Leave No Cash on the Table: An Optimal Approach for Controlling Wireless TCP AIMD

Abstract

The class of additive-increase/multiplicative-decrease (AIMD) algorithms constitutes a key mechanism for congestion control in modern communication networks, such as the current Internet. However, when wireless links are present, the algorithmic behavior of an AIMD algorithm is distorted. Spurious backoffs may be triggered due to packet reordering and non-congestive loss. Prior solutions are developed based on empirical observations without a complete examination of the available design options, thereby overlooking more efficient algorithmic designs (i.e. cash on the table). In this paper, we provide a systematic approach for attaining an optimal enhancement of wireless TCP AIMD. We formulate the design of wireless TCP AIMD as an optimal control problem with the design space of TCP AIMD as the feasible set. With a complete, formal characterization of the design space, we derive a compensation scheme that gives rise to a family of the optimal AIMD algorithms, which can fully filter out the effects due to packet reordering and non-congestive loss. We further identify some properties that help explain the impact of AIMD-based TCP enhancements systematically on the network. We have devised a simple module that realizes the compensation scheme. The module demonstrates a significant performance improvement over existing wireless TCP enhancements through packet level simulation.