WebRTC - Session Description Protocol

The SDP is an important part of the WebRTC. It is a protocol that is intended to describe media communication sessions. It does not deliver the media data but is used for negotiation between peers of various audio and video codecs, network topologies, and other device information. It also needs to be easily transportable. Simply put we need a string-based profile with all the information about the user's device. This is where SDP comes in.
The SDP is a well-known method of establishing media connections as it appeared in the late 90s. It has been used in a vast amount of other types of applications before WebRTC like phone and text-based chatting.
The SDP is string data containing sets of key-value pairs, separated by line breaks −
key = value\n

The key is a single character that sets the type of the value. The value is a machine-readable configuration value.
The SDP covers media description and media constraints for a given user. When we start using RTCPeerConnection object later we will be able easily print this to the javascript console.
The SDP is the first part of the peer connection. Peers have to exchange SDP data with the help of the signaling channel in order to establish a connection.
This is an example of an SDP offer −
v=0 
o=- 487255629242026503 2 IN IP4 127.0.0.1 
s=- 
t=0 0 

a=group:BUNDLE audio video 
a=msid-semantic: WMS 6x9ZxQZqpo19FRr3Q0xsWC2JJ1lVsk2JE0sG 
m=audio 9 RTP/SAVPF 111 103 104 9 0 8 106 105 13 126 
c=IN IP4 0.0.0.0

a=rtcp:9 IN IP4 0.0.0.0 
a=ice-ufrag:8a1/LJqQMzBmYtes 
a=ice-pwd:sbfskHYHACygyHW1wVi8GZM+ 
a=ice-options:google-ice 
a=fingerprint:sha-256 28:4C:19:10:97:56:FB:22:57:9E:5A:88:28:F3:04:
   DF:37:D0:7D:55:C3:D1:59:B0:B2:81 :FB:9D:DF:CB:15:A8 
a=setup:actpass 
a=mid:audio 
a=extmap:1 urn:ietf:params:rtp-hdrext:ssrc-audio-level 
a=extmap:3 http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time 

a=sendrecv 
a=rtcp-mux 
a=rtpmap:111 opus/48000/2 
a=fmtp:111 minptime=10 
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:126 telephone-event/8000 

a=maxptime:60 
a=ssrc:3607952327 cname:v1SBHP7c76XqYcWx 
a=ssrc:3607952327 msid:6x9ZxQZqpo19FRr3Q0xsWC2JJ1lVsk2JE0sG 9eb1f6d5-c3b246fe
   -b46b-63ea11c46c74 
a=ssrc:3607952327 mslabel:6x9ZxQZqpo19FRr3Q0xsWC2JJ1lVsk2JE0sG 
a=ssrc:3607952327 label:9eb1f6d5-c3b2-46fe-b46b-63ea11c46c74 
m=video 9 RTP/SAVPF 100 116 117 96 

c=IN IP4 0.0.0.0 
a=rtcp:9 IN IP4 0.0.0.0 
a=ice-ufrag:8a1/LJqQMzBmYtes
a=ice-pwd:sbfskHYHACygyHW1wVi8GZM+ 
a=ice-options:google-ice 

a=fingerprint:sha-256 28:4C:19:10:97:56:FB:22:57:9E:5A:88:28:F3:04:
   DF:37:D0:7D:55:C3:D1:59:B0:B2:81 :FB:9D:DF:CB:15:A8 
a=setup:actpass 
a=mid:video 
a=extmap:2 urn:ietf:params:rtp-hdrext:toffset 
a=extmap:3 http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time

a=sendrecv 
a=rtcp-mux 
a=rtpmap:100 VP8/90000 
a=rtcp-fb:100 ccm fir 
a=rtcp-fb:100 nack 
a=rtcp-fb:100 nack pli 
a=rtcp-fb:100 goog-remb 
a=rtpmap:116 red/90000 
a=rtpmap:117 ulpfec/90000 
a=rtpmap:96 rtx/90000 

a=fmtp:96 apt=100 
a=ssrc-group:FID 1175220440 3592114481 
a=ssrc:1175220440 cname:v1SBHP7c76XqYcWx 
a=ssrc:1175220440 msid:6x9ZxQZqpo19FRr3Q0xsWC2JJ1lVsk2JE0sG
   43d2eec3-7116-4b29-ad33-466c9358bfb3 
a=ssrc:1175220440 mslabel:6x9ZxQZqpo19FRr3Q0xsWC2JJ1lVsk2JE0sG 
a=ssrc:1175220440 label:43d2eec3-7116-4b29-ad33-466c9358bfb3 
a=ssrc:3592114481 cname:v1SBHP7c76XqYcWx 
a=ssrc:3592114481 msid:6x9ZxQZqpo19FRr3Q0xsWC2JJ1lVsk2JE0sG
   43d2eec3-7116-4b29-ad33-466c9358bfb3 
a=ssrc:3592114481 mslabel:6x9ZxQZqpo19FRr3Q0xsWC2JJ1lVsk2JE0sG 
a=ssrc:3592114481 label:43d2eec3-7116-4b29-ad33-466c9358bfb3
This is taken from my own laptop. It is complex to understand at first glance. It starts with identifying the connection with the IP address, then sets up basic information about my request, audio and video information, encryption type. So the goal is not to understand every line, but to get familiar with it because you will never have to work with it directly.
The following is an SDP answer −
v=0 
o=- 5504016820010393753 2 IN IP4 127.0.0.1 
s=- 
t=0 0 
a=group:BUNDLE audio video 
a=msid-semantic: WMS 
m=audio 9 RTP/SAVPF 111 103 104 9 0 8 106 105 13 126 
c=IN IP4 0.0.0.0 

a=rtcp:9 IN IP4 0.0.0.0 
a=ice-ufrag:RjDpYl08FRKBqZ4A 
a=ice-pwd:wSgwewyvypHhyxrcZELBLOBO 
a=fingerprint:sha-256 28:4C:19:10:97:56:FB:22:57:9E:5A:88:28:
   F3:04:DF:37:D0:7D:55:C3:D1:59:B0:B2:81 :FB:9D:DF:CB:15:A8 
a=setup:active 
a=mid:audio 
a=extmap:1 urn:ietf:params:rtp-hdrext:ssrc-audio-level 
a=extmap:3 http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time 

a=recvonly 
a=rtcp-mux 
a=rtpmap:111 opus/48000/2 
a=fmtp:111 minptime=10 
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:126 telephone-event/8000 

a=maxptime:60 
m=video 9 RTP/SAVPF 100 116 117 96
c=IN IP4 0.0.0.0 
a=rtcp:9 IN IP4 0.0.0.0 
a=ice-ufrag:RjDpYl08FRKBqZ4A 
a=ice-pwd:wSgwewyvypHhyxrcZELBLOBO 
a=fingerprint:sha-256 28:4C:19:10:97:56:FB:22:57:9E:5A:88:28:
   F3:04:DF:37:D0:7D:55:C3:D1:59:B0:B2:81 :FB:9D:DF:CB:15:A8 
a=setup:active 
a=mid:video 
a=extmap:2 urn:ietf:params:rtp-hdrext:toffset 
a=extmap:3 http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time

a=recvonly 
a=rtcp-mux 
a=rtpmap:100 VP8/90000 
a=rtcp-fb:100 ccm fir 
a=rtcp-fb:100 nack 
a=rtcp-fb:100 nack pli 
a=rtcp-fb:100 goog-remb 
a=rtpmap:116 red/90000 
a=rtpmap:117 ulpfec/90000 
a=rtpmap:96 rtx/90000 
a=fmtp:96 apt=100
You can find more SDP examples at https://www.rfc-editor.org/rfc/rfc4317.txt as well as more detailed specification at  http://tools.ietf.org/html/rfc4566.
To sum up, the SDP acts as a text-based profile of your device to other users trying to connect to you.