Transport protocols for Internet-compatible satellite networks

We address the question of how well end-to-end transport connections perfor m in a satellite environment composed of one or more satellites in geostati onary orbit (GEO) or low-altitude earth orbit (LEO), in which the connectio n may traverse a portion of the wired Internet, We first summarize the vari ous ways in which latency and asymmetry can impair the performance of the I nternet's transmission control protocol (TCP), and discuss extensions to st andard TCP that alleviate some of these performance problems. Through analy sis, simulation, and experiments, we quantify the performance of state-of-t he-art TCP implementations in a satellite environment. A key part of the ex perimental method is the use of traffic models empirically derived from Int ernet traffic traces. We identify those TCP implementations that can be exp ected to perform reasonably well, and those that can suffer serious perform ance degradation. An important result is that, even with the best satellite -optimized TCP implementations, moderate levels of congestion in the wide-a rea Internet can seriously degrade performance for satellite connections. F or scenarios in which TCP performance is poor, we investigate the potential improvement of using a satellite gateway, proxy, or Web cache to "split" t ransport connections in a manner transparent to end users. Finally, we desc ribe a new transport protocol for use internally within a satellite network or as part of a split connection. This protocol, which we call the satelli te transport protocol (STP), is optimized for challenging network impairmen ts such as high latency, asymmetry, and high error rates. Among its chief b enefits are up to an order of magnitude reduction in the bandwidth used in the reverse path, as compared to standard TCP, when conducting large file t ransfers. This is a particularly important attribute for the kind of asymme tric connectivity likely to dominate satellite-based Internet access.