Hysteresis-based Active Queue Management for TCP Traffic in Data Centers

Much of the incremental improvement to TCP over the past three decades had the ultimate goal of making it more effective in using the long-fat pipes of the global Internet. This resulted in a rigid set of mechanisms in the protocol that put TCP at a disadvantage in small-delay environments such as data centers. In particular, in the presence of the shallow buffers of commodity switches and the short round trip times in data centers, the continued use of a large TCP initial congestion window and a huge minimum retransmission timeout (both inherited from the Internet-centric design) results in a very short TCP loss cycle that affects particularly the flow completion times of short-lived incast flows. In this paper, we first investigate empirically the TCP loss cycle and discuss its impact on packet losses, recovery and delay; then we propose a switch-based congestion controller with hysteresis (HSCC) that aims to stretch the TCP loss cycle without modifying TCP itself. To protect incast flows from severe congestion, HSCC is designed to transparently induce the TCP source to alternate between its native TCP congestion control algorithm and a slower more conservative constant bit rate flow control mode that is activated when congestion is imminent. We show the stability of HSCC via analytical modelling, and demonstrate its effectiveness via simulation and implementation in a small testbed.