Internet Core Protocols: The Definitive Guide

Echo Request and Echo Reply query messages

One of the simplest tests that a user may wish to perform is verifying that a remote system is up and running on the network. Such a test may be required when basic connectivity appears to be failing.

ICMP provides two query messages that work together to provide just this service. The ICMP Echo Request query message is a probe sent by a user to a destination system, which responds with an ICMP Echo Reply query message.

RFC 1122 states that “every host must implement an ICMP Echo server.” Since this service is mandatory, any user should be able to send an ICMP Echo Request to any host on the Internet and receive an ICMP Echo Reply message in return. However, this is not always the case, as firewalls may be blocking the packets (for security reasons), or the packets may simply fail to be delivered.

Furthermore, RFC 1122 also states that every host should implement an end-user-accessible application interface for sending Echo Request query messages to other hosts. Typically this application interface is implemented as a program called ping. Almost every computing environment—even the most basic network print servers and fax gateways—offers some kind of ping program for testing basic connectivity. This is expected, since RFC 1122 mandates that any device with a TCP/IP stack must have one.

ping works by sending one or more ICMP Echo Request messages to a destination system, and then measuring the amount of time it took for the ICMP Echo Reply ...