Question about DDP Format and associate workflow !!

Hi,
I come from read some papers & notes about DDP format and i have some
questions ?

* I have Eclipse Data DDP File and a file in the DDP package is called
Checksum.chk. But it's not a simple checksum algorithm and i would
know if it's file is very useful in a mastering workflow and i can
change the MPEG2 content without introduce a break into the workflow.

* I suppose that the Eclipse Checksum isn't public ?

* If my change in the Main.dat file and in the MPEG2 PS don't move
MPEG2 frame offset and don't change the file size, the DDP would be
always correct ?

Any help or advice would be welcome.

Alex.