From d25af8ceac786684fed6c801b0f9c5e76443b640 Mon Sep 17 00:00:00 2001 From: Philipp Hancke Date: Fri, 4 Jun 2021 06:58:53 +0200 Subject: [PATCH] doc: document rtp payload type mapping behaviour 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 Reviewed-by: Harald Alvestrand Cr-Commit-Position: refs/heads/master@{#34227} --- g3doc/sitemap.md | 1 + pc/g3doc/rtp.md | 35 +++++++++++++++++++++++++++++++++++ 2 files changed, 36 insertions(+) create mode 100644 pc/g3doc/rtp.md diff --git a/g3doc/sitemap.md b/g3doc/sitemap.md index aa9f09dc23..f54bf4d21a 100644 --- a/g3doc/sitemap.md +++ b/g3doc/sitemap.md @@ -12,6 +12,7 @@ * 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) diff --git a/pc/g3doc/rtp.md b/pc/g3doc/rtp.md new file mode 100644 index 0000000000..271a93973e --- /dev/null +++ b/pc/g3doc/rtp.md @@ -0,0 +1,35 @@ + + + +# 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