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

A.
ACK

B.
PRACK

C.
OPTIONS

D.
CANCEL

E.
REGISTER

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