Section 3.7

Question 1

How is the sending rate typically regulated in a TCP implementation?

Answer 1

By keeping a window of size cwnd over the sequence number space, and making sure that no more than cwnd bytes of data are outstanding (i.e, unACKnowledged). The size of cwnd is regulated by AIMD.

Question 2

In the absence of loss, TCP slow start increases the sending rate …

Answer 2

faster than AIMD. In fact, slowstart increases the sending rate exponentially fast per RTT.

Question 3

Consider the transport-layer flows interacting at a congested link. In the face of such congestion, what happens at this link to a transport-layer flow that does not cut back on its sending rate?

Answer 3

Nothing different from the other flows crossing the congested link.

Question 4

Assuming that the congestion window size,cwnd, has not yet reached Wmax, TCP CUBIC will ..

Answer 4

1. increase its sending rate faster than AIMD when cwnd is far away from Wmax, but increase slower than AIMD when cwnd is closer to Wmax
2. always have a window size, cwnd, and hence a sending rate, higher than that of AIMD (assuming a given window size, Wmax, at which loss would occur).

Question 5

What happens to the sending rate when The currently measured throughput is greater than cwnd/RTTmin

Answer 5

This should never happen.

Question 6

What happens to the sending rate when The currently measured throughput is much less than cwnd/RTTmin

Answer 6

decrease the sending rate

Question 7

What happens to the sending rate when The currently measured throughput is equal to or a bit less than cwnd/RTTmin

Answer 7

increase the sending rate

Question 8

Which of the following statements about TCP’s Additive-increase-multiplicative-decrease (AIMD) algorithm are true?

Answer 8

AIMD is a end-end approach to congestion control.

AIMD cuts the congestion window size, cwnd, in half whenever loss is detected by a triple duplicate ACK.

AIMD cuts the congestion window size, cwnd, i to 1 whenever a timeout occurs.

Question 9

Which of the following best completes this sentence: “In the absence of loss, TCP slow start increases the sending rate … “

Answer 9

” … faster than AIMD. In fact, slowstart increases the sending rate exponentially fast per RTT.”

Question 10

Consider the transport-layer flows interacting at a congested link. In the face of such congestion, what happens at this link to a transport-layer flow that does not cut back on its sending rate?

Answer 10

Nothing different from the other flows crossing the congested link.

Question 11

Assuming that the congestion window size,cwnd, has not yet reached Wmax, TCP CUBIC will … (check all that apply)

Answer 11

… always have a window size, cwnd, and hence a sending rate, higher than that of AIMD (assuming a given window size, Wmax, at which loss would occur).

… increase its sending rate faster than AIMD when cwnd is far away from Wmax, but increase slower than AIMD when cwnd is closer to Wmax