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doc: document rtp payload type mapping behaviour

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documents the various payload type mappers used by WebRTC.

BUG=webrtc:12194,webrtc:12295
No-try: true

Change-Id: I88e2388f54e72db5e5fe0f72fa4fe4c455c99679
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/220936
Commit-Queue: Philipp Hancke <phancke@nvidia.com>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34227}
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Philipp Hancke 2021-06-04 06:58:53 +02:00 committed by WebRTC LUCI CQ
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commit d25af8ceac
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g3doc/sitemap.md
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* STUN
* TURN
* [DTLS](/pc/g3doc/dtls_transport.md)
* [RTP](/pc/g3doc/rtp.md)
* [SRTP](/pc/g3doc/srtp.md)
* [SCTP](/pc/g3doc/sctp_transport.md)
* [Pacing buffer](/modules/pacing/g3doc/index.md)

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<?% config.freshness.reviewed = '2021-06-03' %?>
<?% config.freshness.owner = 'hta' %?>
# RTP in WebRTC
WebRTC uses the RTP protocol described in [RFC3550](https://datatracker.ietf.org/doc/html/rfc3550) for transporting audio and video.
Media is encrypted using [SRTP](./srtp.md).
## Allocation of payload types
RTP packets have a payload type field that describes which media codec can be used to handle a packet. For some (older) codecs like PCMU the payload type is assigned statically as described in [RFC3551](https://datatracker.ietf.org/doc/html/rfc3551). For others, it is assigned dynamically through the SDP.
**Note:** there are no guarantees on the stability of a payload type assignment.
For this allocation, the range from 96 to 127 is used. When this range is exhausted, the allocation falls back to the range from 35 to 63 as permitted by [section 5.1 of RFC3550][1]. Note that older versions of WebRTC failed to recognize payload types in the lower range.
Newer codecs (such as flexfec-03 and AV1) will by default be allocated in that range.
Payload types in the range 64 to 95 are not used to avoid confusion with RTCP as described in [RFC5761](https://datatracker.ietf.org/doc/html/rfc5761).
## Allocation of audio payload types
Audio payload types are assigned from a table by the [PayloadTypeMapper][2] class.
New audio codecs should be allocated in the lower dynamic range [35,63], starting at 63, to reduce collisions with payload types
## Allocation of video payload types
Video payload types are allocated by the [GetPayloadTypesAndDefaultCodecs method][3].
The set of codecs depends on the platform, in particular for H264 codecs and their different profiles.
Payload numbers are assigned ascending from 96 for video codecs and their [associated retransmission format](https://datatracker.ietf.org/doc/html/rfc4588).
Some codecs like flexfec-03 and AV1 are assigned to the lower range [35,63] for reasons explained above.
When the upper range [96,127] is exhausted, payload types are assigned to the lower range [35,63], starting at 35.
## Handling of payload type collisions
Due to the requirement that payload types must be uniquely identifiable when using [BUNDLE](https://datatracker.ietf.org/doc/html/rfc8829) collisions between the assignments of the audio and video payload types may arise. These are resolved by the [UsedPayloadTypes][4] class which will reassign payload type numbers descending from 127.
[1]: https://datatracker.ietf.org/doc/html/rfc3550#section-5.1
[2]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/media/engine/payload_type_mapper.cc;l=25;drc=4f26a3c7e8e20e0e0ca4ca67a6ebdf3f5543dc3f
[3]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/media/engine/webrtc_video_engine.cc;l=119;drc=b412efdb780c86e6530493afa403783d14985347
[4]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/pc/used_ids.h;l=94;drc=b412efdb780c86e6530493afa403783d14985347