FirewallPAPI is a specialised firewall that specifically routes traffic that uses the TCP and UDP protocols. It does this using three key parameters - the IP of origin, destination and the input and output port that's being targeted.
Key to the attraction of this firewall is the fact that is extremely light on resources. In fact, all it really does is send packets of data to the three key areas mentioned above to continually test their integrity. Once it detects a strange or irregular response, it automatically triggers the alarm. The downside of this however is that since it's in continual packet sending mode, it can have a slight slow down effect on your PC and internet connection speed.
This application has mainly been developed for users who want to create their own rules because none are installed by default meaning the firewall needs quite a bit of customisation before you can use it.
For those that are confident of firewall configuration and want a lightweight solution, FirewallPAPI is a solid solution but beginners should stay clear.