H3C Technologies H3C S12500 Series Switches - Manual (Page 109)

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Enabling sending ICMP error packets
Sending error packets is a major function of ICMP. Error packets are usually sent by the network or
transport layer protocols to notify the source device of network failures or errors.
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Functions of sending ICMP error packets
ICMP error packets include redirect, time-exceeded, and destination unreachable packets.
• ICMP redirect packets
A host that has only one default route sends all packets to the default gateway. The default
gateway sends an ICMP redirect packet to inform the host of a correct next hop by following these
rules:
{
The receiving and sending interfaces are the same.
{
The selected route is not created or modified by any ICMP redirect packet.
{
The selected route is not destined for 0.0.0.0.
{
There is no source route option in the received packet.
ICMP redirect packets simplify host management and enable hosts to gradually optimize their
routing table.
• ICMP time-exceeded packets
A device sends ICMP time-exceeded packets by following these rules:
{
If a received packet is not destined for the device and the TTL field of the packet is 1, the device
sends an ICMP TTL Expired in Transit packet to the source.
{
When the device receives the first fragment of an IP datagram destined for it, it starts a timer.
If the timer expires before all the fragments of the datagram are received, the device sends an
ICMP Fragment Reassembly Timeout packet to the source.
• ICMP destination unreachable packets
A device sends ICMP destination unreachable packets by following these rules:
{
If a packet does not match any route and there is no default route in the routing table, the
device sends a Network Unreachable ICMP error packet to the source.
{
If a packet is destined for the device but the transport layer protocol of the packet is not
supported by the device, the device sends a Protocol Unreachable ICMP error packet to the
source.
{
If a UDP packet is destined for the device but the packet's port number does not match the
corresponding process, the device sends the source a Port Unreachable ICMP error packet.
{
If the source uses Strict Source Routing to send packets, but the intermediate device finds that
the next hop specified by the source is not directly connected, the device sends the source a
Source Routing Failure ICMP error packet.
{
If the MTU of the sending interface is smaller than the packet and the packet has DF set, the
device sends the source a Fragmentation Needed and DF-set ICMP error packet.
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Disadvantages of sending ICMP error packets
Sending ICMP error packets facilitates network control and management, but it has the following
disadvantages:
• Sending a lot of ICMP packets increases network traffic.