Suppose two hosts use a TCP connection to transfer a large file. Which of the…
2015
Suppose two hosts use a TCP connection to transfer a large file. Which of the following statements is/are FALSE with respect to the TCP connection?
I. If the sequence number of a segment is \(m\), then the sequence number of the subsequent segment is always \(m+1\).
II. If the estimated round trip time at any given point of time is \(t\) sec, the value of the retransmission timeout is always set to greater than or equal to \(t\) sec.
III. The size of the advertised window never changes during the course of the TCP connection.
IV. The number of unacknowledged bytes at the sender is always less than or equal to the advertised window.
- A.
III only
- B.
I and III only
- C.
I and IV only
- D.
II and IV only
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Correct answer: B
Statement I
“If the sequence number of a segment is X, the next segment’s sequence number is always X + 1.” - FALSE
Reason:
TCP sequence numbers count bytes, not segments.
So if a segment carries N bytes, next sequence number = X + N, not X + 1.
Statement II
“If Estimated RTT = R sec, then RTO ≥ R sec.” - TRUE
Reason:
TCP sets:
RTO = EstimatedRTT + 4 × Deviation
Deviation ≥ 0 ⇒ RTO ≥ EstimatedRTT
So this is true.
Statement III
“The size of the advertised window never changes.” - FALSE
Reason:
Advertised window = receiver’s available buffer space.
It changes dynamically during transfer.
Statement IV
“The number of unacknowledged bytes ≤ advertised window.” - TRUE
Reason:
Sender is not allowed to send more than what receiver advertises (flow control).
So unacknowledged bytes are always ≤ receive window.
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