A TCP sender successfully establishes a connection with a TCP receiver and…
2026
A TCP sender successfully establishes a connection with a TCP receiver and starts the transmission of segments. The TCP congestion control mechanism’s slow-start threshold is set to 10000 segments. Assume that the round-trip time is fixed at 1 millisecond. Assume that the sender always has data to send, the segments are numbered from 1, and no segment is lost. Let 𝑡 denote the time (in milliseconds) at which the transmission of segment number 2000 starts.
Which one of the following options is correct?
- A.
9 ≤ 𝑡 < 10
- B.
10 ≤ 𝑡 < 11
- C.
11 ≤ 𝑡 < 12
- D.
12 ≤ 𝑡 < 13
Attempted by 12 students.
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Correct answer: B
In TCP slow-start, the congestion window (cwnd) doubles every RTT. Starting with cwnd = 1 segment at t=0, the number of segments sent in round k is 2^k. The cumulative segments sent by time t=k is sum(2^i for i=0 to k) = 2^(k+1) - 1. At t=9, cumulative segments = 2^10 - 1 = 1023. At t=10, cumulative segments = 2^11 - 1 = 2047. Since segment 2000 falls between 1023 and 2047, it is transmitted during the round starting at t=10. Thus, transmission starts at t = 10 ms.