TCP Libra: Exploring RTT-fairness for TCP

Abstract

The majority of Internet users rely on the Transmission Control Protocol (TCP1) to download large multimedia files from remote servers (e.g. P2P file sharing). TCP has been advertised as a fair-share protocol. However, when session round-trip-times (RTTs) radically differ from each other, the share (of the bottleneck link) may be anything but fair. This motivates us to explore a new TCP, TCP Libra2, that guarantees fair sharing regardless of RTT. The key element of TCP Libra is the unique window adjustment algorithm that provably leads to RTT-independent throughput, yet converging to the fair share. We position TCP Libra in a non-linear optimization framework, proving that it provides fairness (in the sense of minimum potential delay fairness) among TCP flows that share the same bottleneck link. Equally important are the friendliness of Libra towards legacy TCP and the throughput efficiency. TCP Libra is source only based and thus easy to deploy. Via analytic modeling and simulations we show that TCP Libra achieves fairness while maintaining efficiency and friendliness to TCP New Reno. A comparison with other TCP versions that have been reported as RTT-fair in the literature is also carried out. © IFIP International Federation for Information Processing 2007.