Create a retransmission packet from a lost packet.

def wrap_rtx(packet, payload_type, sequence_number, ssrc):
    """
    Create a retransmission packet from a lost packet.
    """
    rtx = RtpPacket(
        payload_type=payload_type,
        marker=packet.marker,
        sequence_number=sequence_number,
        timestamp=packet.timestamp,
        ssrc=ssrc,
        payload=pack('!H', packet.sequence_number) + packet.payload)
    rtx.csrc = packet.csrc
    rtx.extensions = packet.extensions
    return rtx