5G(NR) RAN Study -- Path Switch Request (1)

In 5G, a path request is a signaling message sent by the target base station to the core network during handover to redirect the path of the terminal (data) session. TS 38.413 defines the following:

I. PDU Session Setup Failures

If any PDU session setup fails, the list of failed sessions shall be included within the “Path Switch Request Setup Failure Transport IE” in the PATH SWITCH REQUEST message. The AMF shall process this information as specified in TS 23.502.

2. User Security and Path Information

• For each PDU session, if its "additional redundant DL QoS flow information IE for each TNL" is included in the PATH SWITCH REQUEST Transfer IE of the Path Switch Request message, Then the SMF can use each included UP transport layer information as the downlink termination point for the associated QoS flows contained in this PDU session, and these QoS flows are split into different tunnels for redundant transmission.
• For each PDU session, if the Path Switch Request Transfer IE of its "Path Switch Request" message contains "Redundant DL NG-U TNL information Reuse IE", then the SMF should (if supported) treat the included DL transport layer address as the DL transport layer address for redundant transfer. As described in TS 23.501.
• For each PDU session, if the Path Switch Request Transfer IE of its "Path Switch Request" message contains the "Global RAN Node ID of the auxiliary NG-RAN node" IE, the SMF should (if supported) handle this information as stipulated in TS 23.501.
• For each PDU session contained in the PATH SWITCH REQUEST message, if the "Path Switch Request Transmission IE" contains the "Current QoS Parameter Set Index IE", the SMF should treat it as the currently implemented QoS parameter set among the alternative QoS parameters of the involved QoS flow.
• The NG-RAN node should (if supported) report the processing indicator IE based on the PDU set in the "PATH SWITCH REQUEST Transmission IE" in the Path Switch Request message.
• If the "PATH SWITCH REQUEST Transfer IE" in the Path Switch Request message contains a processing indicator IE based on the PDU set, the SMF should (if supported) handle this information as stipulated in TS 23.501.
• If the "PATH SWITCH REQUEST Transport IE" in the Path Switch Request message contains the MBS support indicator IE, then the SMF should (if supported) handle this information as stipulated in TS 23.247.
• If supported, the NG-RAN node should report the PATH SWITCH REQUEST transmission of the ECN tag in IE or congestion information report status IE in the Path Switch Request message. If the ECN tag or congestion information reporting status IE is included in the PATH SWITCH REQUEST Transport IE of the Path switch Request message, the SMF should (if supported) use it to infer whether the ECN tag at NG-RAN, the ECN tag at UPF, or congestion information reporting is active. As described in TS 23.501.

3. Upstream Data Processing

• If the PATH SWITCH REQUEST ACKNOWLEDGE Transfer IE of the Path Switch Request Acknowledge message contains UL NG-U UP TNL Information IE Then the NG-RAN node should store this information and use it as the uplink termination point for the user plane data of this PDU session.
• If the PATH SWITCH REQUEST ACKNOWLEDGE Transfer IE of the Path Switch Request Acknowledge message contains additional NG-U UP TNL Information IE, Then the NG-RAN node should store this Information and use the UL NG-U UP TNL Information IE contained therein as the uplink termination point for the user plane data of this PDU session (split into different tunnels).
• If the PATH SWITCH REQUEST ACKNOWLEDGE transmission IE of the path switch request acknowledge message contains redundant UL NG-U UP TNL information IE, the NG-RAN node should (if supported) store this information. And use it as the uplink termination point of the user plane data for the redundant transmission of this PDU session, as described in TS 23.501.
• If the PATH SWITCH REQUEST ACKNOWLEDGE transmission IE of the path switch Request acknowledge message contains additional redundant NG-U UP TNL information IE, the NG-RAN node should (if supported) store this information. And use the included UL NG-U UP TNL information IE as the uplink termination point for the user plane data split in different tunnels of this PDU session