XTPROTO(M)                           UNIX System V



     Name
          xtproto -  multiplexed  channels  protocol  used  by  xt(HW)
          driver


     Description
          The  xt(HW)  driver  contains  routines  which  implement  a
          multiplexed,   multi-buffered,   full-duplex  protocol  with
          guaranteed delivery of ordered data via an 8-bit  byte  data
          stream.   This  protocol  is  used for communication between
          multiple UNIX system host processes and  an  AT&T  windowing
          terminal operating under layers(C).

          The protocol uses packets with a 2-byte header containing  a
          3-bit  sequence  number, 3-bit channel number, control flag,
          and data size.  The data part of a packet may not be  larger
          than  32  bytes.   The  trailer  contains a CRC-16 code in 2
          bytes.  Each channel is double-buffered.

          Correctly received packets in sequence are acknowledged with
          a control packet containing an ACK; however, out of sequence
          packets generate a control packet containing  a  NAK,  which
          will   cause   the   retransmission   in   sequence  of  all
          unacknowledged packets.

          Unacknowledged packets are  retransmitted  after  a  timeout
          interval  which  is dependent on baud rate.  Another timeout
          parameter specifies  the  interval  after  which  incomplete
          receive packets are discarded.


     Files
          /usr/include/sys/xtproto.h   channel  multiplexing  protocol
          definitions


     See Also
          layers(M), layers(C), xt(HW)


     (printed 2/15/90)                                 XTPROTO(M)