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xmodem.py
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xmodem.py
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'''
===============================
XMODEM file transfer protocol
===============================
.. $Id: __init__.py,v 696ce0cd218b 2010/04/22 16:58:05 maze $
This is a literal implementation of XMODEM.TXT_, XMODEM1K.TXT_ and
XMODMCRC.TXT_, support for YMODEM and ZMODEM is pending. YMODEM should
be fairly easy to implement as it is a hack on top of the XMODEM
protocol using sequence bytes ``0x00`` for sending file names (and some
meta data).
.. _XMODEM.TXT: doc/XMODEM.TXT
.. _XMODEM1K.TXT: doc/XMODEM1K.TXT
.. _XMODMCRC.TXT: doc/XMODMCRC.TXT
Data flow example including error recovery
==========================================
Here is a sample of the data flow, sending a 3-block message.
It includes the two most common line hits - a garbaged block,
and an ``ACK`` reply getting garbaged. ``CRC`` or ``CSUM`` represents
the checksum bytes.
XMODEM 128 byte blocks
----------------------
::
SENDER RECEIVER
<-- NAK
SOH 01 FE Data[128] CSUM -->
<-- ACK
SOH 02 FD Data[128] CSUM -->
<-- ACK
SOH 03 FC Data[128] CSUM -->
<-- ACK
SOH 04 FB Data[128] CSUM -->
<-- ACK
SOH 05 FA Data[100] CPMEOF[28] CSUM -->
<-- ACK
EOT -->
<-- ACK
XMODEM-1k blocks, CRC mode
--------------------------
::
SENDER RECEIVER
<-- C
STX 01 FE Data[1024] CRC CRC -->
<-- ACK
STX 02 FD Data[1024] CRC CRC -->
<-- ACK
STX 03 FC Data[1000] CPMEOF[24] CRC CRC -->
<-- ACK
EOT -->
<-- ACK
Mixed 1024 and 128 byte Blocks
------------------------------
::
SENDER RECEIVER
<-- C
STX 01 FE Data[1024] CRC CRC -->
<-- ACK
STX 02 FD Data[1024] CRC CRC -->
<-- ACK
SOH 03 FC Data[128] CRC CRC -->
<-- ACK
SOH 04 FB Data[100] CPMEOF[28] CRC CRC -->
<-- ACK
EOT -->
<-- ACK
YMODEM Batch Transmission Session (1 file)
------------------------------------------
::
SENDER RECEIVER
<-- C (command:rb)
SOH 00 FF foo.c NUL[123] CRC CRC -->
<-- ACK
<-- C
SOH 01 FE Data[128] CRC CRC -->
<-- ACK
SOH 02 FC Data[128] CRC CRC -->
<-- ACK
SOH 03 FB Data[100] CPMEOF[28] CRC CRC -->
<-- ACK
EOT -->
<-- NAK
EOT -->
<-- ACK
<-- C
SOH 00 FF NUL[128] CRC CRC -->
<-- ACK
'''
__author__ = 'Wijnand Modderman <maze@pyth0n.org>'
__copyright__ = ['Copyright (c) 2010 Wijnand Modderman',
'Copyright (c) 1981 Chuck Forsberg']
__license__ = 'MIT'
__url__ = 'http://maze.io/'
__version__ = '0.1'
import time
import sys
# Protocol bytes
SOH = chr(0x01)
STX = chr(0x02)
EOT = chr(0x04)
ACK = chr(0x06)
NAK = chr(0x15)
CAN = chr(0x18)
CRC = chr(0x43)
class XMODEM(object):
'''
XMODEM Protocol handler, expects an object to read from and an object to
write to.
>>> def getc(size, timeout=1):
... return data or None
...
>>> def putc(data, timeout=1):
... return size or None
...
>>> x = XMODEM(getc, putc)
'''
# crctab calculated by Mark G. Mendel, Network Systems Corporation
crctable = [
0x0000, 0x1021, 0x2042, 0x3063, 0x4084, 0x50a5, 0x60c6, 0x70e7,
0x8108, 0x9129, 0xa14a, 0xb16b, 0xc18c, 0xd1ad, 0xe1ce, 0xf1ef,
0x1231, 0x0210, 0x3273, 0x2252, 0x52b5, 0x4294, 0x72f7, 0x62d6,
0x9339, 0x8318, 0xb37b, 0xa35a, 0xd3bd, 0xc39c, 0xf3ff, 0xe3de,
0x2462, 0x3443, 0x0420, 0x1401, 0x64e6, 0x74c7, 0x44a4, 0x5485,
0xa56a, 0xb54b, 0x8528, 0x9509, 0xe5ee, 0xf5cf, 0xc5ac, 0xd58d,
0x3653, 0x2672, 0x1611, 0x0630, 0x76d7, 0x66f6, 0x5695, 0x46b4,
0xb75b, 0xa77a, 0x9719, 0x8738, 0xf7df, 0xe7fe, 0xd79d, 0xc7bc,
0x48c4, 0x58e5, 0x6886, 0x78a7, 0x0840, 0x1861, 0x2802, 0x3823,
0xc9cc, 0xd9ed, 0xe98e, 0xf9af, 0x8948, 0x9969, 0xa90a, 0xb92b,
0x5af5, 0x4ad4, 0x7ab7, 0x6a96, 0x1a71, 0x0a50, 0x3a33, 0x2a12,
0xdbfd, 0xcbdc, 0xfbbf, 0xeb9e, 0x9b79, 0x8b58, 0xbb3b, 0xab1a,
0x6ca6, 0x7c87, 0x4ce4, 0x5cc5, 0x2c22, 0x3c03, 0x0c60, 0x1c41,
0xedae, 0xfd8f, 0xcdec, 0xddcd, 0xad2a, 0xbd0b, 0x8d68, 0x9d49,
0x7e97, 0x6eb6, 0x5ed5, 0x4ef4, 0x3e13, 0x2e32, 0x1e51, 0x0e70,
0xff9f, 0xefbe, 0xdfdd, 0xcffc, 0xbf1b, 0xaf3a, 0x9f59, 0x8f78,
0x9188, 0x81a9, 0xb1ca, 0xa1eb, 0xd10c, 0xc12d, 0xf14e, 0xe16f,
0x1080, 0x00a1, 0x30c2, 0x20e3, 0x5004, 0x4025, 0x7046, 0x6067,
0x83b9, 0x9398, 0xa3fb, 0xb3da, 0xc33d, 0xd31c, 0xe37f, 0xf35e,
0x02b1, 0x1290, 0x22f3, 0x32d2, 0x4235, 0x5214, 0x6277, 0x7256,
0xb5ea, 0xa5cb, 0x95a8, 0x8589, 0xf56e, 0xe54f, 0xd52c, 0xc50d,
0x34e2, 0x24c3, 0x14a0, 0x0481, 0x7466, 0x6447, 0x5424, 0x4405,
0xa7db, 0xb7fa, 0x8799, 0x97b8, 0xe75f, 0xf77e, 0xc71d, 0xd73c,
0x26d3, 0x36f2, 0x0691, 0x16b0, 0x6657, 0x7676, 0x4615, 0x5634,
0xd94c, 0xc96d, 0xf90e, 0xe92f, 0x99c8, 0x89e9, 0xb98a, 0xa9ab,
0x5844, 0x4865, 0x7806, 0x6827, 0x18c0, 0x08e1, 0x3882, 0x28a3,
0xcb7d, 0xdb5c, 0xeb3f, 0xfb1e, 0x8bf9, 0x9bd8, 0xabbb, 0xbb9a,
0x4a75, 0x5a54, 0x6a37, 0x7a16, 0x0af1, 0x1ad0, 0x2ab3, 0x3a92,
0xfd2e, 0xed0f, 0xdd6c, 0xcd4d, 0xbdaa, 0xad8b, 0x9de8, 0x8dc9,
0x7c26, 0x6c07, 0x5c64, 0x4c45, 0x3ca2, 0x2c83, 0x1ce0, 0x0cc1,
0xef1f, 0xff3e, 0xcf5d, 0xdf7c, 0xaf9b, 0xbfba, 0x8fd9, 0x9ff8,
0x6e17, 0x7e36, 0x4e55, 0x5e74, 0x2e93, 0x3eb2, 0x0ed1, 0x1ef0,
]
def __init__(self, getc, putc):
self.getc = getc
self.putc = putc
def abort(self, count=2, timeout=60):
'''
Send an abort sequence using CAN bytes.
'''
for counter in xrange(0, count):
self.putc(CAN, timeout)
def send(self, stream, retry=16, timeout=60, quiet=0):
'''
Send a stream via the XMODEM protocol.
>>> stream = file('/etc/issue', 'rb')
>>> print x.send(stream)
True
Returns ``True`` upon succesful transmission or ``False`` in case of
failure.
'''
# initialize protocol
error_count = 0
crc_mode = 0
cancel = 0
while True:
char = self.getc(1)
if char:
if char == NAK:
crc_mode = 0
break
elif char == CRC:
crc_mode = 1
break
elif char == CAN:
if not quiet:
print >> sys.stderr, 'received CAN'
if cancel:
return False
else:
cancel = 1
else:
if not quiet:
print >> sys.stderr, \
'send ERROR expected NAK/CRC, got', ord(char)
error_count += 1
if error_count >= retry:
self.abort(timeout=timeout)
return False
# send data
error_count = 0
packet_size = 128
sequence = 1
while True:
data = stream.read(packet_size)
if not data:
if not quiet:
print >> sys.stderr, 'send EOS'
# end of stream
break
data = data.ljust(packet_size, '\xff')
if crc_mode:
crc = self.calc_crc(data)
else:
crc = self.calc_checksum(data)
# emit packet
while True:
self.putc(SOH)
self.putc(chr(sequence))
self.putc(chr(0xff - sequence))
self.putc(data)
if crc_mode:
self.putc(chr(crc >> 8))
self.putc(chr(crc & 0xff))
else:
self.putc(chr(crc))
char = self.getc(1, timeout)
if char == ACK:
break
if char == NAK:
error_count += 1
if error_count >= retry:
# excessive amounts of retransmissions requested,
# abort transfer
self.abort(timeout=timeout)
return False
# protocol error
self.abort(timeout=timeout)
return False
# keep track of sequence
sequence = (sequence + 1 ) % 0xff
# end of transmission
self.putc(EOT)
return True
def recv(self, stream, crc_mode=1, retry=16, timeout=60, delay=1, quiet=0):
'''
Receive a stream via the XMODEM protocol.
>>> stream = file('/etc/issue', 'wb')
>>> print x.recv(stream)
2342
Returns the number of bytes received on success or ``None`` in case of
failure.
'''
# initiate protocol
error_count = 0
char = 0
cancel = 0
while True:
# first try CRC mode, if this fails,
# fall back to checksum mode
if error_count >= retry:
self.abort(timeout=timeout)
return None
elif crc_mode and error_count < (retry / 2):
if not self.putc(CRC):
time.sleep(delay)
error_count += 1
else:
crc_mode = 0
if not self.putc(NAK):
time.sleep(delay)
error_count += 1
char = self.getc(1, timeout)
if not char:
error_count += 1
continue
elif char == SOH:
#crc_mode = 0
break
elif char in [STX, CAN]:
break
elif char == CAN:
if cancel:
return None
else:
cancel = 1
else:
error_count += 1
# read data
error_count = 0
income_size = 0
packet_size = 128
sequence = 1
cancel = 0
while True:
while True:
if char == SOH:
packet_size = 128
break
elif char == STX:
packet_size = 1024
break
elif char == EOT:
return income_size
elif char == CAN:
# cancel at two consecutive cancels
if cancel:
return None
else:
cancel = 1
else:
if not quiet:
print >> sys.stderr, \
'recv ERROR expected SOH/EOT, got', ord(char)
error_count += 1
if error_count >= retry:
self.abort()
return None
# read sequence
error_count = 0
cancel = 0
seq1 = ord(self.getc(1))
seq2 = 0xff - ord(self.getc(1))
if seq1 == sequence and seq2 == sequence:
# sequence is ok, read packet
# packet_size + checksum
data = self.getc(packet_size + 1 + crc_mode)
if crc_mode:
csum = (ord(data[-2]) << 8) + ord(data[-1])
data = data[:-2]
if not quiet:
print >> sys.stderr, 'CRC(%04x <> %04x)' % (csum,
self.calc_crc(data)),
valid = csum == self.calc_crc(data)
else:
csum = data[-1]
data = data[:-1]
if not quiet:
print >> sys.stderr, 'checksum(%02x <> %02x)' % \
(ord(csum), self.calc_checksum(data)),
valid = ord(csum) == self.calc_checksum(data)
# valid data, append chunk
if valid:
income_size += len(data)
stream.write(data)
self.putc(ACK)
sequence = (sequence + 1) % 0xff
char = self.getc(1, timeout)
continue
else:
# consume data
self.getc(packet_size + 1 + crc_mode)
if not quiet:
print >> sys.stderr, 'expected sequence %d, got %d/%d' % \
(sequence, seq1, seq2)
# something went wrong, request retransmission
self.putc(NAK)
def calc_checksum(self, data, checksum=0):
'''
Calculate the checksum for a given block of data, can also be used to
update a checksum.
>>> csum = xmodem.calc_checksum('hello')
>>> csum = xmodem.calc_checksum('world', csum)
>>> hex(csum)
'0x3c'
'''
return (sum(map(ord, data)) + checksum) % 256
def calc_crc(self, data, crc=0):
'''
Calculate the Cyclic Redundancy Check for a given block of data, can
also be used to update a CRC.
>>> crc = xmodem.calc_crc('hello')
>>> crc = xmodem.calc_crc('world', crc)
>>> hex(54755)
'0xd5e3'
'''
for char in data:
crc = (crc << 8) ^ self.crctable[((crc >> 8) ^ ord(char)) & 0xff]
return crc & 0xffff