A type of ICMP packet. It is used to dynamically reconfigure routing to be more optimal.

If a host src sends an IP packet to dst via router rtr1, and rtr1 finds that it has to send it back out to router rtr2 from the same interface, then rtr1 might send an ICMPRedirect to src to notify it that it should send packets for dst directly to rtr2 instead of sending them to rtr1.

Obviously, ICMPRedirect messages have great potential for abuse, as they can allow an attacker to cause a machine to send its IP traffic to a host of the attacker's choosing.