Editing Handshake (computing) (section)

=== TCP three-way handshake ===
[[File:Three-way-handshake-example.gif|thumbnail|right|500px|Example of three way handshaking]]
Establishing a normal [[Transmission Control Protocol#Connection establishment|TCP]] connection requires three separate steps:
:# The first host (Alice) sends the second host (Bob) a "synchronize" (SYN) message with its own sequence number <math>x</math>, which Bob receives.
:# Bob replies with a synchronize-acknowledgment ([[SYN-ACK]]) message with its own sequence number <math>y</math> and acknowledgement number <math>x + 1</math>, which Alice receives.
:# Alice replies with an acknowledgment (ACK) message with acknowledgement number <math>y + 1</math>, which Bob receives and to which he doesn't need to reply.
: In this setup, the synchronize messages act as service requests from one server to the other, while the acknowledgement messages return to the requesting server to let it know the message was received.

The reason for the client and server not using a default sequence number such as 0 for establishing the connection is to protect against two incarnations of the same connection reusing the same sequence number too soon, which means a segment from an earlier incarnation of a connection might interfere with a later incarnation of the connection.