Opening of one logon - Three-way handshake

The used procedure in order to establish in reliable way a logon TCP between two host is called three-way handshake (grip of hand to 3 ways), indicating the necessity to exchange 3 messages between host sender and receiving host so that the logon is created correctly. We consider as an example that the host To means to open one logon TCP with the B host; the steps to follow therefore are:

1. To it sendes a segment SYN to B - flag the SYN is set up to 1 and the Sequence field number contains value x that specific the Initial Sequence Number of To;
2. B sendes a segment SYN/ACK to To - flag SYN and ACK are set up you to 1, the Sequence field number contains the value y that specific the Initial Sequence Number di B and the Acknowledgment field number contain the value x+1 confirming the reception of the ISN of To;
3. To it sendes a segment ACK to B - the Acknowledgment field number contains the value y+1 confirming the reception of the ISN of B.

The third segment would not be, ideally, necessary for the opening of the logon in how much already after the reception from part of To of according to segment, both the hosts has expressed their availability R-all'.apertura of the logon. However it turns out necessary to the aim to allow also to the B host an esteem of the timeout begins them, like time elapsed between the shipment of a segment and the reception of correspondent ACK.

Flag the SYN turns out useful in the practical implementazione of the protocol, and in its analysis from part of the firewalls: in traffic TCP segments SYN establish new logons, while those with the flag not active belong already to established logons.

The segments use you during the handshake are usually ' single header', that is phase of synchronization between the two hosts and not of exchange of data has the Given field empty being this one.