def packet(data, seq, ack_field): ## chunk, seq, '00'
flag = "7E"
seq_string = str(seq)
print("Frame num: ", seq_string)
print("Data: ", data)
crc = CRC.calc_checksum(data)
print(
"Checksum (frame) ", seq_string, ": ", crc
) # when the server does a checksum on this chunk, should yield same result - send ack
data = str(data)
header = flag + "-" + seq_string + "-" + ack_field
trailer = crc + "-" + flag
packet = header + "-" + data + "-" + trailer
return str(packet)
if ("101010101010" in data) == False:
# not corrupted
isValid = True # valid for the moment - not corrupted
print"Expected sequence number is ", correct_seq, ", got sequence number ", int(ack_seq)
# -- start error-checking the data -- #
if correct_seq != int(ack_seq): # error has occurred - send a nack to request retransmission
isValid = False
print"Incorrect sequence number - the original packet was dropped"
dropped = True
nack_packet = packet.packet(bytes(payload), correct_seq, "01")
connectionSocket.send(bytes(nack_packet)) # NACK
else: # we have the correct sequence num - continue error-checking
print"Correct sequence number"
# print"CRC received: ", crc_recvd
chk = CRC.calc_checksum(bytes(payload))
if chk != crc_recvd:
isValid = False # request a resend
print"CRC fields do not match"
else:
isValid = True
print"CRC correct"
else:
# packet corrupt
print"Corrupt packet!"
isValid = False
if isValid == True: # no errors - ACK frame
ack_packet = packet.packet(bytes(payload), correct_seq, '10')
connectionSocket.send(bytes(ack_packet))
print"Sent the ack frame (",ack_packet ," ) for seq ", correct_seq, " back to the client"
