RFC 3551 - RTP Profile for Audio and Video
Standard payload types and formats for multimedia RTP sessions
What is RTP A/V Profile?
RFC 3551 defines the RTP Profile for Audio and Video Conferences with Minimal Control (RTP/AVP). It specifies payload type assignments, encoding rules, and session management for multimedia conferences using RTP.
Standard Payload Types
Audio Codecs
- PT 0: PCMU (G.711 μ-law)
- PT 3: GSM
- PT 4: G.723
- PT 5: DVI4 (8kHz)
- PT 6: DVI4 (16kHz)
- PT 7: LPC
- PT 8: PCMA (G.711 A-law)
- PT 9: G.722
- PT 10: L16 (stereo)
- PT 11: L16 (mono)
Video Codecs
- PT 26: JPEG
- PT 31: H.261
- PT 32: MPV (MPEG-1 Video)
- PT 33: MP2T (MPEG-2 TS)
- PT 34: H.263
Key Features
Static Payload Types
Pre-defined payload type numbers for common audio/video formats.
Dynamic Payload Types
Range 96-127 reserved for dynamic assignment via SDP.
Clock Rates
Standardized sampling frequencies for different media types.
Channel Mapping
Audio channel assignments for mono, stereo, and multichannel.
Encoding Considerations
Audio Encoding
Sample-based encoding with consistent timestamp increments based on sampling rate.
Video Encoding
Frame-based encoding with timestamps reflecting presentation time.
Packetization
Media-specific rules for fragmenting data across RTP packets.
Session Management
- SDP Integration: Payload type negotiation via Session Description Protocol
- RTCP Feedback: Quality reporting and congestion control
- Synchronization: Audio/video lip-sync using RTCP sender reports
- Multiplexing: Multiple media streams in a single RTP session
Applications
- Video Conferencing: Multi-party audio/video communication
- VoIP Systems: Internet telephony with various audio codecs
- Streaming Media: Live and on-demand content delivery
- WebRTC: Browser-based real-time communication
- Broadcasting: Professional audio/video distribution