SDP (Session Description Protocol) is the standard describing a peer-to-peer connection. SDP contains the codec, source address, and timing information of audio and video.
Here is a typical SDP message:
o=alice 2890844526 2890844526 IN IP4 host.anywhere.com
c=IN IP4 host.anywhere.com
m=audio 49170 RTP/AVP 0
m=video 51372 RTP/AVP 31
m=video 53000 RTP/AVP 32
SDP is never used alone, but by protocols like RTP and RTSP. SDP is also a component of WebRTC, which uses SDP as a way of describing a session.