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Versions: 00 01

CoRE Working Group                                               E. Dijk
Internet-Draft                                         IoTconsultancy.nl
Updates: 7390, 7641 (if approved)                                C. Wang
Intended status: Standards Track                            InterDigital
Expires: September 11, 2019                                    M. Tiloca
                                                                 RISE AB
                                                          March 10, 2019


  Group Communication for the Constrained Application Protocol (CoAP)
                    draft-dijk-core-groupcomm-bis-00

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 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 September 11, 2019.

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 . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Discovery . . . . . . . . . . . . . . . . . . . . . . . .   4
       2.1.1.  Distributed Device Discovery  . . . . . . . . . . . .   4
       2.1.2.  Distributed Service Discovery . . . . . . . . . . . .   5
       2.1.3.  Directory Discovery . . . . . . . . . . . . . . . . .   5
     2.2.  Operational . . . . . . . . . . . . . . . . . . . . . . .   5
       2.2.1.  Actuator Group Control  . . . . . . . . . . . . . . .   6
       2.2.2.  Device Group Status Request . . . . . . . . . . . . .   6
       2.2.3.  Network-wide Query  . . . . . . . . . . . . . . . . .   6
     2.3.  Software Update . . . . . . . . . . . . . . . . . . . . .   6
   3.  General Group Communication Operation . . . . . . . . . . . .   7
     3.1.  Group Configuration . . . . . . . . . . . . . . . . . . .   7
       3.1.1.  Group Definition  . . . . . . . . . . . . . . . . . .   7
       3.1.2.  Group Naming (DNS)  . . . . . . . . . . . . . . . . .   7
       3.1.3.  Group Creation and Membership . . . . . . . . . . . .   8
       3.1.4.  Group Maintenance . . . . . . . . . . . . . . . . . .   8
     3.2.  CoAP Usage  . . . . . . . . . . . . . . . . . . . . . . .   9
       3.2.1.  Request/Response Model  . . . . . . . . . . . . . . .   9
       3.2.2.  Port and URI Path Selection . . . . . . . . . . . . .  10
       3.2.3.  Proxy Operation . . . . . . . . . . . . . . . . . . .  11
       3.2.4.  Congestion Control  . . . . . . . . . . . . . . . . .  11
       3.2.5.  Observing Resources . . . . . . . . . . . . . . . . .  11
       3.2.6.  Block-Wise Transfer . . . . . . . . . . . . . . . . .  11
     3.3.  Transport . . . . . . . . . . . . . . . . . . . . . . . .  12
       3.3.1.  UDP/IPv6 Multicast Transport  . . . . . . . . . . . .  12
       3.3.2.  UDP/IPv4 Multicast Transport  . . . . . . . . . . . .  12
       3.3.3.  6LoWPAN . . . . . . . . . . . . . . . . . . . . . . .  12
     3.4.  Interworking with Other Protocols . . . . . . . . . . . .  12
       3.4.1.  MLD/MLDv2/IGMP  . . . . . . . . . . . . . . . . . . .  12
       3.4.2.  RPL . . . . . . . . . . . . . . . . . . . . . . . . .  12
       3.4.3.  MPL . . . . . . . . . . . . . . . . . . . . . . . . .  12
   4.  Unsecured Group Communication . . . . . . . . . . . . . . . .  12
   5.  Secured Group Communication using Group OSCORE  . . . . . . .  13
     5.1.  Secure Group Maintenance  . . . . . . . . . . . . . . . .  14
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  15



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     6.1.  CoAP NoSec Mode . . . . . . . . . . . . . . . . . . . . .  15
     6.2.  Group OSCORE  . . . . . . . . . . . . . . . . . . . . . .  15
     6.3.  6LoWPAN . . . . . . . . . . . . . . . . . . . . . . . . .  16
     6.4.  Wi-Fi . . . . . . . . . . . . . . . . . . . . . . . . . .  16
     6.5.  Monitoring  . . . . . . . . . . . . . . . . . . . . . . .  17
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  18
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   This document specifies the use of the Constrained Application
   Protocol (CoAP) [RFC7252] for group communication [RFC7390].  CoAP is
   a RESTful communication protocol that is suited for usage in
   resource-constrained nodes, and in resource-constrained networks.
   This area of use is summarized as Constrained RESTful Environments
   (CoRE).

   One-to-many group communication is achieved in CoAP by using UDP/IP
   multicast, as the underlying data transport to send multicast request
   messages.  Multiple response messages to a single multicast request
   message are sent 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).

   The most common use cases for group communication in resource-
   constrained networks are listed first, in Section 2.  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) [I-D.ietf-core-object-security].  This method
   provides end-to-end application-layer security protection of CoAP
   messages, by using CBOR Object Signing and Encryption (COSE)
   [RFC8152] [RFC7049].

1.1.  Scope

   The guidelines and experimental protocol of [RFC7390] are updated by
   this document with the abovementioned security solution, and with
   other recent protocol developments around CoAP such as Observe
   [RFC7641] and Block-Wise Transfers [RFC7959].



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   For group communication, only solutions that use CoAP over UDP/
   multicast (both IPv6 and IPv4) are considered.  Security solutions
   for group communication other than Group OSCORE are not in scope.
   General principles for secure group configuration are in scope,
   however a specific protocol for secure group configuration is not
   mandated because this is often application-specific.

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.  Section 5 requires readers to be familiar
   with concepts and terminology from OSCORE
   [I-D.ietf-core-object-security] and Group OSCORE
   [I-D.ietf-core-oscore-groupcomm].

2.  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 technical 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.

2.1.  Discovery

   Discovery of physical devices in a network, or discovery of
   information entities hosted on network devices, are operations that
   are particularly 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.

2.1.1.  Distributed Device Discovery

   Device discovery is the discovery and identification of networked
   devices of a particular class, type, model, or brand.  Group
   communication is used for distributed device discovery, where a
   central directory service is not used.  Typically in distributed
   service discovery, a multicast request is sent to a particular



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   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 service is accessed through unicast, in which case group
   communication is not needed.

2.1.2.  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 service not being used.  Technically this is similar to the
   above use case of distributed device discovery (Section 2.1.1).  For
   example, when using CoAP resource discovery (Section 7 of [RFC7252])
   there is no technical distinction between doing distributed device
   discovery and distributed service discovery: both use the same CoAP
   query interface defined in Section 4 of [RFC6690].

2.1.3.  Directory Discovery

   This use case is a specific sub-case of Distributed Service Discovery
   (Section 2.1.2), 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.  One
   particular type of directory is the CoRE Resource Directory
   [I-D.ietf-core-resource-directory]; and there may be other types of
   directories that can be discovered using CoAP.  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 be
   looked up by other devices and/or applications.

2.2.  Operational

   Operational use cases describe those operations that occur most
   frequently in a networked system, during its operational lifetime and
   normal usage.








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2.2.1.  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.

2.2.2.  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.

2.2.3.  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
   (Section 2.1.2) 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.

2.3.  Software Update

   Multicast can be useful to efficiently distribute new software
   (firmware) 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 usually are 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 software blocks.



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3.  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
   configuration (e.g. group creation and maintenance which are usually
   done by an application, user or commissioning entity) is considered
   in Section 3.1.  Then the use of CoAP for group communication
   including support for protocol extensions (Block-Wise, Observe, PATCH
   method) follows in Section 3.2.  How CoAP group messages are carried
   over various transport layers is the subject of Section 3.3.
   Finally, Section 3.4 covers the interworking of CoAP with other
   protocols at the layers below it.

3.1.  Group Configuration

3.1.1.  Group Definition

   Following [RFC7390], a CoAP group is defined here 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.  An endpoint may be a member of multiple 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 group.

   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]).

3.1.2.  Group Naming (DNS)

   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
   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].






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3.1.3.  Group Creation and Membership

   Group membership may be (factory-)preconfigured in devices or
   dynamically configured in a system on-site.

   To create a group, a configuring entity defines an IP multicast
   address (or hostname) and a UDP port number for the group.  Then it
   configures one or more devices as listeners to that IP multicast
   address, with a CoAP server listening on the specific port.  These
   devices are the group members.  The configuring entity can be a local
   application with preconfiguration, a user, a cloud service, or a
   local commissioning tool for example.  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.

   The IETF does not define a mandatory, standardized protocol to
   accomplish these steps.  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 further defined in
   Section 5.  [RFC7390] defines an experimental protocol for
   configuration of group membership for non-secured group
   communication, based on JSON-formatted configuration resources.

3.1.4.  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, 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.

   When group communication is secured using Group OSCORE
   [I-D.ietf-core-oscore-groupcomm] (see Section 5), all CoAP endpoints
   participating to secure group communication MUST be members of a
   corresponding OSCORE group, and thus share a common set of
   cryptographic material.  Additional maintenance operations are
   discussed in Section 5.1.







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3.2.  CoAP Usage

3.2.1.  Request/Response Model

   Editor Note, TBD: this section is strongly based on Section 2.5 in
   [RFC7390].  In case a reference to this section is preferred, we can
   replace most of the following text in this section by a reference to
   it.

   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 as
   detailed in Section 4.5 of [RFC7252].

   A server MAY send back a unicast response to the CoAP group
   communication request - whether it does this or not is selected by
   the server application.  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.

   TBD: the CoAP Option for No Server Response [RFC7967] can be used by
   the client to influence response suppression on the server side.
   Possibly we can include this information here; it specifically
   targets use for multicast use cases also.

   The client can distinguish the origin of multiple server responses by
   the source IP address of the UDP message containing the CoAP response
   or any other available unique identifier (e.g., contained in the CoAP
   response payload).  In case a client has sent multiple group requests
   with concurrent CoAP transactions ongoing, the responses are as usual
   matched to a request using the Token.

   For multicast CoAP requests, there are additional constraints on the
   reuse of Token values, compared to the unicast case.  In the unicast
   case, receiving a response effectively frees up its Token value for
   reuse since no more responses will follow.  However, for multicast
   CoAP, the number of responses is not bounded a priori.  Therefore,
   the reception of a response cannot be used as a trigger to "free up"
   a Token value for reuse.  Reusing a Token value too early could lead
   to incorrect response/request matching in the client and would be a
   protocol error.  Therefore, the time between reuse of Token values
   used in multicast requests MUST be greater than:

   NON_LIFETIME + MAX_LATENCY + MAX_SERVER_RESPONSE_DELAY

   where NON_LIFETIME and MAX_LATENCY are defined in Section 4.8 of
   [RFC7252].  This specification defines MAX_SERVER_RESPONSE_DELAY as



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   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 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.

3.2.2.  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 of 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 use cases where a request only can be successful for a
   subset of the group and errors are returned by those group members
   for which the request was unsuccessful.

   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.

   (TBD: consider if we should say that receiving node/server SHOULD NOT
   send a "ICMP Destination Unreachable - Port Unreachable" in response
   to such request.)



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3.2.3.  Proxy Operation

   TBD: check if draft-ietf-core-multipart-ct-02 can solve the "multiple
   answers" case when a Proxy sends back multiple CoAP responses to a
   multicast request.  Possibly a client may support this.  Is there a
   way to signal multipart support by the client?  Can the multipart
   parts signal the origin/IP address of their origin server?

3.2.4.  Congestion Control

   The measures to reduce network congestion risks are listed in
   Section 2.8 of [RFC7390], including both mandatory protocol elements
   as well as guidelines.  This specification RECOMMENDS to apply the
   guidelines specified in that section.

3.2.5.  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.

   Using the Observe Option in a CoAP multicast GET request is
   explicitly specified here as allowed; it is useful as a means to
   start observing a particular resource on all members of a (multicast)
   group at the same time.  Group members that do not have this resource
   or do not allow the GET method on it will respond with the usual 4.04
   Not Found or 4.05 Method Not Allowed, respectively.

   A client sending a multicast GET with Observe MAY repeat this request
   using the same Token Option and Observe Option value, in order to
   ensure that enough (or all) group members have been reached with the
   request.

3.2.6.  Block-Wise Transfer

   Section 2.8 of [RFC7959] specifies how a server (group member),
   responding to a multicast request with a large resource, can use
   Block-Wise transfer to limit the size of the initial response.

   TBD: investigate use of Block-Wise for PUT/POST/PATCH/iPATCH
   operations e.g. to be used for distributing software blocks over
   multicast.  We can specify its use, or remark that this is undefined.






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3.3.  Transport

   TBD: Mark [RFC8323] (TCP, TLS, WebSockets) as not applicable for this
   form of groupcomm, as well as CoAP-over-SMS.

3.3.1.  UDP/IPv6 Multicast Transport

   TBD: include the "Exceptions" cases here of RFC 7390 Section 2.10.
   State that IPv6 multicast is prerequisite.  Also mention the All-
   CoAP-nodes IPv6 addresses.

3.3.2.  UDP/IPv4 Multicast Transport

   TBD: includes the "Exceptions" cases here of RFC 7390 2.10.  State
   that IPv4 multicast is prerequisite.  mention All-CoAP-nodes IPv4
   addresses and the like

3.3.3.  6LoWPAN

   TBD: 6lowpan-specific considerations to go here.  Specifically, a
   multicast request should preferably fit in one L2 frame to avoid the
   strong performance drop that comes with 6LoWPAN-fragmentation and
   reassembly.  Also reference [RFC7346] for the realm-local scope.

3.4.  Interworking with Other Protocols

3.4.1.  MLD/MLDv2/IGMP

   TBD: see Section 4.2 of [RFC7390] and include the content here or
   refer to it.

3.4.2.  RPL

   TBD: see Section 4.3 of [RFC7390] and include the content here or
   refer to it.

3.4.3.  MPL

   TBD: see Section 4.4.  [RFC7390] and include the content here or
   refer to it.

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].  Before using this mode of operation, the
   security implications (Section 6.1) must be well understood.



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5.  Secured Group Communication using Group OSCORE

   The application-layer protocol Object Security for Constrained
   RESTful Environments (OSCORE) [I-D.ietf-core-object-security]
   provides end-to-end encryption, integrity and replay protection of
   CoAP messages exchanged between two CoAP endpoints.  These can act
   both as CoAP Client as well as CoAP Server, and share an OSCORE
   Security Context used to protect and verify exchanged messages.  The
   use of OSCORE does not affect the URI scheme and OSCORE can therefore
   be used with any URI scheme defined for CoAP.

   OSCORE uses COSE [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 Authenticated Encryption Algorithms with Additional
   Data (AEAD).

   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 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




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   group members; manages, renews and provides keying material in the
   group; and drives the join process for 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
   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 3.1.1 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.

5.1.  Secure Group Maintenance

   Additional key management operations on the OSCORE group are
   required, depending also on the security requirements of the
   application (see Section 6.2).  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



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      secure group communication, while the evicted ones are not able
      to.

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

6.  Security Considerations

   This section provides security considerations for CoAP group
   communication using 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.

   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 non-critical applications (e.g., read-only
   temperature sensors where the client reading out the values 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.

6.2.  Group OSCORE

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

   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




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   from Section 8 of [I-D.ietf-core-oscore-groupcomm] hold for this
   specification.

   A key management scheme for secure revocation and renewal of group
   keying material should be adopted in OSCORE groups.  Also, 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]).

   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.

   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.

6.3.  6LoWPAN

   Editor Note, TBD: identify if multi-fragment multicast requests have
   a negative effect on security and, if so, advice here on trying to
   avoid such requests.  Also an attacker could use multi-fragment to
   occupy reassembly buffers of many routing 6LoWPAN nodes.

6.4.  Wi-Fi

   TBD: Wi-Fi specific security considerations; see also Section 5.3.1
   of [RFC7390].




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6.5.  Monitoring

   TBD: see Section 5.4 of [RFC7390].

7.  IANA Considerations

   This document has no actions for IANA.

8.  References

8.1.  Normative References

   [I-D.ietf-core-object-security]
              Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", draft-ietf-core-object-security-16 (work in
              progress), March 2019.

   [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-04 (work in progress),
              March 2019.

   [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>.

   [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>.





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   [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>.

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

   [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>.

   [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>.

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", draft-ietf-ace-key-groupcomm-
              oscore-01 (work in progress), March 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-22
              (work in progress), March 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-19 (work in progress), January 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-01 (work in progress), January 2019.

   [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>.

   [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>.

Acknowledgments

   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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