In PNNI networks, Crankback is a mechanism that:

A.
Reroutes a call that is rejected at any point in the path specified because the path cannot meet the QoS requirements.

B.
Provides immediate routing without the delay inherent in waiting for topological updates that can take time to propagate through the network.

C.
Reruns a message to the node that generated the DTL, that includes information about the cause and location of the problem.

D.
All of the above.

E.
None of the above.

Explanation:
PNNI employs a crankback mechanism to reroute a call that fails CAC at any point in the path specified by the DTL. Crankback improves the call setup success rate, which can be limited by the lack of a complete view of the network topology, due to summarization of information. Crankback also provides immediate alternate rerouting without the delay inherent in waiting for topological updates that can take time to propagate through the network. For crankback, a message is returned to the node that generated the DTL, which includes crankback information about the cause and location of the problem. If the call is retried, PNNI prevents the failing node or link from being considered when it generates an alternate path. This process might occur several times before a successful path is found, or a determination is made that there is no suitable path.