Dense small cells capable of reciprocity-based massive MIMO is one of the most promising technologies for delivering very large throughputs per unit area. Reciprocity-based MIMO relies on exploiting end-to-end channel reciprocity, and although uplink-downlink radio channel reciprocity holds, the end-to-end channels also include the transmit/receive RF chains, which are, in general, not reciprocal. This paper deals with techniques for RF calibration, i.e., techniques for compensating for the unknown time-varying effects of the transmit/receive RF chains on reciprocity based MIMO. We present Avalanche, a class of novel calibration techniques in order to enable low-overhead calibration of massive arrays for reciprocity based MIMO.