Knowledge Checks

TCP’s AIMD algorithm.

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

AIMD uses the measured RTT delay to detect congestion.

AIMD uses observed packet loss to detect congestion.

AIMD is a network-assisted approach to congestion control.

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

AIMD is a end-end approach to congestion control.

AIMD always cuts the congestion window size, cwnd, in half whenever loss is detected.

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

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1/6

TCP’s AIMD algorithm (2).

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

By using the retransmission timeout timer and counting the number of bytes sent since the last timeout to compute the sending rate since that last timeout, and then making sure its sending rate never exceed the rate set by AIMD.

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.

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2/6

TCP’s Slowstart algorithm.

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

... slower than AIMD, that’s why it’s called Slowstart."

"... at the same rate as AIMD."

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

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3/6

Uncontrolled transport-layer senders.

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?

The router will send a signal to the TCP sender that would force the TCP sender to cut its rate in half.

Nothing different from the other flows crossing the congested link.

That sender’s datagrams will be preferentially dropped at the congested link.

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4/6

TCP CUBIC.

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

... have a sending rate that always increases faster than that of AIMD.

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

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

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5/6

Delay-based congestion control.

For delay-based congestion control, match the sender action to the relationship of the currently measured throughput to the value of cwnd/RTT_min

Question List:

The currently measured throughput is greater than cwnd/RTT_min

The currently measured throughput is equal to or a bit less than than cwnd/RTT_min

The currently measured throughput is much less that than cwnd/RTT_min

Answer List:

increase the sending rate
decrease the sending rate
This should never happen.

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6/6

Knowledge checks

TCP’s AIMD algorithm.

TCP’s AIMD algorithm (2).

TCP’s Slowstart algorithm.

Uncontrolled transport-layer senders.

TCP CUBIC.

Delay-based congestion control.

Question List:

Answer List: