SUMMARY In heterogeneous networks, TCP connections that incorporate a terrestrial or satellite radio link are greatly disadvantaged with respect to entirely wired connections, because of their longer round trip times (RTTs). To cope with this problem, a new TCP proposal, the TCP Hybla, is presented and discussed in the paper. It stems from an analytical evaluation of the congestion window dynamics in the TCP standard versions (Tahoe, Reno, NewReno), which suggests the necessary modifications to remove the performance dependence on RTT. TCP Hybla performance is firstly evaluated in the case of an ideal channel, with good correlation between analytical and simulation data. Then, more realistic situations, which require the adoption of a benchmark network topology and a careful ns-2 simulation set-up, are examined. In particular, TCP Hybla performance is compared with that achievable by TCP standard in the presence of congestion and link losses, either separately or jointly considered. In all the examined cases, the superiority of TCP Hybla is evident, as it greatly reduces the severe penalization suffered by wireless, and especially satellite, TCP connections. Finally, it is worth noting that TCP Hybla does not infringe the end to end semantics of TCP and is compatible with other promising enhancements. Copyright # 2004 John Wiley & Sons, Ltd.

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TCP Hybla: a TCP enhancement for heterogeneous networks

Due to the availability of a wide variety of wireless access technologies, a mobile host can potentially have subscriptions and access to more than one wireless network at a given time. In this paper, we consider such a multi-homed mobile host, and address the problem of achieving bandwidth aggregation by striping data across the multiple interfaces of the mobile host. We show that both link layer striping approaches and application layer techniques that stripe data across multiple TCP sockets do not achieve the optimal bandwidth aggregation due to a variety of factors specific to wireless networks. We propose an end-to-end transport layer approach called pTCP that effectively performs bandwidth aggregation on multi homed mobile hosts. We show through simulations that pTCP achieves the desired goals under a variety of network conditions.

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A transport layer approach for achieving aggregate bandwidths on multi-homed mobile hosts

This document outlines possible TCP enhancements that may allow TCP to better utilize the available bandwidth provided by networks containing satellite links. The algorithms and mechanisms outlined have not been judged to be mature enough to be recommended by the IETF. The goal of this document is to educate researchers as to the current work and progress being done in TCP research related to satellite networks.

Ongoing TCP Research Related to Satellites

We address the question of how well end-to-end transport connections perform in a satellite environment composed of one or more satellites in geostationary orbit (GEO) or low-altitude Earth orbit (LEO), in which the connection may traverse a portion of the wired Internet. We first summarize the various ways in which latency and asymmetry can impair the performance of the Internet's transmission control protocol (TCP), and discuss extensions to standard TCP that alleviate some of these performance problems. Through analysis, simulation, and experiments, we quantify the performance of state-of-the-art TCP implementations in a satellite environment. A key part of the experimental method is the use of traffic models empirically derived from Internet traffic traces. We identify those TCP implementations that can be expected to perform reasonably well, and those that can suffer serious performance degradation. An important result is that, even with the best satellite-optimized TCP implementations, moderate levels of congestion in the wide-area Internet can seriously degrade performance for satellite connections. For scenarios in which TCP performance is poor, we investigate the potential improvement of using a satellite gateway, proxy, or Web cache to "split" transport connections in a manner transparent to end users. Finally, we describe a new transport protocol for use internally within a satellite network or as part of a split connection. This protocol, which we call the satellite transport protocol (STP), is optimized for challenging network impairments such as high latency, asymmetry, and high error rates. Among its chief benefits 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 transfers. This is a particularly important attribute for the kind of asymmetric connectivity likely to dominate satellite-based Internet access.

Transport protocols for Internet-compatible satellite networks

Current TCP protocols have lower throughput performance in satellite networks mainly due to the effects of long propagation delays and high link error rates. In this paper, a new congestion control scheme called TCP-Peach is introduced for satellite networks. TCP-Peach is composed of two new algorithms, namely Sudden Start and Rapid Recovery, as well as the two traditional TCP algorithms, Congestion Avoidance and Fast Retransmit. The new algorithms are based on the novel concept of using dummy segments to probe the availability of network resources without carrying any new information to the sender. Dummy segments are treated as low-priority segments and accordingly they do not effect the delivery of actual data traffic. Simulation experiments show that TCP-Peach outperforms other TCP schemes for satellite networks in terms of goodput. It also provides a fair share of network resources.

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TCP-Peach: a new congestion control scheme for satellite IP networks

Advances in satellite technology are enabling the deployment of large constellations of low Earth Orbiting (LEO) satellites. Next-generation systems will be tailored for broadband, packet-switched services, and therefore require either distributed or centralized packet routing mechanisms. Some researchers have hypothesized that the semi-regular mesh topology of a polar-orbiting constellation admits a simple distributed routing protocol based on using geographic information embedded in the node address. We take a closer look at this hypothesis in the context of commercially-proposed constellation designs. Using simulation, we study a distributed routing protocol that selects the next hop based on a minimization of the remaining distance to the destination. Our numerical results indicate that this routing strategy usually yields good routes, with an average latency degradation of less than 10 ms when compared with the optimal route. However, there are locations in the topology, most notably around the counter-rotating seams, the polar regions, and close to the destination of a packet, where the assumption of a regular mesh topology breaks down and it is difficult to guarantee robustness without adding significant additional complexity to the protocol.

On distributed, geographic-based packet routing for LEO satellite networks

Thanks to both the rapid deployment of the Internet and advances in satellite technology, the market for broadband satellite services is poised for substantial growth in the coming decade. Technological advances are enabling new systems that combine broadband data rates with small terminals, thereby providing more affordable “last-mile” network access to home and small business users worldwide. In particular, two types of broadband satellite systems are under development: high-power satellites deployed at traditional geostationary (GEO) orbits, and large constellations of satellites deployed at much lower (LEO) orbits. 
When using GEO satellites to provide Internet access service, the performance of the Internet's Transmission Control Protocol (TCP) is degraded by the high latency and high degree of bandwidth asymmetry present in such systems. We therefore undertook a comprehensive study of TCP performance in the context of broadband satellite systems used for network access. We demonstrated how minor variations in TCP implementations can have drastic performance implications when used over satellite links and from our observations constructed a satellite-optimized TCP implementation using standardized options. We explored the performance benefits of splitting a TCP connection at a protocol gateway within the satellite network, and found that such an approach can allow the performance of the satellite connection to approach that of a non-satellite connection. We construct a satellite-optimized transport protocol that can be used in such a split-connection environment, and demonstrate how it outperforms TCP in a satellite environment characterized by large amounts of bandwidth asymmetry. 
While LEO systems are being designed specifically to avoid the high latencies found in GEO systems, their design is challenging from a packet routing perspective due to the highly time-varying nature of the LEO network topology. We constructed a detailed packet-level LEO network simulator and identified some fundamental delay performance results of commercially proposed constellations. We then explored whether or not geographic-based network addresses can be constructively used in the design of distributed or centralized packet routing systems. We demonstrated that geographic-based addresses are useful in centralized routing systems. 
Broadband satellite networks are likely to become an important niche of the future Internet because of their unique ability to provide network access from almost any point on the globe, particularly those underserved by terrestrial infrastructure. However, because the design of Internet protocols is driven by the performance of the wired Internet, satellite network engineers must be vigilant in assisting in the design and deployment of satellite-friendly protocols and in considering how satellite networks interwork with the wired Internet. (Abstract shortened by UMI.)

Networking over next-generation satellite systems

Network simulation for LEO satellite networks

Technological advances are enabling new satellite communications systems that combine broadband data rates with small terminals. These novel systems are being designed to provide affordable "last-mile" network access to home and small business users worldwide. In particular, two types of advanced broadband satellite systems are under development: high-power satellites deployed at traditional geostationary (GEO) orbits, and large constellations of satellites deployed at much lower (LEO) orbits. In this report, we explore research problems that have arisen from the attempt to use GEO satellites to provide Internet access. In particular, performance of the Internet''s Transmission Control Protocol (TCP) is degraded by the high latency and high degree of bandwidth asymmetry present in such systems, and the degradation is compounded by the packet-level congestion found in today''s Internet. We first study whether TCP''s congestion avoidance algorithm can be adjusted to provide better fairness when satellite connections are forced to share bottleneck links with other (shorter delay) connections. Our data suggests that adjustments of the policy used in that algorithm may yield substantial fairness benefits without compromizing utilization. We next demonstrate how minor variations in TCP implementations can have drastic performance implications when used over satellite links (such as a reduction in file transfer throughput by over half), and from our observations construct a satellite-optimized TCP implementation, using standardized options, that achieves good file transfer performance when congestion is light. We explore the performance of TCP for short data transfers such as Web traffic, and find that two experimental options relating to how TCP starts a connection, when used together, could reduce the user-perceived latency by a factor of two to three. However, because not all of these options are likely to be deployed on a wide scale, and because even the best satellite-optimized TCP implementation is vulnerable to the fairness problems identified above, we explore the performance benefits of splitting a TCP connection at a protocol gateway within the satellite network, and find that such an approach can allow the performance of the satellite connection to approach that of a non-satellite connection. Our results illustrate many of the remaining challenges facing TCP performance over GEO satellite links while also demonstrating techniques that can be used to optimize performance in many satellite networks.

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Thomas Henderson

Papers

Transport protocols for Internet-compatible satellite networks

On distributed, geographic-based packet routing for LEO satellite networks

Networking over next-generation satellite systems

Network simulation for LEO satellite networks

TCP Performance over Satellite Channels