SYN-ACK Handshake to Establish a TCP Connection

By Stephen Bucaro

Three messages are required to establish a TCP connection between two hosts.

1. Host A sends a sends a SYN (for synchronize) message to Host B. The message contains control bits identifying it as a SYN message and it contains a sequence number.

2. When Host B receives the SYN message, it sends an ACK (for acknowledge) message combined with its own SYN message to Host A. The ACK message contains a sequence number, and a SYN sequence number which is the incremented SYN sequence number it received from Host A.

3. When Host A receives the SYN-ACK message, it sends an ACK message to Host B. The ACK sequence number is the incremented ACK sequence number it received from Host B.

4. When Host B receives the ACK message, a TCP Socket connection is established. A TCP Socket is a "virtual port". A virtual port is a connection identified by the Host's IP address along with a 16-bit port number. Port numbers are standardized for various applications or processes, for example HTTP uses port 80 by default.

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