A Study of TCP Fairness in High-Speed Networks

Under the TCP congestion control regime, heterogeneous flows, i.e., flows with different round-trip times (RTTs), that share the same bottleneck link will not attain equal portions of the available bandwidth. In fact, according to the TCP friendly formula [1], the throughput ratio of two flows is inversely proportional to the ratio of their RTTs. It has also been shown that TCP’s unfairness to flows with longer RTTs is accentuated under loss synchronization. Well-known mechanisms to avoid synchronization are based on injecting randomness into the network, e.g., introducing background traffic, using random drop (as opposed to drop-tail queuing). In this paper, we show that, in high-speed networks, injecting bursty background traffic may actually lead to synchronization and result in unfairness to foreground TCP flows with longer RTTs. We observe that unfairness is especially severe in high-speed variants of TCP such as Scalable TCP (S-TCP) and HighSpeed TCP (HSTCP). We propose three different metrics to characterize traffic burstiness and show that these metrics are reliable predictors of TCP unfairness. Finally, we show that TCP unfairness (including TCP SACK, S-TCP, and HSTCP) in highspeed networks due to bursty background traffic can be mitigated through the use of random drop queuing disciplines (such as RED) at bottleneck routers.