Start RTCP package: add header parsing

pkg/rtcp/packet.go

@@ -0,0 +1,106 @@
+package rtcp
+
+import (
+	"encoding/binary"
+
+	"github.com/pkg/errors"
+)
+
+// RTCP packet types registered with IANA. See: https://www.iana.org/assignments/rtp-parameters/rtp-parameters.xhtml#rtp-parameters-4
+const (
+	TypeSR   = 200 // sender report        RFC 3550, 6.4.1
+	TypeRR   = 201 // receiver report      RFC 3550, 6.4.2
+	TypeSDES = 202 // source description   RFC 3550, 6.5
+	TypeBYE  = 203 // goodbye              RFC 3550, 6.6
+	TypeAPP  = 204 // application-defined  RFC 3550, 6.7
+)
+
+// A Header is the common header shared by all RTCP packets
+type Header struct {
+	// Identifies the version of RTP, which is the same in RTCP packets
+	// as in RTP data packets.
+	Version uint8
+	// If the padding bit is set, this individual RTCP packet contains
+	// some additional padding octets at the end which are not part of
+	// the control information but are included in the length field.
+	Padding bool
+	// The number of reception report blocks contained in this packet.
+	ReportCount uint8
+	// The RTCP packet type for this packet
+	Type uint8
+	// The length of this RTCP packet in 32-bit words minus one,
+	// including the header and any padding.
+	Length uint16
+}
+
+const (
+	headerLength     = 4
+	versionShift     = 6
+	versionMask      = 0x3
+	paddingShift     = 5
+	paddingMask      = 0x1
+	reportCountShift = 0
+	reportCountMask  = 0x1f
+)
+
+var (
+	errInvalidVersion     = errors.New("invalid version")
+	errInvalidReportCount = errors.New("invalid report count")
+)
+
+// Marshal encodes the Header in binary
+func (h Header) Marshal() ([]byte, error) {
+	/*
+	 *  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
+	 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+	 * |V=2|P|    RC   |   PT=SR=200   |             length            |
+	 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+	 */
+	rawPacket := make([]byte, headerLength)
+
+	if h.Version > 3 {
+		return nil, errInvalidVersion
+	}
+	rawPacket[0] |= h.Version << versionShift
+
+	if h.Padding {
+		rawPacket[0] |= 1 << paddingShift
+	}
+
+	if h.ReportCount > 31 {
+		return nil, errInvalidReportCount
+	}
+	rawPacket[0] |= h.ReportCount << reportCountShift
+
+	rawPacket[1] = h.Type
+
+	binary.BigEndian.PutUint16(rawPacket[2:], h.Length)
+
+	return rawPacket, nil
+}
+
+// Unmarshal decodes the Header from binary
+func (h *Header) Unmarshal(rawPacket []byte) error {
+	if len(rawPacket) < headerLength {
+		return errors.Errorf("RTCP header size insufficient; %d < %d", len(rawPacket), headerLength)
+	}
+
+	/*
+	 *  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
+	 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+	 * |V=2|P|    RC   |   PT=SR=200   |             length            |
+	 * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+	 */
+
+	h.Version = rawPacket[0] >> versionShift & versionMask
+	h.Padding = (rawPacket[0] >> paddingShift & paddingMask) > 0
+	h.ReportCount = rawPacket[0] >> reportCountShift & reportCountMask
+
+	h.Type = rawPacket[1]
+
+	h.Length = binary.BigEndian.Uint16(rawPacket[2:])
+
+	return nil
+}