'cksum(data) -> int
Calculates the same checksum as returned by the UNIX-tool ``cksum``.
Arguments:
data(str): The data to checksum.
Example:
>>> print cksum(\'123456789\')
930766865'

@staticmethod
def cksum(data):

    l = len(data)
    data += packing.pack(l, 'all', endian='little', sign=False)
    return crc.crc_32_posix(data)
