RTCP Reception mechanism

Add a channel to the RTCTrack object to
recieve incoming RTCP packets with.
Relates to #119

pkg/rtcp/rapid_resynchronization_request.go

@@ -0,0 +1,65 @@
+package rtcp
+
+import (
+	"encoding/binary"
+)
+
+// The RapidResynchronizationRequest packet informs the encoder about the loss of an undefined amount of coded video data belonging to one or more pictures
+type RapidResynchronizationRequest struct {
+	// SSRC of sender
+	SenderSSRC uint32
+
+	// SSRC of the media source
+	MediaSSRC uint32
+}
+
+const (
+	rrrFMT    = 5
+	rrrLength = 2
+)
+
+// Marshal encodes the RapidResynchronizationRequest in binary
+func (p RapidResynchronizationRequest) Marshal() ([]byte, error) {
+	/*
+	 * RRR does not require parameters.  Therefore, the length field MUST be
+	 * 2, and there MUST NOT be any Feedback Control Information.
+	 *
+	 * The semantics of this FB message is independent of the payload type.
+	 */
+	rawPacket := make([]byte, 8)
+	binary.BigEndian.PutUint32(rawPacket, p.SenderSSRC)
+	binary.BigEndian.PutUint32(rawPacket[4:], p.MediaSSRC)
+
+	h := Header{
+		Count:  rrrFMT,
+		Type:   TypeTransportSpecificFeedback,
+		Length: rrrLength,
+	}
+	hData, err := h.Marshal()
+	if err != nil {
+		return nil, err
+	}
+
+	return append(hData, rawPacket...), nil
+}
+
+// Unmarshal decodes the RapidResynchronizationRequest from binary
+func (p *RapidResynchronizationRequest) Unmarshal(rawPacket []byte) error {
+	
+	if len(rawPacket) < (headerLength + (ssrcLength * 2)) {
+		return errPacketTooShort
+	}
+
+	var h Header
+	if err := h.Unmarshal(rawPacket); err != nil {
+		return err
+	}
+
+	if h.Type != TypeTransportSpecificFeedback || h.Count != 1 {		
+		return errWrongType
+	}
+
+	p.SenderSSRC = binary.BigEndian.Uint32(rawPacket[headerLength:])
+	p.MediaSSRC = binary.BigEndian.Uint32(rawPacket[headerLength+ssrcLength:])
+	return nil
+}