SIP protocol realizes dual stream

Dual stream is a concept in video conferencing, referring to the capability of a client to simultaneously display two video streams. One is the **main video stream** (main), and the other is typically for **content sharing** (slides), also called **share content**. The "content" usually includes screen sharing, PowerPoint presentations, documents, or similar materials.

In SIP protocol implementations of dual streams, the SDP (Session Description Protocol) includes two video media tags: one labeled **main** (representing the primary video stream) and another labeled **slides** (representing the secondary stream). The SDP format distinguishes the two streams using the **`a=content`** attribute, with each stream assigned a separate RTP port.

SIP-based dual streaming requires coordination between the **SIP protocol** and the **BFCP (Binary Floor Control Protocol)** for control implementation. BFCP handles **floor requests** for permission management. For example, when a client wants to share content (e.g., documents), it initiates a BFCP floor request to other clients. Once the request is negotiated and approved, the client triggers the secondary video stream by sending a SIP **re-INVITE** message containing the updated SDP information for the second stream.