From the Economist:
"Governments want to extend wiretapping rules from phones to the internet, but doing so is hard
AMONG the many benefits of the internet's rise over the past decade has been the advent of free phone calls between its users—and much cheaper calls even for people who are not online, since ordinary calls can be partly routed over the internet. For people who work in foreign countries, have friends and relatives spread around the world, or simply have to make a lot of calls, this is great news. But for law-enforcement organisations who are used to being able to tap conventional telephone networks, it is causing increasingly painful headaches. Around the world, the emergence of voice-over-internet-protocol (VoIP) telephony is forcing authorities and communications firms into both conflict and co-operation.
Their shared problem is a fundamental one that results from the very nature of the internet. In the old world of telecoms, the path of a call was easy to follow: a continuous analogue or digital connection was established between the two parties, so it was easy for investigators to select a point somewhere along the line (at the telephone exchange nearest to the caller, for example) to tap the call.
On the internet things are very different. All information, whether e-mails, web pages, music downloads or voice calls, is chopped up into small packets of data and fired off across the network. The path one packet takes across the sprawling network may be different from the path of the next, and packets may arrive at the destination out of order, or not arrive at all. If intercepting a traditional phone call is like apprehending a single suspect at his home, eavesdropping on a VoIP call is more like trying to capture all the members of a gang as they cross a busy city in a fleet of separate vehicles. "