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There are two types of responses defined by SIP. They are provisional and final. Final responses (2xx-6xx) convey the result of the request processing and are sent reliably. SIP Provisional responses do not have an acknowledgement system so they are not reliable. There are certain scenarios in which the provisional SIP responses (1xx) must be delivered reliably. For example in a SIP/ PSTN inter-working scenario it is crucial that the 180 and 183 messages are not missed. To solve this problem the SIP PRACK (Provisional Response Acknowledgement) method described in RFC 3262 guarantees a reliable and ordered delivery of provisional responses in SIP. PRACK Improves network reliability by adding an acknowledgement system to the provisional Responses (1xx).
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