GATE CS 2015 Set 1 Q1 — Transport Layer: Flow Control, Congestion Control, UDP, TCP, Sockets

To determine which statements are FALSE, let us analyze each of them one by one:

• Statement I: "If the sequence number of a segment is $m$, then the sequence number of the subsequent segment is always $m+1$."
  FALSE. TCP sequence numbers are byte-oriented, not segment-oriented. If a segment carries $L$ bytes of data, the sequence number of the next segment will be $m + L$. It is only $m+1$ if the segment contains exactly $1$ byte of data (or is a control segment like SYN/FIN). For a large file transfer, $L$ is typically much larger (e.g., up to the Maximum Segment Size, MSS).
• Statement 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."
  TRUE. The Retransmission Timeout (RTO) is calculated as $\text{RTO} = \text{EstimatedRTT} + 4 \times \text{DevRTT}$. Since the deviation ($\text{DevRTT}$) is non-negative, the timeout interval is always greater than or equal to the estimated RTT ($t$). Setting RTO less than $t$ would cause unnecessary premature retransmissions.
• Statement III: "The size of the advertised window never changes during the course of the TCP connection."
  FALSE. The advertised window (receiver window, $rwnd$) represents the remaining buffer space at the receiver. As the receiver process consumes data and new packets arrive, this available buffer space changes dynamically and is updated in the TCP header of every acknowledgment.
• Statement IV: "The number of unacknowledged bytes at the sender is always less than or equal to the advertised window."
  TRUE. To prevent buffer overflow at the receiver (flow control), the sender's window size is limited by $\min(cwnd, rwnd)$, where $rwnd$ is the advertised window. Thus, the outstanding (unacknowledged) bytes can never exceed the advertised window.

Since statements I and III are false, the correct option is B.