Currently we're focusing on the download portion of the transfer so I'd refrain from commenting on the uplink protocol for the time being. But the general ques

Currently we're focusing on the download portion of the transfer so I'd refrain from commenting on the uplink protocol for the time being. But the general question about router congestion handling is a valid one. And the answer is No, we're not just flooding the network with a blast of packets. Apart from anything else, that strategy is full of fail for throughput anyway. Since a busy router will likely generously drop packets from the UDP queue before it loses stuff from the TCP queues. So we rely on precise knowledge of network conditions at a fine grained level (the grain gets finer as we add more users to our network) to utilize the available bandwidth. We also use other techniques to predict queue congestion on the network path before it gets to the level of dropping packets. Note that (most) TCP implementations rely on packets loss to discover usable link capacity. We don't! One of the advantages of creating your own protocol is that you can use the state of the art research in congestion control strategies without being constrained by legacy concerns. The upshot is, our protocol is actually fairer to competing traffic compared to TCP on the whole. Hope that alleviates your concerns.