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3.1.5.3 Representing new Payload Types

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This protocol adds support for eight new payload types:

  • RTAudio for audio streams.

  • RED for redundant audio streams.

  • G722-Stereo for audio streams.

  • SILK for audio streams.

  • RTVideo for video streams.

  • H.264UC for video streams.

  • RTData for application sharing streams.

  • ULPFEC-UC for video streams.

The media formats of these payload types are described by the parameters in the following table that SHOULD<1> be specified using the a=rtpmap: and a=fmtp: attributes for dynamic payload types, as specified in [RFC4566].

Payload

Encoding name

Clock rate

Bit rate

RTAudio

x-msrta

16000

29000

RTAudio

x-msrta

8000

11800

RED

RED

8000

Not Applicable: the a=fmtp attribute SHOULD NOT be present in an SDP offer for RED.

RTVideo

x-rtvc1

90000

Not applicable: bitrate fmtp parameter is significant only for audio. bitrate SHOULD NOT be present in an SDP offer or SDP answer for RTVideo and MUST be ignored.

RTData

x-data

90000

Not applicable: bitrate fmtp parameter is significant only for audio. bitrate SHOULD NOT be present in an SDP offer or SDP answer for RTData and MUST be ignored.

H.264UC<2>

X-H264UC

90000

Not applicable: bitrate fmtp parameter is significant only for audio. bitrate SHOULD NOT be present in an SDP offer or SDP answer for H.264UC and MUST be ignored.

However, the a=fmtp: attribute for H.264UC MUST specify two parameters: packetization-mode=1 and mst-mode=NI-TC. These two parameters can appear in any order and are case-insensitive.

Also, an a=x-ssrc-range media-level attribute MUST be present. The length of the range given MUST be at least 30.

If either the a=fmtp or a=x-ssrc-range attribute requirement is not met, the H.264UC video codec in an SDP offer or SDP answer MUST be ignored.

G722-Stereo<3>

G722

8000/2

Notes:

1. The actual clock rate of the codec is 16000, but MUST be listed in SDP as 8000.

2. The "/2" is not part of the clock rate but indicates that the codec has 2 channels.

128000

SILK<4>

SILK

16000

Not Applicable: the SILK audio codec uses a variable bitrate.

The a=fmtp attribute SHOULD specify usedtx=0 and either useinbandfec=0 or useinbandfec=1 parameters.

SILK

SILK

8000

Not Applicable: the SILK audio codec uses a variable bitrate.

The a=fmtp attribute SHOULD specify usedtx=0 and either useinbandfec=0 or useinbandfec=1 parameters.

ULPFEC-UC<5>

x-ulpfecuc

90000

Not Applicable. bitrate fmtp parameter is significant only for audio. bitrate SHOULD NOT be present in an SDP offer or SDP answer for ULPFEC-UC and MUST be ignored.

As an example, the following SDP message fragment shows the RTAudio payload type, SILK payload types and G722-Stereo payload type of an audio stream:

m=audio 37632 RTP/AVP 117 104 114 103 ...
…
a=rtpmap:117 G722/8000/2
a=fmtp:117 bitrate=128000
…
a=rtpmap:104 SILK/16000
a=fmtp:104 useinbandfec=1; usedtx=0
…
a=rtpmap:114 x-msrta/16000
a=fmtp:114 bitrate=29000
…
a=rtpmap:103 SILK/8000
a=fmtp:103 useinbandfec=1; usedtx=0
…

Negotiation of these payload types are similar to the negotiation of other payload types, as specified in [RFC3264]. Any dynamic payload type can be chosen for these payloads following the RTP profile for audio and video conferences with minimum control specification in [RFC3551]<6>. Specifying these parameters in the a=rtpmap: attribute in a media description section of an SDP message indicates the preference of these codecs for that payload type.

Applications that do not support these codecs MUST NOT advertise these codecs in an SDP message. In the case of RTP, if a particular codec was referenced with a specific payload type number specified in the a=rtpmap: attribute in the offer, that same payload type number MUST be used for that codec in the answer.

If a user agent supports the H.264UC video codec, it SHOULD also support the ULPFEC-UC video FEC media format. For more information on H.264UC and ULPFEC-UC, refer to [MS-H264PF].