Opening a
TCP connection involves exchanging three packets. These packets are
SYN,
SYN+ACK, and
ACK. They are called this way because the
TCP header has
SYN (for
synchronize) and
ACK (for
acknowledge)
bits. The first packet as the
SYN bit set (
equal to
one), the second packet has both these
bits set, the third packet has only the
ACK bit set.
This exchange is used to establish initial sequence numbers of the connection. After the three-way handshake is complete, connection is established.