Section: 6.10.2.1 SN Addition or modification
Content: When the MN is executing the Secondary Node Addition procedure (i.e. initial offload of one or more radio bearers to the SN), or the Secondary Node Modification procedure (as in clauses 10.2.2 and 10.3.2 in TS 37.340[ Evolved Universal Terrestrial Radio Access (E-UTRA) and NR; Multi-connectivity; Overall Description; Stage-2 ]  [51]) which requires an update of the KSN, the MN shall derive an KSN as defined in clause 6.10.3.2. The MN shall maintain the SN Counter as defined in Clause 6.10.3.1. In the case of Conditional PSCell Change and conditional PSCell addition as specified in TS 37.340[ Evolved Universal Terrestrial Radio Access (E-UTRA) and NR; Multi-connectivity; Overall Description; Stage-2 ]  [51], if there are more than one candidate SNs, for each SN, the MN shall derive a different KSN via using different SN counter as defined in clause 6.10.3.2. When executing the procedure for adding subsequent radio bearer(s) to the same SN, the MN shall, for each new radio bearer, assign a radio bearer identity that has not previously been used since the last KSN change. If the MN cannot allocate an unused radio bearer identity for a new radio bearer in the SN, due to radio bearer identity space exhaustion, the MN shall increment the SN Counter and compute a fresh KSN, and then shall perform a SN Modification procedure to update the KSN. The dual connectivity procedure with activation of encryption/decryption and integrity protection follows the steps outlined on the Figure 6.10.2.1-1. Figure 6.10.2.1-1. Security aspects in SN Addition/Modification procedures (MN initiated) 1. The UE and the MN establish the RRC connection. 2. The MN sends SN Addition/Modification Request to the SN over the Xn-C to negotiate the available resources, configuration, and algorithms at the SN. The MN computes and delivers the KSN to the SN if a new key is needed. The UE security capabilities (see subclause 6.10.4) and the UP security policy received from the SMF shall also be sent to SN. In case of PDU split, UP integrity protection and ciphering activation decision from MN may be also included as described in subclause 6.10.4 When the MN decides to configure CPA or CPC, if there are more than one candidate SNs, for each SN, the MN shall derive a different KSN and delivers the KSN to each SN seperately.. 3. The SN allocates the necessary resources and chooses the ciphering algorithm and integrity algorithm which has the highest priority from its configured list and is also present in the UE security capability. If a new KSN was delivered to the SN then the SN calculates the needed RRC. The UP keys may be derived at the same time when RRC key derived. The SN shall activate the UP security policy as described in subclause 6.10.4. 4. The SN sends SN Addition/Modification Acknowledge to the MN indicating availability of requested resources and the identifiers for the selected algorithm(s) for the requested DRBs and/or SRB for the UE. The UP integrity protection and encryption indications shall be send to the MN. 5. The MN sends the RRC Connection Reconfiguration Request to the UE instructing it to configure the new DRBs and/or SRB for the SN. The MN shall include the SN Counter parameter to indicate a new KSN is needed and the UE shall compute the KSN for the SN. The MN forwards the UE configuration parameters (which contains the algorithm identifier(s) received from the SN in step 4) , and UP integrity protection and encryption indications(received from the SN in step 4) to the UE (see subclause 6.10.3.3 for further details). If an SN sends more than one candidate PScell SCG configuration, the MN signals to the UE that all these configurations are associated with the same SN counter value. NOTE 3: Since the message is sent over the RRC connection between the MN and the UE, it is integrity protected using the KRRCint of the MN. Hence the SN Counter cannot be tampered with. 6. The UE accepts the RRC Connection Reconfiguration Request after validating its integrity. The UE shall compute the KSN for the SN if an SN Counter parameter was included. The UE shall also compute the needed RRC and UP keys and activate the RRC and UP protection as per the indications received for the associated SRB and/or DRBs respectively. The UE sends the RRC Reconfiguration Complete to the MN. The UE activates the chosen encryption/decryption and integrity protection keys with the SN at this point. 7. MN sends SN Reconfiguration Complete to the SN over the Xn-C to inform the SN of the configuration result. On receipt of this message, SN may activate the chosen encryption/decryption and integrity protection with UE. If SN does not activate encryption/decryption and integrity protection with the UE at this stage, SN shall activate encryption/decryption and integrity protection upon receiving the Random Access request from the UE.