Alfalfa is an Internet videoconferencing protocol that works well over cellular wireless networks, where link speeds change dramatically with time, and current transport protocols build up lengthy in-network queues. Alfalfa does not use TCP-style reactive congestion control, but instead uses a stochastic model of the network path to forecast how many bytes are safe to send, while bounding the risk that its packets will be queued inside the network for too long. This forecast is used to govern the coded size of each frame of video, yielding a continuously-variable video compression ratio that achieves high utilization of the varying link without filling up network queues. In evaluations on traces from four commercial LTE and 3G networks, Alfalfa's rate control scheme, compared with Skype, reduced self-inflicted end-to-end delay by a factor of 7.9 and achieved 2.2$\times$ the transmitted bit rate on average. Compared with Google's Hangout, it reduced delay by a factor of 7.2 while achieving 4.4$\times$ the bit rate, and compared with Apple's Facetime, it reduced delay by a factor of 8.7 with 1.9$\times$ the bit rate.