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RL-TCPnet is based on the client/server model of operation, and the TCP connection setup is based on these roles as well. Both, the client and the server, prepare for the connection by performing an open operation. However, there are two different kinds of open:
Active Open: A client process using TCP takes the “active role” and initiates the connection by actually sending a TCP message to start the connection (a SYN message).
Passive Open: A server process, designed to use TCP, takes a more relaxed approach. It performs a passive Open by contacting the TCP, which is like saying “I am here, and I am waiting for clients that may wish to talk to me to send me a message on the following port number”. The Open is called passive because aside from indicating that the process is listening, the server process does nothing.
A passive Open can, in fact, specify that the server is waiting for an Active Open from a specific client. However, not all RL-TCPnet APIs support this capability.
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