How to use the tftpy.TftpPacketOACK function in tftpy

def __init__(self):
        self.classes = {
            1: TftpPacketRRQ,
            2: TftpPacketWRQ,
            3: TftpPacketDAT,
            4: TftpPacketACK,
            5: TftpPacketERR,
            6: TftpPacketOACK
            }

def progresshook(self, pkt):
        if isinstance(pkt, TftpPacketDAT):
            self.progress += len(pkt.data)
            self.out.write("Transferred %d bytes" % self.progress + "\n")
        elif isinstance(pkt, TftpPacketOACK):
            self.out.write("Received OACK, options are: %s" % pkt.options + "\n")

def send_oack(self):
        """This method sends an OACK packet based on current params."""
        logger.debug("Composing and sending OACK packet")
        oack = TftpPacketOACK()
        oack.options = self.options
        self.sock.sendto(oack.encode().buffer,
                         (self.host, self.port))
        self.timesent = time.time()
        self.state.state = 'oack'

dups[curblock] = 1
                    tftpassert(dups[curblock] < MAX_DUPS,
                            "Max duplicates for block %d reached" % curblock)
                    logger.debug("ACKing block %d again, just in case" % curblock)
                    ackpkt = TftpPacketACK()
                    ackpkt.blocknumber = curblock
                    self.sock.sendto(ackpkt.encode().buffer, (self.host, self.port))

                else:
                    msg = "Whoa! Received block %d but expected %d" % (recvpkt.blocknumber, 
                                                                    curblock+1)
                    logger.error(msg)
                    raise TftpException, msg

            # Check other packet types.
            elif isinstance(recvpkt, TftpPacketOACK):
                if not self.state.state == 'rrq':
                    self.errors += 1
                    logger.error("Received OACK in state %s" % self.state.state)
                    continue
                
                self.state.state = 'oack'
                logger.info("Received OACK from server.")
                if recvpkt.options.keys() > 0:
                    if recvpkt.match_options(self.options):
                        logger.info("Successful negotiation of options")
                        for key in self.options:
                            logger.info("    %s = %s" % (key, self.options[key]))
                        logger.debug("sending ACK to OACK")
                        ackpkt = TftpPacketACK()
                        ackpkt.blocknumber = 0
                        self.sock.sendto(ackpkt.encode().buffer, (self.host, self.port))