1PK(4)                      Kernel Interfaces Manual                      PK(4)
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NAME

6       pk - packet driver
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DESCRIPTION

9       The  packet  driver  implements  a  full-duplex end-to-end flow control
10       strategy for machine-to-machine communication.  Packet driver  protocol
11       is established by calling pkon(2) with a character device file descrip‐
12       tor and a desired packet size in bytes.  The  packet  size  must  be  a
13       power  of 2, 32≤size≤4096.  The file descriptor must represent an 8-bit
14       data path.  This is normally obtained by  setting  the  device  in  raw
15       mode (see ioctl(2)).
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17       The  actual  packet  size, which may be smaller than the desired packet
18       size, is arrived at by negotiation with the packet driver at the remote
19       end of the data link.
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21       The  packet  driver  maintains two data areas for incoming and outgoing
22       packets.  The output area is  needed  to  implement  retransmission  on
23       errors, and arriving packets are queued in the input area.  Data arriv‐
24       ing for a file not open for reading is discarded.  Initially  the  size
25       of both areas is set to two packets.
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27       It  is  not  necessary that reads and writes be multiples of the packet
28       size although there is less system overhead if they are.   Read  opera‐
29       tions  return  the maximum amount of data available from the input area
30       up to the number of bytes specified in the  system  call.   The  buffer
31       sizes in write operations are not normally transmitted across the link.
32       However, writes of zero length are treated specially and are  reflected
33       at  the remote end as a zero-length read.  This facilitates marking the
34       serial byte stream, usually for delimiting files.
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36       When one side of a packet driver link is shut down by close(2)or  pkoff
37       (see  pkon(2)),  read(2)  on the other side will return 0, and write on
38       the other side will raise a SIGPIPE signal.
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SEE ALSO

41       pkon(2), pkopen(3)
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45                                     local                               PK(4)
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