Operation Manual – IP Addressing and IP Performance
H3C S5500-EI Series Ethernet Switches
Chapter 2 IP Performance Configuration
2-5
I. Advantage 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)
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 and
notify it to reselect a correct next hop router 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 packets function simplifies host administration and enables a host to
gradually establish a sound routing table to find out the best route
2)
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:
z
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.
z
When the device receives the first fragment of an IP datagram whose destination
is the device itself, it will start 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 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:
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.
z
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.
z
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.