GATE CS 2018 Q.24 (CS Q.14): Transport Layer: UDP, TCP Flow and Congestion Control, Sockets

Consider the following statements regarding the slow start phase of the TCP congestion control algorithm. Note that cwnd stands for the TCP congestion window and MSS denotes the Maximum Segment Size.

(i) The cwnd increases by 2 MSS on every successful acknowledgment.
(ii) The cwnd approximately doubles on every successful acknowledgement.
(iii) The cwnd increases by 1 MSS every round trip time.
(iv) The cwnd approximately doubles every round trip time.

Which one of the following is correct?

Only (ii) and (iii) are true

Only (i) and (iii) are true

Only (iv) is true

Only (i) and (iv) are true

Official answer and solution

Official answer: C
From: Official GATE 2018 answer key

Solution

Recall the dynamics of congestion window growth during TCP slow start.

In the slow start phase of TCP congestion control:

For each individual acknowledgment (ACK) received, the sender increases $cwnd$ by 1 MSS: $cwnd \leftarrow cwnd + 1\text{ MSS}$ .

If a full window of $W$ segments is transmitted and all are successfully acknowledged over one round trip time (RTT), $W$ ACKs are received. Each ACK adds 1 MSS, so the window increases by $W \times 1\text{ MSS} = W\text{ MSS}$ , effectively doubling $cwnd$ across one RTT ( $2W$ MSS).

Therefore, statement (iv) is true: $cwnd$ approximately doubles every round trip time. Statements (i), (ii), and (iii) are false.

Final answer

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2018 Computer Science and Information Technology (CS)
Question: 24 of 65, printed as CS Q.14
Section: Computer Science and Information Technology

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