title: PCMU RTP packet payload has extra length of data
body: <p>I monitor a WebRTC session, found extra unknown(I don't know) data, now I am composing the same RTP audio session, I need to know it why.</p>
  
  <p>Send PCMU (8bit, mono, 8000Hz) frame to WebRTC browser client, each frame is of 20 ms, so the payload size = 8000Hz * 1Byte(8bit) * 20ms/1000ms = 160 bytes, however in client received 170 bytes(can see from WireShark):</p>
  
  <pre><code>Frame 34012: 236 bytes on wire (1888 bits), 236 bytes captured (1888 bits) on interface 0
  Ethernet II, Src: Tp-LinkT_4d:01:68 (e4:d3:32:4d:01:68), Dst: IntelCor_cb:c6:7c (24:77:03:cb:c6:7c)
  Internet Protocol Version 4, Src: 111.198.38.42, Dst: 192.168.1.103
  User Datagram Protocol, Src Port: 48244 (48244), Dst Port: 55003 (55003)
  Real-Time Transport Protocol
      10.. .... = Version: RFC 1889 Version (2)
      ..0. .... = Padding: False
      ...1 .... = Extension: True
      .... 0000 = Contributing source identifiers count: 0
      0... .... = Marker: False
      Payload type: ITU-T G.711 PCMU (0)
      Sequence number: 36871
      Timestamp: 1038276488
      Synchronization Source identifier: 0x4994b249 (1234481737)
      Defined by profile: Unknown (0xbede)
      Extension length: 2
      Header extensions
      Payload: 8037d1facad8535b355510e6b6880e8700eabfaed41c34b7...
  </code></pre>
  
  <p>Where is the extra 10 (170-160) bytes from?
  <img src="https://i.stack.imgur.com/jfV6w.png" alt="where is the extra 10 (170-160) bytes from?"></p>