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Observe Notifications as CoAP Multicast Responses
draft-ietf-core-observe-multicast-notifications-05

Document Type Active Internet-Draft (core WG)
Authors Marco Tiloca , Rikard Höglund , Christian Amsüss , Francesca Palombini
Last updated 2022-10-24
Replaces draft-tiloca-core-observe-multicast-notifications
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draft-ietf-core-observe-multicast-notifications-05
CoRE Working Group                                             M. Tiloca
Internet-Draft                                                R. Höglund
Updates: 7252, 7641 (if approved)                                RISE AB
Intended status: Standards Track                               C. Amsüss
Expires: 27 April 2023                                                  
                                                            F. Palombini
                                                             Ericsson AB
                                                         24 October 2022

           Observe Notifications as CoAP Multicast Responses
           draft-ietf-core-observe-multicast-notifications-05

Abstract

   The Constrained Application Protocol (CoAP) allows clients to
   "observe" resources at a server, and receive notifications as unicast
   responses upon changes of the resource state.  In some use cases,
   such as based on publish-subscribe, it would be convenient for the
   server to send a single notification addressed to all the clients
   observing a same target resource.  This document updates RFC7252 and
   RFC7641, and defines how a server sends observe notifications as
   response messages over multicast, synchronizing all the observers of
   a same resource on a same shared Token value.  Besides, this document
   defines how Group OSCORE can be used to protect multicast
   notifications end-to-end between the server and the observer clients.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the Constrained RESTful
   Environments Working Group mailing list (core@ietf.org), which is
   archived at https://mailarchive.ietf.org/arch/browse/core/.

   Source for this draft and an issue tracker can be found at
   https://github.com/core-wg/observe-multicast-notifications.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   Internet-Drafts are draft documents valid for a maximum of six months
   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 27 April 2023.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Prerequisites . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  High-Level Overview of Available Variants . . . . . . . . . .   6
   4.  Server-Side . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Request . . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.2.  Informative Response  . . . . . . . . . . . . . . . . . .  10
       4.2.1.  Transport-Specific Message Information  . . . . . . .  12
       4.2.2.  Transport-Independent Message Information . . . . . .  15
     4.3.  Notifications . . . . . . . . . . . . . . . . . . . . . .  15
     4.4.  Congestion Control  . . . . . . . . . . . . . . . . . . .  16
     4.5.  Cancellation  . . . . . . . . . . . . . . . . . . . . . .  17
   5.  Client-Side . . . . . . . . . . . . . . . . . . . . . . . . .  17
     5.1.  Request . . . . . . . . . . . . . . . . . . . . . . . . .  17
     5.2.  Informative Response  . . . . . . . . . . . . . . . . . .  18
     5.3.  Notifications . . . . . . . . . . . . . . . . . . . . . .  20
     5.4.  Cancellation  . . . . . . . . . . . . . . . . . . . . . .  20
   6.  Web Linking . . . . . . . . . . . . . . . . . . . . . . . . .  20
   7.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .  21
   8.  Rough Counting of Clients in the Group Observation  . . . . .  23
     8.1.  Multicast-Response-Feedback-Divider Option  . . . . . . .  23
     8.2.  Processing on the Client Side . . . . . . . . . . . . . .  24
     8.3.  Processing on the Server Side . . . . . . . . . . . . . .  25
       8.3.1.  Request for Feedback  . . . . . . . . . . . . . . . .  25
       8.3.2.  Collection of Feedback  . . . . . . . . . . . . . . .  26

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       8.3.3.  Processing of Feedback  . . . . . . . . . . . . . . .  26
   9.  Protection of Multicast Notifications with Group OSCORE . . .  28
     9.1.  Signaling the OSCORE Group in the Informative Response  .  29
     9.2.  Server-Side Requirements  . . . . . . . . . . . . . . . .  31
       9.2.1.  Registration  . . . . . . . . . . . . . . . . . . . .  31
       9.2.2.  Informative Response  . . . . . . . . . . . . . . . .  32
       9.2.3.  Notifications . . . . . . . . . . . . . . . . . . . .  32
       9.2.4.  Cancellation  . . . . . . . . . . . . . . . . . . . .  33
     9.3.  Client-Side Requirements  . . . . . . . . . . . . . . . .  33
       9.3.1.  Informative Response  . . . . . . . . . . . . . . . .  33
       9.3.2.  Notifications . . . . . . . . . . . . . . . . . . . .  34
   10. Example with Group OSCORE . . . . . . . . . . . . . . . . . .  35
   11. Intermediaries  . . . . . . . . . . . . . . . . . . . . . . .  39
   12. Intermediaries Together with End-to-End Security  . . . . . .  40
     12.1.  Listen-To-Multicast-Responses Option . . . . . . . . . .  41
     12.2.  Message Processing . . . . . . . . . . . . . . . . . . .  42
   13. Informative Response Parameters . . . . . . . . . . . . . . .  44
   14. Transport Protocol Information  . . . . . . . . . . . . . . .  45
   15. Security Considerations . . . . . . . . . . . . . . . . . . .  46
     15.1.  Unsecured Multicast Notifications  . . . . . . . . . . .  46
     15.2.  Secured Multicast Notifications  . . . . . . . . . . . .  47
     15.3.  Listen-To-Multicast-Responses Option . . . . . . . . . .  47
   16. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  48
     16.1.  Media Type Registrations . . . . . . . . . . . . . . . .  48
     16.2.  CoAP Content-Formats Registry  . . . . . . . . . . . . .  49
     16.3.  CoAP Option Numbers Registry . . . . . . . . . . . . . .  49
     16.4.  Informative Response Parameters Registry . . . . . . . .  50
     16.5.  CoAP Transport Information Registry  . . . . . . . . . .  50
     16.6.  Expert Review Instructions . . . . . . . . . . . . . . .  51
   17. References  . . . . . . . . . . . . . . . . . . . . . . . . .  52
     17.1.  Normative References . . . . . . . . . . . . . . . . . .  52
     17.2.  Informative References . . . . . . . . . . . . . . . . .  54
   Appendix A.  Different Sources for Group Observation Data . . . .  56
     A.1.  Topic Discovery in Publish-Subscribe Settings . . . . . .  57
     A.2.  Introspection at the Multicast Notification Sender  . . .  58
   Appendix B.  Pseudo-Code for Rough Counting of Clients  . . . . .  59
     B.1.  Client Side . . . . . . . . . . . . . . . . . . . . . . .  59
     B.2.  Client Side - Optimized Version . . . . . . . . . . . . .  60
     B.3.  Server Side . . . . . . . . . . . . . . . . . . . . . . .  61
   Appendix C.  OSCORE Group Self-Managed by the Server  . . . . . .  63
   Appendix D.  Phantom Request as Deterministic Request . . . . . .  66
   Appendix E.  Example with a Proxy . . . . . . . . . . . . . . . .  68
   Appendix F.  Example with a Proxy and Group OSCORE  . . . . . . .  70
   Appendix G.  Example with a Proxy and Deterministic Requests  . .  76
     G.1.  Assumptions and Walkthrough . . . . . . . . . . . . . . .  76
     G.2.  Message Exchange  . . . . . . . . . . . . . . . . . . . .  78
   Appendix H.  Document Updates . . . . . . . . . . . . . . . . . .  83
     H.1.  Version -04 to -05  . . . . . . . . . . . . . . . . . . .  83

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     H.2.  Version -03 to -04  . . . . . . . . . . . . . . . . . . .  84
     H.3.  Version -02 to -03  . . . . . . . . . . . . . . . . . . .  84
     H.4.  Version -01 to -02  . . . . . . . . . . . . . . . . . . .  84
     H.5.  Version -00 to -01  . . . . . . . . . . . . . . . . . . .  84
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  85
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  85

1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] has been
   extended with a number of mechanisms, including resource Observation
   [RFC7641].  This enables CoAP clients to register at a CoAP server as
   "observers" of a resource, and hence being automatically notified
   with an unsolicited response upon changes of the resource state.

   CoAP supports group communication over IP multicast
   [I-D.ietf-core-groupcomm-bis].  This includes support for Observe
   registration requests over multicast, in order for clients to
   efficiently register as observers of a resource hosted at multiple
   servers.

   However, in a number of use cases, using multicast messages for
   responses would also be desirable.  That is, it would be useful that
   a server sends observe notifications for a same target resource to
   multiple observers as responses over IP multicast.

   For instance, in CoAP publish-subscribe [I-D.ietf-core-coap-pubsub],
   multiple clients can subscribe to a topic, by observing the related
   resource hosted at the responsible broker.  When a new value is
   published on that topic, it would be convenient for the broker to
   send a single multicast notification at once, to all the subscriber
   clients observing that topic.

   A different use case concerns clients observing a same registration
   resource at the CoRE Resource Directory [RFC9176].  For example,
   multiple clients can benefit of observation for discovering (to-be-
   created) OSCORE groups [I-D.ietf-core-oscore-groupcomm], by
   retrieving from the Resource Directory updated links and descriptions
   to join them through the respective Group Manager
   [I-D.tiloca-core-oscore-discovery].

   More in general, multicast notifications would be beneficial whenever
   several CoAP clients observe a same target resource at a CoAP server,
   and can be all notified at once by means of a single response
   message.  However, CoAP does not currently define response messages
   over IP multicast.  This document fills this gap and provides the
   following twofold contribution.

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   First, it updates [RFC7252] and [RFC7641], by defining a method to
   deliver Observe notifications as CoAP responses addressed to multiple
   clients, e.g., over IP multicast.  In the proposed method, the group
   of potential observers entrusts the server to manage the Token space
   for multicast notifications.  By doing so, the server provides all
   the observers of a target resource with the same Token value to bind
   to their own observation.  That Token value is then used in every
   multicast notification for the target resource.  This is achieved by
   means of an unicast informative response sent by the server to each
   observer client.

   Second, this document defines how to use Group OSCORE
   [I-D.ietf-core-oscore-groupcomm] to protect multicast notifications
   end-to-end between the server and the observer clients.  This is also
   achieved by means of the unicast informative response mentioned
   above, which additionally includes parameter values used by the
   server to protect every multicast notification for the target
   resource by using Group OSCORE.  This provides a secure binding
   between each of such notifications and the observation of each of the
   clients.

1.1.  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.

   Readers are expected to be familiar with terms and concepts described
   in CoAP [RFC7252], group communication for CoAP
   [I-D.ietf-core-groupcomm-bis], Observe [RFC7641], CBOR [RFC8949],
   OSCORE [RFC8613], and Group OSCORE [I-D.ietf-core-oscore-groupcomm].

   This document additionally defines the following terminology.

   *  Traditional observation.  A resource observation associated with a
      single observer client, as defined in [RFC7641].

   *  Group observation.  A resource observation associated with a group
      of clients.  The server sends notifications for the group-observed
      resource over IP multicast to all the observer clients.

   *  Phantom request.  The CoAP request message that the server would
      have received to start a group observation on one of its
      resources.  A phantom request is generated inside the server and
      does not hit the wire.

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   *  Informative response.  A CoAP response message that the server
      sends to a given client via unicast, providing the client with
      information on a group observation.

2.  Prerequisites

   In order to use multicast notifications as defined in this document,
   the following prerequisites have to be fulfilled.

   *  The server and the clients need to be on a network on which
      multicast notifications can reach a sufficiently large portion of
      the clients.  These may leverage intermediaries such as proxies,
      if not directly able to listen to multicast traffic.

   *  The server needs to be provisioned with multicast addresses whose
      token space is placed under its control.  On general purpose
      networks, unmanaged multicast addresses such as "All CoAP Nodes"
      (see Section 12.8 of [RFC7252]) are not suitable for this purpose.

   *  The server and the clients need to agree out of band that
      multicast notifications may be used.

      This document does not describe a way for a client to influence
      the server's decision to start group observations and thus to use
      multicast notifications.  This is done on purpose.

      That is, the mechanism specified in this document is expected to
      be used in situations where sending individual notifications is
      not feasible, or is not preferred beyond a certain number of
      clients observing a target resource.

      If applications arise where a negotiation between the clients and
      the server does make sense, those applications are welcome to
      specify additional means to opt in to multicast notifications.

3.  High-Level Overview of Available Variants

   The method defined in this document fundamentally enables a server to
   setup a group observation.  This is associated with a phantom
   observation request started by the server, and to which the multicast
   notifications of the group observation are bound.

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   While the server can provide the phantom request in question to the
   interested clients as they reach out for registering to the group
   observation, the server may alternatively distribute the phantom
   request in advance by alternative means (e.g., see Appendix A).
   Clients that have already retrieved the phantom request can
   immediately starts listening to multicast notifications if able to
   directly do so, or rather instruct an assisting intermediary such as
   a proxy to do that on their behalf.

   The following provides an overview of the available variants to
   enforce a group observation, depending on whether a proxy is deployed
   or not, and on whether exchanged messages are protected end-to-end
   between the observer clients and the server.

   *  No proxy - This is simplest network configuration, where the
      clients participating to the group observation are capable to
      listen to multicast traffic.  In such a setup, the clients
      directly receive multicast notifications from the server.

      -  Without end-to-end security - Messages pertaining to the group
         observation are not protected.  This basic case is defined in
         Section 4 and Section 5 from the server and the client side,
         respectively.  An example is provided in Section 7.

      -  With end-to-end security - Messages pertaining to the group
         observation are protected end-to-end between the clients and
         the server, by using the Group OSCORE security protocol
         [I-D.ietf-core-oscore-groupcomm].  This case is defined in
         Section 9.  An example is provided in Section 10.

         If the participating endpoints using Group OSCORE also support
         the concept of Deterministic Client
         [I-D.amsuess-core-cachable-oscore], then the possible early
         distribution of the phantom request can specifically make
         available its smaller, plain version.  Then, all the clients
         are able to compute the same protected phantom request to use
         (see Appendix D).

   *  With proxy - This network configuration is expected in case the
      clients participating to the group observation are not capable to
      listen to multicast traffic.  In such a setup, the proxy directly
      receives multicast notifications from the server, and relays them
      back to the clients.

      -  Without end-to-end security - Messages pertaining to the group
         observation are not protected end-to-end between the clients
         and the server.  This basic case is defined in Section 11.  An
         example is provided in Appendix E.

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      -  With end-to-end security - Messages pertaining to the group
         observation are protected end-to-end between the clients and
         the server, by using the Group OSCORE security protocol
         [I-D.ietf-core-oscore-groupcomm].  In particular, the clients
         are required to separately provide the proxy with the obtained
         phantom request, thus enabling the proxy to receive the
         multicast notifications from the server.  This case is defined
         in Section 12.  An example is provided in Appendix F.

         If the participating endpoints using Group OSCORE also support
         the concept of Deterministic Client
         [I-D.amsuess-core-cachable-oscore], the same advantages
         mentioned above for the case without a proxy applies (see
         Appendix D).  In addition, this allows for a more efficient
         setup and enforcement of the group observation, by reducing the
         amount of message exchanges and allowing the proxy to
         effectively serve protected multicast notifications from its
         cache.  An example is provided in Appendix G.2.

4.  Server-Side

   The server can, at any time, start a group observation on one of its
   resources.  Practically, the server may want to do that under the
   following circumstances.

   *  In the absence of observations for the target resource, the server
      receives a registration request from a first client wishing to
      start a traditional observation on that resource.

   *  When a certain amount of traditional observations has been
      established on the target resource, the server decides to make
      those clients part of a group observation on that resource.

   The server maintains an observer counter for each group observation
   to a target resource, as a rough estimation of the observers actively
   taking part in the group observation.

   The server initializes the counter to 0 when starting the group
   observation, and increments it after a new client starts taking part
   in that group observation.  Also, the server should keep the counter
   up-to-date over time, for instance by using the method described in
   Section 8.  This allows the server to possibly terminate a group
   observation in case, at some point in time, not enough clients are
   estimated to be still active and interested.

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4.1.  Request

   Assuming it is reachable at the address SRV_ADDR and port number
   SRV_PORT, the server starts a group observation on one of its
   resources as defined below.  The server intends to send multicast
   notifications for the target resource to the multicast IP address
   GRP_ADDR and port number GRP_PORT.

   1.  The server builds a phantom observation request, i.e., a GET
       request with an Observe Option set to 0 (register).

   2.  The server selects an available value T, from the Token space of
       a CoAP endpoint used for messages having:

       *  As source address and port number, the IP multicast address
          GRP_ADDR and port number GRP_PORT.

       *  As destination address and port number, the server address
          SRV_ADDR and port number SRV_PORT, intended for accessing the
          target resource.

       This Token space is under exclusive control of the server.

   3.  The server processes the phantom observation request above,
       without transmitting it on the wire.  The request is addressed to
       the resource for which the server wants to start the group
       observation, as if sent by the group of observers, i.e., with
       GRP_ADDR as source address and GRP_PORT as source port.

   4.  Upon processing the self-generated phantom registration request,
       the server interprets it as an observe registration received from
       the group of potential observer clients.  In particular, from
       then on, the server MUST use T as its own local Token value
       associated with that observation, with respect to the (previous
       hop towards the) clients.

   5.  The server does not immediately respond to the phantom
       observation request with a multicast notification sent on the
       wire.  The server stores the phantom observation request as is,
       throughout the lifetime of the group observation.

   6.  The server builds a CoAP response message INIT_NOTIF as initial
       multicast notification for the target resource, in response to
       the phantom observation request.  This message is formatted as
       other multicast notifications (see Section 4.3) and MUST include
       the current representation of the target resource as payload.
       The server stores the message INIT_NOTIF and does not transmit
       it.  The server considers this message as the latest multicast

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       notification for the target resource, until it transmits a new
       multicast notification for that resource as a CoAP message on the
       wire.  After that, the server deletes the message INIT_NOTIF.

4.2.  Informative Response

   After having started a group observation on a target resource, the
   server proceeds as follows.

   For each traditional observation ongoing on the target resource, the
   server MAY cancel that observation.  Then, the server considers the
   corresponding clients as now taking part in the group observation,
   for which it increases the corresponding observer counter
   accordingly.

   The server sends to each of such clients an informative response
   message, encoded as a unicast response with response code 5.03
   (Service Unavailable).  As per [RFC7641], such a response does not
   include an Observe Option.  The response MUST be Confirmable and MUST
   NOT encode link-local addresses.

   The Content-Format of the informative response is set to application/
   informative-response+cbor, defined in Section 16.2.  The payload of
   the informative response is a CBOR map including the following
   parameters, whose CBOR labels are defined in Section 13.

   *  'tp_info', with value a CBOR array.  This includes the transport-
      specific information required to correctly receive multicast
      notifications bound to the phantom observation request.
      Typically, this comprises the Token value associated with the
      group observation, as well as the source and destination
      addressing information of the related multicast notifications.
      The CBOR array is formatted as defined in Section 4.2.1.  This
      parameter MUST be included.

   *  'ph_req', with value the byte serialization of the transport-
      independent information of the phantom observation request (see
      Section 4.1), encoded as a CBOR byte string.  The value of the
      CBOR byte string is formatted as defined in Section 4.2.2.

      This parameter MAY be omitted, in case the phantom request is, in
      terms of transport-independent information, identical to the
      registration request from the client.  Otherwise, this parameter
      MUST be included.

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      Note that the registration request from the client may indeed
      differ from the phantom observation request in terms of transport-
      independent information, but still be acceptable for the server to
      register the client as taking part in the group observation.

   *  'last_notif', with value the byte serialization of the transport-
      independent information of the latest multicast notification for
      the target resource, encoded as a CBOR byte string.  The value of
      the CBOR byte string is formatted as defined in Section 4.2.2.
      This parameter MAY be included.

   *  'next_not_before', with value the amount of seconds that will
      minimally elapse before the server sends the next multicast
      notification for the group observation of the target resource,
      encoded as a CBOR unsigned integer.  This parameter MAY be
      included.

      This information can help a new client to align itself with the
      server's timeline, especially in scenarios where multicast
      notifications are regularly sent.  Also, it can help synchronizing
      different clients when orchestrating a content distribution
      through multicast notifications.

   The CDDL notation [RFC8610] provided below describes the payload of
   the informative response.

   informative_response_payload = {
      0 => array, ; 'tp_info', i.e., transport-specific information
    ? 1 => bstr,  ; 'ph_req' (transport-independent information)
    ? 2 => bstr   ; 'last_notif' (transport-independent information)
    ? 3 => uint   ; 'next_not_before'
   }

            Figure 1: Format of the informative response payload

   Upon receiving a registration request to observe the target resource,
   the server does not create a corresponding individual observation for
   the requesting client.  Instead, the server considers that client as
   now taking part in the group observation of the target resource, of
   which it increments the observer counter by 1.  Then, the server
   replies to the client with the same informative response message
   defined above, which MUST be Confirmable.

   Note that this also applies when, with no ongoing traditional
   observations on the target resource, the server receives a
   registration request from a first client and decides to start a group
   observation on the target resource.

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4.2.1.  Transport-Specific Message Information

   [ This encoding might be replaced by CRIs [I-D.ietf-core-href] in a
   later version of this document. ]

   The CBOR array specified in the 'tp_info' parameter is formatted
   according to the following CDDL notation.

   tp_info = [
       srv_addr  ; Addressing information of the server
     ? req_info  ; Request data extension
   ]

   srv_addr = (
       tp_id : int,  ; Identifier of the used transport protocol
     + elements      ; Number, format and encoding
                     ; based on the value of 'tp_id'
   )

   req_info = (
     + elements  ; Number, format and encoding based on
                 ; the value of 'tp_id' in 'srv_addr'
   )

                   Figure 2: General format of 'tp_info'

   The 'srv_addr' element of 'tp_info' specifies the addressing
   information of the server, and includes at least one element 'tp_id'
   which is formatted as follows.

   *  'tp_id' : this element is a CBOR integer, which specifies the
      transport protocol used to transport the CoAP response from the
      server, i.e., a multicast notification in this document.

      This element takes value from the "Value" column of the "CoAP
      Transport Information" registry defined in Section 16.5 of this
      document.  This element MUST be present.  The value of this
      element determines:

      -  How many elements are required to follow in 'srv_addr', as well
         as what information they convey, their encoding and their
         semantics.

      -  How many elements are required in the 'req_info' element of the
         'tp_info' array, as well as what information they convey, their
         encoding and their semantics.

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      This document registers the integer value 1 ("UDP") to be used as
      value for the 'tp_id' element, when CoAP responses are transported
      over UDP.  In such a case, the full encoding of the 'tp_info' CBOR
      array is as defined in Section 4.2.1.1.

      Future specifications that consider CoAP multicast notifications
      transported over different transport protocols MUST:

      -  Register an entry with an integer value to be used for 'tp_id',
         in the "CoAP Transport Information" registry defined in
         Section 16.5 of this document.

      -  Accordingly, define the elements of the 'tp_info' CBOR array,
         i.e., the elements following 'tp_id' in 'srv_addr' as well as
         the elements in 'req_info', as to what information they convey,
         their encoding and their semantics.

   The 'req_info' element of 'tp_info' specifies transport-specific
   information related to a pertinent request message, i.e., the phantom
   observation request in this document.  The exact format of 'req_info'
   depends on the value of 'tp_id'.

   Given a specific value of 'tp_id', the complete set of elements
   composing 'srv_addr' and 'req_info' in the 'tp_info' CBOR array is
   indicated by the two columns "Srv Addr" and "Req Info" of the "CoAP
   Transport Information" registry defined in Section 16.5,
   respectively.

4.2.1.1.  UDP Transport-Specific Information

   When CoAP multicast notifications are transported over UDP as per
   [RFC7252] and [I-D.ietf-core-groupcomm-bis], the server specifies the
   integer value 1 ("UDP") as value of 'tp_id' in the 'srv_addr' element
   of the 'tp_info' CBOR array in the informative response.  Then, the
   rest of the 'tp_info' CBOR array is defined as follows.

   *  'srv_addr' includes two more elements following 'tp_id':

      -  'srv_host': this element is a CBOR byte string, with value the
         destination IP address of the phantom observation request.
         This parameter is tagged and identified by the CBOR tag 260
         "Network Address (IPv4 or IPv6 or MAC Address)".  That is, the
         value of the CBOR byte string is the IP address SRV_ADDR of the
         server hosting the target resource, from where the server will
         send multicast notifications for the target resource.  This
         element MUST be present.

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      -  'srv_port': this element is a CBOR unsigned integer, with value
         the destination port number of the phantom observation request.
         That is, the specified value is the port number SRV_PORT, from
         where the server will send multicast notifications for the
         target resource.  This element MUST be present.

   *  'req_info' includes the following elements:

      -  'token': this element is a CBOR byte string, with value the
         Token value of the phantom observation request generated by the
         server (see Section 4.1).  Note that the same Token value is
         used for the multicast notifications bound to that phantom
         observation request (see Section 4.3).  This element MUST be
         present.

      -  'cli_host': this element is a CBOR byte string, with value the
         source IP address of the phantom observation request.  This
         parameter is tagged and identified by the CBOR tag 260 "Network
         Address (IPv4 or IPv6 or MAC Address)".  That is, the value of
         the CBOR byte string is the IP multicast address GRP_ADDR,
         where the server will send multicast notifications for the
         target resource.  This element MUST be present.

      -  'cli_port': this element is a CBOR unsigned integer, with value
         the source port number of the phantom observation request.
         That is, the specified value is the port number GRP_PORT, where
         the server will send multicast notifications for the target
         resource.  This element is OPTIONAL.  If not included, the
         default port number 5683 is assumed.

   The CDDL notation provided below describes the full 'tp_info' CBOR
   array using the format above.

 tp_info = [
     tp_id    : 1,             ; UDP as transport protocol
     srv_host : #6.260(bstr),  ; Src. address of multicast notifications
     srv_port : uint,          ; Src. port of multicast notifications
     token    : bstr,          ; Token of the phantom request and
                               ; associated multicast notifications
     cli_host : #6.260(bstr),  ; Dst. address of multicast notifications
   ? cli_port : uint           ; Dst. port of multicast notifications
 ]

      Figure 3: Format of 'tp_info' with UDP as transport protocol

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4.2.2.  Transport-Independent Message Information

   For both the parameters 'ph_req' and 'last_notif' in the informative
   response, the value of the byte string is the concatenation of the
   following components, in the order specified below.

   When defining the value of each component, "CoAP message" refers to
   the phantom observation request for the 'ph_req' parameter, and to
   the corresponding latest multicast notification for the 'last_notif'
   parameter.

   *  A single byte, with value the content of the Code field in the
      CoAP message.

   *  The byte serialization of the complete sequence of CoAP options in
      the CoAP message.

   *  If the CoAP message includes a non-zero length payload, the one-
      byte Payload Marker (0xff) followed by the payload.

4.3.  Notifications

   Upon a change in the status of the target resource under group
   observation, the server sends a multicast notification, intended to
   all the clients taking part in the group observation of that
   resource.  In particular, each of such multicast notifications is
   formatted as follows.

   *  It MUST be Non-confirmable.

   *  It MUST include an Observe Option, as per [RFC7641].

   *  It MUST have the same Token value T of the phantom registration
      request that started the group observation.  This Token value is
      specified in the 'token' element of 'req_info' under the 'tp_info'
      parameter, in the informative response sent to all the observer
      clients.

      That is, every multicast notification for a target resource is not
      bound to the observation requests from the different clients, but
      rather to the phantom registration request associated with the
      whole set of clients taking part in the group observation of that
      resource.

   *  It MUST be sent from the same IP address SRV_ADDR and port number
      SRV_PORT where: i) the original Observe registration requests are
      sent to by the clients; and ii) the corresponding informative
      responses are sent from by the server (see Section 4.2).  These

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      are indicated to the observer clients as value of the 'srv_host'
      and 'srv_port' elements of 'srv_addr' under the 'tp_info'
      parameter, in the informative response (see Section 4.2.1.1).
      That is, redirection MUST NOT be used.

   *  It MUST be sent to the IP multicast address GRP_ADDR and port
      number GRP_PORT.  These are indicated to the observer clients as
      value of the 'cli_host' and 'cli_port' elements of 'req_info'
      under the 'tp_info' parameter, in the informative response (see
      Section 4.2.1.1).

   For each target resource with an active group observation, the server
   MUST store the latest multicast notification.

4.4.  Congestion Control

   In order to not cause congestion, the server should conservatively
   control the sending of multicast notifications.  In particular:

   *  The multicast notifications MUST be Non-confirmable.

   *  In constrained environments such as low-power, lossy networks
      (LLNs), the server should only support multicast notifications for
      resources that are small.  Following related guidelines from
      Section 3.6 of [I-D.ietf-core-groupcomm-bis], this can consist,
      for example, in having the payload of multicast notifications 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.

   *  The server SHOULD provide multicast notifications with the
      smallest possible IP multicast scope that fulfills the application
      needs.  For example, following related guidelines from Section 3.6
      of [I-D.ietf-core-groupcomm-bis], 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.  Ultimately, it is up to the server administrator to
      explicitly configure the most appropriate IP multicast scope.

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   *  Following related guidelines from Section 4.5.1 of [RFC7641], the
      server SHOULD NOT send more than one multicast notification every
      3 seconds, and SHOULD use an even less aggressive rate when
      possible (see also Section 3.1.2 of [RFC8085]).  The transmission
      rate of multicast notifications should also take into account the
      avoidance of a possible "broadcast storm" problem [MOBICOM99].
      This prevents a following, considerable increase of the channel
      load, whose origin would be likely attributed to a router rather
      than the server.

4.5.  Cancellation

   At any point in time, the server may want to cancel a group
   observation of a target resource.  For instance, the server may
   realize that no clients or not enough clients are interested in
   taking part in the group observation anymore.  A possible approach
   that the server can use to assess this is defined in Section 8.

   In order to cancel the group observation, the server sends a
   multicast response with response code 5.03 (Service Unavailable),
   signaling that the group observation has been terminated.  The
   response has the same Token value T of the phantom registration
   request, it has no payload, and it does not include an Observe
   Option.

   The server sends the response to the same multicast IP address
   GRP_ADDR and port number GRP_PORT used to send the multicast
   notifications related to the target resource.  Finally, the server
   releases the resources allocated for the group observation, and
   especially frees up the Token value T used at its CoAP endpoint.

5.  Client-Side

5.1.  Request

   A client sends an observation request to the server as described in
   [RFC7641], i.e., a GET request with an Observe Option set to 0
   (register).  The request MUST NOT encode link-local addresses.  If
   the server is not configured to accept registrations on that target
   resource specifically for a group observation, this would still
   result in a positive notification response to the client as described
   in [RFC7641], in case the server is able and willing to add the
   client to the list of observers.

   In a particular setup, the information typically specified in the
   'tp_info' parameter of the informative response (see Section 4.2) can
   be pre-configured on the server and the clients.  For example, the
   destination multicast address and port number where to send multicast

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   notifications for a group observation, as well as the associated
   Token value to use, can be set aside for particular tasks (e.g.,
   enforcing observations of a specific resource).  Alternative
   mechanisms can rely on using some bytes from the hash of the
   observation request as the last bytes of the multicast address or as
   part of the Token value.

   In such a particular setup, the client may also have an early
   knowledge of the phantom request, i.e., it will be possible for the
   server to safely omit the parameter 'ph_req' from the informative
   response to the observation request (see Section 4.2).  In this case,
   the client can include a No-Response Option [RFC7967] with value 16
   in its Observe registration request, which results in the server
   suppressing the informative response.  As a consequence, the
   observation request only informs the server that there is one
   additional client interested to take part in the group observation.

   While the considered client is able to simply set up its multicast
   address and start receiving multicast notifications for the group
   observation, sending an observation request as above allows the
   server to increment the observer counter.  This helps the server to
   assess the current number of clients interested in the group
   observation over time (e.g., by using the method in Section 8), which
   in turn can play a role in deciding to cancel the group observation.

5.2.  Informative Response

   Upon receiving the informative response defined in Section 4.2, the
   client proceeds as follows.

   1.  The client configures an observation of the target resource.  To
       this end, it relies on a CoAP endpoint used for messages having:

       *  As source address and port number, the server address SRV_ADDR
          and port number SRV_PORT intended for accessing the target
          resource.  These are specified as value of the 'srv_host' and
          'srv_port' elements of 'srv_addr' under the 'tp_info'
          parameter, in the informative response (see Section 4.2.1.1).

       *  As destination address and port number, the IP multicast
          address GRP_ADDR and port number GRP_PORT.  These are
          specified as value of the 'cli_host' and 'cli_port' elements
          of 'req_info' under the 'tp_info' parameter, in the
          informative response (see Section 4.2.1.1).  If the 'cli_port'
          element is omitted in 'req_info', the client MUST assume the
          default port number 5683 as GRP_PORT.

   2.  The client rebuilds the phantom registration request as follows.

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       *  The client uses the Token value T, specified in the 'token'
          element of 'req_info' under the 'tp_info' parameter of the
          informative response.

       *  If the 'ph_req' parameter is not present in the informative
          response, the client uses the transport-independent
          information from its original Observe registration request.

       *  If the 'ph_req' parameter is present in the informative
          response, the client uses the transport-independent
          information specified in the parameter.

   3.  If the informative response includes the parameter 'ph_req', and
       the transport-independent information specified therein differs
       from the one in the original Observe registration request, then
       the client checks whether a response to the rebuilt phantom
       request can, if available in a cache entry, be used to satisfy
       the original observation request.  If this is not the case, the
       client SHOULD explicitly withdraw from the group observation.

   4.  The client stores the phantom registration request, as associated
       with the observation of the target resource.  In particular, the
       client MUST use the Token value T of this phantom registration
       request as its own local Token value associated with that group
       observation, with respect to the server.  The particular way to
       achieve this is implementation specific.

   5.  If the informative response includes the parameter 'last_notif',
       the client rebuilds the latest multicast notification, by using:

       *  The transport-independent information, specified in the
          'last_notif' parameter of the informative response.

       *  The Token value T, specified in the 'token' element of
          'req_info' under the 'tp_info' parameter of the informative
          response.

   6.  If the informative response includes the parameter 'last_notif',
       the client processes the multicast notification rebuilt at step 5
       as defined in Section 3.2 of [RFC7641].  In particular, the value
       of the Observe Option is used as initial baseline for
       notification reordering in this group observation.

   7.  If a traditional observation to the target resource is ongoing,
       the client MAY silently cancel it without notifying the server.

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   If any of the expected fields in the informative response are not
   present or malformed, the client MAY try sending a new registration
   request to the server (see Section 5.1).  Otherwise, the client
   SHOULD explicitly withdraw from the group observation.

   Appendix A describes possible alternative ways for clients to
   retrieve the phantom registration request and other information
   related to a group observation.

5.3.  Notifications

   After having successfully processed the informative response as
   defined in Section 5.2, the client will receive, accept and process
   multicast notifications about the state of the target resource from
   the server, as responses to the phantom registration request and with
   Token value T.

   The client relies on the value of the Observe Option for notification
   reordering, as defined in Section 3.4 of [RFC7641].

5.4.  Cancellation

   At a certain point in time, a client may become not interested in
   receiving further multicast notifications about a target resource.
   When this happens, the client can simply "forget" about being part of
   the group observation for that target resource, as per Section 3.6 of
   [RFC7641].

   When, later on, the server sends the next multicast notification, the
   client will not recognize the Token value T in the message.  Since
   the multicast notification is Non-confirmable, it is OPTIONAL for the
   client to reject the multicast notification with a Reset message, as
   defined in Section 3.5 of [RFC7641].

   In case the server has canceled a group observation as defined in
   Section 4.5, the client simply forgets about the group observation
   and frees up the used Token value T for that endpoint, upon receiving
   the multicast error response defined in Section 4.5.

6.  Web Linking

   The possible use of multicast notifications in a group observation
   may be indicated by a target "grp_obs" attribute in a web link
   [RFC8288] to a resource, e.g., using a link-format document
   [RFC6690].

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   The "grp_obs" attribute is a hint, indicating that the server might
   send multicast notifications for observations of the resource
   targeted by the link.  Note that this is simply a hint, i.e., it does
   not include any information required to participate in a group
   observation, and to receive and process multicast notifications.

   A value MUST NOT be given for the "grp_obs" attribute; any present
   value MUST be ignored by parsers.  The "grp_obs" attribute MUST NOT
   appear more than once in a given link-value; occurrences after the
   first MUST be ignored by parsers.

   The example in Figure 4 shows a use of the "grp_obs" attribute: the
   client does resource discovery on a server and gets back a list of
   resources, one of which includes the "grp_obs" attribute indicating
   that the server might send multicast notifications for observations
   of that resource.  The link-format notation (see Section 5 of
   [RFC6690]) is used.

   REQ: GET /.well-known/core

   RES: 2.05 Content
       </sensors/temp>;grp_obs,
       </sensors/light>;if="sensor"

                           Figure 4: The Web Link

7.  Example

   The following example refers to two clients C_1 and C_2 that register
   to observe a resource /r at a Server S, which has address SRV_ADDR
   and listens to the port number SRV_PORT.  Before the following
   exchanges occur, no clients are observing the resource /r , which has
   value "1234".

   The server S sends multicast notifications to the IP multicast
   address GRP_ADDR and port number GRP_PORT, and starts the group
   observation upon receiving a registration request from a first client
   that wishes to start a traditional observation on the resource /r.

   The following notation is used for the payload of the informative
   responses:

   *  'bstr(X)' denotes a CBOR byte string with value the byte
      serialization of X, with '|' denoting byte concatenation.

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   *  'OPT' denotes a sequence of CoAP options.  This refers to the
      phantom registration request encoded by the 'ph_req' parameter, or
      to the corresponding latest multicast notification encoded by the
      'last_notif' parameter.

   *  'PAYLOAD' denotes a CoAP payload.  This refers to the latest
      multicast notification encoded by the 'last_notif' parameter.

   C_1     ----------------- [ Unicast ] ------------------------> S  /r
    |  GET                                                         |
    |  Token: 0x4a                                                 |
    |  Observe: 0 (Register)                                       |
    |  <Other options>                                             |
    |                                                              |
    |               (S allocates the available Token value 0x7b .) |
    |                                                              |
    |      (S sends to itself a phantom observation request PH_REQ |
    |       as coming from the IP multicast address GRP_ADDR .)    |
    |         ------------------------------------------------     |
    |       /                                                      |
    |       \----------------------------------------------------> |  /r
    |                                       GET                    |
    |                                       Token: 0x7b            |
    |                                       Observe: 0 (Register)  |
    |                                       <Other options>        |
    |                                                              |
    |                      (S creates a group observation of /r .) |
    |                                                              |
    |                          (S increments the observer counter  |
    |                           for the group observation of /r .) |
    |                                                              |
   C_1 <-------------------- [ Unicast ] ---------------------     S
    |  5.03                                                        |
    |  Token: 0x4a                                                 |
    |  Content-Format: application/informative-response+cbor       |
    |  Max-Age: 0                                                  |
    |  <Other options>                                             |
    |  Payload: {                                                  |
    |    tp_info    : [1, bstr(SRV_ADDR), SRV_PORT,                |
    |                  0x7b, bstr(GRP_ADDR), GRP_PORT],            |
    |    last_notif : bstr(0x45 | OPT | 0xff | PAYLOAD)            |
    |  }                                                           |
    |                                                              |
   C_2     ----------------- [ Unicast ] ------------------------> S  /r
    |  GET                                                         |
    |  Token: 0x01                                                 |
    |  Observe: 0 (Register)                                       |
    |  <Other options>                                             |

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    |                                                              |
    |                          (S increments the observer counter  |
    |                           for the group observation of /r .) |
    |                                                              |
   C_2 <-------------------- [ Unicast ] ---------------------     S
    |  5.03                                                        |
    |  Token: 0x01                                                 |
    |  Content-Format: application/informative-response+cbor       |
    |  Max-Age: 0                                                  |
    |  <Other options>                                             |
    |  Payload: {                                                  |
    |    tp_info    : [1, bstr(SRV_ADDR), SRV_PORT,                |
    |                  0x7b, bstr(GRP_ADDR), GRP_PORT],            |
    |    last_notif : bstr(0x45 | OPT | 0xff | PAYLOAD)            |
    |  }                                                           |
    |                                                              |
    |          (The value of the resource /r changes to "5678".)   |
    |                                                              |
   C_1                                                             |
    +  <------------------- [ Multicast ] --------------------     S
   C_2        (Destination address/port: GRP_ADDR/GRP_PORT)        |
    |  2.05                                                        |
    |  Token: 0x7b                                                 |
    |  Observe: 11                                                 |
    |  Content-Format: application/cbor                            |
    |  <Other options>                                             |
    |  Payload: : "5678"                                           |
    |                                                              |

                   Figure 5: Example of group observation

8.  Rough Counting of Clients in the Group Observation

   This section specifies a method that the server can use to keep an
   estimate of still active and interested clients, without creating
   undue traffic on the network.

8.1.  Multicast-Response-Feedback-Divider Option

   In order to enable the rough counting of still active and interested
   clients, a new CoAP option is introduced, which SHOULD be supported
   by clients that listen to multicast responses.

   The option is called Multicast-Response-Feedback-Divider.  As
   summarized in Figure 6, the option is not Critical, not Safe-to-
   Forward, and integer valued.  Since the option is not Safe-to-
   Forward, the column "N" indicates a dash for "not applicable".

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 +-----+---+---+---+---+---------------------+--------+------+---------+
 | No. | C | U | N | R | Name                | Format | Len. | Default |
 +-----+---+---+---+---+---------------------+--------+------+---------+
 | TBD |   | x | - |   | Multicast-Response- | uint   | 0-1  | (none)  |
 |     |   |   |   |   | Feedback-Divider    |        |      |         |
 +-----+---+---+---+---+---------------------+--------+------+---------+

       C = Critical, U = Unsafe, N = NoCacheKey, R = Repeatable

             Figure 6: Multicast-Response-Feedback-Divider

   The Multicast-Response-Feedback-Divider Option is of class E for
   OSCORE [RFC8613][I-D.ietf-core-oscore-groupcomm].

8.2.  Processing on the Client Side

   Upon receiving a response with a Multicast-Response-Feedback-Divider
   Option, a client SHOULD acknowledge its interest in continuing
   receiving multicast notifications for the target resource, as
   described below.

   The client picks an integer random number I, from 0 inclusive to the
   number Z = (2 ** Q) exclusive, where Q is the value specified in the
   option and "**" is the exponentiation operator.  If I is different
   than 0, the client takes no further action.  Otherwise, the client
   should wait a random fraction of the Leisure time (see Section 8.2 of
   [RFC7252]), and then registers a regular unicast observation on the
   same target resource.

   To this end, the client essentially follows the steps that got it
   originally subscribed to group notifications for the target resource.
   In particular, the client sends an observation request to the server,
   i.e., a GET request with an Observe Option set to 0 (register).  The
   request MUST be addressed to the same target resource, and MUST have
   the same destination IP address and port number used for the original
   registration request, regardless the source IP address and port
   number of the received multicast notification.

   Since the Observe registration is only done for its side effect of
   showing as an attempted observation at the server, the client MUST
   send the unicast request in a non confirmable way, and with the
   maximum No-Response setting [RFC7967].  In the request, the client
   MUST include a Multicast-Response-Feedback-Divider Option, whose
   value MUST be empty (Option Length = 0).  The client does not need to
   wait for responses, and can keep processing further notifications on
   the same Token.

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   The client MUST ignore the Multicast-Response-Feedback-Divider
   Option, if the multicast notification is retrieved from the
   'last_notif' parameter of an informative response (see Section 4.2).
   A client includes the Multicast-Response-Feedback-Divider Option only
   in a re-registration request triggered by the server as described
   above, and MUST NOT include it in any other request.

   As the Multicast-Response-Feedback-Divider Option is unsafe to
   forward, a proxy needs to answer it on its own, and is later counted
   as a single client.

   Appendix B.1 and Appendix B.2 provide a description in pseudo-code of
   the operations above performed by the client.

8.3.  Processing on the Server Side

   In order to avoid needless use of network resources, a server SHOULD
   keep a rough, updated count of the number of clients taking part in
   the group observation of a target resource.  To this end, the server
   updates the value COUNT of the associated observer counter (see
   Section 4), for instance by using the method described below.

8.3.1.  Request for Feedback

   When it wants to obtain a new estimated count, the server considers a
   number M of confirmations it would like to receive from the clients.
   It is up to applications to define policies about how the server
   determines and possibly adjusts the value of M.

   Then, the server computes the value Q = max(L, 0), where:

   *  L is computed as L = ceil(log2(N / M)).

   *  N is the current value of the observer counter, possibly rounded
      up to 1, i.e., N = max(COUNT, 1).

   Finally, the server sets Q as the value of the Multicast-Response-
   Feedback-Divider Option, which is sent within a successful multicast
   notification.

   If several multicast notifications are sent in a burst fashion, it is
   RECOMMENDED for the server to include the Multicast-Response-
   Feedback-Divider Option only in the first one of those notifications.

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8.3.2.  Collection of Feedback

   The server collects unicast observation requests from the clients,
   for an amount of time of MAX_CONFIRMATION_WAIT seconds.  During this
   time, the server regularly increments the observer counter when
   adding a new client to the group observation (see Section 4.2).

   It is up to applications to define the value of
   MAX_CONFIRMATION_WAIT, which has to take into account the
   transmission time of the multicast notification and of unicast
   observation requests, as well as the leisure time of the clients,
   which may be hard to know or estimate for the server.

   If this information is not known to the server, it is recommended to
   define MAX_CONFIRMATION_WAIT as follows.

   MAX_CONFIRMATION_WAIT = MAX_RTT + MAX_CLIENT_REQUEST_DELAY

   where MAX_RTT is as defined in Section 4.8.2 of [RFC7252] and has
   default value 202 seconds, while MAX_CLIENT_REQUEST_DELAY is
   equivalent to MAX_SERVER_RESPONSE_DELAY defined in Section 3.1.5 of
   [I-D.ietf-core-groupcomm-bis] and has default value 250 seconds.  In
   the absence of more specific information, the server can thus
   consider a conservative MAX_CONFIRMATION_WAIT of 452 seconds.

   If more information is available in deployments, a much shorter
   MAX_CONFIRMATION_WAIT can be set.  This can be based on a realistic
   round trip time (replacing MAX_RTT) and on the largest leisure time
   configured on the clients (replacing MAX_CLIENT_REQUEST_DELAY), e.g.,
   DEFAULT_LEISURE = 5 seconds, thus shortening MAX_CONFIRMATION_WAIT to
   a few seconds.

8.3.3.  Processing of Feedback

   Once MAX_CONFIRMATION_WAIT seconds have passed, the server counts the
   R confirmations arrived as unicast observation requests to the target
   resource, since the multicast notification with the Multicast-
   Response-Feedback-Divider Option has been sent.  In particular, the
   server considers a unicast observation request as a confirmation from
   a client only if it includes a Multicast-Response-Feedback-Divider
   Option with an empty value (Option Length = 0).

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   Then, the server computes a feedback indicator as E = R * (2 ** Q),
   where "**" is the exponentiation operator.  According to what defined
   by application policies, the server determines the next time when to
   ask clients for their confirmation, e.g., after a certain number of
   multicast notifications has been sent.  For example, the decision can
   be influenced by the reception of no confirmations from the clients,
   i.e., R = 0, or by the value of the ratios (E/N) and (N/E).

   Finally, the server computes a new estimated count of the observers.
   To this end, the server first consider COUNT' as the current value of
   the observer counter at this point in time.  Note that COUNT' may be
   greater than the value COUNT used at the beginning of this process,
   if the server has incremented the observer counter upon adding new
   clients to the group observation (see Section 4.2).

   In particular, the server computes the new estimated count value as
   COUNT' + ((E - N) / D), where D > 0 is an integer value used as
   dampener.  This step has to be performed atomically.  That is, until
   this step is completed, the server MUST hold the processing of an
   observation request for the same target resource from a new client.
   Finally, the server considers the result as the current observer
   counter, and assesses it for possibly canceling the group observation
   (see Section 4.5).

   This estimate is skewed by packet loss, but it gives the server a
   sufficiently good estimation for further counts and for deciding when
   to cancel the group observation.  It is up to applications to define
   policies about how the server takes the newly updated estimate into
   account and determines whether to cancel the group observation.

   As an example, if the server currently estimates that N = COUNT = 32
   observers are active and considers a constant M = 8, it sends out a
   notification with Multicast-Response-Feedback-Divider: 2.  Then, out
   of 18 actually active clients, 5 send a re-registration request based
   on their random draw, of which one request gets lost, thus leaving 4
   re-registration requests received by the server.  Also, no new
   clients have been added to the group observation during this time,
   i.e., COUNT' is equal to COUNT.  As a consequence, assuming that a
   dampener value D = 1 is used, the server computes the new estimated
   count value as 32 + (16 - 32) = 16, and keeps the group observation
   active.

   To produce a most accurate updated counter, a server can include a
   Multicast-Response-Feedback-Divider Option with value Q = 0 in its
   multicast notifications, as if M is equal to N.  This will trigger
   all the active clients to state their interest in continuing
   receiving notifications for the target resource.  Thus, the amount R
   of arrived confirmations is affected only by possible packet loss.

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   Appendix B.3 provides a description in pseudo-code of the operations
   above performed by the server, including example behaviors for
   scheduling the next count update and deciding whether to cancel the
   group observation.

9.  Protection of Multicast Notifications with Group OSCORE

   A server can protect multicast notifications by using Group OSCORE
   [I-D.ietf-core-oscore-groupcomm], thus ensuring they are protected
   end-to-end with the observer clients.  This requires that both the
   server and the clients interested in receiving multicast
   notifications from that server are members of the same OSCORE group.

   In some settings, the OSCORE group to refer to can be pre-configured
   on the clients and the server.  In such a case, a server which is
   aware of such pre-configuration can simply assume a client to be
   already member of the correct OSCORE group.

   In any other case, the server MAY communicate to clients what OSCORE
   group they are required to join, by providing additional guidance in
   the informative response as described in Section 9.1.  Note that
   clients can already be members of the right OSCORE group, in case
   they have previously joined it to securely communicate with the same
   server and/or with other servers to access their resources.

   Both the clients and the server MAY join the OSCORE group by using
   the approach described in [I-D.ietf-ace-key-groupcomm-oscore] and
   based on the ACE framework for Authentication and Authorization in
   constrained environments [RFC9200].  Further details on how to
   discover the OSCORE group and join it are out of the scope of this
   document.

   If multicast notifications are protected using Group OSCORE, the
   original registration requests and related unicast (notification)
   responses MUST also be secured, including and especially the
   informative responses from the server.

   To this end, alternative security protocols than Group OSCORE, such
   as OSCORE [RFC8613] and/or DTLS [RFC9147], can be used to protect
   other exchanges via unicast between the server and each client,
   including the original client registration (see Section 5).

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9.1.  Signaling the OSCORE Group in the Informative Response

   This section describes a mechanism for the server to communicate to
   the client what OSCORE group to join in order to decrypt and verify
   the multicast notifications protected with Group OSCORE.  The client
   MAY use the information provided by the server to start the ACE
   joining procedure described in [I-D.ietf-ace-key-groupcomm-oscore].
   This mechanism is OPTIONAL to support for the client and server.

   Additionally to what defined in Section 4, the CBOR map in the
   informative response payload contains the following fields, whose
   CBOR labels are defined in Section 13.

   *  'join_uri', with value the URI for joining the OSCORE group at the
      respective Group Manager, encoded as a CBOR text string.  If the
      procedure described in [I-D.ietf-ace-key-groupcomm-oscore] is used
      for joining, this field specifically indicates the URI of the
      group-membership resource at the Group Manager.

   *  'sec_gp', with value the name of the OSCORE group, encoded as a
      CBOR text string.

   *  Optionally, 'as_uri', with value the URI of the Authorization
      Server associated with the Group Manager for the OSCORE group,
      encoded as a CBOR text string.

   *  Optionally, 'hkdf', with value the HKDF Algorithm used in the
      OSCORE group, encoded as a CBOR text string or integer.  The value
      is taken from the 'Value' column of the "COSE Algorithms" registry
      [COSE.Algorithms].

   *  Optionally, 'cred_fmt', with value the format of the
      authentication credentials used in the OSCORE group, encoded as a
      CBOR integer.  The value is taken from the 'Label' column of the
      "COSE Header Parameters" Registry [COSE.Header.Parameters].
      Consistently with Section 2.3 of [I-D.ietf-core-oscore-groupcomm],
      acceptable values denote a format that MUST explicitly provide the
      comprehensive set of information related to the public key
      algorithm, including, e.g., the used elliptic curve (when
      applicable).

      At the time of writing this specification, acceptable formats of
      authentication credentials are CBOR Web Tokens (CWTs) and CWT
      Claim Sets (CCSs) [RFC8392], X.509 certificates [RFC7925] and C509
      certificates [I-D.ietf-cose-cbor-encoded-cert].  Further formats
      may be available in the future, and would be acceptable to use as
      long as they comply with the criteria defined above.

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      [ As to CWTs and unprotected CWT claim sets, there is a pending
      registration requested by draft-ietf-lake-edhoc. ]

      [ As to C509 certificates, there is a pending registration
      requested by draft-ietf-cose-cbor-encoded-cert. ]

   *  Optionally, 'sign_enc_alg', with value the Signature Encryption
      Algorithm used in the OSCORE group to encrypt messages protected
      with the group mode, encoded as a CBOR text string or integer.
      The value is taken from the 'Value' column of the "COSE
      Algorithms" registry [COSE.Algorithms].

   *  Optionally, 'sign_alg', with value the Signature Algorithm used to
      sign messages in the OSCORE group, encoded as a CBOR text string
      or integer.  The value is taken from the 'Value' column of the
      "COSE Algorithms" registry [COSE.Algorithms].

   *  Optionally, 'sign_params', encoded as a CBOR array and including
      the following two elements:

      -  'sign_alg_capab': a CBOR array, with the same format and value
         of the COSE capabilities array for the algorithm indicated in
         'sign_alg', as specified for that algorithm in the
         'Capabilities' column of the "COSE Algorithms" Registry
         [COSE.Algorithms].

      -  'sign_key_type_capab': a CBOR array, with the same format and
         value of the COSE capabilities array for the COSE key type of
         the keys used with the algorithm indicated in 'sign_alg', as
         specified for that key type in the 'Capabilities' column of the
         "COSE Key Types" Registry [COSE.Key.Types].

   The values of 'sign_alg', 'sign_params' and 'cred_fmt' provide an
   early knowledge of the format of authentication credentials as well
   as of the type of public keys used in the OSCORE group.  Thus, the
   client does not need to ask the Group Manager for this information as
   a preliminary step before the (ACE) join process, or to perform a
   trial-and-error exchange with the Group Manager upon joining the
   group.  Hence, the client is able to provide the Group Manager with
   its own authentication credential in the correct expected format and
   including a public key of the correct expected type, at the very
   first step of the (ACE) join process.

   The values of 'hkdf', 'sign_enc_alg' and 'sign_alg' provide an early
   knowledge of the algorithms used in the OSCORE group.  Thus, the
   client is able to decide whether to actually proceed with the (ACE)
   join process, depending on its support for the indicated algorithms.

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   As mentioned above, since this mechanism is OPTIONAL, all the fields
   are OPTIONAL in the informative response.  However, the 'join_uri'
   and 'sec_gp' fields MUST be present if the mechanism is implemented
   and used.  If any of the fields are present without the 'join_uri'
   and 'sec_gp' fields present, the client MUST ignore these fields,
   since they would not be sufficient to start the (ACE) join procedure.
   When this happens, the client MAY try sending a new registration
   request to the server (see Section 5.1).  Otherwise, the client
   SHOULD explicitly withdraw from the group observation.

   Appendix C describes a possible alternative approach, where the
   server self-manages the OSCORE group, and provides the observer
   clients with the necessary keying material in the informative
   response.  The approach in Appendix C MUST NOT be used together with
   the mechanism defined in this section for indicating what OSCORE
   group to join.

9.2.  Server-Side Requirements

   When using Group OSCORE to protect multicast notifications, the
   server performs the operations described in Section 4, with the
   following differences.

9.2.1.  Registration

   The phantom registration request MUST be secured, by using Group
   OSCORE.  In particular, the group mode of Group OSCORE defined in
   Section 8 of [I-D.ietf-core-oscore-groupcomm] MUST be used.

   The server protects the phantom registration request as defined in
   Section 8.1 of [I-D.ietf-core-oscore-groupcomm], as if it was the
   actual sender, i.e., by using its Sender Context.  As a consequence,
   the server consumes the current value of its Sender Sequence Number
   SN in the OSCORE group, and hence updates it to SN* = (SN + 1).
   Consistently, the OSCORE Option in the phantom registration request
   includes:

   *  As 'kid', the Sender ID of the server in the OSCORE group.

   *  As 'piv', the previously consumed Sender Sequence Number value SN
      of the server in the OSCORE group, i.e., (SN* - 1).

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9.2.2.  Informative Response

   The value of the CBOR byte string in the 'ph_req' parameter encodes
   the phantom observation request as a message protected with Group
   OSCORE (see Section 9.2.1).  As a consequence: the specified Code is
   always 0.05 (FETCH); the sequence of CoAP options will be limited to
   the outer, non encrypted options; a payload is always present, as the
   authenticated ciphertext followed by the signature.  Note that, in
   terms of transport-independent information, the registration request
   from the client typically differs from the phantom request.  Thus,
   the server has to include the 'ph_req' parameter in the informative
   response.  An exception is the case discussed in Appendix D.

   Similarly, the value of the CBOR byte string in the 'last_notif'
   parameter encodes the latest multicast notification as a message
   protected with Group OSCORE (see Section 9.2.3).  This applies also
   to the initial multicast notification INIT_NOTIF built in step 6 of
   Section 4.1.

   Optionally, the informative response includes information on the
   OSCORE group to join, as additional parameters (see Section 9.1).

9.2.3.  Notifications

   The server MUST protect every multicast notification for the target
   resource with Group OSCORE.  In particular, the group mode of Group
   OSCORE defined in Section 8 of [I-D.ietf-core-oscore-groupcomm] MUST
   be used.

   The process described in Section 8.3 of
   [I-D.ietf-core-oscore-groupcomm] applies, with the following
   additions when building the two OSCORE 'external_aad' to encrypt and
   sign the multicast notification (see Section 4.3 of
   [I-D.ietf-core-oscore-groupcomm]).

   *  The 'request_kid' is the 'kid' value in the OSCORE Option of the
      phantom registration request, i.e., the Sender ID of the server.

   *  The 'request_piv' is the 'piv' value in the OSCORE Option of the
      phantom registration request, i.e., the consumed Sender Sequence
      Number SN of the server.

   *  The 'request_kid_context' is the 'kid context' value in the OSCORE
      Option of the phantom registration request, i.e., the Group
      Identifier value (Gid) of the OSCORE group used as ID Context.

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   Note that these same values are used to protect each and every
   multicast notification sent for the target resource under this group
   observation.

9.2.4.  Cancellation

   When canceling a group observation (see Section 4.5), the multicast
   response with error code 5.03 (Service Unavailable) is also protected
   with Group OSCORE, as per Section 8.3 of
   [I-D.ietf-core-oscore-groupcomm].  The server MUST use its own Sender
   Sequence Number as Partial IV to protect the error response, and
   include it as Partial IV in the OSCORE Option of the response.

9.3.  Client-Side Requirements

   When using Group OSCORE to protect multicast notifications, the
   client performs as described in Section 5, with the following
   differences.

9.3.1.  Informative Response

   Upon receiving the informative response from the server, the client
   performs as described in Section 5.2, with the following additions.

   When performing step 2, the client expects the 'ph_req' parameter to
   be included in the informative response, which is otherwise
   considered malformed.  An exception is the case discussed in
   Appendix D.

   Once completed step 2, the client decrypts and verifies the rebuilt
   phantom registration request as defined in Section 8.2 of
   [I-D.ietf-core-oscore-groupcomm], with the following differences.

   *  The client MUST NOT perform any replay check.  That is, the client
      skips step 3 in Section 8.2 of [RFC8613].

   *  If decryption and verification of the phantom registration request
      succeed:

      -  The client MUST NOT update the Replay Window in the Recipient
         Context associated with the server.  That is, the client skips
         the second bullet of step 6 in Section 8.2 of [RFC8613].

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      -  The client MUST NOT take any further process as normally
         expected according to [RFC7252].  That is, the client skips
         step 8 in Section 8.2 of [RFC8613].  In particular, the client
         MUST NOT deliver the phantom registration request to the
         application, and MUST NOT take any action in the Token space of
         its unicast endpoint, where the informative response has been
         received.

      -  The client stores the values of the 'kid', 'piv' and 'kid
         context' fields from the OSCORE Option of the phantom
         registration request.

   *  If decryption and verification of the phantom registration request
      fail, the client MAY try sending a new registration request to the
      server (see Section 5.1).  Otherwise, the client SHOULD explicitly
      withdraw from the group observation.

   After successful decryption and verification, the client performs
   step 3 in Section 5.2, considering the decrypted phantom registration
   request.

   If the informative response includes the parameter 'last_notif', the
   client also decrypts and verifies the latest multicast notification
   rebuilt at step 5 in Section 5.2, just like it would for the
   multicast notifications transmitted as CoAP messages on the wire (see
   Section 9.3.2).  If decryption and verification succeed, the client
   proceeds with step 6, considering the decrypted latest multicast
   notification.  Otherwise, the client proceeds to step 7.

9.3.2.  Notifications

   After having successfully processed the informative response as
   defined in Section 9.3.1, the client will decrypt and verify every
   multicast notification for the target resource as defined in
   Section 8.4 of [I-D.ietf-core-oscore-groupcomm], with the following
   difference.

   For both decryption and signature verification, the client MUST set
   the 'external_aad' defined in Section 4.3 of
   [I-D.ietf-core-oscore-groupcomm] as follows.  The particular way to
   achieve this is implementation specific.

   *  'request_kid' takes the value of the 'kid' field from the OSCORE
      Option of the phantom registration request (see Section 9.3.1).

   *  'request_piv' takes the value of the 'piv' field from the OSCORE
      Option of the phantom registration request (see Section 9.3.1).

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   *  'request_kid_context' takes the value of the 'kid context' field
      from the OSCORE Option of the phantom registration request (see
      Section 9.3.1).

   Note that these same values are used to decrypt and verify each and
   every multicast notification received for the target resource.

   The replay protection and checking of multicast notifications is
   performed as specified in Section 4.1.3.5.2 of [RFC8613].

10.  Example with Group OSCORE

   The following example refers to two clients C_1 and C_2 that register
   to observe a resource /r at a Server S, which has address SRV_ADDR
   and listens to the port number SRV_PORT.  Before the following
   exchanges occur, no clients are observing the resource /r , which has
   value "1234".

   The server S sends multicast notifications to the IP multicast
   address GRP_ADDR and port number GRP_PORT, and starts the group
   observation upon receiving a registration request from a first client
   that wishes to start a traditional observation on the resource /r.

   Pairwise communication over unicast is protected with OSCORE, while S
   protects multicast notifications with Group OSCORE.  Specifically:

   *  C_1 and S have a pairwise OSCORE Security Context.  In particular,
      C_1 has 'kid' = 0x01 as Sender ID, and SN_1 = 101 as Sender
      Sequence Number.  Also, S has 'kid' = 0x03 as Sender ID, and SN_3
      = 301 as Sender Sequence Number.

   *  C_2 and S have a pairwise OSCORE Security Context.  In particular,
      C_2 has 'kid' = 0x02 as Sender ID, and SN_2 = 201 as Sender
      Sequence Number.  Also, S has 'kid' = 0x04 as Sender ID, and SN_4
      = 401 as Sender Sequence Number.

   *  S is a member of the OSCORE group with name "myGroup", and 'kid
      context' = 0x57ab2e as Group ID.  In the OSCORE group, S has 'kid'
      = 0x05 as Sender ID, and SN_5 = 501 as Sender Sequence Number.

   The following notation is used for the payload of the informative
   responses:

   *  'bstr(X)' denotes a CBOR byte string with value the byte
      serialization of X, with '|' denoting byte concatenation.

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   *  'OPT' denotes a sequence of CoAP options.  This refers to the
      phantom registration request encoded by the 'ph_req' parameter, or
      to the corresponding latest multicast notification encoded by the
      'last_notif' parameter.

   *  'PAYLOAD' denotes an encrypted CoAP payload.  This refers to the
      phantom registration request encoded by the 'ph_req' parameter, or
      to the corresponding latest multicast notification encoded by the
      'last_notif' parameter.

   *  'SIGN' denotes the signature appended to an encrypted CoAP
      payload.  This refers to the phantom registration request encoded
      by the 'ph_req' parameter, or to the corresponding latest
      multicast notification encoded by the 'last_notif' parameter.

   C_1     ------------ [ Unicast w/ OSCORE ]  ------------------> S  /r
    |  0.05 (FETCH)                                                |
    |  Token: 0x4a                                                 |
    |  OSCORE: {kid: 0x01; piv: 101; ...}                          |
    |  <Other class U/I options>                                   |
    |  0xff                                                        |
    |  Encrypted_payload {                                         |
    |    0x01 (GET),                                               |
    |    Observe: 0 (Register),                                    |
    |    <Other class E options>                                   |
    |  }                                                           |
    |                                                              |
    |              (S allocates the available Token value 0x7b .)  |
    |                                                              |
    |      (S sends to itself a phantom observation request PH_REQ |
    |       as coming from the IP multicast address GRP_ADDR .)    |
    |     ------------------------------------------------------   |
    |    /                                                         |
    |    \-------------------------------------------------------> |  /r
    |                         0.05 (FETCH)                         |
    |                         Token: 0x7b                          |
    |                         OSCORE: {kid: 0x05 ; piv: 501;       |
    |                                  kid context: 0x57ab2e; ...} |
    |                         <Other class U/I options>            |
    |                         0xff                                 |
    |                         Encrypted_payload {                  |
    |                           0x01 (GET),                        |
    |                           Observe: 0 (Register),             |
    |                           <Other class E options>            |
    |                         }                                    |
    |                         <Signature>                          |
    |                                                              |
    |   (S steps SN_5 in the Group OSCORE Sec. Ctx : SN_5 <== 502) |

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    |                                                              |
    |                     (S creates a group observation of /r .)  |
    |                                                              |
    |                          (S increments the observer counter  |
    |                           for the group observation of /r .) |
    |                                                              |
   C_1 <--------------- [ Unicast w/ OSCORE ] ----------------     S
    |  2.05 (Content)                                              |
    |  Token: 0x4a                                                 |
    |  OSCORE: {piv: 301; ...}                                     |
    |  Max-Age: 0                                                  |
    |  <Other class U/I options>                                   |
    |  0xff                                                        |
    |  Encrypted_payload {                                         |
    |    5.03 (Service Unavailable),                               |
    |    Content-Format: application/informative-response+cbor,    |
    |    <Other class E options>,                                  |
    |    0xff,                                                     |
    |    CBOR_payload {                                            |
    |      tp_info    : [1, bstr(SRV_ADDR), SRV_PORT,              |
    |                    0x7b, bstr(GRP_ADDR), GRP_PORT],          |
    |      ph_req     : bstr(0x05 | OPT | 0xff | PAYLOAD | SIGN),  |
    |      last_notif : bstr(0x45 | OPT | 0xff | PAYLOAD | SIGN),  |
    |      join_uri   : "coap://myGM/ace-group/myGroup",           |
    |      sec_gp     : "myGroup"                                  |
    |    }                                                         |
    |  }                                                           |
    |                                                              |
   C_2     ------------ [ Unicast w/ OSCORE ]  ------------------> S  /r
    |  0.05 (FETCH)                                                |
    |  Token: 0x01                                                 |
    |  OSCORE: {kid: 0x02; piv: 201; ...}                          |
    |  <Other class U/I options>                                   |
    |  0xff                                                        |
    |  Encrypted_payload {                                         |
    |    0x01 (GET),                                               |
    |    Observe: 0 (Register),                                    |
    |    <Other class E options>                                   |
    |  }                                                           |
    |                                                              |
    |                          (S increments the observer counter  |
    |                           for the group observation of /r .) |
    |                                                              |
   C_2 <--------------- [ Unicast w/ OSCORE ] ----------------     S
    |  2.05 (Content)                                              |
    |  Token: 0x01                                                 |
    |  OSCORE: {piv: 401; ...}                                     |
    |  Max-Age: 0                                                  |

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    |  <Other class U/I options>                                   |
    |  0xff,                                                       |
    |  Encrypted_payload {                                         |
    |    5.03 (Service Unavailable),                               |
    |    Content-Format: application/informative-response+cbor,    |
    |    <Other class E options>,                                  |
    |    0xff,                                                     |
    |    CBOR_payload {                                            |
    |      tp_info    : [1, bstr(SRV_ADDR), SRV_PORT,              |
    |                    0x7b, bstr(GRP_ADDR), GRP_PORT],          |
    |      ph_req     : bstr(0x05 | OPT | 0xff | PAYLOAD | SIGN),  |
    |      last_notif : bstr(0x45 | OPT | 0xff | PAYLOAD | SIGN),  |
    |      join_uri   : "coap://myGM/ace-group/myGroup",           |
    |      sec_gp     : "myGroup"                                  |
    |    }                                                         |
    |  }                                                           |
    |                                                              |
    |            (The value of the resource /r changes to "5678".) |
    |                                                              |
   C_1                                                             |
    +  <----------- [ Multicast w/ Group OSCORE ] ------------     S
   C_2       (Destination address/port: GRP_ADDR/GRP_PORT)         |
    |  2.05 (Content)                                              |
    |  Token: 0x7b                                                 |
    |  OSCORE: {kid: 0x05; piv: 502; ...}                          |
    |  <Other class U/I options>                                   |
    |  0xff                                                        |
    |  Encrypted_payload {                                         |
    |    2.05 (Content),                                           |
    |    Observe: [empty],                                         |
    |    Content-Format: application/cbor,                         |
    |    <Other class E options>,                                  |
    |    0xff,                                                     |
    |    CBOR_Payload: "5678"                                      |
    |  }                                                           |
    |  <Signature>                                                 |
    |                                                              |

          Figure 7: Example of group observation with Group OSCORE

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   The two external_aad used to encrypt and sign the multicast
   notification above have 'request_kid' = 5, 'request_piv' = 501 and
   'request_kid_context' = 0x57ab2e.  These values are specified in the
   'kid', 'piv' and 'kid context' field of the OSCORE Option of the
   phantom observation request, which is encoded in the 'ph_req'
   parameter of the unicast informative response to the two clients.
   Thus, the two clients can build the two same external_aad for
   decrypting and verifying this multicast notification and the
   following ones.

11.  Intermediaries

   This section specifies how the approach presented in Section 4 and
   Section 5 works when a proxy is used between the clients and the
   server.  In addition to what specified in Section 5.7 of [RFC7252]
   and Section 5 of [RFC7641], the following applies.

   A client sends its original observation request to the proxy.  If the
   proxy is not already registered at the server for that target
   resource, the proxy forwards the observation request to the server,
   hence registering itself as an observer.  If the server has an
   ongoing group observation for the target resource or decides to start
   one, the server considers the proxy as taking part in the group
   observation, and replies to the proxy with an informative response.

   Upon receiving an informative response, the proxy performs as
   specified for the client in Section 5, with the peculiarity that
   "consuming" the last notification (if present) means populating its
   cache.

   In particular, by using the information retrieved from the
   informative response, the proxy configures an observation of the
   target resource at the origin server, acting as a client directly
   taking part in the group observation.

   As a consequence, the proxy will listen to the IP multicast address
   and port number indicated by the server in the informative response,
   as 'cli_host' and 'cli_port' element of 'req_info' under the
   'tp_info' parameter, respectively (see Section 4.2.1.1).
   Furthermore, multicast notifications will match the phantom request
   stored at the proxy, based on the Token value specified in the
   'token' element of 'req_info' under the 'tp_info' parameter in the
   informative response.

   Then, the proxy performs the following actions.

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   *  If the 'last_notif' field is not present, the proxy responds to
      the client with an Empty Acknowledgement (if indicated by the
      message type, and if it has not already done so).

   *  If the 'last_notif' field is present, the proxy rebuilds the
      latest multicast notification, as defined in Section 5.  Then, the
      proxy responds to the client, by forwarding back the latest
      multicast notification.

   When responding to an observation request from a client, the proxy
   also adds that client (and its Token) to the list of its registered
   observers for the target resource, next to the older observations.

   Upon receiving a multicast notification from the server, the proxy
   forwards it back separately to each observer client over unicast.
   Note that the notification forwarded back to a certain client has the
   same Token value of the original observation request sent by that
   client to the proxy.

   Note that the proxy configures the observation of the target resource
   at the server only once, when receiving the informative response
   associated with a (newly started) group observation for that target
   resource.

   After that, when receiving an observation request from a following
   new client to be added to the same group observation, the proxy does
   not take any further action with the server.  Instead, the proxy
   responds to the client either with the latest multicast notification
   if available from its cache, or with an Empty Acknowledgement
   otherwise, as defined above.

   An example is provided in Appendix E.

   In the general case with a chain of two or more proxies, every proxy
   in the chain takes the role of client with the (next hop towards the)
   origin server.  Note that the proxy adjacent to the origin server is
   the only one in the chain that receives informative responses and
   listens to an IP multicast address to receive notifications for the
   group observation.  Furthermore, every proxy in the chain takes the
   role of server with the (previous hop towards the) origin client.

12.  Intermediaries Together with End-to-End Security

   As defined in Section 9, Group OSCORE can be used to protect
   multicast notifications end-to-end between the origin server and the
   clients.  In such a case, additional actions are required when also
   the informative responses from the origin server are protected
   specifically end-to-end, by using OSCORE or Group OSCORE.

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   In fact, the proxy adjacent to the origin server is not able to
   access the encrypted payload of such informative responses.  Hence,
   the proxy cannot retrieve the 'ph_req' and 'tp_info' parameters
   necessary to correctly receive multicast notifications and forward
   them back to the clients.

   Then, differently from what defined in Section 11, each proxy
   receiving an informative response simply forwards it back to the
   client that has sent the corresponding observation request.  Note
   that the proxy does not even realize the message to be an actual
   informative response, since the outer Code field is set to 2.05
   (Content).

   Upon receiving the informative response, the client does not
   configure an observation of the target resource.  Instead, the client
   performs a new observe registration request, by transmitting the re-
   built phantom request as intended to reach the proxy adjacent to the
   origin server.  In particular, the client includes the new Listen-To-
   Multicast-Responses CoAP option defined in Section 12.1, to provide
   that proxy with the transport-specific information required for
   receiving multicast notifications for the group observation.

   Details on the additional message exchange and processing are defined
   in Section 12.2.

12.1.  Listen-To-Multicast-Responses Option

   In order to allow the proxy to listen to the multicast notifications
   sent by the server, a new CoAP option is introduced.  This option
   MUST be supported by clients interested to take part in group
   observations through intermediaries, and by proxies that collect
   multicast notifications and forward them back to the observer
   clients.

   The option is called Listen-To-Multicast-Responses and is intended
   only for requests.  As summarized in Figure 8, the option is critical
   and not Safe-to-Forward.  Since the option is not Safe-to-Forward,
   the column "N" indicates a dash for "not applicable".

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 +-----+---+---+---+---+-------------------+--------+--------+---------+
 | No. | C | U | N | R | Name              | Format | Len.   | Default |
 +-----+---+---+---+---+-------------------+--------+--------+---------+
 | TBD | x | x | - |   | Listen-To-        |  (*)   | 3-1024 | (none)  |
 |     |   |   |   |   | Multicast-        |        |        |         |
 |     |   |   |   |   | Responses         |        |        |         |
 +-----+---+---+---+---+-------------------+--------+--------+---------+

       C = Critical, U = Unsafe, N = NoCacheKey, R = Repeatable
       (*) See below.

                Figure 8: Listen-To-Multicast-Responses

   The Listen-To-Multicast-Responses Option includes the serialization
   of a CBOR array.  This specifies transport-specific message
   information required for listening to the multicast notifications of
   a group observation, and intended to the proxy adjacent to the origin
   server sending those notifications.  In particular, the serialized
   CBOR array has the same format specified in Section 4.2.1 for the
   'tp_info' parameter of the informative response (see Section 4.2).

   The Listen-To-Multicast-Responses Option is of class U for OSCORE
   [RFC8613][I-D.ietf-core-oscore-groupcomm].

12.2.  Message Processing

   Compared to Section 11, the following additions apply when
   informative responses are protected end-to-end between the origin
   server and the clients.

   After the origin server sends an informative response, each proxy
   simply forwards it back to the (previous hop towards the) origin
   client that has sent the observation request.

   Once received the informative response, the origin client proceeds in
   a different way than in Section 9.3.1:

   *  The client performs all the additional decryption and verification
      steps of Section 9.3.1 on the phantom request specified in the
      'ph_req' parameter and on the last notification specified in the
      'last_notif' parameter (if present).

   *  The client builds a ticket request (see Appendix B of
      [I-D.amsuess-core-cachable-oscore]), as intended to reach the
      proxy adjacent to the origin server.  The ticket request is
      formatted as follows.

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      -  The Token is chosen as the client sees fit.  In fact, there is
         no reason for this Token to be the same as the phantom
         request's.

      -  The outer Code field, the outer CoAP options and the encrypted
         payload with AEAD tag (protecting the inner Code, the inner
         CoAP options and the possible plain CoAP payload) concatenated
         with the signature are the same of the phantom request used for
         the group observation.  That is, they are as specified in the
         'ph_req' parameter of the received informative response.

      -  An outer Observe Option is included and set to 0 (Register).
         This will usually be set in the phantom request already.

      -  The outer options Proxy-Scheme, Uri-Host and Uri-Port are
         included, and set to the same values they had in the original
         registration request sent by the client.

      -  The new option Listen-To-Multicast-Responses is included as an
         outer option.  The value is set to the serialization of the
         CBOR array specified by the 'tp_info' parameter of the
         informative response.

         Note that, except for transport-specific information such as
         the Token and Message ID values, every different client
         participating to the same group observation (hence rebuilding
         the same phantom request) will build the same ticket request.

         Note also that, identically to the phantom request, the ticket
         request is still protected with Group OSCORE, i.e., it has the
         same OSCORE Option, encrypted payload and signature.

   Then, the client sends the ticket request to the next hop towards the
   origin server.  Every proxy in the chain forwards the ticket request
   to the next hop towards the origin server, until the last proxy in
   the chain is reached.  This last proxy, adjacent to the origin
   server, proceeds as follows.

   *  The proxy MUST NOT further forward the ticket request to the
      origin server.

   *  The proxy removes the Proxy-Scheme, Uri-Host and Uri-Port Options
      from the ticket request.

   *  The proxy removes the Listen-To-Multicast-Responses Option from
      the ticket request, and extracts the conveyed transport-specific
      information.

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   *  The proxy rebuilds the phantom request associated with the group
      observation, by using the ticket request as directly providing the
      required transport-independent information.  This includes the
      outer Code field, the outer CoAP options and the encrypted payload
      with AEAD tag concatenated with the signature.

   *  The proxy configures an observation of the target resource at the
      origin server, acting as a client directly taking part in the
      group observation.  To this end, the proxy uses the rebuilt
      phantom request and the transport-specific information retrieved
      from the Listen-To-Multicast-Responses Option.  The particular way
      to achieve this is implementation specific.

   After that, the proxy will listen to the IP multicast address and
   port number indicated in the Listen-To-Multicast-Responses Option, as
   'cli_host' and 'cli_port' element of the serialized CBOR array,
   respectively.  Furthermore, multicast notifications will match the
   phantom request stored at the proxy, based on the Token value
   specified in the 'token' element of the serialized CBOR array in the
   Listen-To-Multicast-Responses Option.

   An example is provided in Appendix F.

13.  Informative Response Parameters

   This document defines a number of fields used in the informative
   response defined in Section 4.2.

   The table below summarizes them and specifies the CBOR key to use
   instead of the full descriptive name.  Note that the media type
   application/informative-response+cbor MUST be used when these fields
   are transported.

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         +=================+==========+============+=============+
         | Name            | CBOR Key | CBOR Type  | Reference   |
         +=================+==========+============+=============+
         | tp_info         | 0        | array      | Section 4.2 |
         +-----------------+----------+------------+-------------+
         | ph_req          | 1        | bstr       | Section 4.2 |
         +-----------------+----------+------------+-------------+
         | last_notif      | 2        | bstr       | Section 4.2 |
         +-----------------+----------+------------+-------------+
         | next_not_before | 3        | uint       | Section 4.2 |
         +-----------------+----------+------------+-------------+
         | join_uri        | 4        | tstr       | Section 9.1 |
         +-----------------+----------+------------+-------------+
         | sec_gp          | 5        | tstr       | Section 9.1 |
         +-----------------+----------+------------+-------------+
         | as_uri          | 6        | tstr       | Section 9.1 |
         +-----------------+----------+------------+-------------+
         | hkdf            | 7        | int / tstr | Section 9.1 |
         +-----------------+----------+------------+-------------+
         | cred_fmt        | 8        | int        | Section 9.1 |
         +-----------------+----------+------------+-------------+
         | sign_enc_alg    | 9        | int / tstr | Section 9.1 |
         +-----------------+----------+------------+-------------+
         | sign_alg        | 10       | int / tstr | Section 9.1 |
         +-----------------+----------+------------+-------------+
         | sign_params     | 11       | array      | Section 9.1 |
         +-----------------+----------+------------+-------------+
         | gp_material     | 12       | map        | Appendix C  |
         +-----------------+----------+------------+-------------+
         | srv_cred        | 13       | bstr       | Appendix C  |
         +-----------------+----------+------------+-------------+
         | srv_identifier  | 14       | bstr       | Appendix C  |
         +-----------------+----------+------------+-------------+
         | exp             | 15       | uint       | Appendix C  |
         +-----------------+----------+------------+-------------+

                                  Table 1

14.  Transport Protocol Information

   This document defines some values of transport protocol identifiers
   to use within the 'tp_info' parameter of the informative response
   defined in Section 4.2.

   According to the encoding specified in Section 4.2.1, these values
   are used for the 'tp_id' element of 'srv_addr', under the 'tp_info'
   parameter.

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   The table below summarizes them, specifies the integer value to use
   instead of the full descriptive name, and provides the corresponding
   comprehensive set of information elements to include in the 'tp_info'
   parameter.

 +-----------+-------------+-------+----------+-----------+------------+
 | Transport | Description | Value | Srv Addr | Req Info  | Reference  |
 | Protocol  |             |       |          |           |            |
 +-----------+-------------+-------+----------+-----------+------------+
 | Reserved  | This value  | 0     |          |           | [RFC-XXXX] |
 |           | is reserved |       |          |           |            |
 |           |             |       |          |           |            |
 | UDP       | UDP is used | 1     | tp_id    |  token    | [RFC-XXXX] |
 |           | as per      |       | srv_host |  cli_host |            |
 |           | RFC7252     |       | srv_port | ?cli_port |            |
 +-----------+-------------+-------+----------+-----------+------------+

                Figure 9: Transport protocol information

15.  Security Considerations

   In addition to the security considerations from [RFC7252][RFC7641][I-
   D.ietf-core-groupcomm-bis][RFC8613][I-D.ietf-core-oscore-groupcomm],
   the following considerations hold for this document.

15.1.  Unsecured Multicast Notifications

   In case communications are not protected, the server might not be
   able to effectively authenticate a new client when it registers as an
   observer.  Section 7 of [RFC7641] specifies how, in such a case, the
   server must strictly limit the number of notifications sent between
   receiving acknowledgements from the client, as confirming to be still
   interested in the observation; i.e., any notifications sent in Non-
   confirmable messages must be interspersed with confirmable messages.

   This is not possible to achieve by the same means when using the
   communication model defined in this document, since multicast
   notifications are sent as Non-confirmable messages.  Nonetheless, the
   server might obtain such acknowledgements by other means.

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   For instance, the method defined in Section 8 to perform the rough
   counting of still interested clients triggers (some of) them to
   explicitly send a new observation request to acknowledge their
   interest.  Then, the server can decide to terminate the group
   observation altogether, in case not enough clients are estimated to
   be still active.  If the method defined in Section 8 is used, the
   server SHOULD NOT send more than a strict number of multicast
   notifications for a given group observation, without having first
   performed a new rough counting of active clients.

15.2.  Secured Multicast Notifications

   If multicast notifications are protected using Group OSCORE as per
   Section 9, the following applies.

   *  The original registration requests and related unicast
      (notification) responses MUST also be secured, including and
      especially the informative responses from the server.  This
      prevents on-path active adversaries from altering the conveyed IP
      multicast address and serialized phantom registration request.
      Thus, it ensures secure binding between every multicast
      notification for a same observed resource and the phantom
      registration request that started the group observation of that
      resource.

   *  A re-registration request, possibly including the Multicast-
      Response-Feedback-Divider Option to support the rough counting of
      clients (see Section 8), MUST also be secured.

   To this end, clients and servers SHOULD use OSCORE or Group OSCORE,
   so ensuring that the secure binding above is enforced end-to-end
   between the server and each observing client.

15.3.  Listen-To-Multicast-Responses Option

   The CoAP option Listen-To-Multicast-Responses defined in Section 12.1
   is of class U for OSCORE and Group OSCORE
   [RFC8613][I-D.ietf-core-oscore-groupcomm].

   This allows the proxy adjacent to the origin server to access the
   option value conveyed in a ticket request (see Section 12.2), and to
   retrieve from it the transport-specific information about a phantom
   request.  By doing so, the proxy becomes able to configure an
   observation of the target resource and to receive multicast
   notifications matching to the phantom request.

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   Any proxy in the chain, as well as further possible intermediaries or
   on-path active adversaries, are thus able to remove the option or
   alter its content, before the ticket request reaches the proxy
   adjacent to the origin server.

   Removing the option would result in the proxy adjacent to the origin
   server to not configure the group observation, if that has not
   happened yet.  In such a case, the proxy would not receive the
   corresponding multicast notifications to be forwarded back to the
   clients.

   Altering the option content would result in the proxy adjacent to the
   origin server to incorrectly configure a group observation (e.g., by
   indicating a wrong multicast IP address) hence preventing the correct
   reception of multicast notifications and their forwarding to the
   clients; or to configure bogus group observations that are currently
   not active on the origin server.

   In order to prevent what is described above, the ticket requests
   conveying the Listen-To-Multicast-Responses Option can be
   additionally protected hop-by-hop.  This can be achieved by the
   client protecting the ticket request sent to the proxy using OSCORE
   (see [I-D.tiloca-core-oscore-capable-proxies]) and/or DTLS [RFC9147].

16.  IANA Considerations

   This document has the following actions for IANA.

   Note to RFC Editor: Please replace all occurrences of "[RFC-XXXX]"
   with the RFC number of this specification and delete this paragraph.

16.1.  Media Type Registrations

   This document registers the media type 'application/informative-
   response+cbor' for error messages as informative response defined in
   Section 4.2, when carrying parameters encoded in CBOR.  This
   registration follows the procedures specified in [RFC6838].

   *  Type name: application

   *  Subtype name: informative-response+cbor

   *  Required parameters: N/A

   *  Optional parameters: N/A

   *  Encoding considerations: Must be encoded as a CBOR map containing
      the parameters defined in Section 4.2 of [RFC-XXXX].

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   *  Security considerations: See Section 15 of [RFC-XXXX].

   *  Interoperability considerations: N/A

   *  Published specification: [RFC-XXXX]

   *  Applications that use this media type: The type is used by CoAP
      servers and clients that support error messages as informative
      response defined in Section 4.2 of [RFC-XXXX].

   *  Fragment identifier considerations: N/A

   *  Additional information: N/A

   *  Person & email address to contact for further information:
      iesg@ietf.org (mailto:iesg@ietf.org)

   *  Intended usage: COMMON

   *  Restrictions on usage: None

   *  Author: Marco Tiloca marco.tiloca@ri.se
      (mailto:marco.tiloca@ri.se)

   *  Change controller: IESG

   *  Provisional registration?  No

16.2.  CoAP Content-Formats Registry

   IANA is asked to add the following entry to the "CoAP Content-
   Formats" registry within the "Constrained RESTful Environments (CoRE)
   Parameters" registry group.

   Media Type: application/informative-response+cbor

   Encoding: -

   ID: TBD

   Reference: [RFC-XXXX]

16.3.  CoAP Option Numbers Registry

   IANA is asked to enter the following option numbers to the "CoAP
   Option Numbers" registry within the "CoRE Parameters" registry group.

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      +--------+--------------------------------------+------------+
      | Number |                 Name                 | Reference  |
      +--------+--------------------------------------+------------+
      |  TBD   |  Multicast-Response-Feedback-Divider | [RFC-XXXX] |
      +--------+--------------------------------------+------------+
      |  TBD   |  Listen-To-Multicast-Responses       | [RFC-XXXX] |
      +--------+--------------------------------------+------------+

16.4.  Informative Response Parameters Registry

   This document establishes the "Informative Response Parameters"
   registry within the "Constrained RESTful Environments (CoRE)
   Parameters" registry group.  The registry has been created to use the
   "Expert Review" registration procedure [RFC8126].  Expert review
   guidelines are provided in Section 16.6.

   The columns of this registry are:

   *  Name: This is a descriptive name that enables easier reference to
      the item.  The name MUST be unique.  It is not used in the
      encoding.

   *  CBOR Key: This is the value used as CBOR key of the item.  These
      values MUST be unique.  The value can be a positive integer, a
      negative integer, or a string.

   *  CBOR Type: This contains the CBOR type of the item, or a pointer
      to the registry that defines its type, when that depends on
      another item.

   *  Reference: This contains a pointer to the public specification for
      the item.

   This registry has been initially populated by the values in
   Section 13.  The "Reference" column for all of these entries refers
   to sections of this document.

16.5.  CoAP Transport Information Registry

   This document establishes the "CoAP Transport Information" registry
   within the "CoRE Parameters" registry group.  The registry has been
   created to use the "Expert Review" registration procedure [RFC8126].
   Expert review guidelines are provided in Section 16.6.  It should be
   noted that, in addition to the expert review, some portions of the
   Registry require a specification, potentially a Standards Track RFC,
   to be supplied as well.

   The columns of this registry are:

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   *  Transport Protocol: This is a descriptive name that enables easier
      reference to the item.  The name MUST be unique.  It is not used
      in the encoding.

   *  Description: Text giving an overview of the transport protocol
      referred by this item.

   *  Value: CBOR abbreviation for the transport protocol referred by
      this item.  Different ranges of values use different registration
      policies [RFC8126].  Integer values from -256 to 255 are
      designated as "Standards Action With Expert Review".  Integer
      values from -65536 to -257 and from 256 to 65535 are designated as
      "Specification Required".  Integer values greater than 65535 are
      designated as "Expert Review".  Integer values less than -65536
      are marked as Private Use.

   *  Server Addr: List of elements providing addressing information of
      the server.

   *  Req Info: List of elements providing transport-specific
      information related to a pertinent CoAP request.  Optional
      elements are prepended by '?'.

   *  Reference: This contains a pointer to the public specification for
      the item.

   This registry has been initially populated by the values in
   Section 14.  The "Reference" column for all of these entries refers
   to sections of this document.

16.6.  Expert Review Instructions

   The IANA registries established in this document are defined as
   expert review.  This section gives some general guidelines for what
   the experts should be looking for, but they are being designated as
   experts for a reason so they should be given substantial latitude.

   Expert reviewers should take into consideration the following points:

   *  Point squatting should be discouraged.  Reviewers are encouraged
      to get sufficient information for registration requests to ensure
      that the usage is not going to duplicate one that is already
      registered and that the point is likely to be used in deployments.
      The zones tagged as private use are intended for testing purposes
      and closed environments, code points in other ranges should not be
      assigned for testing.

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   *  Specifications are required for the standards track range of point
      assignment.  Specifications should exist for specification
      required ranges, but early assignment before a specification is
      available is considered to be permissible.  Specifications are
      needed for the first-come, first-serve range if they are expected
      to be used outside of closed environments in an interoperable way.
      When specifications are not provided, the description provided
      needs to have sufficient information to identify what the point is
      being used for.

   *  Experts should take into account the expected usage of fields when
      approving point assignment.  The fact that there is a range for
      standards track documents does not mean that a standards track
      document cannot have points assigned outside of that range.  The
      length of the encoded value should be weighed against how many
      code points of that length are left, the size of device it will be
      used on, and the number of code points left that encode to that
      size.

17.  References

17.1.  Normative References

   [COSE.Algorithms]
              IANA, "COSE Algorithms",
              <https://www.iana.org/assignments/cose/
              cose.xhtml#algorithms>.

   [COSE.Header.Parameters]
              IANA, "COSE Header Parameters",
              <https://www.iana.org/assignments/cose/cose.xhtml#header-
              parameters>.

   [COSE.Key.Types]
              IANA, "COSE Key Types",
              <https://www.iana.org/assignments/cose/cose.xhtml#key-
              type>.

   [I-D.ietf-ace-key-groupcomm-oscore]
              Tiloca, M., Park, J., and F. Palombini, "Key Management
              for OSCORE Groups in ACE", Work in Progress, Internet-
              Draft, draft-ietf-ace-key-groupcomm-oscore-15, 5 September
              2022, <https://www.ietf.org/archive/id/draft-ietf-ace-key-
              groupcomm-oscore-15.txt>.

   [I-D.ietf-core-groupcomm-bis]
              Dijk, E., Wang, C., and M. Tiloca, "Group Communication
              for the Constrained Application Protocol (CoAP)", Work in

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              Progress, Internet-Draft, draft-ietf-core-groupcomm-bis-
              07, 11 July 2022, <https://www.ietf.org/archive/id/draft-
              ietf-core-groupcomm-bis-07.txt>.

   [I-D.ietf-core-oscore-groupcomm]
              Tiloca, M., Selander, G., Palombini, F., Mattsson, J. P.,
              and J. Park, "Group OSCORE - Secure Group Communication
              for CoAP", Work in Progress, Internet-Draft, draft-ietf-
              core-oscore-groupcomm-15, 5 September 2022,
              <https://www.ietf.org/archive/id/draft-ietf-core-oscore-
              groupcomm-15.txt>.

   [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>.

   [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>.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13,
              RFC 6838, DOI 10.17487/RFC6838, January 2013,
              <https://www.rfc-editor.org/info/rfc6838>.

   [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>.

   [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>.

   [RFC8085]  Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
              Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
              March 2017, <https://www.rfc-editor.org/info/rfc8085>.

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   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [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>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/info/rfc8288>.

   [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>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

   [RFC9203]  Palombini, F., Seitz, L., Selander, G., and M. Gunnarsson,
              "The Object Security for Constrained RESTful Environments
              (OSCORE) Profile of the Authentication and Authorization
              for Constrained Environments (ACE) Framework", RFC 9203,
              DOI 10.17487/RFC9203, August 2022,
              <https://www.rfc-editor.org/info/rfc9203>.

17.2.  Informative References

   [I-D.amsuess-core-cachable-oscore]
              Amsüss, C. and M. Tiloca, "Cacheable OSCORE", Work in
              Progress, Internet-Draft, draft-amsuess-core-cachable-
              oscore-05, 11 July 2022, <https://www.ietf.org/archive/id/
              draft-amsuess-core-cachable-oscore-05.txt>.

   [I-D.ietf-core-coap-pubsub]
              Koster, M., Keränen, A., and J. Jimenez, "Publish-
              Subscribe Broker for the Constrained Application Protocol
              (CoAP)", Work in Progress, Internet-Draft, draft-ietf-
              core-coap-pubsub-10, 4 May 2022,
              <https://www.ietf.org/archive/id/draft-ietf-core-coap-
              pubsub-10.txt>.

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   [I-D.ietf-core-coral]
              Amsüss, C. and T. Fossati, "The Constrained RESTful
              Application Language (CoRAL)", Work in Progress, Internet-
              Draft, draft-ietf-core-coral-05, 7 March 2022,
              <https://www.ietf.org/archive/id/draft-ietf-core-coral-
              05.txt>.

   [I-D.ietf-core-href]
              Bormann, C. and H. Birkholz, "Constrained Resource
              Identifiers", Work in Progress, Internet-Draft, draft-
              ietf-core-href-11, 7 September 2022,
              <https://www.ietf.org/archive/id/draft-ietf-core-href-
              11.txt>.

   [I-D.ietf-cose-cbor-encoded-cert]
              Mattsson, J. P., Selander, G., Raza, S., Höglund, J., and
              M. Furuhed, "CBOR Encoded X.509 Certificates (C509
              Certificates)", Work in Progress, Internet-Draft, draft-
              ietf-cose-cbor-encoded-cert-04, 10 July 2022,
              <https://www.ietf.org/archive/id/draft-ietf-cose-cbor-
              encoded-cert-04.txt>.

   [I-D.tiloca-core-oscore-capable-proxies]
              Tiloca, M. and R. Höglund, "OSCORE-capable Proxies", Work
              in Progress, Internet-Draft, draft-tiloca-core-oscore-
              capable-proxies-04, 23 September 2022,
              <https://www.ietf.org/archive/id/draft-tiloca-core-oscore-
              capable-proxies-04.txt>.

   [I-D.tiloca-core-oscore-discovery]
              Tiloca, M., Amsüss, C., and P. Van der Stok, "Discovery of
              OSCORE Groups with the CoRE Resource Directory", Work in
              Progress, Internet-Draft, draft-tiloca-core-oscore-
              discovery-12, 5 September 2022,
              <https://www.ietf.org/archive/id/draft-tiloca-core-oscore-
              discovery-12.txt>.

   [MOBICOM99]
              Ni, S., Tseng, Y., Chen, Y., and J. Sheu, "The Broadcast
              Storm Problem in a Mobile Ad Hoc Network", Proceedings of
              the 5th annual ACM/IEEE international conference on Mobile
              computing and networking , August 1999,
              <https://people.eecs.berkeley.edu/~culler/cs294-
              f03/papers/bcast-storm.pdf>.

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

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   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.

   [RFC7925]  Tschofenig, H., Ed. and T. Fossati, "Transport Layer
              Security (TLS) / Datagram Transport Layer Security (DTLS)
              Profiles for the Internet of Things", RFC 7925,
              DOI 10.17487/RFC7925, July 2016,
              <https://www.rfc-editor.org/info/rfc7925>.

   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
              May 2018, <https://www.rfc-editor.org/info/rfc8392>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

   [RFC9147]  Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
              <https://www.rfc-editor.org/info/rfc9147>.

   [RFC9176]  Amsüss, C., Ed., Shelby, Z., Koster, M., Bormann, C., and
              P. van der Stok, "Constrained RESTful Environments (CoRE)
              Resource Directory", RFC 9176, DOI 10.17487/RFC9176, April
              2022, <https://www.rfc-editor.org/info/rfc9176>.

   [RFC9200]  Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments Using the OAuth 2.0 Framework
              (ACE-OAuth)", RFC 9200, DOI 10.17487/RFC9200, August 2022,
              <https://www.rfc-editor.org/info/rfc9200>.

Appendix A.  Different Sources for Group Observation Data

   While the clients usually receive the phantom registration request
   and other information related to the group observation through an
   informative response (see Section 4.2), the server can make the same
   data available through different means, such as the following ones.

   In such a case, the server has to first start the group observation
   (see Section 4.1), before making the corresponding data available.

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A.1.  Topic Discovery in Publish-Subscribe Settings

   In a Publish-Subscribe scenario [I-D.ietf-core-coap-pubsub], a group
   observation can be discovered along with topic metadata.

   To this end, together with topic metadata, the server has to publish
   the same information associated with the group observation that would
   be conveyed in the informative response returned to observer clients
   (see Section 4.2).

   This information especially includes the phantom observation request
   associated with the group observation, as well as the addressing
   information of the server and the addressing information where
   multicast notifications are sent to.

   Figure 10 provides an example where a group observation is
   discovered.  The example assumes a CoRAL namespace
   [I-D.ietf-core-coral], that contains properties analogous to those in
   the content-format application/informative-response+cbor.

   Note that the information about the transport protocol used for the
   group observation is not expressed through a dedicated element
   equivalent to 'tp_id' of the informative response (see
   Section 4.2.1).  Rather, it is expressed through the scheme component
   of the two URIs specified as 'tp_info_srv' and 'tp_info_cli', where
   the former specifies the addressing information of the server (like
   'srv_host' and 'srv_port' in Section 4.2.1.1), while the latter
   specifies the addressing information where multicast notifications
   are sent to (like 'cli_host' and 'cli_port' in Section 4.2.1.1).

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   Request:

       GET </ps/topics?rt=oic.r.temperature>
       Accept: application/coral+cbor

   Response:

       2.05 Content
       Content-Format: application/coral+cbor

       rdf:type [ = <http://example.org/pubsub/topic-list>,
              topic [ = </ps/topics/1234>,
                  tp_info_srv <coap://[2001:db8::1]>,
                  tp_info_token "7b"^^xsd::hexBinary,
                  tp_info_cli <coap://[ff35:30:2001:db8::123]>,
                  ph_req "0160.."^^xsd::hexBinary,
                  last_notif "256105.."^^xsd::hexBinary,
              ]
       ]

        Figure 10: Group observation discovery in a Pub-Sub scenario

   With this information from the topic discovery step, the client can
   already set up its multicast address and start receiving multicast
   notifications for the group observation in question.  Clients that
   are not directly able to listen to multicast notifications can
   instead rely on a proxy to do so on their behalf (see Section 11 and
   Section 12).

   In heavily asymmetric networks like municipal notification services,
   discovery and notifications do not necessarily need to use the same
   network link.  For example, a departure monitor could use its (costly
   and usually-off) cellular uplink to discover the topics it needs to
   update its display to, and then listen on a LoRA-WAN interface for
   receiving the actual multicast notifications.

A.2.  Introspection at the Multicast Notification Sender

   For network debugging purposes, it can be useful to query a server
   that sends multicast responses as matching a phantom registration
   request.

   Such an interface is left for other documents to specify on demand.
   As an example, a possible interface can be as follows, and rely on
   the already known Token value of intercepted multicast notifications,
   associated with a phantom registration request.

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   Request:

       GET </.well-known/core/mc-sender?token=6464>

   Response:

       2.05 Content
       Content-Format: application/informative-response+cbor

       {
           'tp_info': [1, h"7b", h"20010db80100..0001", 5683,
                       h"ff35003020010db8..1234", 5683],
           'ph_req': h"0160..",
           'last_notif' : h"256105.."
       }

      Figure 11: Group observation discovery with server introspection

   For example, a network sniffer could offer sending such a request
   when unknown multicast notifications are seen in a network.
   Consequently, it can associate those notifications with a URI, or
   decrypt them, if member of the correct OSCORE group.

Appendix B.  Pseudo-Code for Rough Counting of Clients

   This appendix provides a description in pseudo-code of the two
   algorithms used for the rough counting of active observers, as
   defined in Section 8.

   In particular, Appendix B.1 describes the algorithm for the client
   side, while Appendix B.2 describes an optimized version for
   constrained clients.  Finally, Appendix B.3 describes the algorithm
   for the server side.

B.1.  Client Side

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   input:  int Q, // Value of the MRFD option
           int LEISURE_TIME, // DEFAULT_LEISURE from RFC 7252,
                             // unless overridden

   output: None

   int RAND_MIN = 0;
   int RAND_MAX = (2**Q) - 1;
   int I = randomInteger(RAND_MIN, RAND_MAX);

   if (I == 0) {
       float fraction = randomFloat(0, 1);

       Timer t = new Timer();
       t.setAndStart(fraction * LEISURE_TIME);
       while(!t.isExpired());

       Request req = new Request();
       // Initialize as NON and with maximum
       // No-Response settings, set options ...

       Option opt = new Option(OBSERVE);
       opt.set(0);
       req.setOption(opt);

       opt = new Option(MRFD);
       req.setOption(opt);

       req.send(SRV_ADDR, SRV_PORT);
   }

B.2.  Client Side - Optimized Version

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   input:  int Q, // Value of the MRFD option
           int LEISURE_TIME, // DEFAULT_LEISURE from RFC 7252,
                             // unless overridden

   output: None

   const unsigned int UINT_BIT = CHAR_BIT * sizeof(unsigned int);

   if (respond_to(Q) == true) {
       float fraction = randomFloat(0, 1);

       Timer t = new Timer();
       t.setAndStart(fraction * LEISURE_TIME);
       while(!t.isExpired());

       Request req = new Request();
       // Initialize as NON and with maximum
       // No-Response settings, set options ...

       Option opt = new Option(OBSERVE);
       opt.set(0);
       req.setOption(opt);

       opt = new Option(MRFD);
       req.setOption(opt);

       req.send(SRV_ADDR, SRV_PORT);
   }

   bool respond_to(int Q) {
       while (Q >= UINT_BIT) {
           if (rand() != 0) return false;
           Q -= UINT_BIT;
       }
       unsigned int mask = ~((~0u) << Q);
       unsigned int masked = mask & rand();
       return masked == 0;
   }

B.3.  Server Side

   input:  int COUNT, // Current observer counter
           int M, // Desired number of confirmations
           int MAX_CONFIRMATION_WAIT,
           Response notification, // Multicast notification to send

   output: int NEW_COUNT // Updated observer counter

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   int D = 4; // Dampener value
   int RETRY_NEXT_THRESHOLD = 4;
   float CANCEL_THRESHOLD = 0.2;

   int N = max(COUNT, 1);
   int Q = max(ceil(log2(N / M)), 0);
   Option opt = new Option(MRFD);
   opt.set(Q);

   notification.setOption(opt);
   <Finalize the notification message>
   notification.send(GRP_ADDR, GRP_PORT);

   Timer t = new Timer();
   t.setAndStart(MAX_CONFIRMATION_WAIT); // Time t1
   while(!t.isExpired());

   // Time t2

   int R = <number of requests to the target resource
            between t1 and t2, with the MRFD option>;

   int E = R * (2**Q);

   // Determine after how many multicast notifications
   // the next count update will be performed
   if ((R == 0) || (max(E/N, N/E) > RETRY_NEXT_THRESHOLD)) {
       <Next count update with the next multicast notification>
   }
   else {
       <Next count update after 10 multicast notifications>
   }

   // Compute the new count estimate
   int COUNT_PRIME = <current value of the observer counter>;
   int NEW_COUNT = COUNT_PRIME + ((E - N) / D);

   // Determine whether to cancel the group observation
   if (NEW_COUNT < CANCEL_THRESHOLD) {
       <Cancel the group observation>;
       return 0;
   }

   return NEW_COUNT;

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Appendix C.  OSCORE Group Self-Managed by the Server

   For simple settings, where no pre-arranged group with suitable
   memberships is available, the server can be responsible to setup and
   manage the OSCORE group used to protect the group observation.

   In such a case, a client would implicitly request to join the OSCORE
   group when sending the observe registration request to the server.
   When replying, the server includes the group keying material and
   related information in the informative response (see Section 4.2).

   Additionally to what is defined in Section 4, the CBOR map in the
   informative response payload contains the following fields, whose
   CBOR labels are defined in Section 13.

   *  'gp_material': this element is a CBOR map, which includes what the
      client needs in order to set up the Group OSCORE Security Context.

      This parameter has as value a subset of the
      Group_OSCORE_Input_Material object, which is defined in
      Section 6.4 of [I-D.ietf-ace-key-groupcomm-oscore] and extends the
      OSCORE_Input_Material object encoded in CBOR as defined in
      Section 3.2.1 of [RFC9203].

      In particular, the following elements of the
      Group_OSCORE_Input_Material object are included, using the same
      CBOR labels from the OSCORE Security Context Parameters Registry,
      as in Section 6.4 of [I-D.ietf-ace-key-groupcomm-oscore].

      -  'ms', 'contexId', 'cred_fmt', 'sign_enc_alg', 'sign_alg' and
         'sign_params'.  These elements MUST be included.

      -  'hkdf' and 'salt'.  These elements MAY be included.

      The 'group_senderId' element of the Group_OSCORE_Input_Material
      object MUST NOT be included.

   *  'srv_cred': this element is a CBOR byte string, with value the
      original binary representation of the server's authentication
      credential used in the OSCORE group.  In particular, the original
      binary representation complies with the format specified by the
      'cred_fmt' element of 'gp_material'.

   *  'srv_identifier': this element MUST be included and is encoded as
      a CBOR byte string, with value the Sender ID that the server has
      in the OSCORE group.

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   *  'exp': with value the expiration time of the keying material of
      the OSCORE group specified in the 'gp_material' parameter, encoded
      as a CBOR unsigned integer.  This field contains a numeric value
      representing the number of seconds from 1970-01-01T00:00:00Z UTC
      until the specified UTC date/time, ignoring leap seconds,
      analogous to what specified for NumericDate in Section 2 of
      [RFC7519].

   Note that the informative response does not require to include an
   explicit proof-of-possession (PoP) of the server's private key.
   Although the server is also acting as Group Manager and a PoP
   evidence of the Group Manager's private key is included in a full-
   fledged Join Response (see Section 6.4 of
   [I-D.ietf-ace-key-groupcomm-oscore]), such proof-of-possession will
   be achieved through every multicast notification, that the server
   sends as protected with the group mode of Group OSCORE and including
   a signature computed with its private key.

   A client receiving an informative response uses the information above
   to set up the Group OSCORE Security Context, as described in
   Section 2 of [I-D.ietf-core-oscore-groupcomm].  Note that the client
   does not obtain a Sender ID of its own, hence it installs a Security
   Context that a "silent server" would, i.e., without Sender Context.
   From then on, the client uses the received keying material to process
   the incoming multicast notifications from the server.

   Since the server is also acting as Group Manager, the authentication
   credential of the server provided in the 'srv_cred' element of the
   informative response is also used in the 'gm_cred' element of the
   external_aad for encrypting and signing the phantom request and
   multicast notifications (see Section 4.3 of
   [I-D.ietf-core-oscore-groupcomm])

   Furthermore, the server complies with the following points.

   *  The server MUST NOT self-manage OSCORE groups and provide the
      related keying material in the informative response for any other
      purpose than the protection of group observations, as defined in
      this document.

      The server MAY use the same self-managed OSCORE group to protect
      the phantom request and the multicast notifications of multiple
      group observations it hosts.

   *  The server MUST NOT provide in the informative response the keying
      material of other OSCORE groups it is or has been a member of.

   After the time indicated in the 'exp' field:

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   *  The server MUST stop using the keying material and MUST cancel the
      group observations for which that keying material is used (see
      Section 4.5 and Section 9.2.4).  If the server creates a new group
      observation as a replacement or follow-up using the same OSCORE
      group:

      -  The server MUST update the Master Secret.

      -  The server MUST update the ID Context used as Group Identifier
         (Gid), consistently with Section 3.2 of
         [I-D.ietf-core-oscore-groupcomm].

      -  The server MAY update the Master Salt.

   *  The client MUST stop using the keying material and MAY re-register
      the observation at the server.

   Before the keying material has expired, the server can send a
   multicast response with response code 5.03 (Service Unavailable) to
   the observing clients, protected with the current keying material.
   In particular, this is an informative response (see Section 4.2),
   which: i) additionally contains the abovementioned parameters for the
   next group keying material to be used; and ii) MAY omit the 'tp_info'
   and 'ph_req' parameters, since the associated information is
   immutable throughout the observation lifetime.  The response has the
   same Token value T of the phantom registration request and it does
   not include an Observe Option.  The server MUST use its own Sender
   Sequence Number as Partial IV to protect the error response, and
   include it as Partial IV in the OSCORE Option of the response.

   When some clients leave the OSCORE group and forget about the group
   observation, the server does not have to provide the remaining
   clients with any stale Sender IDs, as normally required for Group
   OSCORE (see Section 3.2 of [I-D.ietf-core-oscore-groupcomm]).  In
   fact, only two entities in the group have a Sender ID, i.e., the
   server and possibly the Deterministic Client, if the optimization
   defined in this appendix is combined with the use of phantom requests
   as deterministic requests (see Appendix D).  In particular, both of
   them never change their Sender ID during the group lifetime, while
   they both remain part of the group until the group ceases to exist.

   As an alternative to renewing the keying material before it expires,
   the server can simply cancel the group observation (see Section 4.5
   and Section 9.2.4), which results in the eventual re-registration of
   the clients that are still interested in the group observation.

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   Applications requiring backward security and forward security are
   REQUIRED to use an actual group joining process (usually through a
   dedicated Group Manager), e.g., the ACE joining procedure defined in
   [I-D.ietf-ace-key-groupcomm-oscore].  The server can facilitate the
   clients by providing them information about the OSCORE group to join,
   as described in Section 9.1.

Appendix D.  Phantom Request as Deterministic Request

   In some settings, the server can assume that all the approaching
   clients already have the exact phantom observation request to use,
   together with the transport-specific information required to listen
   to corresponding multicast notifications.

   For instance, the clients can be pre-configured with the phantom
   observation request, or they may be expected to retrieve it through
   dedicated means (see Appendix A).  In either case, the server would
   already have started the group observation, before the associated
   phantom observation request was disseminated.

   Then, the clients either set up their multicast address and group
   observation for listening to multicast notifications if directly able
   to, or rely on a proxy to do so on their behalf (see Section 11 and
   Section 12).

   If Group OSCORE is used to protect the group observation (see
   Section 9), and the OSCORE group supports the concept of
   Deterministic Client [I-D.amsuess-core-cachable-oscore], then the
   server and each client in the OSCORE group can also independently
   compute the protected phantom observation request.

   In such a case, the unprotected version of the phantom observation
   request can be made available to the clients as a smaller, plain CoAP
   message.  As above, this can be pre-configured on the clients, or
   they can obtain it through dedicated means (see Appendix A).  In
   either case, the clients and the server can independently protect the
   plain CoAP message by using the approach defined in Section 3 of
   [I-D.amsuess-core-cachable-oscore], thus all computing the same
   protected deterministic request.  The latter is used as the actual
   phantom observation request, against which the protected multicast
   notifications will match for the group observation in question.

   If relying on a proxy, each client sends the deterministic request to
   the proxy as a ticket request (see Section 12).  However, differently
   from what is defined in Section 12 when the ticket request is not a
   deterministic request, the clients do not include a Listen-to-
   Multicast-Responses Option.  This results in the proxy forwarding the
   ticket request (i.e., the phantom observation request) to the server

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   and obtaining the information required to listen to multicast
   notifications, unless the proxy has already set itself to do so.
   Also, the proxy will be able to serve multicast notifications from
   its cache as per [I-D.amsuess-core-cachable-oscore].  An example
   considering such a setup is shown in Appendix G.

   Note that the phantom registration request is, in terms of transport-
   independent information, identical to the same deterministic request
   possibly sent by each client (e.g., if a proxy is deployed).  Thus,
   if the server receives such a phantom registration request, the
   informative response may omit the 'ph_req' parameter (see
   Section 4.2).  If a client receives an informative response that
   includes the 'ph_req' parameter, and this specifies transport-
   independent information different from the one of the sent
   deterministic request, then the client considers the informative
   response malformed.

   If the optimization defined in Appendix C is also used, the
   'gp_material' element in the informative response from the server
   MUST also include the following elements from the
   Group_OSCORE_Input_Material object.

   *  'alg', 'ecdh_alg' and 'ecdh_params', as per Section 6.4 of
      [I-D.ietf-ace-key-groupcomm-oscore].

   *  'det_senderId' and 'det_hash_alg', defined in Section 4 of
      [I-D.amsuess-core-cachable-oscore].  These specify the Sender ID
      of the Deterministic Client in the OSCORE group, and the hash
      algorithm used to compute the deterministic request (see
      Section 3.4.1 of [I-D.amsuess-core-cachable-oscore]).

   If a deterministic request is used as phantom observation request for
   a group observation, the server does not assist clients that are
   interested to take part to the group observation but do not support
   deterministic requests.  This is consistent with the fact that the
   setup in question already relies on a lot of agreed pre-
   configuration.

   Therefore, the following holds when a group observation relies on a
   deterministic request as phantom observation request.

   *  Every client interested to take part to such a group observation:
      has to support deterministic requests; and has to know the phantom
      observation request, as a result of pre-configuration or following
      its retrieval through dedicated means (see Appendix A).

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   *  When running such an observation request, the server does not
      simultaneously run a parallel group observation for the same
      target resource, as associated with a different phantom
      observation request and intended to clients that do not support
      deterministic requests.

      Upon receiving an individual observation request for the same
      target resource, the server MUST reply with a generic 5.03
      (Service Unavailable) response (i.e., not the informative response
      defined in Section 4.2), if the request differs from the specific
      deterministic request associated with the group observation.

Appendix E.  Example with a Proxy

   This section provides an example when a proxy P is used between the
   clients and the server.  The same assumptions and notation used in
   Section 7 are used for this example.  In addition, the proxy has
   address PRX_ADDR and listens to the port number PRX_PORT.

   Unless explicitly indicated, all messages transmitted on the wire are
   sent over unicast.

 C1     C2     P        S
 |      |      |        |
 |      |      |        |  (The value of the resource /r is "1234")
 |      |      |        |
 +------------>|        |  Token: 0x4a
 | GET  |      |        |  Observe: 0 (Register)
 |      |      |        |  Proxy-Uri: coap://sensor.example/r
 |      |      |        |
 |      |      +------->|  Token: 0x5e
 |      |      | GET    |  Observe: 0 (Register)
 |      |      |        |  Uri-Host: sensor.example
 |      |      |        |  Uri-Path: r
 |      |      |        |
 |      |      |        |  (S allocates the available Token value 0x7b)
 |      |      |        |
 |      |      |        |  (S sends to itself a phantom observation
 |      |      |        |  request PH_REQ as coming from the
 |      |      |        |  IP multicast address GRP_ADDR)
 |      |      |        |
 |      |      |  ------+
 |      |      | /      |
 |      |      | \----->|  Token: 0x7b
 |      |      |   GET  |  Observe: 0 (Register)
 |      |      |        |  Uri-Host: sensor.example
 |      |      |        |  Uri-Path: r
 |      |      |        |

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 |      |      |        |  (S creates a group observation of /r)
 |      |      |        |
 |      |      |        |  (S increments the observer counter
 |      |      |        |  for the group observation of /r)
 |      |      |        |
 |      |      |        |
 |      |      |        |
 |      |      |<-------+  Token: 0x5e
 |      |      | 5.03   |  Content-Format: application/
 |      |      |        |     informative-response+cbor
 |      |      |        |  Max-Age: 0
 |      |      |        |  <Other options>
 |      |      |        |  Payload: {
 |      |      |        |    tp_info    : [1, bstr(SRV_ADDR), SRV_PORT,
 |      |      |        |                  0x7b, bstr(GRP_ADDR),
 |      |      |        |                  GRP_PORT],
 |      |      |        |    last_notif : bstr(0x45 | OPT |
 |      |      |        |                      0xff | PAYLOAD)
 |      |      |        |  }
 |      |      |        |
 |      |      |        |  (PAYLOAD in 'last_notif' : "1234")
 |      |      |        |
 |      |      |        |
 |      |      |        |  (The proxy starts listening to the
 |      |      |        |   GRP_ADDR address and the GRP_PORT port.)
 |      |      |        |
 |      |      |        |  (The proxy adds C1 to its list of observers.)
 |      |      |        |
 |<------------+        |  Token: 0x4a
 | 2.05 |      |        |  Observe: 54120
 |      |      |        |  Content-Format: application/cbor
 |      |      |        |  <Other options>
 |      |      |        |  Payload: "1234"
 |      |      |        |
 :      :      :        :
 :      :      :        :
 :      :      :        :
 |      |      |        |
 |      +----->|        |  Token: 0x01
 |      | GET  |        |  Observe: 0 (Register)
 |      |      |        |  Proxy-Uri: coap://sensor.example/r
 |      |      |        |
 |      |      |        |  (The proxy has a fresh cache representation)
 |      |      |        |
 |      |<-----+        |  Token: 0x01
 |      | 2.05 |        |  Observe: 54120
 |      |      |        |  Content-Format: application/cbor
 |      |      |        |  <Other options>

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 |      |      |        |  Payload: "1234"
 |      |      |        |
 :      :      :        :
 :      :      :        :  (The value of the resource
 :      :      :        :  /r changes to "5678".)
 :      :      :        :
 |      |      |        |
 |      |      |  (*)   |
 |      |      |<-------+  Token: 0x7b
 |      |      | 2.05   |  Observe: 11
 |      |      |        |  Content-Format: application/cbor
 |      |      |        |  <Other options>
 |      |      |        |  Payload: "5678"
 |      |      |        |
 |<------------+        |  Token: 0x4a
 | 2.05 |      |        |  Observe: 54123
 |      |      |        |  Content-Format: application/cbor
 |      |      |        |  <Other options>
 |      |      |        |  Payload: "5678"
 |      |      |        |
 |      |<-----+        |  Token: 0x01
 |      | 2.05 |        |  Observe: 54123
 |      |      |        |  Content-Format: application/cbor
 |      |      |        |  <Other options>
 |      |      |        |  Payload: "5678"
 |      |      |        |

 (*) Sent over IP multicast to GROUP_ADDR:GROUP_PORT

          Figure 12: Example of group observation with a proxy

   Note that the proxy has all the information to understand the
   observation request from C2, and can immediately start to serve the
   still fresh values.

   This behavior is mandated by Section 5 of [RFC7641], i.e., the proxy
   registers itself only once with the next hop and fans out the
   notifications it receives to all registered clients.

Appendix F.  Example with a Proxy and Group OSCORE

   This section provides an example when a proxy P is used between the
   clients and the server, and Group OSCORE is used to protect multicast
   notifications end-to-end between the server and the clients.

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   The same assumptions and notation used in Section 10 are used for
   this example.  In addition, the proxy has address PRX_ADDR and
   listens to the port number PRX_PORT.

   Unless explicitly indicated, all messages transmitted on the wire are
   sent over unicast and protected with OSCORE end-to-end between a
   client and the server.

 C1      C2      P         S
 |       |       |         |
 |       |       |         |  (The value of the resource /r is "1234")
 |       |       |         |
 +-------------->|         |  Token: 0x4a
 | FETCH |       |         |  Observe: 0 (Register)
 |       |       |         |  OSCORE: {kid: 0x01; piv: 101; ...}
 |       |       |         |  Uri-Host: sensor.example
 |       |       |         |  Proxy-Scheme: coap
 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  Encrypted_payload {
 |       |       |         |    0x01 (GET),
 |       |       |         |    Observe: 0 (Register),
 |       |       |         |    Uri-Path: r,
 |       |       |         |    <Other class E options>
 |       |       |         |  }
 |       |       |         |
 |       |       +-------->|  Token: 0x5e
 |       |       | FETCH   |  Observe: 0 (Register)
 |       |       |         |  OSCORE: {kid: 0x01; piv: 101; ...}
 |       |       |         |  Uri-Host: sensor.example
 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  Encrypted_payload {
 |       |       |         |    0x01 (GET),
 |       |       |         |    Observe: 0 (Register),
 |       |       |         |    Uri-Path: r,
 |       |       |         |    <Other class E options>
 |       |       |         |  }
 |       |       |         |
 |       |       |         |
 |       |       |         |  (S allocates the available
 |       |       |         |   Token value 0x7b .)
 |       |       |         |
 |       |       |         |  (S sends to itself a phantom observation
 |       |       |         |  request PH_REQ as coming from the
 |       |       |         |  IP multicast address GRP_ADDR)
 |       |       |         |
 |       |       |  -------+

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 |       |       | /       |
 |       |       | \------>|  Token: 0x7b
 |       |       |   FETCH |  Observe: 0 (Register)
 |       |       |         |  OSCORE: {kid: 0x05; piv: 501;
 |       |       |         |           kid context: 0x57ab2e; ...}
 |       |       |         |  Uri-Host: sensor.example
 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  Encrypted_payload {
 |       |       |         |    0x01 (GET),
 |       |       |         |    Observe: 0 (Register),
 |       |       |         |    Uri-Path: r,
 |       |       |         |    <Other class E options>
 |       |       |         |  }
 |       |       |         |  <Signature>
 |       |       |         |
 |       |       |         |  (S steps SN_5 in the Group OSCORE
 |       |       |         |   Security Context : SN_5 <== 502)
 |       |       |         |
 |       |       |         |  (S creates a group observation of /r)
 |       |       |         |
 |       |       |         |
 |       |       |         |  (S increments the observer counter
 |       |       |         |  for the group observation of /r)
 |       |       |         |
 |       |       |<--------+  Token: 0x5e
 |       |       | 2.05    |  OSCORE: {piv: 301; ...}
 |       |       |         |  Max-Age: 0
 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  Encrypted_payload {
 |       |       |         |    5.03 (Service Unavailable),
 |       |       |         |    Content-Format: application/
 |       |       |         |       informative-response+cbor,
 |       |       |         |    <Other class E options>,
 |       |       |         |    0xff,
 |       |       |         |    CBOR_payload {
 |       |       |         |       tp_info : [1, bstr(SRV_ADDR),
 |       |       |         |                  SRV_PORT, 0x7b,
 |       |       |         |                  bstr(GRP_ADDR), GRP_PORT],
 |       |       |         |       ph_req : bstr(0x05 | OPT | 0xff |
 |       |       |         |                     PAYLOAD | SIGN),
 |       |       |         |       last_notif : bstr(0x45 | OPT | 0xff |
 |       |       |         |                         PAYLOAD | SIGN),
 |       |       |         |       join_uri : "coap://myGM/
 |       |       |         |                   ace-group/myGroup",
 |       |       |         |       sec_gp : "myGroup"
 |       |       |         |    }

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 |       |       |         |  }
 |       |       |         |
 |<--------------+         |  Token: 0x4a
 | 2.05  |       |         |  OSCORE: {piv: 301; ...}
 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  (Same Encrypted_payload)
 |       |       |         |
 |       |       |         |
 +-------------->|         |  Token: 0x4b
 | FETCH |       |         |  Observe: 0 (Register)
 |       |       |         |  OSCORE: {kid: 0x05 ; piv: 501;
 |       |       |         |           kid context: 0x57ab2e; ...}
 |       |       |         |  Uri-Host: sensor.example
 |       |       |         |  Proxy-Scheme: coap
 |       |       |         |  Listen-To-
 |       |       |         |  Multicast-Responses: {[1, bstr(SRV_ADDR),
 |       |       |         |                         SRV_PORT, 0x7b,
 |       |       |         |                         bstr(GRP_ADDR),
 |       |       |         |                         GRP_PORT]
 |       |       |         |                       }
 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  Encrypted_payload {
 |       |       |         |    0x01 (GET),
 |       |       |         |    Observe: 0 (Register),
 |       |       |         |    Uri-Path: r
 |       |       |         |    <Other class E options>
 |       |       |         |  }
 |       |       |         |  <Signature>
 |       |       |         |
 |       |       |         |  (The proxy starts listening to the
 |       |       |         |   GRP_ADDR address and the GRP_PORT port.)
 |       |       |         |
 |       |       |         |  (The proxy adds C1 to
 |       |       |         |   its list of observers.)
 |       |       |         |
 |<--------------|         |
 |       |  ACK  |         |
 :       :       :         :
 :       :       :         :
 :       :       :         :
 |       |       |         |
 |       +------>|         |  Token: 0x01
 |       | FETCH |         |  Observe: 0 (Register)
 |       |       |         |  OSCORE: {kid: 0x02; piv: 201; ...}
 |       |       |         |  Uri-Host: sensor.example
 |       |       |         |  Proxy-Scheme: coap

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 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  Encrypted_payload {
 |       |       |         |    0x01 (GET),
 |       |       |         |    Observe: 0 (Register),
 |       |       |         |    Uri-Path: r,
 |       |       |         |    <Other class E options>
 |       |       |         |  }
 |       |       |         |
 |       |       +-------->|  Token: 0x5f
 |       |       | FETCH   |  Observe: 0 (Register)
 |       |       |         |  OSCORE: {kid: 0x02; piv: 201; ...}
 |       |       |         |  Uri-Host: sensor.example
 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  Encrypted_payload {
 |       |       |         |    0x01 (GET),
 |       |       |         |    Observe: 0 (Register),
 |       |       |         |    Uri-Path: r,
 |       |       |         |    <Other class E options>
 |       |       |         |  }
 |       |       |         |
 |       |       |         |  (S increments the observer counter
 |       |       |         |  for the group observation of /r)
 |       |       |         |
 |       |       |<--------+  Token: 0x5f
 |       |       | 2.05    |  OSCORE: {piv: 401; ...}
 |       |       |         |  Max-Age: 0
 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  Encrypted_payload {
 |       |       |         |    5.03 (Service Unavailable),
 |       |       |         |    Content-Format: application/
 |       |       |         |       informative-response+cbor,
 |       |       |         |    <Other class E options>,
 |       |       |         |    0xff,
 |       |       |         |    CBOR_payload {
 |       |       |         |       tp_info : [1, bstr(SRV_ADDR),
 |       |       |         |                  SRV_PORT, 0x7b,
 |       |       |         |                  bstr(GRP_ADDR), GRP_PORT],
 |       |       |         |       ph_req : bstr(0x05 | OPT | 0xff |
 |       |       |         |                     PAYLOAD | SIGN),
 |       |       |         |       last_notif : bstr(0x45 | OPT | 0xff |
 |       |       |         |                         PAYLOAD | SIGN),
 |       |       |         |       join_uri : "coap://myGM/
 |       |       |         |                   ace-group/myGroup",
 |       |       |         |       sec_gp : "myGroup"
 |       |       |         |    }

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 |       |       |         |  }
 |       |       |         |
 |       |<------+         |  Token: 0x01
 |       | 2.05  |         |  OSCORE: {piv: 401; ...}
 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  (Same Encrypted_payload)
 |       |       |         |
 |       +------>|         |  Token: 0x02
 |       | FETCH |         |  Observe: 0 (Register)
 |       |       |         |  OSCORE: {kid: 0x05; piv: 501;
 |       |       |         |           kid context: 57ab2e; ...}
 |       |       |         |  Uri-Host: sensor.example
 |       |       |         |  Proxy-Scheme: coap
 |       |       |         |  Listen-To-
 |       |       |         |  Multicast-Responses: {[1, bstr(SRV_ADDR),
 |       |       |         |                         SRV_PORT, 0x7b,
 |       |       |         |                         bstr(GRP_ADDR),
 |       |       |         |                         GRP_PORT]
 |       |       |         |                       }
 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  Encrypted_payload {
 |       |       |         |    0x01 (GET),
 |       |       |         |    Observe: 0 (Register),
 |       |       |         |    Uri-Path: r
 |       |       |         |    <Other class E options>
 |       |       |         |  }
 |       |       |         |  <Signature>
 |       |       |         |
 |       |       |         |  (The proxy adds C2 to
 |       |       |         |   its list of observers.)
 |       |<------|         |
 |       |  ACK  |         |
 |       |       |         |
 :       :       :         :
 :       :       :         :  (The value of the resource
 :       :       :         :  /r changes to "5678".)
 :       :       :         :
 |       |       |   (*)   |
 |       |       |<--------+  Token: 0x7b
 |       |       | 2.05    |  Observe: 11
 |       |       |         |  OSCORE: {kid: 0x05; piv: 502; ...}
 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  Encrypted_payload {
 |       |       |         |    2.05 (Content),
 |       |       |         |    Observe: [empty],

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 |       |       |         |    Content-Format: application/cbor,
 |       |       |         |    <Other class E options>,
 |       |       |         |    0xff,
 |       |       |         |    CBOR_Payload: "5678"
 |       |       |         |  }
 |       |       |         |  <Signature>
 |       |       |         |
 |<--------------+         |  Token: 0x4b
 | 2.05  |       |         |  Observe: 54123
 |       |       |         |  OSCORE: {kid: 0x05; piv: 502; ...}
 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  (Same Encrypted_payload and Signature)
 |       |       |         |
 |       |<------+         |  Token: 0x02
 |       | 2.05  |         |  Observe: 54123
 |       |       |         |  OSCORE: {kid: 0x05; piv: 502; ...}
 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  (Same Encrypted_payload and signature)
 |       |       |         |

 (*) Sent over IP multicast to GROUP_ADDR:GROUP_PORT and protected
     with Group OSCORE end-to-end between the server and the clients.

 Figure 13: Example of group observation with a proxy and Group OSCORE

   Unlike in the unprotected example in Appendix E, the proxy does _not_
   have all the information to perform request deduplication, and can
   only recognize the identical request once the client sends the ticket
   request.

Appendix G.  Example with a Proxy and Deterministic Requests

   This section provides an example when a proxy P is used between the
   clients and the server, and Group OSCORE is used to protect multicast
   notifications end-to-end between the server and the clients.

   In addition, the phantom request is especially a deterministic
   request (see Appendix D), which is protected with the pairwise mode
   of Group OSCORE as defined in [I-D.amsuess-core-cachable-oscore].

G.1.  Assumptions and Walkthrough

   The example provided in this appendix as reflected by the message
   exchange shown in Appendix G.2 assumes the following.

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   1.   The OSCORE group supports deterministic requests.  Thus, the
        server creates the phantom request as a deterministic request
        [I-D.amsuess-core-cachable-oscore], stores it locally as one of
        its issued phantom requests, and starts the group observation.

   2.   The server makes the phantom request available through other
        means, e.g., a pub-sub broker, together with the transport-
        specific information for listening to multicast notifications
        bound to the phantom request (see Appendix A).

   3.   Since the phantom request is a deterministic request, the server
        can more efficiently make it available in its smaller, plain
        version.  The clients can obtain it from the particular
        alternative source and protect it as per Section 3 of
        [I-D.amsuess-core-cachable-oscore], thus all computing the same
        deterministic request to be used as phantom observation request.

   4.   If the client does not rely on a proxy between itself and the
        server, it simply sets the group observation and starts
        listening to multicast notifications.  Building on point (2)
        above, the same would happen if the phantom request would not be
        specifically a deterministic request.

   5.   If the client relies on a proxy between itself and the server,
        it uses the phantom request as a ticket request (see
        Section 12).  However, unlike the case considered in Section 12
        when the ticket request is not deterministic, the client does
        not include a Listen-to-Multicast-Responses Option in the
        phantom request sent to the proxy.

   6.   Unlike for the case considered in Section 12, here the proxy
        does not know that the request is exactly a ticket request for
        subscribing to multicast notifications.  Thus, the proxy simply
        forwards the ticket request to the server as it normally does
        for any request.

   7.   The server receives the ticket request, which is a deviation
        from the case where the ticket request is not deterministic and
        stops at the proxy (see Section 12).  Then, the server can
        clearly understand what is happening.  In fact, as the result of
        an early check, the server recognizes the phantom request among
        the stored ones.  This happens through a byte-by-byte comparison
        of the incoming message minus the transport-related fields,
        i.e., by considering only: i) the outer REST code; ii) the outer
        options; and iii) the ciphertext from the message payload.

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   8.   Having recognized the incoming request as one of the self-
        generated deterministic phantom requests made available at
        external sources, the server does not perform any OSCORE
        processing on it.  This opens for replying to the proxy with an
        unprotected response, although not signaling any OSCORE-related
        error.

   9.   The server starts the group observation and replies with an
        error response, i.e., the usual 5.03 informative response,
        including: the transport information, the phantom request, and
        (optionally) the latest notification.

   10.  From the received 5.03 response, the proxy retrieves everything
        needed to set itself as an observer in the group observation,
        and it starts listening to multicast notifications.  If the 5.03
        response included a latest notification, the proxy caches it and
        forwards it back to the client, otherwise it replies with an
        empty ACK (if it has not done it already and the request from
        the client was Confirmable).

   11.  Like in the case with a non-deterministic phantom request
        considered in Section 12, the proxy fans out the multicast
        notifications to the origin clients as they come.  Also, as new
        clients following the first one contact the proxy, this does not
        have to contact the server again as in Section 12, since the
        deterministic phantom request would produce a cache hit as per
        [I-D.amsuess-core-cachable-oscore].  Thus, the proxy can serve
        such clients with the latest fresh multicast notification from
        its cache.

G.2.  Message Exchange

   The same assumptions and notation used in Section 10 are used for
   this example.  As a recap of some specific value:

   *  Two clients C_1 and C_2 register to observe a resource /r at a
      Server S, which has address SRV_ADDR and listens to the port
      number SRV_PORT.  Before the following exchanges occur, no clients
      are observing the resource /r , which has value "1234".

   *  The server S sends multicast notifications to the IP multicast
      address GRP_ADDR and port number GRP_PORT, and starts the group
      observation already after creating the deterministic phantom
      request to early disseminate.

   *  S is a member of the OSCORE group with 'kid context' = 0x57ab2e as
      Group ID.  In the OSCORE group, S has 'kid' = 0x05 as Sender ID,
      and SN_5 = 501 as Sender Sequence Number.

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   In addition:

   *  The proxy has address PRX_ADDR and listens to the port number
      PRX_PORT.

   *  The deterministic client in the OSCORE group has 'kid' = 0x09.

   Unless explicitly indicated, all messages transmitted on the wire are
   sent over unicast and protected with Group OSCORE end-to-end between
   a client and the server.

 C1      C2      P         S
 |       |       |         |
 |       |       |         |  (The value of the resource /r is "1234")
 |       |       |         |
 |       |       |         |  (S allocates the available
 |       |       |         |   Token value 0x7b .)
 |       |       |         |
 |       |       |         |  (S sends to itself a phantom observation
 |       |       |         |   request PH_REQ as coming from the
 |       |       |         |   IP multicast address GRP_ADDR.
 |       |       |         |   The OSCORE processing occurs as
 |       |       |         |   specified for a deterministic request)
 |       |       |         |
 |       |       |  -------|
 |       |       | /       |
 |       |       | \------>|  Token: 0x7b
 |       |       |   FETCH |  Uri-Host: sensor.example
 |       |       |         |  Observe: 0 (Register)
 |       |       |         |  OSCORE: {kid: 0x09 ; piv: 0 ;
 |       |       |         |           kid context: 0x57ab2e ; ... }
 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  Encrypted_payload {
 |       |       |         |    0x01 (GET),
 |       |       |         |    Observe: 0 (Register),
 |       |       |         |    Uri-Path: r,
 |       |       |         |    <Other class E options>
 |       |       |         |  }
 |       |       |         |
 |       |       |         |  (S creates a group observation of /r)
 |       |       |         |
 |       |       |         |  (The server does not respond to PH_REQ.
 |       |       |         |   The server stores PH_REQ locally and
 |       |       |         |   makes it available at an external source)
 |       |       |         |
 |       |       |         |
 |       |       |         |  (C1 obtains PH_REQ and sends it to P)

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 |       |       |         |
 |       |       |         |
 +-------------->|         |  Token: 0x4a
 | FETCH |       |         |  Uri-Host: sensor.example
 |       |       |         |  Observe: 0 (Register)
 |       |       |         |  OSCORE: {kid: 0x09 ; piv: 0 ;
 |       |       |         |           kid context: 0x57ab2e ; ... }
 |       |       |         |  Proxy-Scheme: coap
 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  Encrypted_payload {
 |       |       |         |    0x01 (GET),
 |       |       |         |    Observe: 0 (Register),
 |       |       |         |    Uri-Path: r,
 |       |       |         |    <Other class E options>
 |       |       |         |  }
 |       |       |         |
 |       |       +-------->|  Token: 0x5e
 |       |       | FETCH   |  Uri-Host: sensor.example
 |       |       |         |  Observe: 0 (Register)
 |       |       |         |  OSCORE: {kid: 0x09 ; piv: 0 ;
 |       |       |         |           kid context: 0x57ab2e ; ... }
 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  Encrypted_payload {
 |       |       |         |    0x01 (GET),
 |       |       |         |    Observe: 0 (Register),
 |       |       |         |    Uri-Path: r,
 |       |       |         |    <Other class E options>
 |       |       |         |  }
 |       |       |         |
 |       |       |         |  (S recognizes PH_REQ through byte-by-byte
 |       |       |         |   comparison against the stored one, and
 |       |       |         |   skips any OSCORE processing)
 |       |       |         |
 |       |       |         |  (S prepares the "last notification"
 |       |       |         |   response defined below)
 |       |       |         |
 |       |       |         |  0x45 (2.05 Content)
 |       |       |         |  Observe: 10
 |       |       |         |  OSCORE: {kid: 0x05 ; piv: 501 ; ...}
 |       |       |         |  Max-Age: 3000
 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  Encrypted_payload {
 |       |       |         |    0x45 (2.05 Content),
 |       |       |         |    Observe: [empty],
 |       |       |         |    CBOR_Payload: "1234"

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 |       |       |         |  }
 |       |       |         |  <Signature>
 |       |       |         |
 |       |       |         |  (S responds to the proxy with an
 |       |       |         |   unprotected informative response)
 |       |       |         |
 |       |       |<--------|  Token: 0x5e
 |       |       | 5.03    |  Content-Format: application/
 |       |       |         |    informative-response+cbor
 |       |       |         |  Max-Age: 0
 |       |       |         |  0xff,
 |       |       |         |  CBOR_payload {
 |       |       |         |     tp_info : [1, bstr(SRV_ADDR), SRV_PORT,
 |       |       |         |                0x7b, bstr(GRP_ADDR),
 |       |       |         |                GRP_PORT],
 |       |       |         |     last_notif : <the "last notification"
 |       |       |         |                   response prepared above>
 |       |       |         |    }
 |       |       |         |  }
 |       |       |         |
 |       |       |         |  (P extracts PH_REQ and starts listening
 |       |       |         |   to multicast notifications with Token
 |       |       |         |   0x7b at GRP_ADDR:GRP_PORT)
 |       |       |         |
 |       |       |         |  (P extracts the "last notification"
 |       |       |         |   response, caches it and forwards
 |       |       |         |   it back to C1)
 |       |       |         |
 |<--------------+         |  Token: 0x4a
 | 2.05  |       |         |  Observe: 54120
 |       |       |         |  OSCORE: {kid: 0x05 ; piv: 501 ; ...}
 |       |       |         |  Max-Age: 2995
 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  Encrypted_payload {
 |       |       |         |    0x45 (2.05 Content),
 |       |       |         |    Observe: [empty],
 |       |       |         |    CBOR_Payload: "1234"
 |       |       |         |  }
 |       |       |         |  <Signature>
 |       |       |         |
 :       :       :         |
 :       :       :         |
 :       :       :         |
 |       |       |         |
 |       |       |         |  (C2 obtains PH_REQ and sends it to P)
 |       |       |         |
 |       +------>|         |  Token: 0x01

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 |       | FETCH |         |  Uri-Host: sensor.example
 |       |       |         |  Observe: 0 (Register)
 |       |       |         |  OSCORE: {kid: 0x09 ; piv: 0 ;
 |       |       |         |           kid context: 0x57ab2e; ...}
 |       |       |         |  Proxy-Scheme: coap
 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  Encrypted_payload {
 |       |       |         |    0x01 (GET),
 |       |       |         |    Observe: 0 (Register),
 |       |       |         |    Uri-Path: r,
 |       |       |         |    <Other class E options>
 |       |       |         |  }
 |       |       |         |
 |       |       |         |  (P serves C2 from it cache)
 |       |       |         |
 |       |<------+         |  Token: 0x01
 |       | 2.05  |         |  Observe: 54120
 |       |       |         |  OSCORE: {kid: 0x05 ; piv: 501 ; ...}
 |       |       |         |  Max-Age: 1800
 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  Encrypted_payload {
 |       |       |         |    0x45 (2.05 Content),
 |       |       |         |    Observe: [empty],
 |       |       |         |    CBOR_Payload: "1234"
 |       |       |         |  }
 |       |       |         |  <Signature>
 |       |       |         |
 :       :       :         |
 :       :       :         |
 :       :       :         |
 |       |       |         |
 |       |       |         |  (The value of the resource
 |       |       |         |   /r changes to "5678".)
 |       |       |         |
 |       |       |   (*)   |
 |       |       |<--------|  Token: 0x7b
 |       |       | 2.05    |  Observe: 11
 |       |       |         |  OSCORE: {kid: 0x05; piv: 502 ; ...}
 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  Encrypted_payload {
 |       |       |         |    0x45 (2.05 Content),
 |       |       |         |    Observe: [empty],
 |       |       |         |    Content-Format: application/cbor,
 |       |       |         |    <Other class E options>,
 |       |       |         |    0xff,

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 |       |       |         |    CBOR_Payload: "5678"
 |       |       |         |  }
 |       |       |         |  <Signature>
 |       |       |         |
 |       |       |         |  (P updates its cache entry
 |       |       |         |   with this notification)
 |       |       |         |
 |<--------------+         |  Token: 0x4a
 | 2.05  |       |         |  Observe: 54123
 |       |       |         |  OSCORE: {kid: 0x05; piv: 502 ; ...}
 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  (Same Encrypted_payload and signature)
 |       |       |         |
 |       |<------+         |  Token: 0x01
 |       | 2.05  |         |  Observe: 54123
 |       |       |         |  OSCORE: {kid: 0x05; piv: 502 ; ...}
 |       |       |         |  <Other class U/I options>
 |       |       |         |  0xff
 |       |       |         |  (Same Encrypted_payload and signature)
 |       |       |         |

 (*) Sent over IP multicast to GROUP_ADDR:GROUP_PORT and protected
     with Group OSCORE end-to-end between the server and the clients.

Appendix H.  Document Updates

   RFC EDITOR: PLEASE REMOVE THIS SECTION.

H.1.  Version -04 to -05

   *  If the phantom request is an OSCORE deterministic request, there
      is no parallel group observation for clients that lack support.

   *  Clarification on pre-configured clients.

   *  Clarified early publication of phantom request.

   *  Fixes in IANA considerations.

   *  Fixed example with proxy and Group OSCORE.

   *  Editorial improvements.

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H.2.  Version -03 to -04

   *  Added a new section on prerequisites.

   *  Added a new section overviewing alternative variants.

   *  Consistent renaming of 'cli_addr' to 'cli_host' in 'tp_info'.

   *  Added pre-requirements for early retrieval of phantom request.

   *  Revised example with early retrieval of phantom request.

   *  Clarified use, rationale and example of phantom request as
      deterministic request.

   *  Editorial improvements.

H.3.  Version -02 to -03

   *  Distinction between authentication credential and public key.

   *  Fixed processing of informative response at the client, when Group
      OSCORE is used.

   *  Discussed scenarios with pre-configured address/port and Token
      value.

H.4.  Version -01 to -02

   *  Clarifications on client rough counting and IP multicast scope.

   *  The 'ph_req' parameter is optional in the informative response.

   *  New parameter 'next_not_before' for the informative response.

   *  Optimization when rekeying the self-managed OSCORE group.

   *  Security considerations on unsecured multicast notifications.

   *  Protection of the ticket request sent to a proxy.

   *  RFC8126 terminology in IANA considerations.

   *  Editorial improvements.

H.5.  Version -00 to -01

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   *  Simplified cancellation of the group observation, without using a
      phantom cancellation request.

   *  Aligned parameter semantics with core-oscore-groupcomm and ace-
      key-groupcomm-oscore.

   *  Clarifications about self-managed OSCORE group and use of
      deterministic requests for cacheable OSCORE.

   *  New example with a proxy, Group OSCORE and a deterministic phantom
      request.

   *  Fixes in the examples and editorial improvements.

Acknowledgments

   The authors sincerely thank Carsten Bormann, Klaus Hartke, Jaime
   Jiménez, John Preuß Mattsson, Jim Schaad, Ludwig Seitz and Göran
   Selander for their comments and feedback.

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

Authors' Addresses

   Marco Tiloca
   RISE AB
   Isafjordsgatan 22
   SE-16440 Stockholm Kista
   Sweden
   Email: marco.tiloca@ri.se

   Rikard Höglund
   RISE AB
   Isafjordsgatan 22
   SE-16440 Stockholm Kista
   Sweden
   Email: rikard.hoglund@ri.se

   Christian Amsüss
   Hollandstr. 12/4
   1020 Vienna
   Austria
   Email: christian@amsuess.com

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   Francesca Palombini
   Ericsson AB
   Torshamnsgatan 23
   SE-16440 Stockholm Kista
   Sweden
   Email: francesca.palombini@ericsson.com

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