| <?% 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 |