What is Deep Packet Inspection?

PC World — It's easy to turn a deaf ear to the controversy surrounding recent copyright protection bills like the Stop Online Piracy Act (SOPA) or the PROTECT IP Act, which threatened to curtail free speech on the Internet by allowing the U.S. Department of Justice to blacklist and block access to websites suspected of copyright infringement. Most of us don't visit websites suspected of illegally distributing copyrighted material, so blocking us from accessing them seems harmless. But should your ISP ever be legally obligated to prevent you from accessing restricted websites, it will have to find a way to monitor your online activity, and that could cause your privacy to be compromised if your ISP employs deep packet inspection tools to keep tabs on you.

To understand how deep packet inspection works and the potential threat it poses to your privacy, you need to know that your PC packages all the information you send and receive online into packets of data. Internet routers read the labels on those packets to determine what they are, who they're from, and where they're going; this is how most Internet traffic works, and it's how the firewall on your router distinguishes which packets of data make up that email message from your sister and which packets of data are from a spammer in Georgia.

When your Internet service provider engages in deep packet inspection, it uses powerful software from vendors like Procera Networks to scan all of the data packets that pass through its network. The contents of each packet are scanned (and sometimes logged), and then blocked or routed to the appropriate destination. There are plenty of great reasons for your ISP to do this on your behalf: Deep packet inspection helps your ISP block the spread of computer viruses, identify illegal downloads, and prioritize the data transmitted by bandwidth-heavy applications like video chat and VoIP applications to alleviate network congestion and improve your service. Law enforcement officials (with a court order) can use these tools to lawfully intercept communications of suspected criminals.

But deep packet inspection has a dark side, and in the absence of strict legal restrictions, your ISP is free to root through all the information you exchange online and use it as they see fit. Personal data like your age, location, and shopping records can be logged and sold in anonymized batches to advertising companies, and law enforcement agents can monitor and curtail your Internet access without your knowledge. Without strict limitations to preserve user privacy, this sort of deep data filtering can significantly impair your ability to remain anonymous online.

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