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 exponential backoff.
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.