In an ideal Internet all packets would be treated as equal by the Internet Service Providers (ISP) and backbone operators who transport them across cyberspace. Unfortunately, this is not always the case since many ISPs restrict or completely block Internet access to some services by discriminating against certain network protocols.
Several telecommunication companies, who are also offering Internet access, have for example been known to block the Voice-over-IP (VoIP) application Skype in their networks. The underlying reason for this discrimination has in most cases been because the telecommunication providers see Skype as a competitor to their own telephony services. Peer-to-peer (P2P) file sharing applications are also often blocked or bandwidth limited by ISPs.
The principle of network neutrality (also known as "internet openness") advocates that users should be able to send and receive data across the Internet without having the traffic discriminated based on content, application, protocol, source or destination. An ISP who is limiting the bandwidth of one or several P2P protocols is thereby violating the network neutrality principle. The legal requirements for ISPs to comply with the network neutrality principle varies between countries.
However, from an ethical point of view it is pretty obvious that it should be the users, not the ISPs, who decide what protocols and applications can be used on the Internet. The network neutrality principle also protects the concept of an open Internet that allows for democratic communication.
Blocking of P2P filesharing
P2P file sharing is a technology for efficient sharing of data between peers across the Internet. Just as with any other technology for transferring files, P2P file sharing can be used for sharing lawful as well as unlawful content. There is a great deal of lawful content, such as open licensed software and digital media, that can be downloaded through P2P file sharing. Unfortunately, the amount of unlawful content available on P2P file sharing networks is significantly greater.
Copyright violation, however, is not usually a concern for ISPs. The reason many ISPs block P2P traffic is because more than half of the traffic on the Internet is P2P traffic (according to the Ipoque Internet Study 2008/2009), and a small group of active P2P users can typically use up the majority of an ISPs available bandwidth.
A common method for actively controlling the bandwidth of network traffic is to apply "traffic shaping," which is a rate limiting technique that delays packet transmissions when the bandwidth exceeds a predetermined threshold. ISPs can assign differentiated threshold values depending on used application layer protocol and thereby effectively throttle the bandwidth for P2P traffic, or whatever traffic class they want to suppress. But first they need to perform traffic classification of the sessions in their networks to determine what protocols or applications that are being used.
The most simple form of traffic classification uses the server-side TCP and UDP port numbers; HTTP for example typically uses TCP port 80 while DNS relies on UDP port 53. Port number classification is obviously easily dodged by P2P applications using port numbers that are user supplied or randomised. Several port independent methods for classifying traffic have therefore evolved, many use Deep Packet Inspection (DPI) to match payload data in the observed traffic to signatures of known protocols.
Enter protocol obfuscation
Modern P2P file sharing applications such as Vuze, uTorrent and eMule have introduced protocol obfuscation techniques to avoid being fingerprinted by the port independent traffic classification methods. The popular VoIP application Skype applies obfuscation to all of its traffic, which makes the application difficult to identify through network monitoring.