erstand, it would return a 416 response. Since all SIP elements must understand the sip scheme, the request should be retried using a sip uri in the request-uri. 5.4.17 420 Bad Extension This response indicates that the extension specified in the Require header field is not supported by the proxy or user agent. The response should contain Supported header field (Section 6.1.12) listing the extensions that are sup- ported. The UAC could resubmit the same request without the extension in the Require header field (Section 6.2.22) or submit the request to another proxy or user agent. 5.4.18 421 Extension Required The 421 Extension Required response indicates that a server requires an extension to process the request that was not present in a Supported header field in the request. The required extension should be listed in a Required header field in the response. The client should retry the request SIP: Understanding the Session Initiation Protocol adding the extension to a Supported header field, or try the request at a dif- ferent server that may not require the extension. 5.4.19 422 Session Timer Interval Too Small The 422 Session Timer Interval Too Small response [4] is used to reject a request containing a Session-Expires header field (Section 6.2.26) with too short an interval. The ability to reject short durations is important to prevent excessive re-INVITE or UPDATE traffic. The mini- mum allowed interval is indicated in the required Min-SE header field (Section 6.3.4). The requestor may retry the request without the Session- Expires header field or with a value less than or equal to the specified minimum. 5.4.20 423 Interval Too Brief The 423 Interval Too Brief response is returned by a registrar that is rejecting a registration request because the requested expiration time on one or more Contacts is too brief. The response must contain a Min-Expires header field (see Section 6.3.3) listing the minimum expiration interval that the registrar will accept. A client requesting a too short interval can unnecessari