A CCN filter is essentially an "interest in Interests" -- it allows a caller to register to see Interest messages that come in from the network, and optionally generate (or merely provide) data in response. Filters are registered using CCNBase.registerFilter(ContentName, CCNFilterListener). Note that we will only see interests that match the name we registered in our filter -- in other words, Interests in that name or its children; not its parents. We will also only see Interests that were not already satisfied out of ccnd's own cache (or on the network path to our node).