D-Link DSL-G2452GR User Manual page 92

Section 4 - Configuration
NAT
Support rport
Toggle the switch to enable the Symmetric Response Routing function
in accordance with RFC3581. This function allows sending responses
to requests to the port and IP address from which the request was
received via the NAT-enabled router.
NAT keep alive Toggle the switch to allow the router to support the state of
automatically forwarded ports by periodic exchange of service
messages. If the switch is moved to the right, the NAT support interval
field will become available for editing.
NAT support
Specify the time interval between service messages.
interval (in
seconds)
D-Link DSL-G2452GR User Manual 88