add optional abs-send-time support to Packetizer

Relates to #15

packet.go

@@ -211,6 +211,10 @@ func (h *Header) MarshalTo(buf []byte) (n int, err error) {
 	h.PayloadOffset = n
 
 	if h.Extension {
+		if len(h.ExtensionPayload)%4 != 0 {
+			//the payload must be in 32-bit words.
+			return 0, io.ErrShortBuffer
+		}
 		extSize := uint16(len(h.ExtensionPayload) / 4)
 
 		binary.BigEndian.PutUint16(buf[n+0:n+2], h.ExtensionProfile)

packetizer.go

@@ -23,6 +23,7 @@ type packetizer struct {
 	Sequencer   Sequencer
 	Timestamp   uint32
 	ClockRate   uint32
+	AbsSendTime int //if this is nonzero, then it's the extension number that's applied to http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time
 }
 
 // NewPacketizer returns a new instance of a Packetizer for a specific payloader
@@ -41,6 +42,19 @@ func NewPacketizer(mtu int, pt uint8, ssrc uint32, payloader Payloader, sequence
 	}
 }
 
+func toNtpTime(t time.Time) int64 {
+	var s int64
+	var f int64
+	u := t.UnixNano()
+	s = u / 1e9
+	f = u % 1e9
+	f <<= 32
+	f /= 1e9
+	s <<= 32
+
+	return s | f
+}
+
 // Packetize packetizes the payload of an RTP packet and returns one or more RTP packets
 func (p *packetizer) Packetize(payload []byte, samples uint32) []*Packet {
 	// Guard against an empty payload
@@ -68,5 +82,26 @@ func (p *packetizer) Packetize(payload []byte, samples uint32) []*Packet {
 	}
 	p.Timestamp += samples
 
+	if p.AbsSendTime != 0 {
+		t := toNtpTime(time.Now()) >> 14
+		//apply http://www.webrtc.org/experiments/rtp-hdrext/abs-send-time
+		packets[len(payloads)-1].ExtensionProfile = 0xBEDE
+		packets[len(payloads)-1].ExtensionPayload = []byte{
+			//the first byte is
+			// 0 1 2 3 4 5 6 7
+			//+-+-+-+-+-+-+-+-+
+			//|  ID   |  len  |
+			//+-+-+-+-+-+-+-+-+
+			//per RFC65285
+			//Len is the number of bytes in the extension - 1
+
+			byte((p.AbsSendTime << 4) | 2),
+			byte(t & 0xFF0000 >> 16),
+			byte(t & 0xFF00 >> 8),
+			byte(t & 0xFF),
+		}
+
+	}
+
 	return packets
 }