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

Abstract

thing, LBs cannot achieve load balancing fairness by virtue of simple load balancing mechanisms as stated previously. For another, many LBs cannot avoid PCC violations upon frequent DIP pool updates [4-6, 9, 14]. Hence, existing LBs cannot achieve load balancing fairness and PCC at the same time, both of which substantially increase tail latency of services.In response, this paper presents Libra, a stateful LB implemented in a P4 switch, in which a more sophisticated load balancing mechanism (i.e., Weighted M-Least-Connection First (WMLCF)) considering actual states of servers is designed to achieve load balancing fairness and meanwhile PCC is guaranteed in a memory-efficient way in the data plane. Nevertheless, this poses three key challenges: i) it is challenging to implement WMLCF mechanism in the data plane that only supports simple arithmetic computations operating nonfloating point values; ii) implementing the mechanism without violating PCC as the complicated mechanism could incur more PCC violations; and iii) achieving load balancing fairness and PCC without sacrificing performance. Libra copes with these challenges via the following designs: i) we implement WMLCF by means of the collaboration of the data plane and the control plane; ii) we introduce Bloom Filter [17] with small size to avoid PCC violations incurred by WMLCF and DIP pool updates; and iii) WMLCF is implemented in the control plane while Libra makes load balancing decisions in the data plane, which ensures the line-rate packet processing.To summarize, Libra makes four major contributions as follows:• We introduce Libra, a stateful LB that primarily achieves load balancing fairness and PCC. We present the architecture and detailed design of Libra. • We propose and implement a novel load balancing mechanism (i.e., WMLCF) that can fairly spread the load among servers. Libra makes load balancing decisions in the light of WMLCF mechanism. • We ensure PCC even under the more PCC violations induced by the usage of more complicated load balancing mechanism and DIP pool updates, which incurs marginal memory overhead. • We implement Libra on a P4 software switch, and conduct extensive evaluations to demonstrate the capability of Libra in terms of achieving load balancing fairness and PCC. The evaluation shows that, compared with existing LBs, Libra can achieve load balancing fairness above 95% even under heavy load and fully ensuring PCC under various scenes.