The Berkeley Unix operating system provides line printer spooling functionalities via a collection of commands: 'lpr' assigns a queue, 'lpq' lists the printer queue, 'lprm' deletes a job from the queue and 'lpc' controls the queue. The protocol between the printer server and the client is the Line Printer Daemon Protocol (LPD).

A printing environment contains a printer server and several clients. The server provides printing service at port TCP/515 and manages the printer spool to which clients connect and send printing jobs. Every job is assigned a job ID which is a number between 0 and 999; both the client and server use this ID to refer to a given job. The protocol allows the client to send a file to print, maintain the printing queue, remove jobs from the spool and get the status of jobs.

LPD protocol is a request/acknowledgment based protocol. Every REQUEST begins with a single octet code, which represents the binary number of the requested function. The code is followed by the ASCII name of the printer queue. Other operands can follow the queue name separated by whitespace. The request must be closed with an ASCII line feed character. After certain operations (e.g.: receiving a datafile) the server may send an acknowledgment.