Internet Core Protocols: The Definitive Guide

When Not to Send ICMP Messages

Just as it is important to know when to send an error message, it is also important to know when an error message should not be sent. For example, any transient error that causes delivery to fail (such as an invalid checksum or a data-link delivery failure) should not be reported. However, the ICMP specifications also state that ICMP error messages should not be generated when their usage will generate an excessive amount of network traffic. For example, RFC 1122 states that ICMP error messages should not be sent as the result of receiving:

• Another ICMP error message (although ICMP error messages can be sent in response to ICMP query messages)

• An IGMP message of any kind

• An IP datagram with a destination IP address for a broadcast or multicast address

• An IP datagram with a nonspecific source address (all zeroes, a loopback address, or a broadcast or multicast address)

• A data-link frame with a broadcast or multicast address

• Any fragment—other than the first fragment—from a fragmented IP datagram

The first rule is obvious. You would not want to generate ICMP error messages in response to other ICMP error messages, as a message loop could cause a network storm that would prevent any other packets from getting sent. However, RFC 1122 states that devices can send ICMP error messages in response to ICMP query messages. For example, a router can issue an ICMP Redirect error message in response to an ICMP Echo Request query message that was sent to the wrong router. ...