/sɪn-æk/
n. “The TCP handshake’s polite ‘hello back’.”
SYN-ACK is a flag combination in the TCP (Transmission Control Protocol) header used during the three-way handshake to establish a reliable connection between two devices. It signals both synchronization and acknowledgment: the server acknowledges receipt of the client’s initial SYN request and simultaneously sends its own SYN to start communication in the opposite direction.
Key characteristics of SYN-ACK include:
- Dual Purpose: Combines the SYN (synchronize) and ACK (acknowledge) flags in a single packet.
- Handshake Step: Represents the second step in TCP’s three-way handshake: SYN → SYN-ACK → ACK.
- Sequence Number Exchange: Carries the server’s initial sequence number and acknowledges the client’s sequence number.
- Connection Setup: Ensures both client and server are synchronized and ready for reliable data transfer.
Conceptual example of a TCP handshake:
Step 1: Client → Server: SYN (initial sequence number = 100)
Step 2: Server → Client: SYN-ACK (server sequence number = 300, acknowledges 101)
Step 3: Client → Server: ACK (acknowledges 301)Conceptually, SYN-ACK is like the server responding to a handshake by saying, “I see your hello, and here’s mine,” establishing mutual readiness to communicate.
In essence, SYN-ACK is a crucial TCP control signal that confirms the client’s request and synchronizes the server’s sequence numbers, forming the backbone of reliable, ordered network connections.