The TCP (Transmission Control Protocol) three-way handshake is a method used to establish a connection between a client and a server. It is termed a “three-way” handshake because there are three steps or messages exchanged between the two parties. Here’s a breakdown:
- SYN: The client initiates the connection by sending a TCP packet with the SYN (synchronize) flag set to the server. This indicates that the client wishes to establish a connection. The packet also contains an initial sequence number, which will be used to track the sequence of all subsequent packets.
- SYN-ACK: Upon receiving the client’s SYN packet, the server responds with a TCP packet of its own with both the SYN and ACK (acknowledge) flags set. The acknowledgment number in this packet is one more than the initial sequence number received from the client. The server also provides its own initial sequence number.
- ACK: Finally, the client sends back an ACK packet to acknowledge receipt of the server’s SYN-ACK. The acknowledgment number in this packet is one more than the initial sequence number received from the server. After this packet is sent, the connection is considered established, and data transfer can begin.
Significance of the TCP Three-Way Handshake:
- Reliability: The handshake ensures that both the client and server are ready to transmit and receive data, ensuring a reliable connection setup.
- Sequencing: By exchanging initial sequence numbers, both the client and server can track the order of packets, ensuring data is processed in the correct order.
- Flow Control: This mechanism helps establish initial parameters for flow control, which will ensure efficient and orderly communication between the two parties.
- Security Implications: While the three-way handshake establishes a connection reliably, it is not without its vulnerabilities. For instance, it can be exploited in attacks like the SYN flood, where attackers inundate a server with SYN requests without completing the handshake, consuming resources and potentially causing a denial of service.