onse 180 Ringing from the callee to the caller indicates that the callee is being alerted. The provisional response 182 Queued indicates that the call has been placed in a queue. SIP does not transmit provisional responses reliably. When a User Agent Server (UAS) returns a provisional response to the User Agent Client (UAC), it traverses the same proxies as the INVITE did but in the opposite direction. Even if proxies in the path forward this provisional response to the UAC, because UASs do not retransmit provisional responses, any router in the network may discard the IP datagram containing the SIP response. So, it might come about that the UAC never receives the provisional response (Figure 6-3). [Draft-ietf-sip-100rel] is the SIP extension defined to provide the reliable transfer of provisional responses. UASs transmitting reliable provisional responses retransmit until a message from the UAC is received acknowl- edging reception. This mechanism is similar to the one used by core SIP for 200 OK responses, which are retransmitted by the UAS until the ACK is received. A new method was defined to acknowledge the reception of provisional responses. This new method is called Provisional Response ACK (PRACK). A UAS stops retransmitting a provisional response upon reception of a PRACK from the UAC. The PRACK request belongs to a different transaction than the INVITE request. Thus, the UAS also must send a response to the PRACK. In Figure 6-4, the UAS sends a 200 OK response to the PRACK, indicating that the PRACK request succeeded. Extending SIP: The SIP Toolkit Laura Figure 6-3 Provisional responses (1) INVITE might get lost. (2) 180 Ringing (3) 200 OK (4) ACK Conversation Laura Figure 6-4 The PRACK ensures (1) INVITE that the provisional response is (2) 180 Ringing transmitted reliably. (3) 180 Ringing (4) PRACK (5) 200 OK (6) 200 OK (7) ACK Conversation Chapter 6 Figure 6-5 contains the example that was previously described. A cus- tomer is informed about his position in the call queue through reliable pro- vision