Stateful Packet Inspection (SPI) is a simple form of data scanning whereby data is scanned on a packet-by-packet basis according to whether or not the firewall deems the data to be legitimate. Any suspicious or non-requested packets are flagged, logged, or simply denied. Packets are only allowed to pass through the firewall if they are associated with a valid session initiated from within the network.
If a Trojan has managed to breach the other security defences due to a negligent user--the SPI firewall will allow that data through as it seemingly comes from a legitimate request on the LAN. Where SPI firewalls come into their own is in conjunction with other methods of data scanning within the firewall, or with another firewall on the LAN. SPI provides a percentage of coverage while still maintaining performance across the network.
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