The top level directory contains a single directory naming the protocol, plap. It contains a clone file and a set of conversation directories numbered from 0 to 127; only active conversation directories are present.

Opening the clone file allocates a new conversation directory n. The file descriptor will point to the control file, ctl, of that directory. Reading the file returns a text string giving the conversation number.

Each conversation directory contains six files. The connection is controlled by writing text strings to the ctl file. Once a connection has been established, data is exchanged with the remote peer by reading and writing the data file.

A plap address has the form:

[ slot. ] chan!sap

where slot is a chassis slot number, chan is a logical channel (eg, a modem number), and sap is a number representing a `service access point'.

The following control messages are supported:

connect remoteaddr [ localaddr ]

Make a connection to the given remote address (of the form given above). If a local address is not specified, the system will assign an unused address dynamically (the local chassis slot number is of course fixed). Connect fails if the connection is already announced or connected, or localaddr is already in use.

announceaddress

Set the local address to address and accept incoming calls to that address. If no address is specified, all incoming calls to addresses not explicitly announced will be accepted by this connection. Announce fails if the connection is already announced or connected, or the local address specified is already in use.

The remote and local files contain the remote and local addresses for the connection in the full address form slot.chan!sap. The status file returns a line giving the directory name, number of active opens, and the state of the connection.

A process listens for incoming calls by opening the listen file using sys-open(2). The open will block until a call arrives, when it will return a file descriptor open on the ctl file of the newly accepted connection, which can be read to find the directory number and thus the other files.

The device follows the conventions that allow dial, announce and listen of sys-dial(2) to do the work needed to establish and manipulate network connections.