ICMPv4, or simply ICMP, is used alongside IPv4 to communicate network issues that prevent data from being delivered. ICMP error and query messages can alert end systems when there are connectivity issues, and can also obtain diagnostic information from intermediary systems such as the round-trip time.

As powerful as ICMP is, it cannot make IP a reliable protocol; it only assists in data delivery by providing error messages and information. There are times when the causes of delays in data transmission are outside of the messages ICMP can send and report. In that case, it's up to TCP to notify the host of transmission errors during delivery.

To see an example of an error, we can use this example on CloudShark. Go to https://www.cloudshark.org/captures/155db9732c91 and then open the file in Wireshark, as shown in the following screenshot:

I have removed the coloring rules to make the graphic more visible, as Wireshark views this as an error and will show up with black coloring. In this capture, ICMP is reporting an error. In the lower half of the screenshot, we see the IP datagram with an IPv4 header, followed by the ICMP header.

Now that we have reviewed some of the basics of ICMP, let's take a look at ICMPv6, which has many of the same functions, but which also provides additional roles to support IPv6.