Satellites seem an ideal means for offering Internet and intranet access over long distances and to remote locations. However, internet protocols are not optimized for satellite conditions, and consequently, the throughput over satellite networks is restricted to only a fraction of the available bandwidth. Mentat, the leading supplier of TCP/IP to the computer industry, has overcome these restrictions with the development of the SkyX Gateway.

The SkyX Gateway system replaces TCP over the satellite link with a protocol optimized for the long latency, high loss, asymmetric bandwidth conditions typical of satellite communications. Adding the SkyX Gateway to a satellite network allows users to take full advantage of the available bandwidth, increasing the throughput rate by 3 times or more for Web traffic and up to 10 to 100 times for file downloads. The SkyX Gateway offers a solution which is entirely transparent to end users and does not require any modifications to end clients and servers.

For more information on the SkyX Gateway hardware solution, see the SkyX Gateway XR10 Datasheet.

Satellites are an attractive option for carrying Internet and intranet traffic to the many locations across the globe where terrestrial options are limited or the price prohibitive. However, data networking over satellites is faced with overcoming the large latency and high bit error rates typical of satellite communications, as well as the asymmetric bandwidth design of most satellite networks.

Communications over geosynchronous satellites, orbiting at a height of 22,300 miles, have round trip times of approximately 540 ms, an order of magnitude larger than terrestrial networks. The journey through the atmosphere can also introduce bit errors into the data stream. These factors, combined with back channel bandwidth typically much smaller than that available on the forward channel, reduce the effectiveness of TCP which is optimized for short hops over low-loss cable or fiber. Satellite conditions adversely affect a number of elements of the TCP architecture, including its congestion avoidance algorithms, data acknowledgment mechanisms, and window size limitations, which combine to severely constrict the data throughput rate that can be achieved over satellite links.

Congestion Avoidance: In order to avoid the possibility of congestive network meltdown, TCP assumes that all data loss is caused by congestion and responds by reducing the transmission rate. However, over satellite links, TCP misinterprets the long round trip time and bit errors as congestion and responds inappropriately. Similarly, the TCP "Slow Start" algorithm, which over the terrestrial infrastructure prevents new connections from flooding an already congested network, over satellites forces an excessively long ramp-up for each new connection. While these congestion avoidance mechanisms are vital in routed environments, they are ill-suited to point-to-point satellite links.

Data Acknowledgements: The simple, heuristic data acknowledgment scheme used by TCP does not adapt well to long latency or highly asymmetric bandwidth conditions. To provide reliable data transmission, the TCP receiver constantly sends acknowledgments for the data received back to the sender. The sender does not assume any data is lost or corrupted until a multiple of the round trip time has passed without receiving an acknowledgment. If a packet is lost or corrupted, TCP retransmits all of the data starting from the first missing packet. This algorithm does not respond well over satellite networks where the round trip time is long and error rates are high. Further, this constant stream of acknowledgments wastes precious back channel bandwidth. If the back channel bandwidth is small, the return of the acknowledgments to the sender can become the system bottleneck.

Window Size: TCP utilizes a sliding window mechanism to limit the amount of data in flight. When the window becomes full, the sender stops transmitting until it receives new acknowledgments. Over satellite networks, where acknowledgments are slow to return, the TCP window size generally sets a hard limit on the maximum throughput rate. The minimum window size needed to fully utilize an error-free link, known as the "bandwidth-delay product," is 100 KB for a T1 satellite link and 675 KB for a 10 Mbps link. Bit errors increase the required window size further. However, most implementations of TCP are limited to a maximum window size of 64 KB and many common operating systems use a default window size of only 8 KB, imposing a maximum throughput rate over a satellite link of only 128 Kbps per connection, regardless of the bandwidth of the data pipe.

SkyX Gateway Overcomes TCP Performance Limitations

The SkyX Gateway increases the performance of Internet and intranet access over satellites by transparently replacing TCP over the satellite link with a protocol optimized for satellite conditions.

The SkyX Gateway intercepts the TCP connection from the client and converts the data to the SkyX Protocol for transmission over the satellite. The SkyX Gateway on the opposite side of the satellite link translates the data back to TCP for communication with the server. This architecture offers vastly improved performance while remaining entirely transparent to the end user and fully compatible with the Internet infrastructure. No changes are required to the client or server, and all applications continue to function without modification.

The SkyX Gateway splits the single TCP connection into three separate components: A TCP connection on the remote side between the client and SkyX Gateway, a SkyX Protocol connection over the satellite between the two SkyX Gateways, and a TCP connection between the opposite SkyX Gateway and the server.

By splitting the end-to-end TCP connection, the segment over the satellite can use a protocol optimized for satellite conditions while the terrestrial segments of the connection continue to use standard TCP. TCP congestion avoidance mechanisms remain in place over the terrestrial connections to protect the stability of the routed network. The two SkyX Gateways pass control data between each other, allowing the gateway on the opposite side of the satellite to act as a proxy for the original source, thereby maintaining full TCP reliability and end-to-end TCP semantics.

SkyX Performance

Web Benchmark Testing
	SkyX	TCP/IP
Average throughput:	7.7 Mbps	2.7 Mbps
Download time for 5 MB file: (1 of 50 simultaneous connections)	15.3	357.5
Ratio of forward channel to return channel data:	99:1	29.1
Test Conditions: Forward Channel 10 Mbps, BER = 1.0 x 10-7 Reverse Channel: 128 kbps, BER = 1.0 x 10-6

Web Performance

The SkyX Gateway system greatly enhances the performance of all Internet applications, including World Wide Web access. On a typical satellite network with a 10 Mbps link running 50 simultaneous Web connections, the SkyX Gateway provides an aggregate throughput of 7.7 Mbps, compared to only 2.7 Mbps using end-to-end TCP. The performance difference is especially noticeable when downloading larger files. When one of the 50 simultaneous connections is a 5 MB file, downloading the file using end-to-end TCP takes 358 seconds compared to only 15 seconds with the SkyX Gateway.

Single Client Performance

Mentat has run a series of single-client FTP throughput tests to compare the performance of end-to-end TCP to the SkyX Gateway system for a wide variety of different windows sizes, link bandwidths, round trip delay times, and bit error rates.

Window Size and Link Speed vs. Throughput: Without performance enhancements or TCP window size tuning, most clients are limited to a throughput rate of less than 100 Kbps when the round trip time is 540 ms. As the figure below illustrates, even clients with a 32 KB window are only able to reach a throughput of 400 Kbps. However, the addition of the SkyX Gateway to the network allows end users to take full advantage of the available bandwidth regardless of the window size of the client or server.

Round Trip Delay vs. Throughput: The SkyX Gateway removes the dependency of TCP on the round trip time of the network. The figure below shows the measured throughput over symmetric 2 Mbps and 10 Mbps links with no errors and a delay of 540 ms. These results illustrate that TCP is able to fully saturate terrestrial low-delay networks, but as the delay increases, TCP performances drops rapidly. In contrast, a network which includes the SkyX Gateway is able to maintain full usage of the link regardless of the round trip time of the network.

Bit Error Rate vs. Throughput: Networks which incorporate the SkyX Gateway are much less sensitive to the bit error rate of the satellite link. The figure below shows the throughput as a function of the bit error rate for a symmetric 10 Mbps satellite network with a 540 ms round trip time and a 1 MB TCP window. Even at low error rates, TCP is able to deliver only 1.2 Mbps and at an error rate of 1.0E-5, TCP's throughput drops to 0.05 Mbps. A network which includes the SkyX Gateway is able to fully saturate the link at low error rates and delivers 2.7 Mbps even at an error rate of 1.0E-5.

Other Conditions

Mentat has Web and FTP throughput test data available for a wide variety of different conditions. Mentat can also simulate the link speed, bit error rate, and delay conditions of your system on the SkyX Gateway test network. Please contact Mentat for additional data, a simulation of your network with the SkyX Gateway, or a demonstration of the SkyX Gateway running on your network.

SkyX Protocol Design

At the heart of the SkyX Gateway system is the SkyX Protocol, optimized to provide maximum throughput for satellite networks. The SkyX Protocol is designed to respond efficiently to typical satellite latency, bit errors, and asymmetric bandwidth conditions, and to take advantage of optimizations possible on a point-to-point link with fixed bandwidth.

Efficient Acknowledgment Algorithm: The SkyX Protocol utilizes a highly efficient selective retransmission algorithm for the acknowledgment of data. Because the hop over the satellite is a point-to-point link with no intermediate routing, any gaps in the packet sequence can be assumed to be data loss due to corruption. The receiving SkyX Gateway immediately requests retransmission of the missing data from the transmitting SkyX Gateway.

Because the SkyX Protocol does not use acknowledgments as the primary means of identifying lost data, it needs only infrequent acknowledgments to confirm data arrival and clear buffers. In contrast, TCP sends a constant stream of acknowledgments over the reverse channel. The SkyX Protocol reduces back channel usage by 70% or more, thereby dramatically increasing the performance of networks where limited back channel bandwidth is the system bottleneck.

Large Windows: The SkyX Protocol offers essentially unlimited window sizes for transmission of data between the SkyX Gateways. Because the bandwidth-delay product over the satellite between the SkyX Gateways is much larger than that from the SkyX Gateway to the end node, the large SkyX Protocol window effectively removes the dependency of the network on the bandwidth-delay product. The SkyX Gateway becomes a buffer for the network, allowing high throughput independent of the window size of the clients and servers.

Congestion Avoidance: The SkyX Protocol does not use unnecessary congestion avoidance mechanisms for the hop over the satellite between the SkyX Gateways. However, the SkyX Gateway system continues to use Slow Start and all other standard TCP congestion avoidance algorithms for the terrestrial connections between the SkyX Gateways and the end nodes. The SkyX Protocol also uses a streamlined handshake mechanism to reduce the number of round-trip times required for connection set-up.

Rate Control: The SkyX Protocol includes explicit rate control functionality which can be set on both an aggregate and per-connection basis. TCP attempts to determines a safe throughput rate based on how quickly acknowledgments return, but will waste available bandwidth when it transmits at too low a rate and will cause unnecessary retransmissions when it transmits at a rate higher than the bandwidth of the link. Instead, the SkyX Protocol explicitly sets the transmission rate to exactly the bandwidth of the link, thereby providing the maximum throughput possible.

Reliable Multicast: The SkyX Protocol includes transport-layer reliable multicast functionality for point-to-multipoint communications. This functionality is useful for the efficient distribution of cached files and the replication of data from a hub to multiple remote sites.

IP or Link Layer Interface: When integrated with satellite modems or VSAT hardware where the data is delivered directly to the satellite, the SkyX Protocol can run over link layer (ISO Layer 2) protocols for maximum efficiency. When used on routed networks, the SkyX Protocol is encapsulated within IP.

SkyX Protocol Testing: NASA Goddard Space Flight Center has conducted extensive testing which shows the dramatic performance improvement possible using the SkyX Protocol instead of TCP or TCP-SACK in high-speed satellite environments. A summary of the test results, including graphs showing the performance of the SkyX Protocol, TCP, and TCP-SACK as a function of delay and bit error rate, is available at http://www.mentat.com/skyx/skyx-nasa.html.

The SkyX Gateway XR10 can be purchased as stand-alone hardware for use on any standard satellite network. For OEM integration with satellite modems, VSATs and other satellite networking equipment, Mentat also offers the SkyX Gateway in software source code form. SkyX Gateway software is available for Linux and Windows NT and is easily ported to any computer or real time operating system. Contact Mentat to order the SkyX Gateway XR10 or for more information on licensing SkyX Gateway software.

Mentat is the leading provider of high-performance networking solutions to the computer and satellite industries. Based in Los Angeles, California, Mentat was founded in 1987 and is employee owned. Mentat has licensed its networking software to over 50 operating systems vendors and developers throughout North America, Europe, and Japan including Apple, Digital, Hewlett-Packard, IBM, Motorola, Novell, Sony, Sun, and Wind River.

For more information on SkyX Gateway hardware or software solutions, please contact Mentat at:

Mentat Inc.
1145 Gayley Ave. Suite 315
Los Angeles, CA 90024 USA

phone: (310) 208-2650 ext. 28
toll free: 888-4MENTAT (888-463-6828)
fax: (310) 208-3724
email: info@mentat.com