Analysis of Impact of Routing Buffer Size on TCP Variants

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1 Analysis of Impact of Routing Buffer Size on TCP Variants Sana Arif PN Engineering College, National University of Sciences & Technology, Islamabad, Pakistan Hamza Mansoor Manya PN Engineering College, National University of Sciences & Technology, Islamabad, Pakistan Sameer Qazi PN Engineering College, National University of Sciences & Technology, Islamabad, Pakistan Abstract When networks are designed, there are several factors which are taken into account for network performance. The buffer size of the routers at bottle neck (core) links in the network plays a vital role in improving the overall performance of the network. An increase in the buffer size may resolve this problem, however, in some networks, where different types of agents are competing for the network resources, the investment made in increasing the buffer size may lead to negative impacts as well. The agenda of this paper is to present a comparative analysis on different TCP variants (in a mixed TCP agent type environment) on the basis of different performance parameters. Conclusions on the optimal queuing buffer size of the network typology being considered are also presented in this paper. Keywords TCP, Tahoe, Reno, New Reno, Vegas, FACK, NS2, Optimal Buffer Size, Network Performance, Throughput I. INTRODUCTION In highly resource demanding network environments, a need for buffering is a natural phenomenon. Queuing buffers in routers normalize the bursts that naturally occur in such networks, reduce the frequency of packet drops and, especially with TCP traffic, they can avoid under-utilization when TCP connections back off due to packet losses. However, the buffers also add delay and jitters, and they increase the router cost and power consumption. [1] Even today with research experience of decades on packet switching networks, it is surprising that the question of optimal buffer size still remains unclear As explained in [2], the basic question - how much buffering do we need at a given router interface? is still unanswered. Some say a few dozens of packets, a bandwidth-delay product, or a multiple of the number of large TCP flows in that link. [1] However ever most answers disagree with others. Despite the apparent simplicity of the problem it remains a mystery. With today s demanding consumer networks, the issue of buffer sizing becomes increasingly important in practice. The main reason is that IP networks are maturing from just offering reachability to providing performance-centered Service-Level Agreements and delay/loss assurances. Additionally, as the popularity of applications like VOIP and video chats, the tradeoff between oversized and undersized buffers has become more significant. [1] In this paper a network with different variants of TCP flows converging at a single backbone node is considered. The idea is to present the pros and cons of selecting a particular TCP flavor in a given scenario. A consideration made in order to model such a network environment is that the traffic from agents, converging at the network backbone, is highly demanding as compared to the available resources. This will ensure that without buffering or undersized buffering frequent packet drops are observed, which is our goal of study. The TCP variants that were considered for this study are TCP Tahoe, TCP Reno, TCP New Reno, TCP Vegas and TCP FACK. The simulations were performed in a Linux based open source network simulator, ns2. Instead of focusing only on the overall performance of the network individual flow-level performance metrics, such as utilization and loss probability were taken into account in order to draw conclusions. Throughput vs buffer size performance parameter for each of the TCP variant being considered is the central idea of this study. The results of these simulations, illustrating the network performance, are tabulated in a latter part of this paper. Secondly, the optimal buffer size of the network being considered is also discussed. Link utilization or loss probability, can hide what happens at the transport or application layers. For instance, the link may have enough buffers so that it does not suffer from under-utilization, but the per-flow TCP throughput can be awfully low. [1] The topology is such that there are five TCP flows in all, each with the capacity of 500 Mbps, converging at a single bottle neck, with the capacity of 1Mbps, from where the flows are routed to five different sinks on the other side of the bottle neck. Each of the five sinks is capable of handling input traffic of 500 Mbps. This network topology is illustrated in Figure. 1.

2 Figure 1. Network Topology II. RELATED WORK The impact of routing buffer size plays a very significant role in today s demanding applications. The popularity has influenced many researchers and thereforee several papers were found and analyzed. Usually, the networks are modeled with open-loop traffic models, in which the packet arrival process does not depend on the state of the queue. This can be seen in [3], where the model for input traffic is a general Gaussian process, and derives a little for loss probability. In [4] it is recommended that the buffer size should be equal to the Bandwidth-Delay Product of the link. Delay, here refers to the round trip time, RTT, of a single TCP flow that attempts to saturate that link, while the bandwidth term refers to the capacity of the link. This rule requires the bottleneck link to have enough buffer space so that the link can stay fully utilized while the TCP flow recovers from a loss-induced window reduction. However, no recommendations are given, for the more realistic case of multiple TCP flows with different RTTs. Morris [5] investigates buffer requirements for up to 1500 long-lived flows over a link of 10 Mb/s with 25ms latency. He concludes that the minimum amount of buffering needed is a small multiple of the number of flows, and points out that for a bandwidth-delay product of 217 packets, each flow has only a fraction of a packet in transit at any time[6]. In [7] the authors have drawn conclusions showing that depending on the ratio between the edge and core link capacities the buffer requirement can change from a few packets to the bandwidth delay product. In [8] the conclusion is, if the core-to-access-speea buffer of few packets is sufficient at the core routers; ratio is large, then otherwise, larger buffer sizes do improve the flow-level performance of the users. From a modeling point of view, the analysis in the paper offers two new insights. First, it may not be appropriate to derive buffer-sizing rules by studying a network with a fixed number of users. In fact, depending upon the core-to-access-speed ratio, the buffer size itself may affect the number of flows in the system, so these two parameters should not be treated as independent quantities. Second, in the regime where the core-to-accessnoted that the small buffer sizes speed ratio is large it was are sufficient for good performance and that no loss of utilization is observed, as previously believed. In [9], it is observed that the difference in the buffer unit structure impacts the loss performance of routers with very small buffers. When buffer unit are structured in terms of packets, the UDP packet loss rate is higher than the TCP packet loss rate. However, when buffer unit are structured in terms of bytes, the UDP packet loss rate is lower than the TCP packet loss rate.

3 Figure 2: TCP throughput for varying buffer sizes III. METHODOLOGY In order to draw conclusions on the best TCP variant under particular requirements and on selecting an optimal buffer size for the network typology being considered, different simulations were performed on a Linux based open source network simulator, NS2. Firstly, buffer size very throughput simulation was performed. Normally for this particular simulation the approach taken is, the buffer size for particular simulation is kept constant and the simulation is run iteratively with different buffer sizes. At the end of this activity, the average throughput for a particular buffer size is computed and this parameter from all different simulations is plotted against buffer size. The most important consideration for this simulation is, for a particular buffer size the simulation should run for sufficient time in order to allow the network to settle to a stable bandwidth. The novelty of our work to perform this simulation is, this sufficient time was firstly computed by hit and trial practice. Only one simulation was performed in which the queuing buffer size was varying with time, incrementing by 10 packets every fifty seconds. At the end of the fifty seconds the average throughput for this time period was computed and the buffer size was incremented by 10 packets. This increment in buffer size follows equation 1, with t being a multiple of 50 seconds. Buffer_size = f(t)= 10+(0.2t) (Eq. 1)

4 The simulation was run continuously for 800 seconds, meaning that the buffer size vs throughput was plotted for 16 different values of buffer sizes. In this simulation, another important consideration is the each of the five individual flows of five different TCP variants are theoretically assigned the same bandwidth so as to be fair to all flows and making a judgment without any biasness. However, all flows do not practically transmit packets with the same packet rate. IV. PERFORMANCE EVALUATION Figure 2 illustrates the comparison of throughput vs buffer sizes of five considered TCP flavors. When the buffer sizes are small, TCP Vegas outperforms TCP FACK. When the buffer size is large enough, TCP Reno occupy much more bandwidth and leave little for TCP Vegas. Vegas depends heavily on calculating the exact RTT value and uses a conservative algorithm to increase the congestion window, while Reno and NewReno use an aggressive approach to increment the congestion window size. TCP Vegas, when competing with other Reno connections doesn t receive a fair chunk of bandwidth. Hence, due to the aggressive nature of TCP Reno, when buffer sizes are large, TCP Vegas loses to TCP Reno that fills up the available BW, forcing TCP Vegas to back off [10].When buffer size is 40 packets the greatest average throughput for TCP Reno and TCP New Reno is observed, after which the flows enter into a anomalous region. Another set of simulations were performed, in which the buffer size is kept constant for the entire simulation period. to observe the packet drops for all considered TCP variants at a fixed buffer size. From these individual simulations the packet drop parameter was extracted and plotted vs buffer size. Packet Drops TCP Vegas TCP Fack TCP New Reno TCP Tahoe TCP Reno Buffer Size in Packets Figure 3: Packet drop vs Buffer size for TCP Variants

5 0.06 Packet Drops Vs Packets Transmitted Ratio TCP Fack TCP Vegas TCP New Reno TCP Reno TCP Tahoe Buffer Size in Packets Figure 4: Packet drop to packet transmission ratio for various buffer sizes Figure 3 illustrates the packet drop with respect to different buffer sizes. Table 1 illustrates the mean packet drop of for all buffer sizes considered for this simulation. The results show that, TCP Vegas has the lowest value for mean packet drop. It is also observed from these simulations that for a particular queuing buffer size the packet drop approaches zero. Figure 4 illustrates the packet drop to transmitted number of packet ratio (packet drop probability) plotted against buffer size, which again was extracted from the same set of simulations. It is observed for a particular optimal buffer size for this network this probability approaches to zero. In our study this was when the when the buffer size was about 103 packets. Table 1: Mean, variance and standard deviation of TCP variants. TCP Variant Mean Packet Drop Variance Standard Deviation TCP Tahoe TCP Reno TCP Newreno TCP Vegas TCP FACK Table 2: Summary Table 2, is a comparative matrix which illustrates the best choice TCP variant for a particular critical parameter under consideration. The results in Table 2, were extracted from the simulation performed in this research study.

6 V. CONCLUSION This paper summarizes to measure the performance of TCP and its Simulations generated with the help of ns2 network simulator. Several simulations were performed with Ns2 in order to acquire a better understanding of these parameters. We believe that the buffers in backbone routers have to be of the magnitude of 10s of times the number of incoming flows, for gateway routers, where Poisson packet arrival rate is many times greater than the exponential discharge rate. These results significantly lead to the selection of optimal buffer size allocation in the design of the backbone routers. REFERENCES [1] Ravi S. Prasad, Constantine Dovrolis and Marina Thottan. Router Buffer Sizing for TCP Trafficand the Role of the Output/Input Capacity Ratio [2] A. Berger and Y. Kogan, Dimensioning Bandwidth for Elastic Traffic in High-Speed Data Networks, IEEE/ACM Transactions on Networking, 8(5): , [3] H. S. Kim and N. B. Shroff, Loss Probability Calculations and Asymptotic Analysis for Finite Buffer Multiplexers, IEEE/ACM Transactionson Networking, 9(6): , [4] C. Villamizar and C.Song, High Performance TCP in ANSNET ACM CCR, 24(5):45.60, 1994 [5]R. Morris, "TCP Behavior With Many Flows", in Proceedings of the IEEE International Conference on Network Protocols, Atlanta, Georgia, October [6] Guido Appenzeller, "Sizing Router Buffers", Stanford HPNG Technical Report TR04-HPNG [7] A. Lakshmikantha, R. Srikant and C. Beck, Impact of File Arrivals and Departures on Buffer Sizing in Core Routers, in IEEE Infocom, [8]Lakshmikantha, Beck C, Srikant, R, Impact of File Arrivals and Departures on Buffer Sizing in Core Routers, Networking, IEEE/ACM Transactions (Volume:19, Issue: 2 ), April, [9] BotaoBaiand Jinyao Yan, The role of buffer unit in routers with very small buffers, Computing and Networking Technology (ICCNT), Vegas. [10] Jeonghoon Mo, Richard J. La, VenkatAnantharam, and Jean Walrand, "Analysis and Comparison of TCP Reno and Vegas" [11] A. Lakshmikantha, R. Srikant, and C. Beck, Impact of file arrivals and departures on buffer sizing in core routers, in Proc. IEEE INFOCOM, Phoenix, AZ, Apr. 2008, pp [12]ArunVishwanath, Vijay Sivaraman, and George N. Rouskas "Anomalous Loss Performance for Mixed Real-Time and TCP Traffic in Routers With Very Small Buffers," IEEE/ACM TRANSACTIONS ON NETWORKING, VOL. 19, NO. 4, AUGUST 2011