feat: Add playout delay header interceptor

This interceptor adds the playout delay header extension following
https://webrtc.googlesource.com/src/+/refs/heads/main/docs/native-code/rtp-hdrext/playout-delay

pkg/playoutdelay/header_extension_interceptor.go

@@ -0,0 +1,60 @@
+package playoutdelay
+
+import (
+	"github.com/pion/interceptor"
+	"github.com/pion/rtp"
+	"time"
+)
+
+// HeaderExtensionInterceptorFactory is a interceptor.Factory for a HeaderExtensionInterceptor
+type HeaderExtensionInterceptorFactory struct{}
+
+// NewInterceptor constructs a new HeaderExtensionInterceptor
+func (h *HeaderExtensionInterceptorFactory) NewInterceptor(id string, minDelay, maxDelay time.Duration) (interceptor.Interceptor, error) {
+	if minDelay.Milliseconds() < 0 || minDelay.Milliseconds() > 40950 || maxDelay.Milliseconds() < 0 || maxDelay.Milliseconds() > 40950 {
+		return nil, errPlayoutDelayInvalidValue
+	}
+	return &HeaderExtensionInterceptor{minDelay: uint16(minDelay.Milliseconds() / 10), maxDelay: uint16(maxDelay.Milliseconds() / 10)}, nil
+}
+
+// NewHeaderExtensionInterceptor returns a HeaderExtensionInterceptorFactory
+func NewHeaderExtensionInterceptor() (*HeaderExtensionInterceptorFactory, error) {
+	return &HeaderExtensionInterceptorFactory{}, nil
+}
+
+// HeaderExtensionInterceptor adds transport wide sequence numbers as header extension to each RTP packet
+type HeaderExtensionInterceptor struct {
+	interceptor.NoOp
+	minDelay, maxDelay uint16
+}
+
+const playoutDelayURI = "http://www.webrtc.org/experiments/rtp-hdrext/playout-delay"
+
+// BindLocalStream returns a writer that adds a rtp.TransportCCExtension
+// header with increasing sequence numbers to each outgoing packet.
+func (h *HeaderExtensionInterceptor) BindLocalStream(info *interceptor.StreamInfo, writer interceptor.RTPWriter) interceptor.RTPWriter {
+	var hdrExtID uint8
+	for _, e := range info.RTPHeaderExtensions {
+		if e.URI == playoutDelayURI {
+			hdrExtID = uint8(e.ID)
+			break
+		}
+	}
+	if hdrExtID == 0 { // Don't add header extension if ID is 0, because 0 is an invalid extension ID
+		return writer
+	}
+	return interceptor.RTPWriterFunc(func(header *rtp.Header, payload []byte, attributes interceptor.Attributes) (int, error) {
+		tcc, err := (&PlayoutDelayExtension{
+			minDelay: h.minDelay,
+			maxDelay: h.maxDelay,
+		}).Marshal()
+		if err != nil {
+			return 0, err
+		}
+		err = header.SetExtension(hdrExtID, tcc)
+		if err != nil {
+			return 0, err
+		}
+		return writer.Write(header, payload, attributes)
+	})
+}

pkg/playoutdelay/header_extension_interceptor_test.go

@@ -0,0 +1,68 @@
+package playoutdelay
+
+import (
+	"sync"
+	"testing"
+	"time"
+
+	"github.com/pion/interceptor"
+	"github.com/pion/interceptor/internal/test"
+	"github.com/pion/rtp"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestHeaderExtensionInterceptor(t *testing.T) {
+	t.Run("add playout delay to each packet", func(t *testing.T) {
+		factory, err := NewHeaderExtensionInterceptor()
+		assert.NoError(t, err)
+
+		inter, err := factory.NewInterceptor("", 10*time.Millisecond, 20*time.Millisecond)
+		assert.NoError(t, err)
+
+		pChan := make(chan *rtp.Packet, 10*5)
+		go func() {
+			// start some parallel streams using the same interceptor to test for race conditions
+			var wg sync.WaitGroup
+			num := 10
+			wg.Add(num)
+			for i := 0; i < num; i++ {
+				go func(ch chan *rtp.Packet, id uint16) {
+					stream := test.NewMockStream(&interceptor.StreamInfo{RTPHeaderExtensions: []interceptor.RTPHeaderExtension{
+						{
+							URI: playoutDelayURI,
+							ID:  1,
+						},
+					}}, inter)
+					defer func() {
+						wg.Done()
+						assert.NoError(t, stream.Close())
+					}()
+
+					for _, seqNum := range []uint16{id * 1, id * 2, id * 3, id * 4, id * 5} {
+						assert.NoError(t, stream.WriteRTP(&rtp.Packet{Header: rtp.Header{SequenceNumber: seqNum}}))
+						select {
+						case p := <-stream.WrittenRTP():
+							assert.Equal(t, seqNum, p.SequenceNumber)
+							ch <- p
+						case <-time.After(10 * time.Millisecond):
+							panic("written rtp packet not found")
+						}
+					}
+				}(pChan, uint16(i+1))
+			}
+			wg.Wait()
+			close(pChan)
+		}()
+
+		for p := range pChan {
+			// Can't check for increasing transport cc sequence number, since we can't ensure ordering between the streams
+			// on pChan is same as in the interceptor, but at least make sure each packet has a seq nr.
+			extensionHeader := p.GetExtension(1)
+			ext := &PlayoutDelayExtension{}
+			err = ext.Unmarshal(extensionHeader)
+			assert.NoError(t, err)
+			assert.Equal(t, uint16(1), ext.minDelay)
+			assert.Equal(t, uint16(2), ext.maxDelay)
+		}
+	})
+}

pkg/playoutdelay/playout_delay.go

@@ -0,0 +1,52 @@
+// Package playoutdelay implements the playout delay header extension.
+package playoutdelay
+
+import (
+	"encoding/binary"
+	"errors"
+)
+
+const (
+	// transport-wide sequence
+	playoutDelayExtensionSize = 3
+	playoutDelayMaxValue      = (1 << 12) - 1
+)
+
+var (
+	errPlayoutDelayInvalidValue = errors.New("invalid playout delay value")
+	errTooSmall                 = errors.New("playout delay header extension too short")
+)
+
+// PlayoutDelayExtension is a extension payload format in
+// https://webrtc.googlesource.com/src/+/refs/heads/main/docs/native-code/rtp-hdrext/playout-delay
+// 0                   1                   2                   3
+// 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+// +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+// |  ID   | len=2 |       MIN delay       |       MAX delay       |
+// +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+type PlayoutDelayExtension struct {
+	minDelay, maxDelay uint16
+}
+
+// Marshal serializes the members to buffer
+func (p PlayoutDelayExtension) Marshal() ([]byte, error) {
+	if p.minDelay >= playoutDelayMaxValue || p.maxDelay >= playoutDelayMaxValue {
+		return nil, errPlayoutDelayInvalidValue
+	}
+
+	return []byte{
+		byte(p.minDelay >> 4),
+		byte(p.minDelay<<4) | byte(p.maxDelay>>8),
+		byte(p.maxDelay),
+	}, nil
+}
+
+// Unmarshal parses the passed byte slice and stores the result in the members
+func (p *PlayoutDelayExtension) Unmarshal(rawData []byte) error {
+	if len(rawData) < playoutDelayExtensionSize {
+		return errTooSmall
+	}
+	p.minDelay = binary.BigEndian.Uint16(rawData[0:2]) >> 4
+	p.maxDelay = binary.BigEndian.Uint16(rawData[1:3]) & 0x0FFF
+	return nil
+}