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Why the 3-Way Handshake?

Published in TCP/IP Networking 2 mins read

The three-way handshake is crucial for establishing reliable TCP connections because it ensures both communicating devices are ready before data transmission begins. It's not just about initiating contact; it's about verifying the connection's readiness on both ends.

Understanding the Necessity of a Three-Way Handshake

A two-way handshake wouldn't be sufficient because it only allows one party to initiate and the other to acknowledge. This leaves room for ambiguity and potential connection failures. The three-way handshake, however, provides a robust mechanism for reliable connection establishment. As noted in a StackExchange post (https://networkengineering.stackexchange.com/questions/24068/why-do-we-need-a-3-way-handshake-why-not-just-2-way), a two-way handshake lacks the confirmation needed for a stable connection.

  • SYN (Synchronize): The client sends a SYN packet, initiating the connection request and proposing a sequence number.
  • SYN-ACK (Synchronize-Acknowledge): The server acknowledges the client's request, proposes its own sequence number, and confirms its readiness.
  • ACK (Acknowledge): The client acknowledges the server's response, finalizing the connection setup.

This process, as described in numerous sources like GeeksforGeeks (https://www.geeksforgeeks.org/tcp-3-way-handshake-process/), ensures a reliable connection by verifying that both the client and server are active and ready for communication. The final ACK packet from the client acts as a confirmation that the entire process is complete and the connection is established. This is the minimum number of exchanges needed for effective communication (https://serverfault.com/questions/368567/why-is-it-3-way-handshake-and-not-more).

The three-way handshake is fundamental to the TCP protocol because it establishes the connection parameters between two computers before actual data transmission begins. Much of our online activity relies on this reliable connection establishment.

Practical Implications

The three-way handshake is not just a theoretical concept; it is crucial for the smooth operation of countless online applications and services. Without it, network communication would be prone to errors and unreliability. The accept() function, for example, only happens after the three-way handshake is completed (https://stackoverflow.com/questions/18451462/does-accept-event-happen-after-the-three-way-handshake). Even when Network Address Translation (NAT) is involved, the handshake still occurs, albeit with address translation by the router (https://forum.networklessons.com/t/where-does-the-tcp-3-way-handshake-happen-when-nat-is-involved/4134). Therefore, the three-way handshake is not only fundamental but also robust and adaptable to various network configurations.