MPC pad character considerations

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.