Question

Given below are two statements :
one is labelled as
Assertion (A) and the other is labelled as
Reason (R).
Assertion (A) :
TCP uses sequence numbers to ensure reliable, in-order delivery of data segment.
Reason (R) :
Sequence number helps detect packet duplication and reordering in transmission.
In the light of the above statements, choose the most appropriate answer from the options given below :

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A)
• B. Both (A) and (R) are correct but (R) is not the correct explanation of (A)
• C. (A) is correct but (R) is not correct
• D. (A) is not correct but (R) is correct

Hint:

Think of sequence numbers like page numbers in a book. If you drop the book and the pages scatter, you can only put them back in order because the numbers tell you exactly where each page belongs!

Answer 1

The Correct Option is A

Concept:
• TCP (Transmission Control Protocol) is a connection-oriented protocol that provides reliable byte-stream delivery.
• IP (the underlying layer) is best-effort and does not guarantee order or delivery.

Step 1: Explain the role of Sequence Numbers
TCP breaks the application data into segments.
Each byte of data is assigned a unique sequence number.
The sequence number in a segment header represents the number of the first byte in that segment.

Step 2: Analyze how sequence numbers solve network issues

• Reordering: If packets arrive as [Seq 200, Seq 100], the receiver uses the numbers to put 100 before 200.
• Duplication: If two packets with Seq 100 arrive, the receiver keeps one and discards the duplicate.
• Loss: If Seq 100 and 300 arrive, the receiver knows Seq 200 is missing and requests retransmission.

Step 3: Conclusion
Assertion (A) is true as this is the primary mechanism of TCP reliability.
Reason (R) is true as it explains the specific sub-tasks sequence numbers perform.
Since performing these sub-tasks is exactly how reliability is achieved, (R) is the correct explanation.