GATE CS 2024 Set 2 Q.54: Transport Layer: UDP, TCP Flow and Congestion Control, Sockets

Consider a TCP connection operating at a point of time with the congestion window of size 12 MSS (Maximum Segment Size), when a timeout occurs due to packet loss. Assuming that all the segments transmitted in the next two RTTs (Round Trip Time) are acknowledged correctly, the congestion window size (in MSS) during the third RTT will be _________

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Official answer and solution

Official answer: 4
From: Official GATE 2024 answer key

Solution

Apply TCP timeout recovery: set ssthresh to half of cwnd ( $6\text{ MSS}$ ), reset cwnd to $1\text{ MSS}$ , and double cwnd each RTT during slow start.

When a timeout occurs at $\text{cwnd} = 12\text{ MSS}$ $cwnd = 12 MSS$ :
- The slow-start threshold is updated to $\text{ssthresh} = \frac{12}{2} = 6\text{ MSS}$ .
- The congestion window is reset to $\text{cwnd} = 1\text{ MSS}$ , and TCP enters Slow Start.
During the 1st RTT: $\text{cwnd} = 1\text{ MSS}$ . Since the segment is acknowledged and $\text{cwnd} < \text{ssthresh}$ , $\text{cwnd}$ doubles to $2\text{ MSS}$ .
During the 2nd RTT: $\text{cwnd} = 2\text{ MSS}$ . Since both segments are acknowledged and $\text{cwnd} < \text{ssthresh}$ , $\text{cwnd}$ doubles to $4\text{ MSS}$ .
During the 3rd RTT: The congestion window size is $4\text{ MSS}$ .

Final answer

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

Source

Paper: GATE 2024 Computer Science and Information Technology - Set 2 (CS2)
Question: 54 of 65
Section: Computer Science and Information Technology

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