When using the connect function to connect to a remote host, you first need to specify the IP address and port number of the remote host. However, if it is set to 0 when specifying the port number, it means that the operating system will automatically select an appropriate port number according to the protocol and then connect. In fact, for the TCP protocol, three-way handshake will be executed during the connection establishment, and an unoccupied port number will be selected as the local port in the connection.
When connecting to a remote host using the connect function, the peer port is usually 0. It is necessary to negotiate the port number with the peer service program, that is, one party informs the peer to use the port number after establishing the connection.