16.21.2.1 L2 MP Relay using SL indirect path

For multi-path relay operation by using SL indirect path, the protocol stacks for the user plane and control plane of L2 MP Relay architecture are illustrated in Figure 16.21.2.1-1, 16.21.2.1-2.

If PC5 interface is used between L2 MP Remote UE and L2 MP Relay UE, the SRAP sublayer is placed above the RLC sublayer for both CP and UP at both PC5 interface and Uu interface of the indirect path. For the direct path, the Uu SDAP, PDCP, RLC, MAC, PHY, and RRC are terminated at gNB and L2 MP Remote UE. But for the indirect path, only the Uu SDAP, PDCP and RRC are terminated at gNB and L2 MP Remote UE, while SRAP, RLC, MAC, and PHY are terminated in each hop (i.e., the link between L2 MP Remote UE and L2 MP Relay UE and the link between L2 MP Relay UE and the gNB). PDCP duplication is supported for the MP split bearer.