nDPI is an open source LGPLv3 library for deep-packet inspection. Based on OpenDPI it includes ntop extensions.
nDPI is a ntop-maintained superset of the popular OpenDPI library. Released under the LGPL license, its goal is to extend the original library by adding new protocols that are otherwise available only on the paid version of OpenDPI. In addition to Unix platforms, we also support Windows, in order to provide you a cross-platform DPI experience. Furthermore, we have modified nDPI to be more suitable for traffic monitoring applications, by disabling specific features that slow down the DPI engine while being them un-necessary for network traffic monitoring.
You can read about how nDPI is used from one of the resources below: