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RTP and RTCP

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The Real Time Transport Protocol (RTP) transports audio payload between devices.

The Real Time Transport Control Protocol (RTCP) carries the call statistics between endpoints.

Ports

RTP uses random even UDP ports between 16,384 and 32,767

RTCP uses random odd UDP ports between 16,384 and 32,767

As per RFC 3551 (RTP A/V Profile):

As specified in the RTP protocol definition, RTP
data SHOULD be carried on an even UDP port number
and the corresponding RTCP packets SHOULD be carried
on the next higher (odd) port number.
RTP Payload Types
               PT   encoding    media type  clock rate  Chan.
                    name                    (Hz)
               _________________________________________________
               0    PCMU        A            8,000       1
               1    reserved    A
               2    reserved    A
               3    GSM         A            8,000       1
               4    G723        A            8,000       1
               5    DVI4        A            8,000       1
               6    DVI4        A           16,000       1
               7    LPC         A            8,000       1
               8    PCMA        A            8,000       1
               9    G722        A            8,000       1
               10   L16         A           44,100       2
               11   L16         A           44,100       1
               12   QCELP       A            8,000       1
               13   CN          A            8,000       1
               14   MPA         A           90,000
               15   G728        A            8,000       1
               16   DVI4        A           11,025       1
               17   DVI4        A           22,050       1
               18   G729        A            8,000       1
               19   reserved    A
               20   unassigned  A
               21   unassigned  A
               22   unassigned  A
               23   unassigned  A
               dyn  G726-40     A            8,000       1
               dyn  G726-32     A            8,000       1
               dyn  G726-24     A            8,000       1
               dyn  G726-16     A            8,000       1
               dyn  G729D       A            8,000       1
               dyn  G729E       A            8,000       1
               dyn  GSM-EFR     A            8,000       1
               dyn  L8          A            var.        var.
               dyn  RED         A
               dyn  VDVI        A            var.        1

               Table 4: Payload types (PT) for audio encodings

               PT      encoding    media type  clock rate
                       name                    (Hz)
               _____________________________________________
               24      unassigned  V
               25      CelB        V           90,000
               26      JPEG        V           90,000
               27      unassigned  V
               28      nv          V           90,000
               29      unassigned  V
               30      unassigned  V
               31      H261        V           90,000
               32      MPV         V           90,000
               33      MP2T        AV          90,000
               34      H263        V           90,000
               35-71   unassigned  ?
               72-76   reserved    N/A         N/A
               77-95   unassigned  ?
               96-127  dynamic     ?
               dyn     H263-1998   V           90,000

               Table 5: Payload types (PT) for video and combined encodings

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About Behzad Munir

Behzad Munir, P.Eng, is a Voice Solutions Consultant working in Toronto, Canada

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