
       5.2.1.  Group Key Management  . . . . . . . . . . . . . . . .  22
       5.2.2.  Source Authentication . . . . . . . . . . . . . . . .  22
       5.2.3.  Counteraction of Attacks  . . . . . . . . . . . . . .  23
     5.3.  Use of CoAP No Response Option  . . . . . . . . . . . . .  23
     5.4.  6LoWPAN . . . . . . . . . . . . . . . . . . . . . . . . .  23
     5.5.  Wi-Fi . . . . . . . . . . . . . . . . . . . . . . . . . .  24
     5.6.  Monitoring  . . . . . . . . . . . . . . . . . . . . . . .  24
       5.6.1.  General Monitoring  . . . . . . . . . . . . . . . . .  24



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       5.6.2.  Pervasive Monitoring  . . . . . . . . . . . . . . . .  24
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  25
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  25
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  27
   Appendix A.  Use Cases  . . . . . . . . . . . . . . . . . . . . .  29
     A.1.  Discovery . . . . . . . . . . . . . . . . . . . . . . . .  29
       A.1.1.  Distributed Device Discovery  . . . . . . . . . . . .  29
       A.1.2.  Distributed Service Discovery . . . . . . . . . . . .  30
       A.1.3.  Directory Discovery . . . . . . . . . . . . . . . . .  30
     A.2.  Operational Phase . . . . . . . . . . . . . . . . . . . .  30
       A.2.1.  Actuator Group Control  . . . . . . . . . . . . . . .  31
       A.2.2.  Device Group Status Request . . . . . . . . . . . . .  31
       A.2.3.  Network-wide Query  . . . . . . . . . . . . . . . . .  31
       A.2.4.  Network-wide / Group Notification . . . . . . . . . .  31
     A.3.  Software Update . . . . . . . . . . . . . . . . . . . . .  32
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  32

1.  Introduction

   This document specifies group communication using the Constrained
   Application Protocol (CoAP) [RFC7252] together with UDP/IP multicast.
   CoAP is a RESTful communication protocol that is used in resource-
   constrained nodes, and in resource-constrained networks where packet
   sizes should be small.  This area of use is summarized as Constrained
   RESTful Environments (CoRE).

   One-to-many group communication can be achieved in CoAP, by a client
   using UDP/IP multicast data transport to send multicast CoAP request
   messages.  In response, each server in the addressed group sends a
   response message back to the client over UDP/IP unicast.  Notable
   CoAP implementations supporting group communication include the
   framework "Eclipse Californium" 2.0.x [Californium] from the Eclipse
   Foundation and the "Implementation of CoAP Server & Client in Go"
   [Go-OCF] from the Open Connectivity Foundation (OCF).

   Both unsecured and secured CoAP group communication over UDP/IP
   multicast are specified in this document.  Security is achieved by
   using Group Object Security for Constrained RESTful Environments
   (Group OSCORE) [I-D.ietf-core-oscore-groupcomm], which in turn builds
   on Object Security for Constrained Restful Environments (OSCORE)
   [RFC8613].  This method provides end-to-end application-layer
   security protection of CoAP messages, by using CBOR Object Signing
   and Encryption (COSE) [RFC7049][RFC8152].

   All sections in the body of this document are normative, while
   appendices are informative.  For additional background about use



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   cases for CoAP group communication in resource-constrained devices
   and networks, see Appendix A.

1.1.  Scope

   For group communication, only solutions that use CoAP over UDP/
   multicast are in the scope of this document.  There are alternative
   methods to achieve group communication using CoAP, for example
   Publish-Subscribe [I-D.ietf-core-coap-pubsub] which uses a central
   broker server that CoAP clients access via unicast communication.
   The alternative methods may be usable for the same or similar use
   cases as are targeted in this document.

   All guidelines in [RFC7390] are imported by this document which
   replaces [RFC7390] in this respect.  Furthermore, this document adds:
   how to provide security by using Group OSCORE
   [I-D.ietf-core-oscore-groupcomm] as the default group communication
   security solution for CoAP; an updated request/response matching rule
   for multicast CoAP which updates [RFC7252]; the multicast use of CoAP
   Observe which updates [RFC7641]; and an extension of the multicast
   use of CoAP block-wise transfers, which updates [RFC7959].

   Security solutions for group communication and configuration other
   than Group OSCORE are not in scope.  General principles for secure
   group configuration are in scope.  The experimental group
   configuration protocol in Section 2.6.2 of [RFC7390] is not in the
   scope of this document; thus, that remains an experimental protocol.
   Since application protocols defined on top of CoAP often define their
   own specific method of group configuration, the experimental protocol
   of [RFC7390] has not been subject to enough experimentation to
   warrant a change of this status.

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
   [RFC7252] terminology.

2.  General Group Communication Operation

   The general operation of group communication, applicable for both
   unsecured and secured operation, is specified in this section by
   going through the stack from top to bottom.  First, group



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   configuration (e.g. group creation and maintenance which are usually
   done by an application, user or commissioning entity) is considered
   in Section 2.1.  Then the use of CoAP for group communication
   including support for protocol extensions (block-wise, Observe, PATCH
   method) follows in Section 2.2.  How CoAP group messages are carried
   over various transport layers is the subject of Section 2.3.
   Finally, Section 2.4 covers the interworking of CoAP group
   communication with other protocols that may operate in the same
   network.

2.1.  Group Configuration

2.1.1.  Group Definition

   A CoAP group is defined as a set of CoAP endpoints, where each
   endpoint is configured to receive CoAP multicast requests that are
   sent to the group's associated IP multicast address and UDP port.  An
   endpoint may be a member of multiple CoAP groups.  Group
   membership(s) of an endpoint may dynamically change over time.  A
   device sending a CoAP request to a group is not necessarily itself a
   member of this group: it is only a member if it also has a CoAP
   server endpoint listening to requests for this CoAP group.  For
   secure group communication, a receiver also requires the security
   context to successfully decrypt and/or verify group messages in order
   to be a group member.

   A CoAP Group URI has the scheme 'coap' and includes in the authority
   part either an IP multicast address or a group hostname (e.g., 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 (Section 6 of [RFC7252]).

   Besides CoAP groups, that have relevance at the level of networked
   devices, there can also be application groups defined.  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.  There can be a one-to-one or a
   one-to-many relation between CoAP groups and application groups.

2.1.2.  Group Naming

   For clients, it is RECOMMENDED to use by default an IP multicast
   address literal in a configured Group URI, instead of a hostname.
   This is because DNS infrastructure may not be deployed in many
   constrained networks.  In case a group hostname is used in the Group
   URI, it can be uniquely mapped to an IP multicast address via DNS
   resolution - if DNS client functionality is available in the clients



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   and the DNS service is supported in the network.  Some examples of
   hierarchical group FQDN naming (and scoping) for a building control
   application are shown in Section 2.2 of [RFC7390].

   Application groups can be named in many ways, e.g. numbers, IDs,
   strings or URIs.  An application group identifier, if used, is
   typically included in the path component or query component of a
   Group URI.  Appendix A of [I-D.ietf-core-resource-directory] shows
   registration of application groups into a Resource Directory, along
   with the CoAP group it maps to.

2.1.3.  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 server listening on the group's associated UDP
   port.  These devices are the group members.  The configuring entity
   can be, for example, a local application with preconfiguration, a
   user, a cloud service, or a local commissioning tool.  Also, the
   devices sending requests to the group in the role of CoAP clients
   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, possibly together with the required
   security material in case communication is secured in the group.

   For unsecure group communication, any CoAP endpoint may become a
   group member at any time: there is no (central) configuring entity
   that needs to provide the security material for the group.  This
   means that group creation and membership cannot be tightly
   controlled.

   The IETF does not define a mandatory, standardized protocol to
   accomplish these steps.  [RFC7390] defines an experimental protocol
   for configuration of group membership for unsecured group
   communication, based on JSON-formatted configuration resources.  This
   protocol remains experimental.  For secure group communication, the
   part of the process that involves secure distribution of group keys
   MAY use standardized communication with a Group Manager as defined in
   Section 4.

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





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2.1.4.  Group Maintenance

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

   For unsecured group communication (see Section 3), addition/removal
   of group members is simply done by configuring these devices to
   start/stop listening to the group IP multicast address, and to start/
   stop the CoAP server listening to the group IP multicast address and
   port.

   For secured group communication (see Section 4), the protocol Group
   OSCORE [I-D.ietf-core-oscore-groupcomm] is mandatory to implement.
   When using Group OSCORE, CoAP endpoints participating to group
   communication are also members of a corresponding OSCORE group, and
   thus share a common set of cryptographic material.  Additional
   related maintenance operations are discussed in Section 4.1.

2.2.  CoAP Usage

2.2.1.  Request/Response Model

   A CoAP client is an endpoint able to transmit CoAP requests and
   receive CoAP responses.  Since the underlying UDP supports
   multiplexing by means of UDP port number, there can be multiple
   independent CoAP clients operational on a single host.  On each UDP
   port, an independent CoAP client can be hosted.  Each independent
   CoAP client sends requests that use the associated endpoint's UDP
   port number as the UDP source port of the request.

   All CoAP requests that are sent via IP multicast MUST be Non-
   confirmable (Section 8.1 of [RFC7252]).  The Message ID in an IP
   multicast CoAP message is used for optional message deduplication by
   both clients and servers, as detailed in Section 4.5 of [RFC7252].

   A server sends back a unicast response to the CoAP group
   communication request - but it MAY suppress this response if selected
   so by the server application and if permitted by the rules in this
   document.  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.

   Response suppression controlled by a server SHOULD be performed in a
   consistent way, such that if a first request leads to a response that




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   is not suppressed, then a second similar request on the same resource
   that leads to the same response code is also not suppressed.

   The CoAP Option for No Server Response [RFC7967] can be used by a
   client to influence the response suppression on the server side.  It
   is RECOMMENDED for a server to implement this option only on selected
   resources where it is useful in the application context.

   A CoAP client MAY repeat a multicast request using the same Token
   value and same Message ID value, in order to ensure that enough (or
   all) group members have been reached with the request.  This is
   useful in case a number of group members did not respond to the
   initial request.  However, in case one or more servers did receive
   the initial request but the response to that request was lost, this
   repeat may not help to retrieve the lost response(s) if the server
   implements the optional Message ID based deduplication.

   A CoAP client MAY also repeat a multicast request using the same
   Token value but a different Message ID, in which case all servers
   that received the initial request will again process the repeated
   request.  This is useful in case a client needs to collect more, or
   even all, responses from group members.

   The CoAP client can distinguish the origin of multiple server
   responses by the source IP address of the UDP message containing the
   CoAP response and/or any other available application-specific source
   identifiers contained in the CoAP response.  While processing a
   response, the source endpoint of the response is not exactly matched
   to the destination endpoint of the request, since for a multicast
   request these will never match.  This is specified in Section 8.2 of
   [RFC7252].  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 a request using the Token value.  Due
   to UDP level multiplexing, the UDP destination port of the response
   MUST match to the client endpoint's UDP port value, i.e. to the UDP
   source port of the client's request.

   For multicast CoAP requests, there are additional constraints on the
   reuse of Token values at the client, compared to the unicast case.
   In the unicast case, receiving a response effectively frees up its
   Token value for reuse, since no more responses to the same request
   will follow.  However, for multicast CoAP, the number of responses is
   not bound a priori.  Therefore, client cannot use the reception of a
   response as a trigger to "free up" a Token value for reuse.
   Moreover, 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 used in
   multicast requests MUST be greater than:



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   NON_LIFETIME + MAX_LATENCY + MAX_SERVER_RESPONSE_DELAY

   where NON_LIFETIME and MAX_LATENCY are defined in Section 4.8 of
   [RFC7252].  This specification defines MAX_SERVER_RESPONSE_DELAY as
   in [RFC7390], that is: the expected maximum response delay over all
   servers that the client can send a multicast request to.  This delay
   includes the maximum Leisure time period as defined in Section 8.2 of
   [RFC7252].  However, CoAP does not define a time limit for the server
   response delay.  Using the default CoAP parameters, the Token reuse
   time MUST be greater than 250 seconds plus MAX_SERVER_RESPONSE_DELAY.
   A preferred solution to meet this requirement is to generate a new
   unique Token for every new multicast request, such that a Token value
   is never reused.  If a client has to reuse Token values for some
   reason, and also MAX_SERVER_RESPONSE_DELAY is unknown, then using
   MAX_SERVER_RESPONSE_DELAY = 250 seconds is a reasonable guideline.
   The time between Token reuses is in that case set to a value greater
   than 500 seconds.

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

   Since a client sending a multicast request with a Token T will accept
   multiple responses with the same Token T, there is a risk that a same
   server sends multiple responses with the same Token T back to the
   client.  For example, this server might not implement the optional
   CoAP message deduplication based on Message ID, or it might be a
   malicious/compromised server acting out of specification.  To
   mitigate issues with multiple responses from one server bound to a
   same multicast request, the client has to ensure that, as long as the
   the CoAP Token used for a multicast request is retained, at most one
   response to that request per server is accepted, with the exception
   of Observe notifications [RFC7641] (see Section 2.2.5).

   To this end, upon receiving a response corresponding to a multicast
   request, the client MUST perform the following actions.  First, the
   client checks whether it previously received a valid response to this
   request from the same originating server of the just-received
   response.  If the check yields a positive match and the response is
   not an Observe notification (i.e., it does not include an Observe
   option), the client SHALL stop processing the response.  Upon
   eventually freeing up the Token value of a multicast request for
   possible reuse, the client MUST also delete the list of responding
   servers associated to that request.





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2.2.2.  Port and URI Path Selection

   A CoAP server that is a member of a group listens for CoAP messages
   on the group's IP multicast address, usually on the CoAP default UDP
   port 5683, 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 group and
   across all CoAP clients performing the group requests to that group.
   The URI Path used in the request is preferably a path that is known
   to be supported across all group members.  However there are valid
   use cases where a request is known to be successful for a subset of
   the group and those group members for which the request is
   unsuccessful either ignore the multicast request or respond with an
   error status code.

   Using different ports with the same IP multicast address is an
   efficient way to create multiple CoAP groups in constrained devices,
   in case the device's stack only supports a limited number of IP
   multicast group memberships.  However, it must be taken into account
   that this incurs additional processing overhead on each CoAP server
   participating in at least one of these groups: messages to groups
   that are not of interest to the node are only discarded at the higher
   transport (UDP) layer instead of directly at the network (IP) layer.

   Port 5684 is reserved for DTLS-secured CoAP and MUST NOT be used for
   any CoAP group communication.

   For a CoAP server node that supports resource discovery as defined in
   Section 2.4 of [RFC7252], the default port 5683 MUST be supported
   (see Section 7.1 of [RFC7252]) for the "All CoAP Nodes" multicast
   group.

2.2.3.  Proxy Operation

   CoAP (Section 5.7.2 of [RFC7252]) allows a client to request a
   forward-proxy to process its CoAP request.  For this purpose, the
   client specifies either the request group URI as a string in the
   Proxy-URI option or alternatively it uses the Proxy-Scheme option
   with the group URI constructed from the usual Uri-* options.  This
   approach works well for unicast requests.  However, there are certain
   issues and limitations of processing the (unicast) responses to a
   CoAP group communication request made in this manner through a proxy.

   A proxy may buffer all the individual (unicast) responses to a CoAP
   group communication request and then send back only a single
   (aggregated) response to the client.  However, there are some issues
   with this aggregation approach:




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   o  Aggregation of (unicast) responses to a CoAP group communication
      request in a proxy is difficult.  This is because the proxy does
      not know how many members there are in the group or how many group
      members will actually respond.  Also, the proxy does not know how
      long to wait before deciding to send back the aggregated response
      to the client.

   o  There is no default format defined in CoAP for aggregation of
      multiple responses into a single response.  Such a format could be
      defined based on the multipart content-format
      [I-D.ietf-core-multipart-ct] but is not standardized yet
      currently.

   Alternatively, if a proxy does not aggregate responses and follows
   the CoAP Proxy specification (Section 5.7.2 of [RFC7252]), the proxy
   would forward all the individual (unicast) responses to a CoAP group
   communication request to the client.  There are also issues with this
   approach:

   o  The client may be confused as it may not have known that the
      Proxy-URI contained a group URI target.  That is, the client that
      sent a unicast CoAP request to the proxy may be expecting only one
      (unicast) response.  Instead, it receives multiple (unicast)
      responses, potentially leading to fault conditions in the
      application.

   o  Each individual CoAP response 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.

   Due to the above issues, a CoAP Proxy SHOULD NOT support processing
   an IP multicast CoAP request but rather return a 501 (Not
   Implemented) response in such case.  The exception case here (i.e.,
   to support it) is when all the following conditions are met:

   o  The CoAP Proxy MUST be explicitly configured (whitelist) to allow
      proxied IP multicast requests by specific client(s).

   o  The proxy SHOULD return individual (unicast) CoAP responses to the
      client (i.e., not aggregated).  If a (future) standardized
      aggregation format is being used, then aggregated responses may be
      sent.

   o  It MUST be known to the person/entity doing the configuration of
      the proxy, or otherwise verified in some way, that the client



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      configured in the whitelist supports receiving multiple responses
      to a proxied unicast CoAP request.

   The operation of HTTP-to-CoAP proxies for multicast CoAP requests is
   specified in Section 8.4 and 10.1 of [RFC8075].  In this case, the
   "application/http" media type can be 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 needs to
   be aware that it is receiving this format and needs to be able to
   decode from it the responses of multiple servers.  The above
   restrictions listed for CoAP Proxies still apply to HTTP-to-CoAP
   proxies: specifically, the IP address of the sender of each CoAP
   response cannot be determined from the application/http response.

2.2.4.  Congestion Control

   CoAP group communication requests may result in a multitude of
   responses from different nodes, potentially causing congestion.
   Therefore, both the sending of IP multicast requests and the sending
   of the unicast CoAP responses to these multicast requests should be
   conservatively controlled.

   CoAP [RFC7252] reduces IP multicast-specific congestion risks through
   the following measures:

   o  A server may choose not to respond to an IP multicast request if
      there is nothing useful to respond to, e.g., error or empty
      response (see Section 8.2 of [RFC7252]).

   o  A server should limit the support for IP multicast requests to
      specific resources where multicast operation is required
      (Section 11.3 of [RFC7252]).

   o  An IP multicast request MUST be Non-confirmable (Section 8.1 of
      [RFC7252]).

   o  A response to an IP multicast request SHOULD be Non-confirmable
      (Section 5.2.3 of [RFC7252]).

   o  A server does not respond immediately to an IP multicast request
      and should first wait for a time that is randomly picked within a
      predetermined time interval called the Leisure (Section 8.2 of
      [RFC7252]).

   Additional guidelines to reduce congestion risks defined in this
   document are as follows:





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   o  A server in an LLN should only support group communication GET for
      resources that are small.  This can consist, for example, in
      having the payload of the response as limited to approximately 5%
      of the IP Maximum Transmit Unit (MTU) size, so that it fits into a
      single link-layer frame in case IPv6 over Low-Power Wireless
      Personal Area Networks (6LoWPAN) (see Section 4 of [RFC4944]) is
      used.

   o  A server SHOULD minimize the payload length of a response to a
      multicast GET on "/.well-known/core" by using hierarchy in
      arranging link descriptions for the response.  An example of this
      is given in Section 5 of [RFC6690].

   o  A server MAY minimize the payload length of a response to a
      multicast GET (e.g., on "/.well-known/core") by using CoAP block-
      wise transfers [RFC7959] in case the payload is long, returning
      only a first block of the CoRE Link Format description.  For this
      reason, a CoAP client sending an IP multicast CoAP request to
      "/.well-known/core" SHOULD support block-wise transfers.

   o  A client SHOULD use CoAP group communication with the smallest
      possible IP multicast scope that fulfills the application needs.
      As an example, site-local scope is always preferred over global
      scope IP multicast if this fulfills the application needs.
      Similarly, realm-local scope is always preferred over site-local
      scope if this fulfills the application needs.

2.2.5.  Observing Resources

   The CoAP Observe Option [RFC7641] is a protocol extension of CoAP,
   that allows a CoAP client to retrieve a representation of a resource
   and automatically keep this representation up-to-date over a longer
   period of time.  The client gets notified when the representation has
   changed.  [RFC7641] does not mention whether the Observe Option can
   be combined with CoAP multicast.  This section updates [RFC7641] with
   the use of the Observe Option in a CoAP multicast GET request and
   defines normative behavior for both client and server.

   Multicast Observe is a useful way to start observing a particular
   resource on all members of a (multicast) group at the same time.
   Group members that do not have this particular resource or do not
   allow the GET method on it will either respond with an error status -
   4.04 Not Found or 4.05 Method Not Allowed, respectively - or will
   silently suppress the response following the rules of Section 2.2.1,
   depending on server-specific configuration.

   A client that sends a multicast GET request with the Observe Option
   MAY repeat this request using the same Token value and the same



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   Observe Option 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.  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 sent a multicast GET 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.  The client in this case always uses a Message ID that
   differs from the initial multicast message.

   In the above client behaviors, the Token value is kept identical to
   the initial request to avoid that a client is included in more than
   one entry in the list of observers (Section 4.1 of [RFC7641]).

   Before repeating a request as specified above, the client SHOULD wait
   for at least the expected round-trip time plus the Leisure time
   period defined in Section 8.2 of [RFC7252], to give the server time
   to respond.

   A server that receives a legitimate GET request with the Observe
   Option, for which request processing is successful, SHOULD NOT
   suppress the response to this request, because the client is
   obviously interested in the resource representation.  A server that
   adds a client to the list of observers for a resource due to an
   Observe request MUST NOT suppress the response to this request.

   A server SHOULD have a mechanism to verify liveness of its observing
   clients and the continued interest of these clients in receiving the
   observe notifications.  This can be implemented by sending
   notifications occassionally using a Confirmable message.  See
   Section 4.5 of [RFC7641] for details.  This requirement overrides the
   regular behavior of sending Non-Confirmable notifications in response
   to a Non-Confirmable request.





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   For observing a group of servers through a CoAP-to-CoAP proxy or
   HTTP-CoAP proxy, the limitations stated in Section 2.2.3 apply.

2.2.6.  Block-Wise Transfer

   Section 2.8 of [RFC7959] specifies how a client can use block-wise
   transfer (Block2 Option) in a multicast GET request to limit the size
   of the initial response of each server.  The client has to use
   unicast for any further requests, separately addressing each
   different server, in order to retrieve more blocks of the resource
   from that server, if any.  Also, a server (group member) that needs
   to respond to a multicast request with a particularly large resource
   can use block-wise transfer (Block2 Option) at its own initiative, to
   limit the size of the initial response.  Again, a client would have
   to use unicast for any further requests to retrieve more blocks of
   the resource.

   A solution for multicast block-wise transfer using the Block1 Option
   is not specified in [RFC7959] nor in the present document.  Such a
   solution would be useful for multicast 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 multicast group.

2.3.  Transport

   In this document only UDP is considered as a transport protocol, both
   over IPv4 and IPv6.  Therefore, [RFC8323] (CoAP over TCP, TLS, and
   WebSockets) is not in scope as a transport for group communication.

2.3.1.  UDP/IPv6 Multicast Transport

   CoAP group communication can use UDP over IPv6 as a transport
   protocol, provided that IPv6 multicast is enabled.  IPv6 multicast
   MAY be supported in a network only for a limited scope.  For example,
   Section 2.4.2 describes the potential limited support of RPL for
   multicast, depending on how the protocol is configured.

   For a CoAP server node that supports resource discovery as defined in
   Section 2.4 of [RFC7252], the default port 5683 MUST be supported as
   per Section 7.1 and 12.8 of [RFC7252] for the "All CoAP Nodes"
   multicast group.  An IPv6 CoAP server SHOULD support the "All CoAP
   Nodes" groups with at least link-local (2), admin-local (4) and site-
   local (5) scopes.  An IPv6 CoAP server on a 6LoWPAN node (see
   Section 2.3.3) SHOULD also support the realm-local (3) scope.




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

2.3.2.  UDP/IPv4 Multicast Transport

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

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

2.3.3.  6LoWPAN

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

   On 6LoWPAN networks, multicast groups can be defined with realm-local
   scope [RFC7346].  Such a realm-local group is restricted to the local
   6LoWPAN network/subnet.  In other words, a multicast request to that
   group does not propagate beyond the 6LoWPAN network segment where the
   request originated.  For example, a multicast discovery request can
   be sent to the realm-local "All CoAP Nodes" IPv6 multicast group (see
   Section 2.3.1) in order to discover only CoAP servers on the local
   6LoWPAN network.

2.4.  Interworking with Other Protocols

2.4.1.  MLD/MLDv2/IGMP/IGMPv3

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



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   specific (CoAP) multicast group, it requires a way to signal to IP
   multicast routers which IP multicast address(es) it needs to listen
   to.

   The MLDv2 protocol [RFC3810] is the standard IPv6 method to achieve
   this; therefore, this method SHOULD be used by group members to
   subscribe to the multicast group IPv6 address, on IPv6 networks that
   support it.  CoAP server nodes then act in the role of MLD Multicast
   Address Listener.  Constrained IPv6 networks that implement either
   RPL (see Section 2.4.2) or MPL (see Section 2.4.3) typically do not
   support MLD as they have their own mechanisms defined.

   The IGMPv3 protocol [RFC3376] is the standard IPv4 method to signal
   multicast group subscriptions.  This SHOULD be used by group members
   to subscribe to their multicast group 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 LLNs.

2.4.2.  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 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 communication request will be delivered to link-local
   nodes only, for any scope value >= 2 used in the IPv6 destination
   address.

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

   In this mode, RPL DAO can be used by a CoAP node that is either an
   RPL router or RPL Leaf Node, to advertise its IP multicast 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 IP multicast 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



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   Acyclic Graph (DODAG).  This is useful because the edge router then
   learns which IP multicast traffic it needs to pass through from the
   backbone network into the LLN subnet.  In LLNs, such ingress
   filtering helps to avoid congestion of the resource-constrained
   network segment, due to IP multicast traffic from the high-speed
   backbone IP network.

2.4.3.  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.  MPL defines the protocol for a predefined
   group of MPL Forwarders to collectively distribute IPv6 multicast
   packets throughout their MPL Domain.  An MPL Forwarder may be
   associated to multiple MPL Domains at the same time.  Non-Forwarders
   will receive IPv6 multicast packets from one or more of their
   neighboring Forwarders.  Therefore, MPL can be used to propagate a
   CoAP multicast request to all group members.

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

   o  Manual configuration on the ingress MPL Forwarder.

   o  A protocol to register multicast groups at an ingress MPL
      Forwarder.  This could be a protocol offering features similar to
      MLDv2.

3.  Unsecured Group Communication

   CoAP group communication can operate in CoAP NoSec (No Security)
   mode, without using application-layer and transport-layer security
   mechanisms.  The NoSec mode uses the "coap" scheme, and is defined in
   Section 9 of [RFC7252].  Before using this mode of operation, the
   security implications (Section 5.1) must be well understood.

4.  Secured Group Communication using Group OSCORE

   The application-layer protocol Object Security for Constrained
   RESTful Environments (OSCORE) [RFC8613] provides end-to-end
   encryption, integrity and replay protection of CoAP messages
   exchanged between two CoAP endpoints.  These can act both as CoAP



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   Client as well as CoAP Server, and share an OSCORE Security Context
   used to protect and verify exchanged messages.  The use of OSCORE
   does not affect the URI scheme and OSCORE can therefore be used with
   any URI scheme defined for CoAP.

   OSCORE uses COSE [RFC8152] to perform encryption, signing and Message
   Authentication Code operations, and to efficiently encode the result
   as a COSE object.  In particular, OSCORE takes as input an
   unprotected CoAP message and transforms it into a protected CoAP
   message, by using an Authenticated Encryption with Associated Data
   (AEAD) algorithm.

   OSCORE makes it possible to selectively protect different parts of a
   CoAP message in different ways, so still allowing intermediaries
   (e.g., CoAP proxies) to perform their intended funtionalities.  That
   is, some message parts are encrypted and integrity protected; other
   parts are only integrity protected to be accessible to, but not
   modifiable by, proxies; and some parts are kept as plain content to
   be both accessible to and modifiable by proxies.  Such differences
   especially concern the CoAP options included in the unprotected
   message.

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

   In particular, Group OSCORE protects CoAP requests sent over IP
   multicast by a CoAP client, as well as multiple corresponding CoAP
   responses sent over IP unicast by different CoAP servers.  However,
   the same keying 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 uses digital signatures to ensure source authentication
   of all messages exchanged within the OSCORE group.  That is, sender
   devices sign their outgoing messages by means of their own private
   key, and embed the signature in the protected CoAP message.

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

   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




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   for Authentication and Authorization in constrained environments
   [I-D.ietf-ace-oauth-authz].

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

   If security is required, CoAP group communication as described in
   this specification MUST use Group OSCORE.  In particular, a CoAP
   group as defined in Section 2.1.1 and using secure group
   communication is associated to an OSCORE group, which includes:

   o  All members of the CoAP group, i.e. the CoAP endpoints configured
      (also) as CoAP servers and listening to the group's multicast IP
      address.

   o  All further CoAP endpoints configured only as CoAP clients, that
      send (multicast) CoAP requests to the CoAP group.

4.1.  Secure Group Maintenance

   Additional key management operations on the OSCORE group are
   required, depending also on the security requirements of the
   application (see Section 5.2).  That is:

   o  Adding new members to a CoAP group or enabling new client-only
      endpoints to interact with that group require also that each of
      such members/endpoints join the corresponding OSCORE group.  By
      doing so, they are securely provided with the necessary
      cryptographic material.  In case backward security is needed, this
      also requires to first renew such material and distribute it to
      the current members/endpoints, before new ones are added and join
      the OSCORE group.

   o  In case forward security is needed, removing members from a CoAP
      group or stopping client-only endpoints from interacting with that
      group requires removing such members/endpoints from the
      corresponding OSCORE group.  To this end, new cryptographic
      material is generated and securely distributed only to the
      remaining members/endpoints.  This ensures that only the members/
      endpoints intended to remain are able to continue participating to
      secure group communication, while the evicted ones are not able
      to.

   The key management operations mentioned above are entrusted to the
   Group Manager responsible for the OSCORE group
   [I-D.ietf-core-oscore-groupcomm], and it is RECOMMENDED to perform



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   them according to the approach described in
   [I-D.ietf-ace-key-groupcomm-oscore].

5.  Security Considerations

   This section provides security considerations for CoAP group
   communication using IP multicast.

5.1.  CoAP NoSec Mode

   CoAP group communication, if not protected, is vulnerable to all the
   attacks mentioned in Section 11 of [RFC7252] for IP multicast.

   Thus, for sensitive and mission-critical applications (e.g., health
   monitoring systems and alarm monitoring systems), it is NOT
   RECOMMENDED to deploy CoAP group communication in NoSec mode.

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

   Discovery of devices and resources is a typical use case where NoSec
   mode is applied, since the devices involved do not have yet
   configured any mutual security relations at the time the discovery
   takes place.

5.2.  Group OSCORE

   Group OSCORE provides end-to-end application-level security.  This
   has many desirable properties, including maintaining security
   assurances while forwarding traffic through intermediaries (proxies).
   Application-level security also tends to more cleanly separate
   security from the dynamics of group membership (e.g., the problem of
   distributing security keys across large groups with many members that
   come and go).

   For sensitive and mission-critical applications, CoAP group
   communication MUST be protected by using Group OSCORE as specified in
   [I-D.ietf-core-oscore-groupcomm].  The same security considerations
   from Section 8 of [I-D.ietf-core-oscore-groupcomm] hold for this
   specification.






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5.2.1.  Group Key Management

   A key management scheme for secure revocation and renewal of group
   keying material, namely group rekeying, should be adopted in OSCORE
   groups.  In particular, the key management scheme should preserve
   backward and forward security in the OSCORE group, if the application
   requires so (see Section 2.1 of [I-D.ietf-core-oscore-groupcomm]).

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

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

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

   This comes at the cost of not continuously preserving backward and
   forward security, since group rekeying might not occur upon every
   single group membership change.  That is, latest joined nodes would
   have access to the key material used prior to their join, and thus be
   able to access past group communications protected with that key
   material.  Similarly, until the group is rekeyed, latest left nodes
   would preserve access to group communications protected with the
   retained key material.

5.2.2.  Source Authentication

   CoAP endpoints using Group OSCORE countersign their outgoing
   messages, by means of the countersignature algorithm used in the
   OSCORE group.  This ensures source authentication of messages
   exchanged by CoAP endpoints through CoAP group communication.  In
   fact, it allows to verify that a received message has actually been
   originated by a specific and identified member of the OSCORE group.

   Appendix F of [I-D.ietf-core-oscore-groupcomm] discusses a number of
   cases where a recipient CoAP endpoint may skip the verification of
   countersignatures, possibly on a per-message basis.  However, this is
   NOT RECOMMENDED.  That is, a CoAP endpoint receiving a message
   secured with Group OSCORE SHOULD always verify the countersignature.



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5.2.3.  Counteraction of Attacks

   Group OSCORE addresses security attacks mentioned in Sections
   11.2-11.6 of [RFC7252], with particular reference to their execution
   over IP multicast.  That is: it provides confidentiality and
   integrity of request/response data through proxies also in multicast
   settings; it prevents amplification attacks carried out through
   responses to injected requests over IP multicast; it limits the
   impact of attacks based on IP spoofing; it prevents cross-protocol
   attacks; it derives the group key material from, among other things,
   a Master Secret securely generated by the Group Manager and provided
   to CoAP endpoints upon their joining of the OSCORE group;
   countersignatures assure source authentication of exchanged CoAP
   messages, and hence prevent a group member to be used for subverting
   security in the whole group.

5.3.  Use of CoAP No Response Option

   The CoAP No Server Response Option [RFC7967] could be misused by a
   malicious client to evoke as much responses from servers to a
   multicast request as possible, by using the value '0' - Interested in
   all responses.  This can even override 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.  A proposed mitigation is to only
   allow this Option to relax the standard suppression rules for a
   resource in case the Option is sent by an authenticated client.  If
   sent by an unauthenticated client, the Option can be used to expand
   the classes of responses suppressed compared to the default rules but
   not to reduce the classes of responses suppressed.

5.4.  6LoWPAN

   In a 6LoWPAN network, a multicast IPv6 packet may be fragmented prior
   to transmission.  A 6LoWPAN Router that forwards a fragmented packet
   can have a relatively high impact on the occupation of the wireless
   channel and on the memory load of the local node due to packet buffer
   occupation.  For example, the MPL [RFC7731] protocol requires an MPL
   Forwarder to store the packet for a longer duration, to allow
   multiple forwarding transmissions to neighboring Forwarders.  If only
   one of the fragments is not received correctly by an MPL Forwarder,
   the receiver needs to discard all received fragments and it needs to
   receive all the packet fragments again on a future occasion.

   For these reasons, a fragmented IPv6 multicast packet is a possible
   attack vector in a Denial of Service (DoS) amplification attack.  See
   Section 11.3 of [RFC7252] for more details on amplification.  To
   mitigate the risk, applications sending multicast IPv6 requests to



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   6LoWPAN hosted CoAP servers SHOULD limit the size of the request to
   avoid 6LoWPAN fragmentation.  A 6LoWPAN Router or multicast forwarder
   SHOULD deprioritize forwarding for multi-fragment 6LoWPAN multicast
   packets.  Also, a 6LoWPAN Border Router SHOULD implement multicast
   packet filtering to prevent unwanted multicast traffic from entering
   a 6LoWPAN network from the outside.  For example, it could filter out
   all multicast packet for which there is no known multicast listener
   on the 6LoWPAN network.

5.5.  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.

5.6.  Monitoring

5.6.1.  General Monitoring

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

5.6.2.  Pervasive Monitoring

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




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

   One mitigation is to restrict the scope of IP multicast to the
   minimal scope that fulfills the application need.  Thus, for example,
   site-local IP multicast scope is always preferred over global scope
   IP multicast if this fulfills the application needs.

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

6.  IANA Considerations

   This document has no actions for IANA.

7.  References

7.1.  Normative References

   [I-D.ietf-core-oscore-groupcomm]
              Tiloca, M., Selander, G., Palombini, F., and J. Park,
              "Group OSCORE - Secure Group Communication for CoAP",
              draft-ietf-core-oscore-groupcomm-05 (work in progress),
              July 2019.

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

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

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




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

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

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

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

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

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

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

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

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

   [RFC8152]  Schaad, J., "CBOR Object Signing and Encryption (COSE)",
              RFC 8152, DOI 10.17487/RFC8152, July 2017,
              <https://www.rfc-editor.org/info/rfc8152>.



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

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

7.2.  Informative References

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

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

   [I-D.ietf-ace-key-groupcomm-oscore]
              Tiloca, M., Park, J., and F. Palombini, "Key Management
              for OSCORE Groups in ACE", draft-ietf-ace-key-groupcomm-
              oscore-02 (work in progress), July 2019.

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) using the OAuth 2.0
              Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-24
              (work in progress), March 2019.

   [I-D.ietf-core-coap-pubsub]
              Koster, M., Keranen, A., and J. Jimenez, "Publish-
              Subscribe Broker for the Constrained Application Protocol
              (CoAP)", draft-ietf-core-coap-pubsub-09 (work in
              progress), September 2019.

   [I-D.ietf-core-multipart-ct]
              Fossati, T., Hartke, K., and C. Bormann, "Multipart
              Content-Format for CoAP", draft-ietf-core-multipart-ct-04
              (work in progress), August 2019.

   [I-D.ietf-core-resource-directory]
              Shelby, Z., Koster, M., Bormann, C., Stok, P., and C.
              Amsuess, "CoRE Resource Directory", draft-ietf-core-
              resource-directory-23 (work in progress), July 2019.



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   [I-D.tiloca-core-oscore-discovery]
              Tiloca, M., Amsuess, C., and P. Stok, "Discovery of OSCORE
              Groups with the CoRE Resource Directory", draft-tiloca-
              core-oscore-discovery-03 (work in progress), July 2019.

   [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>.

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.

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



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   also be implemented using CoAP resource discovery (Section 7 of
   [RFC7252]) and the CoAP query interface defined in Section 4 of
   [RFC6690] to find these particular resources.  Also, a multicast GET
   request to /.well-known/core can be used to discover all CoAP
   devices.

A.1.2.  Distributed Service Discovery

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

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

A.1.3.  Directory Discovery

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







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A.2.1.  Actuator Group Control

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

A.2.2.  Device Group Status Request

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

A.2.3.  Network-wide Query

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

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.




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A.3.  Software Update

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

Acknowledgments

   The authors sincerely thank Thomas Fossati and Jim Schaad for their
   comments and feedback.

   The work on this document has been partly supported by VINNOVA and
   the Celtic-Next project CRITISEC.

Authors' Addresses

   Esko Dijk
   IoTconsultancy.nl
   \________________\
   Utrecht
   The 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






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   Marco Tiloca
   RISE AB
   Isafjordsgatan 22
   Kista  SE-16440 Stockholm
   Sweden

   Email: marco.tiloca@ri.se












































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