Network Asymmetry on TCP Performance

• Main Authors: Yi-Te Lee, 黎乙德
• Other Authors: Wanjiun Liao
• Format: Others
• Language: en_US
• Published: 2000
• Online Access: http://ndltd.ncl.edu.tw/handle/81213911211730428275

碩士 === 國立臺灣大學 === 電機工程學研究所 === 88 === This thesis proposes a new TCP implementation called Formosa that significantly improves TCP end-to-end performance in terms of high connection throughput and high degree of fairness in resource sharing among multiple competing connections, even in the presence of network asymmetry. Unlike existing TCP implementations based on ack-clock, Formosa TCP pushes the frequency of ACKs per round trip time (RTT) to the lowest limit, using an ACK to cumulatively acknowledge a window’s worth of data packets, rather than an ACK for a single data packet as in the ACK-clocked TCP. The congestion window is updated to reflect the available bandwidth estimated on the forward channel, without the aids of the intermediate routers. As a result, the number of ACK packets is significantly reduced, while the principle of “conservation of packets” is still maintained. Based on “an ACK per window,” Formosa accurately determines RTT to eliminate the needs for exponential timer backoff and Karn’s algorithm, estimates forward delay to implement the additive increase and multiplicative decrease algorithm for better congestion avoidance, and implements the negative acknowledgement (NACK) mechanism for better congestion control during loss recovery. As compared to existing implementations, TCP Formosa demonstrates the best performance in terms of high connection throughput, high degree of fairness among connections and robustness in the face of two-way transfers, in networks with symmetric or asymmetric links.