GATE CS 2015 Set 1 — Question 23
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Computer Networks → Transport Layer → TCP Flow Control
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Question
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 , then the sequence number of the subsequent segment is always .
II. If the estimated round trip time at any given point of time is sec, the value of the retransmission timeout is always set to greater than or equal to 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.
II. If the estimated round trip time at any given point of time is sec, the value of the retransmission timeout is always set to greater than or equal to 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.
Correct answer
(B) I and III only
Solution
Let's evaluate each statement:
- I. FALSE: TCP sequence numbers are byte-oriented, not segment-oriented. If a segment starts with sequence number and contains bytes of data, the next segment's sequence number will be .
- II. TRUE: The Retransmission Timeout (RTO) is calculated as . Since the variation () is non-negative, is always the estimated RTT (, denoted as here).
- III. FALSE: The advertised window (receiver window) is used for flow control and changes dynamically based on the available buffer space at the receiver.
- IV. TRUE: This is the fundamental rule of TCP flow control; the sender cannot have more outstanding (unacknowledged) data than the receiver's advertised window size.
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