6.2 ICMP Message Types

Figure 6.3 lists the different ICMP message types, as determined by the type field and code field in the ICMP message.

Figure 6.3. ICMP message types.

The final two columns in this figure specify whether the ICMP message is a query message or an error message. We need to make this distinction because ICMP error messages are sometimes handled specially. For example, an ICMP error message is never generated in response to an ICMP error message. (If this were not the rule, we could end up with scenarios where an error generates an error, which generates an error, and so on, indefinitely.)

When an ICMP error message is sent, the message always contains the IP header and the first 8 bytes of the IP datagram that caused the ICMP error to be generated. This lets the receiving ICMP module associate the message with one particular protocol (TCP or UDP from the protocol field in the IP header) and one particular user process (from the TCP or UDP port numbers that are in the TCP or UDP header contained in the first 8 bytes of the IP datagram). We'll show an example of this in Section 6.5.

An ICMP error message is never generated in response to

An ICMP error message. (An ICMP error message may, however, be generated in response to an ICMP query message.)
A datagram destined to an IP broadcast address (Figure 3.9) or an IP multicast address (a class D address, Figure 1.5).
A datagram sent as a link-layer broadcast.
A fragment other than the first. (We describe fragmentation in Section 11.5.)
A datagram whose source address does not define a single host. This means the source address cannot be a zero address, a loopback address, a broadcast address, or a multicast address.

These rules are meant to prevent the broadcast storms that have occurred in the past when ICMP errors were sent in response to broadcast packets.