Module 2: Practice Quiz

Question 1

As we have seen, UDP and TCP use port numbers to identify the sending application and destination application. Why don’t UDP and TCP just use process IDs rather than define port numbers?

Answer 1

Process IDs are specific to operating systems and therefore using process IDs rather than a specially defined port would make the protocol operating system dependent. Also, a single process can set up multiple channels of communications and so using the process ID as the destination identifier wouldn’t be able to properly demultiplex, Finally, having processes listen on well-known ports (like 80 for http) is an important convention.

Question 2

UDP and TCP use 1’s complement for their checksums. But why is it that UDP takes the 1’s complement of the sum – why not just use the sum? Exploring this further, using 1’s complement, how does the receiver compute and detect errors? Using 1’s complement, is it possible that a 1-bit error will go undetected? What about a 2-bit error?

Answer 2

To detect errors, the receiver adds the four words (the three original words and the checksum). If the sum contains a zero, the receiver knows there has been an error. While all one-bit errors will be detected, but two-bit errors can be undetected (e.g., if the last digit of the first word is converted to a 0 and the last digit of the second word is converted to a 1).

Question 3

TCP utilizes the Additive Increase Multiplicative Decrease (AIMD) policy for fairness. Other possible policies for fairness in congestion control would be Additive Increase Additive Decrease (AIAD), Multiplicative Increase Additive Decrease (MIAD), and Multiplicative Increase Multiplicative Decrease (MIMD). Would these other policies converge? If so, how would their convergence behavior differ from AIMD?

Answer 3

In AIAD and MIMD, the plotted throughput line will oscillate over the full bandwidth utilization line but will not converge as was shown for AIMD. On the other hand, MIAD will converge.

None of the alternative policies are as stable. The decrease policy in AIAD and MIAD is not as aggressive as AIMD, so those will not effectively address congestion control. In contrast, the increase policy in MIAD and MIMD is too aggressive.

Question 4

Explain how in TCP Cubic the congestion window growth becomes independent of RTTs.

Answer 4

The key feature of CUBIC is that its window growth depends only on the time between two consecutive congestion events. One congestion event is the time when TCP undergoes fast recovery. This feature allows CUBIC flows competing in the same bottleneck to have approximately the same window size independent of their RTTs, achieving good RTT-fairness.