When troubleshooting the connectivity of JNLP build agents, it'd be nice if the TcpSlaveAgentListener provides more diagnosability. Currently, a protocol is binary, so it prevents the standard diagnostic tools like 'nc' and 'telnet' to work.
The situation where this gets frustrating is when you have network middle layers, such as port forwarder, iptables, and other such things. A basic "Ping? Pong!" would help users perform trouble-shooting quickly.