GATE CS 2021 Set 2 Q7 — Transport Layer: Flow Control, Congestion Control, UDP, TCP, Sockets

During the TCP three-way handshake, connection establishment proceeds as follows:

1. Step 1 (P to Q): Host $P$ sends a connection request (SYN segment) to Host $Q$.
   • $\text{SYN bit} = 1$, $\text{ACK bit} = 0$
   • $\text{SEQ number} = X$ (initial sequence number of $P$)
2. Step 2 (Q to P): Host $Q$ accepts the request and sends a combined acknowledgment and connection request (SYN + ACK segment) back to $P$.
   • SYN bit: Since $Q$ is also initiating its connection direction, $\text{SYN bit} = 1$.
   • SEQ number: $Q$ chooses its own random initial sequence number, so $\text{SEQ number} = Y$.
   • ACK bit: Since this segment carries an acknowledgment, $\text{ACK bit} = 1$.
   • ACK number: A SYN segment logically consumes $1$ sequence number. Since $P$'s initial sequence number was $X$, $Q$ acknowledges it by expecting the next sequence number, which is $\text{ACK number} = X + 1$.
   • FIN bit: No connection termination is occurring, so $\text{FIN bit} = 0$.

Thus, the header fields in the segment sent by $Q$ to $P$ are:

$$\text{SYN bit} = 1, \quad \text{SEQ number} = Y, \quad \text{ACK bit} = 1, \quad \text{ACK number} = X + 1, \quad \text{FIN bit} = 0$$

This matches Option C.