Handshaking

In information technology, telecommunications, and related fields, handshaking is an automated process of negotiation that dynamically sets parameters of a communications channel established between two entities before normal communication over the channel begins. It follows the physical establishment of the channel and precedes normal information transfer.

The handshaking process usually takes place in order to establish rules for communication when a computer sets about communicating with a foreign device. When a computer communicates with another device like a modem, printer, or network server, it needs to handshake with it to establish a connection.

Handshaking can negotiate parameters that are acceptable to equipment and systems at both ends of the communication channel, including information transfer rate, coding alphabet, parity, interrupt procedure, and other or hardware features. Handshaking is a technique of communication between two entities. However, within TCP/IP RFCs, the term "handshake" is most commonly used to reference the TCP three-way handshake. For example, the term "handshake" is not present in RFCs covering FTP or SMTP. One exception is Transport Layer Security, TLS, setup, FTP RFC 4217. In place of the term "handshake", FTP RFC 3659 substitutes the term "conversation" for the passing of commands.

A simple handshaking protocol might only involve the receiver sending a message meaning "I received your last message and I am ready for you to send me another one." A more complex handshaking protocol might allow the sender to ask the receiver if it is ready to receive or for the receiver to reply with a negative acknowledgement meaning "I did not receive your last message correctly, please resend it" (e.g., if the data was corrupted en route).

Handshaking facilitates connecting relatively heterogeneous systems or equipment over a communication channel without the need for human intervention to set parameters.

Establishing a normal connection requires three separate steps:

One of the most important factors of three-way handshake is that, in order to exchange the starting sequence number the two sides plan to use, the client first sends a segment with its own initial sequence number ${\displaystyle x}$, then the server responds by sending a segment with its own sequence number ${\displaystyle y}$ and the acknowledgement number ${\displaystyle x+1}$, and finally the client responds by sending a segment with acknowledgement number ${\displaystyle y+1}$.

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