Enables a per-connection TCP keepalive feature; after approximately 3 minutes of network connection idle time, the connection will send a gratuitous ACK to the network peer, either ensuring the connection stays active OR causing a dropped connection condition to be recognized by the reverse raw service. Applications using this feature need to be aware that there might be some considerable delay between a network disconnection and the port being available for the next connection attempt; this is to allow any data sent on prior connections to be transmitted out of the serial port buffer. Application network retry logic needs to accommodate this feature.