The GATE Grind

GATE 2015 CS – Question 23

Computer Networks · TCP: Flow Control, Congestion Control, Socket API · 1 mark · Multiple choice

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.

  1. III only
  2. I and III only
  3. I and IV only
  4. II and IV only

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Correct answer: (B) I and III only

Explanation

TCP numbers bytes, so the next segment's sequence number is $m$ plus the number of data bytes in the segment, not always $m+1$, which makes I false. The advertised window changes as the receiver's buffer fills and empties, so III is false. The retransmission timeout is at least the estimated round trip time (II is true), and the sender never has more unacknowledged bytes than the advertised window (IV is true).