MULTIPLE TCP CONNECTIONSPresented by

Josh Kaltman and Bob Otting

The Question• If multiple TCP connections are opened in parallel,

will the total throughput improve?

>?

Relevance• Aggressiveness of TCP connections

• Would be unfair if opening more connections resulted in greater throughput

• TCP is a dominant transport protocol in the Internet

• Bandwidth restrictions on a single connection

• Download accelerators

Background• “Fair share” in TCP congestion avoidance

• From the slides we have discussed TCP friendliness[1]:• Competition with other TCP flows• Competition with other non-TCP flows

[1]: Chapter 16 Slides

Background• Shorter RTTs obtain a larger share of bandwidth than

those having larger RTTs[1]

• Combats aggressiveness with additive increase and multiplicative decrease[1]

• [1]: Chapter 16 Slides

Hypothesis

1. Increasing the number of TCP connections between two machines will not increase the total throughput.

• Assumptions:• No limit on throughput per connection• Sufficient memory to sustain the connections and their buffers

2. If the window size is set to an unreasonable value, the resulting negative impact on throughput will remain the same as the number of connections increases

.

Requirements for Evaluation• We need a way to accomplish each of the following:

1. Open multiple TCP connections in parallel

2. Adjust TCP window sizes

3. Measure the rate of speed for each connection

4. Ideally plot graphs for each of the experiments

Potential Tools• Program our own diagnostic tool• Use some other existing network diagnostic tool• Use Wireshark to measure and graph connections• Use a different visualization tool for graphs

Selected Tools• iPerf

• Network performance testing tool• Simultaneous TCP connections• Can vary window sizes• Basic throughput measurement

Documentation: https://iperf.fr/

Setup• CSE Machine CSE Machine

• Download and compile iperf on the CSE network• Start one machine as a client and the other as a server• Run the experiment with varying number of connections, window

sizes, and buffer sizes

• CSE Machine External Endpoint• Download and compile iperf on laptop• Address reservation• Port forwarding• Start CSE machine as client and laptop as server

Setup• Shell scripting

• Learned some basic shell scripting and created a script that would test the CSE machines while logged out of SSH

Results

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Throughput with Varying Concurrent TCP Connections: March 12, 2015

1 Connection

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5 Connections

10 Connections

Experiment

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CSE Machines: Gamma BetaWindow Size: 20kb (default)Date: March 12, 2015

Results

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Throughput with Varying Concurrent TCP Connections: March 24, 2015

1 Connection

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5 Connections

10 Connections

Experiment

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CSE Machines: Gamma BetaWindow Size: 20kb (default)Date: March 24, 2015

Results CSE Machines: Gamma BetaDate: March 25, 2015

2Kb 5kb 10kb 20kb 50kb 100kb 200kb

1 Conn. 20.5 194.3 546 554.5 694.8 765 698.5

2 Conn. 41.3 205.3 847.5 867.8 932.3 929.8 629.3

5 Conn. 137.3 439.8 924.8 939.3 939.3 940.8 940.8

10 Conn. 300 600.3 926 938.5 938.5 940 941.3

• A test with each pair of settings was performed 4 times and the mean was taken.

• All results in Mbits/sec

TCP Window Size

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1 Conn.

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Results Machines: Gamma Laptop (home)Date: March 25, 2015

2Kb 5kb 10kb 20kb 50kb 100kb 200kb

1 Conn. .493 .997 1.21 2.54 5.05 9.73 9.92

2 Conn. .983 1.65 2.56 5.06 10.53 15.48 15.63

5 Conn. 2.39 4.22 6.1 12.38 15.98 16.02 16.15

• A test with each pair of settings was performed 4 times and the mean was taken.

• All results in Mbits/sec• Could not test more than 5

connections at once, presumably because of security restrictions somewhere between CSE machine and laptop

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TCP Window Size

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1 Conn.

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The Question• So if multiple TCP connections are opened in

parallel, does the total throughput improve?

>?

Apparently It Does

Analysis• After examining the data, we surprisingly found our

hypotheses to be experimentally incorrect.

• Why might this behavior occur? Some ideas:• Limit on the rate an individual socket or connection can transmit• TCP streams being split up as they travel through the Internet

• Don’t necessarily take up a combined “fair share” of bandwidth

• Stdlinux may be running on a virtual machine• iPerf itself could be faulty (unlikely, but possible)

Analysis• Per iPerf’s documentation,

• “If the total aggregate bandwidth is more than what an individual stream gets, something is wrong. Either the TCP window size is too small, or the OS's TCP implementation has bugs, or the network itself has deficiencies.“

• We varied the TCP window size a lot, so we don’t believe that this is the issue.

• There is most likely something interesting happening with the network itself or the OS’s TCP implementation

Future Extensions• Set up multiple clients and servers on different ports

• Try running tests on an isolated network

• If the unexpected observed behavior is correct, design applications using multiple connections to improve throughput

Concluding Remarks• Based on TCP’s congestion control algorithm, we

assumed that increasing the number of TCP connections would not improve throughput

• We used iPerf as a tool for network analysis• Throughput was very different for single vs multiple

connections

• Theoretically, we believe our hypotheses are correct• Experimentally, we were proven wrong

Thank you. Questions?