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Understanding the Three-Way Handshake

Published in Network Protocols 2 mins read

What is SYN and ACK?

SYN and ACK are control flags in the Transmission Control Protocol (TCP) used to establish and maintain reliable communication between computers. They are part of the three-way handshake, a crucial process for setting up a TCP connection.

The three-way handshake uses three packets:

  1. SYN (Synchronize): The initiating computer sends a SYN packet to the receiving computer, proposing a connection. Think of this as saying, "I want to talk to you." This packet contains an initial sequence number.

  2. SYN-ACK (Synchronize-Acknowledge): The receiving computer responds with a SYN-ACK packet. This acknowledges the initial SYN request and also proposes its own sequence number. This is like saying, "Okay, I'm ready to talk, and here's my starting number."

  3. ACK (Acknowledge): The initiating computer sends an ACK packet, acknowledging the receiving computer's SYN-ACK. This final step confirms the connection is established. This is akin to saying "Got it, let's begin!".

The Role of SYN and ACK Flags

  • SYN: This flag indicates the start of a connection request. It is set to 1 in the SYN packet and 0 in the subsequent packets.
  • ACK: This flag signifies an acknowledgment of a received packet. It's set to 1 to confirm receipt and correct sequencing of data.

Both flags work together to ensure reliable data transmission. Without them, data could be lost or corrupted, leading to unreliable communication.

Practical Applications and Examples

  • Web Browsing: Every time you visit a website using HTTP or HTTPS, a TCP connection is established using the SYN, SYN-ACK, and ACK process.
  • File Transfer: Transferring files often relies on TCP, and SYN and ACK ensure reliable data transmission.
  • Online Gaming: Online games frequently use TCP for low-latency communication; SYN and ACK are essential for maintaining a stable connection.

The SYN and ACK flags are fundamental to reliable internet communication. Understanding their roles is crucial to grasping how data is reliably transmitted across the network. Incorrect handling of these flags can lead to connection failures or security vulnerabilities, such as SYN flood attacks.

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