Deep Packet Inspection: How do mobile carriers detect VoIP streams inside an encrypted connection and are able to interpret (just) them?
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T-Mobile Germany (D1-Netz) disallows the use of VoIP and Skype via the Terms of Service. But it obviously goes much further. With some sort of deep packet inspection T-Mobile monitors the connections and interrupts only the forbidden one even inside of an encrypted and/or tunneled connection. How is that even technically possible? Are there other service providers out there doing the same?
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Answer:
They are not. Most DPI devices have application ready intelligence, i.e. They are able to classify the packets based on either their initial communication with a host and/or in the case they are not encrypted based on the contents of the packet themselves. Unencrypted VOIP packets can very easily be detected. Skype is detected based on the initial connection and communication between two peers based on that initial connection is then disturbed or blocked. Pure encrypted tunnels like VPNs carrying voice or Skype traffic cannot be detected by DPI devices.
Faisal Khan at Quora Visit the source
Other answers
Many carriers use DPI for various purposes - not strictly for blocking. Tethering detection is a common use and the carrier may then measure that separately and charge a separate price for it. Which is fair as a smartphones can only drive a limited amount of data usage - tethering can drive much higher volumes. DPI capabilities vary quite a bit between the engines and DPI is probably the incorrect term for it. Done well it is a combination of techniques including DPI, fingerprinting application behavior and then applying it in the wild. As Faisal states, encryption is a good countermeasure - but using a variety of techniques it is not guaranteed.
Stephen Bryant
Its not about inspecting the payload but building a meta-data profile. You can analyse protocol streams and build a library of unique characteristics about different traffic types. You can use this meta-data to fingerprint what is being used and then block it using devices that analytic and processing power to handle that.
Greg Ferro
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