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:
• Video - Transport Layer (Layer 4) of OSI Networking Model
• Unicast, Multicast, Broadcast. What Does It Mean?
• Basic TCP/IP Networking
• Session Border Controllers - More Than Just a Voice Firewall
• Integration of IPv6 with IPv4
• X.25 and Frame Relay Overview
• Kerberos Authentication Protocol
• IPv6 Payload Length Field and Jumbograms
• Dynamic Host Configuration Protocol (DHCP) Explained
• Comparison of the Layers of the OSI and TCP/IP Models
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