/sɪn/
n. “The TCP handshake’s polite ‘hello’.”
SYN, short for Synchronize, is a flag in the TCP (Transmission Control Protocol) header used to initiate a connection between two devices on a network. It signals the beginning of the TCP three-way handshake, allowing both the client and server to synchronize sequence numbers and prepare for reliable data transfer.
Key characteristics of SYN include:
- Connection Initiation: The client sends a SYN packet to indicate it wants to establish a TCP connection.
- Sequence Number Synchronization: The SYN packet includes an initial sequence number, which is used to track the order of transmitted data.
- Part of TCP Handshake: Forms the first step in the three-way handshake: SYN → SYN-ACK → ACK.
- Lightweight and Stateless: The SYN packet alone carries no payload; its primary role is signaling and establishing session state.
A conceptual example of the TCP handshake using SYN:
Client → Server: SYN (initial sequence number = 100)
Server → Client: SYN-ACK (sequence number = 300, acknowledges 101)
Client → Server: ACK (acknowledges 301)Conceptually, SYN acts like the first handshake in a conversation — it says “I’d like to talk and here’s my starting point” before any actual messages are exchanged.
In essence, SYN is a fundamental TCP control flag that establishes reliable connections, enabling structured, ordered, and error-checked communication over the internet.