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.
More Networking Protocols and Standards:
• What is PPP, PPPoA and PPPoE?
• WAN Network Protocols - DSL, SONET, HDLC, DWDM, DLSW+
• Unicast, Multicast, Broadcast. What Does It Mean?
• OSI Network Model
• TCP Windowing
• Network Cabling and Components
• Basic TCP/IP Networking
• Classless IP Addressing
• DSL (Digital Subscriber Line)
• Wireless Standards - 802.11a 802.11b 802.11g 802.11n 802.11i Explained