@article{ha_rhee_2011, title={Taming the elephants: New TCP slow start}, volume={55}, ISSN={["1872-7069"]}, DOI={10.1016/j.comnet.2011.01.014}, abstractNote={Standard slow start does not work well under large bandwidth-delay product (BDP) networks. We find two reasons for this problem in three popular existing operating systems: Linux, FreeBSD and Windows XP. The first reason is that heavy packet losses occur because of the exponential increase of the congestion window during standard slow start. Recovering from heavy packet losses puts extremely high loads on end systems, rendering the end systems completely unresponsive for a long period of time, and results in a long blackout period without transmission. This problem commonly occurs with all three operating systems. The second reason is that some proprietary protocol optimizations applied to slow start happen to slow down the loss recovery followed by slow start. Although improving the system bottleneck by optimizing data structures is valuable especially for addressing the processing overload with heavy packet losses, it is not effective for the prolonged loss recovery caused by proprietary optimizations. In addition, a large number of packet losses are not desirable since they waste network bandwidth and lead TCP into frequent timeouts and loss synchronization which results in under-utilization of the network. We propose a new slow start algorithm, called Hybrid Start (HyStart), that finds a “safe” exit point for slow start at which it can terminate and safely advance to the congestion avoidance phase without causing heavy packet loss. HyStart uses ACK trains and RTT delay samples to detect whether (1) the forward path is congested or (2) the current size of the congestion window has reached the available capacity of the forward path. HyStart is a plugin to TCP senders and requires no change on TCP receivers. We implement HyStart for TCP-NewReno and TCP-SACK in Linux and compare its performance with five different slow start schemes on the TCP receivers of the three different operating systems on the Internet and also in lab testbeds. Our results indicate that HyStart works consistently well under diverse network environments, including asymmetric links, wireless networks, and high and low BDP networks. Especially with different operating system receivers (Windows XP and FreeBSD), HyStart improves the start-up throughput of TCP significantly by more than 2 to 3 times and is the default slow start algorithm of CUBIC since Linux 2.6.29.}, number={9}, journal={COMPUTER NETWORKS}, author={Ha, Sangtae and Rhee, Injong}, year={2011}, month={Jun}, pages={2092–2110} }
 @article{warrier_janakiraman_ha_rhee_2009, title={DiffQ: Practical Differential Backlog Congestion Control for Wireless Networks}, ISBN={["978-1-4244-3512-8"]}, ISSN={["0743-166X"]}, DOI={10.1109/infcom.2009.5061929}, abstractNote={Congestion control in wireless multi-hop networks is challenging and complicated because of two reasons. First, interference is ubiquitous and causes loss in the shared medium. Second, wireless multihop networks are characterized by the use of diverse and dynamically changing routing paths. Traditional end point based congestion control protocols are ineffective in such a setting resulting in unfairness and starvation. This paper adapts the optimal theoretical work of Tassiulas and Ephremedes on cross-layer optimization of wireless networks involving congestion control, routing and scheduling, for practical solutions to congestion control in multi-hop wireless networks. This work is the first that implements in real off-shelf radios, a differential backlog based MAC scheduling and router-assisted backpressure congestion control for multi-hop wireless networks. Our adaptation, called DiffQ, is implemented between transport and IP and supports legacy TCP and UDP applications. In a network of 46 IEEE 802.11 wireless nodes, we demonstrate that DiffQ far outperforms many previously proposed "practical" solutions for congestion control.}, journal={IEEE INFOCOM 2009 - IEEE CONFERENCE ON COMPUTER COMMUNICATIONS, VOLS 1-5}, author={Warrier, Ajit and Janakiraman, Sankararaman and Ha, Sangtae and Rhee, Injong}, year={2009}, pages={262–270} }
 @article{cai_eun_ha_rhee_xu_2009, title={Stochastic convex ordering for multiplicative decrease internet congestion control}, volume={53}, ISSN={["1872-7069"]}, DOI={10.1016/j.comnet.2008.10.012}, abstractNote={Window growth function for congestion control is a strong determinant of protocol behaviors, especially its second and higher-order behaviors associated with the distribution of transmission rates, its variances, and protocol stability. This paper presents a new stochastic tool, called convex ordering, that provides an ordering of any convex function of transmission rates of two multiplicative-decrease protocols and valuable insights into high-order behaviors of protocols. As the ordering determined by this tool is consistent with any convex function of rates, it can be applied to any unknown metric for protocol performance that consists of some high-order moments of transmission rates, as well as those already known such as rate variance. Using the tool, it is analyzed that a protocol with a growth function that starts off with a concave function and then switches to a convex function (e.g., an odd order function such as x3 and x5) around the maximum window size in the previous loss epoch, gives the smallest rate variation under a variety of network conditions. Among existing protocols, BIC and CUBIC have this window growth function. Experimental and simulation results confirm the analytical findings.}, number={3}, journal={COMPUTER NETWORKS}, author={Cai, Han and Eun, Do Young and Ha, Sangtae and Rhee, Injong and Xu, Lisong}, year={2009}, month={Feb}, pages={365–381} }
 @article{ha_le_rhee_xu_2007, title={Impact of background traffic on performance of high-speed TCP variant protocols}, volume={51}, DOI={10.1016/j.comnet.2006.11.005}, abstractNote={This paper examines the effect of background traffic on the performance of existing high-speed TCP variant protocols, namely BIC-TCP, CUBIC, FAST, HSTCP, H-TCP and Scalable TCP. We demonstrate that the stability, link utilization, convergence speed and fairness of the protocols are clearly affected by the variability of flow sizes and round-trip times (RTTs), and the amount of background flows competing with high-speed flows in a bottleneck router. Our findings include: (1) the presence of background traffic with variable flow sizes and RTTs improves the fairness of most high-speed protocols, (2) all protocols except FAST and HSTCP show good intra-protocol fairness regardless of the types of background traffic, (3) HSTCP needs a larger amount of background traffic and more variable traffic than the other protocols to achieve convergence, (4) H-TCP trades stability for fairness; that is, while its fairness is good independent of background traffic types, larger variance in the flow sizes and RTTs of background flows causes the protocol to induce a higher degree of global loss synchronization among competing flows, lowering link utilization and stability, (5) FAST suffers unfairness and instability in small buffer or long delay networks regardless of background traffic types, and (6) the fairness of high-speed protocols depends more on the amount of competing background traffic rather than its rate variability. We also find that the presence of high-speed flows does not greatly reduce the bandwidth usage of background Web traffic.}, number={7}, journal={Computer Networks (Amsterdam, Netherlands : 1999)}, author={Ha, S. T. and Le, L. and Rhee, I. and Xu, L. S.}, year={2007}, pages={1748–1762} }