Probing the Network
Since ICMP is a generic messaging protocol, it is also useful for determining general characteristics about the network. ICMP query messages provide this service, allowing systems to request information about the network in general.
ICMP queries are conversational by nature, with one system seeking information from another, and with the remote system returning the requested information. This process can be seen with the ping program’s use of Echo Request query messages, which are responded to with Echo Reply query messages. This model is in contrast to the one-way nature of ICMP error messages, which are sent but not responded to.
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