LR2: Accelerating Long-Distance RDMA Recovery via In-Network Retransmission Decoupling

Extending RDMA across datacenters is hindered by inefficient loss recovery. High round-trip time (RTT) and large bandwidth-delay product (BDP) cripple the default Go-back-N (GBN) retransmission, while hardware constraints on RDMA NICs (RNICs) limit the feasibility of practical deployment of selective retransmission (SR). In this paper, we present LR2, an in-network collaborative retransmission mechanism that overcomes RNIC limitations to enable reliable long-distance RDMA. At its core, LR2 decouples retransmission responsibilities, assigning fast-loop GBN at the RNIC and reconstructing long-haul SR at datacenter interconnection (DCI) switches. In addition, DCI switches independently mask intra-DC losses through near-sender fast feedback and near-receiver local retransmission. To realize this design, LR2 deploys lightweight programmable modules at both ends of the long-haul link: (1) a near-receiver Depot module that maintains buffer-efficient reordering and backup pools, and (2) a near-sender Sentry module that generates fast loss feedback and filters out redundant retransmission. We implement LR2 on Tofino switches and evaluate it through testbed experiments and large-scale simulations. LR2 significantly reduces flow completion time (FCT) and improves bandwidth efficiency over baselines, without large system overhead.