Sample 2 - Who-Is/I-Am Counter¶
This sample application builds on the first sample by overriding the default processing for Who-Is and I-Am requests, counting them, then continuing on with the regular processing.
The description of this sample will be about the parts that are different from sample 1.
Python has a excellent defaultdict datatype from the collections module that is perfect for this application. It is very easy to use:
>>> from collections import defaultdict >>> x = defaultdict(int)
The essential idea is that you can treat some key as having a default value if it doesn’t exist, so rather than doing this:
>>> x['a'] = x.get('a', 0) + 1
You can do this:
>>> x['a'] += 1
Processing Service Requests¶
When an instance of the
app.Application receives a request it attempts
to look up a function based on the message. So when a WhoIsRequest APDU is
received, there should be a do_WhoIsRequest function.
The beginning is going to be standard boiler plate function header:
def do_WhoIsRequest(self, apdu): """Respond to a Who-Is request.""" if _debug: SampleApplication._debug("do_WhoIsRequest %r", apdu)
The middle is going to process the data in the request:
# build a key from the source and parameters key = (str(apdu.pduSource), apdu.deviceInstanceRangeLowLimit, apdu.deviceInstanceRangeHighLimit, ) # count the times this has been received whoIsCounter[key] += 1
And the end of the function is going to call back to the standard application processing:
# pass back to the default implementation BIPSimpleApplication.do_WhoIsRequest(self, apdu)
The do_IAmRequest function is similer:
def do_IAmRequest(self, apdu): """Given an I-Am request, cache it.""" if _debug: SampleApplication._debug("do_IAmRequest %r", apdu)
It uses a diferent key, but counts them the same:
# build a key from the source, just use the instance number key = (str(apdu.pduSource), apdu.iAmDeviceIdentifier, ) # count the times this has been received iAmCounter[key] += 1
And has an identical call to the base class:
# pass back to the default implementation BIPSimpleApplication.do_IAmRequest(self, apdu)
By building the key out of elements in a useful order, it is simple enough to sort the dictionary items and print them out, and being able to unpack the key in the for loop is a nice feature of Python:
print "----- Who Is -----" for (src, lowlim, hilim), count in sorted(whoIsCounter.items()): print "%-20s %8s %8s %4d" % (src, lowlim, hilim, count) print
Pairing up the requests and responses can be a useful excersize, but in most cases the I-Am response from a device will be a unicast message directly back to the requestor, so relying on broadcast traffic to analyze device and address binding is not as useful as it used to be.