Consider a TCP connection operating at a point of time with the congestion…
2024
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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Correct answer: 4
Given:
cwnd = 12 MSS
Timeout occurs
Next 2 RTTs all segments acknowledged correctly
Find cwnd in 3rd RTT
Step 1: Timeout in TCP Tahoe
On timeout:
ssthresh = cwnd / 2 = 12 / 2 = 6 MSS
cwnd reset to 1 MSS
Step 2: Slow Start after timeout
Slow start: cwnd doubles every RTT until cwnd ≥ ssthresh
RTT | cwnd at start | cwnd at end |
|---|---|---|
1 | 1 MSS | 2 MSS |
2 | 2 MSS | 4 MSS |
3 | 4 MSS | 6 MSS |
After reaching ssthresh = 6 MSS, TCP enters congestion avoidance (linear growth).
Step 3: cwnd during third RTT
The third RTT starts with cwnd = 4 MSS and ends with cwnd = 6 MSS
Usually, the cwnd at the start of RTT is considered
So, cwnd = 4 MSS at the start of third RTT
Answer:
4 MSS
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