GATE CS 2024 Set 1 Q19 — Transport Layer: Flow Control, Congestion Control, UDP, TCP, Sockets

To determine the correct statements, let's analyze the TCP 3-way handshake process between the client $\text{P}$ and the server $\text{Q}$:

1. Initial Sequence Number ($N_P$):
   • When client $\text{P}$ initiates a connection, it sends a $\text{SYN}$ packet with an Initial Sequence Number (ISN), denoted here as $N_P$.
   • This ISN is chosen randomly (or via a pseudo-random generator) to prevent security vulnerabilities like TCP sequence prediction attacks and to avoid confusion with packets from previous closed connections. It is not always $0$.
   • Thus, Statement A is correct and Statement B is incorrect.
2. Acknowledgement Number in SYN-ACK ($N_Q$):
   • The $\text{SYN}$ flag consumes exactly $1$ sequence number, even though the packet contains no payload data.
   • When server $\text{Q}$ receives the $\text{SYN}$ packet with sequence number $N_P$, it responds with a $\text{SYN-ACK}$ packet.
   • The acknowledgement number ($N_Q$) in this $\text{SYN-ACK}$ packet indicates the next sequence number $\text{Q}$ expects to receive from $\text{P}$. Therefore, $N_Q = N_P + 1$.
   • Thus, Statement D is correct and Statement C is incorrect.

Hence, the correct options are A and D.