6.4.4 N_DAck--Data Ack Request/Indication

Synopsis

#include <stream.h>
 #include <netx25/x25_proto.h >

 struct strbuf ctlb;

 struct xdatacf dack;
 .
 .
 .
 ctlb.len = sizeof(struct xdatacf);
 ctlb.buf = (char *)dack;

 putmsg(x25_fd, &ctlb, NULL, 0);

Description

N_DAck acknowledges a previous Data Acknowledgment Request or Indication which had the D-bit set. The D-bit requests end-to-end, as opposed to local, acknowledgment. There is a one-to-one correspondence between D-bit data and acknowledgments, with one Data Acknowledgment being received/sent for each D-bit data packet sent/received. It is always the oldest outstanding D-bit packet that is being acknowledged.

Refer to "6.3.4 qosformat--Define OSI CONS QOS Parameters" for details of requesting acknowledgment using the reqackservice member of the qosformat structure. For OSI CONS calls, Data Acknowledgment must be negotiated on the connection.

When used with putmsg, N_DAck is a Data Acknowledgment Request, when used with getmsg, it is a Data Acknowledgment Indication. Example 6-1 shows how a getmsg can be constructed. The control part of the Data Acknowledgment Request or Indication is defined by the xdatacf structure. There is no data part.

The xdatacf structure is shown below:

struct xdatacf {
 	unsigned char xl_type; /* Always XL_DAT */
 	unsigned char xl_command; /* Always N_DAck */
 };