Well, in the past we'd use multi-port serial terminal servers and all the hosts had their serial ports as their console hardwired to the terminal server. These

Well, in the past we'd use multi-port serial terminal servers and all the hosts had their serial ports as their console hardwired to the terminal server. These days the servers all include IPMI with KVM redirection. But the serial ports are still there. As far as using any TCP/IP based protocols over the serial port, I suppose you could do something silly with SLIP or PPP, but honestly I don't know why. The whole point of the serial port is that it's damned simple and firmware can easily initialize it and use it for IO extremely early in the boot process.