MPC sends pad bytes in order to maintain 2KB IDAW alignment.
The pad bytes are the bytes in CSM storage that are surrounding the
session data that is being transmitted to the partner LU. Although
the pad bytes are not sent to the end point destination, the pad bytes
are transmitted potentially over an unsecured channel resource. Therefore,
steps must be taken in order to ensure that any residual data in the
CSM storage from the previous use of the CSM storage area are not
transmitted over a potentially unsecured channel facility.
Both the application and VTAM® share
the responsibility for ensuring that residual data is not sent as
MPC pad characters. In this discussion, the term
data image creator denotes
the software entity that originally allocated the CSM storage and
placed data into the storage. The data image creator is responsible
for ensuring that MPC pad characters are cleared. If an application
is the data image creator and is sending data using the XBUFLST option,
the application is responsible for clearing certain residual data
in CSM storage as follows:
- For any data passed in a CSM buffer on an APPCCMD that does not
end on a 2K boundary, the area after the data up to the next 2K boundary
must be cleared (unless the data is known to be non-sensitive).
- If the data start is not 2K boundary-aligned, any residual data
between the previous 2KB boundary and the data start must be cleared
(unless the data is known to be nonsensitive).
An application resending data from CSM storage that was
previously received on an APPCCMD macroinstruction should not clear
residual data because it is not the data image creator.
In this case, the application should copy any 2K portion of data that
does not start or end on a 2K boundary, clearing pad areas as previously
described.
Note: While data sent using the HPDT interface can be aligned
on any storage address boundary, data aligned starting on a 2 KB boundary
and sent in 2 KB increments minimizes the transmission of MPC pad
characters, and therefore increases effective throughput on the channel.