98/99 Darpa Test Dataset

Hi there!
I am now engaged in separating the 98/99 Darpa BSM dataset, that is, I separate the BSM audit data according to sessions. But I can't get the right answer.
For example, when I process the sample dataset, I can only get 38 sessions, while the BSM list of the sample dataset comes up with 64 sessions.
Anyone here has ever come across this problem?
And what if in the BSM audit record, the session id equals '0'? What does it mean?
Thanks so much!