What is the TCP Connection Establishment?

TCP Connection Establishment is the process by which two hosts create a reliable, connection-oriented session before transmitting data. This process ensures both sides are ready to communicate and agree on initial parameters for data exchange.

To make transport services reliable, TCP uses a three-way handshake mechanism to establish connections. This mechanism synchronizes both ends of a network by enabling both sides to agree upon original sequence numbers and confirm readiness for data transmission.

How the Three-Way Handshake Works

The three-way handshake prevents packets from being shared or retransmitted during session establishment. Each host randomly selects a sequence number to track bytes within the data stream it sends and receives.

Step-by-Step Process

• Step 1 − SYN: The requesting host (Client) sends a SYN segment with its initial sequence number (x) to the server's port.

• Step 2 − SYN-ACK: The server acknowledges the client's SYN by sending SYN-ACK with its own sequence number (y) and acknowledgment number (x+1).

• Step 3 − ACK: The client acknowledges the server's SYN by sending ACK with sequence number (x+1) and acknowledgment number (y+1).

Connection Termination Protocol

While three segments establish a connection, four segments are required to terminate a TCP connection. Since TCP connections are full-duplex (data flows independently in each direction), each direction must be shut down separately using a four-way handshake.

Either end can send a FIN (finish) segment when it finishes sending data. The end that sends the first FIN performs an active close, while the receiving end performs a passive close. A FIN indicates no more data will flow in that direction, but the connection can still receive data from the other direction.

Key Features

Conclusion

TCP connection establishment uses a three-way handshake to synchronize sequence numbers and confirm both hosts are ready for data transmission. Connection termination requires a four-way handshake to gracefully close each direction of the full-duplex connection independently.