3-Way Handshake

The client will send a SYN packet to the endpoint, $SYN_F=1$, with an arbitrarily chosen starting sequence number, $Seq=x$.

The endpoint will reply with a SYN/ACK packet, $SYN_F=1, ACK_F=1, ACK=x+1$, with another arbitrarily chosen sequence number, $Seq=y$.

The client will then reply to this SYN/ACK with an ACK, $ACK_F=1, ACK=y+1$. After the server receives this, both hosts are live and the connection is established (ESTAB).