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:
• PoE (Power Over Ethernet)
• IPv6 Neighbor Discovery Protocol (NDP)
• A Simple Description of the IPv6 Header and Datagram
• The OSI Network Layer
• IP Addressing
• Comparing IPv4 and IPv6 at a Glance
• WAN Network Protocols - DSL, SONET, HDLC, DWDM, DLSW+
• Free eBook: IPv6 Addressing
• What Are Private IP Addresses?
• The OSI Data Link Layer
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