NetIPC 3000/XL Programmer's Reference Manual (5958-8600)

38 Chapter1
NetIPC Fundamentals
Direct Access to Level 3 (X.25)
Note that these steps do not show how to synchronize data transfer
between the two programs, and do not include error checking, or the
intrinsic calls required for adding options and special user capabilities.
Access to the Call User Data (CUD) Field
The NetIPC intrinsics IPCCONNECT, IPCRECVCN, IPCCONTROL, IPCRECV
(on connection establishment), and IPCSHUTDOWN (with fast select)
provide access to the call user data (CUD) field in call packets as
follows:
• Specifying a protocol relative address in the CUD.
This field may be present in X.25 call request and incoming call
packets which you can access with IPCCONNECT and IPCRECVCN. The
call user data field can only be accessed over an SVC. The maximum
length of the call user data (CUD) field is normally 16 bytes. The
CUD can be up to 128 bytes if the fast select facility is available. For
NS X.25, the first four bytes of the CUD are reserved for protocol
relative addressing. Figure 1-12 shows the contents of the first four
bytes of the HP 3000 X.25 CUD. The first two bytes, as shown in
Figure 1-12, indicate that the source of the call request packet is an
HP 3000 node using direct access to level 3. Optionally, the last two
bytes contain the protocol relative address that the call request
expects to find (if any).
To access the entire CUD (16 bytes without fast select or 128 bytes
with fast select), the
opt parameter protocol flags bit 17 can be
set in IPCCONNECT. This option is useful for communication with
non-HP nodes.
See the discussion of the Fast Select Facility for examples using
NetIPC intrinsics to send and/or receive call user data using fast
select.
Figure 1-12 NS X.25 Call User Data Field (four bytes)
Byte
FC (hex)
AA (hex)
protocol relative address
protocol relative address
0
1
2
3