RTP Payload Types

A SIP INVITE, which contains an RTP setup, might look like the following:

06:51:13.859941 IP 68.97.22.35.5060 > 215.29.95.2.5060: UDP, length: 793
E..5..@.:..^.O....!......!..INVITE sip:+12125551212@215.29.95.2:5060 SIP/2.0
Record-Route: 
Via: SIP/2.0/UDP 68.97.22.35;branch=z9hG4bK969b.79d3c062.0
Via: SIP/2.0/UDP 68.97.22.48:5060;branch=z9hG4bK506071629460-1138413119228
From: "Raleigh NC" ;tag=VPSF506071629460
To: 
Call-ID: LAXMGC0720060215165205035267@68.97.22.99
CSeq: 1 INVITE
Contact: 
Max-Forwards: 16
Content-Type: application/sdp
Content-Length: 117
Remote-Party-ID: "Raleigh NC" ;party=calling;screen=yes;privacy=off

v=0
o=- 1140022325 1140022326 IN IP4 68.97.22.86
s=-
c=IN IP4 68.97.22.86
t=0 0
m=audio 60888 RTP/AVP 0 18

The last line (m=audio...) sets up the codec types for the RTP streams among other things. In this example, the line ends with 0 and 18. 0 stands for PCM uLaw or G.711 "compression". 18 stands for G.729 compression. The complete list of codecs are below:

RTP Payload types (PT) for standard audio and video encodings - Closed
======================================================================

The RFC "RTP Profile for Audio and Video Conferences with Minimal
Control" [RFC3551] specifies an initial set "payload types".  This
list mantains and extends that list.


      PT         encoding      audio/video    clock rate    channels
                 name          (A/V)          (Hz)          (audio)
      _______________________________________________________________
      0          PCMU          A              8000          1     [RFC3551]
      1          Reserved        
      2          Reserved
      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]

This info from iana rtp parameters. See also RFC 3551.

Once to every man and nation,
comes the moment to decide,
In the strife of truth with falsehood,
for the good or evil side;
Some great cause,
some great decision,
offering each the bloom or blight,
And the choice goes by forever,
'twixt that darkness and that light.

Once to Every Man and Nation.mp3
~ James R. Lowell - 1845

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