Question

The topic is Intro to Networking(congestion sequence number ....etc) and i dont get this homework can anybody please help me out with this and explain so i can perform in test too. Thank you so much !

Answer 1

In order to analyze the results of your NS-2 simulations, you should enable NS-2's trace file by putting the following lines of code in your script:

#Open the trace file (before you start the experiment!)
set tf [open my_experimental_output.tr w]
$ns trace-all $tf
...
# Close the trace file (after you finish the experiment!)
close $tf

These commands will tell NS-2 to create an output file that includes traces of all the packets sent during your experiment. You will need to write other scripts (in whatever language you choose) that parse this data and analyze the results. Your scripts should report things like average bandwidth, packet drops, throughput over time, end-to-end latency (average and over time), sequence numbers of packets sent, and the congestion window of all TCP flows. This information will enable you to analyze the results of your experiments and answer the questions below. You need to understand the format of NS-2s trace file in order to successfully write these scripts.

My suggestion would be to write simple Python scripts to parse/analyze the trace (e.g. compute the average throughput of each flow), output the results to a file, and then use gnuplot or Excel to plot graphs of the data.

Note that many of the NS-2 tutorials refer to tools like nam and xgraph that come bundled with NS-2. I have never used these tools, and I do not recommend that you use them. However, these tools are\ installed in the NS-2 directory and you may use them if you wish. Do not ask for technical support about these tools, because we cannot help you!

Experiment 1: TCP Performance Under Congestion

In the first set of experiments, you will analyze the performance of TCP variants (Tahoe, Reno, NewReno, and Vegas) under the influence of various load conditions. To study the behavior of the four TCP variants, you need to first set up a network topology. Use the following topology for your experiments, and set the bandwidth of each link to 10Mbps:

                         N1                      N4
                           \                    /
                            \                  /
                             N2--------------N3
                            /                  \
                           /                    \
                         N5                      N6

Using this topology, you will perform tests that analyze the performance of different TCP variants in the presence of a Constant Bit Rate (CBR) flow. You can think of the CBR flow as an unresponsive UDP flow: it uses a specific amount of bandwidth, it does not care about dropped packets, and it does not perform congestion control. Basically, it just sends packets blindly at a constant rate.

Add a CBR source at N2 and a sink at N3, then add a single TCP stream from N1 to a sink at N4. Analyze the throughput, packet drop rate, and latency of the TCP stream as a function of the bandwidth used by the CBR flow. That is, the bandwidth of the CBR flow is the parameter you need to vary for this experiment. For example, start the CBR flow at a rate of 1 Mbps and record the performance of the TCP flow. Then change the CBR flow's rate to 2 Mbps and perform the test again. Keep changing the CBR's rate until it reaches the bottleneck capacity. The TCP stream's performance will change depending on the amount of contention with the CBR flow. Conduct this experiment using the following TCP variants: Tahoe, Reno, NewReno, and Vegas.

Based on the results from your tests you should be able to answer the following questions: which TCP variant(s) are able to get higher average throughput? Which has the lowest average latency? Which has the fewest drops? Is there an overall "best" TCP variant in this experiment, or does the "best" variant vary depending on other circumstances?

Experiment 2: Fairness Between TCP Variants

Next, you will conduct experiments to analyze the fairness between different TCP variants. Out on the Internet, there are many different operating systems, each of which may use a different variant of TCP. Ideally, all of these variants should be fair to one another, i.e. no particular variant gets more bandwidth than the others. However, is this ideal assumption actually true?

For these experiments, you will start three flows: one CBR, and two TCP. Add a CBR source at N2 and a sink at N3, then add two TCP streams from N1 to N4 and N5 to N6, respectively. As in Experiment 1, plot the average throughput, packet loss rate, and latency of each TCP flow as a function of the bandwidth used by the CBR flow. Repeat the experiments using the following pairs of TCP variants (i.e. with one flow from N1 to N4, and the other flow from N5 to N6):

• Reno/Reno
• NewReno/Reno
• Vegas/Vegas
• NewReno/Vegas

Based on your results you should be able to answer the following questions: are the different combinations of variants fair to each other? Are there combinations that are unfair, and if so, why is the combination unfair? To explain unfairness, you will need to think critically about how the protocols are implemented and why the different choices in different TCP variants can impact fairness.