Configuring Icmp To Send Error Packet; Advantages Of Sending Icmp Error Packets - HP 830 Series Configuration Manual

Configuring ICMP to send error packet

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.

Advantages of sending ICMP error packets

ICMP error packets include redirect, timeout, and destination unreachable packets.
ICMP redirect packets
•
A host might have only a default route to the default gateway in its routing table after startup. If the
following conditions are satisfied, the default gateway sends ICMP redirect packets to the source
host, telling it to reselect a correct next hop to send the subsequent packets:
The receiving and forwarding interfaces are the same.
The selected route has not been created or modified by an ICMP redirect packet.
The selected route is not the default route of the device.
There is no source route option in the packet.
The ICMP redirect packets function simplifies host administration and enables a host to gradually
optimize the routing table.
• ICMP timeout packets
If the device receives an IP packet with a timeout error, it drops the packet and sends an ICMP
timeout packet to the source.
The device sends an ICMP timeout packet under the following conditions:
If the device finds that 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.
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 sends a "reassembly timeout" ICMP error packet.
• ICMP destination unreachable packets
If the device receives an IP packet with the destination unreachable, it drops the packet and sends
an ICMP destination unreachable error packet to the source.
Conditions for sending an ICMP destination unreachable packet:
If neither a route nor the default route for forwarding a packet is available, the device sends a
"network unreachable" ICMP error packet.
If the destination of a packet is local, but 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 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.
When forwarding a packet, if the MTU of the sending interface is smaller than the packet, but
the packet has been set as "Don't Fragment," the device sends the source a "fragmentation
needed and Don't Fragment (DF)-set" ICMP error packet.
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