Question 29

Evaluating each statement:

• Statement I: 'If the sequence number of a segment is $m$, then the sequence number of the subsequent segment is always $m+1$.' TCP sequence numbers count bytes, not packets. If a segment carries $L$ bytes of payload, the sequence number of the next segment will be $m + L$ (not $m + 1$, unless $L = 1$). (FALSE)
• Statement II: 'If the estimated round trip time at any given point of time is $t\text{ sec}$, the value of the retransmission timeout is always set to greater than or equal to $t\text{ sec}$.' By RFC 6298, $RTO = \text{SRTT} + \max(G, 4 \times \text{RTTVAR})$. Because RTTVAR $\ge 0$, RTO is always greater than or equal to estimated RTT $t$. (TRUE)
• Statement III: 'The size of the advertised window never changes during the course of the TCP connection.' The advertised receive window ($rwnd$) is dynamic; it expands and contracts depending on application read rates and buffer occupancy. (FALSE)
• Statement IV: 'The number of unacknowledged bytes at the sender is always less than or equal to the advertised window.' Standard TCP flow control ensures that the sender never transmits more unacknowledged data than the receiver's advertised window. (TRUE)

Therefore, statements I and III are FALSE (option B).