Research of TCP ssthresh Dynamical Adjustment Algorithm Based on Available Bandwidth in Mixed Networks

Research of TCP ssthresh Dynamical Adjustment Algorithm Based on Available Bandwidth in Mixed Networks 1 Wang Zhanjie, 2 Zhang Yunyang 1, First Author Department of Computer Science,Dalian University of Technology of China, Dalian 116023,China, wangzhj@dlut.edu.cn *2,Corresponding Author Department of Computer Science,Dalian University of Technology of China, Dalian 116023, China, zhangyunyang302@126.com Abstract Slow start threshold only makes adjustment if congestion happens in current TCP protocol, although TCP works well, when running in wireless environment and other environment with dynamical changing bandwidth, TCP slow-start threshold still makes adjustment according to the traditional way, therefore cannot make full use of the available bandwidth and resulting in a significant decline in its performance. Recently, a lot of research for TCP congestion control mechanism in wired/wireless mixed network has been done to improve the performance of TCP in links with dynamically changed bandwidth. Those researches had achieved some results, but there are still much to be desired. In this regard, this paper proposes a new mechanism with dynamically changed slow start threshold of TCP congestion control protocol. The mechanism dynamically adjusts the slow start threshold based on the estimated available bandwidth and gives the control algorithm. Simulation results show that the algorithm proposed by this paper can increase the utilization of effective bandwidth in the environment that with dynamically changed bandwidth, greatly improve the performance of TCP and achieves the desired effect. Keywords: Slow Start Threshold Dynamical Adjustment, Wireless Network, TCP Congestion Control 1. Introduction With the development of network applications, TCP protocol currently has become the most used network transport protocol and one of the cornerstones of the rapid development of the Internet. Wireless communication technology has become more sophisticated [1-4], and mixed network is now developing rapidly. The so-called mixed network is constructed by the seamless integration of wired and wireless communication networks. However, the efficiency of TCP protocol is very low in the mixed network, far less than its efficiency in traditional wired networks [5]. The main reason is that TCP congestion control algorithm is too simple [6]. Most researches of TCP protocol are for wired networks, in that situation, the underlying link is considered to be relatively reliable and error rate can be ignored. Compared to wired link, wireless channel has its own characteristics, such as long delay, high error rate, asymmetry of uplink and downlink, small and obviously changed bandwidth and so on[7-9]. Traditional TCP protocol designed for wired network which has more stable link quality, has many limitations [10]. In particular, the control strategies of TCP protocol are still used in wireless network environment. For example, the slow start threshold only makes adjustment when congestion happens, and the send rate cannot adjust according to the dynamical change of bandwidth in wireless network [11]. As a result, the bandwidth is not fully utilized, the throughput is reduced, and ultimately reduces bandwidth utilization and TCP performance in wireless network. Therefore, how to improve the mixed network bandwidth utilization effectively and the performance of TCP is an important issue in today's network research. This paper makes a deep study for low bandwidth utilization and poor TCP performance in mixed network. By modifying the congestion control algorithm, this paper proposes a scheme to improve endto-end TCP performance which can dynamically adjust slow-start threshold based on estimated available bandwidth. Results obtained by simulation experiments are ideal and the throughput is significantly increased. International Journal of Advancements in Computing Technology(IJACT) Volume 5, Number 9, May 2013 doi:10.4156/ijact.vol5.issue9.49 412

2. Research background In recent years, with the development of Internet, wireless networks have become more sophisticated and the network shows characteristics of high-bandwidth, long delay and easily changed bandwidth. These have put forward higher requirements for the performance of TCP. Researchers have done a lot of research and exploration in terms of performance, especially how to make effective use of network bandwidth has become a hot issue. The research has achieved some results, such as typical TCP congestion control algorithm, Reno, Westwood, Vegas, etc., which effective improves TCP performance and network utilization to some extent. 2.1. TCP Reno TCP Reno is the most widely used TCP protocol on Internet currently [12]. It improves the traditional congestion control by adding the fast recovery mechanism. TCP Reno congestion control is divided into five states: slow start-up, congestion avoidance, fast retransmit, fast recovery and timeout retransmission. In slow start-up phase, the cwnd (congestion window) of the sender increases exponentially with RTT (Round-Trip Time) of message segment. When the congestion window is greater than slow start threshold (ssthresh), the congestion control algorithm enters the congestion avoidance phase, namely, cwnd increases linearly with RTT. When RTT exceeds RTO, the sender considers that the packet has lost, and starts the timeout retransmission mechanism, sets the slow start threshold to cwnd / 2, then sets the cwnd to l and re-enters the slow start phase. If the sender receives three duplicate ACKs, the sender considers that network congestion has occurred. Then the sender starts fast recovery algorithm, rather than the slow start algorithm, namely, halves the slow start threshold, sets the congestion window to the after halving value of slow start threshold and then enters the congestion avoidance phase. TCP Reno considers packet timeout as a sign of severe network congestion, and three duplicate ACKs as mild network congestion [13]. The fast recovery mechanism of TCP Reno maintains high cwnd in mild congestion. It improves the performance of congestion control algorithm when multiple packets are lost, is able to prevent emergence of idle communication link after fast retransmit, and therefore avoids increased occupancy of communication link caused by reusing the slow start when packet loss happened. By introducing fast recovery mechanism, TCP Reno transmits packets in the network more efficiently, improves the bandwidth utilization and the TCP performance is significantly improved. 2.2. TCP Westwood TCP Westwood (TCPW) controls the traffic by estimating end-to-end available bandwidth and therefore improves utilization of network bandwidth and TCP performance. TCP Westwood [14] is an algorithm for wireless network, and is very effective in wired / wireless mixed network. It strictly follows the TCP end-to-end design principles [15] and does not require support of middle node. The congestion control mechanism used in TCPW is called AIAD (Additive Increase Adaptive Decrease), and its main purpose is to increase connection throughput by slightly reducing the congestion window when a small amount of packet loss happens. The window control method of TCPW in the slow start and congestion avoidance phase has not changed, namely, is still the same as the traditional TCP Reno. When network congestion is detected, TCPW uses estimation of bandwidth to set the congestion window and the slow start threshold instead of using traditional congestion control method. TCPW is made of two parts: first, the estimation of available bandwidth; second, adjustment of congestion window and slow start threshold. The estimation of available bandwidth uses ABSE (Adaptive Bandwidth Share Estimation) filtering mechanism to estimate the available bandwidth by detecting returning speed of the reply ACK. 413

TCPW uses the estimated available bandwidth to adjust sender's congestion window and the slow start threshold, improves the throughput and can effectively handle packet loss in wireless network. When sender receives n duplicate ACKs, ssthresh is set to (BWE*RTTmin) / seg_size according to the estimated bandwidth, and the cwnd is set equal to ssthresh only if cwnd > ssthresh. After a timeout, cwnd and ssthresh are set equal to 1 and BWE respectively, while BWE represents the estimated connection bandwidth, RTTmin is the minimum value of measured round-trip time RTT of message segment, and seg_size is size of the packet. Simulation results show that TCP Westwood outperforms TCP Reno and has higher throughput. Based on Reno, TCPW makes appropriate adjustment to the congestion window cwnd by estimating the available bandwidth of the network and achieves a faster recovery. 2.3. Reno and Westwood comparison TCPW control mechanism effectively improves the performance of TCP in the wired and wireless networks, and doubles the throughput compared with the Reno [14] TCPW uses the estimated bandwidth to modify slow start threshold and congestion window, makes more rational use of network resources after the retransmission, controls the sending window and sending rate better, and improves the transmission performance of the network. The throughput of a wired/wireless mixed network can be increased by 550%. To a large extent, TCPW eliminates negative impacts on the utilization of network bandwidth caused by random packet loss, and fully complies with the end-to-end TCP semantics. In wired network environment which has relatively stable bandwidth, TCP Reno plays a good performance. However, under wireless or mixed networks that bandwidth is prone to change dynamically, Reno only simply halves the slow-start threshold when congestion occurs for controlling congestion and regardless of changes of network bandwidth. This will inevitably affects the sending rate and therefore the network bandwidth cannot be fully utilized. TCP in wireless or mixed networks cannot give full play to its performance and the network utilization is declined. Due to the instability of wireless network transmissions, although TCPW makes some improvements of setting slow-start threshold based on the change of bandwidth, the slow start threshold remains the same without duplicate ACK and timeout and obviously not takes into account current situation of network bandwidth, therefore the utilization of network bandwidth remains low. 3. Slow start threshold dynamical adjustment To address this issue, this paper carried out a large number of theoretical studies and simulation experiments, and found that slow start threshold settings play a serious impact on network bandwidth utilization and TCP performance. Especially in the network with easily changed bandwidth, slow start threshold still changes in the traditional way, regardless of bandwidth changes, and certainly will seriously affect the use of effective bandwidth. Therefore, this paper proposes a dynamic control strategy of slow-start threshold. For dynamically changed bandwidth in mixed network, the program can set the slow start threshold according to current available bandwidth, and therefore achieves full utilization of bandwidth and improves TCP performance. 3.1. Bandwidth detection and analysis Under mixed network environment, the network bandwidth may be changed frequently with various media and interference. Therefore, in order to give full play to the performance, available network bandwidth should be considered in TCP congestion control algorithm. Bandwidth detection is an important part of the congestion control mechanism. The receiver provides feedback on network conditions to the sender, and the sender makes decisions of controlling congestion based on the feedback. This article uses ABSE (Adaptive Bandwidth Share Estimation) filtering mechanisms to detect and analyze the bandwidth, namely, the same bandwidth estimation method as TCPW. By analysis and 414

observation of ACK s return interval, the bandwidth estimation method is able to estimate current bandwidth more accurately. It can be applied to networks with dynamically changed bandwidth, such as wired and wireless mixed network, to estimate current available bandwidth and is very effective in mixed networks. 3.2. Dynamical adjustment of slow start threshold In mixed network environment, of which available bandwidth is dynamically changed, how to set the slow start threshold according to current load capacity of the network is the key to improve TCP performance. The setting of TCP slow-start threshold is inappropriate and is prone to cause two potential problems in congestion control: 1. First, if the slow start threshold is set far higher than available bandwidth of current link, it will cause TCP too much time to stay in the slow start phase, the congestion window increases exponentially, generates a large number of packets, seriously exceeds load capacity of current network, makes network conditions deteriorate rapidly and causes faster network congestion. And then leads to continuous packet loss and timeout of many packets on bottleneck router, creates more packet retransmission, increases the restart time and causes overload of the entire network traffic. Finally the frequency of network congestion is increased, the performance of networks is deteriorated and the utilization of the network is reduced. 2.Second, if the slow start threshold is too low, there will be a large part of available bandwidth and will enter the congestion avoidance phase too early instead of effectively using the exponential growth of congestion window in slow start phase. And congestion window turns to grow linearly, which cannot make full use of bandwidth, wastes a lot of available bandwidth and bandwidth utilization is reduced. Based on the above analysis, this paper improves TCP congestion control algorithm and gives a new control strategy for networks with dynamically changed bandwidth. The strategy and algorithm obtain load capacity by estimating available bandwidth at the sending end and dynamically change the slow start threshold based on load capacity of current network. Namely, regardless of whether the sender is in slow start phase or congestion avoidance phase, re-adjust the slow start threshold according to network load capacity, and determines sender s phase by new value of the slow start threshold. Especially when the network load capacity is high, the slow-start threshold can be increased according to the load capacity and then the delivery volume of the sender can be increased. When network load capacity declines, the sender can enter the congestion avoidance phase as soon as possible by reducing the slow start threshold. The congestion frequency and the restart time caused by the congestion can be reduced, and the bandwidth and network utilization are increased. When n repeated ACKs are received in original TCPW, ssthresh is set to BWE*RTTmin, cwnd is set to the same value as ssthresh only if cwnd > ssthresh, and then increases linearly. In the improved proposal, ssthresh can be adjusted based on current situation of network bandwidth. When network condition improves, ssthresh can be increased, and then cwnd can achieve exponential growth and therefore uses the bandwidth better. This often happens especially in the wireless/wired mixed network. This study fully considers the adjustment of slow start threshold in the case with no timeout and no repeated ACKs, and retains handling of duplicate ACKs and timeout in TCPW. The strategy proposed by this paper improves the congestion control algorithm based on the sending end. So it does not need support of middle node and fully complies with end-to-end design principles. Process of improved proposal is as follows. Every time the sender receives a new ACK (nonduplicate ACK) which is not overtime, available bandwidth of current network BWE can be obtained by using the bandwidth estimation algorithm. Estimate the current slow start threshold ssthresh_eva by combining BWE with RTT, rather than RTTmin, and set it to BWE*RTT / seg_size. When ssthresh_eva > ssthresh, ssthresh is set equal to ssthresh_eva; when ssthresh_eva < ssthresh, ssthresh is set equal to ssthresh_eva only if cwnd < ssthresh_eva. The pseudo code of the algorithm is as follows: if (current_ack > last_ack_) {/*new ACK*/ 415

ssthresh_eva = BWE*RTT / seg_size; /*evaluate the slow start threshold*/ if (ssthresh_eva < ssthresh) {/*low*/ if (cwnd < ssthresh_eva) { ssthresh = ssthresh_eva; } } else {/*high*/ ssthresh = ssthresh_eva; } } Among them, current_ack is the confirmation number of current ACK, and last_ack is the confirmation number of last ACK. In this process, the slow-start threshold is adjusted based on load capacity obtained by available bandwidth of current network, thus ensures that the sending rate adapts to available bandwidth of current network and improves the efficient use of network bandwidth. The specific process of TCP on the sender is as follows: (1) The connection begins. (2) In the case of no timeout, obtain the slow-start threshold ssthresh_eva for every new received ACK (non-duplicate ACK) according to the estimated bandwidth BWE, and the value is BWE*RTT/seg_size. (3) Compare ssthresh_eva with ssthresh. If ssthresh_eva < ssthresh, go to (4); otherwise, go to (5). (4) Compare cwnd with ssthresh_eva. If cwnd < ssthresh_eva, go to (5); otherwise, go to (6). (5) Set ssthresh equal to ssthresh_eva. (6) Compare cwnd with ssthresh. If cwnd < ssthresh, go to (7); otherwise, go to (8). (7) Available bandwidth of current network is in good condition, and cwnd is in exponential growth. (8) Available bandwidth of current network is close to the limit, and cwnd is in linear growth. (9) Determine whether to terminate the connection, if not, then go to (2); otherwise, go to (10). (10) Terminate the connection Flowchart of TCP implementation is as follows. Figure 1. Flowchart of TCP implementation 416

4. Simulation and analysis In order to verify the congestion control strategy and new algorithm, this study does simulation experiments and evaluates the performance of the improved TCP from the perspective of throughput. Meanwhile, compares and analyzes with Reno and TCPW under the same conditions. 4.1. Simulation environments This paper uses NS2 (Network Simulator version 2) [16] [17] as the simulation platform. NS2 is an object-oriented, discrete event driven network simulator and is mainly used to solve problems in network research. Simulation experiments establish network model in NS2 to simulate the performance of TCP with influences of wireless packet loss rate and wireless bandwidth respectively, and compares the performance with Reno and TCPW. The topology used in simulation experiments is the wired and wireless mixed link. In wired part, the link between wired sending terminal and base station is 100Mbps, and the propagation delay is assumed to be 10ms Wireless receiver is a mobile terminal which is connected to the wired link through the base station. The wireless link in the simulation is 20Mbps, and the propagation delay is assumed to be 0.1ms. The topology is shown in Figure 2. S wired network wireless network R 20Mbps 100Mbps 0.1ms 10ms Figure 2. The topology used for simulation D 4.2. Simulation results and analysis This paper conducts simulation experiments of Reno, TCPW and the improved proposal proposed by this paper under wired and wireless mixed networks by using simulation tools. 4.2.1. Impact of packet loss rate In the network with bit error, error rate increased from 0 to 6.25%, and we made simulation and comparison of throughput for these three TCP protocol models. Figure 3. Throughput vs. the packet loss rate in the wireless link 417

Figure 4. Throughput vs. the capacity in the wireless link Figure 3 shows throughput changes with packet loss rate in three models. Simulation results show that with the growth of wireless packet loss rate, the proposed scheme makes the slow start threshold correspond with dynamically changed bandwidth by adjusting the slow start threshold dynamically and achieves great improvement in throughput. 4.2.2. Impact of bandwidth Assume that the wireless packet loss rate is 0.3% and bandwidth increases from 20Mbps to 100Mbps in wireless networks with dynamically changed bandwidth, simulation experiments have been done to compare the throughput of three TCP protocol models. Throughput changes in three protocol models over the wireless bandwidth are shown in Figure 4. The experimental results in the figure show that the improved controlling method proposed by this paper has better throughput by dynamically adjusting slow start threshold according to the bandwidth as bandwidth continues to increase, and can adapt to changes of available network bandwidth better. 5. Conclusion In wired and wireless mixed network environment, congestion control mechanism is the focus of current research. Especially in the wireless link, noise or signal attenuation is likely to cause dynamical change of the bandwidth. Current TCP does not take into account changes of network bandwidth when adjusting slow-start threshold. Congestion control mechanism of traditional TCP seriously affects the effective use of bandwidth in mixed networks. The new control strategy proposed in this paper of dynamically setting the slow start threshold according to the available network bandwidth gives a method of dynamically controlling TCP slow-start threshold and the performance of TCP is improved. The control strategy can enable TCP to enter the congestion avoidance phase at an appropriate time, avoids the effects caused by prematurely entering the congestion avoidance phase, such as not full use of the bandwidth, lower throughput and decreased bandwidth utilization. And also avoids the effects of entering the congestion avoidance phase too late, such as deterioration of network conditions, network congestion and reduced network utilization. In short, adjust the slow start threshold according to current available bandwidth can take full advantage of the bandwidth, avoid congestion and increase network utilization. The scheme better solves the problem of dynamical changes of network bandwidth. TCP can make better use of bandwidth by setting the slow start threshold 418

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