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diff --git a/third_party/libwebrtc/docs/native-code/rtp-hdrext/video-layers-allocation00/README.md b/third_party/libwebrtc/docs/native-code/rtp-hdrext/video-layers-allocation00/README.md new file mode 100644 index 0000000000..c4454d8ee1 --- /dev/null +++ b/third_party/libwebrtc/docs/native-code/rtp-hdrext/video-layers-allocation00/README.md @@ -0,0 +1,86 @@ +# Video Layers Allocation + +The goal of this extension is for a video sender to provide information about +the target bitrate, resolution and frame rate of each scalability layer in order +to aid a selective forwarding middlebox to decide which layer to relay. + +**Name:** "Video layers allocation version 0" + +**Formal name:** +<http://www.webrtc.org/experiments/rtp-hdrext/video-layers-allocation00> + +**Status:** This extension is defined here to allow for experimentation. + +In a conference scenario, a video from a single sender may be received by +several recipients with different downlink bandwidth constraints and UI +requirements. To allow this, a sender can send video with several scalability +layers and a middle box can choose a layer to relay for each receiver. + +This extension support temporal layers, multiple spatial layers sent on a single +rtp stream (SVC), or independent spatial layers sent on multiple rtp streams +(simulcast). + +## RTP header extension format + +### Data layout + +``` +// +-+-+-+-+-+-+-+-+ +// |RID| NS| sl_bm | +// +-+-+-+-+-+-+-+-+ +// Spatial layer bitmask |sl0_bm |sl1_bm | +// up to 2 bytes |---------------| +// when sl_bm == 0 |sl2_bm |sl3_bm | +// +-+-+-+-+-+-+-+-+ +// Number of temporal layers |#tl|#tl|#tl|#tl| +// per spatial layer | | | | | +// +-+-+-+-+-+-+-+-+ +// Target bitrate in kpbs | | +// per temporal layer : ... : +// leb128 encoded | | +// +-+-+-+-+-+-+-+-+ +// Resolution and framerate | | +// 5 bytes per spatial layer + width-1 for + +// (optional) | rid=0, sid=0 | +// +---------------+ +// | | +// + height-1 for + +// | rid=0, sid=0 | +// +---------------+ +// | max framerate | +// +-+-+-+-+-+-+-+-+ +// : ... : +// +-+-+-+-+-+-+-+-+ +``` + +RID: RTP stream index this allocation is sent on, numbered from 0. 2 bits. + +NS: Number of RTP streams minus one. 2 bits, thus allowing up-to 4 RTP streams. + +sl_bm: BitMask of the active Spatial Layers when same for all RTP streams or 0 +otherwise. 4 bits, thus allows up to 4 spatial layers per RTP streams. + +slX_bm: BitMask of the active Spatial Layers for RTP stream with index=X. +When NS < 2, takes one byte, otherwise uses two bytes. Zero-padded to byte +alignment. + +\#tl: 2-bit value of number of temporal layers-1, thus allowing up-to 4 temporal +layers. Values are stored in ascending order of spatial id. Zero-padded to byte +alignment. + +Target bitrate in kbps. Values are stored using leb128 encoding [1]. One value per +temporal layer. Values are stored in (RTP stream id, spatial id, temporal id) +ascending order. All bitrates are total required bitrate to receive the +corresponding layer, i.e. in simulcast mode they include only corresponding +spatial layers, in full-svc all lower spatial layers are included. All lower +temporal layers are also included. + +Resolution and framerate. Optional. Presence is inferred from the rtp header +extension size. Encoded (width - 1), 16-bit, (height - 1), 16-bit, max frame +rate 8-bit per spatial layer per RTP stream. Values are stored in (RTP stream +id, spatial id) ascending order. + +An empty layer allocation (i.e nothing sent on ssrc) is encoded as +special case with a single 0 byte. + +[1] https://aomediacodec.github.io/av1-spec/#leb128 |