XTPROTO(5)           INTERACTIVE UNIX System           XTPROTO(5)



          NAME
               xtproto - multiplexed channels protocol used by xt(7) driver

          DESCRIPTION
               The xt(7) driver contains routines which implement a multi-
               plexed, 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(1).

               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 unack-
               nowledged 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(5).
               layers(1), xt(7) in the INTERACTIVE UNIX System
               User's/System Administrator's Reference Manual.



















          Rev. C Software Development Set                            Page 1