Alcatel-Lucent NRS I Composite Exam — Question 5

TCP is sending data in 5 segments with SEQ numbers 27000, 27500.28000, 28500, and 29000 respectively. During transmission, the segment with SEQ=28500 is dropped in the network. How does TCP handle this situation?

Answer options

• A. The receiving end station sends an ACK flag with SEQ=28000 to indicate it has received a segment out of order.
• B. The receiving station sends a NACK frame with SEQ=28500 requesting that the frame be retransmitted
• C. Once the reciever's window is full. TCP assembles the payload and determines that SEQ=28500 is missing. At this point, the reciever sends the RST flag with SEQ=28500 to Request Single Transmission of the missing segment.
• D. TCP will collect the segments and forward them to the application layer where the SEQ numbers will be processed and a retransmit request will occur for the entire packet.

Correct answer: A

Explanation

The correct answer is A because TCP uses acknowledgments to inform the sender about received segments, and an ACK for SEQ=28000 indicates that the receiver has received segments up to that point but has not yet received SEQ=28500. Options B and C are incorrect as TCP does not use NACK frames or RST flags for retransmission requests. Option D is also incorrect since TCP does not forward incomplete segments to the application layer; it requires all segments to be received in order.