The following information is provided as is, and the authors take no responsibility for the correctness.
The Line Printer Daemon protocol/Line Printer Remote protocol (or LPD, LPR) also known as the Berkeley printing system, is a set of programs that provide printer spooling and network print server functionality for Unix-like systems.
The most common implementations of LPD are the official BSD UNIX operating system and the LPRng project. The Common Unix Printing System (or CUPS) is more common on modern Linux distributions and optionally supports LPD.
Please see RFC 1179 (Line Printer Daemon Protocol)