Discovery of OSCORE Groups with the CoRE Resource Directory
Internet-Draft draft-tiloca-core-oscore-discovery-12
Marco Tiloca
RISE AB
Isafjordsgatan 22
Kista
16440 Stockholm
Sweden
marco.tiloca@ri.se
Christian Amsuess
Hollandstr. 12/4
Vienna
1020
Austria
christian@amsuess.com
Peter van der Stok
Consultant
+31-492474673 (Netherlands), +33-966015248 (France)
stokcons@bbhmail.nl
05 September 2022
Applications
CoRE Working Group
Internet-Draft
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.
Discussion Venues
Discussion of this document takes place on the Constrained RESTful Environments Working Group mailing list (core@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/core/.
Source for this draft and an issue tracker can be found at https://gitlab.com/crimson84/draft-tiloca-core-oscore-discovery.
Introduction
The Constrained Application Protocol (CoAP) [RFC7252] supports group communication over IP multicast [I-D.ietf-core-groupcomm-bis] to improve efficiency and latency of communication and reduce bandwidth requirements. A set of CoAP endpoints constitutes an application group by sharing a common pool of resources, that can be efficiently accessed through group communication. The members of an application group may be members of a security group, thus sharing a common set of keying material to secure group communication.
The security protocol Group Object Security for Constrained RESTful Environments (Group OSCORE) [I-D.ietf-core-oscore-groupcomm] builds on OSCORE [RFC8613] and protects CoAP messages end-to-end in group communication contexts through CBOR Object Signing and Encryption (COSE) [RFC9052][RFC9053]. An application group may rely on one or more security groups, and a same security group may be used by multiple application groups at the same time.
A CoAP endpoint relies on a Group Manager (GM) to join a security group and get the group keying material. The joining process in [I-D.ietf-ace-key-groupcomm-oscore] is based on the ACE framework for Authentication and Authorization in constrained environments [RFC9200], with the joining endpoint and the GM acting as ACE Client and Resource Server, respectively. That is, the joining endpoint accesses the group-membership resource exported by the GM and associated with the security group to join.
Typically, devices store a static X509 IDevID certificate installed at manufacturing time [RFC8995]. This is used at deployment time during an enrollment process that provides the devices with an Operational Certificate, possibly updated during the device lifetime. Operational Certificates may specify information to join security groups, especially a reference to the group-membership resources to access at the respective GMs.
However, it is usually impossible to provide such precise information to freshly deployed devices, as part of their (early) Operational Certificate. This can be due to a number of reasons: (1) the security group(s) to join and the responsible GM(s) are generally unknown at manufacturing time; (2) a security group of interest is created, or the responsible GM is deployed, only after the device is enrolled and fully operative in the network; (3) information related to existing security groups or to their GMs has changed. This requires a method for CoAP endpoints to dynamically discover security groups and their GM, and to retrieve relevant information about deployed groups.
To this end, CoAP endpoints can use descriptions and links of group-membership resources at GMs, to discover security groups and retrieve the information required for joining them. With the discovery process of security groups expressed in terms of links to resources, the remaining problem is the discovery of those links. The CoRE Resource Directory (RD) [RFC9176] allows such discovery in an efficient way, and it is expected to be used in many setups that would benefit of security group discovery.
This specification builds on this approach and describes how CoAP endpoints can use the RD to perform the link discovery steps, in order to discover security groups and retrieve the information required to join them through their GM. In short, the GM registers as an endpoint with the RD. The resulting registration resource includes one link per security group under that GM, specifying the path to the related group-membership resource to access for joining that group.
Additional descriptive information about the security group is stored with the registered link. In a RD based on Link Format [RFC6690] as defined in [RFC9176], this information is specified as target attributes of the registered link, and includes the identifiers of the application groups which use that security group. This enables a lookup of those application groups at the RD, where they are separately announced by a Commissioning Tool (see Appendix A of [RFC9176]).
When querying the RD for security groups, a CoAP endpoint can use CoAP observation [RFC7641]. This results in automatic notifications on the creation of new security groups or the update of existing groups. Thus, it facilitates the early deployment of CoAP endpoints, i.e., even before the GM is deployed and security groups are created.
Interaction examples are provided in Link Format, as well as in the Constrained RESTful Application Language CoRAL [I-D.ietf-core-coral] with reference to a CoRAL-based RD [I-D.hartke-t2trg-coral-reef]. While all the CoRAL examples show the CoRAL textual serialization format, its binary serialization format is used on the wire.
[ NOTE:
The reported CoRAL examples are based on the textual representation used until version -03 of [I-D.ietf-core-coral]. These will be revised to use the CBOR diagnostic notation instead.
]
The approach in this document is consistent with, but not limited to, the joining of security groups defined in [I-D.ietf-ace-key-groupcomm-oscore].
Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
This specification requires readers to be familiar with the terms and concepts discussed in [RFC9176] and [RFC6690], as well as in [I-D.ietf-core-coral]. Readers should also be familiar with the terms and concepts discussed in [RFC7252][I-D.ietf-core-groupcomm-bis], [I-D.ietf-core-oscore-groupcomm] and [I-D.ietf-ace-key-groupcomm-oscore].
Terminology for constrained environments, such as "constrained device" and "constrained-node network", is defined in [RFC7228].
Consistently with the definitions from Section 2.1 of [I-D.ietf-core-groupcomm-bis], this document also refers to the following terminology.
- CoAP group: a set of CoAP endpoints all configured to receive CoAP multicast messages sent to the group's associated IP multicast address and UDP port. An endpoint may be a member of multiple CoAP groups by subscribing to multiple IP multicast addresses.
- Security group: a set of CoAP endpoints that share the same security material, and use it to protect and verify exchanged messages. A CoAP endpoint may be a member of multiple security groups. There can be a one-to-one or a one-to-many relation between security groups and CoAP groups.
This document especially considers a security group to be an OSCORE group, where all members share one OSCORE Security Context to protect group communication with Group OSCORE [I-D.ietf-core-oscore-groupcomm]. However, the approach defined in this document can be used to support the discovery of different security groups than OSCORE groups.
- Application group: a set of CoAP endpoints that share a common set of resources. An endpoint may be a member of multiple application groups. An application group can be associated with one or more security groups, and multiple application groups can use the same security group. Application groups are announced in the RD by a Commissioning Tool, according to the RD-Groups usage pattern (see Appendix A of [RFC9176]).
Like [I-D.ietf-core-oscore-groupcomm], this document also uses the following term.
- Authentication credential: set of information associated with an entity, including that entity's public key and parameters associated with the public key. Examples of authentication credentials are CBOR Web Tokens (CWTs) and CWT Claims Sets (CCSs) [RFC8392], X.509 certificates [RFC7925] and C509 certificates [I-D.ietf-cose-cbor-encoded-cert].
Registration of Group Manager Endpoints
During deployment, a Group Manager (GM) can find the CoRE Resource Directory (RD) as described in Section 4 of [RFC9176].
Afterwards, the GM registers as an endpoint with the RD, as described in Section 5 of [RFC9176]. The GM SHOULD NOT use the Simple Registration approach described in Section 5.1 of [RFC9176].
When registering with the RD, the GM also registers the links to all the group-membership resources it has at that point in time, i.e., one for each of its security groups.
In the registration request, each link to a group-membership resource has as target the URI of that resource at the GM. Also, it specifies a number of descriptive parameters as defined in Section 2.1.
Furthermore, the GM MAY additionally register the link to its resource implementing the ACE authorization information endpoint (see Section 5.10.1 of [RFC9200]). A joining node can provide the GM with its own access token by sending it in a request targeting that resource, thus proving to be authorized to join certain security groups (see Section 6.1 of [I-D.ietf-ace-key-groupcomm-oscore]). In such a case, the link MUST include the parameter 'rt' with value "ace.ai", defined in Section 8.2 of [RFC9200].
Parameters
For each registered link to a group-membership resource at a GM, the following parameters are specified together with the link.
In the RD defined in [RFC9176] and based on Link Format, each parameter is specified in a target attribute with the same name.
In a RD based on CoRAL, such as the one defined in [I-D.hartke-t2trg-coral-reef], each parameter is specified in a nested element with the same name.
- 'ct', specifying the content format used with the group-membership resource at the Group Manager, with value "application/ace-groupcomm+cbor" registered in Section 8.2 of [I-D.ietf-ace-key-groupcomm].
Note: The examples in this document use the provisional value 65000 from the range "Reserved for Experimental Use" of the "CoAP Content-Formats" registry, within the "CoRE Parameters" registry.
- 'rt', specifying the resource type of the group-membership resource at the Group Manager, with value "core.osc.gm" registered in Section 25.11 of [I-D.ietf-ace-key-groupcomm-oscore].
- 'if', specifying the interface description for accessing the group-membership resource at the Group Manager, with value "ace.group" registered in Section 8.10 of [I-D.ietf-ace-key-groupcomm].
- 'sec-gp', specifying the name of the security group of interest, as a stable and invariant identifier, such as the group name used in [I-D.ietf-ace-key-groupcomm-oscore]. This parameter MUST specify a single value.
- 'app-gp', specifying the name(s) of the application group(s) associated with the security group of interest indicated by 'sec-gp'. This parameter MUST occur once for each application group, and MUST specify only a single application group.
When a security group is created at the GM, the names of the application groups using it are also specified as part of the security group configuration (see [I-D.ietf-ace-oscore-gm-admin]). Thus, when registering the links to its group-membership resource, the GM is aware of the application groups and their names.
If a different entity than the GM registers the security groups to the RD, e.g., a Commissioning Tool, this entity has to also be aware of the application groups and their names to specify. To this end, it can obtain them from the GM or from the Administator that created the security groups at the GM (see [I-D.ietf-ace-oscore-gm-admin]).
Optionally, the following parameters can also be specified. If Link Format is used, the value of each of these parameters is encoded as a text string.
- 'hkdf', specifying the HKDF Algorithm used in the security group. If present, this parameter MUST specify a single value, which is taken from the 'Value' column of the "COSE Algorithms" Registry [COSE.Algorithms].
- 'cred_fmt', specifying the format of the authentication credentials used in the security group. If present, this parameter MUST specify a single value, which is taken from the 'Label' column of the "COSE Header Parameters" Registry [COSE.Header.Parameters]. Acceptable values denote a format that MUST explicitly provide the public key as well as the comprehensive set of information related to the public key algorithm, including, e.g., the used elliptic curve (when applicable).
At the time of writing this specification, acceptable formats of authentication credentials are CBOR Web Tokens (CWTs) and CWT Claim Sets (CCSs) [RFC8392], X.509 certificates [RFC7925] and C509 certificates [I-D.ietf-cose-cbor-encoded-cert]. Further formats may be available in the future, and would be acceptable to use as long as they comply with the criteria defined above.
[ 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. ]
- 'sign_enc_alg', specifying the encryption algorithm used to encrypt messages in the security group, when these are also signed, e.g., as in the group mode of Group OSCORE (see Section 8 of [I-D.ietf-core-oscore-groupcomm]). If present, this parameter MUST specify a single value, which is taken from the 'Value' column of the "COSE Algorithms" Registry [COSE.Algorithms].
- 'sign_alg', specifying the algorithm used to sign messages in the security group. If present, this parameter MUST specify a single value, which is taken from the 'Value' column of the "COSE Algorithms" Registry [COSE.Algorithms].
- 'sign_alg_crv', specifying the elliptic curve (if applicable) for the algorithm used to sign messages in the security group. If present, this parameter MUST specify a single value, which is taken from the 'Value' column of the "COSE Elliptic Curves" Registry [COSE.Elliptic.Curves].
- 'sign_key_kty', specifying the key type of countersignature keys used to sign messages in the security group. If present, this parameter MUST specify a single value, which is taken from the 'Value' column of the "COSE Key Types" Registry [COSE.Key.Types].
- 'sign_key_crv', specifying the elliptic curve (if applicable) of countersignature keys used to sign messages in the security group. If present, this parameter MUST specify a single value, which is taken from the 'Value' column of the "COSE Elliptic Curves" Registry [COSE.Elliptic.Curves].
- 'alg', specifying the encryption algorithm used to encrypt messages in the security group, when these are encrypted with pairwise encryption keys, e.g., as in the pairwise mode of Group OSCORE (see Section 9 of [I-D.ietf-core-oscore-groupcomm]). If present, this parameter MUST specify a single value, which is taken from the 'Value' column of the "COSE Algorithms" Registry [COSE.Algorithms].
- 'ecdh_alg', specifying the ECDH algorithm used to derive pairwise encryption keys in the security group, e.g., as for the pairwise mode of Group OSCORE (see Section 2.4 of [I-D.ietf-core-oscore-groupcomm]). If present, this parameter MUST specify a single value, which is taken from the 'Value' column of the "COSE Algorithms" Registry [COSE.Algorithms].
- 'ecdh_alg_crv', specifying the elliptic curve for the ECDH algorithm used to derive pairwise encryption keys in the security group. If present, this parameter MUST specify a single value, which is taken from the 'Value' column of the "COSE Elliptic Curves" Registry [COSE.Elliptic.Curves].
- 'ecdh_key_kty', specifying the key type of keys used with an ECDH algorithm to derive pairwise encryption keys in the security group. If present, this parameter MUST specify a single value, which is taken from the 'Value' column of the "COSE Key Types" Registry [COSE.Key.Types].
- 'ecdh_key_crv', specifying the elliptic curve of keys used with an ECDH algorithm to derive pairwise encryption keys in the security group. If present, this parameter MUST specify a single value, which is taken from the 'Value' column of the "COSE Elliptic Curves" Registry [COSE.Elliptic.Curves].
- 'det_hash_alg', specifying the hash algorithm used in the security group when producing deterministic requests, as defined in [I-D.amsuess-core-cachable-oscore]. If present, this parameter MUST specify a single value, which is taken from the 'Value' column of the "COSE Algorithms" Registry [COSE.Algorithms]. This parameter MUST NOT be present if the security group does not use deterministic requests.
- 'rekeying_scheme', specifying the rekeying scheme used in the security group for distributing new group keying meterial to the group members. If present, this parameter MUST specify a single value, which is taken from the 'Value' column of the "ACE Groupcomm Rekeying Schemes" registry defined in Section 11.14 of [I-D.ietf-ace-key-groupcomm].
If the security group does not recur to message signing, then the parameters 'sign_enc_alg', 'sign_alg', 'sign_alg_crv', 'sign_key_kty' and 'sign_key_crv' MUST NOT be present. For instance, this is the case for a security group that uses Group OSCORE and uses only the pairwise mode (see Section 9 of [I-D.ietf-core-oscore-groupcomm]).
If the security group does not recur to message encryption through pairwise encryption keys, then the parameters 'alg', 'ecdh_alg', 'ecdh_alg_crv', 'ecdh_key_kty' and 'ecdh_key_crv' MUST NOT be present. For instance, this is the case for a security group that uses Group OSCORE and uses only the group mode see Section 8 of [I-D.ietf-core-oscore-groupcomm]).
Note that the values registered in the COSE Registries [COSE.Algorithms][COSE.Elliptic.Curves][COSE.Key.Types] are strongly typed. On the contrary, Link Format is weakly typed and thus does not distinguish between, for instance, the string value "-10" and the integer value -10.
Thus, in RDs that return responses in Link Format, string values which look like an integer are not supported. Therefore, such values MUST NOT be advertised through the corresponding parameters above.
A CoAP endpoint that queries the RD to discover security groups and their group-membership resource to access (see Section 4) would benefit from the information above as follows.
- The values of 'cred_fmt', 'sign_alg', 'sign_alg_crv', 'sign_key_kty', 'sign_key_crv', 'ecdh_alg', 'ecdh_alg_crv', 'ecdh_key_kty' and 'ecdh_key_crv' related to a group-membership resource provide an early knowledge of the format of authentication credentials as well as of the type of public keys used in the security group.
Thus, the CoAP endpoint does not need to ask the GM for this information as a preliminary step before the joining process, or to perform a trial-and-error joining exchange with the GM. Hence, at the very first step of the joining process, the CoAP endpoint is able to provide the GM with its own authentication credential in the correct expected format and including a public key of the correct expected type.
- The values of 'hkdf', 'sign_enc_alg', 'sign_alg', 'alg' and 'ecdh_alg' related to a group-membership resource provide an early knowledge of the algorithms used in the security group.
Thus, the CoAP endpoint is able to decide whether to actually proceed with the joining process, depending on its support for the indicated algorithms.
Relation Link to Authorization Server
For each registered link to a group-membership resource, the GM MAY additionally specify the link to the ACE Authorization Server (AS) [RFC9200] associated with the GM, and issuing authorization credentials to join the security group as described in [I-D.ietf-ace-key-groupcomm-oscore].
The link to the AS has as target the URI of the resource where to send an authorization request to.
In the RD defined in [RFC9176] and based on Link Format, the link to the AS is separately registered with the RD, and includes the following parameters as target attributes.
- 'rel', with value "authorization_server".
- 'anchor', with value the target of the link to the group-membership resource at the GM.
In a RD based on CoRAL, such as the one defined in [I-D.hartke-t2trg-coral-reef], this is mapped (as describe there) to a link from the registration resource to the AS, using the <http://www.iana.org/assignments/relation/authorization_server> link relation type.
Registration Example
The example below shows a GM with endpoint name "gm1" and address 2001:db8::ab that registers with the RD.
The GM specifies the value of the 'sec-gp' parameter for accessing the security group with name "feedca570000", and used by the application group with name "group1" specified with the value of the 'app-gp' parameter.
The countersignature algorithm used in the security group is EdDSA (-8), with elliptic curve Ed25519 (6). Authentication credentials used in the security group are X.509 certificates [RFC7925], which is signaled through the COSE Header Parameter "x5chain" (33). The ECDH algorithm used in the security group is ECDH-SS + HKDF-256 (-27), with elliptic curve X25519 (4).
In addition, the GM specifies the link to the ACE Authorization Server associated with the GM, to which a CoAP endpoint should send an Authorization Request for joining the corresponding security group (see [I-D.ietf-ace-key-groupcomm-oscore]).
Example in Link Format
Request: GM -> RD

Req: POST coap://rd.example.com/rd?ep=gm1
Content-Format: 40
Payload:
</ace-group/feedca570000>;ct=65000;rt="core.osc.gm";if="ace.group";
                             sec-gp="feedca570000";app-gp="group1";
                             cred_fmt="33";sign_enc_alg="10";
                             sign_alg="-8";sign_alg_crv="6";
                             alg="10";ecdh_alg="-27";ecdh_alg_crv="4",
<coap://as.example.com/token>;rel="authorization-server";
      anchor="coap://[2001:db8::ab]/ace-group/feedca570000"

Response: RD -> GM

Res: 2.01 Created
Location-Path: /rd/4521

Example in CoRAL
Request: GM -> RD

Req: POST coap://rd.example.com/rd?ep=gm1
Content-Format: TBD123456 (application/coral+cbor)

Payload:
#using <http://coreapps.org/core.oscore-discovery#>
#using reef = <http://coreapps.org/reef#>
#using iana = <http://www.iana.org/assignments/relation/>

#base <coap://[2001:db8::ab]/>
reef:rd-item </ace-group/feedca570000> {
   reef:ct 65000
   reef:rt "core.osc.gm"
   reef:if "ace.group"
   sec-gp "feedca570000"
   app-gp "group1"
   cred_fmt 33
   sign_enc_alg 10
   sign_alg -8
   sign_alg_crv 6
   alg 10
   ecdh_alg -27
   ecdh_alg_crv 4
   iana:authorization-server <coap://as.example.com/token>
}

Response: RD -> GM

Res: 2.01 Created
Location-Path: /rd/4521

Addition and Update of Security Groups
The GM is responsible to refresh the registration of all its group-membership resources in the RD. This means that the GM has to update the registration within its lifetime as per Section 5.3.1 of [RFC9176], and has to change the content of the registration when a group-membership resource is added/removed, or if its parameters have to be changed, such as in the following cases.
- The GM creates a new security group and starts exporting the related group-membership resource.
- The GM dismisses a security group and stops exporting the related group-membership resource.
- Information related to an existing security group changes, e.g., the list of associated application groups.
To perform an update of its registrations, the GM can re-register with the RD and fully specify all links to its group-membership resources.
Alternatively, the GM can perform a PATCH/iPATCH [RFC8132] request to the RD, as per Section 5.3.3 of [RFC9176]. This requires new media-types to be defined in future standards, to apply a new document as a patch to an existing stored document.
Addition Example
The example below shows how the GM from Section 2 re-registers with the RD. When doing so, it specifies:
- The same previous group-membership resource associated with the security group with name "feedca570000".
- An additional group-membership resource associated with the security group with name "ech0ech00000" and used by the application group "group2".
- A third group-membership resource associated with the security group with name "abcdef120000" and used by two application groups, namely "group3" and "group4".
Furthermore, the GM relates the same Authorization Server also to the security groups "ech0ech00000" and "abcdef120000".
Example in Link Format
Request: GM -> RD

Req: POST coap://rd.example.com/rd?ep=gm1
Content-Format: 40
Payload:
</ace-group/feedca570000>;ct=65000;rt="core.osc.gm";if="ace.group";
                             sec-gp="feedca570000";app-gp="group1";
                             cred_fmt="33";sign_enc_alg="10";
                             sign_alg="-8";sign_alg_crv="6";
                             alg="10";ecdh_alg="-27";ecdh_alg_crv="4",
</ace-group/ech0ech00000>;ct=65000;rt="core.osc.gm";if="ace.group";
                             sec-gp="ech0ech00000";app-gp="group2";
                             cred_fmt="33";sign_enc_alg="10";
                             sign_alg="-8";sign_alg_crv="6";
                             alg="10";ecdh_alg="-27";ecdh_alg_crv="4",
</ace-group/abcdef120000>;ct=65000;rt="core.osc.gm";if="ace.group";
                             sec-gp="abcdef120000";app-gp="group3";
                             app-gp="group4";cred_fmt="33";
                             sign_enc_alg="10";sign_alg="-8";
                             sign_alg_crv="6";alg="10";ecdh_alg="-27";
                             ecdh_alg_crv="4",
<coap://as.example.com/token>;rel="authorization-server";
      anchor="coap://[2001:db8::ab]/ace-group/feedca570000",
<coap://as.example.com/token>;rel="authorization-server";
      anchor="coap://[2001:db8::ab]/ace-group/ech0ech00000",
<coap://as.example.com/token>;rel="authorization-server";
      anchor="coap://[2001:db8::ab]/ace-group/abcdef120000"

Response: RD -> GM

Res: 2.04 Changed
Location-Path: /rd/4521

Example in CoRAL
Request: GM -> RD

Req: POST coap://rd.example.com/rd?ep=gm1
Content-Format: TBD123456 (application/coral+cbor)

Payload:
#using <http://coreapps.org/core.oscore-discovery#>
#using reef = <http://coreapps.org/reef#>
#using iana = <http://www.iana.org/assignments/relation/>

#base <coap://[2001:db8::ab]/>
reef:rd-item </ace-group/feedca570000> {
   reef:ct 65000
   reef:rt "core.osc.gm"
   reef:if "ace.group"
   sec-gp "feedca570000"
   app-gp "group1"
   cred_fmt 33
   sign_enc_alg 10
   sign_alg -8
   sign_alg_crv 6
   alg 10
   ecdh_alg -27
   ecdh_alg_crv 4
   iana:authorization-server <coap://as.example.com/token>
}
reef:rd-item </ace-group/ech0ech00000> {
   reef:ct 65000
   reef:rt "core.osc.gm"
   reef:if "ace.group"
   sec-gp "ech0ech00000"
   app-gp "group2"
   cred_fmt 33
   sign_enc_alg 10
   sign_alg -8
   sign_alg_crv 6
   alg 10
   ecdh_alg -27
   ecdh_alg_crv 4
   iana:authorization-server <coap://as.example.com/token>
}
reef:rd-item </ace-group/abcdef120000> {
   reef:ct 65000
   reef:rt "core.osc.gm"
   reef:if "ace.group"
   sec-gp "abcdef120000"
   app-gp "group3"
   app-gp "group4"
   cred_fmt 33
   sign_enc_alg 10
   sign_alg -8
   sign_alg_crv 6
   alg 10
   ecdh_alg -27
   ecdh_alg_crv 4
   iana:authorization-server <coap://as.example.com/token>
}

Response: RD -> GM

Res: 2.04 Changed
Location-Path: /rd/4521

Discovery of Security Groups
A CoAP endpoint that wants to join a security group, hereafter called the joining node, might not have all the necessary information at deployment time. Also, it might want to know about possible new security groups created afterwards by the respective Group Managers.
To this end, the joining node can perform a resource lookup at the RD as per Section 6.1 of [RFC9176], to retrieve the missing pieces of information needed to join the security group(s) of interest. The joining node can find the RD as described in Section 4 of [RFC9176].
The joining node uses the following parameter value for the lookup filtering.
- 'rt' = "core.osc.gm", specifying the resource type of the group-membership resource at the Group Manager, with value "core.osc.gm" registered in Section 25.11 of [I-D.ietf-ace-key-groupcomm-oscore].
The joining node may additionally consider the following parameters for the lookup filtering, depending on the information it has already available.
- 'sec-gp', specifying the name of the security group of interest. This parameter MUST specify a single value.
- 'ep', specifying the registered endpoint of the GM.
- 'app-gp', specifying the name(s) of the application group(s) associated with the security group of interest. This parameter MAY be included multiple times, and each occurrence MUST specify the name of one application group.
- 'if', specifying the interface description for accessing the group-membership resource at the Group Manager, with value "ace.group" registered in Section 8.10 of [I-D.ietf-ace-key-groupcomm].
The response from the RD may include links to a group-membership resource specifying multiple application groups, as all using the same security group. In this case, the joining node is already expected to know the exact application group of interest.
Furthermore, the response from the RD may include the links to different group-membership resources, all specifying a same application group of interest for the joining node, if the corresponding security groups are all used by that application group.
In this case, application policies on the joining node should define how to determine the exact security group to join (see Section 2.1 of [I-D.ietf-core-groupcomm-bis]). For example, different security groups can reflect different security algorithms to use. Hence, a client application can take into account what the joining node supports and prefers, when selecting one particular security group among the indicated ones, while a server application would need to join all of them. Later on, the joining node will be anyway able to join only security groups for which it is actually authorized to be a member (see [I-D.ietf-ace-key-groupcomm-oscore]).
Note that, with RD-based discovery, including the 'app-gp' parameter multiple times would result in finding only the group-membership resource that serves all the specified application groups, i.e., not any group-membership resource that serves either. Therefore, a joining node needs to perform N separate queries with different values for 'app-gp', in order to safely discover the (different) group-membership resource(s) serving the N application groups.
The discovery of security groups as defined in this document is applicable and useful to other CoAP endpoints than the actual joining nodes. In particular, other entities can be interested to discover and interact with the group-membership resource at the Group Manager. These include entities acting as signature checkers, e.g., intermediary gateways, that do not join a security group, but can retrieve authentication credentials of group members from the Group Manager, in order to verify counter signatures of group messages (see Section 3 of [I-D.ietf-core-oscore-groupcomm]).
Discovery Example #1
Consistently with the examples in Section 2 and Section 3, the examples below consider a joining node that wants to join the security group associated with the application group "group1", but that does not know the name of the security group, the responsible GM and the group-membership resource to access.
Example in Link Format
Request: Joining node -> RD

Req: GET coap://rd.example.com/rd-lookup/res
  ?rt=core.osc.gm&app-gp=group1

Response: RD -> Joining node

Res: 2.05 Content
Payload:
<coap://[2001:db8::ab]/ace-group/feedca570000>;ct=65000;
    rt="core.osc.gm";if="ace.group";sec-gp="feedca570000";
    app-gp="group1";cred_fmt="33";sign_enc_alg="10";
    sign_alg="-8";sign_alg_crv="6";alg="10";
    ecdh_alg="-27";ecdh_alg_crv="4"

By performing the separate resource lookup below, the joining node can retrieve the link to the ACE Authorization Server associated with the GM, where to send an Authorization Request for joining the corresponding security group (see [I-D.ietf-ace-key-groupcomm-oscore]).
Request: Joining node -> RD

Req: GET coap://rd.example.com/rd-lookup/res
  ?rel=authorization-server&
   anchor=coap://[2001:db8::ab]/ace-group/feedca570000

Response: RD -> Joining node

Res: 2.05 Content
Payload:
<coap://as.example.com/token>;rel=authorization-server;
      anchor="coap://[2001:db8::ab]/ace-group/feedca570000"

To retrieve the multicast IP address of the CoAP group used by the application group "group1", the joining node performs an endpoint lookup as shown below. The following assumes that the application group "group1" had been previously registered as per Appendix A of [RFC9176], with ff35:30:2001:db8::23 as multicast IP address of the associated CoAP group.
Request: Joining node -> RD

Req: GET coap://rd.example.com/rd-lookup/ep
  ?et=core.rd-group&ep=group1

Response: RD -> Joining node

Res: 2.05 Content
Payload:
</rd/501>;ep="group1";et="core.rd-group";
          base="coap://[ff35:30:2001:db8::23]";rt="core.rd-ep"

Example in CoRAL
Request: Joining node -> RD

Req: GET coap://rd.example.com/rd-lookup/res
  ?rt=core.osc.gm&app-gp=group1
Accept: TBD123456 (application/coral+cbor)

Response: RD -> Joining node

Res: 2.05 Content
Content-Format: TBD123456 (application/coral+cbor)

Payload:
#using <http://coreapps.org/core.oscore-discovery#>
#using reef = <http://coreapps.org/reef#>
#using iana = <http://www.iana.org/assignments/relation/>

#base <coap://[2001:db8::ab]/>
reef:rd-item </ace-group/feedca570000> {
   reef:ct 65000
   reef:rt "core.osc.gm"
   reef:if "ace.group"
   sec-gp "feedca570000"
   app-gp "group1"
   cred_fmt 33
   sign_enc_alg 10
   sign_alg -8
   sign_alg_crv 6
   alg 10
   ecdh_alg -27
   ecdh_alg_crv 4
   iana:authorization-server <coap://as.example.com/token>
}

To retrieve the multicast IP address of the CoAP group used by the application group "group1", the joining node performs an endpoint lookup as shown below. The following assumes that the application group "group1" had been previously registered, with ff35:30:2001:db8::23 as multicast IP address of the associated CoAP group.
Request: Joining node -> RD

Req: GET coap://rd.example.com/rd-lookup/ep
  ?et=core.rd-group&ep=group1
Accept: TBD123456 (application/coral+cbor)

Response: RD -> Joining node

Res: 2.05 Content
Content-Format: TBD123456 (application/coral+cbor)

Payload:
#using <http://coreapps.org/core.oscore-discovery#>
#using reef = <http://coreapps.org/reef#>

reef:rd-unit <./rd/501> {
   reef:ep "group1"
   reef:et "core.rd-group"
   reef:base <coap://[ff35:30:2001:db8::23]>
   reef:rt "core.rd-ep"
}

Discovery Example #2
Consistently with the examples in Section 2 and Section 3, the examples below consider a joining node that wants to join the security group with name "feedca570000", but that does not know the responsible GM, the group-membership resource to access, and the associated application groups.
The examples also show how the joining node uses CoAP observation [RFC7641], in order to be notified of possible changes to the parameters of the group-membership resource. This is also useful to handle the case where the security group of interest has not been created yet, so that the joining node can receive the requested information when it becomes available.
Example in Link Format
Request: Joining node -> RD

Req: GET coap://rd.example.com/rd-lookup/res
  ?rt=core.osc.gm&sec-gp=feedca570000
Observe: 0

Response: RD -> Joining node

Res: 2.05 Content
Observe: 24
Payload:
<coap://[2001:db8::ab]/ace-group/feedca570000>;ct=65000;
    rt="core.osc.gm";if="ace.group";sec-gp="feedca570000";
    app-gp="group1";cred_fmt="33";sign_enc_alg="10";
    sign_alg="-8";sign_alg_crv="6";alg="10";
    ecdh_alg="-27";ecdh_alg_crv="4"

Depending on the search criteria, the joining node performing the resource lookup can get large responses. This can happen, for instance, when the lookup request targets all the group-membership resources at a specified GM, or all the group-membership resources of all the registered GMs, as in the example below.
Request: Joining node -> RD

Req: GET coap://rd.example.com/rd-lookup/res?rt=core.osc.gm

Response: RD -> Joining node

Res: 2.05 Content
Payload:
<coap://[2001:db8::ab]/ace-group/feedca570000>;ct=65000;
    rt="core.osc.gm";if="ace.group";sec-gp="feedca570000";
    app-gp="group1";cred_fmt="33";sign_enc_alg="10";
    sign_alg="-8";sign_alg_crv="6";alg="10";ecdh_alg="-27";
    ecdh_alg_crv="4",
<coap://[2001:db8::ab]/ace-group/ech0ech00000>;ct=65000;
    rt="core.osc.gm";if="ace.group";sec-gp="ech0ech00000";
    app-gp="group2";cred_fmt="33";sign_enc_alg="10";
    sign_alg="-8";sign_alg_crv="6";alg="10";ecdh_alg="-27";
    ecdh_alg_crv="4",
<coap://[2001:db8::ab]/ace-group/abcdef120000>;ct=65000;
    rt="core.osc.gm";if="ace.group";sec-gp="abcdef120000";
    app-gp="group3";app-gp="group4";cred_fmt="33";
    sign_enc_alg="10";sign_alg="-8";sign_alg_crv="6";alg="10";
    ecdh_alg="-27";ecdh_alg_crv="4"

Therefore, it is RECOMMENDED that a joining node which performs a resource lookup with the CoAP Observe option specifies the value of the parameter 'sec-gp' in its GET request sent to the RD.
Example in CoRAL
Request: Joining node -> RD

Req: GET coap://rd.example.com/rd-lookup/res
  ?rt=core.osc.gm&sec-gp=feedca570000
Accept: TBD123456 (application/coral+cbor)
Observe: 0

Response: RD -> Joining node

Res: 2.05 Content
Observe: 24
Content-Format: TBD123456 (application/coral+cbor)

Payload:
#using <http://coreapps.org/core.oscore-discovery#>
#using reef = <http://coreapps.org/reef#>
#using iana = <http://www.iana.org/assignments/relation/>

#base <coap://[2001:db8::ab]/>
reef:rd-item </ace-group/feedca570000> {
   reef:ct 65000
   reef:rt "core.osc.gm"
   reef:if "ace.group"
   sec-gp "feedca570000"
   app-gp "group1"
   cred_fmt 33
   sign_enc_alg 10
   sign_alg -8
   sign_alg_crv 6
   alg 10
   ecdh_alg -27
   ecdh_alg_crv 4
   iana:authorization-server <coap://as.example.com/token>
}

Use Case Example With Full Discovery
In this section, the discovery of security groups is described to support the installation process of a lighting installation in an office building. The described process is a simplified version of one of many processes.
The process described in this section is intended as an example and does not have any particular ambition to serve as recommendation or best practice to adopt. That is, it shows a possible workflow involving a Commissioning Tool (CT) used in a certain way, while it is not meant to prescribe how the workflow should necessarily be.
Assume the existence of four luminaires that are members of two application groups. In the first application group, the four luminaires receive presence messages and light intensity messages from sensors or their proxy. In the second application group, the four luminaires and several other pieces of equipment receive building state schedules.
Each of the two application groups is associated with a different security group and to a different CoAP group with its own dedicated multicast IP address.
The Fairhair Alliance describes how a new device is accepted and commissioned in the network [Fairhair], by means of its certificate stored during the manufacturing process. When commissioning the new device in the installation network, the new device gets a new identity defined by a newly allocated certificate, following the BRSKI specification.
Section 7.3 of [RFC9176] describes how the CT assigns an endpoint name based on the CN field, (CN=ACME) and the serial number of the certificate (serial number = 123x, with 3 < x < 8). Corresponding ep-names ACME-1234, ACME-1235, ACME-1236 and ACME-1237 are also assumed.
It is common practice that locations in the building are specified according to a coordinate system. After the acceptance of the luminaires into the installation network, the coordinate of each device is communicated to the CT. This can be done manually or automatically.
The mapping between location and ep-name is calculated by the CT. For instance, on the basis of grouping criteria, the CT assigns: i) application group "grp_R2-4-015" to the four luminaires; and ii) application group "grp_schedule" to all schedule requiring devices. Also, the device with ep name ACME-123x has been assigned IP address: [2001:db8:4::x]. The RD is assigned IP address: [2001:db8:4:ff]. The used multicast addresses are: [ff05::5:1] and [ff05::5:2].
The following assumes that each device is pre-configured with the name of the two application groups it belongs to. Additional mechanisms can be defined in the RD, for supporting devices to discover the application groups they belong to.
Appendix A provides this same use case example in CoRAL.
*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***

The CT defines the application group "grp_R2-4-015", with resource /light and base address [ff05::5:1], as follows.
Request: CT -> RD

Req: POST coap://[2001:db8:4::ff]/rd
  ?ep=grp_R2-4-015&et=core.rd-group&base=coap://[ff05::5:1]
Content-Format: 40
Payload:
</light>;rt="oic.d.light"

Response: RD -> CT

Res: 2.01 Created
Location-Path: /rd/501

Also, the CT defines a second application group "grp_schedule", with resource /schedule and base address [ff05::5:2], as follows.
Request: CT -> RD

Req: POST coap://[2001:db8:4::ff]/rd
  ?ep=grp_schedule&et=core.rd-group&base=coap://[ff05::5:2]
Content-Format: 40
Payload:
</schedule>;rt="oic.r.time.period"

Response: RD -> CT

Res: 2.01 Created
Location-Path: /rd/502

*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***

Finally, the CT defines the corresponding security groups. In particular, assuming a Group Manager responsible for both security groups and with address [2001:db8::ab], the CT specifies:
Request: CT -> RD

Req: POST coap://[2001:db8:4::ff]/rd
  ?ep=gm1&base=coap://[2001:db8::ab]
Content-Format: 40
Payload:
</ace-group/feedca570000>;ct=65000;rt="core.osc.gm";if="ace.group";
                          sec-gp="feedca570000";
                          app-gp="grp_R2-4-015",
</ace-group/feedsc590000>;ct=65000;rt="core.osc.gm";if="ace.group";
                          sec-gp="feedsc590000";
                          app-gp="grp_schedule"

Response: RD -> CT

Res: 2.01 Created
Location-Path: /rd/4521

*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***

The device with IP address [2001:db8:4::x] can retrieve the multicast IP address of the CoAP group used by the application group "grp_R2-4-015", by performing an endpoint lookup as shown below.
Request: Joining node -> RD

Req: GET coap://[2001:db8:4::ff]/rd-lookup/ep
  ?et=core.rd-group&ep=grp_R2-4-015

Response: RD -> Joining node

Res: 2.05 Content
Content-Format: 40
Payload:
</rd/501>;ep="grp_R2-4-015";et="core.rd-group";
          base="coap://[ff05::5:1]";rt="core.rd-ep"

Similarly, to retrieve the multicast IP address of the CoAP group used by the application group "grp_schedule", the device performs an endpoint lookup as shown below.
Request: Joining node -> RD

Req: GET coap://[2001:db8:4::ff]/rd-lookup/ep
  ?et=core.rd-group&ep=grp_schedule

Response: RD -> Joining node

Res: 2.05 Content
Content-Format: 40
Payload:
</rd/502>;ep="grp_schedule";et="core.rd-group";
          base="coap://[ff05::5:2]";rt="core.rd-ep"

*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***

Consequently, the device learns the security groups it has to join. In particular, it does the following for app-gp="grp_R2-4-015".
Request: Joining node -> RD

Req: GET coap://[2001:db8:4::ff]/rd-lookup/res
  ?rt=core.osc.gm&app-gp=grp_R2-4-015

Response: RD -> Joining Node

Res: 2.05 Content
Content-Format: 40
Payload:
<coap://[2001:db8::ab]/ace-group/feedca570000>;ct=65000;
    rt="core.osc.gm";if="ace.group";sec-gp="feedca570000";
    app-gp="grp_R2-4-015"

Similarly, the device does the following for app-gp="grp_schedule".

Req: GET coap://[2001:db8:4::ff]/rd-lookup/res
  ?rt=core.osc.gm&app-gp=grp_schedule

Response: RD -> Joining Node

Res: 2.05 Content
Content-Format: 40
Payload:
<coap://[2001:db8::ab]/ace-group/feedsc590000>;ct=65000;
    rt="core.osc.gm";if="ace.group";sec-gp="feedsc590000";
    app-gp="grp_schedule"

*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***

After this last discovery step, the device can ask permission to join the security groups, and effectively join them through the Group Manager, e.g., according to [I-D.ietf-ace-key-groupcomm-oscore].
Security Considerations
The security considerations as described in Section 8 of [RFC9176] apply here as well.
IANA Considerations
This document has no actions for IANA.
References
Normative References
[I-D.ietf-core-oscore-groupcomm]

Group OSCORE - Secure Group Communication for CoAP
Marco Tiloca
RISE AB
Göran Selander
Ericsson AB
Francesca Palombini
Ericsson AB
John Preuss Mattsson
Ericsson AB
Jiye Park
Universitaet Duisburg-Essen
7 March 2022
Internet-Draft draft-ietf-core-oscore-groupcomm-14
[I-D.ietf-core-coral]

The Constrained RESTful Application Language (CoRAL)
Christian Amsüss
Thomas Fossati
ARM
7 March 2022
Internet-Draft draft-ietf-core-coral-05
[I-D.ietf-core-groupcomm-bis]

Group Communication for the Constrained Application Protocol (CoAP)
Esko Dijk
IoTconsultancy.nl
Chonggang Wang
InterDigital
Marco Tiloca
RISE AB
11 July 2022
Internet-Draft draft-ietf-core-groupcomm-bis-07
[RFC2119]

Key words for use in RFCs to Indicate Requirement Levels
S. Bradner
March 1997
BCP 14
RFC 2119
DOI 10.17487/RFC2119
[RFC6690]

Constrained RESTful Environments (CoRE) Link Format
Z. Shelby
August 2012
RFC 6690
DOI 10.17487/RFC6690
[RFC7252]

The Constrained Application Protocol (CoAP)
Z. Shelby
K. Hartke
C. Bormann
June 2014
RFC 7252
DOI 10.17487/RFC7252
[RFC8174]

Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words
B. Leiba
May 2017
BCP 14
RFC 8174
DOI 10.17487/RFC8174
[RFC9052]

CBOR Object Signing and Encryption (COSE): Structures and Process
J. Schaad
August 2022
STD 96
RFC 9052
DOI 10.17487/RFC9052
[RFC9053]

CBOR Object Signing and Encryption (COSE): Initial Algorithms
J. Schaad
August 2022
RFC 9053
DOI 10.17487/RFC9053
[RFC9176]

Constrained RESTful Environments (CoRE) Resource Directory
C. Amsüss, Ed.
Z. Shelby
M. Koster
C. Bormann
P. van der Stok
April 2022
RFC 9176
DOI 10.17487/RFC9176
[COSE.Algorithms]

COSE Algorithms
IANA
[COSE.Key.Types]

COSE Key Types
IANA
[COSE.Elliptic.Curves]

COSE Elliptic Curves
IANA
[COSE.Header.Parameters]

COSE Header Parameters
IANA
Informative References
[I-D.ietf-ace-key-groupcomm-oscore]

Key Management for OSCORE Groups in ACE
Marco Tiloca
RISE AB
Jiye Park
Universitaet Duisburg-Essen
Francesca Palombini
Ericsson AB
28 April 2022
Internet-Draft draft-ietf-ace-key-groupcomm-oscore-14
[I-D.ietf-ace-key-groupcomm]

Key Provisioning for Group Communication using ACE
Francesca Palombini
Ericsson AB
Marco Tiloca
RISE AB
23 December 2021
Internet-Draft draft-ietf-ace-key-groupcomm-15
[I-D.ietf-ace-oscore-gm-admin]

Admin Interface for the OSCORE Group Manager
Marco Tiloca
RISE AB
Rikard Höglund
RISE AB
Peter van der Stok
Consultant
Francesca Palombini
Ericsson AB
11 July 2022
Internet-Draft draft-ietf-ace-oscore-gm-admin-06
[I-D.hartke-t2trg-coral-reef]

Resource Discovery in Constrained RESTful Environments (CoRE) using the Constrained RESTful Application Language (CoRAL)
Klaus Hartke
Ericsson
9 May 2020
Internet-Draft draft-hartke-t2trg-coral-reef-04
[I-D.ietf-cose-cbor-encoded-cert]

CBOR Encoded X.509 Certificates (C509 Certificates)
John Preuß Mattsson
Ericsson AB
Göran Selander
Ericsson AB
Shahid Raza
RISE AB
Joel Höglund
RISE AB
Martin Furuhed
Nexus Group
10 July 2022
Internet-Draft draft-ietf-cose-cbor-encoded-cert-04
[I-D.amsuess-core-cachable-oscore]

Cacheable OSCORE
Christian Amsüss
Marco Tiloca
RISE AB
11 July 2022
Internet-Draft draft-amsuess-core-cachable-oscore-05
[RFC7228]

Terminology for Constrained-Node Networks
C. Bormann
M. Ersue
A. Keranen
May 2014
RFC 7228
DOI 10.17487/RFC7228
[RFC7641]

Observing Resources in the Constrained Application Protocol (CoAP)
K. Hartke
September 2015
RFC 7641
DOI 10.17487/RFC7641
[RFC7925]

Transport Layer Security (TLS) / Datagram Transport Layer Security (DTLS) Profiles for the Internet of Things
H. Tschofenig, Ed.
T. Fossati
July 2016
RFC 7925
DOI 10.17487/RFC7925
[RFC8132]

PATCH and FETCH Methods for the Constrained Application Protocol (CoAP)
P. van der Stok
C. Bormann
A. Sehgal
April 2017
RFC 8132
DOI 10.17487/RFC8132
[RFC8392]

CBOR Web Token (CWT)
M. Jones
E. Wahlstroem
S. Erdtman
H. Tschofenig
May 2018
RFC 8392
DOI 10.17487/RFC8392
[RFC8613]

Object Security for Constrained RESTful Environments (OSCORE)
G. Selander
J. Mattsson
F. Palombini
L. Seitz
July 2019
RFC 8613
DOI 10.17487/RFC8613
[RFC8995]

Bootstrapping Remote Secure Key Infrastructure (BRSKI)
M. Pritikin
M. Richardson
T. Eckert
M. Behringer
K. Watsen
May 2021
RFC 8995
DOI 10.17487/RFC8995
[RFC9200]

Authentication and Authorization for Constrained Environments Using the OAuth 2.0 Framework (ACE-OAuth)
L. Seitz
G. Selander
E. Wahlstroem
S. Erdtman
H. Tschofenig
August 2022
RFC 9200
DOI 10.17487/RFC9200
[Fairhair]

Security Architecture for the Internet of Things (IoT) in Commercial Buildings
FairHair Alliance
March 2018
White Paper, ed. Piotr Polak
Use Case Example With Full Discovery (CoRAL)
This section provides the same use case example of Section 5, but specified in CoRAL [I-D.ietf-core-coral].
*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***

The CT defines the application group "grp_R2-4-015", with resource /light and base address [ff05::5:1], as follows.
Request: CT -> RD

Req: POST coap://[2001:db8:4::ff]/rd
Content-Format: TBD123456 (application/coral+cbor)

Payload:
#using reef = <http://coreapps.org/reef#>

#base <coap://[ff05::5:1]/>
reef:ep "grp_R2-4-015"
reef:et "core.rd-group"
reef:rd-item </light> {
   reef:rt "oic.d.light"
}

Response: RD -> CT

Res: 2.01 Created
Location-Path: /rd/501

Also, the CT defines a second application group "grp_schedule", with resource /schedule and base address [ff05::5:2], as follows.
Request: CT -> RD

Req: POST coap://[2001:db8:4::ff]/rd?ep=grp_schedule&et=core.rd-group
Content-Format: TBD123456 (application/coral+cbor)

Payload:
#using reef = <http://coreapps.org/reef#>

#base <coap://[ff05::5:2]/>
reef:rd-item </schedule> {
   reef:rt "oic.r.time.period"
}

Response: RD -> CT

Res: 2.01 Created
Location-Path: /rd/502

*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***

Finally, the CT defines the corresponding security groups. In particular, assuming a Group Manager responsible for both security groups and with address [2001:db8::ab], the CT specifies:
Request: CT -> RD

Req: POST coap://[2001:db8:4::ff]/rd?ep=gm1
Content-Format: TBD123456 (application/coral+cbor)

Payload:
#using <http://coreapps.org/core.oscore-discovery#>
#using reef = <http://coreapps.org/reef#>

#base <coap://[2001:db8::ab]/>
reef:rd-item </ace-group/feedca570000> {
   reef:ct 65000
   reef:ct 41
   reef:rt "core.osc.gm"
   reef:if "ace.group"
   sec-gp "feedca570000"
   app-gp "grp_R2-4-015"
}
reef:rd-item </ace-group/feedsc590000> {
   reef:ct 65000
   reef:ct 41
   reef:rt "core.osc.gm"
   reef:if "ace.group"
   sec-gp "feedsc590000"
   app-gp "grp_schedule"
}

Response: RD -> CT

Res: 2.01 Created
Location-Path: /rd/4521

*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***

The device with IP address [2001:db8:4::x] can retrieve the multicast IP address of the CoAP group used by the application group "grp_R2-4-015", by performing an endpoint lookup as shown below.
Request: Joining node -> RD

Req: GET coap://[2001:db8:4::ff]/rd-lookup/ep
  ?et=core.rd-group&ep=grp_R2-4-015

Response: RD -> Joining node

Res: 2.05 Content
Content-Format: TBD123456 (application/coral+cbor)

Payload:
#using reef = <http://coreapps.org/reef#>

#base <coap://[2001:db8:4::ff]/rd/>
reef:rd-unit <501> {
   reef:ep "grp_R2-4-015"
   reef:et "core.rd-group"
   reef:base <coap://[ff05::5:1]/>
   reef:rt "core.rd-ep"
}

Similarly, to retrieve the multicast IP address of the CoAP group used by the application group "grp_schedule", the device performs an endpoint lookup as shown below.
Request: Joining node -> RD

Req: GET coap://[2001:db8:4::ff]/rd-lookup/ep
  ?et=core.rd-group&ep=grp_schedule

Response: RD -> Joining node

Res: 2.05 Content
Content-Format: TBD123456 (application/coral+cbor)

Payload:
#using reef = <http://coreapps.org/reef#>

#base <coap://[2001:db8:4::ff]/rd/>
reef:rd-unit <502> {
   reef:ep "grp_schedule"
   reef:et "core.rd-group"
   reef:base <coap://[ff05::5:2]/>
   reef:rt "core.rd-ep"
}

*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***

Consequently, the device learns the security groups it has to join. In particular, it does the following for app-gp="grp_R2-4-015".
Request: Joining node -> RD

Req: GET coap://[2001:db8:4::ff]/rd-lookup/res
  ?rt=core.osc.gm&app-gp=grp_R2-4-015

Response: RD -> Joining Node

Res: 2.05 Content
Content-Format: TBD123456 (application/coral+cbor)

Payload:
#using <http://coreapps.org/core.oscore-discovery#>
#using reef = <http://coreapps.org/reef#>

#base <coap://[2001:db8::ab]/>
reef:rd-item </ace-group/feedca570000> {
   reef:ct 65000
   reef:rt "core.osc.gm"
   reef:if "ace.group"
   sec-gp "feedca570000"
   app-gp "grp_R2-4-015"
}

Similarly, the device does the following for app-gp="grp_schedule".

Req: GET coap://[2001:db8:4::ff]/rd-lookup/res
  ?rt=core.osc.gm&app-gp=grp_schedule

Response: RD -> Joining Node

Res: 2.05 Content
Content-Format: TBD123456 (application/coral+cbor)

Payload:
#using <http://coreapps.org/core.oscore-discovery#>
#using reef = <http://coreapps.org/reef#>

#base <coap://[2001:db8::ab]/>
reef:rd-item </ace-group/feedsc590000> {
   reef:ct 65000
   reef:rt "core.osc.gm"
   reef:if "ace.group"
   sec-gp "feedsc590000"
   app-gp "grp_schedule"
}

*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***

After this last discovery step, the device can ask permission to join the security groups, and effectively join them through the Group Manager, e.g., according to [I-D.ietf-ace-key-groupcomm-oscore].
Acknowledgments
The authors sincerely thank Carsten Bormann, Klaus Hartke, Jaime Jimenez, Francesca Palombini, Dave Robin and Jim Schaad for their comments and feedback.
The work on this document has been partly supported by VINNOVA and the Celtic-Next project CRITISEC; by the H2020 project SIFIS-Home (Grant agreement 952652); and by the EIT-Digital High Impact Initiative ACTIVE.