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The BSP application is an odd-ball when debugging is concerned... We're not dealing with SAP screens...First of all: debugging for internet services (external debugging) or BSP applications can be done as usual: set your break-point in the ABAP coding you expect to be called. However: do make sure you visit Utilities -> Settings from transaction SE80. Tab “ABAP Editor” then “Debugging”. External debugging should be switched on. 

There is an OSS note on debugging ITS applications:

OSS note number: 722735 Description: This note provides information about debugging IAC

Also: From transaction SICF, quite a variety of tools for debugging and trace options are available to you. When you run into any trouble, find the tools to help you pin-point what went wrong..