Skip to content
Free public beta until December 4, 2026: every course is open, no card needed. After that, the first chapter of each course stays free and full access is a one-time $39 ($19 for up to 100 founding members chosen for helpful feedback). Your progress is kept. How the beta works

three-way handshake

The SYN, SYN-ACK, ACK exchange that opens a TCP connection.

TCP opens a connection with SYN, SYN-ACK and ACK, which agree on initial sequence numbers in both directions before any data flows.

Examples

  • The sensor sends SYN with seq=1000, the laptop answers SYN-ACK with seq=5000 and ack=1001, and the sensor's ACK carries ack=5001.

  • If the sensor's SYN is lost, no connection exists yet, so the sensor waits for its timer and sends the SYN again.

See also

TCP sequence number socket
Read the full section →