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.