hange options are supported, as well as bearer termination point changes for all bearer types. Whenever a bearer termination point is changed, user data forwarding may be performed between NG-RAN nodes following the behavior of handover, that is the node from which data are forwarded behaves as a "source NG-RAN node" and the node to which data are forwarded behaves as a "target NG-RAN node" during handover. 4.3.4.3 Procedures Different procedures are used to set up a MR-DC configuration, to modify the characteristics of an MR-DC configuration between an MN and an SN, and to change the SN involved in the MR-DC configuration. The procedures described in the present section are defined in generic terms and are applicable to all MR-DC architectures. However, in practice, different (albeit quite similar) protocols are used on the network interface between MN and SN, that is X2-AP for EN-DC and Xn-AP for MR-DC with 5GC. The former is defined in 3GPP TS 36.423 and the latter in 3GPP TS 38.423. Note that in the procedure diagrams below EPC entities (S-GW and MME) and therefore X2-AP protocol on the X2 interface are used. The procedures for MR-DC with 5GC are very similar. 7 This is also because the DRB ID management by a single node makes security updates due to wraparound less frequent (DRB ID is used as an input to the PDCP encryption algorithm of a DRB). 4 NG-RAN Architecture 4.3.4.3.1 Secondary Node Addition The Secondary Node Addition procedure is initiated by the node serving the UE (that plays the role of the MN) and is used to add an SN providing additional resources to the UE. For bearers requiring SCG radio resources, this procedure is used to add at least the first cell of the SCG. But this procedure can also be used to configure an SN terminated MCG bearer (where no SCG configuration is needed). Figure 4.3.12 shows the message flow for the Secondary Node Addition procedure in the EN-DC case. 1. The procedure is triggered by the MN based on the measurement reports provided by the UE and also based on QoS and t