TCP

Create a connection

It uses a three-way handshake

1. The client [a machine/device requesting a service/connection] sends a SYN flag & the server port number of the service it wants to communicate (i.e. Port 80 for http/web services)
   The packet also includes the client’s Initial Sequence Number (ISN)
2. Server – Resplies with it’s own SYN flag & ISN to the client’s TCP port
   An ACK flag is included in the packet in reply to the client’s SYN flag
3. Client replies with an ACK flag that acknowledges the server’s SYN Flag

Terminate a connection

After all data is sent or session no longer needs the session is terminated. This can be done either by client or server. Here is an example of what is called an active close

1. The application (i.e. Web browser –Firefore) on the client sends a close command to the application (i.e. Web Server – Apache) on the server
2. Server sends a FIN flag to client
3. Client sends an ACK flag to acknowledge the FIN flag
4. Client sends a FIN flag to server
5. Server replies with ACK flag –acknowledging the termination of the TCP connection

Flags used in TCP Connections?

SYN Synchronized sequence number
FIN Sender FINished sending data –end connection
RST Reset Connection
PSH Push the data
ACK Acknowledgment
URG Urgent