7.2.4 5G ProSe direct link identifier update procedure

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

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

7.2.4.1 General

The 5G ProSe direct link identifier update procedure is used to update and exchange the new identifiers (e.g., application layer ID, layer-2 ID, security information and IP address/prefix) between two UEs for a 5G ProSe direct link before using the new identifiers. The UE sending the PROSE DIRECT LINK IDENTIFIER UPDATE REQUEST message is called the "initiating UE" and the other UE is called the "target UE".

7.2.4.2 5G ProSe direct link identifier update procedure initiation by initiating UE

The initiating UE shall initiate the procedure if:

a) the initiating UE receives a request from upper layers to change the application layer ID and there is an existing 5G ProSe direct link associated with this application layer ID; or

b) the privacy timer (see clause 5.2.4) of the initiating UE’s layer-2 ID expires for an existing 5G ProSe direct link.

If the 5G ProSe direct link identifier update procedure is triggered by a change of the initiating UE’s application layer ID, the initiating UE shall create a PROSE DIRECT LINK IDENTIFIER UPDATE REQUEST message. In this message, the initiating UE:

a) shall include the initiating UE’s new application layer ID received from upper layer;

b) shall include the initiating UE’s new layer-2 ID assigned by itself;

c) shall include the new MSB of K_NRP-sess ID; and

d) shall include the new IP address/prefix if IP communication is used and the 5G ProSe direct link is not for 5G ProSe direct communication between 5G ProSe layer-2 remote UE and 5G ProSe layer-2 UE-to-network relay UE.

If the 5G ProSe direct link identifier update procedure is triggered by the expiry of the initiating UE’s privacy timer T5090 as specified in clause 5.2.4 and clause 5.2.5, the initiating UE shall create a PROSE DIRECT LINK IDENTIFIER UPDATE REQUEST message. In this message, the initiating UE:

a) shall include the initiating UE’s new layer-2 ID assigned by itself;

b) shall include the new MSB of K_NRP-sess ID;

c) may include the initiating UE’s new application layer ID if received from upper layer; and

d) shall include the new IP address/prefix if IP communication is used and changed and the 5G ProSe direct link is not for 5G ProSe direct communication between 5G ProSe layer-2 remote UE and 5G ProSe layer-2 UE-to-network relay UE.

After the PROSE DIRECT LINK IDENTIFIER UPDATE REQUEST message is generated, the initiating UE shall pass this message to the lower layers for transmission along with the initiating UE’s old layer-2 ID for 5G ProSe direct communication and the target UE’s layer-2 ID for 5G ProSe direct communication and start timer T5082. The UE shall not send a new PROSE DIRECT LINK IDENTIFIER UPDATE REQUEST message to the same target UE while timer T5082 is running.

Figure 7.2.4.2.1: 5G ProSe direct link identifier update procedure

7.2.4.3 5G ProSe direct link identifier update procedure accepted by the target UE

Upon receipt of a PROSE DIRECT LINK IDENTIFIER UPDATE REQUEST message, if the target UE determines:

a) the 5G ProSe direct link associated with this request message is still valid; and

b) the timer T5083 for the 5G ProSe direct link identified by this request message is not running,

then the target UE accepts this request and responds with a PROSE DIRECT LINK IDENTIFIER UPDATE ACCEPT message.

The target UE shall create the PROSE DIRECT LINK IDENTIFIER UPDATE ACCEPT message. In this message, the target UE:

a) shall include the target UE’s new layer-2 ID assigned by itself;

b) shall include the new LSB of K_NRP-sess ID;

c) shall include the initiating UE’s new MSB of K_NRP-sess ID;

d) shall include the initiating UE’s new layer-2 ID;

e) shall include the target UE’s new application layer ID if received from upper layer;

f) shall include the initiating UE’s new IP address/prefix if received from the initiating UE and IP communication is used;

g) shall include the initiating UE’s new application layer ID if received from the initiating UE; and

h) shall include the target UE’s new IP address/prefix if IP communication is used and changed and the 5G ProSe direct link is not for 5G ProSe direct communication between 5G ProSe layer-2 remote UE and 5G ProSe layer-2 UE-to-network relay UE.

After the PROSE DIRECT LINK IDENTIFIER UPDATE ACCEPT message is generated, the target UE shall pass this message to the lower layers for transmission along with the initiating UE’s old layer-2 ID for 5G ProSe direct communication and the target UE’s old layer-2 ID for 5G ProSe direct communication and start timer T5083. The UE shall not send a new PROSE DIRECT LINK IDENTIFIER UPDATE ACCEPT message to the same initiating UE while timer T5083 is running.

Before target UE receives the traffic using the new layer-2 IDs, the target UE shall continue to receive the traffic with the old layer-2 IDs (i.e., initiating UE’s old layer-2 ID and target UE’s old layer-2 ID) from initiating UE.

Before target UE receives the PROSE DIRECT LINK IDENTIFIER UPDATE ACK message from initiating UE, the target UE shall keep sending traffic to the initiating UE using the old layer-2 IDs (i.e., initiating UE’s old layer-2 ID for 5G ProSe direct communication and target UE’s old layer-2 ID for 5G ProSe direct communication).

7.2.4.4 5G ProSe direct link identifier update procedure acknowledged by the initiating UE

Upon receipt of the PROSE DIRECT LINK IDENTIFIER UPDATE ACCEPT message, the initiating UE shall stop timer T5082 and respond with a PROSE DIRECT LINK IDENTIFIER UPDATE ACK message. In this message, the initiating UE:

a) shall include the target UE’s new layer-2 ID;

b) shall include the target UE’s new LSB of K_NRP-sess ID;

c) shall include the target UE’s new application layer ID, if received; and

d) shall include the target UE’s new IP address/prefix, if received.

After the PROSE DIRECT LINK IDENTIFIER UPDATE ACK message is generated, the initiating UE shall pass this message to the lower layers for transmission along with the initiating UE’s old layer-2 ID for 5G ProSe direct communication and the target UE’s old layer-2 ID for 5G ProSe direct communication and shall stop timer T5090 if running and start a timer T5090 as configured if at least one of ProSe identifiers for the 5G ProSe direct link satisfying the privacy requirements as specified in clause 5.2.4 or if satifying the privacy requirements as specified in clause 5.2.5.

Upon sending the PROSE DIRECT LINK IDENTIFIER UPDATE ACK message, the initiating UE shall update the associated 5G ProSe direct link context with the new identifiers and pass the new layer-2 IDs (i.e., initiating UE’s new layer-2 ID for 5G ProSe direct communication and target UE’s new layer-2 ID for 5G ProSe direct communication) along with the PC5 link identifier down to the lower layer. Then the initiating UE shall use the new layer-2 IDs (i.e., initiating UE’s new layer-2 ID for 5G ProSe direct communication and target UE’s new layer-2 ID for 5G ProSe direct communication) to transmit the PC5 signalling message and PC5 user plane data.

The initiating UE shall continue to receive traffic with the old layer-2 IDs (i.e., initiating UE’s old layer-2 ID for 5G ProSe direct communication and target UE’s old layer-2 ID for 5G ProSe direct communication) from the target UE until it receives traffic with the new layer-2 IDs (i.e., initiating UE’s new layer-2 ID and target UE’s new layer-2 ID) from the target UE.

7.2.4.5 5G ProSe direct link identifier update procedure completion by the target UE

Upon receipt of the PROSE DIRECT LINK IDENTIFIER UPDATE ACK message, the target UE shall update the associated 5G ProSe direct link context with the new identifiers, pass the new layer-2 IDs (i.e., initiating UE’s new layer-2 ID and target UE’s new layer-2 ID) down to the lower layer, stop timer T5083 and timer T5090 if running and start a timer T5090 as configured if at least one of ProSe identifiers for the 5G ProSe direct link satisfying the privacy requirements as specified in clause 5.2.4 or if satifying the privacy requirements as specified in clause 5.2.5. Then the target UE shall use the new layer-2 IDs (i.e., initiating UE’s new layer-2 ID for 5G ProSe direct communication and target UE’s new layer-2 ID for 5G ProSe direct communication) to transmit the PC5 signalling message and PC5 user plane data.

7.2.4.6 5G ProSe direct link identifier update procedure not accepted by the target UE

If the PROSE DIRECT LINK IDENTIFIER UPDATE REQUEST message cannot be accepted, the target UE shall send a PROSE DIRECT LINK IDENTIFIER UPDATE REJECT message. The PROSE DIRECT LINK IDENTIFIER UPDATE REJECT message contains a PC5 signalling protocol cause IE set to one of the following cause values:

#3 conflict of layer-2 ID for 5G ProSe direct communication is detected; or

#111 protocol error, unspecified.

For a received PROSE DIRECT LINK IDENTIFIER UPDATE REQUEST message from a layer-2 ID (for 5G ProSe direct communication), if the target UE already has an existing link using this layer-2 ID or is currently processing a PROSE DIRECT LINK IDENTIFIER UPDATE REQUEST message from the same layer-2 ID, but with user info different from the user info IE included in this new incoming message, the target UE shall send a PROSE DIRECT LINK IDENTIFIER UPDATE REJECT message with PC5 signalling protocol cause value #3 "conflict of layer-2 ID for 5G ProSe direct communication is detected".

NOTE: After receiving the PROSE DIRECT LINK IDENTIFIER UPDATE REJECT message, whether the initiating UE initiates the 5G ProSe direct link release procedure or initiates another 5G ProSe direct link identifier update procedure with a new layer-2 ID depends on UE implementation.

For other reasons causing the failure of link identifier update, the target UE shall send a PROSE DIRECT LINK IDENTIFIER UPDATE REJECT message with PC5 signalling protocol cause value #111 "protocol error, unspecified".

Upon receipt of the PROSE DIRECT LINK IDENTIFIER UPDATE REJECT message, the initiating UE shall stop timer T5082 and abort this 5G ProSe direct link identifier update procedure.

7.2.4.7 Abnormal cases

7.2.4.7.1 Abnormal cases at the initiating UE

The following abnormal cases can be identified:

a) If timer T5082 expires, the initiating UE shall retransmit the PROSE DIRECT LINK IDENTIFIER UPDATE REQUEST message and restart timer T5082. After reaching the maximum number of allowed retransmissions, the initiating UE shall abort the 5G ProSe direct link identifier update procedure and may notify the upper layer that the target UE is unreachable.

NOTE 1: The maximum number of allowed retransmissions is UE implementation specific.

NOTE 2: After reaching the maximum number of allowed retransmissions, whether the initiating UE releases this 5G ProSe direct link depends on its implementation.

b) For the same 5G ProSe direct link, if the initiating UE receives a PROSE DIRECT LINK IDENTIFIER UPDATE REQUEST message during the 5G ProSe direct link identifier update procedure, the initiating UE shall stop the timer T5082 and abort the 5G ProSe direct link identifier update procedure. Following handling is implementation dependent, e.g., the initiating UE waits for an implementation dependent time for initiating a new 5G ProSe direct link identifier update procedure, if still needed.

NOTE 3: The implementation dependent timer value needs to be set to avoid further collisions (e.g., random timer value).

c) For the same 5G ProSe direct link, if the initiating UE receives a PROSE DIRECT LINK REKEYING REQUEST message after initiating the 5G ProSe direct link identifier update procedure, the initiating UE shall ignore the PROSE DIRECT LINK REKEYING REQUEST message and proceed with the 5G ProSe direct link identifier update procedure.

d) For the same 5G ProSe direct link, if the initiating UE receives a PROSE DIRECT LINK RELEASE REQUEST message after the initiation of 5G ProSe direct link identifier update procedure, the initiating UE shall stop the timer T5082 and abort the 5G ProSe direct link identifier update procedure and proceed with the 5G ProSe direct link release procedure.

e) After sending the PROSE DIRECT LINK IDENTIFIER UPDATE ACK message to the target UE, if another PROSE DIRECT LINK IDENTIFIER UPDATE ACCEPT message from the target UE is received before the traffic from the target UE with the new layer-2 IDs is received, the initiating UE shall retransmit the PROSE DIRECT LINK IDENTIFIER UPDATE ACK message along with the initiating UE’s old layer-2 ID and the target UE’s old layer-2 ID.

NOTE 4: It is up to implementation to handle the failure of traffic delivery for new layer-2 IDs if such traffic has been sent before the initiating UE retransmits the PROSE DIRECT LINK IDENTIFIER UPDATE ACK message.

f) After sending the PROSE DIRECT LINK IDENTIFIER UPDATE ACK message to the target UE, if the initiating UE keeps receiving traffic from the target UE with the old layer-2 IDs and traffic from the target UE with the new layer-2 IDs is not received during an implementation specific time which is greater than the value of timer T5083, the initiating UE shall abort the 5G ProSe direct link identifier update procedure and may release the 5G ProSe direct link.

7.2.4.7.2 Abnormal cases at the target UE

The following abnormal cases can be identified:

a) If timer T5083 expires, the target UE shall retransmit the PROSE DIRECT LINK IDENTIFIER UPDATE ACCEPT message and restart timer T5083. After reaching the maximum number of allowed retransmissions, the target UE shall abort the 5G ProSe direct link identifier update procedure and may notify the upper layer that the initiating UE is unreachable.

NOTE 1: The maximum number of allowed retransmissions is UE implementation specific.

NOTE 2: After reaching the maximum number of allowed retransmissions, whether the target UE releases this 5G ProSe direct link depends on its implementation.

b) If PROSE DIRECT LINK IDENTIFIER UPDATE REQUEST is received when the timer T5083 is running, the target UE shall stop the timer T5083 and abort the ongoing 5G ProSe direct link identifier update procedure. The target UE shall handle the new PROSE DIRECT LINK IDENTIFIER UPDATE REQUEST as specified in clause 7.2.4.3.

c) For the same 5G ProSe direct link, if the target UE receives a PROSE DIRECT LINK RELEASE REQUEST message during the 5G ProSe direct link identifier update procedure, the target UE shall stop the timer T5083, abort the 5G ProSe direct link identifier update procedure and proceed with the 5G ProSe direct link release procedure.