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SYN-ACK Handshake to Establish a TCP Connection

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

TCP SYN message

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.

TCP SYN-ACK message

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.

TCP ACK message

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.

TCP Socket connection

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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