SIP协议是会话发起协议，是一个应用层协议。

SIP建立和终止多媒体通信的5个方面：

——用户位置：确定用于通信的终端系统

——用户可用性：确定被呼叫者参加通信的意愿

——用户能力：确定使用的媒体和媒体参数

——会话建立：“响铃”方式建立会话

——会话管理：包括传输和终止会话、修改会话参数以及调用服务

SIP同时和其他协议一起使用形成多媒体架构：

——RTP：实时传输协议

——RTSP：实时传输流协议

——SDP：会话描述协议

SIP会话流程：

其中，在2个会话主体之间，间隔着两个SIP代理，代理服务器是用来代理多个SIP终端的服务器，进行客户端信令的重定向转发。

注册请求不建立对话，是对SIP代理服务器（或注册服务器）进行注册本地的URI地址绑定。

该请求的答复为200 OK信令。

Request-Line: REGISTER sip:172.16.80.80 SIP/2.0

Method: REGISTER

Request-URI: sip:172.16.80.80

Request-URI Host Part: 172.16.80.80

[Resent Packet: False]

Message Header

From: ;tag=11c587c0-0-13c4-525d6966-566380ee-525d6966

To:

Call-ID: 11c673a0-0-13c4-525d6966-3fa1b031-525d6966

CSeq: 1 REGISTER

Via: SIP/2.0/UDP 172.16.178.25:5060;branch=z9hG4bK-525d6966-bce3b6f1-180cfdf

User-Agent: 7920 2010 3 26 0.1

Max-Forwards: 70

Contact:

Expires: 300

Content-Length: 0

Status-Line: SIP/2.0 200 OK

Status-Code: 200

[Resent Packet: False]

[Request Frame: 48]

[Response Time (ms): 26]

Message Header

From: ;tag=11c587c0-0-13c4-525d6966-566380ee-525d6966

To: ;tag=2360c00-0-13c4-62b9-52fe56a4-62b9

Call-ID: 11c673a0-0-13c4-525d6966-3fa1b031-525d6966

CSeq: 1 REGISTER

Contact: ;expires=30

Via: SIP/2.0/UDP 172.16.178.25:5060;branch=z9hG4bK-525d6966-bce3b6f1-180cfdf

Content-Length: 0

Request-Line: INVITE sip:[email protected]:5060 SIP/2.0

Method: INVITE

Request-URI: sip:[email protected]:5060;transport=UDP

Request-URI User Part: 17826

Request-URI Host Part: 172.16.178.26

Request-URI Host Port: 5060

[Resent Packet: False]

Message Header

From: ;tag=11c2d510-0-13c4-521dff22-6a332c0d-521dff22

To:

Call-ID: [email protected]

CSeq: 1 INVITE

Via: SIP/2.0/UDP 172.16.178.24:5060;branch=z9hG4bK-521dff22-c52c9d49-700a1e12

Allow: INVITE,BYE,CANCEL,ACK,INFO,REGISTER,OPTIONS

User-Agent: 7920 2010 3 26 0.1

Max-Forwards: 70

Contact:

Content-Type: application/SDP

Content-Length: 792

Message Body

Session Description Protocol Version (v): 0

Owner/Creator, Session Id (o): 7920 1377697570 1 IN IP4 172.16.178.24

Session Information (i): 7920 2010 3 26 0.1

Connection Information (c): IN IP4 172.16.178.24

Bandwidth Information (b): AS:2048

Time Description, active time (t): 0 0

Media Description, name and address (m): audio 60040 RTP/AVP 96 0 98 9 15 8 18 102 103 99

Media Attribute (a): rtpmap:96 mp3/8000

Media Attribute (a): rtpmap:0 PCMU/8000

Media Attribute (a): rtpmap:98 G7221.C/8000

Media Attribute (a): rtpmap:9 g722-64k/8000

Media Attribute (a): rtpmap:15 G728/8000

Media Attribute (a): rtpmap:8 PCMA/8000

Media Attribute (a): rtpmap:18 G729/8000

Media Attribute (a): rtpmap:102 MP4A-LATM/8000

Media Attribute (a): rtpmap:103 MP4A-LATM/8000

Media Attribute (a): rtpmap:99 G719/8000

Media Attribute (a): fmtp:98 bitrate=24000

Media Attribute (a): fmtp:102 object=2

Media Attribute (a): fmtp:103 object=23

Media Attribute (a): rtcp:60049

Media Attribute (a): sendrecv

Media Description, name and address (m): video 60042 RTP/AVP 106 97 34

Bandwidth Information (b): AS:2048

Media Attribute (a): rtpmap:106 H264/90000

Media Attribute (a): rtpmap:97 MPEG4/90000

Media Attribute (a): rtpmap:34 H263/90000

Media Attribute (a): fmtp:106 profile-level-id=40804e; max-mbps=243000; max-fs=8192;

Media Attribute (a): fmtp:97 AUTO=1;CIF16=1;CIF4=1;CIF2=1;CIF=1;

Media Attribute (a): fmtp:0 CIF=1;

Media Attribute (a): rtcp:60051

Media Attribute (a): sendrecv

Status-Line: SIP/2.0 100 Trying

Status-Code: 100

[Resent Packet: False]

[Request Frame: 927]

[Response Time (ms): 34]

Message Header

From: ;tag=11c2d510-0-13c4-521dff22-6a332c0d-521dff22

To:

Call-ID: [email protected]

CSeq: 2 INVITE

Via: SIP/2.0/UDP 172.16.178.24:5060;branch=z9hG4bK-521dff22-c52c9db3-67c76d03

Contact:

Content-Length: 0

Status-Line: SIP/2.0 180 Ringing

Status-Code: 180

[Resent Packet: False]

[Request Frame: 927]

[Response Time (ms): 34]

Message Header

From: ;tag=11c2d510-0-13c4-521dff22-6a332c0d-521dff22

To: ;tag=3143fe8-0-13c4-521dff2a-6f440c17-521dff2a

Call-ID: [email protected]

CSeq: 2 INVITE

Via: SIP/2.0/UDP 172.16.178.24:5060;branch=z9hG4bK-521dff22-c52c9db3-67c76d03

Contact:

Content-Length: 0

Status-Line: SIP/2.0 200 OK

Status-Code: 200

[Resent Packet: False]

[Request Frame: 927]

[Response Time (ms): 55]

Message Header

From: ;tag=11c2d510-0-13c4-521dff22-6a332c0d-521dff22

To: ;tag=3143fe8-0-13c4-521dff2a-6f440c17-521dff2a

Call-ID: [email protected]

CSeq: 2 INVITE

Allow: INVITE,BYE,CANCEL,ACK,INFO,REGISTER,OPTIONS,UPDATE

Supported: 100rel

User-Agent:   UNKNOWN  

Level: Level_County

ScreenNum: 0

Via: SIP/2.0/UDP 172.16.178.24:5060;branch=z9hG4bK-521dff22-c52c9db3-67c76d03

Contact:

Content-Type: application/SDP

Content-Length: 798

Message Body

Session Description Protocol Version (v): 0

Owner/Creator, Session Id (o): 7920B 1377697570 1 IN IP4 172.16.178.26

Session Information (i): 7920B V1.4

Connection Information (c): IN IP4 172.16.178.26

Bandwidth Information (b): AS:12590

Time Description, active time (t): 0 0

Media Description, name and address (m): audio 60040 RTP/AVP 8 0 98 9 15 96 18 99

Connection Information (c): IN IP4 172.16.178.26

Media Attribute (a): fmtp:98 bitrate=24000

Media Attribute (a): rtcp:60049

Media Attribute (a): tpattrib:capsign:0 capscreen:-1

Media Attribute (a): rtpmap:8 PCMA/8000

Media Attribute (a): rtpmap:0 PCMU/8000

Media Attribute (a): rtpmap:98 G7221.C/8000

Media Attribute (a): rtpmap:9 g722-64k/8000

Media Attribute (a): rtpmap:15 G728/8000

Media Attribute (a): rtpmap:96 mp3/8000

Media Attribute (a): rtpmap:18 G729/8000

Media Attribute (a): rtpmap:99 G719/8000

Media Attribute (a): sendrecv

Media Description, name and address (m): video 60042 RTP/AVP 106 97 34

Connection Information (c): IN IP4 172.16.178.26

Bandwidth Information (b): AS:8192

Media Attribute (a): fmtp:106 profile-level-id=42801f; max-mbps=216000; max-fs=3840;

Media Attribute (a): fmtp:97 AUTO=1;CIF16=1;CIF4=1;CIF2=1;CIF=1;

Media Attribute (a): fmtp:34 CIF=1;

Media Attribute (a): rtcp:60051

Media Attribute (a): tpattrib:capsign:0 capscreen:-1

Media Attribute (a): rtpmap:106 H264/90000

Media Attribute (a): rtpmap:97 MPEG4/90000

Media Attribute (a): rtpmap:34 H263/90000

Media Attribute (a): sendrecv

Request-Line: ACK sip:[email protected]:5060;transport=UDP SIP/2.0

Method: ACK

Request-URI: sip:[email protected]:5060;transport=UDP

[Resent Packet: False]

[Request Frame: 927]

[Response Time (ms): 159]

Message Header

From: ;tag=11c2d510-0-13c4-521dff22-6a332c0d-521dff22

To: ;tag=3143fe8-0-13c4-521dff2a-6f440c17-521dff2a

Call-ID: [email protected]

CSeq: 2 ACK

Via: SIP/2.0/UDP 172.16.178.24:5060;branch=z9hG4bK-521dff22-c52c9e56-4838bad0

Max-Forwards: 70

Contact:

Content-Length: 0

Request-Line: INFO sip:[email protected]:5060;transport=UDP SIP/2.0

Method: INFO

Request-URI: sip:[email protected]:5060;transport=UDP

[Resent Packet: False]

Message Header

From: ;tag=3143fe8-0-13c4-521dff2a-6f440c17-521dff2a

To: ;tag=11c2d510-0-13c4-521dff22-6a332c0d-521dff22

Call-ID: [email protected]

CSeq: 1 INFO

Via: SIP/2.0/UDP 172.16.178.26:5060;branch=z9hG4bK-521dff2a-c52cbeae-5f3840e7

Max-Forwards: 70

Contact:

Content-Type: application/media_control+xml

Content-Length: 198

Message Body

eXtensible Markup Language

Status-Line: SIP/2.0 200 OK

Status-Code: 200

[Resent Packet: False]

Message Header

From: ;tag=3143fe8-0-13c4-521dff2a-6f440c17-521dff2a

To: ;tag=11c2d510-0-13c4-521dff22-6a332c0d-521dff22

Call-ID: [email protected]

CSeq: 1 INFO

Via: SIP/2.0/UDP 172.16.178.26:5060;branch=z9hG4bK-521dff2a-c52cbeae-5f3840e7

Contact:

Content-Length: 0