GATE CS 2021 Set 2 Q.17 (CS Q.7): Transport Layer: UDP, TCP Flow and Congestion Control, Sockets

Consider the three-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 segment 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 $P$ ?

SYN bit = 1, SEQ number = $X+1$ , ACK bit = 0, ACK number = $Y$ , FIN bit = 0

SYN bit = 0, SEQ number = $X+1$ , ACK bit = 0, ACK number = $Y$ , FIN bit = 1

SYN bit = 1, SEQ number = $Y$ , ACK bit = 1, ACK number = $X+1$ , FIN bit = 0

SYN bit = 1, SEQ number = $Y$ , ACK bit = 1, ACK number = $X$ , FIN bit = 0

Official answer and solution

Official answer: C
From: Official GATE 2021 answer key

Solution

Follow the sequence numbers and control flags used in the second message (SYN-ACK) of the TCP three-way handshake.

In TCP connection establishment:

Host $P$ sends a SYN segment: $\text{SYN} = 1, \text{ACK} = 0, \text{SEQ} = X$ .
Host $Q$ responds with a SYN + ACK segment: it acknowledges $P$ 's SYN by setting $\text{ACK} = 1$ and $\text{ACK number} = X + 1$ (since the SYN consumes one sequence number). At the same time, $Q$ initiates its own sequence stream by setting $\text{SYN} = 1$ with its initial sequence number $\text{SEQ} = Y$ . The connection is not terminating, so $\text{FIN} = 0$ .

Thus, the segment sent by $Q$ has $\text{SYN bit} = 1$ , $\text{SEQ number} = Y$ , $\text{ACK bit} = 1$ , $\text{ACK number} = X + 1$ , and $\text{FIN bit} = 0$ . This matches option C.

Final answer

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2021 Computer Science and Information Technology - Session 2 (CS2)
Question: 17 of 65, printed as CS Q.7
Section: Computer Science and Information Technology

Report an error in this question

GATE questions are the work of the organising institutes. This site is independent and is not affiliated with them.

Question

Solution

Source