Libra: A Stateful Layer-4 Load Balancer with Fair Load Distribution

Layer-4 (L4) load balancers (LBs) are essential for data centers, dispatching incoming connections among thousands of servers. There are two critical requirements for L4 LBs: i) load balancing fairness, i.e., the ability to assign the load to servers in proportion to their capacity; ii) per-connection consistency (PCC), i.e., all the packets belonging to the same connection should be forwarded to the same server. Howbeit, existing LBs at large sacrifice load balancing fairness to mitigate PCC violations, which cannot satisfy both requirements in the meantime. In this paper, we present Libra, a stateful L4 LB that supports fair load distribution, PCC, memory efficiency, and resilience to resource depletion attacks. Libra makes load balancing decisions resorting to the proposed Weighted M-Least-Connection First (WMLCF) mechanism considering the real-time load and available processing capacity of servers, hence enabling load balancing fairness. We prototype Libra in a programmable software switch—BMv2 using P4 language and conduct extensive flow-level simulation to evaluate the performance. The evaluation indicates that Libra significantly improves load balancing fairness (over 95%), fully ensures PCC, and reduces the average flow completion time by 17.27–42.55% compared to existing mechanisms.