6.2.6 Discoveree request procedure for restricted 5G ProSe direct discovery model B

24.5543GPPProximity-services (ProSe) in 5G System (5GS) protocol aspectsRelease 17Stage 3TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

6.2.6.1 General

The purpose of the discoveree request procedure for restricted 5G ProSe direct discovery model B is for the UE to obtain discovery query filter(s) to be used for monitoring a model B query for a RPAUID over the PC5 interface and a ProSe response code to be announced over the PC5 interface as a response to a model B query, as defined in 3GPP TS 23.304 [2].

Before initiating the discoveree request procedure, the UE shall be authorised for restricted 5G ProSe direct discovery model B discoveree operation in the registered PLMN or the local PLMN based on the service authorization procedure as specified in clause 5.

As the result of successful completion of this procedure, the UE obtains one or more discovery query filters and applies them to the monitoring operation in PC5 interface. The UE shall also include the ProSe response code in a PROSE PC5 DISCOVERY message and passes the message to the lower layers for transmission over the PC5 interface when there is a match of the discovery query filter(s).

6.2.6.2 Discoveree request procedure initiation

Before initiating the discoveree request procedure, the user sets the permissions for the restricted discovery using application layer mechanisms. The application client in the UE retrieves the PDUID provisioned to the UE as part of the service authorization procedure as specified in clause 5 and obtains a RPAUID associated with the UE’s PDUID from the ProSe application server. The UE can provide metadata to be associated with the RPAUID and the ProSe application server stores the metadata. This step is performed using mechanisms that are out of scope of the present specification.

If the UE is authorised to perform restricted 5G ProSe direct discovery model B discoveree operation in the PLMN operating the radio resources signalled from the serving PLMN, it shall initiate a discoveree request procedure:

a) when the UE is triggered by an upper layer application to announce a RPAUID in Model B and the UE has no valid corresponding ProSe response code and discovery query filter(s) for that RPAUID of the upper layer application;

b) when the validity timer T5068 assigned by the 5G DDNMF to a ProSe response code and the corresponding discovery query filter(s) has expired and the request from upper layers to announce the RPAUID corresponding to that ProSe response code is still in place;

c) when the UE selects a new PLMN while announcing or waiting for announcing a ProSe response code and intends to announce in the new PLMN and the UE is authorised for restricted 5G ProSe direct discovery model B discoveree operation in the new PLMN, and the UE does not have a valid allocated ProSe response code for this new PLMN yet; or

d) when the UE needs to update a previously sent restricted 5G ProSe direct discovery model B discoveree request.

When the UE selects a new PLMN while announcing or waiting for announcing a ProSe response code and the UE is not yet authorised for restricted 5G ProSe direct discovery model B discoveree operation in the new PLMN, the UE shall initiate a discoveree request procedure only after the UE is authorised for restricted 5G ProSe direct discovery model B discoveree operation in the new PLMN.

NOTE 1: To ensure service continuity if the UE needs to keep announcing in Model B a ProSe response code corresponding to the same RPAUID, the UE can initiate the discoveree request procedure before the validity timer T5068 assigned by the 5G DDNMF for a ProSe response code expires.

The UE shall initiate the discoveree request procedure by sending a DISCOVERY_REQUEST message with:

a) a new transaction ID not used in any other direct discovery procedures in PC3a interface;

b) the RPAUID set to the RPAUID received from upper layers;

c) the command set to "response";

d) the application identity set to the ProSe identifier of the upper layer application that requested the announcing as specified in clause 5.2.3;

e) the discovery type set to "Restricted discovery";

f) the discovery model set to "Model B";

g) the discovery entry ID set to a 0 if the discoveree request is a new request and set to the discovery entry ID received from the 5G DDNMF if the discoveree request is to update a previously sent discoveree request;

h) optionally the announcing PLMN ID set to the PLMN ID of the local PLMN operating the radio resources that the UE intends to use for announcing the RPAUID; and

i) the PC5 UE ciphering algorithm capability set to the UE supported ciphering algorithm(s) for ciphering the PROSE PC5 DISCOVERY message.

NOTE 2: A UE can include one or multiple transactions in one DISCOVERY_REQUEST message for different RPAUIDs (e.g., for different applications) and receive corresponding <restricted-discoveree-response> element or <response-reject> element in a DISCOVERY_RESPONSE message for each respective transaction. In the following description of the discoveree request procedure, only one transaction is included.

Figure 6.2.6.2.1 illustrates the interaction of the UE and the 5G DDNMF in the discoveree request procedure.

Figure 6.2.6.2.1: Discoveree request procedure for restricted 5G ProSe direct discovery model B

6.2.6.3 Discoveree request procedure accepted by the 5G DDNMF

Upon receiving a DISCOVERY_REQUEST message, the 5G DDNMF shall check that the application corresponding to the ProSe identifier contained in the DISCOVERY_REQUEST message is authorised for restricted 5G ProSe direct discovery model B discoveree operation. If the application is authorised for restricted 5G ProSe direct discovery model B discoveree operation, the 5G DDNMF shall check whether there is an existing context for the UE.

If there is no associated UE context, the 5G DDNMF checks with the UDM whether the UE is authorised for restricted 5G ProSe direct discovery model B discoveree operation as described in 3GPP TS 29.503 [10]. If the check indicates that the UE is authorised, the 5G DDNMF creates a UE context that contains the UE’s subscription parameters obtained from the UDM. The UDM also provides to the 5G DDNMF the PLMN ID of the PLMN in which the UE is currently registered.

If the UE context exists, the 5G DDNMF shall check whether the UE is authorized for restricted 5G ProSe direct discovery model B discoveree operation in the currently registered PLMN or the local PLMN identified by the Announcing PLMN ID included in the DISCOVERY_REQUEST message.

If the UE is authorized and the discovery entry ID included in the DISCOVERY_REQUEST message is set to 0 then:

a) the 5G DDNMF shall check whether the UE is authorised to announce the RPAUID contained in the DISCOVERY_REQUEST message. Optionally this can include checking with the ProSe application server as described in 3GPP TS 29.557 [19] to obtain the binding between the RPAUID and PDUID and then verifying that the PDUID belongs to the requesting UE;

b) if the UE is authorised to announce the RPAUID, the 5G DDNMF shall allocate the corresponding ProSe response code and ProSe query code for the RPAUID. It shall also allocate discovery query filter(s) based on the allocated ProSe query code. Then it shall assign a value for validity timer T5068, which is associated with the ProSe response code, ProSe query code and discovery query filter(s); and

c) the 5G DDNMF associates the allocated ProSe response code, ProSe query code and discovery query filter with a new discovery entry ID in the UE context and starts timer T5069. For a given ProSe response code, timer T5069 shall be longer than timer T5068. By default, the value of timer T5069 is 4 minutes greater than the value of timer T5068.

If the discovery entry ID included in the DISCOVERY_REQUEST message is not set to 0 and if there is an existing discovery entry for this discovery entry ID value in the UE context, the 5G DDNMF shall either update the discovery entry with a new validity timer T5068, or allocate a new ProSe response code, ProSe query code and the discovery query filter(s) for the requested RPAUID with a new validity timer T5068, restart timer T5069.

If the discovery entry ID contained in the DISCOVERY_REQUEST message is not found in the UE context or there is no UE context in the 5G DDNMF, the 5G DDNMF shall behave as if the discovery entry ID included in the DISCOVERY_REQUEST message was set to 0 and the 5G DDNMF shall allocate a new non-zero discovery entry ID for this entry.

If a new UE context was created or an existing UE context was updated and the UE is currently roaming or the Announcing PLMN ID is included in the DISCOVERY_REQUEST message, the 5G DDNMF checks with the 5G DDNMF of the VPLMN or the local PLMN identified by the Announcing PLMN ID whether the UE is authorised for restricted 5G ProSe direct discovery model B discoveree operation as described in 3GPP TS 29.555 [9].

The 5G DDNMF shall then send a DISCOVERY_RESPONSE message containing a <restricted-discoveree-response> element with:

a) the transaction ID set to the value of the transaction ID received in the DISCOVERY_REQUEST message from the UE;

b) the ProSe response code set to the ProSe response code allocated for the RPAUID received in the DISCOVERY_REQUEST message;

c) one or more discovery query filters set to the ProSe query filter(s) used to match a query for the RPAUID received in the DISCOVERY_REQUEST message;

d) a validity timer T5068 set to the T5068 timer value assigned by the 5G DDNMF to the ProSe response code and the discovery query filter(s);

e) the code-sending security parameter containing the security-related information for the discoveree UE to protect the transmission of the ProSe response code;

f) the code-receiving security parameter containing the security-related information needed by the discoveree UE to undo the protection applied by the discoverer UE;

g) the discovery entry ID set to the ID of the discovery entry associated with this discoveree request in the UE context;

h) the current time set to the current UTC-based time at the 5G DDNMF and the max offset;

i) optionally, the PC5 security policies that are associated with the ProSe response code and used for 5G ProSe direct link establishment procedure instead of any PC5 security policies configured in the UE as specified in clause 5; and

j) the selected PC5 ciphering algorithm set to the PC5 ciphering algorithm selected by the 5G DDNMF if the received PC5 UE ciphering algorithm capability includes the selected PC5 ciphering algorithm associated with the ProSe response code corresponding to the discovery query filters.

If timer T5069 expires, the 5G DDNMF shall remove the discovery entry associated with the corresponding RPAUID from the UE’s context.

The 5G DDNMF may associate the ProSe response code with the PC5 security policies.

The 5G DDNMF uses the information (e.g. application identity) received from the UE in the DISCOVERY_REQUEST message, UE identity in GBA or AKMA information related to TLS tunnel transporting the DISCOVERY_REQUEST message, and other information for charging purposes as specified in 3GPP TS 32.277 [45].

6.2.6.4 Discoveree request procedure completion by the UE

Upon receipt of the DISCOVERY_RESPONSE message, if the transaction ID contained in the <restricted-discoveree-response> element matches the value sent by the UE in a DISCOVERY_REQUEST message with the command set to "response", the UE shall create a new discovery entry or update an existing discovery entry with the ProSe response code and discovery query filter(s) received in the DISCOVERY_RESPONSE message and the PLMN ID of the intended announcing PLMN. For this discovery entry, the UE shall stop the validity timer T5068 if running and start the validity timer T5068 with the received value. The UE shall also use the received ProSe response code and discovery query filter(s) to replace the old counterparts if they are currently used. This may involve notifying the lower layers to stop announcing the old ProSe response code or to stop monitoring with the old discovery query filter(s). Otherwise, the UE shall discard the DISCOVERY_RESPONSE message and shall not perform the procedures described in clause 6.2.14.2.2.3. The UE shall set a ProSe clock (see 3GPP TS 33.503 [34]) to the value of the received current time parameter and store the received max offset parameter.

The UE shall store the selected PC5 ciphering algorithm received in the DISCOVERY_RESPONSE message for the ProSe response code and the ProSe query code and the UE shall use it for protection of the restricted 5G ProSe direct discovery messages over the PC5 interface as specified in clause 6.1.3.2.3 of 3GPP TS 33.503 [34].

6.2.6.5 Discoveree request procedure not accepted by the 5G DDNMF

If the DISCOVERY_REQUEST message cannot be accepted by the 5G DDNMF, the 5G DDNMF sends a DISCOVERY_RESPONSE message containing a <response-reject> element to the UE including an appropriate PC3a control protocol cause value.

If the application corresponding to the ProSe identifier contained in the DISCOVERY_REQUEST message is not authorised for ProSe direct discovery Model B discoveree operation, the 5G DDNMF shall send the DISCOVERY_RESPONSE message containing a <response-reject> element with PC3a control protocol cause value #1 "Invalid application".

If the RPAUID contained in the DISCOVERY_REQUEST message is unknown to the 5G DDNMF or ProSe application server, the 5G DDNMF shall send a DISCOVERY_RESPONSE message containing a <response-reject> element with PC3a control protocol cause value #9 "Unknown RPAUID".

If the RPAUID contained in the DISCOVERY_REQUEST message does not match the stored RPAUID for the requested discovery entry ID, the 5G DDNMF shall send a DISCOVERY_RESPONSE message containing a <response-reject> element with PC3a control protocol cause value #10 "Unknown or invalid discovery entry ID".

If the UE is not authorised for restricted 5G ProSe direct discovery model B discoveree operation, the 5G DDNMF shall send the DISCOVERY_RESPONSE message containing a <response-reject> element with PC3a control protocol cause value #3 "UE authorization failure".

If the RPAUID contained in the DISCOVERY_REQUEST message is not associated with a PDUID belonging to the requesting UE, the 5G DDNMF shall send a DISCOVERY_RESPONSE message containing a <response-reject> element with PC3a control protocol cause value #3 "UE authorization failure".

If the PC5 UE ciphering algorithm capability contained in the DISCOVERY_REQUEST message is not compatible with the selected PC5 ciphering algorithm for the ProSe response code and the ProSe query code, the 5G DDNMF shall send a DISCOVERY_RESPONSE message containing a <response-reject> element with PC3a control protocol cause value #19 "Not compatible PC5 UE ciphering algorithm capability".

6.2.6.6 Abnormal cases

6.2.6.6.1 Abnormal cases in the UE

The following abnormal cases can be identified:

a) Indication from the transport layer of transmission failure of DISCOVERY_REQUEST message (e.g., after TCP retransmission timeout)

The UE shall close the existing secure connection to the 5G DDNMF, establish a new secure connection and then restart the discoveree request procedure.

NOTE 1: The UE can abort this procedure when detecting loss of NG-RAN coverage in its serving PLMN.

b) No response from the 5G DDNMF after the DISCOVERY_REQUEST message has been successfully delivered (e.g., TCP ACK has been received for the DISCOVERY_REQUEST message)

The UE shall retransmit the DISCOVERY_REQUEST message.

NOTE 2: The UE can abort this procedure when detecting loss of NG-RAN coverage in its serving PLMN.

NOTE 3: The timer to trigger retransmission and the maximum number of allowed retransmissions are UE implementation specific.

c) Indication from upper layers that the request to announce the RPAUID in model B is no longer in place after sending the DISCOVERY_REQUEST message, but before the discoveree request procedure is completed

The UE shall acknowledge the DISCOVERY_RESPONSE message received from the 5G DDNMF but discard its contents and then abort the procedure.

d) Change of PLMN

If a PLMN change occurs before the discoveree request procedure is completed, the procedure shall be aborted. If the UE is authorized to perform restricted 5G ProSe direct discovery model B discoveree operation in the new PLMN, the procedure shall be restarted once the UE is registered on the new PLMN.

6.2.6.6.2 Abnormal cases in the 5G DDNMF

The following abnormal cases can be identified:

a) Indication from the lower layer of transmission failure of DISCOVERY_RESPONSE message

After receiving an indication from lower layer that the DISCOVERY_RESPONSE message has not been successfully acknowledged (e.g. TCP ACK is not received), the 5G DDNMF shall abort the procedure and stop any associated timer(s) T5069, if running.