11-4The actual length of the finwait timer is determined by the following formula:Actual length of the finwait timer = (Configured length of the finwait timer – 75) + configured length ofthe synwait timerConfiguring ICMP to Send Error PacketsSending error packets is a major function of ICMP. In case of network abnormalities, ICMP packets areusually sent by the network or transport layer protocols to notify corresponding devices so as tofacilitate control and management.Advantages of sending ICMP error packetsThere are three kinds of ICMP error packets: redirect packets, timeout packets and destinationunreachable packets. Their sending conditions and functions are as follows.1) Sending ICMP redirect packetsA host may have only a default route to the default gateway in its routing table after startup. The defaultgateway will send ICMP redirect packets to the source host, telling it to reselect a correct next hop tosend the subsequent packets, if the following conditions are satisfied:z The receiving and forwarding interfaces are the same.z The selected route has not been created or modified by ICMP redirect packet.z The selected route is not the default route of the device.z There is no source route option in the packet.ICMP redirect packets function simplifies host administration and enables a host to gradually establisha sound routing table to find out the best route.2) Sending ICMP timeout packetsIf the device received an IP packet with a timeout error, it drops the packet and sends an ICMP timeoutpacket to the source.The device will send an ICMP timeout packet under the following conditions:z If the device finds the destination of a packet is not itself and the TTL field of the packet is 1, it willsend a “TTL timeout” ICMP error message.z When the device receives the first fragment of an IP datagram whose destination is the deviceitself, it starts a timer. If the timer times out before all the fragments of the datagram are received,the device will send a “reassembly timeout” ICMP error packet.3) Sending ICMP destination unreachable packetsIf the device receives an IP packet with the destination unreachable, it will drop the packet and send anICMP destination unreachable error packet to the source.Conditions for sending this ICMP packet:z If neither a route nor the default route for forwarding a packet is available, the device will send a“network unreachable” ICMP error packet.
