protocol/rtp: add Timestamp parsing function

protocol/rtp/parse.go

@@ -82,6 +82,16 @@ func Sequence(d []byte) (uint16, error) {
 	return binary.BigEndian.Uint16(d[2:]), nil
 }
 
+// Timestamp returns the RTP timestamp of an RTP packet. An error is returned
+// if the packet is not valid.
+func Timestamp(d []byte) (uint32, error) {
+	err := checkPacket(d)
+	if err != nil {
+		return 0, err
+	}
+	return binary.BigEndian.Uint32(d[4:]), nil
+}
+
 // checkPacket checks the validity of the packet, firstly by checking size and
 // then also checking that version is compatible with these utilities.
 func checkPacket(d []byte) error {