Shallow packet inspection (SPI) refers to the inspection of packet headers for optimization of packet routing, detection of network abuse, and statistical analysis. Such inspection does not disclose the contents of data packets.
It is a less sophisticated version of the deep packet inspection (DPI) technique that can be used to block packets based on their content. Unlike DPI, SPI makes broad generalities about traffic based solely on evaluating the packet header. Although shallow packet inspection cannot provide the same refined/detailed traffic assessments as DPI, it is much better at handling a large volume than DPI.
SPI is much less refined than DPI, but it is capable of handling a greater volume of traffic much more quickly. SPI is akin to judging a book by its cover. This method is prone to exploitation by users because they can disguise their packets to look like a different kind of traffic.