TCP Connection Establishment: Three-Way Handshake

TCP is connection-oriented, so both sides synchronize sequence numbers before data transfer using three segments.

Steps

1. SYN: the client (active open) sends a segment with SYN=1 and its initial sequence number $x$.
2. SYN + ACK: the server (passive open, in LISTEN) replies with SYN=1, ACK=1, its own initial sequence number $y$ and acknowledgement $x+1$.
3. ACK: the client sends ACK=1 with acknowledgement $y+1$. Both sides are now in ESTABLISHED and data transfer begins.

Points to note

• The SYN segment consumes one sequence number.
• The handshake also negotiates parameters such as window size and MSS.
• Using random initial sequence numbers protects against old duplicate segments and spoofing.