When lpd is started it makes a single pass through the printcap database restarting any printers that have jobs.
Clients communicate with lpd using a simple transaction oriented protocol.
The following table shows the requests understood by lpd . Additional qualifiers may follow, depending on the request. Lpd either schedules the job to be printed locally, or if printing remotely, attempts to forward the job to the appropriate machine. If the printer cannot be opened or the destination machine is unreachable, the job will remain queued until it is possible to complete the work. If necessary, lprm will first kill off a running daemon that is servicing the queue and restart it after the required files are removed.