Which SIP request method enables reliability of SIP 1xx response types?

Which SIP request method enables reliability of SIP 1xx response types?
A . ACK
B . PRACK
C . OPTIONS
D . CANCEL
E . REGISTER

Answer: B

Explanation:

In order to achieve reliability for provisional responses, we do nearly the same thing. Reliable provisional responses are retransmitted by the TU with an exponentialbackoff. Those retransmissions cease when a PRACK message is received. The PRACK request plays the same role as ACK, but for provisional responses. There is an important difference, however. PRACK is a normal SIP message, like BYE. As such, its own