ACC X.25 Protocol User's Guide

X.25 Application Programming
Sending and Receiving Data
Chapter 4 81
• For received messages or message fragments with the D-bit set, the
X.25 firmware does not send an X.25 level 3 acknowledgment for the
message. This is now the responsibility of the application. The
application must determine whether the D-bit was set by examining
the tag byte. For each received message or message fragment with
the D-bit set, the application must call the function zx25dbit_ack() to
cause a level 3 RR (D-bit acknowledgment) to be transmitted by the
firmware for the data packet. If the T25 timer has been configured,
the application should call zx25dbit_ack() within the T25 timer value
or the VC is reset.
Q-Bit Handling
A complete packet sequence received with the Q-bit set in every packet is
passed to the “Alternate Receiver” for the logical channel (refer to the
man page on zset_rcvr in the ACC Programmer’s Reference Guide). If the
same application is Receiver and Alternate Receiver for the logical
channel, it can identify a message with the Q-bit set by checking bit 1 of
the tag byte. Bit 1 will be set if the message had the Q-bit set.
To send a message with the Q-bit set, the application should set bit 7 of
the tag byte.