详细参考：WebRTC SDP 详解和剖析
这里已一个webrtc的offer和answer为例研究一下：
offer
v=0 o=- 2661928673431850918 2 IN IP4 127.0.0.1 s=- t=0 0 a=group:BUNDLE 0 1 a=extmap-allow-mixed a=msid-semantic: WMS m=audio 9 UDP/TLS/RTP/SAVPF 111 103 104 9 0 8 106 105 13 110 112 113 126 c=IN IP4 0.0.0.0 a=rtcp:9 IN IP4 0.0.0.0 a=ice-ufrag:PA7e a=ice-pwd:F1o3tHlhk6OPBtXo8IdhZCRH a=ice-options:trickle a=fingerprint:sha-256 D4:50:20:EA:EE:A6:86:59:77:3B:88:84:95:69:8A:AE:79:1A:C0:35:D9:25:EE:3F:0E:02:CB:2B:AF:99:F5:9E a=setup:actpass a=mid:0 a=extmap:1 urn:ietf:params:rtp-hdrext:ssrc-audio-level a=extmap:2 http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time a=extmap:3 http://www.ietf.org/id/draft-holmer-rmcat-transport-wide-cc-extensions-01 a=extmap:4 urn:ietf:params:rtp-hdrext:sdes:mid a=extmap:5 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id a=extmap:6 urn:ietf:params:rtp-hdrext:sdes:repaired-rtp-stream-id a=recvonly a=rtcp-mux 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 m=video 9 UDP/TLS/RTP/SAVPF 96 97 98 99 100 101 122 102 121 127 120 125 107 108 109 35 36 124 119 123 118 114 115 116 37 c=IN IP4 0.0.0.0 a=rtcp:9 IN IP4 0.0.0.0 a=ice-ufrag:PA7e a=ice-pwd:F1o3tHlhk6OPBtXo8IdhZCRH a=ice-options:trickle a=fingerprint:sha-256 D4:50:20:EA:EE:A6:86:59:77:3B:88:84:95:69:8A:AE:79:1A:C0:35:D9:25:EE:3F:0E:02:CB:2B:AF:99:F5:9E a=setup:actpass a=mid:1 a=extmap:14 urn:ietf:params:rtp-hdrext:toffset a=extmap:2 http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time a=extmap:13 urn:3gpp:video-orientation a=extmap:3 http://www.ietf.org/id/draft-holmer-rmcat-transport-wide-cc-extensions-01 a=extmap:12 http://www.webrtc.org/experiments/rtp-hdrext/playout-delay a=extmap:11 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://www.webrtc.org/experiments/rtp-hdrext/color-space a=extmap:4 urn:ietf:params:rtp-hdrext:sdes:mid a=extmap:5 urn:ietf:params:rtp-hdrext:sdes:rtp-stream-id a=extmap:6 urn:ietf:params:rtp-hdrext:sdes:repaired-rtp-stream-id a=recvonly a=rtcp-mux a=rtcp-rsize a=rtpmap:96 VP8/90000 a=rtcp-fb:96 goog-remb a=rtcp-fb:96 transport-cc a=rtcp-fb:96 ccm fir a=rtcp-fb:96 nack a=rtcp-fb:96 nack pli a=rtpmap:97 rtx/90000 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:122 VP9/90000 a=rtcp-fb:122 goog-remb a=rtcp-fb:122 transport-cc a=rtcp-fb:122 ccm fir a=rtcp-fb:122 nack a=rtcp-fb:122 nack pli a=fmtp:122 profile-id=1 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:121 rtx/90000 a=fmtp:121 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:120 rtx/90000 a=fmtp:120 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:35 AV1X/90000 a=rtcp-fb:35 goog-remb a=rtcp-fb:35 transport-cc a=rtcp-fb:35 ccm fir a=rtcp-fb:35 nack a=rtcp-fb:35 nack pli a=rtpmap:36 rtx/90000 a=fmtp:36 apt=35 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=4d001f a=rtpmap:119 rtx/90000 a=fmtp:119 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=64001f a=rtpmap:118 rtx/90000 a=fmtp:118 apt=123 a=rtpmap:114 red/90000 a=rtpmap:115 rtx/90000 a=fmtp:115 apt=114 a=rtpmap:116 ulpfec/90000 a=rtpmap:37 flexfec-03/90000 a=rtcp-fb:37 goog-remb a=rtcp-fb:37 transport-cc a=fmtp:37 repair-window=10000000View Code
answer：
v=0 o=SRS/4.0.161(Leo) 41744832 2 IN IP4 0.0.0.0 s=SRSPlaySession t=0 0 a=ice-lite a=group:BUNDLE 0 1 a=msid-semantic: WMS live/livestream m=audio 9 UDP/TLS/RTP/SAVPF 111 c=IN IP4 0.0.0.0 a=ice-ufrag:8p42d118 a=ice-pwd:ok61un195fg8q8083yy06247w0xg483s a=fingerprint:sha-256 06:5D:44:D5:6A:62:A9:2E:5F:C5:5E:1E:99:3A:9F:11:20:7B:71:B1:D3:DF:CA:70:2D:82:0F:7D:AC:DC:0C:CC a=setup:passive a=mid:0 a=extmap:3 http://www.ietf.org/id/draft-holmer-rmcat-transport-wide-cc-extensions-01 a=sendonly a=rtcp-mux a=rtcp-rsize a=rtpmap:111 opus/48000/2 a=rtcp-fb:111 transport-cc a=ssrc:216302819 cname:85n6wsoy1t00k452 a=ssrc:216302819 label:audio-68w76144 a=candidate:0 1 udp 2130706431 10.151.3.77 8000 typ host generation 0 m=video 9 UDP/TLS/RTP/SAVPF 125 c=IN IP4 0.0.0.0 a=ice-ufrag:8p42d118 a=ice-pwd:ok61un195fg8q8083yy06247w0xg483s a=fingerprint:sha-256 06:5D:44:D5:6A:62:A9:2E:5F:C5:5E:1E:99:3A:9F:11:20:7B:71:B1:D3:DF:CA:70:2D:82:0F:7D:AC:DC:0C:CC a=setup:passive a=mid:1 a=extmap:3 http://www.ietf.org/id/draft-holmer-rmcat-transport-wide-cc-extensions-01 a=sendonly a=rtcp-mux a=rtcp-rsize a=rtpmap:125 H264/90000 a=rtcp-fb:125 transport-cc 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=ssrc:216302820 cname:85n6wsoy1t00k452 a=ssrc:216302820 label:video-35j24601 a=candidate:0 1 udp 2130706431 10.151.3.77 8000 typ host generation 0View Code