在 IMS/SIP中的未注册前转（Communication Forwarding on Not Logged-in -CFNL) 是指被叫未完成在CSCF中注册的呼叫前转；如下图所示：参与终端UA1,UA2和UA3中UA1,UA3已完成注册，UA2未注册；

1.未注册前转流程

        根据3GPP TS24.604中的A.1.3（呼叫）未注册前转的主要流程如下：

2.未注册前转主要消息

2.1  (A) INVITE sip:UA2@test.3gpp.com SIP/2.0

Via: SIP/2.0/UDP [2001:0:0:1::2]:50329;branch=z9hG4bK996978744;rport

From: <sip:UA1@test.3gpp.com>;tag=996990843

To: <sip:UA2@test.3gpp.com>

Contact: <sip:UA1@[2001:0:0:1::2]:50329;transport=udp>;+g.oma.sip-im;language="en,fr";+g.3gpp.icsi-ref="urn%3Aurn-7%3A3gpp-service.ims.icsi.mmtel"

Call-ID: 8c8faa31-bf9c-3adf-fb9e-1571d98e4073

CSeq: 8139 INVITE

Content-Type: application/sdp

Content-Length: 740

Max-Forwards: 70

Route: <sip:[2001:0:0:1::2]:5060;lr;transport=udp>

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

P-Preferred-Service: urn:urn-7:3gpp-service.ims.icsi.mmtel

Allow: INVITE, ACK, CANCEL, BYE, MESSAGE, OPTIONS, NOTIFY, PRACK, UPDATE, REFER

Privacy: none

P-Access-Network-Info: ADSL;utran-cell-id-3gpp=00000000

User-Agent: IM-client/OMA1.0 Boghe/v2.0.112.744

P-Preferred-Identity: <sip:123456789@test.3gpp.com>

Supported: 100rel

v=0

o=doubango 1983 678901 IN IP6 2001:0:0:1::2

s=-

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

t=0 0

m=audio 26306 RTP/AVP 112 111 3 8 0 9 97 98 99 101

a=ptime:20

a=rtpmap:112 AMR/8000/1

a=imageattr:112 octet-align=1

a=fmtp:112 octet-align=1

a=rtpmap:111 AMR/8000/1

a=imageattr:111 octet-align=0

a=fmtp:111 octet-align=0

a=rtpmap:3 GSM/8000/1

a=rtpmap:8 PCMA/8000/1

a=rtpmap:0 PCMU/8000/1

a=rtpmap:9 G722/8000/1

a=rtpmap:97 SPEEX/8000/1

a=rtpmap:98 SPEEX/16000/1

a=rtpmap:99 SPEEX/32000/1

a=rtpmap:101 telephone-event/8000/1

a=fmtp:101 0-15

a=tcap:1 RTP/AVPF

a=pcfg:1 t=1

a=sendrecv

a=rtcp-mux

a=ssrc:1146886926 cname:ldjWoB60jbyQlR6e

a=ssrc:1146886926 mslabel:6994f7d1-6ce9-4fbd-acfd-84e5131ca2e2

a=ssrc:1146886926 label:Doubango

2.2(B) 181 Call Is Being Forwarded

Max-Forwards: 70

Via: SIP/2.0/UDP [2001:0:0:1::2]:50329;branch=z9hG4bK996978744;rport=50329

From: <sip:UA1@test.3gpp.com>;tag=996990843

To: <sip:UA2@test.3gpp.com>;tag=987654321

Call-ID: 8c8faa31-bf9c-3adf-fb9e-1571d98e4073

CSeq: 8139 INVITE

History-Info: <sip:223456789@test.3gpp.com>; index=1

Content-Length: 0

2.3(C) INVITE sip:UA3@[2001:0:0:1::11]:5060;cause=404 SIP/2.0

Via: SIP/2.0/UDP [2001:0:0:1::2]:5060;branch=z9hG4bKe5a451b9d19443a79242368f24597c6111d;rport;transport=udp

Via: SIP/2.0/UDP [2001:0:0:1::2]:50329;branch=z9hG4bK996978744;rport=50329

From: <sip:UA1@test.3gpp.com>;tag=996990843

To: <sip:UA3@ims.mnc246.mcc081.3gppnetwork.org>

Contact: <sip:UA1@[2001:0:0:1::2]:50329;transport=udp>;+g.oma.sip-im;language="en,fr";+g.3gpp.icsi-ref="urn%3Aurn-7%3A3gpp-service.ims.icsi.mmtel"

Call-ID: 8c8faa31-bf9c-3adf-fb9e-1571d98e4073

CSeq: 8139 INVITE

Content-Type: application/sdp

Content-Length: 740

Max-Forwards: 69

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

P-Preferred-Service: urn:urn-7:3gpp-service.ims.icsi.mmtel

Allow: INVITE, ACK, CANCEL, BYE, MESSAGE, OPTIONS, NOTIFY, PRACK, UPDATE, REFER

Privacy: none

P-Access-Network-Info: ADSL;utran-cell-id-3gpp=00000000

User-Agent: IM-client/OMA1.0 Boghe/v2.0.112.744

Supported: 100rel

P-Asserted-Identity: <sip:123456789@test.3gpp.com>

History-Info: <sip:223456789@test.3gpp.com>; index=1

History-Info: <sip:001010123456789@ims.mnc246.mcc081.3gppnetwork.org?cause=404>; index=1.1; MP=1

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

v=0

o=doubango 1983 678901 IN IP6 2001:0:0:1::2

s=-

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

t=0 0

m=audio 26306 RTP/AVP 112 111 3 8 0 9 97 98 99 101

a=ptime:20

a=rtpmap:112 AMR/8000/1

a=imageattr:112 octet-align=1

a=fmtp:112 octet-align=1

a=rtpmap:111 AMR/8000/1

a=imageattr:111 octet-align=0

a=fmtp:111 octet-align=0

a=rtpmap:3 GSM/8000/1

a=rtpmap:8 PCMA/8000/1

a=rtpmap:0 PCMU/8000/1

a=rtpmap:9 G722/8000/1

a=rtpmap:97 SPEEX/8000/1

a=rtpmap:98 SPEEX/16000/1

a=rtpmap:99 SPEEX/32000/1

a=rtpmap:101 telephone-event/8000/1

a=fmtp:101 0-15

a=tcap:1 RTP/AVPF

a=pcfg:1 t=1

a=sendrecv

a=rtcp-mux

a=ssrc:1146886926 cname:ldjWoB60jbyQlR6e

a=ssrc:1146886926 mslabel:6994f7d1-6ce9-4fbd-acfd-84e5131ca2e2

a=ssrc:1146886926 label:Doubango

原文来自：http://www.sharetechnote.com

整理：kangguoying20201018

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