def [](id)
      interfaces_hash.fetch(id.to_s) do
        raise UnknownInterface.new(id.to_s),
              "Unknown interface: #{id}"
      end
    end
def [](id)
      interfaces_hash.fetch(id.to_s) do
        raise UnknownInterface.new(id.to_s),
              "Unknown interface: #{id}"
      end
    end
Returns traffic information for a certain interface.

 @param [String] id The name of the interface.
 @raise [UnknownInterface] An error that is raised if the
   specified interface is not tracked.
 @return [Interface]