mpletion. If first informational or provisional response. Success Request has succeeded. If for an INVITE, ACK should be sent; otherwise, stop retransmissions of request. Redirection Server has returned possible locations. The client should retry request at another server. Client error The request has failed due to an error by the client. The client may retry the request if reformulated according to response. Server failure The request has failed due to an error by the server. The request may be retried at another server. Global failure The request has failed. The request should not be tried again at this or other servers. created for SIP typically start at x80 to try to avoid collisions with HTTP response codes. Unless otherwise referenced, the responses described here are defined in RFC 3261 [1]. 5.1 Informational The informational class of responses 1xx are used to indicate call progress. Informational responses are end-to-end responses and may contain message bodies. The exception to this is the 100 Trying response, which is only a hop-by-hop response and may not contain a message body. Any number of informational responses can be sent by a UAS prior to a final response (2xx, 3xx, 4xx, 5xx, or 6xx class response) being sent. The first informational response received by the UAC confirms receipt of the INVITE, and stops retransmission of the INVITE, as described in Section 3.5. For this reason, servers returning 100 Trying responses minimizes INVITE retransmissions in the network. Further informational responses have no effect on INVITE retransmissions. A stateful proxy receiving a retransmission of an INVITE will resend the last provisional response sent to date. Informational responses are optional-a UAS can send a final response without first sending an informa- tional response. While final responses to an INVITE receive an ACK to confirm SIP Response Messages receipt, provisional responses are not acknowledged, except using the PRACK method described in Section 4.1.12. All provisional responses with the exc