Observe Notifications as CoAP Multicast Responses

Observe Notifications as CoAP Multicast Responses RISE AB

Isafjordsgatan 22 Kista 16440 Stockholm Sweden marco.tiloca@ri.se

RISE AB

Isafjordsgatan 22 Kista 16440 Stockholm Sweden rikard.hoglund@ri.se

Hollandstr. 12/4 Vienna 1020 Austria christian@amsuess.com

Ericsson AB

Torshamnsgatan 23 Kista 16440 Stockholm Sweden francesca.palombini@ericsson.com

Internet CoRE Working Group Internet-Draft The Constrained Application Protocol (CoAP) allows clients to "observe" resources at a server, and receive notifications as unicast responses upon changes of the resource state. In some use cases, such as based on publish-subscribe, it would be convenient for the server to send a single notification addressed to all the clients observing a same target resource. This document updates RFC7252 and RFC7641, and defines how a server sends observe notifications as response messages over multicast, synchronizing all the observers of a same resource on a same shared Token value. Besides, this document defines how Group OSCORE can be used to protect multicast notifications end-to-end between the server and the observer clients. Discussion Venues Discussion of this document takes place on the Constrained RESTful Environments Working Group mailing list (core@ietf.org), which is archived at . Source for this draft and an issue tracker can be found at .

Introduction The Constrained Application Protocol (CoAP) has been extended with a number of mechanisms, including resource Observation . 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 . 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 , 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 . For example, multiple clients can benefit of observation for discovering (to-be-created) OSCORE groups , by retrieving from the Resource Directory updated links and descriptions to join them through the respective Group Manager . More in general, multicast notifications would be beneficial whenever several CoAP clients observe a same target resource at a CoAP server, and can be all notified at once by means of a single response message. However, CoAP does not currently define response messages over IP multicast. This document fills this gap and provides the following twofold contribution. First, it updates and , by defining a method to deliver Observe notifications as CoAP responses addressed to multiple clients, e.g., over IP multicast. In the proposed method, the group of potential observers entrusts the server to manage the Token space for multicast notifications. By doing so, the server provides all the observers of a target resource with the same Token value to bind to their own observation. That Token value is then used in every multicast notification for the target resource. This is achieved by means of an informative unicast response sent by the server to each observer client. Second, this document defines how to use Group OSCORE 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.

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 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 , group communication for CoAP , Observe , CBOR , OSCORE , and Group OSCORE . This document additionally defines the following terminology.

Traditional observation. A resource observation associated with a single observer client, as defined in .
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 or cancel 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.

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 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 . This allows the server to possibly terminate a group observation in case, at some point in time, not enough clients are estimated to be still active and interested. This document does not describe a way for the client to influence the server's decision to start group observations. That is done on purpose: the specified mechanism is expected to be used in situations where sending individual notifications is not feasible, or not preferred beyond a certain number of clients observing a target resource. If applications arise where negotiation does make sense, they are welcome to specify additional means to opt in to multicast notifications.

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

The server builds a phantom observation request, i.e., a GET request with an Observe option set to 0 (register).
The server selects an available value T, from the Token space of a CoAP endpoint used for messages having:
- As source address and port number, the IP multicast address GRP_ADDR and port number GRP_PORT.
- As destination address and port number, the server address SRV_ADDR and port number SRV_PORT, intended for accessing the target resource.
This Token space is under exclusive control of the server.
The server processes the phantom observation request above, without transmitting it on the wire. The request is addressed to the resource for which the server wants to start the group observation, as if sent by the group of observers, i.e., with GRP_ADDR as source address and GRP_PORT as source port.
Upon processing the self-generated phantom registration request, the server interprets it as an observe registration received from the group of potential observer clients. In particular, from then on, the server MUST use T as its own local Token value associated with that observation, with respect to the (previous hop towards the) clients.
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.
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 ) 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.

Informative Response After having started a group observation on a target resource, the server proceeds as follows. For each traditional observation ongoing on the target resource, the server MAY cancel that observation. Then, the server considers the corresponding clients as now taking part in the group observation, for which it increases the corresponding observer counter accordingly. The server sends to each of such clients an informative response message, encoded as a unicast response with response code 5.03 (Service Unavailable). As per , such a response does not include an Observe option. The response MUST be Confirmable and MUST NOT encode link-local addresses. The Content-Format of the informative response is set to application/informative-response+cbor, defined in . The payload of the informative response is a CBOR map including the following parameters, whose CBOR labels are defined in .

'tp_info', with value a CBOR array. This includes the transport-specific information required to correctly receive multicast notifications bound to the phantom observation request. Typically, this comprises the Token value associated with the group observation, as well as the source and destination addressing information of the related multicast notifications. The CBOR array is formatted as defined in . This parameter MUST be included.
'ph_req', with value the byte serialization of the transport-independent information of the phantom observation request (see ), encoded as a CBOR byte string. The value of the CBOR byte string is formatted as defined in . This parameter MAY be omitted, in case the phantom request is, in terms of transport-independent information, identical to the registration request from the client. Otherwise, this parameter MUST be included. 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 . This parameter MAY be included.
'next_not_before', with value the amount of seconds that will minimally elapse before the server sends the next multicast notification for the group observation of the target resource, encoded as a CBOR unsigned integer. This parameter MAY be included. This information can help a new client to align itself with the server's timeline, especially in scenarios where multicast notifications are regularly sent. Also, it can help synchronizing different clients when orchestrating a content distribution through multicast notifications.

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

Format of the informative response payload array, ; 'tp_info', i.e., transport-specific information ? 1 => bstr, ; 'ph_req' (transport-independent information) ? 2 => bstr ; 'last_notif' (transport-independent information) ? 3 => uint ; 'next_not_before' } ]]> Upon receiving a registration request to observe the target resource, the server does not create a corresponding individual observation for the requesting client. Instead, the server considers that client as now taking part in the group observation of the target resource, of which it increments the observer counter by 1. Then, the server replies to the client with the same informative response message defined above, which MUST be Confirmable. Note that this also applies when, with no ongoing traditional observations on the target resource, the server receives a registration request from a first client and decides to start a group observation on the target resource.

Encoding of Transport-Specific Message Information [ This encoding might be replaced by CRIs in a later version of this document. ] The CBOR array specified in the 'tp_info' parameter is formatted according to the following CDDL notation.

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

'tp_id' : this element is a CBOR integer, which specifies the transport protocol used to transport the CoAP response from the server, i.e., a multicast notification in this document. This element takes value from the "Value" column of the "CoAP Transport Information" registry defined in of this document. This element MUST be present. The value of this element determines:
- How many elements are required to follow in 'srv_addr', as well as what information they convey, their encoding and their semantics.
- How many elements are required in the 'req_info' element of the 'tp_info' array, as well as what information they convey, their encoding and their semantics.
This document registers the integer value 1 ("UDP") to be used as value for the 'tp_id' element, when CoAP responses are transported over UDP. In such a case, the full encoding of the 'tp_info' CBOR array is as defined in . Future specifications that consider CoAP multicast notifications transported over different transport protocols MUST:
- Register an entry with an integer value to be used for 'tp_id', in the "CoAP Transport Information" registry defined in of this document.
- Accordingly, define the elements of the 'tp_info' CBOR array, i.e., the elements following 'tp_id' in 'srv_addr' as well as the elements in 'req_info', as to what information they convey, their encoding and their semantics.

The 'req_info' element of 'tp_info' specifies transport-specific information related to a pertinent request message, i.e., the phantom observation request in this document. The exact format of 'req_info' depends on the value of 'tp_id'. Given a specific value of 'tp_id', the complete set of elements composing 'srv_addr' and 'req_info' in the 'tp_info' CBOR array is indicated by the two columns "Srv Addr" and "Req Info" of the "CoAP Transport Information" registry defined in , respectively.

UDP Transport-Specific Information When CoAP multicast notifications are transported over UDP as per and , the server specifies the integer value 1 ("UDP") as value of 'tp_id' in the 'srv_addr' element of the 'tp_info' CBOR array in the error informative response. Then, the rest of the 'tp_info' CBOR array is defined as follows.

'srv_addr' includes two more elements following 'tp_id':
- 'srv_host': this element is a CBOR byte string, with value the destination IP address of the phantom observation request. This parameter is tagged and identified by the CBOR tag 260 "Network Address (IPv4 or IPv6 or MAC Address)". That is, the value of the CBOR byte string is the IP address SRV_ADDR of the server hosting the target resource, from where the server will send multicast notifications for the target resource. This element MUST be present.
- 'srv_port': this element is a CBOR unsigned integer, with value the destination port number of the phantom observation request. That is, the specified value is the port number SRV_PORT, from where the server will send multicast notifications for the target resource. This element MUST be present.
'req_info' includes the following elements:
- 'token': this element is a CBOR byte string, with value the Token value of the phantom observation request generated by the server (see ). Note that the same Token value is used for the multicast notifications bound to that phantom observation request (see ). This element MUST be present.
- 'cli_addr': this element is a CBOR byte string, with value the source IP address of the phantom observation request. This parameter is tagged and identified by the CBOR tag 260 "Network Address (IPv4 or IPv6 or MAC Address)". That is, the value of the CBOR byte string is the IP multicast address GRP_ADDR, where the server will send multicast notifications for the target resource. This element MUST be present.
- 'cli_port': this element is a CBOR unsigned integer, with value the source port number of the phantom observation request. That is, the specified value is the port number GRP_PORT, where the server will send multicast notifications for the target resource. This element is OPTIONAL. If not included, the default port number 5683 is assumed.

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

Format of 'tp_info' with UDP as transport protocol

Encoding of Transport-Independent Message Information For both the parameters 'ph_req' and 'last_notif' in the informative response, the value of the byte string is the concatenation of the following components, in the order specified below. When defining the value of each component, "CoAP message" refers to the phantom observation request for the 'ph_req' parameter, and to the corresponding latest multicast notification for the 'last_notif' parameter.

A single byte, with value the content of the Code field in the CoAP message.
The byte serialization of the complete sequence of CoAP options in the CoAP message.
If the CoAP message includes a non-zero length payload, the one-byte Payload Marker (0xff) followed by the payload.

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

It MUST be Non-confirmable.
It MUST include an Observe option, as per .
It MUST have the same Token value T of the phantom registration request that started the group observation. This Token value is specified in the 'token' element of 'req_info' under the 'tp_info' parameter, in the informative response message sent to all the observer clients. That is, every multicast notification for a target resource is not bound to the observation requests from the different clients, but rather to the phantom registration request associated with the whole set of clients taking part in the group observation of that resource.
It MUST be sent from the same IP address SRV_ADDR and port number SRV_PORT where: i) the original Observe registration requests are sent to by the clients; and ii) the corresponding informative responses are sent from by the server (see ). These are indicated to the observer clients as value of the 'srv_host' and 'srv_port' elements of 'srv_addr' under the 'tp_info' parameter, in the informative response message (see ). That is, redirection MUST NOT be used.
It MUST be sent to the IP multicast address GRP_ADDR and port number GRP_PORT. These are indicated to the observer clients as value of the 'cli_addr' and 'cli_port' elements of 'req_info' under the 'tp_info' parameter, in the informative response message (see ).

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

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

The multicast notifications MUST be Non-confirmable.
In constrained environments such as low-power, lossy networks (LLNs), the server should only support multicast notifications for resources that are small. Following related guidelines from , 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 ) is used.
The server SHOULD provide multicast notifications with the smallest possible IP multicast scope that fulfills the application needs. For example, following related guidelines from , 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 , 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 ). The transmission rate of multicast notifications should also take into account the avoidance of a possible "broadcast storm" problem . This prevents a following, considerable increase of the channel load, whose origin would be likely attributed to a router rather than the server.

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 . In order to cancel the group observation, the server sends a multicast response with response code 5.03 (Service Unavailable), signaling that the group observation has been terminated. The response has the same Token value T of the phantom registration request, it has no payload, and it does not include an Observe option. The server sends the response to the same multicast IP address GRP_ADDR and port number GRP_PORT used to send the multicast notifications related to the target resource. Finally, the server releases the resources allocated for the group observation, and especially frees up the Token value T used at its CoAP endpoint.

Client-Side Requirements

Request A client sends an observation request to the server as described in , 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 . In a particular setup, the information typically specified in the 'tp_info' parameter of the informative response (see ) can be preconfigured 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 ). In this case, the client can include a No-Response option 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. This still helps the server to assess the current number of clients interested in a group observation (e.g., by using the method defined in ), which in turn can play a role in deciding to cancel the group observation.

Informative Response Upon receiving the informative response defined in , the client proceeds as follows.

The client configures an observation of the target resource. To this end, it relies on a CoAP endpoint used for messages having:
- As source address and port number, the server address SRV_ADDR and port number SRV_PORT intended for accessing the target resource. These are specified as value of the 'srv_host' and 'srv_port' elements of 'srv_addr' under the 'tp_info' parameter, in the informative response (see ).
- As destination address and port number, the IP multicast address GRP_ADDR and port number GRP_PORT. These are specified as value of the 'cli_addr' and 'cli_port' elements of 'req_info' under the 'tp_info' parameter, in the informative response (see ). If the 'cli_port' element is omitted in 'req_info', the client MUST assume the default port number 5683 as GRP_PORT.
The client rebuilds the phantom registration request as follows.
- The client uses the Token value T, specified in the 'token' element of 'req_info' under the 'tp_info' parameter of the informative response.
- If the 'ph_req' parameter is not present in the informative response, the client uses the transport-independent information from its original Observe registration request.
- If the 'ph_req' parameter is present in the informative response, the client uses the transport-independent information specified in the parameter.
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. In case no such response is available, the client SHOULD explicitly withdraw from the group observation.
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.
If the informative response includes the parameter 'last_notif', the client rebuilds the latest multicast notification, by using:
- The transport-independent information, specified in the 'last_notif' parameter of the informative response.
- The Token value T, specified in the 'token' element of 'req_info' under the 'tp_info' parameter of the informative response.
If the informative response includes the parameter 'last_notif', the client processes the multicast notification rebuilt at step 5 as defined in . In particular, the value of the Observe option is used as initial baseline for notification reordering in this group observation.
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 in the informative response are not present or malformed, the client MAY try sending a new registration request to the server (see ). Otherwise, the client SHOULD explicitly withdraw from the group observation. describes possible alternative ways for clients to retrieve the phantom registration request and other information related to a group observation.

Notifications After having successfully processed the informative response as defined in , 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 .

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 . 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 . In case the server has canceled a group observation as defined in , 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 .

Web Linking The possible use of multicast notifications in a group observation may be indicated by a target "grp_obs" attribute in a web link to a resource, e.g., using a link-format document . The "grp_obs" attribute is a hint, indicating that the server might send multicast notifications for observations of the resource targeted by the link. Note that this is simply a hint, i.e., it does not include any information required to participate in a group observation, and to receive and process multicast notifications. A value MUST NOT be given for the "grp_obs" attribute; any present value MUST be ignored by parsers. The "grp_obs" attribute MUST NOT appear more than once in a given link-value; occurrences after the first MUST be ignored by parsers. The example in shows a use of the "grp_obs" attribute: the client does resource discovery on a server and gets back a list of resources, one of which includes the "grp_obs" attribute indicating that the server might send multicast notifications for observations of that resource. The link-format notation (see ) is used.

The Web Link ;grp_obs, ;if="sensor" ]]>

Example The following example refers to two clients C_1 and C_2 that register to observe a resource /r at a Server S, which has address SRV_ADDR and listens to the port number SRV_PORT. Before the following exchanges occur, no clients are observing the resource /r , which has value "1234". The server S sends multicast notifications to the IP multicast address GRP_ADDR and port number GRP_PORT, and starts the group observation upon receiving a registration request from a first client that wishes to start a traditional observation on the resource /r. The following notation is used for the payload of the informative responses:

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

Example of group observation S /r | GET | | Token: 0x4a | | Observe: 0 (Register) | | | | | | (S allocates the available Token value 0x7b .) | | | | (S sends to itself a phantom observation request PH_REQ | | as coming from the IP multicast address GRP_ADDR .) | | ------------------------------------------------ | | / | | \----------------------------------------------------> | /r | GET | | Token: 0x7b | | Observe: 0 (Register) | | | | | | (S creates a group observation of /r .) | | | | (S increments the observer counter | | for the group observation of /r .) | | | C_1 <-------------------- [ Unicast ] --------------------- S | 5.03 | | Token: 0x4a | | Content-Format: application/informative-response+cbor | | Max-Age: 0 | | | | Payload: { | | tp_info : [1, bstr(SRV_ADDR), SRV_PORT, | | 0x7b, bstr(GRP_ADDR), GRP_PORT], | | last_notif : bstr(0x45 | OPT | 0xff | PAYLOAD) | | } | | | C_2 ----------------- [ Unicast ] ------------------------> S /r | GET | | Token: 0x01 | | Observe: 0 (Register) | | | | | | (S increments the observer counter | | for the group observation of /r .) | | | C_2 <-------------------- [ Unicast ] --------------------- S | 5.03 | | Token: 0x01 | | Content-Format: application/informative-response+cbor | | Max-Age: 0 | | | | Payload: { | | tp_info : [1, bstr(SRV_ADDR), SRV_PORT, | | 0x7b, bstr(GRP_ADDR), GRP_PORT], | | last_notif : bstr(0x45 | OPT | 0xff | PAYLOAD) | | } | | | | (The value of the resource /r changes to "5678".) | | | C_1 | + <------------------- [ Multicast ] -------------------- S C_2 (Destination address/port: GRP_ADDR/GRP_PORT) | | 2.05 | | Token: 0x7b | | Observe: 11 | | Content-Format: application/cbor | | | | Payload: : "5678" | | | ]]>

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.

Multicast-Response-Feedback-Divider Option In order to enable the rough counting of still active and interested clients, a new CoAP option is introduced, which SHOULD be supported by clients that listen to multicast responses. The option is called Multicast-Response-Feedback-Divider. As summarized in , the option is not Critical, not Safe-to-Forward, and integer valued. Since the option is not Safe-to-Forward, the column "N" indicates a dash for "not applicable".

Multicast-Response-Feedback-Divider The Multicast-Response-Feedback-Divider option is of class E for OSCORE .

Processing on the Client Side Upon receiving a response with a Multicast-Response-Feedback-Divider option, a client SHOULD acknowledge its interest in continuing receiving multicast notifications for the target resource, as described below. The client picks an integer random number I, from 0 inclusive to the number Z = (2 ** Q) exclusive, where Q is the value specified in the option and "**" is the exponentiation operator. If I is different than 0, the client takes no further action. Otherwise, the client should wait a random fraction of the Leisure time (see ), and then registers a regular unicast observation on the same target resource. To this end, the client essentially follows the steps that got it originally subscribed to group notifications for the target resource. In particular, the client sends an observation request to the server, i.e., a GET request with an Observe option set to 0 (register). The request MUST be addressed to the same target resource, and MUST have the same destination IP address and port number used for the original registration request, regardless the source IP address and port number of the received multicast notification. Since the Observe registration is only done for its side effect of showing as an attempted observation at the server, the client MUST send the unicast request in a non confirmable way, and with the maximum No-Response setting . 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 ). 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. and provide a description in pseudo-code of the operations above performed by the client.

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 ), for instance by using the method described below.

Request for Feedback When it wants to obtain a new estimated count, the server considers a number M of confirmations it would like to receive from the clients. It is up to applications to define policies about how the server determines and possibly adjusts the value of M. Then, the server computes the value Q = max(L, 0), where:

L is computed as L = ceil(log2(N / M)).
N is the current value of the observer counter, possibly rounded up to 1, i.e., N = max(COUNT, 1).

Finally, the server sets Q as the value of the Multicast-Response-Feedback-Divider option, which is sent within a successful multicast notification. If several multicast notifications are sent in a burst fashion, it is RECOMMENDED for the server to include the Multicast-Response-Feedback-Divider option only in the first one of those notifications.

Collection of Feedback The server collects unicast observation requests from the clients, for an amount of time of MAX_CONFIRMATION_WAIT seconds. During this time, the server regularly increments the observer counter when adding a new client to the group observation (see ). 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 and has default value 202 seconds, while MAX_CLIENT_REQUEST_DELAY is equivalent to MAX_SERVER_RESPONSE_DELAY defined in 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.

Processing of Feedback Once MAX_CONFIRMATION_WAIT seconds have passed, the server counts the R confirmations arrived as unicast observation requests to the target resource, since the multicast notification with the Multicast-Response-Feedback-Divider option has been sent. In particular, the server considers a unicast observation request as a confirmation from a client only if it includes a Multicast-Response-Feedback-Divider option with an empty value (Option Length = 0). Then, the server computes a feedback indicator as E = R * (2 ** Q), where "**" is the exponentiation operator. According to what defined by application policies, the server determines the next time when to ask clients for their confirmation, e.g., after a certain number of multicast notifications has been sent. For example, the decision can be influenced by the reception of no confirmations from the clients, i.e., R = 0, or by the value of the ratios (E/N) and (N/E). Finally, the server computes a new estimated count of the observers. To this end, the server first consider COUNT' as the current value of the observer counter at this point in time. Note that COUNT' may be greater than the value COUNT used at the beginning of this process, if the server has incremented the observer counter upon adding new clients to the group observation (see ). In particular, the server computes the new estimated count value as COUNT' + ((E - N) / D), where D > 0 is an integer value used as dampener. This step has to be performed atomically. That is, until this step is completed, the server MUST hold the processing of an observation request for the same target resource from a new client. Finally, the server considers the result as the current observer counter, and assesses it for possibly canceling the group observation (see ). This estimate is skewed by packet loss, but it gives the server a sufficiently good estimation for further counts and for deciding when to cancel the group observation. It is up to applications to define policies about how the server takes the newly updated estimate into account and determines whether to cancel the group observation. As an example, if the server currently estimates that N = COUNT = 32 observers are active and considers a constant M = 8, it sends out a notification with Multicast-Response-Feedback-Divider: 2. Then, out of 18 actually active clients, 5 send a re-registration request based on their random draw, of which one request gets lost, thus leaving 4 re-registration requests received by the server. Also, no new clients have been added to the group observation during this time, i.e., COUNT' is equal to COUNT. As a consequence, assuming that a dampener value D = 1 is used, the server computes the new estimated count value as 32 + (16 - 32) = 16, and keeps the group observation active. To produce a most accurate updated counter, a server can include a Multicast-Response-Feedback-Divider option with value Q = 0 in its multicast notifications, as if M is equal to N. This will trigger all the active clients to state their interest in continuing receiving notifications for the target resource. Thus, the amount R of arrived confirmations is affected only by possible packet loss. 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.

Protection of Multicast Notifications with Group OSCORE A server can protect multicast notifications by using Group OSCORE , thus ensuring they are protected end-to-end with the observer clients. This requires that both the server and the clients interested in receiving multicast notifications from that server are members of the same OSCORE group. In some settings, the OSCORE group to refer to can be pre-configured on the clients and the server. In such a case, a server which is aware of such pre-configuration can simply assume a client to be already member of the correct OSCORE group. In any other case, the server MAY communicate to clients what OSCORE group they are required to join, by providing additional guidance in the informative response as described in . Note that clients can already be members of the right OSCORE group, in case they have previously joined it to securely communicate with the same server and/or with other servers to access their resources. Both the clients and the server MAY join the OSCORE group by using the approach described in and based on the ACE framework for Authentication and Authorization in constrained environments . Further details on how to discover the OSCORE group and join it are out of the scope of this document. If multicast notifications are protected using Group OSCORE, the original registration requests and related unicast (notification) responses MUST also be secured, including and especially the informative responses from the server. To this end, alternative security protocols than Group OSCORE, such as OSCORE and/or DTLS , can be used to protect other exchanges via unicast between the server and each client, including the original client registration (see ).

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 . This mechanism is OPTIONAL to support for the client and server. Additionally to what defined in , the CBOR map in the informative response payload contains the following fields, whose CBOR labels are defined in .

'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 is used for joining, this field specifically indicates the URI of the group-membership resource at the Group Manager.
'sec_gp', with value the name of the OSCORE group, encoded as a CBOR text string.
Optionally, 'as_uri', with value the URI of the Authorization Server associated with the Group Manager for the OSCORE group, encoded as a CBOR text string.
Optionally, 'hkdf', with value the HKDF Algorithm used in the OSCORE group, encoded as a CBOR text string or integer. The value is taken from the 'Value' column of the "COSE Algorithms" registry .
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 . Consistently with , acceptable values denote a format that MUST explicitly provide the comprehensive set of information related to the public key algorithm, including, e.g., the used elliptic curve (when applicable). At the time of writing this specification, acceptable formats of authentication credentials are CBOR Web Tokens (CWTs) and CWT Claim Sets (CCSs) , X.509 certificates and C509 certificates . Further formats may be available in the future, and would be acceptable to use as long as they comply with the criteria defined above. [ As to CWTs and unprotected CWT claim sets, there is a pending registration requested by draft-ietf-lake-edhoc. ] [ As to C509 certificates, there is a pending registration requested by draft-ietf-cose-cbor-encoded-cert. ]
Optionally, 'sign_enc_alg', with value the Signature Encryption Algorithm used in the OSCORE group to encrypt messages protected with the group mode, encoded as a CBOR text string or integer. The value is taken from the 'Value' column of the "COSE Algorithms" registry .
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 .
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 .
- '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 .

The values of 'sign_alg', 'sign_params' and 'cred_fmt' provide an early knowledge of the format of authentication credentials as well as of the type of public keys used in the OSCORE group. Thus, the client does not need to ask the Group Manager for this information as a preliminary step before the (ACE) join process, or to perform a trial-and-error exchange with the Group Manager upon joining the group. Hence, the client is able to provide the Group Manager with its own authentication credential in the correct expected format and including a public key of the correct expected type, at the very first step of the (ACE) join process. The values of 'hkdf', 'sign_enc_alg' and 'sign_alg' provide an early knowledge of the algorithms used in the OSCORE group. Thus, the client is able to decide whether to actually proceed with the (ACE) join process, depending on its support for the indicated algorithms. 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 ). Otherwise, the client SHOULD explicitly withdraw from the group observation. 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 MUST NOT be used together with the mechanism defined in this section for indicating what OSCORE group to join.

Server-Side Requirements When using Group OSCORE to protect multicast notifications, the server performs the operations described in , with the following differences.

Registration The phantom registration request MUST be secured, by using Group OSCORE. In particular, the group mode of Group OSCORE defined in MUST be used. The server protects the phantom registration request as defined in , as if it was the actual sender, i.e., by using its Sender Context. As a consequence, the server consumes the current value of its Sender Sequence Number SN in the OSCORE group, and hence updates it to SN* = (SN + 1). Consistently, the OSCORE option in the phantom registration request includes:

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

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 ). As a consequence: the specified Code is always 0.05 (FETCH); the sequence of CoAP options will be limited to the outer, non encrypted options; a payload is always present, as the authenticated ciphertext followed by the signature. Note that, in terms of transport-independent information, the registration request from the client typically differs from the phantom request. Thus, the server has to include the 'ph_req' parameter in the informative response. An exception is the case discussed in . 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 ). This applies also to the initial multicast notification INIT_NOTIF built in step 6 of . Optionally, the informative response includes information on the OSCORE group to join, as additional parameters (see ).

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 MUST be used. The process described in applies, with the following additions when building the two OSCORE 'external_aad' to encrypt and sign the multicast notification (see ).

The 'request_kid' is the 'kid' value in the OSCORE option of the phantom registration request, i.e., the Sender ID of the server.
The 'request_piv' is the 'piv' value in the OSCORE option of the phantom registration request, i.e., the consumed Sender Sequence Number SN of the server.
The 'request_kid_context' is the 'kid context' value in the OSCORE option of the phantom registration request, i.e., the Group Identifier value (Gid) of the OSCORE group used as ID Context.

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

Cancellation When canceling a group observation (see ), the multicast response with error code 5.03 (Service Unavailable) is also protected with Group OSCORE, as per . The server MUST use its own Sender Sequence Number as Partial IV to protect the error response, and include it as Partial IV in the OSCORE option of the response.

Client-Side Requirements When using Group OSCORE to protect multicast notifications, the client performs as described in , with the following differences.

Informative Response Upon receiving the informative response from the server, the client performs as described in , 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 . Once completed step 2, the client decrypts and verifies the rebuilt phantom registration request as defined in , with the following differences.

The client MUST NOT perform any replay check. That is, the client skips step 3 in .
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 .
- The client MUST NOT take any further process as normally expected according to . That is, the client skips step 8 in . In particular, the client MUST NOT deliver the phantom registration request to the application, and MUST NOT take any action in the Token space of its unicast endpoint, where the informative response has been received.
- The client stores the values of the 'kid', 'piv' and 'kid context' fields from the OSCORE option of the phantom registration request.
If decryption and verification of the phantom registration request fail, the client MAY try sending a new registration request to the server (see ). Otherwise, the client SHOULD explicitly withdraw from the group observation.

After successful decryption and verification, the client performs step 3 in , 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 , just like it would for the multicast notifications transmitted as CoAP messages on the wire (see ). If decryption and verification succeed, the client proceeds with step 6, considering the decrypted latest multicast notification. Otherwise, the client proceeds to step 7.

Notifications After having successfully processed the informative response as defined in , the client will decrypt and verify every multicast notification for the target resource as defined in , with the following difference. For both decryption and signature verification, the client MUST set the 'external_aad' defined in as follows. The particular way to achieve this is implementation specific.

'request_kid' takes the value of the 'kid' field from the OSCORE option of the phantom registration request (see ).
'request_piv' takes the value of the 'piv' field from the OSCORE option of the phantom registration request (see ).
'request_kid_context' takes the value of the 'kid context' field from the OSCORE option of the phantom registration request (see ).

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 .

Example with Group OSCORE The following example refers to two clients C_1 and C_2 that register to observe a resource /r at a Server S, which has address SRV_ADDR and listens to the port number SRV_PORT. Before the following exchanges occur, no clients are observing the resource /r , which has value "1234". The server S sends multicast notifications to the IP multicast address GRP_ADDR and port number GRP_PORT, and starts the group observation upon receiving a registration request from a first client that wishes to start a traditional observation on the resource /r. Pairwise communication over unicast is protected with OSCORE, while S protects multicast notifications with Group OSCORE. Specifically:

C_1 and S have a pairwise OSCORE Security Context. In particular, C_1 has 'kid' = 0x01 as Sender ID, and SN_1 = 101 as Sender Sequence Number. Also, S has 'kid' = 0x03 as Sender ID, and SN_3 = 301 as Sender Sequence Number.
C_2 and S have a pairwise OSCORE Security Context. In particular, C_2 has 'kid' = 0x02 as Sender ID, and SN_2 = 201 as Sender Sequence Number. Also, S has 'kid' = 0x04 as Sender ID, and SN_4 = 401 as Sender Sequence Number.
S is a member of the OSCORE group with name "myGroup", and 'kid context' = 0x57ab2e as Group ID. In the OSCORE group, S has 'kid' = 0x05 as Sender ID, and SN_5 = 501 as Sender Sequence Number.

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

'bstr(X)' denotes a CBOR byte string with value the byte serialization of X, with '|' denoting byte concatenation.
'OPT' denotes a sequence of CoAP options. This refers to the phantom registration request encoded by the 'ph_req' parameter, or to the corresponding latest multicast notification encoded by the 'last_notif' parameter.
'PAYLOAD' denotes an encrypted CoAP payload. This refers to the phantom registration request encoded by the 'ph_req' parameter, or to the corresponding latest multicast notification encoded by the 'last_notif' parameter.
'SIGN' denotes the signature appended to an encrypted CoAP payload. This refers to the phantom registration request encoded by the 'ph_req' parameter, or to the corresponding latest multicast notification encoded by the 'last_notif' parameter.

Example of group observation with Group OSCORE S /r | 0.05 (FETCH) | | Token: 0x4a | | OSCORE: {kid: 0x01; piv: 101; ...} | | | | 0xff | | Encrypted_payload { | | 0x01 (GET), | | Observe: 0 (Register), | | | | } | | | | (S allocates the available Token value 0x7b .) | | | | (S sends to itself a phantom observation request PH_REQ | | as coming from the IP multicast address GRP_ADDR .) | | ------------------------------------------------------ | | / | | \-------------------------------------------------------> | /r | 0.05 (FETCH) | | Token: 0x7b | | OSCORE: {kid: 0x05 ; piv: 501; | | kid context: 0x57ab2e; ...} | | | | 0xff | | Encrypted_payload { | | 0x01 (GET), | | Observe: 0 (Register), | | | | } | | | | | | (S steps SN_5 in the Group OSCORE Sec. Ctx : SN_5 <== 502) | | | | (S creates a group observation of /r .) | | | | (S increments the observer counter | | for the group observation of /r .) | | | C_1 <--------------- [ Unicast w/ OSCORE ] ---------------- S | 2.05 (Content) | | Token: 0x4a | | OSCORE: {piv: 301; ...} | | Max-Age: 0 | | | | 0xff | | Encrypted_payload { | | 5.03 (Service Unavailable), | | Content-Format: application/informative-response+cbor, | | , | | 0xff, | | CBOR_payload { | | tp_info : [1, bstr(SRV_ADDR), SRV_PORT, | | 0x7b, bstr(GRP_ADDR), GRP_PORT], | | ph_req : bstr(0x05 | OPT | 0xff | PAYLOAD | SIGN), | | last_notif : bstr(0x45 | OPT | 0xff | PAYLOAD | SIGN), | | join_uri : "coap://myGM/ace-group/myGroup", | | sec_gp : "myGroup" | | } | | } | | | C_2 ------------ [ Unicast w/ OSCORE ] ------------------> S /r | 0.05 (FETCH) | | Token: 0x01 | | OSCORE: {kid: 0x02; piv: 201; ...} | | | | 0xff | | Encrypted_payload { | | 0x01 (GET), | | Observe: 0 (Register), | | | | } | | | | (S increments the observer counter | | for the group observation of /r .) | | | C_2 <--------------- [ Unicast w/ OSCORE ] ---------------- S | 2.05 (Content) | | Token: 0x01 | | OSCORE: {piv: 401; ...} | | Max-Age: 0 | | | | 0xff, | | Encrypted_payload { | | 5.03 (Service Unavailable), | | Content-Format: application/informative-response+cbor, | | , | | 0xff, | | CBOR_payload { | | tp_info : [1, bstr(SRV_ADDR), SRV_PORT, | | 0x7b, bstr(GRP_ADDR), GRP_PORT], | | ph_req : bstr(0x05 | OPT | 0xff | PAYLOAD | SIGN), | | last_notif : bstr(0x45 | OPT | 0xff | PAYLOAD | SIGN), | | join_uri : "coap://myGM/ace-group/myGroup", | | sec_gp : "myGroup" | | } | | } | | | | (The value of the resource /r changes to "5678".) | | | C_1 | + <----------- [ Multicast w/ Group OSCORE ] ------------ S C_2 (Destination address/port: GRP_ADDR/GRP_PORT) | | 2.05 (Content) | | Token: 0x7b | | OSCORE: {kid: 0x05; piv: 502; ...} | | | | 0xff | | Encrypted_payload { | | 2.05 (Content), | | Observe: [empty], | | Content-Format: application/cbor, | | , | | 0xff, | | CBOR_Payload: "5678" | | } | | | | | ]]> The two external_aad used to encrypt and sign the multicast notification above have 'request_kid' = 5, 'request_piv' = 501 and 'request_kid_context' = 0x57ab2e. These values are specified in the 'kid', 'piv' and 'kid context' field of the OSCORE option of the phantom observation request, which is encoded in the 'ph_req' parameter of the unicast informative response to the two clients. Thus, the two clients can build the two same external_aad for decrypting and verifying this multicast notification and the following ones.

Intermediaries This section specifies how the approach presented in and works when a proxy is used between the clients and the server. In addition to what specified in and , the following applies. A client sends its original observation request to the proxy. If the proxy is not already registered at the server for that target resource, the proxy forwards the observation request to the server, hence registering itself as an observer. If the server has an ongoing group observation for the target resource or decides to start one, the server considers the proxy as taking part in the group observation, and replies to the proxy with an informative response. Upon receiving an informative response, the proxy performs as specified for the client in , with the peculiarity that "consuming" the last notification (if present) means populating its cache. In particular, by using the information retrieved from the informative response, the proxy configures an observation of the target resource at the origin server, acting as a client directly taking part in the group observation. As a consequence, the proxy will listen to the IP multicast address and port number indicated by the server in the informative response, as 'cli_addr' and 'cli_port' element of 'req_info' under the 'tp_info' parameter, respectively (see ). Furthermore, multicast notifications will match the phantom request stored at the proxy, based on the Token value specified in the 'token' element of 'req_info' under the 'tp_info' parameter in the informative response. Then, the proxy performs the following actions.

If the 'last_notif' field is not present, the proxy responds to the client with an Empty Acknowledgement (if indicated by the message type, and if it has not already done so).
If the 'last_notif' field is present, the proxy rebuilds the latest multicast notification, as defined in . Then, the proxy responds to the client, by forwarding back the latest multicast notification.

When responding to an observation request from a client, the proxy also adds that client (and its Token) to the list of its registered observers for the target resource, next to the older observations. Upon receiving a multicast notification from the server, the proxy forwards it back separately to each observer client over unicast. Note that the notification forwarded back to a certain client has the same Token value of the original observation request sent by that client to the proxy. Note that the proxy configures the observation of the target resource at the server only once, when receiving the informative response associated with a (newly started) group observation for that target resource. After that, when receiving an observation request from a following new client to be added to the same group observation, the proxy does not take any further action with the server. Instead, the proxy responds to the client either with the latest multicast notification if available from its cache, or with an Empty Acknowledgement otherwise, as defined above. An example is provided in . In the general case with a chain of two or more proxies, every proxy in the chain takes the role of client with the (next hop towards the) origin server. Note that the proxy adjacent to the origin server is the only one in the chain that receives informative responses and listens to an IP multicast address to receive notifications for the group observation. Furthermore, every proxy in the chain takes the role of server with the (previous hop towards the) origin client.

Intermediaries Together with End-to-End Security As defined in , Group OSCORE can be used to protect multicast notifications end-to-end between the origin server and the clients. In such a case, additional actions are required when also the informative responses from the origin server are protected specifically end-to-end, by using OSCORE or Group OSCORE. In fact, the proxy adjacent to the origin server is not able to access the encrypted payload of such informative responses. Hence, the proxy cannot retrieve the 'ph_req' and 'tp_info' parameters necessary to correctly receive multicast notifications and forward them back to the clients. Then, differently from what defined in , each proxy receiving an informative response simply forwards it back to the client that has sent the corresponding observation request. Note that the proxy does not even realize the message to be an actual informative response, since the outer Code field is set to 2.05 (Content). Upon receiving the informative response, the client does not configure an observation of the target resource. Instead, the client performs a new observe registration request, by transmitting the re-built phantom request as intended to reach the proxy adjacent to the origin server. In particular, the client includes the new Listen-To-Multicast-Responses CoAP option defined in , to provide that proxy with the transport-specific information required for receiving multicast notifications for the group observation. Details on the additional message exchange and processing are defined in .

Listen-To-Multicast-Responses Option In order to allow the proxy to listen to the multicast notifications sent by the server, a new CoAP option is introduced. This option MUST be supported by clients interested to take part in group observations through intermediaries, and by proxies that collect multicast notifications and forward them back to the observer clients. The option is called Listen-To-Multicast-Responses and is intended only for requests. As summarized in , the option is critical and not Safe-to-Forward. Since the option is not Safe-to-Forward, the column "N" indicates a dash for "not applicable".

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

Message Processing Compared to , the following additions apply when informative responses are protected end-to-end between the origin server and the clients. After the origin server sends an informative response, each proxy simply forwards it back to the (previous hop towards the) origin client that has sent the observation request. Once received the informative response, the origin client proceeds in a different way than in :

The client performs all the additional decryption and verification steps of on the phantom request specified in the 'ph_req' parameter and on the last notification specified in the 'last_notif' parameter (if present).
The client builds a ticket request (see Appendix B of ), as intended to reach the proxy adjacent to the origin server. The ticket request is formatted as follows.
- The Token is chosen as the client sees fit. In fact, there is no reason for this Token to be the same as the phantom request's.
- The outer Code field, the outer CoAP options and the encrypted payload with AEAD tag (protecting the inner Code, the inner CoAP options and the possible plain CoAP payload) concatenated with the signature are the same of the phantom request used for the group observation. That is, they are as specified in the 'ph_req' parameter of the received informative response.
- An outer Observe option is included and set to 0 (Register). This will usually be set in the phantom request already.
- The outer options Proxy-Scheme, Uri-Host and Uri-Port are included, and set to the same values they had in the original registration request sent by the client.
- The new option Listen-To-Multicast-Responses is included as an outer option. The value is set to the serialization of the CBOR array specified by the 'tp_info' parameter of the informative response. Note that, except for transport-specific information such as the Token and Message ID values, every different client participating to the same group observation (hence rebuilding the same phantom request) will build the same ticket request. Note also that, identically to the phantom request, the ticket request is still protected with Group OSCORE, i.e., it has the same OSCORE option, encrypted payload and signature.

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

The proxy MUST NOT further forward the ticket request to the origin server.
The proxy removes the Proxy-Scheme, Uri-Host and Uri-Port options from the ticket request.
The proxy removes the Listen-To-Multicast-Responses option from the ticket request, and extracts the conveyed transport-specific information.
The proxy rebuilds the phantom request associated with the group observation, by using the ticket request as directly providing the required transport-independent information. This includes the outer Code field, the outer CoAP options and the encrypted payload with AEAD tag concatenated with the signature.
The proxy configures an observation of the target resource at the origin server, acting as a client directly taking part in the group observation. To this end, the proxy uses the rebuilt phantom request and the transport-specific information retrieved from the Listen-To-Multicast-Responses Option. The particular way to achieve this is implementation specific.

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

Informative Response Parameters This document defines a number of fields used in the informative response message defined in . 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.

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

Transport Protocol Information This document defines some values of transport protocol identifiers to use within the 'tp_info' parameter of the informative response message defined in . According to the encoding specified in , these values are used for the 'tp_id' element of 'srv_addr', under the 'tp_info' parameter. The table below summarizes them, specifies the integer value to use instead of the full descriptive name, and provides the corresponding comprehensive set of information elements to include in the 'tp_info' parameter.

Transport protocol information

Security Considerations In addition to the security considerations from , the following considerations hold for this document.

Unsecured Multicast Notifications In case communications are not protected, the server might not be able to effectively authenticate a new client when it registers as an observer. 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 to perform the rough counting of still interested clients triggers (some of) them to explicitly send a new observation request to acknowledge their interest. Then, the server can decide to terminate the group observation altogether, in case not enough clients are estimated to be still active. If the method defined in 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.

Secured Multicast Notifications If multicast notifications are protected using Group OSCORE as per , the following applies.

The original registration requests and related unicast (notification) responses MUST also be secured, including and especially the informative responses from the server. This prevents on-path active adversaries from altering the conveyed IP multicast address and serialized phantom registration request. Thus, it ensures secure binding between every multicast notification for a same observed resource and the phantom registration request that started the group observation of that resource.
A re-registration request, possibly including the Multicast-Response-Feedback-Divider option to support the rough counting of clients (see ), MUST also be secured.

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

Listen-To-Multicast-Responses Option The CoAP option Listen-To-Multicast-Responses defined in is of class U for OSCORE and Group OSCORE . This allows the proxy adjacent to the origin server to access the option value conveyed in a ticket request (see ), and to retrieve from it the transport-specific information about a phantom request. By doing so, the proxy becomes able to configure an observation of the target resource and to receive multicast notifications matching to the phantom request. Any proxy in the chain, as well as further possible intermediaries or on-path active adversaries, are thus able to remove the option or alter its content, before the ticket request reaches the proxy adjacent to the origin server. Removing the option would result in the proxy adjacent to the origin server to not configure the group observation, if that has not happened yet. In such a case, the proxy would not receive the corresponding multicast notifications to be forwarded back to the clients. Altering the option content would result in the proxy adjacent to the origin server to incorrectly configure a group observation (e.g., by indicating a wrong multicast IP address) hence preventing the correct reception of multicast notifications and their forwarding to the clients; or to configure bogus group observations that are currently not active on the origin server. In order to prevent what is described above, the ticket requests conveying the Listen-To-Multicast-Responses option can be additionally protected hop-by-hop. This can be achieved by the client protecting the ticket request sent to the proxy using OSCORE (see ) and/or DTLS .

IANA Considerations This document has the following actions for IANA.

Media Type Registrations This document registers the media type 'application/informative-response+cbor' for error messages as informative response defined in , when carrying parameters encoded in CBOR. This registration follows the procedures specified in .

Type name: application
Subtype name: informative-response+cbor
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: Must be encoded as a CBOR map containing the parameters defined in of [this document].
Security considerations: See of [this document].
Interoperability considerations: N/A
Published specification: [this document]
Applications that use this media type: The type is used by CoAP servers and clients that support error messages as informative response defined in of [this document].
Fragment identifier considerations: N/A
Additional information: N/A
Person & email address to contact for further information: iesg@ietf.org
Intended usage: COMMON
Restrictions on usage: None
Author: Marco Tiloca marco.tiloca@ri.se
Change controller: IESG
Provisional registration? No

CoAP Content-Formats Registry IANA is asked to add the following entry to the "CoAP Content-Formats" registry within the "Constrained RESTful Environments (CoRE) Parameters" registry group. Media Type: application/informative-response+cbor Encoding: - ID: TBD Reference: [this document]

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

Informative Response Parameters Registry This document establishes the "Informative Response Parameters" registry. The registry has been created to use the "Expert Review Required" registration procedure . Expert review guidelines are provided in . The columns of this registry are:

Name: This is a descriptive name that enables easier reference to the item. The name MUST be unique. It is not used in the encoding.
CBOR Key: This is the value used as CBOR key of the item. These values MUST be unique. The value can be a positive integer, a negative integer, or a string.
CBOR Type: This contains the CBOR type of the item, or a pointer to the registry that defines its type, when that depends on another item.
Reference: This contains a pointer to the public specification for the item.

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

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

Transport Protocol: This is a descriptive name that enables easier reference to the item. The name MUST be unique. It is not used in the encoding.
Description: Text giving an overview of the transport protocol referred by this item.
Value: CBOR abbreviation for the transport protocol referred by this item. Different ranges of values use different registration policies . Integer values from -256 to 255 are designated as Standards Action. Integer values from -65536 to -257 and from 256 to 65535 are designated as Specification Required. Integer values greater than 65535 are designated as Expert Review. Integer values less than -65536 are marked as Private Use.
Server Addr: List of elements providing addressing information of the server.
Req Info: List of elements providing transport-specific information related to a pertinent CoAP request. Optional elements are prepended by '?'.
Reference: This contains a pointer to the public specification for the item.

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

Expert Review Instructions The IANA registries established in this document are defined as expert review. This section gives some general guidelines for what the experts should be looking for, but they are being designated as experts for a reason so they should be given substantial latitude. Expert reviewers should take into consideration the following points:

Point squatting should be discouraged. Reviewers are encouraged to get sufficient information for registration requests to ensure that the usage is not going to duplicate one that is already registered and that the point is likely to be used in deployments. The zones tagged as private use are intended for testing purposes and closed environments, code points in other ranges should not be assigned for testing.
Specifications are required for the standards track range of point assignment. Specifications should exist for specification required ranges, but early assignment before a specification is available is considered to be permissible. Specifications are needed for the first-come, first-serve range if they are expected to be used outside of closed environments in an interoperable way. When specifications are not provided, the description provided needs to have sufficient information to identify what the point is being used for.
Experts should take into account the expected usage of fields when approving point assignment. The fact that there is a range for standards track documents does not mean that a standards track document cannot have points assigned outside of that range. The length of the encoded value should be weighed against how many code points of that length are left, the size of device it will be used on, and the number of code points left that encode to that size.

References Normative References Group Communication for the Constrained Application Protocol (CoAP) IoTconsultancy.nl InterDigital RISE AB This document specifies the use of the Constrained Application Protocol (CoAP) for group communication, including the use of UDP/IP multicast as the default underlying data transport. Both unsecured and secured CoAP group communication are specified. Security is achieved by use of the Group Object Security for Constrained RESTful Environments (Group OSCORE) protocol. The target application area of this specification is any group communication use cases that involve resource-constrained devices or networks that support CoAP. This document replaces RFC 7390, while it updates RFC 7252 and RFC 7641. Group OSCORE - Secure Group Communication for CoAP RISE AB Ericsson AB Ericsson AB Ericsson AB Universitaet Duisburg-Essen This document defines Group Object Security for Constrained RESTful Environments (Group OSCORE), providing end-to-end security of CoAP messages exchanged between members of a group, e.g., sent over IP multicast. In particular, the described approach defines how OSCORE is used in a group communication setting to provide source authentication for CoAP group requests, sent by a client to multiple servers, and for protection of the corresponding CoAP responses. Group OSCORE also defines a pairwise mode where each member of the group can efficiently derive a symmetric pairwise key with any other member of the group for pairwise OSCORE communication. Key Management for OSCORE Groups in ACE RISE AB Universitaet Duisburg-Essen Ericsson AB This document defines an application profile of the ACE framework for Authentication and Authorization, to request and provision keying material in group communication scenarios that are based on CoAP and secured with Group Object Security for Constrained RESTful Environments (OSCORE). This application profile delegates the authentication and authorization of Clients that join an OSCORE group through a Resource Server acting as Group Manager for that group. This application profile leverages protocol-specific transport profiles of ACE to achieve communication security, server authentication and proof-of-possession for a key owned by the Client and bound to an OAuth 2.0 Access Token. OSCORE Profile of the Authentication and Authorization for Constrained Environments Framework Ericsson AB Combitech Ericsson AB RISE This document specifies a profile for the Authentication and Authorization for Constrained Environments (ACE) framework. It utilizes Object Security for Constrained RESTful Environments (OSCORE) to provide communication security and proof-of-possession for a key owned by the client and bound to an OAuth 2.0 access token. Key words for use in RFCs to Indicate Requirement Levels In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. Transmission of IPv6 Packets over IEEE 802.15.4 Networks This document describes the frame format for transmission of IPv6 packets and the method of forming IPv6 link-local addresses and statelessly autoconfigured addresses on IEEE 802.15.4 networks. Additional specifications include a simple header compression scheme using shared context and provisions for packet delivery in IEEE 802.15.4 meshes. [STANDARDS-TRACK] Media Type Specifications and Registration Procedures This document defines procedures for the specification and registration of media types for use in HTTP, MIME, and other Internet protocols. This memo documents an Internet Best Current Practice. The Constrained Application Protocol (CoAP) The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained (e.g., low-power, lossy) networks. The nodes often have 8-bit microcontrollers with small amounts of ROM and RAM, while constrained networks such as IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) often have high packet error rates and a typical throughput of 10s of kbit/s. The protocol is designed for machine- to-machine (M2M) applications such as smart energy and building automation. CoAP provides a request/response interaction model between application endpoints, supports built-in discovery of services and resources, and includes key concepts of the Web such as URIs and Internet media types. CoAP is designed to easily interface with HTTP for integration with the Web while meeting specialized requirements such as multicast support, very low overhead, and simplicity for constrained environments. Observing Resources in the Constrained Application Protocol (CoAP) The Constrained Application Protocol (CoAP) is a RESTful application protocol for constrained nodes and networks. The state of a resource on a CoAP server can change over time. This document specifies a simple protocol extension for CoAP that enables CoAP clients to "observe" resources, i.e., to retrieve a representation of a resource and keep this representation updated by the server over a period of time. The protocol follows a best-effort approach for sending new representations to clients and provides eventual consistency between the state observed by each client and the actual resource state at the server. Constrained Application Protocol (CoAP) Option for No Server Response There can be machine-to-machine (M2M) scenarios where server responses to client requests are redundant. This kind of open-loop exchange (with no response path from the server to the client) may be desired to minimize resource consumption in constrained systems while updating many resources simultaneously or performing high-frequency updates. CoAP already provides Non-confirmable (NON) messages that are not acknowledged by the recipient. However, the request/response semantics still require the server to respond with a status code indicating "the result of the attempt to understand and satisfy the request", per RFC 7252. This specification introduces a CoAP option called 'No-Response'. Using this option, the client can explicitly express to the server its disinterest in all responses against the particular request. This option also provides granular control to enable expression of disinterest to a particular response class or a combination of response classes. The server MAY decide to suppress the response by not transmitting it back to the client according to the value of the No-Response option in the request. This option may be effective for both unicast and multicast requests. This document also discusses a few examples of applications that benefit from this option. UDP Usage Guidelines The User Datagram Protocol (UDP) provides a minimal message-passing transport that has no inherent congestion control mechanisms. This document provides guidelines on the use of UDP for the designers of applications, tunnels, and other protocols that use UDP. Congestion control guidelines are a primary focus, but the document also provides guidance on other topics, including message sizes, reliability, checksums, middlebox traversal, the use of Explicit Congestion Notification (ECN), Differentiated Services Code Points (DSCPs), and ports. Because congestion control is critical to the stable operation of the Internet, applications and other protocols that choose to use UDP as an Internet transport must employ mechanisms to prevent congestion collapse and to establish some degree of fairness with concurrent traffic. They may also need to implement additional mechanisms, depending on how they use UDP. Some guidance is also applicable to the design of other protocols (e.g., protocols layered directly on IP or via IP-based tunnels), especially when these protocols do not themselves provide congestion control. This document obsoletes RFC 5405 and adds guidelines for multicast UDP usage. Guidelines for Writing an IANA Considerations Section in RFCs Many protocols make use of points of extensibility that use constants to identify various protocol parameters. To ensure that the values in these fields do not have conflicting uses and to promote interoperability, their allocations are often coordinated by a central record keeper. For IETF protocols, that role is filled by the Internet Assigned Numbers Authority (IANA). To make assignments in a given registry prudently, guidance describing the conditions under which new values should be assigned, as well as when and how modifications to existing values can be made, is needed. This document defines a framework for the documentation of these guidelines by specification authors, in order to assure that the provided guidance for the IANA Considerations is clear and addresses the various issues that are likely in the operation of a registry. This is the third edition of this document; it obsoletes RFC 5226. Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. Web Linking This specification defines a model for the relationships between resources on the Web ("links") and the type of those relationships ("link relation types"). It also defines the serialisation of such links in HTTP headers with the Link header field. Object Security for Constrained RESTful Environments (OSCORE) This document defines Object Security for Constrained RESTful Environments (OSCORE), a method for application-layer protection of the Constrained Application Protocol (CoAP), using CBOR Object Signing and Encryption (COSE). OSCORE provides end-to-end protection between endpoints communicating using CoAP or CoAP-mappable HTTP. OSCORE is designed for constrained nodes and networks supporting a range of proxy operations, including translation between different transport protocols. Although an optional functionality of CoAP, OSCORE alters CoAP options processing and IANA registration. Therefore, this document updates RFC 7252. Concise Binary Object Representation (CBOR) The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code size, fairly small message size, and extensibility without the need for version negotiation. These design goals make it different from earlier binary serializations such as ASN.1 and MessagePack. This document obsoletes RFC 7049, providing editorial improvements, new details, and errata fixes while keeping full compatibility with the interchange format of RFC 7049. It does not create a new version of the format. COSE Algorithms IANA COSE Key Types IANA COSE Header Parameters IANA Informative References Authentication and Authorization for Constrained Environments (ACE) using the OAuth 2.0 Framework (ACE-OAuth) Combitech Ericsson Spotify AB Arm Ltd. This specification defines a framework for authentication and authorization in Internet of Things (IoT) environments called ACE- OAuth. The framework is based on a set of building blocks including OAuth 2.0 and the Constrained Application Protocol (CoAP), thus transforming a well-known and widely used authorization solution into a form suitable for IoT devices. Existing specifications are used where possible, but extensions are added and profiles are defined to better serve the IoT use cases. Publish-Subscribe Broker for the Constrained Application Protocol (CoAP) SmartThings Ericsson Ericsson The Constrained Application Protocol (CoAP), and related extensions are intended to support machine-to-machine communication in systems where one or more nodes are resource constrained, in particular for low power wireless sensor networks. This document defines a publish- subscribe Broker for CoAP that extends the capabilities of CoAP for supporting nodes with long breaks in connectivity and/or up-time. There is work in progress to resolve some of the transfer layer issues by using a more RESTful approach. Please see https://github.com/core-wg/pubsub/blob/master/proposal.txt CoRE Resource Directory ARM SmartThings Universitaet Bremen TZI consultant In many IoT applications, direct discovery of resources is not practical due to sleeping nodes, or networks where multicast traffic is inefficient. These problems can be solved by employing an entity called a Resource Directory (RD), which contains information about resources held on other servers, allowing lookups to be performed for those resources. The input to an RD is composed of links and the output is composed of links constructed from the information stored in the RD. This document specifies the web interfaces that an RD supports for web servers to discover the RD and to register, maintain, lookup and remove information on resources. Furthermore, new target attributes useful in conjunction with an RD are defined. Discovery of OSCORE Groups with the CoRE Resource Directory RISE AB Consultant Group communication over the Constrained Application Protocol (CoAP) can be secured by means of Group Object Security for Constrained RESTful Environments (Group OSCORE). At deployment time, devices may not know the exact security groups to join, the respective Group Manager, or other information required to perform the joining process. This document describes how a CoAP endpoint can use descriptions and links of resources registered at the CoRE Resource Directory to discover security groups and to acquire information for joining them through the respective Group Manager. A given security group may protect multiple application groups, which are separately announced in the Resource Directory as sets of endpoints sharing a pool of resources. This approach is consistent with, but not limited to, the joining of security groups based on the ACE framework for Authentication and Authorization in constrained environments. The Datagram Transport Layer Security (DTLS) Protocol Version 1.3 This document specifies Version 1.3 of the Datagram Transport Layer Security (DTLS) protocol. DTLS 1.3 allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. The DTLS 1.3 protocol is intentionally based on the Transport Layer Security (TLS) 1.3 protocol and provides equivalent security guarantees with the exception of order protection/non-replayability. Datagram semantics of the underlying transport are preserved by the DTLS protocol. This document obsoletes RFC 6347. The Constrained RESTful Application Language (CoRAL) ARM The Constrained RESTful Application Language (CoRAL) defines a data model and interaction model as well as a compact serialization formats for the description of typed connections between resources on the Web ("links"), possible operations on such resources ("forms"), and simple resource metadata. Cacheable OSCORE RISE AB Group communication with the Constrained Application Protocol (CoAP) can be secured end-to-end using Group Object Security for Constrained RESTful Environments (Group OSCORE), also across untrusted intermediary proxies. However, this sidesteps the proxies' abilities to cache responses from the origin server(s). This specification restores cacheability of protected responses at proxies, by introducing consensus requests which any client in a group can send to one server or multiple servers in the same group. CBOR Encoded X.509 Certificates (C509 Certificates) Ericsson AB Ericsson AB RISE AB RISE AB Nexus Group This document specifies a CBOR encoding of X.509 certificates. The resulting certificates are called C509 Certificates. The CBOR encoding supports a large subset of RFC 5280 and all certificates compatible with the RFC 7925, IEEE 802.1AR (DevID), CNSA, RPKI, GSMA eUICC, and CA/Browser Forum Baseline Requirements profiles. When used to re-encode DER encoded X.509 certificates, the CBOR encoding can in many cases reduce the size of RFC 7925 profiled certificates with over 50%. The CBOR encoded structure can alternatively be signed directly ("natively signed"), which does not require re- encoding for the signature to be verified. The document also specifies C509 COSE headers, a C509 TLS certificate type, and a C509 file format. OSCORE-capable Proxies RISE AB RISE AB Object Security for Constrained RESTful Environments (OSCORE) can be used to protect CoAP messages end-to-end between two endpoints at the application layer, also in the presence of intermediaries such as proxies. This document defines how OSCORE is used to protect CoAP messages also between an origin application endpoint and an intermediary, or between two intermediaries. Besides, it defines how a CoAP message can be double-protected through "OSCORE-in-OSCORE", i.e., both end-to-end between origin application endpoints, as well as between an application endpoint and an intermediary or between two intermediaries. Thus, this document updates RFC 8613. The same approach applies to Group OSCORE, for protecting CoAP messages when group communication with intermediaries is used. Datagram Transport Layer Security Version 1.2 This document specifies version 1.2 of the Datagram Transport Layer Security (DTLS) protocol. The DTLS protocol provides communications privacy for datagram protocols. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. The DTLS protocol is based on the Transport Layer Security (TLS) protocol and provides equivalent security guarantees. Datagram semantics of the underlying transport are preserved by the DTLS protocol. This document updates DTLS 1.0 to work with TLS version 1.2. [STANDARDS-TRACK] Constrained RESTful Environments (CoRE) Link Format This specification defines Web Linking using a link format for use by constrained web servers to describe hosted resources, their attributes, and other relationships between links. Based on the HTTP Link Header field defined in RFC 5988, the Constrained RESTful Environments (CoRE) Link Format is carried as a payload and is assigned an Internet media type. "RESTful" refers to the Representational State Transfer (REST) architecture. A well-known URI is defined as a default entry point for requesting the links hosted by a server. [STANDARDS-TRACK] JSON Web Token (JWT) JSON Web Token (JWT) is a compact, URL-safe means of representing claims to be transferred between two parties. The claims in a JWT are encoded as a JSON object that is used as the payload of a JSON Web Signature (JWS) structure or as the plaintext of a JSON Web Encryption (JWE) structure, enabling the claims to be digitally signed or integrity protected with a Message Authentication Code (MAC) and/or encrypted. Transport Layer Security (TLS) / Datagram Transport Layer Security (DTLS) Profiles for the Internet of Things A common design pattern in Internet of Things (IoT) deployments is the use of a constrained device that collects data via sensors or controls actuators for use in home automation, industrial control systems, smart cities, and other IoT deployments. This document defines a Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) 1.2 profile that offers communications security for this data exchange thereby preventing eavesdropping, tampering, and message forgery. The lack of communication security is a common vulnerability in IoT products that can easily be solved by using these well-researched and widely deployed Internet security protocols. Concise Data Definition Language (CDDL): A Notational Convention to Express Concise Binary Object Representation (CBOR) and JSON Data Structures This document proposes a notational convention to express Concise Binary Object Representation (CBOR) data structures (RFC 7049). Its main goal is to provide an easy and unambiguous way to express structures for protocol messages and data formats that use CBOR or JSON. CBOR Web Token (CWT) CBOR Web Token (CWT) is a compact means of representing claims to be transferred between two parties. The claims in a CWT are encoded in the Concise Binary Object Representation (CBOR), and CBOR Object Signing and Encryption (COSE) is used for added application-layer security protection. A claim is a piece of information asserted about a subject and is represented as a name/value pair consisting of a claim name and a claim value. CWT is derived from JSON Web Token (JWT) but uses CBOR rather than JSON. The Broadcast Storm Problem in a Mobile Ad Hoc Network Constrained Resource Identifiers Universität Bremen TZI Fraunhofer SIT The Constrained Resource Identifier (CRI) is a complement to the Uniform Resource Identifier (URI) that serializes the URI components in Concise Binary Object Representation (CBOR) instead of a sequence of characters. This simplifies parsing, comparison and reference resolution in environments with severe limitations on processing power, code size, and memory size.

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, the same data can be made available through different services, such as the following ones.

Topic Discovery in Publish-Subscribe Settings In a Publish-Subscribe scenario , a group observation can be discovered along with topic metadata. For instance, a discovery step can make the following metadata available. This example assumes a CoRAL namespace , that contains properties analogous to those in the content-format application/informative-response+cbor. [ The reported CoRAL example is based on the textual representation used until version -03 of . This will be revised to use the CBOR diagnostic notation instead. ]

Group observation discovery in a Pub-Sub scenario Accept: CoRAL Response: 2.05 Content Content-Format: CoRAL rdf:type topic { tp_info [1, h"7b", h"20010db80100..0001", 5683, h"ff35003020010db8..1234", 5683], ph_req h"0160..", last_notif h"256105.." } ]]> 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.

Introspection at the Multicast Notification Sender For network debugging purposes, it can be useful to query a server that sends multicast responses as matching a phantom registration request. Such an interface is left for other documents to specify on demand. As an example, a possible interface can be as follows, and rely on the already known Token value of intercepted multicast notifications, associated with a phantom registration request.

Group observation discovery with server introspection Response: 2.05 Content Content-Format: application/informative-response+cbor { 'tp_info': [1, h"7b", h"20010db80100..0001", 5683, h"ff35003020010db8..1234", 5683], 'ph_req': h"0160..", 'last_notif' : h"256105.." } ]]> For example, a network sniffer could offer sending such a request when unknown multicast notifications are seen in a network. Consequently, it can associate those notifications with a URI, or decrypt them, if member of the correct OSCORE group.

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 . In particular, describes the algorithm for the client side, while describes an optimized version for constrained clients. Finally, describes the algorithm for the server side.

Client Side

Client Side - Optimized Version = UINT_BIT) { if (rand() != 0) return false; Q -= UINT_BIT; } unsigned int mask = ~((~0u) << Q); unsigned int masked = mask & rand(); return masked == 0; } ]]>

Server Side notification.send(GRP_ADDR, GRP_PORT); Timer t = new Timer(); t.setAndStart(MAX_CONFIRMATION_WAIT); // Time t1 while(!t.isExpired()); // Time t2 int R = ; 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)) { } else { } // Compute the new count estimate int COUNT_PRIME = ; int NEW_COUNT = COUNT_PRIME + ((E - N) / D); // Determine whether to cancel the group observation if (NEW_COUNT < CANCEL_THRESHOLD) { ; return 0; } return NEW_COUNT; ]]>

OSCORE Group Self-Managed by the Server For simple settings, where no pre-arranged group with suitable memberships is available, the server can be responsible to setup and manage the OSCORE group used to protect the group observation. In such a case, a client would implicitly request to join the OSCORE group when sending the observe registration request to the server. When replying, the server includes the group keying material and related information in the informative response (see ). Additionally to what defined in , the CBOR map in the informative response payload contains the following fields, whose CBOR labels are defined in .

'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 and extends the OSCORE_Input_Material object encoded in CBOR as defined in . In particular, the following elements of the Group_OSCORE_Input_Material object are included, using the same CBOR labels from the OSCORE Security Context Parameters Registry, as in .
- 'ms', 'contexId', 'cred_fmt', 'sign_enc_alg', 'sign_alg' and 'sign_params'. These elements MUST be included.
- 'hkdf' and 'salt'. These elements MAY be included.
The 'group_senderId' element of the Group_OSCORE_Input_Material object MUST NOT be included.
'srv_cred': this element is a CBOR byte string, with value the original binary representation of the server's authentication credential used in the OSCORE group. In particular, the original binary representation complies with the format specified by the 'cred_fmt' element of 'gp_material'.
'srv_identifier': this element MUST be included and is encoded as a CBOR byte string, with value the Sender ID that the server has in the OSCORE group.
'exp': with value the expiration time of the keying material of the OSCORE group specified in the 'gp_material' parameter, encoded as a CBOR unsigned integer. This field contains a numeric value representing the number of seconds from 1970-01-01T00:00:00Z UTC until the specified UTC date/time, ignoring leap seconds, analogous to what specified for NumericDate in .

Note that the informative response does not require to include an explicit proof-of-possession (PoP) of the server's private key. Although the server is also acting as Group Manager and a PoP evidence of the Group Manager's private key is included in a full-fledged Joining Response (see ), 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 . Note that the client does not obtain a Sender ID of its own, hence it installs a Security Context that a "silent server" would, i.e., without Sender Context. From then on, the client uses the received keying material to process the incoming multicast notifications from the server. Since the server is also acting as Group Manager, the authentication credential of the server provided in the 'srv_cred' element of the informative response is also used in the 'gm_cred' element of the external_aad for encrypting and signing the phantom request and multicast notifications (see ) Furthermore, the server complies with the following points.

The server MUST NOT self-manage OSCORE groups and provide the related keying material in the informative response for any other purpose than the protection of group observations, as defined in this document. The server MAY use the same self-managed OSCORE group to protect the phantom request and the multicast notifications of multiple group observations it hosts.
The server MUST NOT provide in the informative response the keying material of other OSCORE groups it is or has been a member of.

After the time indicated in the 'exp' field:

The server MUST stop using the keying material and MUST cancel the group observations for which that keying material is used (see and ). If the server creates a new group observation as a replacement or follow-up using the same OSCORE group:
- The server MUST update the Master Secret.
- The server MUST update the ID Context used as Group Identifier (Gid), consistently with .
- The server MAY update the Master Salt.
The client MUST stop using the keying material and MAY re-register the observation at the server.

Before the keying material has expired, the server can send a multicast response with response code 5.03 (Service Unavailable) to the observing clients, protected with the current keying material. In particular, this is an informative response (see ), which: i) additionally contains the abovementioned parameters for the next group keying material to be used; and ii) MAY omit the 'tp_info' and 'ph_req' parameters, since the associated information is immutable throughout the observation lifetime. The response has the same Token value T of the phantom registration request and it does not include an Observe option. The server MUST use its own Sender Sequence Number as Partial IV to protect the error response, and include it as Partial IV in the OSCORE option of the response. When some clients leave the OSCORE group and forget about the group observation, the server does not have to provide the remaining clients with any stale Sender IDs, as normally required for Group OSCORE (see ). 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 ). In particular, both of them never change their Sender ID during the group lifetime, while they both remain part of the group until the group ceases to exist. As an alternative to renewing the keying material before it expires, the server can simply cancel the group observation (see and ), which results in the eventual re-registration of the clients that are still interested in the group observation. Applications requiring backward security and forward security are REQUIRED to use an actual group joining process (usually through a dedicated Group Manager), e.g., the ACE joining procedure defined in . The server can facilitate the clients by providing them information about the OSCORE group to join, as described in .

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. 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 ), before sending an observe registration request to the server. If Group OSCORE is used to protect the group observation (see ), and the OSCORE group supports the concept of Deterministic Client , then the server and each client in the OSCORE group can independently protect the phantom observation request possibly available as plain CoAP message. To this end, they use the approach defined in to compute a protected deterministic request, against which the protected multicast notifications will match for the group observation in question. Note that the same deterministic request sent by each client as registration request is, in terms of transport-independent information, identical to the phantom registration request. Thus, the informative response sent by the server may omit the 'ph_req' parameter (see ). If a client receives an informative response that includes the 'ph_req' parameter, and this specifies transport-independent information different from the one of the sent deterministic request, then the client considers the informative response malformed. If the optimization defined in is also used, the 'gp_material' element in the error informative response from the server MUST also include the following elements from the Group_OSCORE_Input_Material object.

'alg', 'ecdh_alg' and 'ecdh_params', as per .
'det_senderId' and 'det_hash_alg', defined in . These specify the Sender ID of the Deterministic Client in the OSCORE group, and the hash algorithm used to compute the deterministic request (see ).

Example with a Proxy This section provides an example when a proxy P is used between the clients and the server. The same assumptions and notation used in are used for this example. In addition, the proxy has address PRX_ADDR and listens to the port number PRX_PORT. Unless explicitly indicated, all messages transmitted on the wire are sent over unicast.

Example of group observation with a proxy | | Token: 0x4a | GET | | | Observe: 0 (Register) | | | | Proxy-Uri: coap://sensor.example/r | | | | | | +------->| Token: 0x5e | | | GET | Observe: 0 (Register) | | | | Uri-Host: sensor.example | | | | Uri-Path: r | | | | | | | | (S allocates the available Token value 0x7b) | | | | | | | | (S sends to itself a phantom observation | | | | request PH_REQ as coming from the | | | | IP multicast address GRP_ADDR) | | | | | | | ------+ | | | / | | | | \----->| Token: 0x7b | | | GET | Observe: 0 (Register) | | | | Uri-Host: sensor.example | | | | Uri-Path: r | | | | | | | | (S creates a group observation of /r) | | | | | | | | (S increments the observer counter | | | | for the group observation of /r) | | | | | | | | | | | | | | |<-------+ Token: 0x5e | | | 5.03 | Content-Format: application/ | | | | informative-response+cbor | | | | Max-Age: 0 | | | | | | | | Payload: { | | | | tp_info : [1, bstr(SRV_ADDR), SRV_PORT, | | | | 0x7b, bstr(GRP_ADDR), | | | | GRP_PORT], | | | | last_notif : bstr(0x45 | OPT | | | | | 0xff | PAYLOAD) | | | | } | | | | | | | | (PAYLOAD in 'last_notif' : "1234") | | | | | | | | | | | | (The proxy starts listening to the | | | | GRP_ADDR address and the GRP_PORT port.) | | | | | | | | (The proxy adds C1 to its list of observers.) | | | | |<------------+ | Token: 0x4a | 2.05 | | | Observe: 54120 | | | | Content-Format: application/cbor | | | | | | | | Payload: "1234" | | | | : : : : : : : : : : : : | | | | | +----->| | Token: 0x01 | | GET | | Observe: 0 (Register) | | | | Proxy-Uri: coap://sensor.example/r | | | | | | | | (The proxy has a fresh cache representation) | | | | | |<-----+ | Token: 0x01 | | 2.05 | | Observe: 54120 | | | | Content-Format: application/cbor | | | | | | | | Payload: "1234" | | | | : : : : : : : : (The value of the resource : : : : /r changes to "5678".) : : : : | | | | | | | (*) | | | |<-------+ Token: 0x7b | | | 2.05 | Observe: 11 | | | | Content-Format: application/cbor | | | | | | | | Payload: "5678" | | | | |<------------+ | Token: 0x4a | 2.05 | | | Observe: 54123 | | | | Content-Format: application/cbor | | | | | | | | Payload: "5678" | | | | | |<-----+ | Token: 0x01 | | 2.05 | | Observe: 54123 | | | | Content-Format: application/cbor | | | | | | | | Payload: "5678" | | | | (*) Sent over IP multicast to GROUP_ADDR:GROUP_PORT ]]> Note that the proxy has all the information to understand the observation request from C2, and can immediately start to serve the still fresh values. This behavior is mandated by , i.e., the proxy registers itself only once with the next hop and fans out the notifications it receives to all registered clients.

Example with a Proxy and Group OSCORE This section provides an example when a proxy P is used between the clients and the server, and Group OSCORE is used to protect multicast notifications end-to-end between the server and the clients. The same assumptions and notation used in are used for this example. In addition, the proxy has address PRX_ADDR and listens to the port number PRX_PORT. Unless explicitly indicated, all messages transmitted on the wire are sent over unicast and protected with OSCORE end-to-end between a client and the server.

Example of group observation with a proxy and Group OSCORE | | Token: 0x4a | FETCH | | | Observe: 0 (Register) | | | | OSCORE: {kid: 0x01; piv: 101; ...} | | | | Uri-Host: sensor.example | | | | Proxy-Scheme: coap | | | | | | | | 0xff | | | | Encrypted_payload { | | | | 0x01 (GET), | | | | Observe: 0 (Register), | | | | Uri-Path: r, | | | | | | | | } | | | | | | +-------->| Token: 0x5e | | | FETCH | Observe: 0 (Register) | | | | OSCORE: {kid: 0x01; piv: 101; ...} | | | | Uri-Host: sensor.example | | | | | | | | 0xff | | | | Encrypted_payload { | | | | 0x01 (GET), | | | | Observe: 0 (Register), | | | | Uri-Path: r, | | | | | | | | } | | | | | | | | | | | | (S allocates the available | | | | Token value 0x7b .) | | | | | | | | (S sends to itself a phantom observation | | | | request PH_REQ as coming from the | | | | IP multicast address GRP_ADDR) | | | | | | | -------+ | | | / | | | | \------>| Token: 0x7b | | | FETCH | Observe: 0 (Register) | | | | OSCORE: {kid: 0x05; piv: 501; | | | | kid context: 0x57ab2e; ...} | | | | Uri-Host: sensor.example | | | | | | | | 0xff | | | | Encrypted_payload { | | | | 0x01 (GET), | | | | Observe: 0 (Register), | | | | Uri-Path: r, | | | | | | | | } | | | | | | | | | | | | (S steps SN_5 in the Group OSCORE | | | | Security Context : SN_5 <== 502) | | | | | | | | (S creates a group observation of /r) | | | | | | | | | | | | (S increments the observer counter | | | | for the group observation of /r) | | | | | | |<--------+ Token: 0x5e | | | 2.05 | OSCORE: {piv: 301; ...} | | | | Max-Age: 0 | | | | | | | | 0xff | | | | Encrypted_payload { | | | | 5.03 (Service Unavailable), | | | | Content-Format: application/ | | | | informative-response+cbor, | | | | , | | | | 0xff, | | | | CBOR_payload { | | | | tp_info : [1, bstr(SRV_ADDR), | | | | SRV_PORT, 0x7b, | | | | bstr(GRP_ADDR), GRP_PORT], | | | | ph_req : bstr(0x05 | OPT | 0xff | | | | | PAYLOAD | SIGN), | | | | last_notif : bstr(0x45 | OPT | 0xff | | | | | PAYLOAD | SIGN), | | | | join_uri : "coap://myGM/ | | | | ace-group/myGroup", | | | | sec_gp : "myGroup" | | | | } | | | | } | | | | |<--------------+ | Token: 0x4a | 2.05 | | | OSCORE: {piv: 301; ...} | | | | | | | | 0xff | | | | (Same Encrypted_payload) | | | | | | | | +-------------->| | Token: 0x4b | FETCH | | | Observe: 0 (Register) | | | | OSCORE: {kid: 0x05 ; piv: 501; | | | | kid context: 0x57ab2e; ...} | | | | Uri-Host: sensor.example | | | | Proxy-Scheme: coap | | | | Listen-To- | | | | Multicast-Responses: {[1, bstr(SRV_ADDR), | | | | SRV_PORT, 0x7b, | | | | bstr(GRP_ADDR), | | | | GRP_PORT] | | | | } | | | | | | | | 0xff | | | | Encrypted_payload { | | | | 0x01 (GET), | | | | Observe: 0 (Register), | | | | Uri-Path: r | | | | | | | | } | | | | | | | | | | | | (The proxy starts listening to the | | | | GRP_ADDR address and the GRP_PORT port.) | | | | | | | | (The proxy adds C1 to | | | | its list of observers.) | | | | |<--------------| | | | ACK | | : : : : : : : : : : : : | | | | | +------>| | Token: 0x01 | | FETCH | | Observe: 0 (Register) | | | | OSCORE: {kid: 0x02; piv: 201; ...} | | | | Uri-Host: sensor.example | | | | Proxy-Scheme: coap | | | | | | | | 0xff | | | | Encrypted_payload { | | | | 0x01 (GET), | | | | Observe: 0 (Register), | | | | Uri-Path: r, | | | | | | | | } | | | | | | +-------->| Token: 0x5f | | | FETCH | Observe: 0 (Register) | | | | OSCORE: {kid: 0x02; piv: 201; ...} | | | | Uri-Host: sensor.example | | | | | | | | 0xff | | | | Encrypted_payload { | | | | 0x01 (GET), | | | | Observe: 0 (Register), | | | | Uri-Path: r, | | | | | | | | } | | | | | | |<--------+ Token: 0x5f | | | 2.05 | OSCORE: {piv: 401; ...} | | | | Max-Age: 0 | | | | | | | | 0xff | | | | Encrypted_payload { | | | | 5.03 (Service Unavailable), | | | | Content-Format: application/ | | | | informative-response+cbor, | | | | , | | | | 0xff, | | | | CBOR_payload { | | | | tp_info : [1, bstr(SRV_ADDR), | | | | SRV_PORT, 0x7b, | | | | bstr(GRP_ADDR), GRP_PORT], | | | | ph_req : bstr(0x05 | OPT | 0xff | | | | | PAYLOAD | SIGN), | | | | last_notif : bstr(0x45 | OPT | 0xff | | | | | PAYLOAD | SIGN), | | | | join_uri : "coap://myGM/ | | | | ace-group/myGroup", | | | | sec_gp : "myGroup" | | | | } | | | | } | | | | | |<------+ | Token: 0x01 | | 2.05 | | OSCORE: {piv: 401; ...} | | | | | | | | 0xff | | | | (Same Encrypted_payload) | | | | | +------>| | Token: 0x02 | | FETCH | | Observe: 0 (Register) | | | | OSCORE: {kid: 0x05; piv: 501; | | | | kid context: 57ab2e; ...} | | | | Uri-Host: sensor.example | | | | Proxy-Scheme: coap | | | | Listen-To- | | | | Multicast-Responses: {[1, bstr(SRV_ADDR), | | | | SRV_PORT, 0x7b, | | | | bstr(GRP_ADDR), | | | | GRP_PORT] | | | | } | | | | | | | | 0xff | | | | Encrypted_payload { | | | | 0x01 (GET), | | | | Observe: 0 (Register), | | | | Uri-Path: r | | | | | | | | } | | | | | | | | | | | | (The proxy adds C2 to | | | | its list of observers.) | |<------| | | | ACK | | | | | | : : : : : : : : (The value of the resource : : : : /r changes to "5678".) : : : : | | | (*) | | | |<--------+ Token: 0x7b | | | 2.05 | Observe: 11 | | | | OSCORE: {kid: 0x05; piv: 502; ...} | | | | | | | | 0xff | | | | Encrypted_payload { | | | | 2.05 (Content), | | | | Observe: [empty], | | | | Content-Format: application/cbor, | | | | , | | | | 0xff, | | | | CBOR_Payload: "5678" | | | | } | | | | | | | | |<--------------+ | Token: 0x4b | 2.05 | | | Observe: 54123 | | | | OSCORE: {kid: 0x05; piv: 502; ...} | | | | | | | | 0xff | | | | (Same Encrypted_payload and Signature) | | | | | |<------+ | Token: 0x02 | | 2.05 | | Observe: 54123 | | | | OSCORE: {kid: 0x05; piv: 502; ...} | | | | | | | | 0xff | | | | (Same Encrypted_payload and signature) | | | | (*) Sent over IP multicast to GROUP_ADDR:GROUP_PORT and protected with Group OSCORE end-to-end between the server and the clients. ]]> Unlike in the unprotected example in , the proxy does not have all the information to perform request deduplication, and can only recognize the identical request once the client sends the ticket request.

Example with a Proxy and Deterministic Requests This section provides an example when a proxy P is used between the clients and the server, and Group OSCORE is used to protect multicast notifications end-to-end between the server and the clients. In addition, the phantom request is especially a deterministic request (see ), which is protected with the pairwise mode of Group OSCORE as defined in .

Assumptions and Walkthrough The example provided in this appendix as reflected by the message exchange shown in assumes the following.

The OSCORE group supports deterministic requests. Thus, the server creates the phantom request as a deterministic request , and stores it locally as one of its issued phantom requests, without starting the group observation yet.
The server makes the phantom request available through other means, e.g., a pub-sub broker, together with the transport-specific information for listening to multicast notifications bound to the phantom request (see ).
Since the phantom request is a deterministic request, the server can more efficiently make it available in its smaller, plain version. The clients can obtain it from the particular alternative source, and compute the protected version to use from the retrieved plain version.
If the client does not rely on a proxy between itself and the server, it simply sets the group observation and starts listening to multicast notifications. Building on point (2) above, the same would happen if the phantom request would not be specifically a deterministic request.
If the client relies on a proxy between itself and the server, it uses the phantom request as a ticket request. However, unlike the case considered in when the ticket request is not deterministic, the client does not include a Listen-to-Multicast-Responses option in the phantom request sent to the proxy.
Unlike for the case considered in , here the proxy does not know that the request is exactly a ticket request for subscribing to multicast notifications. Thus, the proxy simply forwards the ticket request to the server as it normally does for any request.
The server receives the ticket request, which is a deviation from the case where the ticket request is not deterministic and stops at the proxy (see ). Then, the server can clearly understand what is happening. In fact, as the result of an early check, the server recognizes the phantom request among the stored ones. This happens through a byte-by-byte comparison of the incoming message minus the transport-related fields, i.e., by considering only: i) the outer REST code; ii) the outer options; and iii) the ciphertext and signature from the message payload.
Having recognized the incoming request as one of the self-generated deterministic phantom requests made available at external sources, the server does not perform any OSCORE processing on it. This opens for replying to the proxy with an unprotected response, although not signaling any OSCORE-related error.
The server starts the group observation and replies with an error response, i.e., the usual 5.03 informative response, including: the transport information, the phantom request, and (optionally) the latest notification.
From the received 5.03 response, the proxy retrieves everything needed to set itself as an observer in the group observation, and it starts listening to multicast notifications. If the 5.03 response included a latest notification, the proxy caches it and forwards it back to the client, otherwise it replies with an empty ACK (if it has not done it already and the request from the client was Confirmable).
Like in the case with a non-deterministic phantom request considered in , the proxy fans out the multicast notifications to the origin clients as they come. Also, as new clients following the first one contact the proxy, this does not have to contact the server again as in , since the deterministic phantom request would produce a cache hit as per . Thus, the proxy can serve such clients with the latest fresh multicast notification from its cache.

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

Two clients C_1 and C_2 register to observe a resource /r at a Server S, which has address SRV_ADDR and listens to the port number SRV_PORT. Before the following exchanges occur, no clients are observing the resource /r , which has value "1234".
The server S sends multicast notifications to the IP multicast address GRP_ADDR and port number GRP_PORT, and starts the group observation upon receiving a registration request from a first client that wishes to start a traditional observation on the resource /r.
S is a member of the OSCORE group with 'kid context' = 0x57ab2e as Group ID. In the OSCORE group, S has 'kid' = 0x05 as Sender ID, and SN_5 = 501 as Sender Sequence Number.

In addition:

The proxy has address PRX_ADDR and listens to the port number PRX_PORT.
The deterministic client in the OSCORE group has 'kid' = 0x09.

Unless explicitly indicated, all messages transmitted on the wire are sent over unicast and protected with Group OSCORE end-to-end between a client and the server. | | Token: 0x4a | FETCH | | | Uri-Host: sensor.example | | | | Observe: 0 (Register) | | | | OSCORE: {kid: 0x09 ; piv: 0 ; | | | | kid context: 0x57ab2e ; ... } | | | | Proxy-Scheme: coap | | | | | | | | 0xff | | | | Encrypted_payload { | | | | 0x01 (GET), | | | | Observe: 0 (Register), | | | | Uri-Path: r, | | | | | | | | } | | | | | | +-------->| Token: 0x5e | | | FETCH | Uri-Host: sensor.example | | | | Observe: 0 (Register) | | | | OSCORE: {kid: 0x09 ; piv: 0 ; | | | | kid context: 0x57ab2e ; ... } | | | | | | | | 0xff | | | | Encrypted_payload { | | | | 0x01 (GET), | | | | Observe: 0 (Register), | | | | Uri-Path: r, | | | | | | | | } | | | | | | | | (S recognizes PH_REQ through byte-by-byte | | | | comparison against the stored one, and | | | | skips any OSCORE processing) | | | | | | | | (S allocates the available | | | | Token value 0x7b .) | | | | | | | | (S sends to itself PH_REQ, with Token 0x7b | | | | and as coming from the IP multicast | | | | address GRP_ADDR; now the OSCORE | | | | processing does happen, as specified | | | | for a deterministic request) | | | | | | | -------| | | | / | | | | \------>| Token: 0x7b | | | FETCH | Uri-Host: sensor.example | | | | Observe: 0 (Register) | | | | OSCORE: {kid: 0x09 ; piv: 0 ; | | | | kid context: 0x57ab2e ; ... } | | | | | | | | 0xff | | | | Encrypted_payload { | | | | 0x01 (GET), | | | | Observe: 0 (Register), | | | | Uri-Path: r, | | | | | | | | } | | | | | | | | (S prepares the "last notification" | | | | response defined below) | | | | | | | | 0x45 (2.05 Content) | | | | Observe: 10 | | | | OSCORE: {kid: 0x05 ; piv: 501 ; ...} | | | | Max-Age: 3000 | | | | | | | | 0xff | | | | Encrypted_payload { | | | | 0x45 (2.05 Content), | | | | Observe: [empty], | | | | CBOR_Payload: "1234" | | | | } | | | | | | | | | | | | (S responds to the proxy with an | | | | unprotected informative response) | | | | | | |<--------| Token: 0x5e | | | 5.03 | Content-Format: application/ | | | | informative-response+cbor | | | | Max-Age: 0 | | | | 0xff, | | | | CBOR_payload { | | | | tp_info : [1, bstr(SRV_ADDR), SRV_PORT, | | | | 0x7b, bstr(GRP_ADDR), | | | | GRP_PORT], | | | | last_notif : | | | | } | | | | } | | | | | | | | (P extracts PH_REQ and starts listening | | | | to multicast notifications with Token | | | | 0x7b at GRP_ADDR:GRP_PORT) | | | | | | | | (P extracts the "last notification" | | | | response, caches it and forwards | | | | it back to C1) | | | | |<--------------+ | Token: 0x4a | 2.05 | | | Observe: 54120 | | | | OSCORE: {kid: 0x05 ; piv: 501 ; ...} | | | | Max-Age: 2995 | | | | | | | | 0xff | | | | Encrypted_payload { | | | | 0x45 (2.05 Content), | | | | Observe: [empty], | | | | CBOR_Payload: "1234" | | | | } | | | | | | | | : : : | : : : | : : : | | | | | | | | | (C2 obtains PH_REQ and sends it to P) | | | | | +------>| | Token: 0x01 | | FETCH | | Uri-Host: sensor.example | | | | Observe: 0 (Register) | | | | OSCORE: {kid: 0x09 ; piv: 0 ; | | | | kid context: 0x57ab2e; ...} | | | | Proxy-Scheme: coap | | | | | | | | 0xff | | | | Encrypted_payload { | | | | 0x01 (GET), | | | | Observe: 0 (Register), | | | | Uri-Path: r, | | | | | | | | } | | | | | | | | (P serves C2 from it cache) | | | | | |<------+ | Token: 0x01 | | 2.05 | | Observe: 54120 | | | | OSCORE: {kid: 0x05 ; piv: 501 ; ...} | | | | Max-Age: 1800 | | | | | | | | 0xff | | | | Encrypted_payload { | | | | 0x45 (2.05 Content), | | | | Observe: [empty], | | | | CBOR_Payload: "1234" | | | | } | | | | | | | | : : : | : : : | : : : | | | | | | | | | (The value of the resource | | | | /r changes to "5678".) | | | | | | | (*) | | | |<--------| Token: 0x7b | | | 2.05 | Observe: 11 | | | | OSCORE: {kid: 0x05; piv: 502 ; ...} | | | | | | | | 0xff | | | | Encrypted_payload { | | | | 0x45 (2.05 Content), | | | | Observe: [empty], | | | | Content-Format: application/cbor, | | | | , | | | | 0xff, | | | | CBOR_Payload: "5678" | | | | } | | | | | | | | | | | | (P updates its cache entry | | | | with this notification) | | | | |<--------------+ | Token: 0x4a | 2.05 | | | Observe: 54123 | | | | OSCORE: {kid: 0x05; piv: 502 ; ...} | | | | | | | | 0xff | | | | (Same Encrypted_payload and signature) | | | | | |<------+ | Token: 0x01 | | 2.05 | | Observe: 54123 | | | | OSCORE: {kid: 0x05; piv: 502 ; ...} | | | | | | | | 0xff | | | | (Same Encrypted_payload and signature) | | | | (*) Sent over IP multicast to GROUP_ADDR:GROUP_PORT and protected with Group OSCORE end-to-end between the server and the clients. ]]>

Document Updates RFC EDITOR: PLEASE REMOVE THIS SECTION.

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.

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.

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 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; and by the H2020 project SIFIS-Home (Grant agreement 952652).