Certain Internet service providers have begun to interfere with their users' communications by injecting forged or spoofed packets - data that appears to come from the other end but was actually generated by an Internet service provider (ISP) in the middle. This spoofing is one means (although not the only means) of blocking, jamming, or degrading users' ability to use particular applications, services, or protocols. One important means of holding ISPs accountable for this interference is the ability of some subscribers to detect and document it reliably. We have to learn what ISPs are doing before we can try to do something about it. Internet users can often detect interference by comparing data sent at one end with data received at the other end of a connection.