A firewall technology that ensures that all inbound packets are the result of an outbound request. Also called "stateful packet inspection" (SPI), it was designed to prevent harmful or unrequested packets from entering the computer. For example, if you click on a link to a Web page, an HTTP request is being made to a specific URL address. All packets coming back from that URL would pass the stateful inspection and be accepted. Every so many minutes, your e-mail program queries the mail server, and returning packets from that server are allowed.
Essential and Problematic
Stateful inspection is the norm and generally a major component in every network firewall and personal firewall. However, stateful inspection causes problems with videoconferencing and VoIP, in which a user outside the network wants to initiate a communication with a user inside the network. Various techniques are used to work around this (see STUN, UPnP and port forwarding). See firewall.
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