1) A tri-directional communications establishment process.
1) A handshake 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.
2) It follows the physical establishment of the channel and precedes normal information transfer.
To establish a connection, TCP uses a three-way handshake. Before a client attempts to connect with a server, the server must first bind to a port to open it up for connections: this is called a passive open. Once the passive open is established, a client may initiate an active open. To establish a connection, the three-way (or 3-step) handshake occurs:
1. The active open is performed by sending a SYN to the server.
2. In response, the server replies with a SYN-ACK.
3. Finally the client sends an ACK (usually called SYN-ACK-ACK) back to the server.
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At this point, both the client and server have received an acknowledgement of the connection.
1. The initiating host (client) sends a synchronization (SYN flag set) packet to initiate a connection. Any SYN packet holds a Sequence Number. The Sequence Number is a 32-bit field in TCP segment header. For example let the Sequence Number value for this session be x.
2. The other host receives the packet, records the Sequence Number of x from the client, and replies with an acknowledgment and synchronization (SYN-ACK). The Acknowledgment Number is a 32-bit field in TCP segment header. It contains the next sequence number that this host is expecting to receive (x + 1). The host also initiates a return session. This includes a TCP segment with its own initial Sequence Number value of y.
3. The initiating host responds with the next Sequence Number (x+1) and a simple Acknowledgment Number value of y + 1, which is the Sequence Number value of the other host + 1 and InfoSec Institute Data Recovery.
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Last Modified 09.14.09