There are quite a few ways in which ISP’s can block or obstruct internet traffic.
- Simple: block the port. That way, you stop any application that wants to use that port.
- A little harder: packet inspection. Many protocols can use more than just their standard ports. Therefore, use dpi, deep packet inspection. Depending on the contents, the provider decides whether a packet is allowed to continue its journey to its destination.
- For pros only: allow packets to pass only some of the time. An example is obstructing peer2peer for Skype. This forces Skype to use a proxy for a bandwidth intensive service such as video calling, with all sorts of legal and financial consequences.
The Open Internet NL app looks for the first two tactics, simple port blocks, and obstruction through packet inspection.