Type: message/isup Content-Length: 16 51a6324134527 The mandatory headers in an INFO request are shown in Table 4.12. 4.1.12 PRACK The PRACK [17] 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 deter- mining 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 (see Section 6.3.10) and a Supported: 100rel header. The PRACK echoes the number in the RSeq and the CSeq of the response in a RACK header. The message flow is as shown in Figure 4.5. In this example, the UAC sends the 180 Ringing response reliably by including the RSeq header. When no PRACK is received from the UAC after the expiration of a timer, the response is retransmitted. The receipt of the PRACK confirms the delivery of the response and stops all further transmis- sions. The 200 OK response to the PRACK stops retransmissions of the PRACK request. The call completes when the UAC sends the ACK in response to the 200 OK. Table 4.12 Mandatory Headers in an INFO Request Mandatory Headers Call-ID Max-Forwards SIP Request Messages Reliable responses are retransmitted using the same exponential backoff mechanism used for final responses to an INVITE. The combination of Call - ID, CSeq number, and RACK number allows the UAC to match the PRACK to the provisional response it is acknowledging. As shown in Figure 4.5, the PRACK receives a 200 OK response, which can be distinguished from the 200 OK to the INVITE by the method contained in the CSeq header. The detailed use of the method (see Figure 4.11) is described in Sections 6.2.29 and 6.3.10 (where the RAC