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DPI software extracts metadata (as an additional function apart from classifying applications / protocols)

But how does it do this? For classification, I get that you can use pattern matching or port-based matching or heuristic analysis. But for metadata extraction, do they use these same methods? Or do they just read the metadata from the contents and headers of packets?

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There is no single, rigid definition for deep packet inspection. It's usually understood as doing all those things: extracting transport-layer and application-layer data and analyzing it using various methods, including pattern matching, heuristics, scanning content for data-loss, malware protection, unwanted content, and so on.

Generally, DPI is known to use all information it can get at: headers, content, even breaking encryption (SSL inspection). Some products may even logically add metadata (from a central database) to a packet (e.g. geo location, provider, source ASN, ...) for analysis.

The capabilities of the many products vary widely, however. How those products exactly work is often a trade secret.

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    Thanks. I think the last line is especially correct, given that the definition of network traffic metadata itself varies by product. Jun 2, 2021 at 9:29
  • Indeed - I've expanded the answer a bit on the metadata aspect.
    – Zac67
    Jun 2, 2021 at 11:12

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