Plan B is an IETF draft proposal for signaling multiple media sources in SDP.

Signaling multiple media sources is needed to be able to send a video along with screen sharing, multiple camera sources or routing multiple streams from an SFU to the users.

In Plan B, an m= line is an “envelope” that includes multiple media sources per one defined transport.

The mechanism chosen for the official WebRTC specification isn’t Plan B but rather Unified Plan.

Plan B has been deprecated and removed from browsers.

About WebRTC Glossary

The WebRTC Glossary is an ongoing project where users can learn more about WebRTC related terms. It is maintained by Tsahi Levent-Levi of

Looking to learn more about WebRTC? 

Check my WebRTC training courses