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TCP 3-Way Handshake (SYN,SYN-ACK,ACK)

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The TCP three-way handshake in Transmission Control Protocol (also called the three message handshake) is the method used to establish TCP socket connections and tear down TCP socket connections over the network. TCP's three way handshaking technique is referred to as the 3-way handshake or as "SYN-SYN-ACK" (or more accurately SYN, SYN-ACK, ACK). The TCP handshaking mechanism is designed so that two computers attempting to communicate can negotiate the parameters of the network TCP socket connection before beginning communication. This three way handshaking process is also designed so that both ends can initiate and negotiate separate TCP socket connections at the same time. Being able to negotiate multiple TCP socket connections in both directions at the same time allows a single physical network interface (such as ethernet) to be multiplexed.

3-Way Handshake Description

Below is a (very) simplified description of the TCP 3-way handshake process. Have a look at the diagram on the right as you examine the list of events on the left.

EVENT DIAGRAM

Host A sends a TCP SYNchronize packet to Host B

Host B receives A's SYN

Host B sends a SYNchronize-ACKnowledgement

Host A receives B's SYN-ACK

Host A sends ACKnowledge

Host B receives ACK.
TCP socket connection is ESTABLISHED.

 tcp three-way handshake,syn,syn-ack,ack
TCP Three Way Handshake
(SYN,SYN-ACK,ACK)

SYNchronize and ACKnowledge messages are indicated by a bit inside the header of the TCP segment.

TCP knows whether the network TCP socket connection is opening, synchronizing, established by using the SYNchronize and ACKnowledge messages when establishing a network TCP socket connection.

When the communication between two computers ends, another 3-way communication is performed to tear down the TCP socket connection. This setup and teardown of a TCP socket connection is part of what qualifies TCP a reliable protocol. TCP also acknowledges that data is successfully received and guarantees the data is reassenbled in the correct order.

Note that UDP is connectionless. That means UDP doesn't establish connections as TCP does, so UDP does not perform this 3-way handshake and for this reason, it is referred to as an unreliable protocol.

Protocols Encapsulated in TCP

Note that FTP, Telnet, HTTP, HTTPS, SMTP, POP3, IMAP, SSH and any other protocol that rides over TCP also has a three way handshake performed as connection is opened. HTTP web requests, SMTP emails, FTP file transfers all manage the messages they each send. TCP handles the transmission of those messages.

TCP 'rides' on top of Internet Protocol (IP) in the protocol stack, which is why the combined pair of Internet protocols is called TCP/IP (TCP over IP). TCP segments are passed inside the payload section of the IP packets. IP handles IP addressing and routing and gets the packets from one place to another, but TCP manages the actual communication sockets between endpoints (computers at either end of the network or internet connection).

 

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