D-Link, formally known as Datex Systems Inc., is a Taiwan-based company that manufactures networking and communications products geared towards home users, as well as small and medium-sized businesses. D-Link products include wireless routers, switches and firewalls, media players, security cameras and other surveillance devices. Many of these devices feature a Stateful Packet Inspection, or SPI, mode option.
SPI, also known as stateful inspection or stateful firewall, is a setting available on many D-Link devices that tracks various packets or network connections. If your D-Link device includes an SPI mode, you can enable or disable this protective setting through the device's main Settings menu. When enabled, SPI mode blocks unrecognized network connections, and approaches each packet as an isolated connection, unlike a stateless firewall.
What the Router Checks
When you enable SPI mode, the D-Link device checks several pieces of information for each incoming data packet. This checked information is more comprehensive than the information checked by devices that do not have SPI mode enabled. Without SPI mode, devices limit the validity of information checked to a packet's header; with SPI mode, they check information that includes the port address, the IP address, and the sequence number and acknowledgment code of each packet.
Enabled Vs. Disabled
SPI mode is not enabled by default on a D-Link device. This means that if you want to use this security setting, you need to enable SPI mode through the device's Settings menu. While enabling SPI mode does increase the security related to incoming connections, your D-Link device can function properly without SPI mode. SPI mode does, however, help to ensure that packets of information entering the system originate at the port where you've established the network connection.
A potential benefit of not enabling SPI mode is that you may have an easier time using some applications that require peer-to-peer communication, such as games. In most cases, if you want to use these applications and simultaneously leave SPI mode enabled, you must manually open ports specifically for these applications, so that they can continue to run in SPI mode without interruption.