Some Simulation Studies to Characterize TCP Window ... - IEEE Xplore

Some Simulation Studies to Characterize TCP Window Control Behavior in Wired/WireIess Internetworks 'Anup K Ghosh, 'Amitava Mukherjee, 3Debashis Saha 'ISM Global Services India, Salt Lake, Calcutta 700 091 India, a n m g h o s h ( @ i n . jbm.coni 2PrescntIy at KTH, Royal Institute of Technology, 164 40 Stockholm, Sweden 'IBM Global Services India, Salt Lake, Calcutta700091 India, a m i t a v a . m u k h e r i e e i ~ i n . i h mc o s 'MIS and Computer Science Group, Indian Institute of Management (1IM)-Calcutta, Joka, Calcutta 700104, India, M i ~ m c a l . a c . i n

ABSTRACT

11. HETEROGENEOUS ENVIRONMENT

This paper explores the possibility of developing a new variant of TCP, which would be performing equally well under wireless as in wired connections. In order to get this new variant of TCP, this work primarily looks into the behavior of TCP under various loads and error conditions and different window sizes (both variabk and fixed), and compares them with TCP Reno. The simulation is carried out in ns simulator.

Heterogeneity (in terms of transmission media) i s an attribute that most characterizes the evolution of modern communications networks, The heterogeneous internetwork topology i s as shown in Figure 1, where N different sources are connected to the router R,, which in turn is connected to the router RR over Some wireless and wired links. is connected to b+] via a bottleneck link. &+I is finally connected to the router RMover again some wireless and wired links. N different receivers are connected to the router RM by wired and wireless links. Several studies [3]-[7] have indicated that the number of Bows in the network significantly impacts TCP performance evaluation in heterogeneous networks. To emulate real life scenarios, we consider various traffic conditions together. Most previous studies have considered the so-calIed greedy users assumption whereby sources are saturated. Here, in our model, multiple TCP flows are intermingled with multiple UDP flows. All flows are passing over a combination of wired and wireless links, Moreover, some flows for both TCP and UDP are on last hop wireless.

Keywords: TCP/IP, congestion control, wireless loss, multiple flows

I. INTRODUCTION TCP was designed to work well in networks with low channel error rates. Wireless networks on the other hand are characterized by frequent transmission losses. As a result, when TCP is used in wired/wirekss internetworks, the losses due to channel errors are mistaken as congestion losses and the sending rate is unnecessarily reduced in an attempt to relieve the congestion, resulting in a degraded perfamancc [ 11. There are several studies to model the behavior of TCP in such environments, typically under last-hop wireless scenarios. The consensus is that TCP needs some form of intimations to segregate wireless loss from congestion loss and "have accordingly in its window control. However, it is not an easy task to detect the type of loss from TCP behavior. Till now acknowledgement has been used as a measure of congestion in the system and used as the trigger for congestion control. To be able to distinguish between congestion loss and wireless loss, some other parameter needs to be found out which would help in this distinction. Instead of changing the TCP Window size based on ACKs received it will depend on the Network load. The TCP performs best under a Fixed Window the size of which varies depending on the network load. In order to further extend the model for more accuracy in capturing the exact TCP window control 121. we carry out a series of simulation studies for a synthetic heterogeneous environment with multiple TCP flows.

m Bottleneck

Apart fiom Introduction section, Section I1 defines the Heterogeneous environment. In Section 111, we describe the Simulation Environment. In Section IV, we discuss the results and observations of the simulation studies performed. Section V concludes the paper.

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Figure 1. Heterogeneous network under consideration

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In this work, we consider only multiple TCP flows in heterogeneous network environment.

variables +ut- & gput- to keep a track of the throughput & goodput of the individual TCP sources.

111. SIMULATION ENVIRONMENT

The file nsdefault.tcl (ns-allinone-2.27/ns-2.27/tclilib) was also modified in order to incorporate the newly introduced variables in ns provide them with the default values.

A . Assumptions The network topology shown in Figure 2 is simulated using ns-2.27 where all the nodes are TCP-New Reno agents. These nodes are connected to a router via wired links each having a link BW of lOMbps and a delay of 0.5 ms. The router is in turn connected to a base station via a lMbps 20ms wired link. This link acts as the bottleneck link. To the base station are connected the TCP Receivers each of which are connected via a wireless link.

The packet size for the TCP agents is 1000 bytes (+ 40 bytes of TCP header). The queues used in ail the links are drop tail. The queue size, queue delay, source-receiver pairs, and wireless error have been taken as the parameters during the experiment. Varying the set of parameters has performed various sets of experiments and the data extracted are enclosed. The first 100 cases are for a variable window size as available with TCP. The next set of 100 experiments are performed with a constant size decided statically as equal to window (RTT*Bandwidth)*0.8. RTT is the average RTT of TCP Reno. A further set of 100 experiments have been performed with a constant window size of (RTT*Bandwidth)*0.2. Encouraged by the results of the previous two sets of experiments which clearly show a better control on the throughput by the fixed window, another set of experiments were performed by fixing the window sizes in the range between the first two sets of experiments i.e. within 80% to 10% of BW x RTT product.

i: A .

We have analyzed based only on the average throughput of the system, and the performance of the system is evaluated with this regard. Figure 2: Network Topology used in Simulation

IV. RESULTS AND OBSERVATIONS

B. Various Scenarios

SET 1

We performed various experiments under various conditions, which are listed as follows:

In the first set of experiments, the performance (measured by average throughput) of 3 different variants of TCP is compared. In the first variant TCP New Reno without any modification is used. In the figures this variant is called as ‘Variable Window”. in the next variant, the ns source code i s modified to change the TCP behavior so that the window size does not change even when a packet is dropped. The window size is kept fixed at 80% & 20% of BW x RTT in two different variants. The results are shown below in Figures 3.

Number of Source-Receiver Pairs The experiments have been carried out for 1, 5, 10, 15 & 20 pairs of Sources & Receivers. For later experiments only 1 , 10 & 20 S/R pairs are used. Error rates in the wireless links The experiments had been carried out for 0.0 % i.e. no wireless error, 0.01% i.e. small wireless error and 0.1% i.e. large wireless error. . Variable & Constant cwnd for TCP sources The experiments had been carried out for all the above cases in both the cases of variable cwnd i.e. the original TCP Reno flavor, as well as for the TCP with fixed window size.

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SET 2 The experiments performed in Set I ate found that for a singIe Source-Receiver pair, constant window of 80% performed better than 20%. The next set of experiments would try to find out what window size works best under different conditions. Thus the window size is varied from 10% to 100% of BW x RTT product, and the throughput is compared and shown in Figures 4.

C. Implementation For running the cases where the variable cwnd is used, no changes are made to the ns-code, but in order to simulate the cases with constant cwnd, the following changes are being made to the ns-codes, and ns is recompiled: The files tcp.h, tcpcc & tcp-reno.cc (ns-allinone-2.27/ns2.27/tcp/) have been modified in order that the TCP cwnd variable remains constant. The files tcpsink,h & tcpsink.cc (ns-allinone-2.27/ns2.27/tcp/) were also modified in order to add two

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Figure 3: Queue size- Queue delay vs. average throughput for a single sourcelreceiver pair

Figure 4:Comparison of throughput for given source/receiver pair(s) for the various values in percentage

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0bservation A comparison of the 3 different variants of TCP shows (from Figures 3a-3c) that for single source-pair combination, constant cwnd of 80% performs better than TCP Reno at large error condition and equally well for less and no wireless errors.

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Observations The abovc graphs (Figures 4a-41) show that with the increase in SIR pairs the system performs better at lower cwnd size. We have found that the system performs best when the window size is about 10% of the BW and RTT product for S/R pairs 2 5.For single S R pair the system performance is better at the higher end.

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V. CONCLUSION The above simulation results show that for a given network load condition, as determined by the number of source-receiver pairs, a fixed window TCP performs better than variable window TCP under large and small wireless errors while performing equally well under no wireless loss condition.. This interesting development leads to immense possibilities of dynamically calcuIating the fixed window size in a changing load scenario, which would enable the TCP to move away from an ACK driven window control mechanism leading to equal performance under both wired and wireless conditions. In our h h r e work, the theoretical analysis of these findings and the different possibilities of using this phenomenon to develop a robust variant of TCP will be presented.

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