ACC X.25 Protocol User's Guide

Chapter 4 79
X.25 Application Programming
Sending and Receiving Data
Application End-to-End Acknowledgments Disabled
(APD=0)
End-to-end application acknowledgment handling is controlled through
the VC ZLU’s option word APD bit. When this bit is set to zero,
application end-to-end acknowledgments are disabled and all data
packet D-bit handling is performed by the X .25 firmware running on the
ACC card. The implications for the application pro gram are as follows:
• For transmitted messages with the D-bit set, the write completion
(definite) status is returned when a level two acknowledgment (RR) is
received for that data message. That is, if the application called
zsend() with a mode parameter of ZcMODE_DEF_STATUS (2), the
application would received a write completion status when the
HDLC/LAP-B protocol RR’d (ack’d) the data message. There is no way
for the application to determine when X.25 level 3 acknowledges the
data message (e.g. D-bit ack).
• For received messages or message fragments with the D-bit set, the
applicationcandetermine that the D-bit was set by examining the tag
byte. The application has no control over the transmission of the
acknowledgment to the D-bit data packets. In fact, the X.25 firmware
generally sends the D-bit acknowledgment immediately.
Using Application End-to-End Acknowledgments
(APD=1)
Application end-to-end D-bit handling is enabled by setting the APD bit
in the VC ZLU’s option word. This alters how D-bit data packets are
handled by the firmware, imposes certain restrictions on transmission of
data messages with the D-bit set, and ma ndates programming rules by
the application in regards to D-bit handling. The implications for the
application are as follows:
• For transmitted messages with the D-bit set, the write completion
(definite) status is returned when the messages receives a level three
acknowledgment (RR). O nly one message at a time, per VC, may be
transmitted with the D-bit set. The application must wait for the
D-bit acknowledgment before another zsend() is issued with the D-bit
set on that VC. This implies that applications should always use a
mode of ZcMODE_DEF_STATUS, ZcMODE_WAIT, or
ZcMODE_DEF_STATUS_WBUF when sending data messages with
the D-bit.