. This successful out- come is communicated back to the UAC using a NOTIFY method (described in Section 4.1.9). The message body of the NOTIFY contains a partial copy of the final response to the triggered request. In this case, it contains the start-line SIP/2.0 200 OK. This part of a SIP message is described in the Content-Type header field as a message/sipfrag [3]. An example of a REFER message is shown below: REFER sip h.rejewski@biuroszyfrow.p. SIP/2.0 Via SIP/2.0/UDP lab34.bletchleypark.co.uk 5060 branch=z9hG4bK932039 Max-Forwards: 69 To: <sip:m.rejewski@biuroszyfrow.pl>; tag=ACEBDC From: Alan Turing sip:turing@bletchleypark.co.uk> ; tag=213424 Call-ID: 3419fak3kFD23s1A9dkl CSeq: 5412 REFER Refer-To: <sip:info@scherbius-ritter.com> Content-Length: 0 Another example is the use of REFER to "push" a Web page. In the exam- ple of Figure 4.6, the UAC sends a REFER to the UAS with a Refer-To set to an HTTP URL or a Web page. This causes the UAS to send a 202 Accepted then send a HTTP GET request to the Web server identified by the SIP: Understanding the Session Initiation Protocol Chebychev Poisson REFER 202 Accepted HTTP GET NOTIFY 200 OK 200 OK NOTIFY 200 OK Figure 4.6 REFER example used to push Web page. URL. After the Web page has loaded, the UAS sends a NOTIFY containing a body and HTTP/1.0 200 OK. A REFER and the SIP request triggered by the REFER may contain the Referred-By header field (see Section 6.2.17), which contains information about who requested the request. Figure 4.7 shows an advanced use of REFER to implement a common PSTN or PBX feature known as attended transfer [4]. In this feature, the trans- feror is assumed to be in a dialog (in a session) with the transferee. The trans- feror places the transferee on hold, then sends an INVITE to another party, called the transfer target. After the session is established between the transferor and the transfer target, the transferor then puts the target on hold. Now the transferor has two on hold sessions. The transferor then sends a REFER to the transfe