A bit mask of interfaces that support the defined rate limit.
Each interface is represented by a single bit. If that bit
is set then the corresponding interface is a member of this
control. The bits are assigned with the least significant bit
of the first byte corresponding to ifIndex 1, the next
bit corresponding to ifIndex 2 and so on.
Note that each port can only be associated with a single rate
limit per control. An error will be returned if the user tries
to assign a port that already has a rate limit associated with
it.