From original post:
Establishing a normal TCP connection requires three separate steps:
- The first host (Alice) sends the second host (Bob) a “synchronize” (SYN) message with its own sequence number x, which Bob receives.
- Bob replies with a synchronize-acknowledgment (SYN-ACK) message with its own sequence number y and acknowledgement number x + 1, which Alice receives.
- Alice replies with an acknowledgment message with acknowledgement number y + 1, which Bob receives and to which he doesn’t need to reply.
Additional image (in more details):