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packet capture guide

RTP Payload Types Explained

RTP payload type numbers identify what media format is inside RTP packets. Some payload types are static, but many VoIP calls use dynamic mappings declared in SIP SDP.

Static Examples

  • Payload type 0 is commonly G.711 PCMU.
  • Payload type 8 is commonly G.711 PCMA.
  • Payload type 18 is commonly G.729 when supported.

Dynamic Payloads

Dynamic payload types such as 96 and above need rtpmap lines in SDP. Without the SDP, a packet capture may show RTP timing but not enough information to identify the codec confidently.

Troubleshooting Clue

If endpoints disagree about a payload mapping, audio can fail even though packets are present. Compare the SIP SDP offer and answer with the RTP stream.