This page describes the various linux syscontrols and what they do, these can be viewed/edited via the sysctl(8) command, or via /proc/sys/. eg, kernel.panic can be viewed by

kernel.panic

or

cat /proc/sys/kernel/panic

kernel

kernel.panic

This is the number of seconds to wait after a kernel has paniced before the machine will reboot itself automatically. Very useful for unattended servers, or machines that are difficult to get physical access to.

net

These sysctl's control the networking. See also http://bec.at/support/ipsysctl-tutorial/tcpvariables.html

net.ipv4

These control most of the IPv4 options

net.ipv4.ip_forwarding

enable global IP forwarding. Very imporant.

net.ipv4.tcp_fack

Enable Forward Acknowledgement congestion avoidance and fast retransmission. The value is not used, if net.ipv4.tcp_sack is not enabled. See http://www.psc.edu/networking/papers/fack_abstract.html for details on how this option works. Basically it seems to assume that missing sequence ranges are dropped (ie, implies no reordering). Linux will disable fack on a per connection basis if it detects reordering.