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diff --git a/third_party/libwebrtc/pc/g3doc/rtp.md b/third_party/libwebrtc/pc/g3doc/rtp.md new file mode 100644 index 0000000000..38c1702ad3 --- /dev/null +++ b/third_party/libwebrtc/pc/g3doc/rtp.md @@ -0,0 +1,56 @@ +<?% 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 |