The Linux ping utility has the nice feature that fills the ICMPv6 echo
request payload with a pattern `payload_index & 0xFF`.
Then the ICMPv6 echo response payload is checked to verify that the pattern
is still intact.
This way corrupted messages can be detected.
In the past that revealed some 6lo-fragmentation bugs in Linux when
corrupted replies arrived.
This feature is also useful for RIOT, so implement it in RIOTs `ping`
command.