Sending ICMP messages

ICMP messages are grouped into two categories: error reporting and queries. Some messages are specific to each version; however, a few are common to both versions, as shown here:

Figure 12.10 – ICMP messages

For both categories, each ICMP packet has type, code, and checksum fields. The payload for queries is different from error messages, as each has a different purpose, as we'll see in the following sections. 

Let's start with a review of how ICMP reports errors.

Reporting errors on the network

ICMP error messages report on network issues that prevent data from being delivered. Commonly sent error messages are grouped into categories that have a specific purpose, and include the following:

• Destination unreachable is where a router informs the host that the requested destination address can't be reached.
• Time exceeded is sent when either...