RPL is the IPv6 routing protocol for low-power and lossy networks in the Internet of Things which supports point-to-point (P2P) communication. However, the partition of two modes of operations (MOPs) in downward routing complicates achieving high performance. In the non-storing mode, a downward route with the longest path length is often picked. In the storing mode, the downward routes to some child nodes cannot be stored by their parent because of the limitation of memory space, which makes some nodes unreachable. In addition, there are extra performance costs of mixing or switching the two modes in the existing hybrid-MOPs works. Therefore, this article proposes TB-RPL to achieve an enhancement of RPL with a better performance of P2P communication. It allows all nodes to behave in a single and uniformly fused MOP that solves the problems mentioned above. The proposed mode uses a modified routing header format and introduces a threshold to the number of route entries. We implemented and compared TB-RPL with related mechanisms in Cooja simulator based on the Contiki-NG operating system. Simulation results verify that TB-RPL eliminates the three identified problems. Consequently, it significantly improves the performance of P2P communication in LLN.