livekit/pkg/sfu/buffer/buffer.go

// Copyright 2023 LiveKit, Inc.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package buffer

import (
	"encoding/binary"
	"errors"
	"io"
	"math/rand/v2"
	"os"
	"strconv"

	"github.com/pion/rtcp"
	"github.com/pion/rtp"
	"github.com/pion/webrtc/v4"

	sutils "github.com/livekit/livekit-server/pkg/utils"
	"github.com/livekit/mediatransportutil/pkg/bucket"
	"github.com/livekit/mediatransportutil/pkg/twcc"
	"github.com/livekit/protocol/livekit"
	"github.com/livekit/protocol/utils/mono"
)

const (
	rtcpReceiverReportDelta = 1e9

	InitPacketBufferSizeVideo = 300
	InitPacketBufferSizeAudio = 70

	flexFECStatsLogInterval = int64(30 * 1e9) // 30s
)

var (
	errInvalidCodec = errors.New("invalid codec")
)

// debugRxDropPercent enables receive-side fault injection for testing loss
// recovery (NACK/RTX, FlexFEC) without OS-level traffic shaping. Packets are
// dropped before any processing, like wire loss. Test harnesses only, see
// scripts/fec/README.md.
var debugRxDropPercent = func() float64 {
	v := os.Getenv("LIVEKIT_DEBUG_RX_DROP_PCT")
	if v == "" {
		return 0
	}
	pct, err := strconv.ParseFloat(v, 64)
	if err != nil || pct <= 0 {
		return 0
	}
	return pct
}()

var _ BufferProvider = (*Buffer)(nil)

type pendingPacket struct {
	arrivalTime int64
	packet      []byte
}

// Buffer contains all packets
type Buffer struct {
	*BufferBase

	pPackets     []pendingPacket
	lastReportAt int64
	isBound      bool

	twcc      *twcc.Responder
	twccExtID uint8

	enableAudioLossProxying  bool
	lastFractionLostToReport uint8 // Last fraction lost from subscribers, should report to publisher; Audio only

	lastPacketRead int

	// callbacks
	onClose         func()
	onRtcpFeedback  func([]rtcp.Packet)
	onFinalRtpStats func(*livekit.RTPStats)
	onNotifyRTX     func(uint32, uint32, string)

	primaryBufferForRTX *Buffer
	rtxPktBuf           []byte

	primaryBufferForFEC *Buffer
	fecDecoder          *flexFECDecoder
	fecInjecting        bool
	lastFECStatsLogAt   int64
	lastFECStatsLogged  FECStreamStats
}

func NewBuffer(ssrc uint32, maxVideoPkts, maxAudioPkts int) *Buffer {
	b := &Buffer{}
	b.BufferBase = NewBufferBase(BufferBaseParams{
		SSRC:               ssrc,
		MaxVideoPkts:       maxVideoPkts,
		MaxAudioPkts:       maxAudioPkts,
		LoggerComponents:   []string{sutils.ComponentPub, sutils.ComponentSFU},
		SendPLI:            b.sendPLI,
		IsReportingEnabled: true,
	})
	return b
}

func (b *Buffer) SetTWCCAndExtID(twcc *twcc.Responder, extID uint8) {
	b.Lock()
	defer b.Unlock()

	b.twcc = twcc
	b.twccExtID = extID
}

func (b *Buffer) SetAudioLossProxying(enable bool) {
	b.Lock()
	defer b.Unlock()

	b.enableAudioLossProxying = enable
}

func (b *Buffer) Bind(params webrtc.RTPParameters, codec webrtc.RTPCodecCapability, bitrates int) error {
	b.Lock()
	if b.isBound {
		b.Unlock()
		return nil
	}

	if err := b.BufferBase.BindLocked(params, codec, bitrates); err != nil {
		b.Unlock()
		return err
	}

	b.lastReportAt = mono.UnixNano()

	if len(b.pPackets) != 0 {
		b.logger.Debugw("releasing queued packets on bind", "count", len(b.pPackets))
	}
	var rtcpPackets []rtcp.Packet
	for _, pp := range b.pPackets {
		rtcpPackets = append(rtcpPackets, b.calc(pp.packet, nil, pp.arrivalTime, true, false)...)
	}
	b.pPackets = nil

	b.isBound = true
	b.Unlock()

	if len(rtcpPackets) != 0 {
		if cb := b.getOnRtcpFeedback(); cb != nil {
			cb(rtcpPackets)
		}
	}

	return nil
}

// Write adds an RTP Packet, ordering is not guaranteed, newer packets may arrive later
func (b *Buffer) Write(pkt []byte) (n int, err error) {
	var rtpPacket rtp.Packet
	err = rtpPacket.Unmarshal(pkt)
	if err != nil {
		return
	}

	if debugRxDropPercent > 0 && rand.Float64()*100 < debugRxDropPercent {
		return len(pkt), nil
	}

	b.Lock()
	if b.BufferBase.IsClosed() {
		b.Unlock()
		err = io.EOF
		return
	}

	now := mono.UnixNano()
	if b.twcc != nil && b.twccExtID != 0 {
		if ext := rtpPacket.GetExtension(b.twccExtID); ext != nil {
			b.twcc.Push(rtpPacket.SSRC, binary.BigEndian.Uint16(ext[0:2]), now, rtpPacket.Marker)
		}
	}

	// libwebrtc will use 0 ssrc for probing, don't push the packet to pending queue to avoid memory increasing since
	// the Bind will not be called to consume the pending packets. More details in https://github.com/pion/webrtc/pull/2816
	if rtpPacket.SSRC == 0 {
		b.Unlock()
		return
	}

	// handle RTX packet
	if pb := b.primaryBufferForRTX; pb != nil {
		b.Unlock()

		// skip padding only packets
		if rtpPacket.Padding && len(rtpPacket.Payload) == 0 {
			return
		}

		pb.writeRTX(&rtpPacket, now)
		return
	}

	// handle FlexFEC packet
	if pb := b.primaryBufferForFEC; pb != nil {
		b.Unlock()

		// skip padding only packets
		if rtpPacket.Padding && len(rtpPacket.Payload) == 0 {
			return
		}

		pb.writeFEC(&rtpPacket, now)
		return
	}

	if !b.isBound {
		packet := make([]byte, len(pkt))
		copy(packet, pkt)

		if len(b.pPackets) == 0 {
			b.logger.Debugw("received first packet")
		}

		startIdx := 0
		overflow := len(b.pPackets) - max(b.BufferBase.MaxVideoPkts(), b.BufferBase.MaxAudioPkts())
		if overflow > 0 {
			startIdx = overflow
		}
		b.pPackets = append(b.pPackets[startIdx:], pendingPacket{
			packet:      packet,
			arrivalTime: now,
		})

		b.BufferBase.NotifyRead()
		b.Unlock()
		return
	}

	rtcpPackets := b.calc(pkt, &rtpPacket, now, false, false)
	b.Unlock()

	if len(rtcpPackets) != 0 {
		if cb := b.getOnRtcpFeedback(); cb != nil {
			cb(rtcpPackets)
		}
	}
	return
}

func (b *Buffer) SetPrimaryBufferForRTX(primaryBuffer *Buffer) {
	b.Lock()
	b.primaryBufferForRTX = primaryBuffer
	pkts := b.pPackets
	b.pPackets = nil
	b.Unlock()

	for _, pp := range pkts {
		var rtpPacket rtp.Packet
		err := rtpPacket.Unmarshal(pp.packet)
		if err != nil {
			continue
		}
		if rtpPacket.Padding && len(rtpPacket.Payload) == 0 {
			continue
		}
		primaryBuffer.writeRTX(&rtpPacket, pp.arrivalTime)
	}
}

func (b *Buffer) NotifyRTX(ssrc uint32, repairSSRC uint32, rsid string) {
	if onNotifyRTX := b.getOnNotifyRTX(); onNotifyRTX != nil {
		onNotifyRTX(ssrc, repairSSRC, rsid)
	}
}

func (b *Buffer) SetPrimaryBufferForFEC(primaryBuffer *Buffer) {
	b.Lock()
	b.primaryBufferForFEC = primaryBuffer
	pkts := b.pPackets
	b.pPackets = nil
	b.Unlock()

	for _, pp := range pkts {
		var rtpPacket rtp.Packet
		err := rtpPacket.Unmarshal(pp.packet)
		if err != nil {
			continue
		}
		if rtpPacket.Padding && len(rtpPacket.Payload) == 0 {
			continue
		}
		primaryBuffer.writeFEC(&rtpPacket, pp.arrivalTime)
	}
}

func (b *Buffer) writeRTX(rtxPkt *rtp.Packet, arrivalTime int64) {
	b.Lock()
	defer b.Unlock()
	if !b.isBound {
		return
	}

	if rtxPkt.PayloadType != b.rtxPayloadType {
		b.logger.Debugw("unexpected rtx payload type", "expected", b.rtxPayloadType, "actual", rtxPkt.PayloadType)
		return
	}

	if b.rtxPktBuf == nil {
		b.rtxPktBuf = make([]byte, bucket.RTPMaxPktSize)
	}

	if len(rtxPkt.Payload) < 2 {
		b.logger.Warnw("rtx payload too short", nil, "size", len(rtxPkt.Payload))
		return
	}

	repairedPkt := *rtxPkt
	repairedPkt.PayloadType = b.payloadType
	repairedPkt.SequenceNumber = binary.BigEndian.Uint16(rtxPkt.Payload[:2])
	repairedPkt.SSRC = b.BufferBase.SSRC()
	repairedPkt.Payload = rtxPkt.Payload[2:]
	n, err := repairedPkt.MarshalTo(b.rtxPktBuf)
	if err != nil {
		b.logger.Errorw("could not marshal repaired packet", err, "ssrc", b.BufferBase.SSRC(), "sn", repairedPkt.SequenceNumber)
		return
	}

	b.calc(b.rtxPktBuf[:n], &repairedPkt, arrivalTime, false, true)
}

// writeFEC processes a packet of this buffer's FlexFEC repair flow, recovering
// and injecting lost media packets when possible
func (b *Buffer) writeFEC(fecPkt *rtp.Packet, arrivalTime int64) {
	b.Lock()
	defer b.Unlock()
	if !b.isBound {
		return
	}

	// FEC packets carry transport-wide sequence numbers and count toward the
	// publisher's send side bandwidth estimate. The FEC stream is not bound by
	// pion, so it is reported on the primary's responder, the streams share a
	// media section and with it the extension IDs.
	if b.twcc != nil && b.twccExtID != 0 {
		if ext := fecPkt.GetExtension(b.twccExtID); ext != nil {
			b.twcc.Push(fecPkt.SSRC, binary.BigEndian.Uint16(ext[0:2]), arrivalTime, fecPkt.Marker)
		}
	}

	// the payload type is learned from negotiated parameters at bind time, when
	// unknown the FEC-FR pairing from the SDP and the header validation of the
	// decoder are relied upon instead
	if b.fecPayloadType != 0 && fecPkt.PayloadType != b.fecPayloadType {
		b.logger.Debugw("unexpected fec payload type", "expected", b.fecPayloadType, "actual", fecPkt.PayloadType)
		return
	}

	if b.fecDecoder == nil {
		b.logger.Infow("first flexfec packet received", "fecSSRC", fecPkt.SSRC, "payloadType", fecPkt.PayloadType)
		b.fecDecoder = newFlexFECDecoder(flexFECDecoderParams{
			Logger:       b.logger,
			SSRC:         b.BufferBase.SSRC(),
			GetPacket:    b.BufferBase.getWirePacketLocked,
			ExtHighestSN: b.BufferBase.extHighestWireSNLocked,
			WindowSize:   b.BufferBase.bucketCapacityLocked,
		})
	}

	b.injectRecoveredLocked(b.fecDecoder.AddFEC(fecPkt.SequenceNumber, fecPkt.Payload, arrivalTime), arrivalTime)
	b.maybeLogFECStatsLocked(arrivalTime)
}

// injectRecoveredLocked feeds FEC recovered packets through the regular packet
// path. An injected packet can complete other buffered FEC packets, those
// chained recoveries are processed in the same loop.
func (b *Buffer) injectRecoveredLocked(recovered [][]byte, arrivalTime int64) {
	if len(recovered) == 0 || b.fecInjecting {
		return
	}

	b.fecInjecting = true
	for len(recovered) > 0 {
		raw := recovered[0]
		recovered = recovered[1:]

		var rtpPacket rtp.Packet
		if err := rtpPacket.Unmarshal(raw); err != nil {
			continue
		}
		if rtpPacket.Padding && len(rtpPacket.Payload) == 0 {
			// a padding only packet would have been dropped on arrival as well
			b.logger.Debugw("discarding recovered padding only packet", "sn", rtpPacket.SequenceNumber)
			continue
		}

		wireSN := rtpPacket.SequenceNumber
		b.calc(raw, &rtpPacket, arrivalTime, false, true)
		if b.fecDecoder.HasPending() {
			recovered = append(recovered, b.fecDecoder.OnMediaPacket(wireSN)...)
		}
	}
	b.fecInjecting = false
}

func (b *Buffer) maybeLogFECStatsLocked(now int64) {
	if b.fecDecoder == nil || now-b.lastFECStatsLogAt < flexFECStatsLogInterval {
		return
	}

	stats := b.fecDecoder.Stats()
	if stats == b.lastFECStatsLogged {
		return
	}

	b.lastFECStatsLogAt = now
	b.lastFECStatsLogged = stats
	b.logger.Debugw("flexfec stats", "stats", stats.String())
}

// GetFECStreamStats returns FlexFEC counters for this buffer's repair flow,
// false if no FlexFEC packet has been received
func (b *Buffer) GetFECStreamStats() (FECStreamStats, bool) {
	b.RLock()
	defer b.RUnlock()

	if b.fecDecoder == nil {
		return FECStreamStats{}, false
	}
	return b.fecDecoder.Stats(), true
}

func (b *Buffer) Read(buff []byte) (n int, err error) {
	b.Lock()
	for {
		if b.BufferBase.IsClosed() {
			b.Unlock()
			return 0, io.EOF
		}

		if b.pPackets != nil && len(b.pPackets) > b.lastPacketRead {
			if len(buff) < len(b.pPackets[b.lastPacketRead].packet) {
				b.Unlock()
				return 0, bucket.ErrBufferTooSmall
			}

			n = copy(buff, b.pPackets[b.lastPacketRead].packet)
			b.lastPacketRead++
			b.Unlock()
			return
		}
		b.BufferBase.WaitRead()
	}
}

func (b *Buffer) Close() error {
	stats, err := b.BufferBase.CloseWithReason("close")
	if err != nil {
		return err
	}

	if fecStats, ok := b.GetFECStreamStats(); ok {
		b.logger.Infow("flexfec final stats", "stats", fecStats.String())
	}

	if stats != nil {
		if cb := b.getOnFinalRtpStats(); cb != nil {
			cb(stats)
		}
	}

	if cb := b.getOnClose(); cb != nil {
		cb()
	}

	return nil
}

func (b *Buffer) OnClose(fn func()) {
	b.Lock()
	b.onClose = fn
	b.Unlock()
}

func (b *Buffer) getOnClose() func() {
	b.RLock()
	defer b.RUnlock()

	return b.onClose
}

func (b *Buffer) sendPLI() {
	ssrc := b.BufferBase.SSRC()
	if ssrc == 0 {
		return
	}

	b.logger.Debugw("send pli", "mediaSSRC", ssrc)
	pli := []rtcp.Packet{
		&rtcp.PictureLossIndication{
			SenderSSRC: ssrc,
			MediaSSRC:  ssrc,
		},
	}

	if cb := b.getOnRtcpFeedback(); cb != nil {
		cb(pli)
	}
}

func (b *Buffer) calc(rawPkt []byte, rtpPacket *rtp.Packet, arrivalTime int64, isBuffered bool, isRTX bool) []rtcp.Packet {
	// HandleIncomingPacketLocked rewrites the sequence number for bucket storage,
	// rtpPacket is nil for packets queued before bind, no FEC state exists then
	var wireSN uint16
	if rtpPacket != nil {
		wireSN = rtpPacket.SequenceNumber
	}

	b.BufferBase.HandleIncomingPacketLocked(
		rawPkt,
		rtpPacket,
		arrivalTime,
		isBuffered,
		isRTX,
		nil,
		0,
	)

	// an arriving packet can complete buffered FEC packets waiting on it
	if rtpPacket != nil && b.fecDecoder != nil && !b.fecInjecting && b.fecDecoder.HasPending() {
		b.injectRecoveredLocked(b.fecDecoder.OnMediaPacket(wireSN), arrivalTime)
	}

	return b.getRTCPPackets(arrivalTime)
}

func (b *Buffer) getRTCPPackets(arrivalTime int64) []rtcp.Packet {
	var pkts []rtcp.Packet
	if nackPkt := b.getNACKPacket(); nackPkt != nil {
		pkts = append(pkts, nackPkt)
	}
	if receiverReport := b.getRTCPReceiverReport(arrivalTime); receiverReport != nil {
		pkts = append(pkts, receiverReport)
	}

	return pkts
}

func (b *Buffer) getNACKPacket() *rtcp.TransportLayerNack {
	if nacks := b.BufferBase.GetNACKPairsLocked(); len(nacks) > 0 {
		ssrc := b.BufferBase.SSRC()
		return &rtcp.TransportLayerNack{
			SenderSSRC: ssrc,
			MediaSSRC:  ssrc,
			Nacks:      nacks,
		}
	}
	return nil
}

func (b *Buffer) getRTCPReceiverReport(arrivalTime int64) *rtcp.ReceiverReport {
	if arrivalTime-b.lastReportAt < rtcpReceiverReportDelta {
		return nil
	}
	b.lastReportAt = arrivalTime

	proxyLoss := b.lastFractionLostToReport
	if b.codecType == webrtc.RTPCodecTypeAudio && !b.enableAudioLossProxying {
		proxyLoss = 0
	}

	if receptionReport := b.BufferBase.GetRtcpReceptionReportLocked(proxyLoss); receptionReport != nil {
		return &rtcp.ReceiverReport{
			SSRC:    b.BufferBase.SSRC(),
			Reports: []rtcp.ReceptionReport{*receptionReport},
		}
	}

	return nil
}

func (b *Buffer) SetLastFractionLostReport(lost uint8) {
	b.Lock()
	defer b.Unlock()

	b.lastFractionLostToReport = lost
}

func (b *Buffer) OnRtcpFeedback(fn func(fb []rtcp.Packet)) {
	b.Lock()
	b.onRtcpFeedback = fn
	b.Unlock()
}

func (b *Buffer) getOnRtcpFeedback() func(fb []rtcp.Packet) {
	b.RLock()
	defer b.RUnlock()

	return b.onRtcpFeedback
}

func (b *Buffer) OnFinalRtpStats(fn func(*livekit.RTPStats)) {
	b.Lock()
	b.onFinalRtpStats = fn
	b.Unlock()
}

func (b *Buffer) getOnFinalRtpStats() func(*livekit.RTPStats) {
	b.RLock()
	defer b.RUnlock()

	return b.onFinalRtpStats
}

func (b *Buffer) OnNotifyRTX(fn func(ssrc uint32, repairSSRC uint32, rsid string)) {
	b.Lock()
	b.onNotifyRTX = fn
	b.Unlock()
}

func (b *Buffer) getOnNotifyRTX() func(ssrc uint32, repairSSRC uint32, rsid string) {
	b.RLock()
	defer b.RUnlock()

	return b.onNotifyRTX
}