A conceptual row in the inetIcmpTable.
A row with an inetIcmpIfIndex value of zero indicates a
system-wide value; a row with a non-zero inetIcmpIfIndex
indicates an interface-specific value. A system may
provide both system-wide and interface-specific values,
in which case it is important to note that the
system-wide value may not be equal to the sum of the
interface-specific value across all interfaces due to
e.g. dynamic interface creation/deletion.