PK(4)							     Kernel Interfaces Manual							     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 reading 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 opera-
       tions 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 SIGPIPE signal.

SEE ALSO

       pkon(2), pkopen(3)

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