IP Filters
Depending on whether there is an existing Internet connection, or in other words “the WAN link status is up or down”, the IP filter architecture categorizes traffic into two: Call Filter and Data Filter.
zCall Filter - When there is no existing Internet connection, Call Filter is applied to all traffic, all of which should be outgoing. It will check packets according to the filter rules. If legal, the packet will pass. Then the router shall “initiate a call” to build the Internet connection and send the packet to Internet.
zData Filter - When there is an existing Internet connection, Data Filter is applied to incoming and outgoing traffic. It will check packets according to the filter rules. If legal, the packet will pass the router.
The following illustrations are flow charts explaining how router will treat incoming traffic and outgoing traffic respectively.
Stateful Packet Inspection (SPI)
Stateful inspection is a firewall architecture that works at the network layer. Unlike legacy static packet filtering, which examines a packet based on the information in its header, stateful inspection builds up a state machine to track each connection traversing all interfaces of the firewall and makes sure they are valid. The stateful firewall of Vigor router not just examine the header information also monitor the state of the connection.
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