ACC X.25 Protocol User's Guide

X.25 Status and Completion Messages
Unsolicited Status Messages
Appendix A 133
Unsolicited Status Messages
The X.25 subsystem generates many different types of X.25 unsolicited
status messages. Some of the status messages provide X.25 link specific
information, that is, some of the status codes are only generated by the
X.25 link and will never be generated by a Virtual Circuit ZLU. Other
codes pertain to various VC related protocol errors. Typically these are
only delivered to the zx25d X.25 protocol driver. Finally, a third set of
status codes are returned which pertain explicitly to the various states
and protocol packets received by a Virtual Circuit ZLU. In addition, the
VC status codes may also return a data buffer containing additional data
about the event. Note that status messages 64 through 84 and 88
through 109 are delivered by default to the X.25 link ZLU receivers.
The format of the status messages listed below is the symbolic name
(define) for the specific status code following by the numeric value of the
status code in parenthesis and then a description of the meaning of the
status code.
X.25 Link Status
These status messages all pertain solely to X.25 link ZLUs. None of these
status messages provide an additional data buffer. For status codes 64
through 74, bit 7 of the mrqstat field in the ZCOM message header is
used to indicate whether the HDLC/LAP-B protocol is up. This is, if the
X.25 link at level 2 has been established. If the bit is set, than level 2 is
down. Otherwise, if clear, then the HDLC/LAP-B handshaking has been
completed and level 2 is up. Note that although the level 2 protocol is up,
you may not use the link until the X.25 packet layer (level 3) restart
processing has been completed (indicated by status code 85). These
unsolicited status messages may be received by an application program,
if requested through a call to zx25l2stat_rcvr(). Status codes 64 through
84 are also delivered to all applications that have set themselves up as
receivers on the X.25 link ZLU.
ST25ENBL (64) X.25 Link status after ENABLE. This status is used to
indicate whether the HDLC/LAP-B (X.25 level 2) link
has been established following an enable of the X.25
link ZLU. If bit 7 of the mrqstat field is set, the link is
down indicating that the MUX card was unable to
establish the HDLC/LAP-B link (X.25 level 2) following