RTP opens two ports for communication. One for the media stream (an even port number) and one for control (QoS feedback and media control) - RTCP. The port numbers are not hard defined, it depends very much upon the application.

  • RTP (Real-time Transport Protocol)
  • RTCP (Real-time Control Protocol)
    • Adds information for:
    • Packet Loss
    • Jitter
    • Delay
    • Signal Level
    • Call Quality Metrics
    • Echo Return Loss
    • etc.
  • RTCP XR (Real-time Control Protocol Extended Reports)
    • All of the RTCP list above plus:
    • R Factor
    • MOS
    • and more

Actual voice packets are sent using RTP/RTCP for SIP VOIP calls. RTP is able to carry media identified by parameters registred by the Internet assigned numbers authority, IANA. These are also used for SDP descriptions in SIP and MGCP messages.

Some of these payloads:

PT encoding name audio/video (A/V) clock rate (Hz) channelsRef
0 G711 PCMU A 8000 1 RFC3551
3 GSM A 8000 1 RFC3551
4 G723 A 8000 1 Kumar
5 DVI4 A 8000 1 RFC3551
6 DVI4 A 16000 1 RFC3551
7 LPC A 8000 1 RFC3551
8 PCMA A 8000 1 RFC3551
9 G722 A 8000 1 RFC3551
10 L16 A 44100 2 RFC3551
11 L16 A 44100 1 RFC3551
12 QCELP A 8000 1 -
13 CN A 8000 1 RFC3389
14 MPA A 90000 RFC3551,RFC2250
15 G728 A 8000 1 RFC3551
16 DVI4 A 11025 1 DiPol
17 DVI4 A 22050 1 DiPol
18 G729 A 8000 1
25CelBV90000 RFC2029
26JPEGV90000 RFC2435
28nvV90000 RFC3551
31H261V90000 RFC2032
32MPVV90000 RFC2250
33MP2TAV90000 RFC2250
34H263V90000 Zhu
72--76reserved for RTCP conflict avoidance RFC3550
96--127dynamic? RFC3551


In a VOIP session, there are two RTP streams, one in each direction. If one of the parties involved in the session is on a private IP address, that stream from the public client to the NAT box, will not be allowed to reach the client on the inside of the NAT. To handle this, Symmetric RTP is often used. For more information on NAT and VOIP, see NAT and VOIP.



  • IETF RFC 3550 RTP: A Transport Protocol for Real-Time Applications
  • IETF RFC 3611 RTP Control Protocol Extended Reports (RTCP XR)
  • IETF RFC 1890 RTP Profile for Audio and Video Conferences with Minimal Control
  • IETF RFC 2508 Compressing IP/UDP/RTP Headers for Low-Speed Serial Links
  • IETF RFC 3545 Enhanced Compressed RTP (CRTP) for Links with High Delay, Packet Loss and Reordering

See also

Created by: jht2, Last modification: Sun 06 of May, 2012 (20:55 UTC) by admin
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