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TCP Performance over Wireless Multihop Networks

Abhishek KonaComputer Systems and TelematicsInstitute of Computer ScienceFreie Universität Berlinhttp://cst.mi.fu-berlin.de

2Prof. Dr. Mesut Güneş www.inf.fu-berlin.de/inst/ag-tech TI II - 2008

Introduction

• Increase in research in the fields of WSN and WMN• Performance of TCP not up to par in wireless scenarios.• TCP does not recognize between losses to congestion and

losses due to non availability of medium and other causes• This study tries to analyze the performance of TCP in a

real life deployment of a static multi hop network in the FU Berlin DES Mesh Test bed.

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Methodology

• Nodes used DES Test bed Nodes t9-154,155,163,165,166,169,k004c,k004a,137,136

• Experiments performed by logging on to the nodes using SSH and not using Test Bed Control Center

• Experiments performed at day and night time

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Methodology

• Each experiment performed for 10 trials.

• Each Iperf measurement lasts for 180 s.

• Results saved in log files.

• All experiments were automated using shell scripts, so that multiple trials could be performed without intervention

• All graphs have been plotted using Matplotlib Python library

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Configuration

• The MTU of all the participating nodes set to 1064 bytes using 'ifconfig' command This enables an MSS of 1024 Bytes.

• Chain topology set using 'route' command.Eg: prompt$ sudo route add -net 192.168.18.22 netmask

255.255.255.255 gw 192.168.18.22 dev wlan0

• TCP congestion control algorithms: RENO WESTWOOD CUBIC

• Settings changed using sysctl

• Window size set using iperf -w parameter.

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Observations

1. Degradation of TCP performance with increase in hopcount.

The transmission medium has to be acquired at each participating node by sending RTS and CTS frame and this causes more and more delay as hop count increases. Thisa increases the RTT and hence decreases the throughput.

The signals are more prone to fading effects.

The entire path can be slowed by one slow performing node

TCP is an end to end protocol and cannot differentiate between losses, so congestion control measures can be initiated.

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2.TCP Performance wrt Window Size

Performance of TCP is sensitive to the window size and that significant performance improvements are obtained if 'good' window size is used

TCP delay jitter is least when the window size is 3/2 times the hop-count.[4]

TCP through put will be maximal at a window size of hop-count/4.[3]

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Observations

• Throughput becomes constant after a particular window size.

• For 1 Hop network the maximum throughput is achieved at a window size of 2KB

• For a 2 Hop network the maximum throughput is achieved at a window size of 6KB.

• For a 3 Hop network the maximum throughput is achieved at a window size of 4KB

• There is no particular observed relation between hop count and through put.

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Observation

2. Comparision of CUBIC, RENO, WESTWOOD

RENO allows for fast retransmission and fast recovery

CUBIC default Linux TCP congestion algorithm.

CUBIC is expected to ensure the maximum fairness.

TCP WESTWOOD is a congestion algorithm designed for wireless networks. TCP WESTWOOD adjusts congestion window to a calculated bandwidth of the network using ACK timings.

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1 Hop comparision 2 hop comparision

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Observation

no observable better performing TCP variant in the above performed tests.

For smaller window sizes (upto 4 KB) WESTWOOD marginally outperforms the other two.

13Prof. Dr. Mesut Güneş www.inf.fu-berlin.de/inst/ag-tech TI II - 2008

TCP Sack performance

• TCP Sack (selective acknowledgement ) expected to have no visible effect on the TCP performance in wireless networks

• In this observation turning off TCP Sack option seems to give a better performance over the 3 hop network.

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Limitations

Unavailability of same nodes or paths to perform tests. Restarting of TEST Bed due by the Test Bed Control center

without notification. Unable to set up a 4 (or more) hop route which lasts long

enough for the experiments to run. High variance in Data generated from the experiments on

the test bed. Previos studies on TCP performance on wireless networks

have been performed in controlled environments and hence the results do not completely match the previos studies.

15Prof. Dr. Mesut Güneş www.inf.fu-berlin.de/inst/ag-tech TI II - 2008

Conclusion

• A lot of previous studies on the performance of TCP on wireless networks have used simulation experiments using simulators like NS2 and qual net simulator.

• A few studies performed over real hardware have used standardised hardware and have sensitized the settings with respect to the environment

• It is difficult to emulate the TCP behaviour observed in previous studies in a real life deployment scenario

• A set statistical tests may have to be developed to analyze the highly variable data generated from TCP performance parameters on real life test bed. Some statistical analysis of TCP results was performed in one study.

• A centralized testing model can be developed to analyze TCP performance in various scenarios.

16Prof. Dr. Mesut Güneş www.inf.fu-berlin.de/inst/ag-tech TI II - 2008

Conclusion

• A lot of papers suggest many alterations to TCP for better performance in wireless networks. e.g. I TCP Snoop protocol.

• All of them require modifying the TCP implementations.

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About Me

• I am Undergraduate Engineering student ( Bachelors in Technology) in Computer Science in National Institute of Technology Karnataka near Mangalore

• I had a wonderful time here in Germany, Danke schön!

• If any one ever comes to India contact me at abhishek.kona@gmail.com