RoSR: A Novel Selective Retransmission FPGA Architecture for RDMA NICs

Abstract

Remote Direct Memory Access (RDMA) enables low-latency datacenter networks but suffers from inefficient loss recovery using Go-Back-N (GBN). GBN retransmits entire packet windows, degrading Flow Completion Time (FCT) under congestion. We introduce RoSR, a novel selective retransmission architecture for Field-Programmable Gate Array (FPGA)-based RDMA NICs that supports hardware-accelerated direct writes of out-of-order (OoO) packets. RoSR supports efficient OoO packet reception and enables fine-grained retransmission using a dynamic shared bitmap for packet tracking. By extending the RDMA over Converged Ethernet version 2 (RoCEv2) packet format, RoSR facilitates selective retransmission. It triggers retransmissions via timeouts using bitmap blocks and introduces new Nack-bitmap and rd-req-bitmap messages for loss reporting. Under 1% packet loss, RoSR achieves up to 13.5× (RDMA Write) and 15.6× (RDMA Read) higher throughput than Xilinx ERNIC. In NS-3 simulations using the HPCC RDMA stack, RoSR reduces FCT slowdown by 3× to 6× compared to GBN across various packet loss rates, congestion control algorithms (DCQCN, HPCC, Timely), and traffic patterns, while maintaining robustness under high round-trip time (RTT) conditions.