PK(4) (local) PK(4)
NAME
pk - packet driver
DESCRIPTION
The packet driver implements a full-duplex end-to-end flow
control strategy for machine-to-machine communication.
Packet driver protocol is established by calling pkon(2)
with a character device file descriptor and a desired packet
size in bytes. The packet size must be a power of 2,
32≦size≦4096. The file descriptor must represent an 8-bit
data path. This is normally obtained by setting the device
in raw mode (see ioctl(2)).
The actual packet size, which may be smaller than the
desired packet size, is arrived at by negotiation with the
packet driver at the remote end of the data link.
The packet driver maintains two data areas for incoming and
outgoing packets. The output area is needed to implement
retransmission on errors, and arriving packets are queued in
the input area. Data arriving for a file not open for read-
ing is discarded. Initially the size of both areas is set
to two packets.
It is not necessary that reads and writes be multiples of
the packet size although there is less system overhead if
they are. Read operations return the maximum amount of data
available from the input area up to the number of bytes
specified in the system call. The buffer sizes in write
operations are not normally transmitted across the link.
However, writes of zero length are treated specially and are
reflected at the remote end as a zero-length read. This
facilitates marking the serial byte stream, usually for
delimiting files.
When one side of a packet driver link is shut down by
close(2)or pkoff (see pkon(2)), read(2) on the other side
will return 0, and write on the other side will raise a SIG-
PIPE signal.
SEE ALSO
pkon(2), pkopen(3)