GATE CSE 2021 Set 2 | Question: 7

Question

Consider the three-way handshake mechanism followed during $\text{TCP}$ 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 $\text{TCP}$ connection request message to $Q$ with a $\text{TCP}$ segment having $\text{SYN}$ bit $=1$, $\text{SEQ}$ number $=X$, and $\text{ACK}$ bit $=0$. Suppose $Q$ accepts the connection request. Which one of the following choices represents the information present in the $\text{TCP}$ segment header that is sent by $Q$ to $P$?

• A. $\text{SYN}$ bit $=1$, $\text{SEQ}$ number $=X+1$, $\text{ACK}$ bit $=0$, $\text{ACK}$ number $=Y$, $\text{FIN}$ bit $=0$
• B. $\text{SYN}$ bit $=0$, $\text{SEQ}$ number $=X+1$, $\text{ACK}$ bit $=0$, $\text{ACK}$ number $=Y$, $\text{FIN}$ bit $=1$
• C. $\text{SYN}$ bit $=1$, $\text{SEQ}$ number $=Y$, $\text{ACK}$ bit $=1$, $\text{ACK}$ number $=X+1$, $\text{FIN}$ bit $=0$
• D. $\text{SYN}$ bit $=1$, $\text{SEQ}$ number $=Y$, $\text{ACK}$ bit $=1$, $\text{ACK}$ number $=X$, $\text{FIN}$ bit $=0$

Comments

Answer C:

Since sender will send its sQno. x and reciver will send ACK no. x+1 to make sure the sender that i recieved x and send now x+1 Byte

SYN bit = 1 for synchronise

ACK bit = 1 because reciever giving ackonwledgement to sender,   

FIN bit = 0 because it is used for termination connection

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SYN and FIN consumes one sequence number whereas pure ACK not consumes any sequence number.

So by this conclusion we can easily derive that SYN segment sent by P to Q having sequence number X will be consumed and next sequence number of X+1 is expected by the receiver Q.

So Q will sent ACK number as X+1 with ACK flag set.

Also, it needs to SYN its sequence number so it sends it Y sequence number with its SYN flag set.

Answer 1

This is related to 3 way handshake where  in first handshake, sender send SYN bit. 

In second handshake reciever sends to sender SYN + ACK bit ( remember here the connection gets established)

In third handshake, sender sends ACK bit