Actual length of the finwait timer = (Configured length of the finwait timer – 75) + configured length of the
Configuring ICMP to Send Error Packets
Sending error packets is a major function of ICMP. In case of network abnormalities, ICMP packets are
usually sent by the network or transport layer protocols to notify corresponding devices so as to
facilitate control and management.
Advantages of sending ICMP error packets
There are three kinds of ICMP error packets: redirect packets, timeout packets and destination
unreachable packets. Their sending conditions and functions are as follows.
Sending ICMP redirect packets
A host may have only a default route to the default gateway in its routing table after startup. The default
gateway will send ICMP redirect packets to the source host, telling it to reselect a correct next hop to
send the subsequent packets, if the following conditions are satisfied:
The receiving and forwarding interfaces are the same.
The selected route has not been created or modified by ICMP redirect packet.
The selected route is not the default route of the device.
There is no source route option in the packet.
ICMP redirect packets function simplifies host administration and enables a host to gradually establish a
sound routing table to find out the best route.
Sending ICMP timeout packets
If the device received an IP packet with a timeout error, it drops the packet and sends an ICMP timeout
packet to the source.
The device will send an ICMP timeout packet under the following conditions:
If the device finds the destination of a packet is not itself and the TTL field of the packet is 1, it will
send a "TTL timeout" ICMP error message.
When the device receives the first fragment of an IP datagram whose destination is the device itself,
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.
Sending ICMP destination unreachable packets
If the device receives an IP packet with the destination unreachable, it will drop the packet and send an
ICMP destination unreachable error packet to the source.
Conditions for sending this ICMP packet:
If neither a route nor the default route for forwarding a packet is available, the device will send a
"network unreachable" ICMP error packet.
If the destination of a packet is local while the transport layer protocol of the packet is not supported
by the local device, the device sends a "protocol unreachable" ICMP error packet to the source.
When receiving a packet with the destination being local and transport layer protocol being UDP, if
the packet's port number does not match the running process, the device will send the source a
"port unreachable" ICMP error packet.