关于IMS/SIP中的无应答前转（Communication Forwarding on no Reply-CFNR)是当被叫未做出应答时处理方式之一，其具体在3GPP TS24.604有具体定义；

1.终端注册，参与无应答前转的终端(UA)需要：

          A.在CSCF进行注册并预设前转条件；

          B.当某一终端（UA）发起的呼叫响应（180 Ringing）超时后，根据预设进行呼叫前转；

2.无应答前转信令流程：

3.无应答前转主要消息内容

(A) INVITE sip:UA1@ims.mnc246.mcc081.3gppnetwork.org SIP/2.0

Via: SIP/2.0/TCP [2001:0:0:1::2]:59417;branch=z9hG4bK62bc484e8cdd4ddc8e563c19815bf51aac5bd1e1;transport=tcp

Max-Forwards: 70

Call-ID: cf01b053c04a414ab24990140d4d2297

CSeq: 38001 INVITE

To: sip:UA1@ims.mnc246.mcc081.3gppnetwork.org

From: sip:UA2@ims.mnc246.mcc081.3gppnetwork.org;tag=1111111111

Feature-Caps: *;+g.3gpp.srvcc-alerting;+g.3gpp.mid-call

Allow: ACK, BYE, CANCEL, INVITE, PRACK, UPDATE

Accept-Contact: *;+g.3gpp.icsi-ref="urn%3Aurn-7%3A3gpp-service.ims.icsi.mmtel";require;explicit

Privacy: none

User-Agent: Sharetechnote-VirtualUA/7e501f1

Content-Type: application/sdp

Content-Length: 728

Route: <sip:[2001:0:0:1::2]:28101;lr>

Contact: <sip:UA2@[2001:0:0:1::2]:59417;transport=tcp>;+g.3gpp.icsi-ref="urn%3Aurn-7%3A3gpp-service.ims.icsi.mmtel";video;+g.3gpp.srvcc-alerting;+g.3gpp.mid-call

v=0

o=Sharetechnote 583 102583 IN IP6 2001:0:0:1::2

s=-

i=A VOIP Session

c=IN IP6 2001:0:0:1::2

t=0 0

m=audio 60000 RTP/AVP 96 97 98 99 100 8 0 101 102

b=AS:153

b=RS:800

b=RR:2400

a=ptime:20

a=maxptime:240

a=rtpmap:96 EVS/16000/1

a=fmtp:96 max-red=220

a=rtpmap:97 AMR-WB/16000/1

a=fmtp:97 mode-change-capability=2

a=rtpmap:98 AMR-WB/16000/1

a=fmtp:98 octet-align=1; mode-change-capability=2

a=rtpmap:99 AMR/8000/1

a=fmtp:99 mode-change-capability=2

a=rtpmap:100 AMR/8000/1

a=fmtp:100 octet-align=1; mode-change-capability=2

a=rtpmap:8 PCMA/8000/1

a=rtpmap:0 PCMU/8000/1

a=rtpmap:101 telephone-event/16000

a=fmtp:101 0-15

a=rtpmap:102 telephone-event/8000

a=fmtp:102 0-15

a=mid:0

a=rtcp:60001

a=sendrecv

(C) 180 Ringing（UA1送回铃音给CSCF)

Via: SIP/2.0/UDP [2001:0:0:1::2]:5060;branch=z9hG4bKc7d5dede360c4bb5a88be02f843e7c4598;rport;transport=udp

Via: SIP/2.0/TCP [2001:0:0:1::2]:59417;branch=z9hG4bK62bc484e8cdd4ddc8e563c19815bf51aac5bd1e1;transport=tcp

Max-Forwards: 70

Record-Route: <sip:[2001:0:0:1::2]:5060;lr>

Contact: <sip:UA1@[2001:0:0:1::11]:5060>;+sip.instance="<urn:gsma:imei:35975507-290776-0>";+g.3gpp.icsi-ref="urn%3Aurn-7%3A3gpp-service.ims.icsi.mmtel";+g.3gpp.mid-call;+g.3gpp.srvcc-alerting;+g.3gpp.ps2cs-srvcc-orig-pre-alerting;video

To: <sip:UA1@ims.mnc246.mcc081.3gppnetwork.org>;tag=c0eff945

From: <sip:UA2@ims.mnc246.mcc081.3gppnetwork.org>;tag=1111111111

Call-ID: cf01b053c04a414ab24990140d4d2297

CSeq: 38001 INVITE

Supported: timer, 100rel

User-Agent: T-Mobile VoLTE-ePDG-IR94 SEC/G930TUVU4Z 7.0

P-Access-Network-Info: 3GPP-E-UTRAN-FDD;utran-cell-id-3gpp=0010100010000000

Content-Length: 0

(D) 180 Ringing（CSCF送回铃音给UA2)

Via: SIP/2.0/TCP [2001:0:0:1::2]:59417;branch=z9hG4bK62bc484e8cdd4ddc8e563c19815bf51aac5bd1e1;transport=tcp

Max-Forwards: 70

Record-Route: <sip:[2001:0:0:1::2]:28101;lr>

Contact: <sip:UA1@[2001:0:0:1::11]:5060>;+sip.instance="<urn:gsma:imei:35975507-290776-0>";+g.3gpp.icsi-ref="urn%3Aurn-7%3A3gpp-service.ims.icsi.mmtel";+g.3gpp.mid-call;+g.3gpp.srvcc-alerting;+g.3gpp.ps2cs-srvcc-orig-pre-alerting;video

To: <sip:UA1@ims.mnc246.mcc081.3gppnetwork.org>;tag=c0eff945

From: <sip:UA2@ims.mnc246.mcc081.3gppnetwork.org>;tag=1111111111

Call-ID: cf01b053c04a414ab24990140d4d2297

CSeq: 38001 INVITE

Supported: timer, 100rel

User-Agent: T-Mobile VoLTE-ePDG-IR94 SEC/G930TUVU4Z 7.0

P-Access-Network-Info: 3GPP-E-UTRAN-FDD;utran-cell-id-3gpp=0010100010000000

Content-Length: 0

(E) CANCEL sip:UA1@[2001:0:0:1::11]:5060 SIP/2.0(呼叫超时，CSCF撤消UA1发起的呼叫）

Max-Forwards: 70

Call-ID: cf01b053c04a414ab24990140d4d2297

CSeq: 38001 CANCEL

To: sip:UA1@ims.mnc246.mcc081.3gppnetwork.org

From: sip:UA2@ims.mnc246.mcc081.3gppnetwork.org;tag=1111111111

Via: SIP/2.0/UDP [2001:0:0:1::2]:5060;branch=z9hG4bKc7d5dede360c4bb5a88be02f843e7c4598;rport;transport=udp

Via: SIP/2.0/TCP [2001:0:0:1::2]:59417;branch=z9hG4bK62bc484e8cdd4ddc8e563c19815bf51aac5bd1e1;transport=tcp

Content-Length: 0  

(F) 181 Call Is Being Forwarded（呼叫前转）

Max-Forwards: 70

Via: SIP/2.0/TCP [2001:0:0:1::2]:59417;branch=z9hG4bK62bc484e8cdd4ddc8e563c19815bf51aac5bd1e1;transport=tcp

From: sip:UA2@ims.mnc246.mcc081.3gppnetwork.org;tag=1111111111

To: sip:UA1@ims.mnc246.mcc081.3gppnetwork.org;tag=c0eff945

Call-ID: cf01b053c04a414ab24990140d4d2297

CSeq: 38001 INVITE

History-Info: <sip:UA1@ims.mnc246.mcc081.3gppnetwork.org>; index=1

History-Info: <sip:UA1@ims.mnc246.mcc081.3gppnetwork.org>; index=1.1; NP=1

Content-Length: 0 

(G) INVITE sip:UA3@[2001:0:0:1::2]:54990;transport=udp;cause=408 SIP/2.0（呼叫INVITE预设对象UA3)

Via: SIP/2.0/UDP [2001:0:0:1::2]:5060;branch=z9hG4bKc7d5dede360c4bb5a88be02f843e7c459d;rport;transport=udp

Via: SIP/2.0/TCP [2001:0:0:1::2]:59417;branch=z9hG4bK62bc484e8cdd4ddc8e563c19815bf51aac5bd1e1;transport=tcp

Max-Forwards: 69

Call-ID: cf01b053c04a414ab24990140d4d2297

CSeq: 38001 INVITE

To: sip:UA3@test.3gpp.com

From: sip:UA2@ims.mnc246.mcc081.3gppnetwork.org;tag=1111111111

Feature-Caps: *;+g.3gpp.srvcc-alerting;+g.3gpp.mid-call

Allow: ACK, BYE, CANCEL, INVITE, PRACK, UPDATE

Accept-Contact: *;+g.3gpp.icsi-ref="urn%3Aurn-7%3A3gpp-service.ims.icsi.mmtel";require;explicit

User-Agent: Sharetechnote-VirtualUA/7e501f1

Content-Type: application/sdp

Content-Length: 728

Contact: <sip:UA2@[2001:0:0:1::2]:59417;transport=tcp>;+g.3gpp.icsi-ref="urn%3Aurn-7%3A3gpp-service.ims.icsi.mmtel";video;+g.3gpp.srvcc-alerting;+g.3gpp.mid-call

Privacy: none

P-Asserted-Identity: <sip:UA2@ims.mnc246.mcc081.3gppnetwork.org>

History-Info: <sip:UA1@ims.mnc246.mcc081.3gppnetwork.org>; index=1

History-Info: <sip:UA1@ims.mnc246.mcc081.3gppnetwork.org>; index=1.1; NP=1

原文来自：http://www.sharetechnote.com

整理：kangguoying20201016

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