Probably implemented using
PassiveRequests .
PassiveRequests for a
TUK could for example be used as a stream, but practically this can be attacked and has limited capacity; streams should usually use either a cascade of
PassiveRequests (-1, -2, -3, -4, -5, when get 5, add for the next few...), much like
UpdatableSubspaceKeys , or aggregate a bundle of
PassiveRequests for
TimeUpdatableKeys.