Transaction Commit Protocol (no replies)

Hi all,

As I know, each transaction in mysql cluster consists of number of operations. Each write operation is going to 1 fragment and Ndb kernel runs linear 2 phase commit to guarantee the consistency of write operations on all nodes (primary and backup) in a fragment.
But as a transaction might have operations on several fragments and physical nodes, what is the protocol that NDb uses for committing a transaction? How does it guarantee transaction atomicity?
Is there another 2PC protocol for transaction commit or not?
Is TC responsible for collecting operations responses and committing the transaction or the mysql server?

Thanks,
Nave