propriate signature goes here) -divider42- -unique-boundary-1- An unsigned Referred-By header field may be rejected requesting that the Referred-By security token be included using the 429 Provide Referror Identity response code (see Section 5.4.22). An example using the compact form is: b: <sips:friend@neighbor.org> 6.2.22 Reply-To The Reply-To header field is used to indicate a sip or sips URI, which should be used in replying to this request. Normally, this URI is present in the From header field (the Contact is not used as it is only assumed valid for the duration of the dialog). However, in some cases, the From cannot be populated SIP: Understanding the Session Initiation Protocol with this information, SO the URI in this header field should be used instead of the From URI. An example is: Reply-To: <sip:1.tolstoy@stpetersburg.ru> 6.2.23 Replaces The Replaces header field [11] is used in SIP call control applications. A user agent in an established dialog receiving another INVITE with a Replaces header field that matches the existing dialog must accept the INVITE, termi- nate the existing dialog with a BYE, and transfer all resources and state from the existing dialog to the newly established dialog. If the Replaces header field matches no dialog, the INVITE must be rejected with a 481 Dialog Does Not Exist response. In addition, Replaces has one application in pending dialogs. A UAC that has sent an INVITE but has not yet received a final response may receive an INVITE containing a Replaces header field that matches the pending INVITE. The UAC must terminate the pending dialog with a CANCEL (and be prepared to send a ACK and BYE if a 200 OK eventually arrives) and accept the new INVITE. For an INVITE containing both a Require: replaces and Replaces header field, this results in the return of one of the following set of responses: 200 (if a match is found); 481 (if no match is found); 420 Replaces is not supported). Figure 6.4 shows a call flow using Replaces to implement a feature called "call pickup." Figur