Appropriate determinations in a series of tests, which transition from more passive tests to more active tests, control the admission of data streams onto a network data path. More passive tests can include promiscuous mode and packet-pair tests. When the results of more passive tests indicate a reduce likelihood of a data stream causing a network data path to transition into a congested state, the network data path can be actively probed to make a more precise determination on the likelihood of congestion. A train of diagnostic data packets is transmitted at a diagnostic data transfer rate having a significantly reduced chance of causing congestion. A train of target data packets is transmitted at a requested application transfer data rate. The number of target data packets received within a specified delay threshold is compared to the number of diagnostic data packets receive with the delay threshold.