Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 178
ICMP Informational Messages

•  Echo Request

Type: 128

Code: 0
The first 16 bits of the ICMP message are an identifier to aid in the construction of an Echo Reply to an Echo Request. It may contain a 0. The next 16 bits are a sequence number to aid in matching specifically multiple echo requests from the same source. It, too, may contain a 0.
The remaining message is option data that may have been typed in on the request and is echoed on the reply.
This is the PING command that you may have typed in. For example, PING 192.1.1.1 is an echo request to an echo server residing on host 192.1.1.1.

•  Echo Reply

Type: 129

Code: 0
The first 16 bits (after the ICMP header) comprise an identifier field from the previously received Echo Request message, which is used to match replies with requests. The next 16 bits comprise a sequence number from the node that sent the echo request number message. This is useful to match multiple requests from the same host.
The rest of the message contains echoed data copied from the received echo request.