LPD(Line Printer Daemon) is a widely used print server protocol on TCP/IP network computing environment. LPD uses TCP protocol; the default port number is 515. The detailed information about LPD protocol is described in RFC1179 (L. McLaughlin III, 1990.8).

For a print job, an LPD server program should be running on your PC. The LPD program runs on TCP port 515 and waits for print jobs. In a UNIX system, a user requests a print job using an lp, lpr or pr command, and the job will be sent to your PC over the network. Then the LPD server program running on your PC receives this job request and processes it using the printing options set inside the program. At this time, LPD protocol is used for communication.