The three-way handshake is a fundamental process in networking, particularly in the context of the Transmission Control Protocol (TCP), one of the core protocols of the Internet Protocol (IP) suite. It is used to establish a reliable and connection-oriented communication between two devices, typically a client and a server. The three-way handshake ensures that both ends agree on the initial sequence numbers and other parameters before starting the actual data transfer.
Here’s how the three-way handshake works:
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SYN (Synchronize): The process begins with the client sending a TCP segment to the server with the SYN (synchronize) flag set. This initial segment contains a randomly generated sequence number (let’s call it client_seq) that the client chooses.
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SYN-ACK (Synchronize-Acknowledge): Upon receiving the SYN segment, the server responds with its own TCP segment. The server sets the SYN flag and the ACK (acknowledge) flag, acknowledging the receipt of the client’s SYN. The server also generates its own randomly chosen sequence number (server_seq) and sends it back to the client.
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ACK (Acknowledge): Finally, the client acknowledges the server’s response by sending a TCP segment with the ACK flag set. The ACK segment also includes an acknowledgment number, which is the server’s sequence number (server_seq) incremented by 1. At this point, both the client and server have completed the three-way handshake, and a reliable connection has been established.
The purpose of the three-way handshake is to achieve several important goals:
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Synchronization of Sequence Numbers: The exchange of sequence numbers ensures that both ends of the connection agree on the initial sequence numbers for the data transfer.
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Connection Establishment: The process establishes a reliable connection between the client and server, allowing them to exchange data in a way that guarantees delivery and order.
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Security: The three-way handshake helps prevent unauthorized connections by requiring both parties to exchange specific information before starting the data transfer.
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Flow Control: The acknowledgment number in the ACK segment can be used to implement flow control, allowing the sender to know which data has been successfully received by the other end.
Once the three-way handshake is complete, data transfer can occur between the client and server. After the data transfer is finished, a similar process (four-way handshake) is used to gracefully terminate the connection.
It’s worth noting that the three-way handshake is specific to TCP. Other transport layer protocols, like User Datagram Protocol (UDP), do not establish connections in the same way since they are connectionless and do not provide the same reliability mechanisms as TCP.