PK(4)               UNIX Programmer's 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 _p_k_o_n(2)
     with a character device file descriptor and a desired packet
     size in bytes.  The packet size must be a power of 2,
     32<__s_i_z_e<_4096.  The file descriptor must represent an 8-bit
     data path.  This is normally obtained by  setting the device
     in raw mode (see _i_o_c_t_l(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 are 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 del-
     imiting files.

     When one side of a packet driver link is shut down by
     _c_l_o_s_e(2) or _p_k_o_f_f (see _p_k_o_n(2)), _r_e_a_d(2) on the other side
     will return 0, and _w_r_i_t_e on the other side will raise a SIG-
     PIPE signal.

SEE ALSO
     pkon(2), pkopen(3)

Printed 5/17/83                                                 1