The PRACK method is used to acknowledge receipt of RELIABLY transported provisional responses (1XX). The reliability of 2xx, 3xx, 4xx, 5xx, and 6xx responses to INVITEs is achieved using the ACK method. However, in cases where a provisional RESPONSE, such as 180 Ringing, is critical in determining the call state, it may be necessary for the receipt of a provisional response to be confirmed. The PRACK method applies to all provisional responses except the 100 Trying response, which is never reliably transported. A PRACK is generated by a UAC when a provisional response has been received containing a RSeq reliable sequence number and a Supported: 100rel HEADER. The PRACK COPY the number in the RSeq and the CSeq of the response in a RAck header.

The PRACK method is used to acknowledge receipt of reliably transported provisional responses (1xx). The reliability of 2xx, 3xx, 4xx, 5xx, and 6xx responses to INVITEs is achieved using the ACK method. However, in cases where a provisional response, such as 180 Ringing, is critical in determining the call state, it may be necessary for the receipt of a provisional response to be confirmed. The PRACK method applies to all provisional responses except the 100 Trying response, which is never reliably transported. A PRACK is generated by a UAC when a provisional response has been received containing a RSeq reliable sequence number and a Supported: 100rel header. The PRACK copy the number in the RSeq and the CSeq of the response in a RAck header.