The reliability mechanism works by mirroring the current reliability
mechanisms for 2xx final responses to INVITE. Those requests are
transmitted periodically by the Transaction User (TU) until a
separate transaction, ACK, is received that indicates reception of
the 2xx by the UAC. The reliability for the 2xx responses to INVITE
and ACK messages are end-to-end. 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. As such, its own
reliability is ensured hop-by-hop through each stateful proxy. Also
like BYE, but unlike ACK, PRACK has its own response. If this were
not the case, the PRACK message could not traverse proxy servers
compliant to RFC 2543 [4].

;maxexpiry=3600 ; Maximum allowed time of incoming registrations
; and subscriptions (seconds)
;minexpiry=60 ; Minimum length of registrations/subscriptions (default 60)
;defaultexpiry=120 ; Default length of incoming/outgoing registration