Modeling Network Coded TCP : Analysis of Throughput and Energy Cost Citation

We analyze the performance of TCP and TCP with network coding (TCP/NC) in lossy networks. We build upon the framework introduced by Padhye et al. and characterize the throughput behavior of classical TCP and TCP/NC as a function of erasure probability, round-trip time, maximum window si ze, and duration of the connection. Our analytical results show that network coding masks random erasures from TCP, thus preventing TCP’s performance degradation in lossy networks. It is further seen that TCP/NC has significant throughput gains over TCP. In addition, we show that TCP/NC may lead to cost reduction for wireless network providers while maintaining a certain quality of service to their users. We measure the cost in terms of number of base stations, which is highly correlated to the en ergy, capital, and operational costs of a network provider. We sho w that increasing the available bandwidth may not necessaril y lead to increase in throughput, particularly in lossy networks in which TCP does not perform well. We show that using protocols such as TCP/NC, which are more resilient to erasures, may lead to a throughput commensurate the bandwidth dedicated to each us er. I. I NTRODUCTION The Transmission Control Protocol (TCP) is one of the core protocols of today’s Internet Protocol Suite. TCP was designed for reliable transmission over wired networks, in which losses are generally indication of congestion. This i s not the case in wireless networks, where losses are often due to fading, interference, and other physical phenomena. Consequently, TCP’s performance in wireless networks is po or when compared to the wired counterparts as shown e.g. in [1], [2]. There has been extensive research to combat these harmf ul effects of erasures and failures [3]–[5]; however, TCP even with modifications does not achieve significant improvement . For example, there has been suggestions to allow TCP sender to maintain a large transmission window to overcome the random losses within the network. However, as we shall show in this paper, just keeping the window open does not lead to improvements in TCP’s performance. Even if the transmissio n window is kept open, the sender can not transmit additional packets into the network without receiving acknowledgment s. References [5], [6] give an overview and a comparison of various TCP versions over wireless links. ∗M. Kim and M. Médard are with the RLE at the Massachusetts Ins titute of Technology, MA USA (e-mail:{minjikim, medard}@mit.edu). †T. Klein and E. Soljanin are with the Alcatel-Lucent Bell Lab oratories, NJ USA (e-mail: thierry.klein@alcatel-lucent.com, emina@research.belllabs.com). §J. Barros is with the Department of Electrical and Computer E ngineering at the University of Porto, Portugal (e-mail: jbarros@fe.u p.pt). This work is supported by the MIT-Portugal Program under awa rd No: 014098-153. Some relief may come from network coding [7], which has been introduced as a potential paradigm to operate communication networks, in particular wireless networks. Netwo rk coding allows and encourages mixing of data at intermediate nodes, which has been shown to increase throughput and robustness against failures and erasures [8]. There are sev eral practical protocols that take advantage of network coding i n wireless networks [9]–[12]. In order to combine the benefits of TCP and network coding, [13] proposes a new protocol called TCP/NC. In this paper, we present a performance evaluation of TCP as well as TCP/NC in lossy networks. We adopt and extend the TCP model in [2] – i.e. we consider standard TCP with GoBack-N pipe lining. Thus, the standard TCP discards packets that are out-of-order. We analytically show the throughput gains of TCP/NC over standard TCP. We characterize the steady state throughput behavior of both TCP and TCP/NC as a function of erasure rate, round-trip time (RTT), and maximum window size. Furthermore, we use NS-2 (Network Simulator [14]) to verify our analytical results for TCP and TCP/NC. Our analysis and simulations show that TCP/NC is robust against erasures and failures. TCP/NC is not only abl e to increase its window size faster but also maintain a large window size despite losses within the network. Thus, TCP/NC is well suited for reliable communication in lossy networks . In contrast, standard TCP experiences window closing as losse s are mistaken to be congestion. We use the model for TCP/NC’s and TCP’s performance to study their effect on cost of operating a network. In particu lar, we show that maintaining or even improving users’ quality of experience may be achieved without installing additiona l network infrastructure, e.g. base stations. We measure use rs’ quality of experience using the throughput perceived by the user. We make a clear distinction between the terms throughp ut and bandwidth, where throughput is the number of useful bits over unit time received by the user and bandwidth is the number of bits transmitted by the base station per unit time. In essence, bandwidth is indicative of the resources provisioned by the service providers; while throughput is indicative of the user’s quality of experience. For example , the base station, taking into account the error correction c odes, may be transmitting bits at 10 megabits per second (Mbps), i.e. bandwidth is 10 Mbps. However, the user may only receive useful information at 5 Mbps, i.e. throughput is 5 Mbps. The disparity between throughput and bandwidth used can be reduced by using a transport protocol that is more resilie nt to losses. One method is to use multiple base stations sip1 p2 p3 Lost ACK(p1)