HP X.25/9000 Programmer's Guide

122 AppendixA
X.25 Packet Formats
DATA Packet
DATA Packet
A client process sends a DATA packet to terminate an SVC with a remote
server. The subsystem sends a DATA packet when a send() or write()
system call is issued.
Figure A-4 DATA packet
Access to the fields in the DATA packet is described below:
General Format
Identifier
Contains the D bit and the packet sequence count (modulo 8 or modulo
128). The packet sequence window size cannot be read or written by an
X.25 application.
To read this field on an incoming packet:
This field cannot be directly read by the application. The arrival of a
DATA packet with the D bit set can be detected with an
ioctl(X25_NEXT_MSG_STAT) system call. The X.25 subsystem
automatically responds to an incoming DATA packet with the D bit set
when data is read by the application (D bit confirmation indicates that
the data has been read by the remote X.25 application).
To write this field on an outgoing packet:
Use ioctl(X25_SEND_TYPE) before issuing a send() or write() system
call.
Logical Channel
Identifier
Contains the logical channel identifier. It is controlled by the X.25
subsystem. It can be read using ioctl(X25_RD_LCI) after the call has
been established.
Logical Channel Identifier
87654321
1
2
3
Bits
Receive
General Format Identifier
Octets
User Data
0M Bit
Sequence Number
Receive
Sequence Number
36960-90061.bk Page 122 Friday, November 10, 2000 3:42 PM