Application and summary of CRC

At present, there are two kinds of Crc calculation methods: Crc32 and Crc32mpg2.

CRC error detection technology of cyclic redundancy test can prove that the data is complete and error free (it is only very similar to think that it is error free).

The methods to ensure reliable data transmission are as follows:

1. Inspection and

   Crc is used to check the integrity of the received data. The result of Crc calculation shows that the data is complete without dislocation.

2. ACK mechanism

   TCP realizes reliable data transmission through acknowledgement and response mechanism. In the head of TCP, there is a flag bit - ACK, which indicates whether the confirmation number is valid. The receiver will confirm the data arrived in sequence. When the flag bit ACK=1, the confirmation field of the header is valid. When confirming, the value of the confirmation field indicates that the data before the value has arrived in order. If the sender receives the confirmation message of the sent data, it will continue to transmit the next part of the data; if it has not received the confirmation message for a certain time, it will start the retransmission mechanism.

3. Timeout retransmission mechanism

   When the data is sent without receiving the confirmation of the receiver within a certain period of time, the sender will retransmit (usually a specific time interval is set after sending the message segment, and retransmission will be carried out when the time is up and no response is received).

The process of calculating Crc32 by using the look-up table method implemented by python is as follows.

1. table defining crc

origin_crc32_table = [
#/*Generation polynomial of CRC32 adopts 0x04C11DB7*/
0x00000000, 0x04c11db7, 0x09823b6e, 0x0d4326d9, 0x130476dc, 0x17c56b6b, 0x1a864db2, 0x1e475005, 0x2608edb8, 0x22c9f00f, 0x2f8ad6d6, 0x2b4bcb61, 0x350c9b64, 0x31cd86d3, 0x3c8ea00a, 0x384fbdbd, 0x4c11db70, 0x48d0c6c7, 0x4593e01e, 0x4152fda9, 0x5f15adac, 0x5bd4b01b, 0x569796c2, 0x52568b75, 0x6a1936c8, 0x6ed82b7f, 0x639b0da6, 0x675a1011, 0x791d4014, 0x7ddc5da3, 0x709f7b7a, 0x745e66cd, 0x9823b6e0, 0x9ce2ab57, 0x91a18d8e, 0x95609039, 0x8b27c03c, 0x8fe6dd8b, 0x82a5fb52, 0x8664e6e5, 0xbe2b5b58, 0xbaea46ef, 0xb7a96036, 0xb3687d81, 0xad2f2d84, 0xa9ee3033, 0xa4ad16ea, 0xa06c0b5d, 0xd4326d90, 0xd0f37027, 0xddb056fe, 0xd9714b49, 0xc7361b4c, 0xc3f706fb, 0xceb42022, 0xca753d95, 0xf23a8028, 0xf6fb9d9f, 0xfbb8bb46, 0xff79a6f1, 0xe13ef6f4, 0xe5ffeb43, 0xe8bccd9a, 0xec7dd02d, 0x34867077, 0x30476dc0, 0x3d044b19, 0x39c556ae, 0x278206ab, 0x23431b1c, 0x2e003dc5, 0x2ac12072, 0x128e9dcf, 0x164f8078, 0x1b0ca6a1, 0x1fcdbb16, 0x018aeb13, 0x054bf6a4, 0x0808d07d, 0x0cc9cdca, 0x7897ab07, 0x7c56b6b0, 0x71159069, 0x75d48dde, 0x6b93dddb, 0x6f52c06c, 0x6211e6b5, 0x66d0fb02, 0x5e9f46bf, 0x5a5e5b08, 0x571d7dd1, 0x53dc6066, 0x4d9b3063, 0x495a2dd4, 0x44190b0d, 0x40d816ba, 0xaca5c697, 0xa864db20, 0xa527fdf9, 0xa1e6e04e, 0xbfa1b04b, 0xbb60adfc, 0xb6238b25, 0xb2e29692, 0x8aad2b2f, 0x8e6c3698, 0x832f1041, 0x87ee0df6, 0x99a95df3, 0x9d684044, 0x902b669d, 0x94ea7b2a, 0xe0b41de7, 0xe4750050, 0xe9362689, 0xedf73b3e, 0xf3b06b3b, 0xf771768c, 0xfa325055, 0xfef34de2, 0xc6bcf05f, 0xc27dede8, 0xcf3ecb31, 0xcbffd686, 0xd5b88683, 0xd1799b34, 0xdc3abded, 0xd8fba05a, 0x690ce0ee, 0x6dcdfd59, 0x608edb80, 0x644fc637, 0x7a089632, 0x7ec98b85, 0x738aad5c, 0x774bb0eb, 0x4f040d56, 0x4bc510e1, 0x46863638, 0x42472b8f, 0x5c007b8a, 0x58c1663d, 0x558240e4, 0x51435d53, 0x251d3b9e, 0x21dc2629, 0x2c9f00f0, 0x285e1d47, 0x36194d42, 0x32d850f5, 0x3f9b762c, 0x3b5a6b9b, 0x0315d626, 0x07d4cb91, 0x0a97ed48, 0x0e56f0ff, 0x1011a0fa, 0x14d0bd4d, 0x19939b94, 0x1d528623, 0xf12f560e, 0xf5ee4bb9, 0xf8ad6d60, 0xfc6c70d7, 0xe22b20d2, 0xe6ea3d65, 0xeba91bbc, 0xef68060b, 0xd727bbb6, 0xd3e6a601, 0xdea580d8, 0xda649d6f, 0xc423cd6a, 0xc0e2d0dd, 0xcda1f604, 0xc960ebb3, 0xbd3e8d7e, 0xb9ff90c9, 0xb4bcb610, 0xb07daba7, 0xae3afba2, 0xaafbe615, 0xa7b8c0cc, 0xa379dd7b, 0x9b3660c6, 0x9ff77d71, 0x92b45ba8, 0x9675461f, 0x8832161a, 0x8cf30bad, 0x81b02d74, 0x857130c3, 0x5d8a9099, 0x594b8d2e, 0x5408abf7, 0x50c9b640, 0x4e8ee645, 0x4a4ffbf2, 0x470cdd2b, 0x43cdc09c, 0x7b827d21, 0x7f436096, 0x7200464f, 0x76c15bf8, 0x68860bfd, 0x6c47164a, 0x61043093, 0x65c52d24, 0x119b4be9, 0x155a565e, 0x18197087, 0x1cd86d30, 0x029f3d35, 0x065e2082, 0x0b1d065b, 0x0fdc1bec, 0x3793a651, 0x3352bbe6, 0x3e119d3f, 0x3ad08088, 0x2497d08d, 0x2056cd3a, 0x2d15ebe3, 0x29d4f654, 0xc5a92679, 0xc1683bce, 0xcc2b1d17, 0xc8ea00a0, 0xd6ad50a5, 0xd26c4d12, 0xdf2f6bcb, 0xdbee767c, 0xe3a1cbc1, 0xe760d676, 0xea23f0af, 0xeee2ed18, 0xf0a5bd1d, 0xf464a0aa, 0xf9278673, 0xfde69bc4, 0x89b8fd09, 0x8d79e0be, 0x803ac667, 0x84fbdbd0, 0x9abc8bd5, 0x9e7d9662, 0x933eb0bb, 0x97ffad0c, 0xafb010b1, 0xab710d06, 0xa6322bdf, 0xa2f33668, 0xbcb4666d, 0xb8757bda, 0xb5365d03, 0xb1f740b4]

1. Write a function to create a CRC32 table.

def generate_crc32_table(_poly):
    global custom_crc32_table

    for i in range(256):
        c = i << 24

        for j in range(8):
            if (c & 0x80000000):
                c = (c << 1) ^ _poly
            else:
                c = c << 1

        custom_crc32_table[i] = c & 0xffffffff

1. Write a function to evaluate Crc32.

def getCrc32(bytes_arr):
    length = len(bytes_arr)

    if bytes_arr != None:
        crc = 0xffffffff

        for i in range(0, length):
            #print i, getReverse(bytes_arr[i], 8)
            crc = (crc << 8) ^ custom_crc32_table[(getReverse(bytes_arr[i], 8) ^ (crc >> 24)) & 0xff]
    else:
        crc = 0xffffffff

    # /*-Returns the calculated CRC value*/
    crc = getReverse(crc ^ 0xffffffff, 32)
    return crc

def getReverse(tempData, byte_length):
    reverseData = 0
    for i in range(0, byte_length):
        reverseData += ((tempData>>i)&1)<<(byte_length-1-i)

    return reverseData

When the above functions are completed, check with the following code

if __name__ == '__main__':
    a = 20
    b = 400
    s = struct.pack('>ii', a, b)
    print(s, type(s))

    s = struct.pack('<ii', a, b)
    print(s, type(s))

    print 'Reversal mode LSB First，XOR value is 0 xFFFFFFFF, test:'
    s = struct.pack('>i', 400)
    print ' current CRC Enter initial value:', (s, type(s))
    test = binascii.crc32(s) & 0xffffffff
    print 'Calculated CRC value:', '0x'+"{:0>8s}".format(str('%x'%test))
    test = zlib.crc32(s) & 0xffffffff
    print 'Calculated CRC value:', '0x'+"{:0>8s}".format(str('%x'%test))

    crc32mpeg2_poly = 0x04C11DB7
    #buf = [0x31, 0x32, 0x33, 0x34, 0x35, 0x36]
    #buf = [0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38, 0x39, 0x30, 0x31, 0x32]
    #buf = [0x31, 0x32, 0x33]
    #buf = [0x31, 0x32]
    #buf = [0x31]
    buf_s = [0x00, 0x00, 0x01, 0x90]
    #buf_s = [0x90, 0x01, 0x00, 0x00]

    generate_crc32_table(crc32mpeg2_poly)
    print 'create_crc32_table:', " ".join('0x'+"{:0>8s}".format(str('%x'%i)) for i in custom_crc32_table)
    print 'origin_crc32_table:', " ".join('0x'+"{:0>8s}".format(str('%x'%i)) for i in origin_crc32_table)
    crc_stm = getCrc32(bytearray(buf_s)) & 0xffffffff
    print ' Calculated CRC Value:', '0x' + "{:0>8s}".format(str('%x' % crc_stm))