ly load a registrar server with registration refresh requests-this response allows a registrar to protect against this. 5.4.21 428 Use Authentication Token The 428 Use Authentication Token response [5] is used by a UAS that is requiring the use of an Authentication Information Body (AIB) [6]. The AIB is a S/MIME body that is an encrypted message/sip or message/sipfrag body. At a minimum, an AIB must contain From, Date, and Call-ID header fields, and they should contain the To, Contact, and CSeq header fields as well. (The selection of these header fields is to prevent cut-and-paste attacks and hijacking.) The SIP request should contain a Content-Type: - message/sipfrag header field and a Content-Disposition aib; handling=optional header field. For more information on S/MIME signatures, see Section 3.7. 5.4.22 429 Provide Referror Identity The 429 Provide Referror Identity response [7] is used to request that a Referred-By header field be re-sent with a valid Referred-By SIP Response Messages security token. The security token is carried as an S/MIME message body. The recipient of this error message (the UA that received and accepted the REFER) should relay this request back to the originator of the REFER by including it in a NOTIFY. The sender of the REFER can then generate the Referred-By security token and include it in the REFER, which would then be copied into the triggered request. 5.4.23 480 Temporarily Unavailable This response indicates that the request has reached the correct destination, but the called party is not available for some reason. The reason phrase should be modified for this response to give the caller a better understanding of the situa- tion. The response should contain a Retry-After header indicating when the request may be able to be fulfilled. For example, this response could be sent when a telephone has its ringer turned off, or a "do not disturb" button has been pressed. This response can also be sent by a redirect server. 5.4.24 481 Dialog/Transaction Does Not Exist This response ind