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pk - packet driver
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 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
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
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