ICMP error messages report on network issues that prevent data from being delivered. The more commonly sent error messages are grouped into categories that have a specific purpose, as discussed here:

• Destination unreachable is where a router informs the host that the requested destination address can't be reached.
• Time exceeded is sent when the hop limit reaches zero.
• Parameter problems can be reported when there is an issue in determining a field value in the header or extension header.

As discussed, ICMPv6 has many of the same functions as ICMP, but also provides additional roles to support IPv6. As a result, when reporting errors, you may see this on an IPv6 network:

• Packet too big is sent when a device cannot send the data, as the packet is larger than the Maximum Transmission Unit (MTU) of the outgoing link.

In either version, when there is an error, ICMP helps reconstruct the possible reasons why the data did not get to its final destination. If appropriate, the sending host will adjust the payload so that the data can be delivered successfully.

When an error message is sent, the message includes the header, as well as the IP header, and then the first eight bytes (or 64 bits) of the original datagram that caused the error, as shown in this screenshot:

Within the redirect message, you see an IP header, and then an ICMP message followed by the first eight bytes (or 64 bits) of the original IP and ICMP header. 

Common error messages include destination unreachable and redirection. Errors are received and acted upon by TCP, IP, or user applications. In some cases, ICMP messages are ignored. However, some error messages must not be ignored, such as redirect messages, which will cause an automatic update to the host's routing table.

As we can see, ICMP error messages provide additional information so that the host can see exactly what happened. However, ICMP can also request and provide information, as discussed in the following section.