13-5
To do…
Use the command…
Remarks
Configure the TCP finwait timer
tcp timer fin-timeout time-value
Optional
675 seconds by default.
Configure the size of TCP
receive/send buffer
tcp window window-size
Optional
8 KB by default.
The 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 of
the synwait timer
Configuring ICMP to Send Error Packets
Sending error packets is a major function of ICMP. In case of network abnormalities, error 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.
1) ICMP redirect packets
A host may have only a default route to the 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:
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 simplifies host administration and enables a host to gradually establish a sound routing
table to find the best route.
2) ICMP timeout packets
If a device receives 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:
z
If the device finds the destination of a packet is not itself and the TTL field of the packet is 1, it
sends a “TTL timeout” ICMP error message.
z
The device starts a timer when it receives the first fragment of an IP datagram destined to it. If the
timer times out before all fragments of the datagram are received, the device sends a
“reassembly timeout” ICMP error packet.
