//------------------------------- 会话层 -------------------------------
// version <sdp版本号，不包括次版本号>
v=0
// 过程中有改变编码之类的操作,重新生成sdp时,session id不变,session version加1
// owner <user name> <session id> <session version> <net type> <addr type> <addr>
o=- 4571619080104764276 2 IN IP4 127.0.0.1
// session <session name>
s=-
// webrtc始终是0
// time <开始时间> <结束时间>
t=0 0
// attribute,0 1绑定一个传输通道传输,与下方的a=mid相匹配
a=group:BUNDLE 0 1
// media stream id缩写msid,webrtc media stream缩写WMS
a=msid-semantic: WMS 9TH6yxQRgyF6Yna81tiidOMebGqKms24SCEU
//------------------------------- 媒体层:以下为音频描述信息 -------------------------------
// 端口9表示不接收数据;secret audio video protocol family缩写SAVPF
// media <媒体类型> <port> <传输协议> <payload type集>
m=audio 9 UDP/TLS/RTP/SAVPF 111 103 104 9 0 8 106 105 13 110 112 113 126
// 该属性webrtc并没有使用
// connection
c=IN IP4 0.0.0.0
a=rtcp:9 IN IP4 0.0.0.0
// 以下两行ice协商过程中的安全验证信息
a=ice-ufrag:FvYc
a=ice-pwd:st4MHVcrFq+pnZgFNV2qSmW0
// trickle,即sdp里面描述媒体信息和ice候选项的信息可以分开传输
a=ice-options:trickle
// dtls协商过程中需要的认证信息,sha-256加密算法
a=fingerprint:sha-256 94:85:06:47:AD:89:09:7D:AF:E0:48:B7:58:55:1D:44:48:34:8F:4A:39:98:62:44:B0:19:BA:03:00:B9:94:66
// actpass既可以是客户端,也可以是服务器;active客户端;passive服务器
a=setup:actpass
// 前面a=group:BUNDLE中用到的媒体标识
a=mid:0
// 要在rtp头部中加入音量信息
a=extmap:1 urn:ietf:params:rtp-hdrext:ssrc-audio-level
a=extmap:2 http://www.ietf.org/id/draft-holmer-rmcat-transport-wide-cc-extensions-01
a=extmap:3 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:4 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
a=extmap:5 urn:ietf:params:rtp-hdrext:sdes:repaired-rtp-stream-id
// 仅发送,其他类型sendrecv,recvonly,sendonly,inactive
a=sendonly
// 与前面的msid相同,第二个为track id
a=msid:9TH6yxQRgyF6Yna81tiidOMebGqKms24SCEU a1987158-6a67-4b19-9931-5cd2a8d2f6f8
// rtp,rtcp使用同一个端口来传输
a=rtcp-mux
// payload type的描述
a=rtpmap:111 opus/48000/2
a=rtcp-fb:111 transport-cc
a=fmtp:111 minptime=10;useinbandfec=1
a=rtpmap:103 ISAC/16000
a=rtpmap:104 ISAC/32000
a=rtpmap:9 G722/8000
a=rtpmap:0 PCMU/8000
a=rtpmap:8 PCMA/8000
a=rtpmap:106 CN/32000
a=rtpmap:105 CN/16000
a=rtpmap:13 CN/8000
a=rtpmap:110 telephone-event/48000
a=rtpmap:112 telephone-event/32000
a=rtpmap:113 telephone-event/16000
a=rtpmap:126 telephone-event/8000
a=ssrc:1196321802 cname:kDl3rfnZtJEKUJsa
a=ssrc:1196321802 msid:9TH6yxQRgyF6Yna81tiidOMebGqKms24SCEU a1987158-6a67-4b19-9931-5cd2a8d2f6f8
a=ssrc:1196321802 mslabel:9TH6yxQRgyF6Yna81tiidOMebGqKms24SCEU
a=ssrc:1196321802 label:a1987158-6a67-4b19-9931-5cd2a8d2f6f8
//------------------------------- 媒体层:以下为视频描述信息 -------------------------------
m=video 9 UDP/TLS/RTP/SAVPF 96 97 98 99 100 101 102 122 127 121 125 107 108 109 124 120 123 119 114 115 116
c=IN IP4 0.0.0.0
// CT是设置整个会议的带宽，AS是设置单个会话的带宽，它们的单位都是kbit/s。setRemoteDescription之前修改
// bandwidth <类型>:<带宽>
b=AS:100
a=rtcp:9 IN IP4 0.0.0.0
a=ice-ufrag:FvYc
a=ice-pwd:st4MHVcrFq+pnZgFNV2qSmW0
a=ice-options:trickle
a=fingerprint:sha-256 94:85:06:47:AD:89:09:7D:AF:E0:48:B7:58:55:1D:44:48:34:8F:4A:39:98:62:44:B0:19:BA:03:00:B9:94:66
a=setup:actpass
a=mid:1
a=extmap:14 urn:ietf:params:rtp-hdrext:toffset
a=extmap:13 http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time
a=extmap:12 urn:3gpp:video-orientation
a=extmap:2 http://www.ietf.org/id/draft-holmer-rmcat-transport-wide-cc-extensions-01
a=extmap:11 http://www.webrtc.org/experiments/rtp-hdrext/playout-delay
a=extmap:6 http://www.webrtc.org/experiments/rtp-hdrext/video-content-type
a=extmap:7 http://www.webrtc.org/experiments/rtp-hdrext/video-timing
a=extmap:8 http://tools.ietf.org/html/draft-ietf-avtext-framemarking-07
a=extmap:9 http://www.webrtc.org/experiments/rtp-hdrext/color-space
a=extmap:3 urn:ietf:params:rtp-hdrext:sdes:mid
a=extmap:4 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id
a=extmap:5 urn:ietf:params:rtp-hdrext:sdes:repaired-rtp-stream-id
a=sendonly
a=msid:9TH6yxQRgyF6Yna81tiidOMebGqKms24SCEU f2b07fae-01be-4c8e-ac69-895c1d44ff87
a=rtcp-mux
// 尽可能的减少rtcp包的发送，只发丢包
a=rtcp-rsize
// payload type的描述,编码器VP8,时钟采样率90000
a=rtpmap:96 VP8/90000
// 对rtpmap 96的描述,google标准的接收端带宽评估
a=rtcp-fb:96 goog-remb
// 对rtpmap 96的描述,传输端的带宽评估
a=rtcp-fb:96 transport-cc
// 对rtpmap 96的描述,支持客户端请求i帧,codec control using RTCP feedback message缩写ccm,Full Intra Request缩写fir
a=rtcp-fb:96 ccm fir
// 支持丢包重传
a=rtcp-fb:96 nack
// 支持i帧重传
a=rtcp-fb:96 nack pli
// rtx丢包重传
a=rtpmap:97 rtx/90000
// apt关联
a=fmtp:97 apt=96
a=rtpmap:98 VP9/90000
a=rtcp-fb:98 goog-remb
a=rtcp-fb:98 transport-cc
a=rtcp-fb:98 ccm fir
a=rtcp-fb:98 nack
a=rtcp-fb:98 nack pli
a=fmtp:98 profile-id=0
a=rtpmap:99 rtx/90000
a=fmtp:99 apt=98
a=rtpmap:100 VP9/90000
a=rtcp-fb:100 goog-remb
a=rtcp-fb:100 transport-cc
a=rtcp-fb:100 ccm fir
a=rtcp-fb:100 nack
a=rtcp-fb:100 nack pli
a=fmtp:100 profile-id=2
a=rtpmap:101 rtx/90000
a=fmtp:101 apt=100
a=rtpmap:102 H264/90000
a=rtcp-fb:102 goog-remb
a=rtcp-fb:102 transport-cc
a=rtcp-fb:102 ccm fir
a=rtcp-fb:102 nack
a=rtcp-fb:102 nack pli
a=fmtp:102 level-asymmetry-allowed=1;packetization-mode=1;profile-level-id=42001f
a=rtpmap:122 rtx/90000
a=fmtp:122 apt=102
a=rtpmap:127 H264/90000
a=rtcp-fb:127 goog-remb
a=rtcp-fb:127 transport-cc
a=rtcp-fb:127 ccm fir
a=rtcp-fb:127 nack
a=rtcp-fb:127 nack pli
a=fmtp:127 level-asymmetry-allowed=1;packetization-mode=0;profile-level-id=42001f
a=rtpmap:121 rtx/90000
a=fmtp:121 apt=127
a=rtpmap:125 H264/90000
a=rtcp-fb:125 goog-remb
a=rtcp-fb:125 transport-cc
a=rtcp-fb:125 ccm fir
a=rtcp-fb:125 nack
a=rtcp-fb:125 nack pli
a=fmtp:125 level-asymmetry-allowed=1;packetization-mode=1;profile-level-id=42e01f
a=rtpmap:107 rtx/90000
a=fmtp:107 apt=125
a=rtpmap:108 H264/90000
a=rtcp-fb:108 goog-remb
a=rtcp-fb:108 transport-cc
a=rtcp-fb:108 ccm fir
a=rtcp-fb:108 nack
a=rtcp-fb:108 nack pli
a=fmtp:108 level-asymmetry-allowed=1;packetization-mode=0;profile-level-id=42e01f
a=rtpmap:109 rtx/90000
a=fmtp:109 apt=108
a=rtpmap:124 H264/90000
a=rtcp-fb:124 goog-remb
a=rtcp-fb:124 transport-cc
a=rtcp-fb:124 ccm fir
a=rtcp-fb:124 nack
a=rtcp-fb:124 nack pli
a=fmtp:124 level-asymmetry-allowed=1;packetization-mode=1;profile-level-id=4d0032
a=rtpmap:120 rtx/90000
a=fmtp:120 apt=124
a=rtpmap:123 H264/90000
a=rtcp-fb:123 goog-remb
a=rtcp-fb:123 transport-cc
a=rtcp-fb:123 ccm fir
a=rtcp-fb:123 nack
a=rtcp-fb:123 nack pli
a=fmtp:123 level-asymmetry-allowed=1;packetization-mode=1;profile-level-id=640032
a=rtpmap:119 rtx/90000
a=fmtp:119 apt=123
// fec冗余编码，rtp头部负载类型116，否则就是各编码原生负责类型
a=rtpmap:114 red/90000
a=rtpmap:115 rtx/90000
a=fmtp:115 apt=114
//支持ULP FEC
a=rtpmap:116 ulpfec/90000
// 一个cname可以对应多个ssrc
a=ssrc-group:FID 790880351 2784909276
a=ssrc:790880351 cname:kDl3rfnZtJEKUJsa
a=ssrc:790880351 msid:9TH6yxQRgyF6Yna81tiidOMebGqKms24SCEU f2b07fae-01be-4c8e-ac69-895c1d44ff87
a=ssrc:790880351 mslabel:9TH6yxQRgyF6Yna81tiidOMebGqKms24SCEU
a=ssrc:790880351 label:f2b07fae-01be-4c8e-ac69-895c1d44ff87
a=ssrc:2784909276 cname:kDl3rfnZtJEKUJsa
a=ssrc:2784909276 msid:9TH6yxQRgyF6Yna81tiidOMebGqKms24SCEU f2b07fae-01be-4c8e-ac69-895c1d44ff87
a=ssrc:2784909276 mslabel:9TH6yxQRgyF6Yna81tiidOMebGqKms24SCEU
a=ssrc:2784909276 label:f2b07fae-01be-4c8e-ac69-895c1d44ff87
candiadte
{"candidate":"candidate:3575467457 1 udp 2113937151 10.15.83.23 57857 typ host generation 0 ufrag 6R0z network-cost 999","sdpMid":"0","sdpMLineIndex":0}
{"candidate":"candidate:842163049 1 udp 1677729535 xxx.xxx.xxx.xxx 57857 typ srflx raddr 10.15.83.23 rport 57857 generation 0 ufrag 6R0z network-cost 999","sdpMid":"0","sdpMLineIndex":0}
{"candidate":"candidate:888962573 1 udp 33562367 xxx.xxx.xxx.xxx 57153 typ relay raddr xxx.xxx.xxx.xxx rport 57857 generation 0 ufrag 6R0z network-cost 999","sdpMid":"0","sdpMLineIndex":0}
- host 主机候选者，主机自己可以提供。优先级高
- srflx 反射候选者，STUN提供。优先级中
- relay 中继候选者，TURN提供。优先级低
- 10.15.83.23 57857 ip和端口
- sdpMid 与sdp中a=mid:0一致
扩展阅读
ICE： 用于Offer/Answer协议的NAT穿越的协议 rfc5245
RTP：实时应用程序的传输协议 rfc3550
SDP：会话描述协议 rfc4566
STUN rfc5389
