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

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 provisional responses. There is an important difference, however. PRACK is a normal SIP message, like BYE. As such, its own