Section: 10.10.2 MR-DC with 5GC
Content: The RRC Transfer procedure is used to deliver an RRC message, encapsulated in a PDCP PDU between the MN and the SN (and vice versa) so that it may be forwarded to/from the UE using split SRB. The RRC transfer procedure is also used for: - providing a SN measurement report, failure information report, SN UE assistance information or intra-SN CPC execution completion from the UE to the SN. If UE is IAB-MT, providing NR IAB other information from the IAB-MT to the SN when the IAB-donor is the SN; - providing MCG failure information from the UE to the MN via the SN and an RRC reconfiguration, or release, or an inter-RAT handover command from the MN to the UE via the SN; - providing F1-C traffic from an IAB-node to the MN via the SN, or F1-C traffic from the MN to an IAB-node via the SN. Additional details of the RRC transfer procedure are defined in TS 38.423[ NG-RAN; Xn Application Protocol (XnAP) ]  [5]. Split SRB: Figure 10.10.2-1: RRC Transfer procedure for split SRB (DL operation) Figure 10.10.2-1 shows an example signaling flow for DL RRC Transfer in case of the split SRB: 1. The MN, when it decides to use the split SRBs, starts the procedure by initiating the RRC Transfer procedure. The MN encapsulates the RRC message in a PDCP PDU and ciphers with own keys. NOTE: The usage of the split SRBs shall be indicated in the Secondary Node Addition procedure or Modification procedure. 2. The SN forwards the RRC message to the UE. 3. The SN may send PDCP delivery acknowledgement of the RRC message forwarded in step 2. Figure 10.10.2-2: RRC Transfer procedure for split SRB (UL operation) Figure 10.10.2-2 shows an example signaling flow for UL RRC Transfer in case of the split SRB: 1. When the UE provides response to the RRC message, it sends it to the SN. 2. The SN initiates the RRC Transfer procedure, in which it transfers the received PDCP PDU with encapsulated RRC message. SN measurement report, failure information report, SN UE assistance information, intra-SN CPC execution completion or IAB other information: Figure 10.10.2-3: RRC Transfer procedure for SN measurement report, failure information report, SN UE assistance information, intra-SN CPC execution completion or IAB other information Figure 10.10.2-3 shows an example signaling flow for RRC Transfer in case of the forwarding of the SN measurement report, failure information report, SN UE assistance information, intra-SN CPC execution completion or IAB other information from the UE: 1. When the UE sends an SN measurement report, failure information report, SN UE assistance information, intra-SN CPC execution completion or IAB other information it sends it to the MN in a container called ULInformationTransferMRDC message as specified in TS 38.331[ NR; Radio Resource Control (RRC); Protocol specification ]  [4]. 2. The MN initiates the RRC Transfer procedure, in which it transfers the received SN measurement report, failure information, SN UE assistance information, intra-SN CPC execution completion or IAB other information as an octet string. MCG failure information and RRC Reconfiguration / RRC Release / inter-RAT handover command over SRB3: Figure 10.10.2-4: RRC Transfer procedure for MCG failure information Figure 10.10.2-4 shows an example signaling flow for RRC Transfer in case of the forwarding of the MCG failure information from the UE: 1. When the UE sends MCGFailureInformation message over SRB3, it sends it to the SN in a container called ULInformationTransferMRDC message as specified in TS 38.331[ NR; Radio Resource Control (RRC); Protocol specification ]  [4]. 2. The SN initiates the RRC Transfer procedure, in which it transfers the received MCGFailureInformation message as an octet string. 3. The MN initiates the RRC Transfer procedure, in which it transfers the RRCConnectionReconfiguration message, or RRCReconfiguration message, or RRCConnectionRelease message, or RRCRelease message, or MobilityFromNRCommand message, or MobilityFromEUTRACommand message as an octet string. 4. The SN sends the received RRC message to the UE in a container called DLInformationTransferMRDC message, as specified in TS 38.331[ NR; Radio Resource Control (RRC); Protocol specification ]  [4]. F1-C traffic transfer: Figure 10.10.2-5: Scenario 2: F1-C Traffic Transfer procedure between IAB-MT and MN (F1-terminating node) in NR-DC 1. The IAB-MT sends a F1-AP message encapsulated in SCTP/IP or F1-C related (SCTP/)IP packet to the SN (non-F1-terminating IAB-donor) via split SRB2 in a container within ULInformationTransfer message encapsulated in a PDCP PDU as specified in TS 38.331[ NR; Radio Resource Control (RRC); Protocol specification ]  [4]. 2. The SN initiates the RRC Transfer procedure, in which it transfers the received PDCP PDU (ULInformationTransfer message) including F1-AP message. 3. When the MN (F1-terminating IAB-donor) sends a F1-AP message encapsulated in SCTP/IP or F1-C related (SCTP/)IP packet, it starts the procedure by initiating the RRC Transfer procedure, if split SRB2 is determined to be used and usage of SCG path is determined. The MN sends the F1-AP message to the SN in a container within DLInformationTransfer message encapsulated in a PDCP PDU specified in TS 38.331[ NR; Radio Resource Control (RRC); Protocol specification ]  [4]. 4. The SN forwards the encapsulated DLInformationTransfer message in a PDCP PDU as specified in TS 38.331[ NR; Radio Resource Control (RRC); Protocol specification ]  [4] to IAB-MT.