RTP — Real-time Transport Protocol Deep Dive

What RTP Provides

RTP doesn't guarantee timely delivery — UDP doesn't either. What RTP provides is the information receivers need to reconstruct real-time streams even when packets arrive out of order, late, or not at all.

RTP Header (12 bytes minimum)

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|V=2|P|X|  CC   |M|     PT      |       Sequence Number         |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                           Timestamp                           |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           Synchronization Source (SSRC) identifier            |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|       Contributing Source (CSRC) identifiers (optional)       |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Key fields:

• Version (V): Always 2
• Padding (P): Last bytes are padding (for block ciphers)
• Extension (X): Header extension present
• CSRC Count (CC): Number of contributing sources
• Marker (M): Interpretation depends on payload type (often "start of talkspurt")
• Payload Type (PT): Codec identifier (0 = PCMU, 8 = PCMA, etc.)
• Sequence Number: 16-bit, increments by 1 per packet, detects loss/reorder
• Timestamp: Sampling instant, used for jitter buffer timing
• SSRC: Random 32-bit identifier for this stream source

Sequence Numbers: Detecting Loss and Reordering

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