 called a forking proxy. In this example the first location responds with a 404 Not Found response. The second location responds with a 180 Ringing response, while the third location returns a 180 Ringing then a 200 OK response. The caller then sends an ACK to the third location to establish the call. Because one successful response has been received, a CANCEL is sent to the second location to terminate the search. The second location sends a 200 OK to the CANCEL and a 487 Request Terminated to the INVITE. This example shows some customized reason phrases in messages M7, M10, and M11. INVITE sip :faraday@effect.org;user=ip SIP/2.0 Via: SIP/2.0/UDP 7.9.18.12:60000;branch=z9hG4bK3 <Port 60000 is used Max-Forwards: 70 From: J.C. Maxwell sip:james.maxwell@kings.cambridge.edu.uk>; tag=4 To: sip:faraday@effect.org;user=ip> Call-ID: iNjdwWjkBfWrd@7.9.18 CSeq: 54 INVITE <CSeq initialized to 54 Contact:< sip:james.maxwell@kings.cambridge.edu.uk: Content-Type: application/sdp Content-Length: 129 o=max 2890844521 2890844521 IN IP4 7.9.18.12 t=0 0 C=IN IP4 7.9.18.12 m=audio 32166 RTP/AVP 4 a=rtpmap 4 DVI/8000 SIP/2.0 300 Multiple locations <Redirect server returns Via: SIP/2.0/UDP 7.9.18.12:60000;branch=z9hG4bk3 three locations SIP: Understanding the Session Initiation Protocol Babbage Redirect Royal SIP Common room server society phone (Faraday) INVITE faraday@effect.org 300 M2 ACK M3 INVITEfaraday@lab.royalsoc.org M4 VITE+44.555.1212@sip-phone.effect.org M5 VITEfaraday@commonroom.club.gb M6 404 M7 ACK M8 180 M9 180 M10 200 M11 ACK M12 RTP media packets CANCEL M13 200 M14 487 M15 Figure 10.5 Parallel search example. From: J.C. Maxwell k<sip:james.maxwell@kings.cambridge.edu.uk> ;tag=4 To:<sip: :faraday@effect.org;user=ip>;tag=10 Call-ID: NjdwWjkBfWrd@7.9.18.12 CSeq: 54 INVITE Contact:- ip:faraday@lab.royalsoc.gb> Contact:<sip:+44.555.1212@sip-phone.effect.org;user=phon Contact: <sip:michael.faraday@commonroom.club.gb> ACK sip:faraday@effect.org;user=ip Via: SIP/2.0/UDP 7.9.18.12:60000;branch=z9hG4bk3 Max-Forwards 70 From: J