



CoRE Working Group                                             M. Tiloca
Internet-Draft                                                R. Höglund
Updates: 7252, 7641 (if approved)                                RISE AB
Intended status: Standards Track                               C. Amsüss
Expires: 24 October 2026                                                
                                                            F. Palombini
                                                             Ericsson AB
                                                           22 April 2026


           Observe Notifications as CoAP Multicast Responses
           draft-ietf-core-observe-multicast-notifications-14

Abstract

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







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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Prerequisites . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  High-Level Overview of Available Variants . . . . . . . . . .   7
   4.  Server-Side Requirements  . . . . . . . . . . . . . . . . . .   8
     4.1.  Request . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Informative Response  . . . . . . . . . . . . . . . . . .   9
       4.2.1.  Transport-Specific Message Information  . . . . . . .  12
       4.2.2.  Transport-Independent Message Information . . . . . .  16
     4.3.  Notifications . . . . . . . . . . . . . . . . . . . . . .  16
     4.4.  Congestion Control  . . . . . . . . . . . . . . . . . . .  18
     4.5.  Cancellation  . . . . . . . . . . . . . . . . . . . . . .  18
   5.  Client-Side Requirements  . . . . . . . . . . . . . . . . . .  19
     5.1.  Request . . . . . . . . . . . . . . . . . . . . . . . . .  19
     5.2.  Informative Response  . . . . . . . . . . . . . . . . . .  20
     5.3.  Notifications . . . . . . . . . . . . . . . . . . . . . .  22
     5.4.  Cancellation  . . . . . . . . . . . . . . . . . . . . . .  22
   6.  Web Linking . . . . . . . . . . . . . . . . . . . . . . . . .  23
   7.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .  24
   8.  Rough Counting of Clients in the Group Observation  . . . . .  26



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     8.1.  Feedback-Divider Option . . . . . . . . . . . . . . . . .  26
     8.2.  Processing on the Client Side . . . . . . . . . . . . . .  27
     8.3.  Processing on the Server Side . . . . . . . . . . . . . .  28
       8.3.1.  Request for Feedback  . . . . . . . . . . . . . . . .  28
       8.3.2.  Collection of Feedback  . . . . . . . . . . . . . . .  29
       8.3.3.  Processing of Feedback  . . . . . . . . . . . . . . .  30
   9.  Protection of Multicast Notifications with Group OSCORE . . .  31
     9.1.  Signaling the OSCORE Group in the Informative Response  .  32
     9.2.  Server-Side Requirements  . . . . . . . . . . . . . . . .  34
       9.2.1.  Registration  . . . . . . . . . . . . . . . . . . . .  35
       9.2.2.  Informative Response  . . . . . . . . . . . . . . . .  35
       9.2.3.  Notifications . . . . . . . . . . . . . . . . . . . .  36
       9.2.4.  Cancellation  . . . . . . . . . . . . . . . . . . . .  36
     9.3.  Client-Side Requirements  . . . . . . . . . . . . . . . .  37
       9.3.1.  Informative Response  . . . . . . . . . . . . . . . .  37
       9.3.2.  Notifications . . . . . . . . . . . . . . . . . . . .  38
   10. Example with Group OSCORE . . . . . . . . . . . . . . . . . .  38
   11. Informative Response Parameters . . . . . . . . . . . . . . .  43
   12. Transport Protocol Information  . . . . . . . . . . . . . . .  44
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  45
     13.1.  Unprotected Communications . . . . . . . . . . . . . . .  45
     13.2.  Protected Communications . . . . . . . . . . . . . . . .  46
   14. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  46
     14.1.  Media Type Registrations . . . . . . . . . . . . . . . .  46
     14.2.  CoAP Content-Formats Registry  . . . . . . . . . . . . .  47
     14.3.  CoAP Option Numbers Registry . . . . . . . . . . . . . .  48
     14.4.  Target Attributes Registry . . . . . . . . . . . . . . .  48
     14.5.  Informative Response Parameters Registry . . . . . . . .  48
     14.6.  CoAP Transport Information Registry  . . . . . . . . . .  49
     14.7.  Expert Review Instructions . . . . . . . . . . . . . . .  50
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  51
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  51
     15.2.  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 . . . . . .  67
   Appendix E.  Document Updates . . . . . . . . . . . . . . . . . .  70
     E.1.  Version -13 to -14  . . . . . . . . . . . . . . . . . . .  70
     E.2.  Version -12 to -13  . . . . . . . . . . . . . . . . . . .  71
     E.3.  Version -11 to -12  . . . . . . . . . . . . . . . . . . .  71
     E.4.  Version -10 to -11  . . . . . . . . . . . . . . . . . . .  71
     E.5.  Version -09 to -10  . . . . . . . . . . . . . . . . . . .  72



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     E.6.  Version -08 to -09  . . . . . . . . . . . . . . . . . . .  72
     E.7.  Version -07 to -08  . . . . . . . . . . . . . . . . . . .  72
     E.8.  Version -06 to -07  . . . . . . . . . . . . . . . . . . .  73
     E.9.  Version -05 to -06  . . . . . . . . . . . . . . . . . . .  73
     E.10. Version -04 to -05  . . . . . . . . . . . . . . . . . . .  73
     E.11. Version -03 to -04  . . . . . . . . . . . . . . . . . . .  74
     E.12. Version -02 to -03  . . . . . . . . . . . . . . . . . . .  74
     E.13. Version -01 to -02  . . . . . . . . . . . . . . . . . . .  74
     E.14. Version -00 to -01  . . . . . . . . . . . . . . . . . . .  75
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  75
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  75

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 to be automatically notified
   with an unsolicited response upon changes of the resource state.

   CoAP supports group communication [I-D.ietf-core-groupcomm-bis],
   e.g., over IP multicast.  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 the same target resource to
   multiple observers as responses over IP multicast.

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

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




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

   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 particular, the group of
   potential observers entrusts the server to manage the Token space for
   multicast notifications.  Building on that, the server provides all
   the observers of a target resource with the same Token value to bind
   to their own observation, by sending a unicast informative response
   to each observer client.  That Token value is then used in every
   multicast notification for the target resource under that
   observation.

   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], Concise Data
   Definition Language (CDDL) [RFC8610], Concise Binary Object
   Representation (CBOR) [RFC8949], Object Security for Constrained
   RESTful Environments (OSCORE) [RFC8613], Group OSCORE
   [I-D.ietf-core-oscore-groupcomm], and Constrained Resource
   Identifiers (CRIs) [I-D.ietf-core-href].

   This document additionally defines the following terminology.




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   *  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.

   *  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 where multicast
      notifications can reach a sufficiently large portion of the
      clients.

      This document focuses on a network setup where the clients are
      capable of listening to multicast traffic and can directly receive
      multicast notifications.

      Alternative network setups may leverage intermediaries such as
      proxies, e.g., in order to accommodate clients that are not able
      to directly listen to multicast traffic.  How the method specified
      in this document can be used in such setups is discussed in
      [I-D.ietf-core-multicast-notifications-proxy].

   *  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.





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      That is, the method 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 for clients to opt in for receiving
      multicast notifications.

3.  High-Level Overview of Available Variants

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

   Consistent with the scope of this document, the following assumes a
   network setup where the clients participating in the group
   observation are capable of listening to multicast traffic.  In such a
   setup, the clients directly receive multicast notifications from the
   server.  Alternative network setups that rely on intermediaries such
   as proxies are discussed in
   [I-D.ietf-core-multicast-notifications-proxy].

   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 start listening to multicast notifications.

   The rest of this section provides an overview of the available
   variants to enforce a group observation, which differ as to whether
   exchanged messages are protected end-to-end between the observer
   clients and the server.

   *  Variant 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.

   *  Variant 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 security protocol Group OSCORE
      [I-D.ietf-core-oscore-groupcomm].  This case is defined in
      Section 9.  An example is provided in Section 10.




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

4.  Server-Side Requirements

   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 number of traditional observations has been
      established on the target resource, the server decides to make the
      corresponding observer 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.  The server is expected to 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 if, at some point in time, not enough clients are
   estimated to be still active and interested.

4.1.  Request

   Assuming that the server 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 that have:




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       *  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 it was 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 request 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 the
       initial multicast notification for the target resource, in
       response to the phantom observation request.  This message is
       formatted like other multicast notifications (see Section 4.3)
       and MUST include the current representation of the target
       resource as its payload.

       The server stores the message INIT_NOTIF and does not transmit
       it.  The server considers this message as the latest multicast
       notification for the target resource, until it transmits a new
       multicast notification for that resource as a CoAP message on the
       wire, after which 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.







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   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 a Confirmable
   message sent as a separate response (see Section 5.2.2 of [RFC7252]),
   MUST NOT have link-local source or destination addresses, and MUST
   NOT provide link-local or site-local addresses in the transport-
   specific information specified in its payload (see below).

   The Content-Format of the informative response is set to
   "application/informative-response+cbor", which is registered in
   Section 14.2.  The payload of the informative response is a CBOR map,
   whose fields use the CBOR abbreviations that are defined in
   Section 11.

   When using the method specified in this document, the CBOR map
   conveyed as the payload of the informative response includes the
   following parameters with the semantics defined below.  Other
   specifications may define different uses of the informative response
   for providing alternative information that is relevant to other
   protocols and applications.

   *  'tp_info', with value a CBOR array.  This includes the transport-
      specific information required to correctly receive multicast
      notifications that are 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 if 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 number 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 content distribution through
      multicast notifications.

   *  'ending', with value the time when the group observation of the
      target resource is planned to be canceled, encoded as a CBOR
      integer or as a CBOR floating-point number.  The value is the
      number of seconds from 1970-01-01T00:00:00Z UTC until the
      specified UTC date/time, ignoring leap seconds, analogous to what
      is specified for NumericDate in Section 2 of [RFC7519].  This
      parameter MAY be included.

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

   informative_response_payload = {
      0 => array, ; 'tp_info' (transport-specific information)
    ? 1 => bstr,  ; 'ph_req' (transport-independent information)
    ? 2 => bstr,  ; 'last_notif' (transport-independent information)
    ? 3 => uint,  ; 'next_not_before'
    ? 4 => ~time  ; 'ending'
   }

            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



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   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 a Confirmable message sent as a separate
   response (see Section 5.2.2 of [RFC7252]).

   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.

4.2.1.  Transport-Specific Message Information

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

   tp_info = [
       tpi_server: CRI-no-local, ; Addressing information of the server
     ? tpi_details               ; Further information about the request
   ]

   tpi_details = (
     + elements ; The number, format, and encoding of the elements
                ; depend on the scheme-id and authority of the CRI
                ; specified as tpi_server
   )

   CRI-no-local = [
     scheme-id,
     authority
   ]

   scheme-id = nint  ; -1 - scheme-number

   authority = [?userinfo, host, ?port]
   userinfo  = (false, text .feature "userinfo")
   host      = (host-ip // host-name)
   host-name = (*text) ; lowercase, NFC labels
   host-ip   = (bytes .size 4 //
                  (bytes .size 16, ?zone-id))
   zone-id   = text
   port      = 0..65535

                   Figure 2: General Format of 'tp_info'

   The following holds for the two elements 'tpi_server' and
   'tpi_details'.

   *  The 'tpi_server' element MUST be present and specifies:



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      -  The transport protocol used to transport a CoAP response from
         the server, i.e., a multicast notification in this document;
         and

      -  The addressing information of the server, i.e., the source
         addressing information of the multicast notifications that are
         sent for the group observation.

         Such addressing information MUST be equal to the source
         addressing information of the informative response sent by the
         server (see Section 4.3).

      This element specifies a CRI [I-D.ietf-core-href], of which both
      'scheme' and 'authority' are given, while 'path', 'query', and
      'fragment' are not given.

      Consistent with Section 5.1.1 of [I-D.ietf-core-href], the CRI
      scheme is given as a negative integer 'scheme-id'.  In particular,
      a 'scheme-id' with value ID denotes the CRI scheme that has CRI
      scheme number equal to (-1 - ID).  The latter identifies the
      corresponding registered URI scheme, per the associated entry in
      the "Uniform Resource Identifier (URI) Schemes" registry defined
      in [RFC7595] and updated in Section 11.1 of [I-D.ietf-core-href].

      The combination of URI scheme and 'authority' component determines
      the CoAP transport used to distribute multicast notifications for
      the group observation.  Note that:

      -  If the 'authority' component specifies a host-ip, then the
         'scheme-id' (hence the URI scheme) is sufficient to identify
         the transport.

      -  If the 'authority' component specifies a host-name, then the
         consumer of the CRI has to resolve the host-name and consider
         the result together with the 'scheme-id' (hence the URI scheme)
         in order to identify the transport.  For instance, DNS
         resolution can be used (e.g., as defined in [RFC9953]).

      The identified transport determines what elements are required in
      the 'tpi_details' element of the 'tp_info' array, as well as what
      information they convey, their encoding, and their semantics.
      Those elements are specified in the 'Transport Information
      Details' column of the "CoAP Transport Information" registry for
      the entry associated with the identified CoAP transport (see
      Section 14.6)






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   *  The 'tpi_details' element MAY be present and specifies transport-
      specific information related to a pertinent request message, i.e.,
      the phantom observation request in this document.

      The exact format of 'tpi_details' depends on the CoAP transport,
      which is identified according to the CRI conveyed by the
      'tpi_server' element, as described above.

      In the "CoAP Transport Information" registry defined in
      Section 14.6 of this document, the entry corresponding to the
      identified CoAP transport specifies the list of elements composing
      'tpi_details' for that transport, as indicated in the 'Transport
      Information Details' column.  Within 'tpi_details', its elements
      MUST be ordered according to what is specified in the 'Transport
      Information Details' column of the "CoAP Transport Information"
      registry.

   Section 12 defines an entry to be registered in the "CoAP Transport
   Information" registry, for the transport "CoAP over UDP".  When such
   a transport is used, i.e., CoAP responses are transported over UDP as
   per [RFC7252] and [I-D.ietf-core-groupcomm-bis], the full encoding of
   the 'tp_info' CBOR array is as defined in Section 4.2.1.1.

   If a future specification defines the use of CoAP multicast
   notifications transported over different transport protocols, then
   that specification must perform the following actions, unless those
   have been already performed for different reasons:

   *  Define the elements in 'tpi_details', as to what information they
      convey, their encoding, and their semantics.

   *  Register an entry in the "CoAP Transport Information" registry
      defined in Section 14.6 of this document.

   *  Register an entry in the "Uniform Resource Identifier (URI)
      Schemes" registry defined in [RFC7595] and updated in Section 11.1
      of [I-D.ietf-core-href], where the value in the 'CRI Scheme
      Number' column is (-1 - ID).  In particular, ID is the negative
      integer to be used as 'scheme-id' for CRIs conveyed by the
      'tpi_server' element and by elements in 'tpi_details'.

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
   'tp_info' CBOR array as follows.





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   *  In the 'tpi_server' element, the CRI has 'scheme-id' with value -1
      ("coap"), while 'authority' conveys addressing information of the
      server, i.e., the source addressing information of the multicast
      notifications that are sent for the group observation.

      This information consists of the IP address SRV_ADDR (expressed as
      a literal or as a host-name to be resolved) and the port number
      SRV_PORT of the server hosting the target resource, from where the
      server will send multicast notifications for the target resource.

   *  The 'tpi_details' element MUST be present and in turn includes the
      following two elements:

      -  'tpi_client' is a CRI, with the same format of 'tpi_server'
         (see Section 4.2.1).  In particular, the CRI has 'scheme-id'
         with value -1 ("coap"), while 'authority' conveys the
         destination addressing information of the multicast
         notifications that the server sends for the group observation.

         This information consists of the IP multicast address GRP_ADDR
         (expressed as a literal or as a host-name to be resolved) and
         the port number GRP_PORT, where the server will send multicast
         notifications for the target resource.

      -  'tpi_token' is a CBOR byte string, whose value is 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).

   The CDDL notation in Figure 3 describes the format of the 'tp_info'
   CBOR array when CoAP is transported over UDP.

   tp_info_coap_udp = [
     tpi_server: CRI-no-local, ; Source addressing information
                               ; of the multicast notifications
     tpi_client: CRI-no-local, ; Destination addressing information
                               ; of the multicast notifications
     tpi_token: bstr           ; Token value of the phantom request and
                               ; associated multicast notifications
   ]

        Figure 3: Format of 'tp_info' with UDP as Transport Protocol

   The CBOR diagnostic notation in Figure 4 provides an example of the
   'tp_info' CBOR array when CoAP is transported over UDP.





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   In the example, SRV_ADDR is 2001:db8::ab, SRV_PORT is 5683 (omitted
   in the CRI of 'tpi_server' as it is the default port number when CoAP
   is transported over UDP), GRP_ADDR is ff35:30:2001:db8::23, and
   GRP_PORT is 61616.

   [ / tp_info /
     [ / tpi_server /
       -1,        / scheme-id -- equivalent to "coap" /
       h'20010db80000000000000000000000ab'  / host-ip /
     ],
     [ / tpi_client /
       -1,        / scheme-id -- equivalent to "coap" /
       h'ff35003020010db80000000000000023', / host-ip /
       61616                                   / port /
     ],
     h'7b'                                / tpi_token /
   ]

       Figure 4: Example of 'tp_info' with UDP as Transport Protocol

4.2.2.  Transport-Independent Message Information

   For both the parameters 'ph_req' and 'last_notif' in the informative
   response, the value of the CBOR 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 such multicast notification is
   formatted as follows.




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   *  It MUST be a Non-confirmable message.

   *  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.

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

      The Token value T is specified by an element of 'tpi_details'
      within the 'tp_info' parameter, in the informative response sent
      to the observer clients.  In particular, when transporting CoAP
      over UDP, the Token value is specified by the element 'tpi_token'
      (see Section 4.2.1.1).

   *  It MUST be sent from the same IP address SRV_ADDR and port number
      SRV_PORT where the corresponding informative responses are sent
      from by the server (see Section 4.2).  That is, redirection MUST
      NOT be used.

      Note that, in most cases, such SRV_ADDR and SRV_PORT are those to
      which original observation requests are sent to by clients (see
      Section 5.1), unless those requests are sent to a multicast
      address (see [I-D.ietf-core-groupcomm-bis]).

      The addressing information above is provided to the observer
      clients through the CRI specified by the element 'tpi_server'
      within the 'tp_info' parameter, in the informative response (see
      Section 4.2.1).

   *  It MUST be sent to the IP multicast address GRP_ADDR and port
      number GRP_PORT.

      The addressing information above is provided to the observer
      clients through the CRI specified by an element of 'tpi_details'
      within the 'tp_info' parameter, in the informative response.  In
      particular, when transporting CoAP over UDP, the CRI is conveyed
      by the element 'tpi_client' (see Section 4.2.1.1).

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






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4.4.  Congestion Control

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

   *  The multicast notifications MUST be Non-confirmable messages.

   *  In constrained environments such as low-power, lossy networks
      (LLNs), the server SHOULD only support multicast notifications for
      resources whose representation is small in size.  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 when using IPv6 over
      Low-Power Wireless Personal Area Networks (6LoWPAN) (see Section 4
      of [RFC4944]).

   *  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.

   *  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]).  Furthermore, a
      goal for an appropriate transmission rate of multicast
      notifications is 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 to the server.

4.5.  Cancellation

   At a certain point in time, the server might want to cancel a group
   observation of a target resource.  For instance, the server realizes
   that no clients or not enough clients are interested in taking part
   in the group observation anymore.  Section 8 defines a possible
   approach that the server can use to make an assessment in this
   respect.  Another reason is that the group observation has reached
   its ending time, as originally scheduled by the server.




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   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 memory and network resources allocated for the group
   observation, and it especially frees up the Token value T used at its
   CoAP endpoint.

5.  Client-Side Requirements

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 have link-local source or
   destination 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 the case that
   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
   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.



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   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 defined in
   Section 8), which in turn can play a role in deciding to cancel the
   group observation (see Section 4.5).

5.2.  Informative Response

   Upon receiving the informative response defined in Section 4.2, the
   client has to identify the CoAP transport used to distribute
   multicast notifications for the group observation.

   To this end, the client relies on the element 'tpi_server' within the
   'tp_info' parameter of the informative response (see Section 4.2.1).

   In particular, the client considers the CRI conveyed by 'tpi_server'
   and identifies the CoAP transport, by assessing together the
   'authority' component and the URI scheme determined from 'scheme-id'
   (see Section 4.2.1).

   After that, the client parses the remainder of the 'tp_info' array,
   i.e., the information conveyed by 'tpi_details', according to what is
   specified in the 'Transport Information Details' column of the "CoAP
   Transport Information" registry for the entry associated with the
   identified CoAP transport (see Section 14.6).

   Then, the client performs the following steps.

   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 by the CRI conveyed by the
          element 'tpi_server' within the 'tp_info' parameter, in the
          informative response (see Section 4.2.1).

          If the port number is not present in the CRI, the client MUST
          use as SRV_PORT the default port number defined for the
          identified CoAP transport (e.g., the default port number is
          5683 when the transport is CoAP over UDP).

       *  As destination address and port number, the IP multicast
          address GRP_ADDR and port number GRP_PORT.  These are
          specified by the CRI conveyed by a dedicated element of



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          'tpi_details' within the 'tp_info' parameter, in the
          informative response.  In particular, when transporting CoAP
          over UDP, the CRI is conveyed by the element 'tpi_client' (see
          Section 4.2.1.1).

          If the port number is not present in the CRI, the client MUST
          use as GRP_PORT the default port number defined for the
          identified CoAP transport (e.g., the default port number is
          5683 when the transport is CoAP over UDP).

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

       *  The client uses the Token value T that is specified by a
          dedicated element of 'tpi_details' within the 'tp_info'
          parameter, in the informative response.  In particular, when
          transporting CoAP over UDP, the Token value is specified by
          the element 'tpi_token' (see Section 4.2.1.1).

       *  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 same Token value T used at Step 2; and

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



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   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.

   In addition to 'tpi_server', further elements of the 'tp_info' array
   can convey a CRI.  The client MUST treat any CRI within the 'tp_info'
   array as invalid, if the 'authority' component is a host-name such
   that, when resolved, its combination with the URI scheme indicates
   multiple transports (see Section 4.2.1).  As a possible way to verify
   if that is the case, the client can rely on DNS resolution (e.g., as
   defined in [RFC9953]).

   If any of the expected fields in the informative response are absent,
   malformed, or invalid, the client MAY try sending a new registration
   request to the server (see Section 5.1).  If the client chooses not
   to, then 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 no longer be 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].






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   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 a Non-confirmable message, it is
   optional for the client to reject it with a Reset message (see
   Section 3.5 of [RFC7641]).

   If 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 attribute "gp-obs" in a web link
   [RFC8288] to a resource, e.g., using a link-format document
   [RFC6690].

   The "gp-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 "gp-obs" attribute and any present
   value MUST be ignored by the recipient.  The "gp-obs" attribute MUST
   NOT appear more than once in a given link-value; occurrences after
   the first MUST be ignored by the recipient.

   The example in Figure 5 shows a use of the "gp-obs" attribute: the
   client does resource discovery on a server and gets back a list of
   resources, one of which includes the "gp-obs" attribute indicating
   that the server might send multicast notifications for observations
   of that resource.  The CoRE Link-Format notation from Section 5 of
   [RFC6690] is used.

   REQ: GET /.well-known/core

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

                           Figure 5: The Web Link








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7.  Example

   The following example refers to two clients C1 and C2 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.  The server 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:

   *  The application-extension identifier "cri" defined in Section 3.4
      of [I-D.ietf-cbor-edn-literals] is used to notate a CBOR Extended
      Diagnostic Notation (EDN) literal for a CRI.

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

   *  '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.

   C1 --------------------- [ Unicast ] ------------------------> S  /r
   |  GET                                                         |
   |  Token: 0x4a                                                 |
   |  Observe: 0 (register)                                       |
   |  Uri-Path: "r"                                               |
   |  <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                    |



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   |                                       Token: 0x7b            |
   |                                       Observe: 0 (register)  |
   |                                       Uri-Path: "r"          |
   |                                       <Other options>        |
   |                                                              |
   |                      ( S creates a group observation of /r ) |
   |                                                              |
   |                          ( S increments the observer counter |
   |                            for the group observation of /r ) |
   |                                                              |
   C1 <-------------------- [ Unicast ] ------------------------- S
   |  5.03                                                        |
   |  Token: 0x4a                                                 |
   |  Content-Format: application/informative-response+cbor       |
   |  Max-Age: 0                                                  |
   |  <Other options>                                             |
   |  Payload: {                                                  |
   |    / tp_info /    0 : [                                      |
   |                        cri'coap://SRV_ADDR:SRV_PORT/',       |
   |                          cri'coap://GRP_ADDR:GRP_PORT/',     |
   |                            0x7b                              |
   |                       ],                                     |
   |    / last_notif / 2 : bstr(0x45 | OPT | 0xff | PAYLOAD)      |
   |  }                                                           |
   |                                                              |
   C2 --------------------- [ Unicast ] ------------------------> S  /r
   |  GET                                                         |
   |  Token: 0x01                                                 |
   |  Observe: 0 (register)                                       |
   |  Uri-Path: "r"                                               |
   |  <Other options>                                             |
   |                                                              |
   |                         ( S increments the observer counter  |
   |                           for the group observation of /r )  |
   |                                                              |
   C2 <-------------------- [ Unicast ] ------------------------- S
   |  5.03                                                        |
   |  Token: 0x01                                                 |
   |  Content-Format: application/informative-response+cbor       |
   |  Max-Age: 0                                                  |
   |  <Other options>                                             |
   |  Payload: {                                                  |
   |    / tp_info /    0 : [                                      |
   |                        cri'coap://SRV_ADDR:SRV_PORT/',       |
   |                          cri'coap://GRP_ADDR:GRP_PORT/',     |
   |                            0x7b                              |
   |                       ],                                     |
   |    / last_notif / 2 : bstr(0x45 | OPT | 0xff | PAYLOAD)      |



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   |  }                                                           |
   |                                                              |
   |           ( The value of the resource /r changes to "5678" ) |
   |                                                              |
   +--+                                                           |
   C1 |                                                           |
      | <------------------ [ Multicast ] ----------------------- S
   C2 |      ( Destination address/port: GRP_ADDR/GRP_PORT )      |
   +--+                                                           |
   |    2.05                                                      |
   |    Token: 0x7b                                               |
   |    Observe: 11                                               |
   |    <Other options>                                           |
   |    Payload: "5678"                                           |
   |                                                              |

                   Figure 6: 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.  Feedback-Divider Option

   This section defines the new CoAP option Feedback-Divider.  By
   including the option in an outgoing message, a sender endpoint
   elicits a stochastic reaction and thereby a feedback from the message
   recipient(s) that would otherwise not react.

   If the sender endpoint triggering a feedback is a client sending a
   request, the Feedback-Divider Option included in the request triggers
   the recipient server(s) to send a response with some probability.  In
   particular, the Feedback-Divider Option stochastically overrides the
   possible response suppression performed by the server(s).  Such an
   influence affects response suppression that can otherwise be
   performed, e.g., according to: default processing; the No-Response
   Option [RFC7967], if present in the request; an unmatching filter
   from the URI query component, when the request is sent over multicast
   and targets the /.well-known/core resource (see Section 4.1 of
   [RFC6690]).

   If the sender endpoint triggering a feedback is a server sending a
   response, the feedback elicitation is currently limited to using
   observe notifications.  In particular, the Feedback-Divider Option
   included in an observe notification triggers the recipient client(s)
   to send with some probability a new request to the server.  Such a



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   request includes the Observe Option set to 0 (register) and is
   addressed to the same target resource for which the observe
   notification was sent.  That is, a reacting client (re-)registers a
   regular unicast observation on the same target resource.

   This document specifically defines how the Feedback-Divider Option is
   used when the sender endpoint triggering a feedback is a server
   sending multicast notifications.  Note that it is not so useful to
   include the Feedback-Divider Option in an observe notification sent
   over unicast to a single client.  That is, the server can more
   efficiently query a single client by means of an observe notification
   sent as a Confirmable message, thereby eliciting an Acknowledgement
   message in return.

   The Feedback-Divider Option has the properties summarized in Table 1,
   which extends Table 4 of [RFC7252].  The option is not Critical, not
   Safe-to-Forward, and integer valued.  Since the option is not Safe-
   to-Forward, the 'N' column indicates a dash for "not applicable".

   +=======+=+=+===+===+==================+========+========+=========+
   | No.   |C|U| N | R | Name             | Format | Length | Default |
   +=======+=+=+===+===+==================+========+========+=========+
   | TBD18 | |x| - |   | Feedback-Divider | uint   | 0-1    | (none)  |
   +-------+-+-+---+---+------------------+--------+--------+---------+

       Table 1: The Feedback-Divider Option.  C=Critical, U=Unsafe,
                        N=NoCacheKey, R=Repeatable

   Note to RFC Editor: In the table above, please replace TBD18 with the
   registered option number.  Then, please delete this paragraph.

   The 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 Feedback-Divider Option, a client
   that supports the option and is interested in continuing receiving
   multicast notifications for the target resource SHOULD acknowledge
   its interest, as described below.

   The client picks an integer random number I, from 0 inclusive to Z =
   (2^Q) exclusive, where Q is the value specified in the option.  If I
   is different from 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 observation on
   the same target resource.




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   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 of the source IP address and port
   number of the received multicast notification.

   Since this Observe registration is only done for its side effect of
   looking as an attempted observation at the server, the client MUST
   send the unicast request as a Non-confirmable message and with the
   maximum No-Response setting [RFC7967].  In the request, the client
   MUST include a Feedback-Divider Option, whose value MUST be set to 0.
   As per Section 3.2 of [RFC7252], this is represented with an empty
   option value (a zero-length sequence of bytes).  The client does not
   need to wait for responses and can keep processing further
   notifications on the same Token.

   The client MUST ignore the 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
   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.

   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 that 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)).




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   *  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 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 Feedback-Divider Option
   only in the first notification of such a burst.

8.3.2.  Collection of Feedback

   The server collects 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 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.










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8.3.3.  Processing of Feedback

   Once MAX_CONFIRMATION_WAIT seconds have passed, the server counts the
   R confirmations that have arrived as observation requests to the
   target resource, since the time when the latest multicast
   notification with the Feedback-Divider Option has been sent.  In
   particular, the server considers an observation request as a
   confirmation from a client only if the request includes a Feedback-
   Divider Option with value 0.

   Then, the server computes a feedback indicator as E = R * (2^Q).
   According to what is 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 considers 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, which can be taken into account 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.












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   As an example, if the server currently estimates that N = COUNT = 32
   observers are active and considers a constant M = 8, it sends a
   notification with Feedback-Divider with value Q = 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
   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.

   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 the security
   protocol Group OSCORE [I-D.ietf-core-oscore-groupcomm], thus ensuring
   that they are protected end-to-end for 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 already
   be a 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 could already be members of the right OSCORE group, if they
   previously joined it to securely communicate with the same server or
   with other servers to access their resources.

   Both the clients and the server MAY join the OSCORE group by using
   the approach defined in [I-D.ietf-ace-key-groupcomm-oscore] and based
   on the ACE framework for Authentication and Authorization in
   constrained environments [RFC9200].  When doing so, the server has to



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   join the group (also) with the roles of Requester and Responder.
   Instead, a client can join the group with any permitted role or
   combination of roles, unless it intends to send its original
   observation requests (see Section 5.1) protected with Group OSCORE.
   In such a case, the client has to join the group (also) as a
   Requester.  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, then the
   original registration requests and related unicast (notification)
   responses MUST also be protected, including and especially the
   informative responses from the server.  An exception is the case
   discussed in Appendix D, where the informative response from the
   server is not protected.

   In order to protect unicast messages exchanged between the server and
   each client, including the original client registration (see
   Section 5), alternative security protocols than Group OSCORE can be
   used, such as OSCORE [RFC8613] and/or DTLS [RFC9147].  However, it is
   RECOMMENDED to use OSCORE or Group OSCORE, in order to reduce the
   number of libraries that the clients and the server have to support.

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].
   The mechanism defined in this section is OPTIONAL to support for the
   client and server.

   In addition to what is defined in Section 4, the CBOR map in the
   informative response payload contains the following fields, whose
   CBOR abbreviations are defined in Section 11.

   *  '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.



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   *  Optionally, 'hkdf', with value the HKDF Algorithm used in the
      OSCORE group, encoded as a CBOR text string or integer.  The HKDF
      Algorithm is specified by the HMAC Algorithm value, which is taken
      from the 'Value' column of the "COSE Algorithms" registry
      [COSE.Algorithms].  For example, the HKDF Algorithm HKDF SHA-256
      is specified as the HMAC Algorithm HMAC 256/256.

   *  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].
      Consistent with Section 2.4 of [I-D.ietf-core-oscore-groupcomm],
      acceptable values denote a format that provides the public key and
      a 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 [RFC5280], and
      C509 certificates [I-D.ietf-cose-cbor-encoded-cert].  Further
      formats may be available in the future, and they would be
      acceptable to use as long as they comply with the criteria above.

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

   *  Optionally, 'gp_enc_alg', with value the Group 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].





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      -  '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', 'gp_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.

   As mentioned above, since this mechanism is OPTIONAL, all the
   corresponding fields are OPTIONAL in the informative response.
   However, the 'join_uri' and 'sec_gp' fields MUST be present if this
   mechanism is 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).  If the client
   chooses not to, then 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.






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9.2.1.  Registration

   The phantom registration request MUST be protected, by using Group
   OSCORE.  In particular, the group mode of Group OSCORE defined in
   Section 7 of [I-D.ietf-core-oscore-groupcomm] MUST be used.

   The server protects the phantom registration request as defined in
   Section 7.1 of [I-D.ietf-core-oscore-groupcomm] by using its Sender
   Context, i.e., like if it was the actual sender.  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).
   Consistent with that, the OSCORE Option value in the phantom
   registration request specifies:

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

   *  In the 'Partial IV' field, the Partial IV encoding the previously
      consumed Sender Sequence Number value SN of the server in the
      OSCORE group, i.e., (SN* - 1).

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).  Consequently, the following applies:

   *  The specified Code is 0.05 (FETCH) [RFC8132].

   *  The sequence of CoAP options will be limited to the outer, non
      encrypted options.

   *  A payload is always present, as the ciphertext followed by the
      countersignature.

   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 at Step 6 of
   Section 4.1.





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   Optionally, the informative response includes additional parameters
   that provide information about the OSCORE group to join (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 7 of [I-D.ietf-core-oscore-groupcomm] MUST
   be used.

   The process described in Section 7.3 of
   [I-D.ietf-core-oscore-groupcomm] applies, with the following
   additions when building the two OSCORE external_aad structures to
   encrypt and sign the multicast notification (see Section 3.4 of
   [I-D.ietf-core-oscore-groupcomm]).

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

   *  The 'request_piv' element contains the value of the 'Partial IV'
      field in the OSCORE Option value of the phantom registration
      request, i.e., the Partial IV encoding the consumed Sender
      Sequence Number SN of the server.

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

   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 as defined in Section 4.5, the
   multicast response with error code 5.03 (Service Unavailable) is
   protected with Group OSCORE, as per Section 7.3 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 MUST
   include the Partial IV in the OSCORE Option value of the response.








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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 the same steps defined 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 7.2 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].

      -  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', 'Partial IV', and
         'kid context' fields from the OSCORE Option value 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).  If the client chooses not to, then the
      client SHOULD explicitly withdraw from the group observation.




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   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 7.4 of [I-D.ietf-core-oscore-groupcomm], with the following
   difference.

   For both decryption and signature verification, the client MUST set
   the external_aad structure defined in Section 3.4 of
   [I-D.ietf-core-oscore-groupcomm] as follows.  The particular way to
   achieve this is implementation specific.

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

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

   *  The 'request_kid_context' element takes the value of the 'kid
      context' field from the OSCORE Option value 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 under
   this group observation.

10.  Example with Group OSCORE

   The following example refers to two clients C1 and C2 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".



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   The server S sends multicast notifications to the IP multicast
   address GRP_ADDR and port number GRP_PORT.  The server 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:

   *  C1 and S have a pairwise OSCORE Security Context.  In particular,
      C1 has 'kid' = 0x01 as Sender ID and SN_1 = 101 (i.e., 0x65) as
      Sender Sequence Number.

   *  C2 and S have a pairwise OSCORE Security Context.  In particular,
      C2 has 'kid' = 0x02 as Sender ID and SN_2 = 201 (i.e., 0xc9) as
      Sender Sequence Number.

   *  S is a member of the OSCORE group with name "myGroup" and with
      'kid context' = 0x57ab2e as Group ID.  In the OSCORE group, S has
      'kid' = 0x05 as Sender ID and SN_5 = 501 (i.e., 0x01f5) as Sender
      Sequence Number.

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

   *  The application-extension identifier "cri" defined in Section 3.4
      of [I-D.ietf-cbor-edn-literals] is used to notate a CBOR Extended
      Diagnostic Notation (EDN) literal for a CRI.

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

   *  '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 countersignature 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.





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   C1 ---------------- [ Unicast w/ OSCORE ]  ------------------> S  /r
   |  0.05 (FETCH)                                                |
   |  Token: 0x4a                                                 |
   |  Observe: 0 (register)                                       |
   |  OSCORE: [kid:0x01, Partial IV:0x65]                         |
   |  <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  )    |
   |     .------------------------------------------------------- |
   |    /                                                         |
   |    \                                                         |
   |     `------------------------------------------------------> |  /r
   |                        0.05 (FETCH)                          |
   |                        Token: 0x7b                           |
   |                        Observe: 0 (register)                 |
   |                        OSCORE: [kid:0x05, Partial IV:0x01f5, |
   |                                 kid context:0x57ab2e]        |
   |                        <Other class U/I options>             |
   |                        0xff                                  |
   |                        Encrypted_payload {                   |
   |                          0x01 (GET),                         |
   |                          Observe: 0 (register),              |
   |                          Uri-Path: "r",                      |
   |                          <Other class E options>             |
   |                        }                                     |
   |                        <Countersignature>                    |
   |                                                              |
   |                           ( 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 ) |
   |                                                              |
   C1 <--------------- [ Unicast w/ OSCORE ] -------------------- S
   |  2.05 (Content)                                              |



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   |  Token: 0x4a                                                 |
   |  OSCORE: - (empty)                                           |
   |  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,                                                     |
   |    Payload {                                                 |
   |      / tp_info /    0 : [                                    |
   |                          cri'coap://SRV_ADDR:SRV_PORT/',     |
   |                            cri'coap://GRP_ADDR:GRP_PORT/',   |
   |                              0x7b                            |
   |                         ],                                   |
   |      / ph_req /     1 : bstr(0x05 | OPT | 0xff |             |
   |                              PAYLOAD | SIGN),                |
   |      / last_notif / 2 : bstr(0x45 | OPT | 0xff |             |
   |                              PAYLOAD | SIGN),                |
   |      / join_uri /   4 : "coap://myGM/ace-group/myGroup",     |
   |      / sec_gp /     5 : "myGroup"                            |
   |    }                                                         |
   |  }                                                           |
   |                                                              |
   C2 ---------------- [ Unicast w/ OSCORE ]  ------------------> S  /r
   |  0.05 (FETCH)                                                |
   |  Token: 0x01                                                 |
   |  Observe: 0 (register)                                       |
   |  OSCORE: [kid:0x02, Partial IV:0xc9]                         |
   |  <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 ) |
   |                                                              |
   C2 <--------------- [ Unicast w/ OSCORE ] -------------------- S
   |  2.05 (Content)                                              |
   |  Token: 0x01                                                 |
   |  OSCORE: - (empty)                                           |
   |  Max-Age: 0                                                  |
   |  <Other class U/I options>                                   |



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   |  0xff,                                                       |
   |  Encrypted_payload {                                         |
   |    5.03 (Service Unavailable),                               |
   |    Content-Format: application/informative-response+cbor,    |
   |    <Other class E options>,                                  |
   |    0xff,                                                     |
   |    Payload {                                                 |
   |      / tp_info /    0 : [                                    |
   |                          cri'coap://SRV_ADDR:SRV_PORT/',     |
   |                            cri'coap://GRP_ADDR:GRP_PORT/',   |
   |                              0x7b                            |
   |                         ],                                   |
   |      / ph_req /     1 : bstr(0x05 | OPT | 0xff |             |
   |                              PAYLOAD | SIGN),                |
   |      / last_notif / 2 : bstr(0x45 | OPT | 0xff |             |
   |                              PAYLOAD | SIGN),                |
   |      / join_uri /   4 : "coap://myGM/ace-group/myGroup",     |
   |      / sec_gp /     5 : "myGroup"                            |
   |    }                                                         |
   |  }                                                           |
   |                                                              |
   |           ( The value of the resource /r changes to "5678" ) |
   |                                                              |
   +--+                                                           |
   C1 |                                                           |
      | <----------- [ Multicast w/ Group OSCORE ] -------------- S
   C2 |       (Destination address/port: GRP_ADDR/GRP_PORT)       |
   +--+                                                           |
   |    2.05 (Content)                                            |
   |    Token: 0x7b                                               |
   |    Observe: 2                                                |
   |    OSCORE: [kid:0x05, Partial IV:0x01f6]                     |
   |    Max-Age: 0                                                |
   |    <Other class U/I options>                                 |
   |    0xff                                                      |
   |    Encrypted_payload {                                       |
   |      2.05 (Content),                                         |
   |      Observe: - (empty),                                     |
   |      <Other class E options>,                                |
   |      0xff,                                                   |
   |      Payload: "5678"                                         |
   |    }                                                         |
   |    <Signature>                                               |
   |                                                              |

          Figure 7: Example of Group Observation with Group OSCORE





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   The two external_aad structures used to encrypt and sign the
   multicast notification above include 'request_kid' = 0x05,
   'request_piv' = 0x01f5, and 'request_kid_context' = 0x57ab2e.  These
   values are specified in the 'kid', 'Partial IV', and 'kid context'
   fields of the OSCORE Option value 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 structures for decrypting and verifying this
   multicast notification and the following ones in the group
   observation.

11.  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 as
   abbreviation, instead of the full descriptive name.  Note that the
   media type "application/informative-response+cbor" MUST be used when
   these fields are transported.

       +=================+==========+=============+===============+
       | Name            | CBOR Key | CBOR Type   | Reference     |
       +=================+==========+=============+===============+
       | tp_info         | 0        | array       | Section 4.2   |
       |                 |          |             | of [RFC-XXXX] |
       +-----------------+----------+-------------+---------------+
       | ph_req          | 1        | byte string | Section 4.2   |
       |                 |          |             | of [RFC-XXXX] |
       +-----------------+----------+-------------+---------------+
       | last_notif      | 2        | byte string | Section 4.2   |
       |                 |          |             | of [RFC-XXXX] |
       +-----------------+----------+-------------+---------------+
       | next_not_before | 3        | unsigned    | Section 4.2   |
       |                 |          | integer     | of [RFC-XXXX] |
       +-----------------+----------+-------------+---------------+
       | ending          | 4        | integer or  | Section 4.2   |
       |                 |          | float       | of [RFC-XXXX] |
       +-----------------+----------+-------------+---------------+
       | join_uri        | 5        | text string | Section 9.1   |
       |                 |          |             | of [RFC-XXXX] |
       +-----------------+----------+-------------+---------------+
       | sec_gp          | 6        | text string | Section 9.1   |
       |                 |          |             | of [RFC-XXXX] |
       +-----------------+----------+-------------+---------------+
       | as_uri          | 7        | text string | Section 9.1   |
       |                 |          |             | of [RFC-XXXX] |
       +-----------------+----------+-------------+---------------+



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       | hkdf            | 8        | integer or  | Section 9.1   |
       |                 |          | text string | of [RFC-XXXX] |
       +-----------------+----------+-------------+---------------+
       | cred_fmt        | 9        | integer     | Section 9.1   |
       |                 |          |             | of [RFC-XXXX] |
       +-----------------+----------+-------------+---------------+
       | gp_enc_alg      | 10       | integer or  | Section 9.1   |
       |                 |          | text string | of [RFC-XXXX] |
       +-----------------+----------+-------------+---------------+
       | sign_alg        | 11       | integer or  | Section 9.1   |
       |                 |          | text string | of [RFC-XXXX] |
       +-----------------+----------+-------------+---------------+
       | sign_params     | 12       | array       | Section 9.1   |
       |                 |          |             | of [RFC-XXXX] |
       +-----------------+----------+-------------+---------------+
       | gp_material     | 13       | map         | Appendix C of |
       |                 |          |             | [RFC-XXXX]    |
       +-----------------+----------+-------------+---------------+
       | srv_cred        | 14       | byte string | Appendix C of |
       |                 |          |             | [RFC-XXXX]    |
       +-----------------+----------+-------------+---------------+
       | srv_identifier  | 15       | byte string | Appendix C of |
       |                 |          |             | [RFC-XXXX]    |
       +-----------------+----------+-------------+---------------+
       | exi             | 16       | unsigned    | Appendix C of |
       |                 |          | integer     | [RFC-XXXX]    |
       +-----------------+----------+-------------+---------------+
       | exp             | 17       | integer or  | Appendix C of |
       |                 |          | float       | [RFC-XXXX]    |
       +-----------------+----------+-------------+---------------+

                Table 2: Informative Response Parameters.

   Note to RFC Editor: In the table above, please replace "[RFC-XXXX]"
   with the RFC number of this specification and delete this paragraph.

12.  Transport Protocol Information

   Section 4.2.1.1 defines the transport-specific information that the
   server has to specify as elements of 'tpi_details' within the
   'tp_info' parameter of the informative response defined in
   Section 4.2, when CoAP responses are transported over UDP.

   Table 3 defines the corresponding entry that Section 14.6 registers
   in the "CoAP Transport Information" registry defined in this
   document.





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   +================+===============================+=================+
   | CoAP Transport | Transport Information Details | Reference       |
   +================+===============================+=================+
   | CoAP over UDP  | tpi_client,tpi_token          | Section 4.2.1.1 |
   |                |                               | of [RFC-XXXX]   |
   +----------------+-------------------------------+-----------------+

          Table 3: CoAP Transport Information for CoAP over UDP.

   Note to RFC Editor: In the table above, please replace "[RFC-XXXX]"
   with the RFC number of this specification and delete this paragraph.

13.  Security Considerations

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

13.1.  Unprotected Communications

   If 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.

   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 the case that not enough clients are
   estimated to be still active.











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   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.  Note that, when using the method defined
   in Section 8 with unprotected communications, an adversary can craft
   and inject multiple new observation requests including the Feedback-
   Divider Option, hence inducing the server to overestimate the number
   of still interested clients and thus to inappropriately continue the
   group observation.

13.2.  Protected Communications

   If multicast notifications for an observed resource are protected
   using Group OSCORE as per Section 9, it is ensured that those are
   securely bound to the phantom registration request that started the
   group observation of that resource.  Furthermore, the following
   applies.

   *  The original registration requests and related unicast
      (notification) responses MUST also be protected, including and
      especially the informative responses from the server.  An
      exception is the case discussed in Appendix D, where the
      informative response from the server is not protected.

      Protecting informative responses from the server prevents on-path
      active adversaries from altering the conveyed IP multicast address
      and serialized phantom registration request.

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

14.  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.

14.1.  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




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   *  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].

   *  Security considerations: See Section 13 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: CoRE WG
      mailing list (core@ietf.org) or IETF Applications and Real-Time
      Area (art@ietf.org)

   *  Intended usage: COMMON

   *  Restrictions on usage: None

   *  Author/Change controller: IETF

   *  Provisional registration: No

14.2.  CoAP Content-Formats Registry

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

   Content Type: application/informative-response+cbor

   Content Coding: -

   ID: TBD (value between 0 and 255)

   Reference: [RFC-XXXX]



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14.3.  CoAP Option Numbers Registry

   IANA is asked to enter the following option number to the "CoAP
   Option Numbers" registry [CoAP.Option.Numbers] within the
   "Constrained RESTful Environments (CoRE) Parameters" registry group.

                +========+==================+============+
                | Number | Name             | Reference  |
                +========+==================+============+
                | TBD18  | Feedback-Divider | [RFC-XXXX] |
                +--------+------------------+------------+

                    Table 4: Registrations in the CoAP
                         Option Numbers Registry

   For the Feedback-Divider Option, the preferred value range is 0-255.
   In particular, 18 is the preferred option number.

   Note to RFC Editor: In the table above, please replace TBD18 with the
   registered option number.  Then, please delete this paragraph and the
   previous paragraph.

14.4.  Target Attributes Registry

   IANA is asked to register the following entry in the "Target
   Attributes" registry [Target.Attributes] within the "Constrained
   RESTful Environments (CoRE) Parameters" registry group.

   *  Attribute Name: gp-obs

   *  Brief Description: Observable resource supporting group
      observation

   *  Change Controller: IETF

   *  Reference: Section 6 of [RFC-XXXX]

14.5.  Informative Response Parameters Registry

   This document establishes the "Informative Response Parameters"
   registry within the "Constrained RESTful Environments (CoRE)
   Parameters" registry group.

   The registration policy is either "Private Use", "Standards Action
   with Expert Review", or "Specification Required" or "Expert Review"
   per [RFC8126].  "Expert Review" guidelines are provided in
   Section 14.7.




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   All assignments according to "Standards Action with Expert Review"
   are made on a "Standards Action" basis per Section 4.9 of [RFC8126]
   with "Expert Review" additionally required per Section 4.5 of
   [RFC8126].  The procedure for early IANA allocation of "standards
   track code points" defined in [RFC7120] also applies.  When such a
   procedure is used, IANA will ask the designated expert(s) to approve
   the early allocation before registration.  In addition, working group
   chairs are encouraged to consult the expert(s) early during the
   process outlined in Section 3.1 of [RFC7120].

   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 of the item.

   *  CBOR Key: This is the value used as the CBOR map key of the item.
      These values MUST be unique.  The value can be a positive integer,
      a negative integer, or a text string.  Different ranges of values
      use different registration policies [RFC8126].  Integer values
      from -256 to 255 as well as text strings of length 1 are
      designated as "Standards Action With Expert Review".  Integer
      values from -65536 to -257 and from 256 to 65535 as well as text
      strings of length 2 are designated as "Specification Required".
      Integer values greater than 65535 as well as text strings of
      length greater than 2 are designated as "Expert Review".  Integer
      values less than -65536 are marked as "Private Use".

   *  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, if one exists.

   This registry has been initially populated by the entries in Table 2.

14.6.  CoAP Transport Information Registry

   This document establishes the "CoAP Transport Information" registry
   within the "Constrained RESTful Environments (CoRE) Parameters"
   registry group.

   The registration policy is "Expert Review" [RFC8126].  "Expert
   Review" guidelines are provided in Section 14.7.

   The columns of this registry are:




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   *  CoAP Transport: This field contains a text string.  The value MUST
      be unique and it uniquely identifies the transport used for CoAP
      messages.

   *  Transport Information Details: This field contains a list of text
      strings, where two adjacent strings are separated by a single
      comma character.  Each text string is the name of an element that
      provides transport-specific information related to a pertinent
      CoAP request.  Optional elements are prepended by '?' and MUST be
      specified next to each other as last ones.

   *  Reference: This contains a pointer to the public specification for
      the item, if one exists.

   This registry has been initially populated by the entry in Table 3.

14.7.  Expert Review Instructions

   "Standards Action with Expert Review", "Specification Required", and
   "Expert Review" are three of the registration policies defined for
   the IANA registries established in this document.  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.

   *  Specifications are required for the "Standards Action With Expert
      Review" range of point assignment.  Specifications should exist
      for "Specification Required" ranges, but early assignment before a
      specification is available is considered to be permissible.  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



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      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.

15.  References

15.1.  Normative References

   [CoAP.Content.Formats]
              IANA, "CoAP Content-Formats",
              <https://www.iana.org/assignments/core-parameters/core-
              parameters.xhtml#content-formats>.

   [CoAP.Option.Numbers]
              IANA, "CoAP Option Numbers",
              <https://www.iana.org/assignments/core-parameters/core-
              parameters.xhtml#option-numbers>.

   [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. and F. Palombini, "Key Management for Group
              Object Security for Constrained RESTful Environments
              (Group OSCORE) Using Authentication and Authorization for
              Constrained Environments (ACE)", Work in Progress,
              Internet-Draft, draft-ietf-ace-key-groupcomm-oscore-21, 14
              March 2026, <https://datatracker.ietf.org/doc/html/draft-
              ietf-ace-key-groupcomm-oscore-21>.

   [I-D.ietf-cbor-edn-literals]
              Bormann, C., "CBOR Extended Diagnostic Notation (EDN)",
              Work in Progress, Internet-Draft, draft-ietf-cbor-edn-
              literals-22, 6 April 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-cbor-
              edn-literals-22>.



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   [I-D.ietf-core-groupcomm-bis]
              Dijk, E. and M. Tiloca, "Group Communication for the
              Constrained Application Protocol (CoAP)", Work in
              Progress, Internet-Draft, draft-ietf-core-groupcomm-bis-
              18, 10 February 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              groupcomm-bis-18>.

   [I-D.ietf-core-href]
              Bormann, C. and H. Birkholz, "Constrained Resource
              Identifiers", Work in Progress, Internet-Draft, draft-
              ietf-core-href-30, 21 November 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              href-30>.

   [I-D.ietf-core-oscore-groupcomm]
              Tiloca, M., Selander, G., Palombini, F., Mattsson, J. P.,
              and R. Höglund, "Group Object Security for Constrained
              RESTful Environments (Group OSCORE)", Work in Progress,
              Internet-Draft, draft-ietf-core-oscore-groupcomm-28, 23
              December 2025, <https://datatracker.ietf.org/doc/html/
              draft-ietf-core-oscore-groupcomm-28>.

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

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

   [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/rfc/rfc6838>.

   [RFC7120]  Cotton, M., "Early IANA Allocation of Standards Track Code
              Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
              2014, <https://www.rfc-editor.org/rfc/rfc7120>.

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





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   [RFC7595]  Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
              and Registration Procedures for URI Schemes", BCP 35,
              RFC 7595, DOI 10.17487/RFC7595, June 2015,
              <https://www.rfc-editor.org/rfc/rfc7595>.

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

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

   [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/rfc/rfc8085>.

   [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/rfc/rfc8126>.

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

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

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

   [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/rfc/rfc8610>.

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




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   [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/rfc/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/rfc/rfc9203>.

   [Target.Attributes]
              IANA, "Target Attributes",
              <https://www.iana.org/assignments/core-parameters/core-
              parameters.xhtml#target-attributes>.

15.2.  Informative References

   [I-D.ietf-core-cacheable-oscore]
              Amsüss, C. and M. Tiloca, "End-to-End Protected and
              Cacheable Responses for the Constrained Application
              Protocol (CoAP) using Group Object Security for
              Constrained RESTful Environments (Group OSCORE)", Work in
              Progress, Internet-Draft, draft-ietf-core-cacheable-
              oscore-01, 2 March 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              cacheable-oscore-01>.

   [I-D.ietf-core-coap-pubsub]
              Jimenez, J., Koster, M., and A. Keränen, "A publish-
              subscribe architecture for the Constrained Application
              Protocol (CoAP)", Work in Progress, Internet-Draft, draft-
              ietf-core-coap-pubsub-19, 2 March 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              coap-pubsub-19>.

   [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-06, 4 March 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              coral-06>.

   [I-D.ietf-core-multicast-notifications-proxy]
              Tiloca, M., Höglund, R., Amsüss, C., and F. Palombini,
              "Using Proxies for Observe Notifications as CoAP Multicast
              Responses", Work in Progress, Internet-Draft, draft-ietf-



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              core-multicast-notifications-proxy-00, 20 October 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              multicast-notifications-proxy-00>.

   [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-17, 2 March 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-cose-
              cbor-encoded-cert-17>.

   [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-19, 2 March 2026,
              <https://datatracker.ietf.org/doc/html/draft-tiloca-core-
              oscore-discovery-19>.

   [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>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/rfc/rfc5280>.

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

   [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/rfc/rfc7519>.

   [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/rfc/rfc8392>.






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   [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/rfc/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/rfc/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/rfc/rfc9200>.

   [RFC9953]  Lenders, M. S., Amsüss, C., Gündoğan, C., Schmidt, T. C.,
              and M. Wählisch, "DNS over CoAP (DoC)", RFC 9953,
              DOI 10.17487/RFC9953, March 2026,
              <https://www.rfc-editor.org/rfc/rfc9953>.

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 also make the
   same group observation data available through different means, such
   as those described in Appendix A.1 and Appendix A.2.

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

   When distributed through different means than informative responses,
   the group observation data has to specify the time when the group
   observation is planned to be canceled by the server.  In particular,
   the server commits to keeping the group observation ongoing until the
   scheduled cancellation time is reached.  Before that time, the server
   might however retract the advertised group observation data and thus
   make it not available to new clients.

   After a client has obtained the group observation data from different
   sources than an informative response, the client can simply set up
   the right multicast address and start receiving multicast
   notifications for the group observation.  Consequently, the server
   will not receive an observation request from that client, will not
   follow-up with a corresponding informative response, and thus its
   observer counter (see Section 4) is not incremented to reflect the
   presence of the new client.



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A.1.  Topic Discovery in Publish-Subscribe Settings

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

   To this end, together with the 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 8 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).  Instead, it is expressed through the scheme and
   authority components of the two URIs specified as 'tp_info_server'
   and 'tp_info_client', where the former specifies the addressing
   information of the server (like 'tpi_server' in Section 4.2.1.1), and
   the latter specifies the addressing information where multicast
   notifications are sent to (like 'tpi_client' in Section 4.2.1.1).





















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   Request:

       GET </ps/topics?rt=oic.r.temperature>
       Accept: 65087 (application/coral+cbor)

   Response:

       2.05 Content
       Content-Format: 65087 (application/coral+cbor)

       rdf:type [ = <http://example.org/pubsub/topic-list>,
              topic [ = </ps/topics/1234>,
                  tp_info_server <coap://[2001:db8::1]>,
                  tp_info_client <coap://[ff35:30:2001:db8::123]>,
                  tp_info_token "7b"^^xsd::hexBinary,
                  ph_req "0160.."^^xsd::hexBinary,
                  last_notif "256105.."^^xsd::hexBinary,
                  ending "2051251201"^^xsd::unsignedLong,
              ]
       ]

        Figure 8: 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.

   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 switched off) cellular uplink to discover the topics it
   needs to update its display to, and then listen on a LoRaWAN
   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.
   Figure 9 shows an example of a possible interface, as relying 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 /    0 : [
                       [ / tpi_server /
                        -1, / scheme-id -- equivalent to "coap" /
                         h'20010db80000000000000000000000ab' / host-ip /
                       ],
                       [ / tpi_client /
                        -1, / scheme-id -- equivalent to "coap" /
                        h'ff35003020010db80000000000000023', / host-ip /
                        61616 / port /
                       ],
                       h'7b' / tpi_token /
                      ],
   / ph_req /     1 : h'0160...528c', / elided for brevity /
   / last_notif / 2 : h'256105...4fa1', / elided for brevity /
   / ending /     4 : 2051251201

 }

    Figure 9: Group Observation Discovery with Server Introspection

   For example, a network sniffer could offer sending such a request
   when unknown multicast notifications are seen in the network.
   Consequently, it can associate those notifications with a URI, or
   decrypt them if it is a 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 and 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 FEEDBACK-DIVIDER 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(FEEDBACK-DIVIDER);
       opt.set(0);
       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 FEEDBACK-DIVIDER 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(FEEDBACK-DIVIDER);
       opt.set(0);
       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









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   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


   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(FEEDBACK-DIVIDER);
   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
            received between t1 and t2, and including
            the FEEDBACK-DIVIDER Option with value 0>;

   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) {



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       <Cancel the group observation>;
       return 0;
   }

   return NEW_COUNT;

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 set up 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).

   In addition to what is defined in Section 4, the CBOR map in the
   informative response payload contains the following fields, whose
   CBOR abbreviations are defined in Section 11.

   *  '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 6.3 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 abbreviations from the OSCORE Security Context Parameters
      Registry, as in Section 6.3 of
      [I-D.ietf-ace-key-groupcomm-oscore].

      -  'ms', 'contexId', 'cred_fmt', 'gp_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.








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      Note that no information is provided as related to the AEAD
      Algorithm, or to the Pairwise Key Agreement Algorithm and its
      parameters.  In fact, the clients and the server will never use
      the pairwise mode of Group OSCORE as per Section 8 of
      [I-D.ietf-core-oscore-groupcomm] and will not need to compute a
      cofactor Diffie-Hellman shared secret in this OSCORE group.

      It follows that:

      -  In the Common Context of the Group OSCORE Security Context, the
         parameter AEAD Algorithm and the parameter Pairwise Key
         Agreement Algorithm are not set (see Section 2.1.1 of
         [I-D.ietf-core-oscore-groupcomm] and Section 2.1.10 of
         [I-D.ietf-core-oscore-groupcomm]).

      -  Aligned with the previous point, when building the two OSCORE
         external_aad structures to process messages protected with
         Group OSCORE in this OSCORE group, (see Section 3.4 of
         [I-D.ietf-core-oscore-groupcomm]), the elements 'alg_aead' and
         'alg_pairwise_key_agreement' within the 'algorithms' arrays are
         set to the CBOR simple value null (0xf6).

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

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

   *  'exi': this element has as value the residual lifetime of the
      keying material of the OSCORE group specified in the 'gp_material'
      parameter, encoded as a CBOR unsigned integer.

      The value represents the residual lifetime of the keying material
      in seconds, i.e., the number of seconds between the current time
      at the server and the time when the keying material expires.  Upon
      receiving the informative response containing the 'exi' parameter,
      a client determines the expiration time of the keying material by
      adding the number of seconds specified in the 'exi' parameter to
      its current time.

   If the server has a reliable way to synchronize its internal clock
   with UTC, then the server includes also the following field:






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   *  'exp': this element has as value the expiration time of the keying
      material of the OSCORE group specified in the 'gp_material'
      parameter, encoded as a CBOR integer or as a CBOR floating-point
      number.

      The value is the number of seconds from 1970-01-01T00:00:00Z UTC
      until the specified UTC date/time, ignoring leap seconds,
      analogous to what is specified for NumericDate in Section 2 of
      [RFC7519].

   If a client has a reliable way to synchronize its internal clock with
   UTC and the 'exp' parameter is present in the informative response,
   then the client MUST use the 'exp' parameter value as expiration time
   for the group keying material.

   Note that the informative response does not require to include an
   explicit proof of possession of the server's private key.  Although
   the server is also acting as Group Manager and a proof-of-possession
   evidence of the Group Manager's private key is included in a full-
   fledged Join Response (see Section 6.3 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 the
   Security Context like a "silent server" would, i.e., without a 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 the 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 structure for encrypting and signing the
   phantom request and multicast notifications (see Section 3.4 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 the phantom requests and the
      multicast notifications in group observations that it hosts,
      according to the method defined in this document.




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

   Upon expiration of the group keying material as indicated in the
   informative response by the 'exp' parameter (if present) and the
   'exi' parameter:

   *  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, then the following applies:

      -  The server MUST update the OSCORE Master Secret.

      -  The server MUST update the ID Context used as Group Identifier
         (Gid), consistent with Section 12.2 of
         [I-D.ietf-core-oscore-groupcomm].

      -  The server MAY update the OSCORE 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, while it is analogous to the informative response
   defined in Section 4.2, this response has the following differences:

   *  it additionally contains the parameters mentioned above, for the
      next group keying material to be used; and

   *  it 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 MUST include the Partial IV in the OSCORE Option value
   of the response.





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   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 12.2 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 and 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.

   Applications requiring backward security or 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 with 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 can set up the multicast address and group
   observation for listening to multicast notifications.

   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.ietf-core-cacheable-oscore], then the server and clients
   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



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   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.ietf-core-cacheable-oscore], thus all computing the same
   protected Deterministic Request.  The latter is used as the actual
   phantom observation request that the protected multicast
   notifications will match under the group observation in question.

   When receiving the Deterministic Request, 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 relies on 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.

   If the server recognizes the received Deterministic Request as one of
   its self-generated deterministic phantom requests, then the server
   does not perform any Group OSCORE processing on it.  This opens for
   replying with an unprotected response, although not indicating any
   OSCORE-related error.  In particular, the server MUST reply with an
   informative response that MUST NOT be protected.

   Note that the phantom registration request is, in terms of transport-
   independent information, identical to the same Deterministic Request
   sent by any client that wishes to take part in the group observation.
   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.

   When using a Deterministic Request as a phantom observation request,
   the observer counter at the server (see Section 4) is not reliably
   incremented when new clients start participating in the group
   observation.

   That is, the clients can simply set up the right multicast address
   and port number, and then start listening to multicast notifications
   bound to the Deterministic Request.  Hence, the observer counter at
   the server is not incremented as new clients start listening to
   multicast notifications.







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   Furthermore, the security identity associated with the sender of any
   Deterministic Request in the OSCORE group is exactly the same one,
   i.e., the pair (SID, OSCORE ID Context), where SID is the OSCORE
   Sender ID of the Deterministic Client in the OSCORE group, which
   clients in the group rely on to produce Deterministic Requests.

   The setting described above requires the server and all the clients
   interested in taking part in the group observation to support the
   approach defined in [I-D.ietf-core-cacheable-oscore].  On the other
   hand, its use allows clients to start from a smaller, unprotected
   version of the phantom observation request, which does not need to be
   verified as a request generated by the server.  Therefore, in
   applications where the OSCORE group is configured for the use of
   Deterministic Requests and observer clients are expected to support
   the approach defined in [I-D.ietf-core-cacheable-oscore], servers
   that start group observations are encouraged to do so by using
   Deterministic Requests as corresponding phantom observation requests.

   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', as per Section 6.3 of [I-D.ietf-ace-key-groupcomm-oscore].

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

   Note that, like in Appendix C, no information is provided as related
   to the Pairwise Key Agreement Algorithm and its parameters.  In fact,
   the clients and the server will not need to compute a cofactor
   Diffie-Hellman shared secret in this OSCORE group.  It follows that:

   *  In the Common Context of the Group OSCORE Security Context, the
      parameter Pairwise Key Agreement Algorithm is not set (see
      Section 2.1.10 of [I-D.ietf-core-oscore-groupcomm]).

   *  Aligned with the previous point, when building the two OSCORE
      external_aad structures to process messages protected with Group
      OSCORE in this OSCORE group, (see Section 3.4 of
      [I-D.ietf-core-oscore-groupcomm]), the element
      'alg_pairwise_key_agreement' within the 'algorithms' arrays is set
      to the CBOR simple value null (0xf6).





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   If a Deterministic Request is used as a phantom observation request
   for a group observation, the server does not assist clients that are
   interested in taking part in 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 for a target
   resource relies on a Deterministic Request as a phantom observation
   request.

   *  Every client that is interested to take part in 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).

   *  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.

   *  If the server receives an observation request for the target
      resource that differs from the specific Deterministic Request
      associated with the group observation for that target resource,
      then the server replies as usual with an informative response,
      including: the transport-specific information, the phantom request
      (i.e., the expected Deterministic Request), and (optionally) the
      latest notification.

Appendix E.  Document Updates

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

E.1.  Version -13 to -14

   *  Clarified expected roles in the OSCORE group for clients and
      server.

   *  The HKDF Algorithm of Group OSCORE is specified as the
      corresponding HMAC Algorithm.

   *  Generalized semantics of the Multicast-Response-Feedback-Divider
      Option and renamed it as Feedback-Divider.

   *  Clarified pre-conditions and advantages of using deterministic
      phantom requests.




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   *  Removed unnecessary normative language.

   *  Fixes and simplifications in the example with Group OSCORE.

   *  Updated references.

   *  Clarifications and editorial improvements.

E.2.  Version -12 to -13

   *  Content on proxies moved out to the new document draft-ietf-core-
      multicast-notifications-proxy.

   *  Clarified avoidance of link-local addresses.

   *  Alignment with the update to the "Uniform Resource Identifier
      (URI) Schemes" IANA registry made by draft-ietf-core-href-26.

   *  Revised value syntax for the 'Transport Information Details'
      column of the new IANA registry "CoAP Transport Information".

   *  Suggested range and value for the registration in the IANA
      registry "CoAP Option Numbers".

   *  Updated references.

   *  Editorial improvements.

E.3.  Version -11 to -12

   *  Changed CBOR type of 'ending' and 'exp' to be integer or float.

   *  Avoid limiting the informative response to this protocol.

   *  Transport identified by (scheme-id, authority) in the CRI of
      'tpi_server'.

   *  Renamed 'sign_enc_alg' to 'gp_enc_alg', now aligned with draft-
      ietf-ace-key-groupcomm-oscore.

   *  Editorial improvements.

E.4.  Version -10 to -11

   *  Do not rule out original observation requests sent over multicast.

   *  Defined 'ending' parameter for the informative response payload.




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   *  Group observation data available on different sources can be
      removed.

   *  Initial description of a scenario with a reverse-proxy.

   *  Minor fixes in examples.

   *  Clarifications and editorial improvements.

E.5.  Version -09 to -10

   *  Fixed section numbers of referred documents.

   *  Revised registration policies in the IANA considerations.

   *  Clarifications and editorial improvements.

E.6.  Version -08 to -09

   *  Revised 'tp_info' also to use CRIs for transport information.

   *  Section restructuring: impact from proxies on rough counting of
      clients.

   *  Revised and repositioned text on deterministic phantom requests.

   *  Fixes in the examples with message exchanges.

   *  Clarifications and editorial improvements.

E.7.  Version -07 to -08

   *  Fixed the CDDL definition 'srv_addr' in 'tp_info'.

   *  Early mentioning that 'srv_addr' cannot instruct redirection.

   *  The replay protection of multicast notifications is as per Group
      OSCORE.

   *  Consistently use the format uint for the Multicast-Response-
      Feedback-Divider Option.

   *  Fixed consumption of proxy-related options in a ticket request
      sent to the proxy.

   *  Possible use of the option Proxy-Cri or Proxy-Scheme-Number in a
      ticket request.




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   *  Explained non-provisioning of some parameters in self-managed
      OSCORE groups.

   *  Use of 'exi' for relative expiration time in self-managed OSCORE
      groups.

   *  Improved notation in the examples of message exchanges with proxy.

   *  Clarifications and editorial improvements.

E.8.  Version -06 to -07

   *  Added more details on proxies that do not support the Multicast-
      Response-Feedback-Divider Option.

   *  Added more details on the reliability of the client rough
      counting.

   *  Added more details on the unreliability of counting new clients,
      when the phantom request is obtained from other sources or is an
      OSCORE Deterministic Request.

   *  Revised parameter naming.

   *  Fixes in IANA considerations.

   *  Editorial improvements.

E.9.  Version -05 to -06

   *  Clarified rough counting of clients when a proxy is used

   *  IANA considerations: registration of target attribute "gp-obs"

   *  Editorial improvements.

E.10.  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.



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   *  Editorial improvements.

E.11.  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.

E.12.  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.

E.13.  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.





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E.14.  Version -00 to -01

   *  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, Matthias Kovatsch⁩, 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 the Sweden's
   Innovation Agency VINNOVA and the Celtic-Next projects CRITISEC and
   CYPRESS; and by the H2020 project SIFIS-Home (Grant agreement
   952652).

Authors' Addresses

   Marco Tiloca
   RISE AB
   Isafjordsgatan 22
   SE-164 40 Kista
   Sweden
   Email: marco.tiloca@ri.se


   Rikard Höglund
   RISE AB
   Isafjordsgatan 22
   SE-164 40 Kista
   Sweden
   Email: rikard.hoglund@ri.se








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


   Francesca Palombini
   Ericsson AB
   Torshamnsgatan 23
   SE-164 40 Kista
   Sweden
   Email: francesca.palombini@ericsson.com






































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