ICMP tunneling works by injecting arbitrary data into an echo packet sent to a remote computer.
The remote computer replies in the same manner, injecting an answer into another ICMP packet and sending it back.
Depending on the implementation of the ICMP tunneling software, this type of connection can also be categorized as an encrypted communication channel between two computers.
[2] One way to prevent this type of tunneling is to block ICMP traffic, at the cost of losing some network functionality that people usually take for granted (e.g. it might take tens of seconds to determine that a peer is offline, rather than almost instantaneously).
If the network operator made the erroneous assumption that it is enough to only block normal transport protocols like TCP and UDP, but not core protocols such as ICMP, then it is sometimes possible to use an ICMP-tunnel to access the internet despite not having been authorized for network access.