Retransmissions and reordering in 6G user plane protocols

Streamlining the user plane protocols for the 6G radio access network (RAN) is the key to enabling more efficient implementations while still improving performance. Therefore, mechanisms for packet loss and out-of-order handling across the different layers of end-to-end (E2E) network need to be revisited. As such, in this paper, we investigate how transmission control protocol (TCP) equipped with various congestion control (CC) algorithms is impacted by different RAN configurations for loss recovery and reordering. In RAN, losses and reordering on the air interface are locally repaired by Hybrid Automatic Repeat Request (HARQ), Radio Link Control (RLC) and Packet Data Convergence Protocol (PDCP) protocols. On the other hand, modern E2E transport protocols are also equipped to handle losses and reordering. The appropriate interplay of these protocols is thus crucial in achieving performance gains. We show that, as expected, loss-based CC algorithms are impacted the most when packet losses are not recovered in the RAN. For out-of-order delivery, we observe that performance is negatively impacted, particularly for scenarios with very low E2E latency, where CC interprets extensive reordering erroneously as loss.