Question

If 5 TCP segments of 100-byte MSS are sent consecutively, starting with sequence number 101, 201, 301, 401, and 501, and if the first segment is lost, the ACKs returned will have ACK numbers as:

• A. 101,101,101,101
• B. 201,301,401,501
• C. 201,201,201,201
• D. 101,201,301,401

Hint:

- TCP uses cumulative ACKs; when a segment is lost, the receiver keeps sending the same ACK number for the missing segment.
- The sender detects duplicate ACKs and triggers fast retransmission.

Answer 1

The Correct Option is C

Step 1: Understanding TCP ACK Mechanism
- In TCP, the ACK number indicates the next expected byte from the sender.
- If a packet is lost, the receiver keeps sending the same ACK for the last correctly received segment.
Step 2: Sequence Number Breakdown
- Packets are sent in order with sequence numbers: \[ 101, 201, 301, 401, 501 \] - If 101 is lost, the receiver correctly gets 201, 301, 401, 501, but cannot acknowledge them until it receives 101.
Step 3: ACK Behavior
- Since 101 is missing, the receiver keeps sending ACK 201, requesting 101 to be retransmitted.
- This results in the following repeated ACKs: \[ 201, 201, 201, 201 \]
Step 4: Evaluating Options
- (A) Incorrect: ACK should not remain at 101.
- (B) Incorrect: This assumes normal delivery without loss.
- (C) Correct: Repeated ACK of 201 due to loss of 101.
- (D) Incorrect: ACK numbers must remain the same if loss is detected.