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)	channels	Ref
0	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
19	reserved	A
20	unassigned	A
21	unassigned	A
22	unassigned	A
23	unassigned	A
24	unassigned	V
25	CelB	V	90000		RFC2029
26	JPEG	V	90000		RFC2435
27	unassigned	V
28	nv	V	90000		RFC3551
29	unassigned	V
30	unassigned	V
31	H261	V	90000		RFC2032
32	MPV	V	90000		RFC2250
33	MP2T	AV	90000		RFC2250
34	H263	V	90000		Zhu
35--71	unassigned	?
72--76	reserved for RTCP conflict avoidance				RFC3550
77--95	unassigned	?
96--127	dynamic	?			RFC3551

• IANA: Registred RTP Parameters : http://www.iana.org/assignments/rtp-parameters

RTP and NAT

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.

Articles

• RTCP XR measures VoIP performance Network World 11/17/03

RFCs

• 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

• Open Source VOIP Software RTP protocol stacks and related software
• RTP Packet Format with field descriptions
• http://www.cs.columbia.edu/~hgs/rtp/: All you want to or need to know about RTP
• SRTP: Secure RTP
• IETF
• SIP: Session initiation protocol
• MGCP
• RTP Silence Suppression Also known as VAD
• What is SIP?