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Versions: 00 01 02 03

CoRE Working Group                                             M. Tiloca
Internet-Draft                                               R. Hoeglund
Updates: 7252, 7641 (if approved)                                RISE AB
Intended status: Standards Track                              C. Amsuess
Expires: January 14, 2021
                                                            F. Palombini
                                                             Ericsson AB
                                                           July 13, 2020


           Observe Notifications as CoAP Multicast Responses
          draft-tiloca-core-observe-multicast-notifications-03

Abstract

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

Status of This Memo

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

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

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

   This Internet-Draft will expire on January 14, 2021.








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Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Server-Side Requirements  . . . . . . . . . . . . . . . . . .   5
     2.1.  Request . . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Informative Response  . . . . . . . . . . . . . . . . . .   6
     2.3.  Notifications . . . . . . . . . . . . . . . . . . . . . .   8
     2.4.  Congestion Control  . . . . . . . . . . . . . . . . . . .   9
     2.5.  Cancellation  . . . . . . . . . . . . . . . . . . . . . .   9
       2.5.1.  Rough Counting of Clients in the Group Observation  .  10
   3.  Client-Side Requirements  . . . . . . . . . . . . . . . . . .  13
     3.1.  Request . . . . . . . . . . . . . . . . . . . . . . . . .  13
     3.2.  Informative Response  . . . . . . . . . . . . . . . . . .  14
     3.3.  Notifications . . . . . . . . . . . . . . . . . . . . . .  14
     3.4.  Cancellation  . . . . . . . . . . . . . . . . . . . . . .  15
   4.  Example . . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   5.  Protection of Multicast Notifications with Group OSCORE . . .  17
     5.1.  Signaling the OSCORE Group in the Informative Response  .  17
     5.2.  Server-Side Requirements  . . . . . . . . . . . . . . . .  19
       5.2.1.  Registration  . . . . . . . . . . . . . . . . . . . .  19
       5.2.2.  Informative Response  . . . . . . . . . . . . . . . .  20
       5.2.3.  Notifications . . . . . . . . . . . . . . . . . . . .  20
       5.2.4.  Cancellation  . . . . . . . . . . . . . . . . . . . .  21
     5.3.  Client-Side Requirements  . . . . . . . . . . . . . . . .  21
       5.3.1.  Informative Response  . . . . . . . . . . . . . . . .  21
       5.3.2.  Notifications . . . . . . . . . . . . . . . . . . . .  22
   6.  Example with Group OSCORE . . . . . . . . . . . . . . . . . .  22
   7.  Informative Response Parameters . . . . . . . . . . . . . . .  25
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  26
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
     9.1.  Media Type Registrations  . . . . . . . . . . . . . . . .  27
     9.2.  CoAP Content-Formats Registry . . . . . . . . . . . . . .  28



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     9.3.  Informative Response Parameters Registry  . . . . . . . .  28
     9.4.  CoAP Option Numbers Registry  . . . . . . . . . . . . . .  29
     9.5.  Expert Review Instructions  . . . . . . . . . . . . . . .  29
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  30
     10.2.  Informative References . . . . . . . . . . . . . . . . .  32
     10.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  33
   Appendix A.  Pseudo-Code for Rough Counting of Clients  . . . . .  33
     A.1.  Client Side . . . . . . . . . . . . . . . . . . . . . . .  33
     A.2.  Server Side . . . . . . . . . . . . . . . . . . . . . . .  34
   Appendix B.  Different Sources for Group Observation Data . . . .  35
     B.1.  PubSub  . . . . . . . . . . . . . . . . . . . . . . . . .  35
     B.2.  Sender Introspection  . . . . . . . . . . . . . . . . . .  36
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  37
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  37

1.  Introduction

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

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

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

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

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



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   through the respective Group Manager
   [I-D.tiloca-core-oscore-discovery].

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

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

   Second, this specification 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 informative unicast response mentioned
   above, which additionally includes parameter values used by the
   server to protect every multicast notification for the target
   resource by using Group OSCORE.  This provides a secure binding
   between each of such notifications and the observation of each of the
   clients.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Readers are expected to be familiar with terms and concepts described
   in CoAP [RFC7252], group communication for CoAP
   [I-D.ietf-core-groupcomm-bis], Observe [RFC7641], CBOR [RFC7049],
   OSCORE [RFC8613], and Group OSCORE [I-D.ietf-core-oscore-groupcomm].

   This specification additionally defines the following terminology.

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




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

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

   o  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.  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.

   o  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.

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

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

   The server initializes the counter to 0 when starting the group
   observation, and increments it after a new client starts taking part
   in that group observation.  Also, the server should keep the counter
   up-to-date over time, for instance by using the method described in
   Section 2.5.1.

2.1.  Request

   Assuming it is reachable at the address SERVER_ADDR and port number
   SERVER_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
   GROUP_ADDR and port number GROUP_PORT.

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




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

       *  As source address and port number, the IP multicast address
          GROUP_ADDR and port number GROUP_PORT.

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

       This Token space is under exclusive control of the server.

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

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

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

   6.  The server builds a CoAP response message INIT_NOTIF as initial
       multicast notification for the target resource, in response to
       the phantom observation request.  This message is formatted as
       other multicast notifications (see Section 2.3) and MUST include
       the current representation of the target resource as payload.
       The server stores the message INIT_NOTIF and does not transmit
       it.  The server considers this message as the latest multicast
       notification for the target resource, until it transmits a new
       multicast notification for that resource as a CoAP message on the
       wire.  After that, the server deletes the message INIT_NOTIF.

2.2.  Informative Response

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

   For each traditional observation ongoing on the target resource, the
   server MAY cancel that observation.  Then, the server considers the N




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   corresponding clients as now taking part in the group observation, of
   which it increases the corresponding observer counter by N.

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

   The Content-Format of the informative response is set to application/
   informative-response+cbor, as defined in Section 9.2.  The payload of
   the informative response is a CBOR map which MUST include all the
   following parameters, whose CBOR labels are defined in Section 7.

   o  'ph_req', with value the byte serialization of the CoAP message
      received by the server as phantom observation request (see
      Section 2.1), encoded as a CBOR byte string.  Specifically, the
      value of the byte string is the byte serialization of what
      intended as payload for the transport layer underlying CoAP.

   o  'last_notif', with value the byte serialization of the CoAP
      message stored by the server as the latest multicast notification
      for the target resource, encoded as a CBOR byte string.
      Specifically, the value of the byte string is the byte
      serialization of what intended as payload for the transport layer
      underlying CoAP.

   o  'cli_addr', with value the source IP address of the phantom
      observation request, encoded as a CBOR byte string.  This
      parameter is tagged and identified by the CBOR tag 260 "Network
      Address (IPv4 or IPv6 or MAC Address)".  The specified value is
      the IP multicast address GROUP_ADDR, where the server will send
      multicast notifications for the target resource.

   o  'cli_port', with value the source port number of the phantom
      observation request, encoded as a CBOR unsigned integer.  The
      specified value is the port number GROUP_PORT, where the server
      will send multicast notifications for the target resource.

   o  'srv_addr', with value the destination IP address of the phantom
      observation request, encoded as a CBOR byte string.  This
      parameter is tagged and identified by the CBOR tag 260 "Network
      Address (IPv4 or IPv6 or MAC Address)".  The specified value is
      the IP address SERVER_ADDR of the server hosting the target
      resource.

   o  'srv_port', with value the destination port number of the phantom
      observation request, encoded as a CBOR unsigned integer.  The



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      specified value is the port number SERVER_PORT of the server
      hosting the target resource has been listening to.

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

   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.

2.3.  Notifications

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

   o  It MUST be Non-confirmable.

   o  It MUST include an Observe option, as per [RFC7641].

   o  It MUST have the same Token value T of the phantom registration
      request that started the group observation, also specified in the
      'ph_req' parameter of the informative response message to the
      observer clients.  That is, every multicast notification for a
      target resource is not bound to the observation requests from the
      different clients, but rather to the phantom registration request
      associated to the whole set of clients taking part in the group
      observation of that resource.

   The server sends a multicast notification to the IP multicast address
   GROUP_ADDR and port number GROUP_PORT indicated to the observer
   clients, as value of the 'cli_addr' and 'cli_port' parameters of the
   informative response message (see Section 2.2).

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






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2.4.  Congestion Control

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

   o  The multicast notifications MUST be Non-confirmable.

   o  In constrained environments such as low-power, lossy networks
      (LLNs), the server should only support multicast notifications for
      resources that are small.  Following related guidelines from
      Section 2.2.4 of [I-D.ietf-core-groupcomm-bis], this can consist,
      for example, in having the payload of multicast notifications as
      limited to approximately 5% of the IP Maximum Transmit Unit (MTU)
      size, so that it fits into a single link-layer frame in case IPv6
      over Low-Power Wireless Personal Area Networks (6LoWPAN) (see
      Section 4 of [RFC4944]) is used.

   o  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 2.2.4 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.

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

2.5.  Cancellation

   At any point in time, the server may want to cancel a group
   observation of a target resource.  For instance, the server may
   realize that no clients or not enough clients are interested in
   taking part in the group observation anymore.  A possible approach
   that the server can use to assess this is defined in Section 2.5.1.

   In order to cancel the group observation, the server sends to itself
   a phantom cancellation request, i.e. a GET request with an Observe
   option set to 1 (deregister), without transmitting it on the wire.
   As per Section 3.6 of [RFC7641], all other options MUST be identical



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   to those in the phantom registration request, except for the set of
   ETag Options.  This request has the same Token value T of the phantom
   registration request, and is addressed to the resource for which the
   server wants to end the group observation, as if sent from the group
   of observers, i.e. with the multicast IP address GROUP_ADDR as source
   address and the port number GROUP_PORT as source port.

   After that, the server sends a multicast response with response code
   5.03 (Service Unavailable), signaling that the group observation has
   been terminated.  The response has no payload, and is sent to the
   same multicast IP address GROUP_ADDR and port number GROUP_PORT used
   to send the multicast notifications related to the target resource.
   As per [RFC7641], this response does not include an Observe option.
   Finally, the server releases the resources allocated for the group
   observation, and especially frees up the Token value T used at its
   CoAP endpoint.

2.5.1.  Rough Counting of Clients in the Group Observation

   To allow the server to keep an estimate of interested clients without
   creating undue traffic on the network, a new CoAP option is
   introduced, which SHOULD be supported by clients that listen to
   multicast responses.

   The option is called Multicast-Response-Feedback-Divider, and is only
   used in responses.  As summarized in Figure 1, the option is not
   critical but proxy-unsafe, and integer valued.

+-----+---+---+---+---+---------------------+--------+-------+---------+
| No. | C | U | N | R | Name                | Format | Len.  | Default |
+-----+---+---+---+---+---------------------+--------+-------+---------+
| TBD |   | x |   |   | Multicast-Response- | uint   | 0-8 B | (none)  |
|     |   |   |   |   | Feedback-Divider    |        |       |         |
+-----+---+---+---+---+---------------------+--------+-------+---------+

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

               Figure 1: Multicast-Response-Feedback-Divider

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

2.5.1.1.  Client Processing

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



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

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

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

   The client MUST ignore the Multicast-Response-Feedback-Divider
   option, if the multicast notification is retrieved from the
   'last_notif' parameter of an informative response (see Section 2.2).
   A client includes the Multicast-Response-Feedback-Divider option only
   in a re-registration request triggered by the server as described
   above, and MUST NOT include it in any other request.

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

   Appendix A.1 provides a description in pseudo-code of the operations
   above performed by the client.

2.5.1.2.  Client Counting

   In order to avoid needless use of network resources, a server SHOULD
   keep a rough count of the number of clients taking part in the group
   observation of a target resource.  To this end, the server updates
   the associated observer counter (see Section 2), for instance by
   using the method described below.





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   When it wants to obtain a new estimated count, the server picks a
   number M of confirmations it would like to receive from the clients.
   It is up to applications to define policies about how the server
   determines and adjusts the value of M.  The following example will be
   done with M = 5.

   Then, the server considers its current estimate of listeners N, and
   divides it by M.  The resulting quotient Q = ceil(N / M) is set as
   value in the Multicast-Response-Feedback-Divider option, which is
   sent within a successful multicast notification.  If several
   multicast notifications are sent in a burst fashion, it is
   RECOMMENDED for the server to include the Multicast-Response-
   Feedback-Divider option only in the first one of those notifications.

   The server collects unicast observation requests from the clients,
   for an amount of time of MAX_CONFIRMATION_WAIT seconds.  The server
   MUST NOT update the observer counter N associated to the group
   observation, until MAX_CONFIRMATION_WAIT seconds have elapsed.

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

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

   MAX_CONFIRMATION_WAIT = MAX_RTT + MAX_CLIENT_REQUEST_DELAY

   where MAX_RTT is as defined in Section 4.8.2 of [RFC7252] and has
   default value 202 seconds, while MAX_CLIENT_REQUEST_DELAY is
   equivalent to MAX_SERVER_RESPONSE_DELAY defined in Section 2.3.1 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, based on a realistic round trip
   time (replacing MAX_RTT) and on the largest leisure time configured
   on the clients (e.g.  DEFAULT_LEISURE = 5 replacing
   MAX_CLIENT_REQUEST_DELAY), thus shortening MAX_CONFIRMATION_WAIT to a
   few seconds.

   Once MAX_CONFIRMATION_WAIT seconds have passed, the server counts the
   R confirmations arrived as unicast observation requests to the target
   resource, after the multicast notification has been sent.  In
   particular, the server considers a unicast observation request as a



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   confirmation from a client only if it includes a Multicast-Response-
   Feedback-Divider option with an empty value (Option Length = 0).
   Then, the server multiplies R by the original Multicast-Response-
   Feedback-Divider value Q, to get an updated client estimate.

   If X new clients are added to the group observation while the process
   above is occurring, the server MUST first complete the counting
   process and update N based on the received re-registration requests.
   Only after that, the server further increments N by X, and considers
   the result as the current observer counter to assess for possibly
   cancelling the group observation (see Section 2.5).

   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 updated value of N into
   account and determines whether to cancel the group observation.

   As an example, if the server currently estimates that N = 20
   observers are active, it sends a notification out with Multicast-
   Response-Feedback-Divider: 4.  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 four re-registration
   requests received by the server.  Also, no new clients have been
   added to the group observation in the meanwhile.  As a consequence,
   the server updates the observer counter to N = (4 * 4) + 0 = 16, and
   continues sending notifications to the group of observers.

   Note that a server can send Multicast-Response-Feedback-Divider: 1 in
   the last notifications, before cancelling a group observation.  This
   will trigger all the active clients to state their interest in
   continuing receiving notifications for the target resource.

   Appendix A.2 provides a description in pseudo-code of the operations
   above performed by the server.

3.  Client-Side Requirements

3.1.  Request

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




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3.2.  Informative Response

   Upon receiving the informative response defined in Section 2.2, the
   client proceeds as follows.

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

       *  As source address and port number, the server address
          SERVER_ADDR and port number SERVER_PORT intended for accessing
          the target resource.

       *  As destination address and port number, the IP multicast
          address GROUP_ADDR and port number GROUP_PORT, specified in
          the 'cli_addr' and 'cli_port' parameter.

   2.  The client retrieves and stores the phantom registration request
       specified in the 'ph_req' parameter.  The group observation is
       bound to this phantom registration request.  In particular, the
       client MUST use its Token value T as its own local Token value
       associated to that group observation, with respect to the (next
       hop towards the) server.  The particular way to achieve this is
       implementation specific.

   3.  The client retrieves the multicast notification specified in the
       'last_notif' parameter, and processes it 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.

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

   If any of the expected fields are not present, the client MAY try
   sending a new registration request to the server (see Section 3.1).
   Otherwise, the client SHOULD explicitly withdraw from the group
   observation.

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

3.3.  Notifications

   After having successfully processed the informative response as
   defined in Section 3.2, the client will receive, accept and process
   multicast notifications about the state of the target resource from




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   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].

3.4.  Cancellation

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

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

   In case the server has cancelled a group observation as defined in
   Section 2.5, 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 2.5.

4.  Example

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

   In the informative responses, 'bstr(X)' denotes a byte string with
   value the byte serialization of X.  Also, the notation Y.CoAP denotes
   the CoAP-layer part of a message Y, i.e. the part of Y that becomes
   payload for the transport layer underlying CoAP.

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





   C_1     ------------------ [ Unicast ] --------------------> S  /r



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    |  GET                                                      |
    |  Token: 0x4a                                              |
    |  Observe: 0 (Register)                                    |
    |                                                           |
    |            (S allocates the available Token value 0xff .) |
    |                                                           |
    |                                                           |
    |                                                           |
    |   (S sends to itself a phantom observation request PH_REQ |
    |    as coming from the IP multicast address GROUP_ADDR .)  |
    |     -------------------------------------------------     |
    |    /                                                      |
    |    \----------------------------------------------------> |  /r
    |                                    GET                    |
    |                                    Token: 0xff            |
    |                                    Observe: 0 (Register)  |
    |                                                           |
    |                   (S creates a group observation of /r .) |
    |                                                           |
    |                       (S increments the observer counter  |
    |                        for the group observation of /r .) |
    |                                                           |
   C_1 <--------------------- [ Unicast ] -----------------     S
    |  5.03                                                     |
    |  Token: 0x4a                                              |
    |  Payload: { ph_req     : bstr(PH_REQ.CoAP),               |
    |             last_notif : bstr(LAST_NOTIF.CoAP)            |
    |             cl_addr    : bstr(GROUP_ADDR),                |
    |             cl_port    : GROUP_PORT,                      |
    |             srv_addr   : bstr(SERVER_ADDR),               |
    |             srv_port   : SERVER_PORT,                     |
    |           }                                               |
    |                                                           |
   C_2     ------------------ [ Unicast ] --------------------> S  /r
    |  GET                                                      |
    |  Token: 0x01                                              |
    |  Observe: 0 (Register)                                    |
    |                                                           |
    |                       (S increments the observer counter  |
    |                        for the group observation of /r .) |
    |                                                           |
   C_2 <--------------------- [ Unicast ] -----------------     S
    |  5.03                                                     |
    |  Token: 0x01                                              |
    |  Payload: { ph_req     : bstr(PH_REQ.CoAP),               |
    |             last_notif : bstr(LAST_NOTIF.CoAP)            |
    |             cl_addr    : bstr(GROUP_ADDR),                |
    |             cl_port    : GROUP_PORT,                      |



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    |             srv_addr   : bstr(SERVER_ADDR),               |
    |             srv_port   : SERVER_PORT,                     |
    |           }                                               |
    |                                                           |
    |         (The value of the resource /r changes to "5678".) |
    |                                                           |
   C_1                                                          |
    +  <-------------------- [ Multicast ] ----------------     S
   C_2   (Destination address/port: GROUP_ADDR/GROUP_PORT)      |
    |  2.05                                                     |
    |  Token: 0xff                                              |
    |  Observe: 11                                              |
    |  Payload: "5678"                                          |
    |                                                           |

5.  Protection of Multicast Notifications with Group OSCORE

   A server can protect multicast notifications by using Group OSCORE
   [I-D.ietf-core-oscore-groupcomm].  Both the server and the clients
   interested in receiving multicast notifications from that server have
   to be members of the same OSCORE group.

   Clients MAY discover the OSCORE group to refer to by using the method
   in [I-D.tiloca-core-oscore-discovery], based on the CoRE Resource
   Directory (RD) [I-D.ietf-core-resource-directory].  Alternatively,
   the server MAY communicate to the client what OSCORE group to join,
   as described in Section 5.1.  Furthermore, both the clients and the
   server MAY join the OSCORE group by using the approach described in
   [I-D.ietf-ace-key-groupcomm-oscore] and based on the ACE framework
   for Authentication and Authorization in constrained environments
   [I-D.ietf-ace-oauth-authz].  Further details on how to discover the
   OSCORE group and join it are out of the scope of this specification.

   Alternative security protocols than Group OSCORE, such as OSCORE
   [RFC8613] and/or DTLS [RFC6347][I-D.ietf-tls-dtls13], can be used to
   protect other exchanges via unicast between the server and each
   client, including the original client registration (see Section 3).

5.1.  Signaling the OSCORE Group in the Informative Response

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





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   Additionally to what defined in Section 2, the CBOR map in the
   informative response payload contains the following fields, whose
   CBOR labels are defined in Section 7.

   o  '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.

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

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

   o  Optionally, 'cs_alg', with value the COSE algorithm
      [I-D.ietf-cose-rfc8152bis-algs] used to countersign 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].

   o  Optionally, 'cs_alg_crv', with value the elliptic curve (if
      applicable) for the COSE algorithm [I-D.ietf-cose-rfc8152bis-algs]
      used to countersign messages in the OSCORE group, encoded as a
      CBOR text string or integer.  The value is taken from the 'Value'
      column of the "COSE Elliptic Curve" Registry
      [COSE.Elliptic.Curves].

   o  Optionally, 'cs_key_kty', with value the COSE key type
      [I-D.ietf-cose-rfc8152bis-struct] of countersignature keys used to
      countersign messages in the OSCORE group, encoded as a CBOR text
      string or a integer.  The value is taken from the 'Value' column
      of the "COSE Key Types" Registry [COSE.Key.Types].

   o  Optionally, 'cs_key_crv', with value the elliptic curve (if
      applicable) of countersignature keys used to countersign messages
      in the OSCORE group, encoded as a CBOR text string or integer.
      The value is taken from the 'Value' column of the "COSE Elliptic
      Curve" Registry [COSE.Elliptic.Curves].

   o  Optionally, 'cs_kenc', with value the encoding of the public keys
      used in the OSCORE group, encoded as a CBOR integer.  The value is
      taken from the 'Confirmation Key' column of the "CWT Confirmation
      Method" registry defined in [RFC8747].  Future specifications may
      define additional values for this parameter.




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   o  Optionally, 'alg', with value the COSE AEAD algorithm
      [I-D.ietf-cose-rfc8152bis-algs], encoded as a CBOR text string or
      integer.  The value is taken from the 'Value' column of the "COSE
      Algorithms" Registry [COSE.Algorithms].

   o  Optionally, 'hkdf', with value the COSE HKDF algorithm
      [I-D.ietf-cose-rfc8152bis-algs], encoded as a CBOR text string or
      integer.  The value is taken from the 'Value' column of the "COSE
      Algorithms" Registry [COSE.Algorithms].

   The values of 'cs_alg', 'cs_alg_crv', 'cs_key_kty', 'cs_key_crv' and
   'cs_key_kenc' provide an early knowledge of the format and encoding
   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 public key
   in the correct expected format and encoding, at the very first step
   of the (ACE) join process.

   The values of 'cs_alg', 'alg' and 'hkdf' 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 fields
   are OPTIONAL in the informative response.  However, the 'join_uri'
   and 'sec_gp' fields MUST be present if the mechanism is implemented
   and used.  If any of the fields are present without the 'join_uri'
   and 'sec_gp' fields present, the client MUST ignore these fields,
   since they would not be sufficient to start the (ACE) join procedure.
   When this happens, the client MAY try sending a new registration
   request to the server (see Section 3.1).  Otherwise, the client
   SHOULD explicitly withdraw from the group observation.

5.2.  Server-Side Requirements

   When using Group OSCORE to protect multicast notifications, the
   server performs the operations described in Section 2, with the
   following differences.

5.2.1.  Registration

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





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

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

   o  As 'piv', the previously consumed sender sequence number value SN
      of the server in the OSCORE group, i.e. (SN* - 1).

5.2.2.  Informative Response

   The phantom observation request specified in the 'ph_req' parameter
   is protected with Group OSCORE (see Section 5.2.1).

   The multicast notification specified in the 'last_notif' parameter is
   also protected with Group OSCORE, just like for the multicast
   notifications transmitted as CoAP messages on the wire (see
   Section 5.2.3).  This applies also to the initial multicast
   notification INIT_NOTIF built in step 6 of Section 2.1.

   Optionally, the informative response includes information on the
   OSCORE group to join, as additional parameters (see Section 5.1).

5.2.3.  Notifications

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

   The process described in Section 8.3 of
   [I-D.ietf-core-oscore-groupcomm] applies, with the following
   additions when building the two OSCORE 'external_aad' to encrypt and
   countersign the multicast notification (see Sections 4.3.1 and 4.3.2
   of [I-D.ietf-core-oscore-groupcomm]).

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

   o  The 'request_piv' is the 'piv' value in the OSCORE option of the
      phantom registration request, i.e. the consumed sender sequence
      number SN of the server.





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

5.2.4.  Cancellation

   When cancelling a group observation (see Section 2.5), the phantom
   cancellation request MUST be secured, by using Group OSCORE.  In
   particular, the group mode of Group OSCORE defined in Section 8 of
   [I-D.ietf-core-oscore-groupcomm] MUST be used.

   Like defined in Section 5.2.1 for the phantom registration request,
   the server protects the phantom cancellation request as per
   Section 8.1 of [I-D.ietf-core-oscore-groupcomm], by using its Sender
   Context and consuming its current Sender Sequence number in the
   OSCORE group, from its Sender Context.  The following, corresponding
   multicast error response defined in Section 2.5 is also protected
   with Group OSCORE, as per Section 8.3 of
   [I-D.ietf-core-oscore-groupcomm].

   Note that, differently from the multicast notifications, this
   multicast error response will be the only one securely paired with
   the phantom cancellation request.

5.3.  Client-Side Requirements

   When using Group OSCORE to protect multicast notifications, the
   client performs as described in Section 3, with the following
   differences.

5.3.1.  Informative Response

   Upon receiving the informative response from the server, the client
   retrieves the phantom registration request specified in the 'ph_req'
   parameter.

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

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

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

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



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

      *  The client stores the values of the 'kid' and 'piv' fields from
         the OSCORE option of the phantom registration request.

   The client also decrypts and verifies the multicast notification
   specified in the 'last_notif' parameter, just like for the multicast
   notifications transmitted as CoAP messages on the wire (see
   Section 5.3.2).

5.3.2.  Notifications

   After having successfully processed the informative response as
   defined in Section 5.3.1, the client will decrypt and verify every
   multicast notification for the target resource as defined in
   Section 8.4 of [I-D.ietf-core-oscore-groupcomm], with the following
   difference.

   The client MUST set the two 'external_aad' defined in Sections 4.3.1
   and 4.3.2 of [I-D.ietf-core-oscore-groupcomm] as follows.  The
   particular way to achieve this is implementation specific.

   o  'request_kid' takes the value of the 'kid' field from the OSCORE
      option of the phantom registration request (see Section 5.3.1).

   o  'request_piv' takes the value of the 'piv' field from the OSCORE
      option of the phantom registration request (see Section 5.3.1).

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

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

6.  Example with Group OSCORE

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




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   In the informative responses, 'bstr(X)' denotes a byte string with
   value the byte serialization of X.  Also, the notation Y.CoAP denotes
   the CoAP-layer part of a message Y, i.e. the part of Y that becomes
   payload for the transport layer underlying CoAP.

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

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

   o  C_1 and S have a pairwise OSCORE Security Context.  In particular,
      C_1 has 'kid' = 1 as Sender ID, and SN_1 = 101 as Sequence Number.
      Also, S has 'kid' = 3 as Sender ID, and SN_3 = 301 as Sequence
      Number.

   o  C_2 and S have a pairwise OSCORE Security Context.  In particular,
      C_2 has 'kid' = 2 as Sender ID, and SN_2 = 201 as Sequence Number.
      Also, S has 'kid' = 4 as Sender ID, and SN_4 = 401 as Sequence
      Number.

   o  S is a member of the OSCORE group with name "myGroup", and
      'kid_context' = "feedca57ab2e" as Group ID.  In the OSCORE group,
      S has 'kid' = 5 as Sender ID, and SN_5 = 501 as Sequence Number.

  C_1     --------------- [ Unicast w/ OSCORE ]  ----------------> S  /r
   |  GET                                                          |
   |  Token: 0x4a                                                  |
   |  Observe: 0 (Register)                                        |
   |  OSCORE: {kid: 1 ; piv: 101 ; ...}                            |
   |                                                               |
   |               (S allocates the available Token value 0xff .)  |
   |                                                               |
   |       (S sends to itself a phantom observation request PH_REQ |
   |        as coming from the IP multicast address GROUP_ADDR .)  |
   |     -------------------------------------------------------   |
   |    /                                                          |
   |    \--------------------------------------------------------> |  /r
   |                             GET                               |
   |                             Token: 0xff                       |
   |                             Observe: 0 (Register)             |
   |                             OSCORE: {kid: 5 ; piv: 501 ; ...} |
   |                                                               |
   |    (S steps SN_5 in the Group OSCORE Sec. Ctx : SN_5 <== 502) |
   |                                                               |
   |                      (S creates a group observation of /r .)  |



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   |                                                               |
   |                           (S increments the observer counter  |
   |                            for the group observation of /r .) |
   |                                                               |
   |                                                               |
  C_1 <---------------- [ Unicast w/ OSCORE ] ----------------     S
   |  5.03                                                         |
   |  Token: 0x4a                                                  |
   |  OSCORE: {piv: 301; ...}                                      |
   |  Payload: { ph_req     : bstr(PH_REQ.CoAP),                   |
   |             last_notif : bstr(LAST_NOTIF.CoAP)                |
   |             cl_addr    : bstr(GROUP_ADDR),                    |
   |             cl_port    : GROUP_PORT,                          |
   |             srv_addr   : bstr(SERVER_ADDR),                   |
   |             srv_port   : SERVER_PORT,                         |
   |             join_uri   : "coap://myGM/group-oscore/myGroup",  |
   |             sec_gp     : "myGroup"                            |
   |           }                                                   |
   |                                                               |
   |                                                               |
  C_2     --------------- [ Unicast w/ OSCORE ]  ----------------> S  /r
   |  GET                                                          |
   |  Token: 0x01                                                  |
   |  Observe: 0 (Register)                                        |
   |  OSCORE: {kid: 2 ; piv: 201 ; ...}                            |
   |                                                               |
   |                           (S increments the observer counter  |
   |                            for the group observation of /r .) |
   |                                                               |
  C_2 <------------------ [ Unicast w/ OSCORE ] --------------     S
   |  5.03                                                         |
   |  Token: 0x01                                                  |
   |  OSCORE: {piv: 401; ...}                                      |
   |  Payload: { ph_req     : bstr(PH_REQ.CoAP),                   |
   |             last_notif : bstr(LAST_NOTIF.CoAP)                |
   |             cl_addr    : bstr(GROUP_ADDR),                    |
   |             cl_port    : GROUP_PORT,                          |
   |             srv_addr   : bstr(SERVER_ADDR),                   |
   |             srv_port   : SERVER_PORT,                         |
   |             join_uri   : "coap://myGM/group-oscore/myGroup",  |
   |             sec_gp     : "myGroup"                            |
   |           }                                                   |
   |                                                               |
   |                                                               |
   |          (The value of the resource /r changes to "5678".)    |
   |                                                               |
  C_1                                                              |
   +  <-------------- [ Multicast w/ Group OSCORE ] ----------     S



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  C_2     (Destination address/port: GROUP_ADDR/GROUP_PORT)        |
   |  2.05                                                         |
   |  Token: 0xff                                                  |
   |  Observe: 11                                                  |
   |  OSCORE: {kid: 5; piv: 502 ; ...}                             |
   |  Payload: "5678"                                              |
   |                                                               |

   The two external_aad used to encrypt and countersign the multicast
   notification above have 'req_kid' = 5 and 'req_iv' = 501.  These are
   indicated in the 'kid' and 'iv' field of the OSCORE option of the
   phantom observation request, which is included in the 'ph_req'
   parameter of the unicast informative response to the two clients.
   Thus, the two clients can build the two same external_aad for
   decrypting and verifying this multicast notification and the
   following ones.

7.  Informative Response Parameters

   This specification defines a number of fields used in error messages
   as informative response defined in Section 2.2 of this specification.

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

























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        +------------+----------+-------------------+-------------+
        | Name       | CBOR Key | CBOR Type         | Reference   |
        +------------+----------+-------------------+-------------+
        | ph_req     | TBD      | byte string       | Section 2.2 |
        |            |          |                   |             |
        | last_notif | TBD      | byte string       | Section 2.2 |
        |            |          |                   |             |
        | cli_addr   | TBD      | byte string       | Section 2.2 |
        |            |          |                   |             |
        | cli_port   | TBD      | unsigned int      | Section 2.2 |
        |            |          |                   |             |
        | srv_addr   | TBD      | byte string       | Section 2.2 |
        |            |          |                   |             |
        | srv_port   | TBD      | unsigned int      | Section 2.2 |
        |            |          |                   |             |
        | join_uri   | TBD      | text string       | Section 5.1 |
        |            |          |                   |             |
        | sec_gp     | TBD      | text string       | Section 5.1 |
        |            |          |                   |             |
        | as_uri     | TBD      | text string       | Section 5.1 |
        |            |          |                   |             |
        | cs_alg     | TBD      | int / text string | Section 5.1 |
        |            |          |                   |             |
        | cs_crv     | TBD      | int / text string | Section 5.1 |
        |            |          |                   |             |
        | cs_kty     | TBD      | int / text string | Section 5.1 |
        |            |          |                   |             |
        | cs_kenc    | TBD      | int               | Section 5.1 |
        |            |          |                   |             |
        | alg        | TBD      | int / text string | Section 5.1 |
        |            |          |                   |             |
        | hkdf       | TBD      | int / text string | Section 5.1 |
        +------------+----------+-------------------+-------------+

8.  Security Considerations

   The same security considerations from [RFC7252][RFC7641][I-D.ietf-cor
   e-groupcomm-bis][RFC8613][I-D.ietf-core-oscore-groupcomm] hold for
   this document.

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



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

9.  IANA Considerations

   This document has the following actions for IANA.

9.1.  Media Type Registrations

   This specification registers the media type 'application/informative-
   response+cbor' for error messages as informative response defined in
   Section 2.2 of this specification, when carrying parameters encoded
   in CBOR.  This registration follows the procedures specified in
   [RFC6838].

   o  Type name: application

   o  Subtype name: informative-response+cbor

   o  Required parameters: none

   o  Optional parameters: none

   o  Encoding considerations: Must be encoded as a CBOR map containing
      the parameters defined in Section 2.2 of [this document].

   o  Security considerations: See Section 8 of [this document].

   o  Interoperability considerations: n/a

   o  Published specification: [this document]

   o  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 2.2 of [this document].

   o  Additional information:

      *  Magic number(s): n/a

      *  File extension(s): .informative-response

      *  Macintosh file type code(s): n/a

   o  Person & email address to contact for further information:
      iesg@ietf.org [1]




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   o  Intended usage: COMMON

   o  Restrictions on usage: None

   o  Author: Marco Tiloca marco.tiloca@ri.se [2]

   o  Change controller: IESG

   o  Provisional registration? (standards tree only): No

9.2.  CoAP Content-Formats Registry

   IANA is asked to add the following entry to the "CoAP Content-
   Formats" subregistry defined in Section 12.3 of [RFC7252], within the
   "Constrained RESTful Environments (CoRE) Parameters" registry.

   Media Type: application/informative-response+cbor

   Encoding: -

   ID: TBD

   Reference: [this document]

9.3.  Informative Response Parameters Registry

   This specification establishes the "Informative Response Parameters"
   IANA Registry.  The Registry has been created to use the "Expert
   Review Required" registration procedure [RFC8126].  Expert review
   guidelines are provided in Section 9.5.

   The columns of this Registry are:

   o  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.

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

   o  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.

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




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   This Registry has been initially populated by the values in
   Section 7.  The "Reference" column for all of these entries refers to
   sections of this document.

9.4.  CoAP Option Numbers Registry

   IANA is asked to enter the following option numbers to the "CoAP
   Option Numbers" registry defined in [RFC7252] within the "CoRE
   Parameters" registry.

   +--------+--------------------------------------+-------------------+
   | Number |                 Name                 |     Reference     |
   +--------+--------------------------------------+-------------------+
   |  TBD   |  Multicast-Response-Feedback-Divider | [[this document]] |
   +--------+--------------------------------------+-------------------+

9.5.  Expert Review Instructions

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

   Expert reviewers should take into consideration the following points:

   o  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.

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

   o  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.

10.  References

10.1.  Normative References

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

   [COSE.Elliptic.Curves]
              IANA, "COSE Elliptic Curves",
              <https://www.iana.org/assignments/cose/
              cose.xhtml#elliptic-curves>.

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

   [I-D.ietf-core-groupcomm-bis]
              Dijk, E., Wang, C., and M. Tiloca, "Group Communication
              for the Constrained Application Protocol (CoAP)", draft-
              ietf-core-groupcomm-bis-00 (work in progress), March 2020.

   [I-D.ietf-core-oscore-groupcomm]
              Tiloca, M., Selander, G., Palombini, F., and J. Park,
              "Group OSCORE - Secure Group Communication for CoAP",
              draft-ietf-core-oscore-groupcomm-09 (work in progress),
              June 2020.

   [I-D.ietf-cose-rfc8152bis-algs]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Initial Algorithms", draft-ietf-cose-rfc8152bis-algs-11
              (work in progress), July 2020.

   [I-D.ietf-cose-rfc8152bis-struct]
              Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", draft-ietf-cose-rfc8152bis-
              struct-11 (work in progress), July 2020.

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



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

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

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

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

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

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

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

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

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

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





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10.2.  Informative References

   [I-D.ietf-ace-key-groupcomm-oscore]
              Tiloca, M., Park, J., and F. Palombini, "Key Management
              for OSCORE Groups in ACE", draft-ietf-ace-key-groupcomm-
              oscore-07 (work in progress), June 2020.

   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) using the OAuth 2.0
              Framework (ACE-OAuth)", draft-ietf-ace-oauth-authz-35
              (work in progress), June 2020.

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

   [I-D.ietf-core-resource-directory]
              Shelby, Z., Koster, M., Bormann, C., Stok, P., and C.
              Amsuess, "CoRE Resource Directory", draft-ietf-core-
              resource-directory-24 (work in progress), March 2020.

   [I-D.ietf-tls-dtls13]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-38 (work in progress), May
              2020.

   [I-D.tiloca-core-oscore-discovery]
              Tiloca, M., Amsuess, C., and P. Stok, "Discovery of OSCORE
              Groups with the CoRE Resource Directory", draft-tiloca-
              core-oscore-discovery-05 (work in progress), March 2020.

   [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>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.




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   [RFC8747]  Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
              Tschofenig, "Proof-of-Possession Key Semantics for CBOR
              Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March
              2020, <https://www.rfc-editor.org/info/rfc8747>.

10.3.  URIs

   [1] mailto:iesg@ietf.org

   [2] mailto:marco.tiloca@ri.se

Appendix A.  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 2.5.1.

A.1.  Client Side

































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

   output: None


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

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

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

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

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

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

       req.send(SERVER_ADDR, SERVER_PORT);
   }


A.2.  Server Side

















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

   output: int N // Updated observer counter


   int Q = ceil(N / M);
   Option opt = new Option(MRFD);
   opt.set(Q);

   notification.setOption(opt);
   <Finalize the notification message>
   notification.send(GROUP_ADDR, GROUP_PORT);

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

   // Time t2

   int R = <number of requests to the resource between t1 and t2,
            with the MRFD option>;
   int X = <number of requests to the resource between t1 and t2,
            without the MRFD option>;

   int N = (R * Q) + X;
   return N;

Appendix B.  Different Sources for Group Observation Data

   While the clients usually receive the phantom request and other
   information related to the group observation through an Informative
   Response, the same data can be made available through different
   services, such as the following ones.

B.1.  PubSub

   In a pubsub case ([I-D.ietf-core-coap-pubsub]), a group observation
   can be discovered, along with topic metadata.  For instance, a
   discovery step can make the following metadata available.

   This examples assumes a CoRAL namespace that contains properties
   analogous to those in the content-format application/informative-
   response+cbor.





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   Request:

       GET </ps/topics?rt=oic.r.temperature>
       Accept: CoRAL

   Response:

       2.05 Content
       Content-Format: CoRAL

       rdf:type <http://example.org/pubsub/topic-list>
       topic </ps/topics/1234> {
           ph_req h"120100006464b431323334"
           last_notif h"120100006464b431324321"
           cli_addr h"ff35003020010db8..1234"
           cli_port 5683
           srv_addr h"20010db80100..0001"
           srv_port 5683
       }

   With this information from the topic discovery step, the client can
   already set up its multicast address and start receiving multicast
   notifications.

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

B.2.  Sender Introspection

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

   Such an interface is left for other documents to specify on demand,
   but could look like this as relying on the already known token value
   of multicast notifications associated to a phantom request:












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   Request:

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

   Response:

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

       {
           'ph_req': h"120100006464b431323334"
           'last_notif' : h"120100006464b431324321"
           'cli_addr': h"ff35003020010db8..1234"
           'cli_port': 5683
           'srv_addr': h"20010db80100..0001"
           'srv_port': 5683
       }

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

Acknowledgments

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

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

Authors' Addresses

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

   Email: marco.tiloca@ri.se










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   Rikard Hoeglund
   RISE AB
   Isafjordsgatan 22
   Kista  SE-16440 Stockholm
   Sweden

   Email: rikard.hoglund@ri.se


   Christian Amsuess
   Hollandstr. 12/4
   Vienna  1020
   Austria

   Email: christian@amsuess.com


   Francesca Palombini
   Ericsson AB
   Torshamnsgatan 23
   Kista  SE-16440 Stockholm
   Sweden

   Email: francesca.palombini@ericsson.com



























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