Coordinate ifIndex value allocation for entries in usdSonetVTTable.
A GET of this object returns the next available ifIndex value to be used
to create an entry in the associated interface table; or zero, if no
valid ifIndex value is available. This object also returns a value of
zero when it is the lexicographic successor of a varbind presented in an
SNMP GETNEXT or GETBULK request, for which circumstance it is assumed
that ifIndex allocation is unintended.
Successive GETs will typically return different values, thus avoiding
collisions among cooperating management clients seeking to create table
entries simultaneously.