Length fields are not being checked

[erickvermot]

length fields are not beeing checked.
(pg 50) - "OpenFlow is a simple binary protocol, therefore invalid OpenFlow messages
generated by OpenFlow implementations will in general result in the message
having the wrong length or some fields having invalid values (see 7.1.3). It
is recommended to monitor those conditions to detect non-compliant
implementations."

• when length is passed as a parameter, as in flow_match, maybe length
  should be computed locally, or checked for validity before message is sent.

[erickvermot]

I propose some simple length and bit checks on unpack methods like in the examples bellow. Do you guys want this kind of implementation? @diraol @beraldoleal ?

• check Pad unpacking. Check if length is correct and bits are 0 simply checking if buffer slice equals result of pack() method.

    def unpack(self, buff, offset=0):
        if not buff[offset:offset + self._length] == self.pack():
            raise UnpackException

• reimplement DPID unpacking. Check if buffer length is correct by trying to unpack the hole buffer slice. ( update: I realized now buffer is not beeing sliced prior to unpacking the attributes... So with this approach, my method may not work very wel...)
  old method:

    def unpack(self, buff, offset=0):
        begin = offset
        hexas = []
        while begin < offset + 8:
            number = struct.unpack("!B", buff[begin:begin + 1])[0]
            hexas.append("%.2x" % number)
            begin += 1
        self._value = ':'.join(hexas)

proposition:

    def unpack(self, buff, offset=0):
        hexa_values = struct.unpack("!8B", buff[offset:offset + 8])
        self._value = ':'.join(('{:02x}'.format(number)
                                for number in hexa_values))

should I continue making theese kind of changes? or is this not what you have in mind?

[erickvermot]

• maybe groups similar methods/classes #315 (group similar classes/methods) could be done before this, or at the same moment. I believe it would lead to a easier dev.

[beraldoleal]

I agree, we must check this validations.

However we have already an issue for a more open discussion, please see #96 .

We need to find a solution for a more generic implementation. I'm removing the milestone for now. But for sure we need to think this soon.

[erickvermot]

( update: I realized now buffer is not beeing sliced prior to unpacking the attributes... So with this approach, my method may not work very well...)
I'll leave this for now so we can discuss later.