irection response must retry the request to the specified address. The UAC can be configured to automatically generate a new INVITE upon receipt of a redirection class response without requiring user intervention. In addition, proxies may also automatically send an ACK to a redirect and proxy the INVITE to the new location provided in the Contact URI of the redirec- tion. To prevent looping, the server must not return any addresses contained in the request Via header field, and the client must check the address returned in the Contact header field against all other addresses tried in an earlier call attempt. Note that this type of transaction looping is different from request looping. SIP Response Messages 5.3.1 300 Multiple Choices This redirection response contains multiple Contact header fields, which indicate that the location service has returned multiple possible locations for the sip or sips URI in the Request-URI. The order of the Contact header fields is assumed to be significant. That is, they should be tried in the order in which they were listed in the response. 5.3.2 301 Moved Permanently This redirection response contains a Contact header field with the new per- manent URI of the called party. The address can be saved and used in future INVITE requests. 5.3.3 302 Moved Temporarily This redirection response contains a URI that is currently valid but that is not permanent. As a result, the Contact header field should not be cached across calls unless an Expires header field is present, in which case the location is valid for the duration of the time specified. 5.3.4 305 Use Proxy This redirection response contains a URI that points to a proxy server who has authoritative information about the calling party. The caller should resend the request to the proxy for forwarding. This response could be sent by a UAS that is using a proxy for incoming call screening. Because the proxy makes the deci- sions for the UAS on acceptance of the call, the UAS will only respond to INVITE requests that come from the sc