xtproto(M)                      19 June 1992                      xtproto(M)


 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 communica-
    tion 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 con-
    tains 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.

 File

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

 See also

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