Discussion
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?
Answer: B
Segment 2000 starts in the interval \(10 \le t \lt 11\).
The threshold of 10000 segments is never reached, so the sender stays in slow start and the window doubles every RTT of 1 ms: 1, 2, 4, ... segments in the rounds starting at t = 0, 1, 2, ...
\[ \begin{aligned} \text{sent before } t = 10 &: 1+2+\dots+512 = 1023 \\ \text{round at } t = 10 &: \text{segments } 1024 \text{ to } 2047 \end{aligned} \]
Segment 2000 lies in the round that starts at t = 10 ms and so starts transmission before t = 11 ms.
The round starting at t = 9 covers only segments 512 to 1023, and by t = 11 segment 2000 has already been sent.
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