Question

Consider the tree-way handshake mechanism followed during TCP connection establishment between hosts P and Q. Let X and Y be two random 32-bit starting sequence numbers chosen by P and Q respectively. Suppose P sends a TCP connection request message to Q with a TCP segments having SYN bit =1. SEQ number = X, and ACK bit = 0. Suppose Q accepts the connection request. Which one of the following choices represents the information present in the TCP segment header that is sent by Q to P?

• A. SYN bit =1, SEQ number =Y, ACK bit =1, ACK number =X, FIN bit = 0
• B. SYN bit =0, SEQ number =X+1, ACK bit =0, ACK number =Y, FIN bit = 1
• C. SYN bit =1, SEQ number =X+1, ACK bit =0, ACK number =Y, FIN bit = 0
• D. SYN bit = 1, SEQ number =Y, ACK bit =1, ACK number = X+1 , FIN bit =0

Answer 1

The correct option is D SYN bit = 1, SEQ number =Y, ACK bit =1, ACK number = X+1 , FIN bit =0

Q will send the SYN bit=1 to the connection establishment.
Q seq number will be Y different form X.
ACK bit = 1 because sending the ACK.
ACK number = X+1 (Next seq number id)
FIN bit = 0 (Because establishing the connection)