Mark Allman / ICSI @mallman_icsi

Mark Allman, Shawn Ostermann, Hans Kruse. Data Transfer Efficiency Over Satellite Circuits Using a Multi-Socket Extension to the File Transfer Protocol (FTP). Proceedings of the ACTS Results Conference, NASA Lewis Research Center, Cleveland, OH, September 11-13, 1995.


In several experiments using NASA's Advanced Communications Technology Satellite (ACTS), investigators have reported disappointing throughput using the TCP/IP protocol suite over 1.536Mbit/sec (T1) satellite circuits. A detailed analysis of FTP file transfers reveals that both the TCP window size, and the TCP "Slow Start" algorithm contribute to the observed limits in throughput.

While it is tempting to approach a solution to this issue by raising the TCP window size, there are several issues which can not be addressed in that way: (1) In order to raise the TCP window size sufficiently to allow full utilization of a T1 circuit, the TCP Extended Window Option is required[4]. Commercial implementation of this option in major operating systems has not been completed. (2) The use of very large windows may actually hurt throughput if a moderate bit error rate is present on the satellite channel. (3) A change in the window size does not address the effect of the TCP Slow Start algorithm. We therefore propose an application-layer solution by adding an option to the standard FTP which uses multiple data connections. The use of multiple TCP connections allows the effective utilization of the channel bandwidth without an increase in the TCP window size. A similar approach has been suggested by Long et al for the transfer of specialized image databases, both via the Internet and over satellite links [7].

In this paper we summarize the experimental and theoretical analysis of the throughput limit imposed by TCP on the satellite circuit. We then discuss in detail the implementation of a multi-socket FTP (XFTP) client and server. XFTP has been tested using the ACTS system. We present results from these runs and discuss the interaction between the multi-socket application and the TCP/IP network, especially the queues in the IP routers. Our results show that a careful choice of the number of connections, or sockets, must be made. Too few connections result in wasted bandwidth, while too many connections lead to dropped packets due to queue overflows in the router; in this case the overall throughput is reduced. The optimal choice of the number of connections leads to a better than 90% utilization of the satellite circuit.

Finally, we discuss a preliminary set of tests on a link with non-zero bit error rates. XFTP shows promising performance under these circumstances, suggesting the possibility that a multi-socket application may be less effected by bit errors than a single, large-window TCP connection.


    author  =       "Mark Allman and Shawn Ostermann and Hans Kruse",
    title   =       "{Data Transfer Efficiency Over Satellite Circuits Using a Multi-Socket Extension to the File Transfer Protocol (FTP)}",
    booktitle =     "Proceedings of the ACTS Results Conference",
    year    =       1995,
    publisher =     "NASA Lewis Research Center",
    month   =       sep,
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