The Naval Research Laboratory (NRL) is conducting a simulation and experimental effort to document and optimize the use of Internet Protocol (IP) over satellite based highly asymmetric networks. Within the DoD, the emergence of the Global Broadcast Service (GBS) with its ability to deliver data to the warfighter at data rates approaching 25 Mbps is creating unprecedented opportunities. Commercial very small aperture terminal (VSAT) networks, while not typically offering quite as much throughput, often support Mbps capabilities. However, it is often the case that high data rates to the warfighter are not available in the reverse direction. NRL has demonstrated asymmetric networking using GBS as the forward channel and low-cost organic backchannels for the return paths. Typical backchannel solutions include telephone (POTS), cellular phone, satellite mobile phone, commercially available VSAT, UHF FLTSAT, Inmarsat B and tactical radio. Each of the backchannel solutions presents different data rate capabilities and bit error rates, but typical backchannel data rates range from 1.2 kbps to 160 kbps. A typical GBS or VSAT "forward" channel operates in the 1-5 Mbps range. A network built on such asymmetry presents challenges not typical of full duplex physical layer links. NRL has investigated the effect such asymmetry has on TCP, TCP extended window option, selective acknowledgements, and reliable multicast. The investigations have focused on the ability of each to efficiently transfer large data files to the warfighter. The investigative effort began with simulation and experimental efforts to document the effect on forward channel throughput for FTP and HTTP file transfers using backchannels ranging in data rate from 1.2 kbps to 2 Mbps. Standard round trip time calculations for throughput based on window size limitations are not adequate for describing performance in highly asymmetric networks. Extended window option and selective acknowledgement enhancements to TCP simulated and experimental results are discussed. The use of reliable multicast to overcome the basic windowing challenges and deliver data to multiple users simultaneously is addressed.
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