TCP Three-Way Handshake

TCP promises a *reliable, ordered byte stream* — but it runs over IP, which loses, reorders, and duplicates packets freely. To deliver on that promise, both sides must first agree on where the stream *starts*. That agreement is the three-way handshake: SYN, SYN-ACK, ACK. It costs exactly one round trip before the first byte of real data can flow.

Sequence numbers — the thing being agreed

TCP labels every byte with a sequence number so the receiver can reorder what arrives and detect what's missing. Each side picks its *own* starting sequence number (an ISN, chosen unpredictably for safety). The handshake exists so each side learns the other's starting number and confirms it was heard.

The three packets

1. 1SYN — the client sends a packet with the SYN flag set and its starting sequence number, say x. (Client: SYN-SENT.)
2. 2SYN-ACK — the server replies with SYN carrying *its* starting number y, plus an ACK of x+1 (the next byte it expects from the client). One packet does both jobs. (Server: SYN-RECEIVED.)
3. 3ACK — the client acknowledges the server's number with ACK y+1. The connection is now ESTABLISHED and data can flow.

Client                         Server
  |  --- SYN  seq=x ----------->  |
  |  <-- SYN-ACK seq=y ack=x+1 -  |
  |  --- ACK  ack=y+1 -------->   |
  |        (ESTABLISHED)          |