Section: 7.5 SRB3
Content: SRB3 is supported in EN-DC, NGEN-DC and NR-DC, but not in NE-DC. The decision to establish SRB3 is taken by the SN, which provides the SRB3 configuration using an SN RRC message. SRB3 establishment and release can be done at Secondary Node Addition and Secondary Node Change. SRB3 reconfiguration can be done at Secondary Node Modification procedure. SRB3 may be used to send SN RRC Reconfiguration, SN RRC Reconfiguration Complete, SN Measurement Report, SN Failure Information (i.e., in case of failure for an SCG RLC bearer), SN UE Assistance Information message and SN IABOtherInformation, only in procedures where the MN is not involved. SN RRC Reconfiguration Complete messages are mapped to the same SRB as the message initiating the procedure. SN Measurement Report messages are mapped to SRB3, if configured, regardless of whether the configuration is received directly from the SN or via the MN. No MN RRC messages are mapped to SRB3. If split SRB1 is not configured, SRB3 may be used by the UE to transmit to the MN an encapsulated MCG Failure Information message in the ULInformationTransferMRDC message and receive in response an encapsulated RRC reconfiguration message, MobilityFromNRCommand message, MobilityFromEUTRACommand message or RRC release message in the DLInformationTransferMRDC message. SRB3 is modelled as one of the SRBs defined in TS 38.331[ NR; Radio Resource Control (RRC); Protocol specification ]  [4] and uses the NR-DCCH logical channel type. RRC PDUs on SRB3 are ciphered and integrity protected using NR PDCP, with security keys derived from S-KgNB. The SN selects ciphering and integrity protection algorithms for the SRB3 and provides them to the MN within the SCG Configuration for transmission to the UE. NOTE: A NR SCG RRC message sent via E-UTRA MCG SRB is protected by E-UTRA MCG SRB security (NR security is not used in this case). SRB3 is of higher scheduling priority than all DRBs. The default scheduling priorities of split SRB1 and SRB3 are the same. There is no requirement on the UE to perform any reordering of RRC messages between SRB1 and SRB3. When SCG is released, SRB3 is released.