Three-way Handshake Continued
Even with four steps, only three packets need to be exchanged, because the SYN packet from Host 2 also serves as the acknowledgement to the SYN packet from Host 1.
Each host computer must acknowledge the sequence number of the sender and include its own sequence number in the following packet.
If a given packet does not arrive within a given time interval, the source computer will retransmit it.
If the destination computer receives packets out of order, it can use the sequence numbers to reassemble those packets into the correct order.