Advantages of sending ICMP error packets

ICMP error packets include the following types:

• ICMP redirect packets

  A host may have only a default route to the default gateway in its routing table after startup. If the following conditions are satisfied, 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:

  • 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 establish a sound routing table to find the best route.

• 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 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.

• 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 an ICMP destination unreachable 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 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 will send 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 will send 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 will send the source a "fragmentation needed and Don't Fragment (DF)-set" ICMP error packet.