PCMU RTP数据包有效载荷具有额外的数据长度

Question

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.

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):

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...

Where is the extra 10 (170-160) bytes from?

Answer 1

The rtp header extension bit was set, so the 10 extra bytes are probably that. Good chance its the "urn:ietf:params:rtp-hdrext:ssrc-audio-level" extension if your getting audio from a webrtc endpoint. Have a look at https://tools.ietf.org/html/rfc6464

Answer 2

可能是SRTP身份验证标签，如果使用80位，则为10个字节。