Lloyd Wood

IP ROUTING ISSUES IN SATELLITE CONSTELLATION NETWORKS

SUMMARY The growth in use of Internet-based applications in recent years has led to telecommunication networks transporting an increasingly large amount of Internet Protocol (IP)-based traffic. Proposed broadband satellite constellation networks, currently under development, will be required to transport IP traffic. A case can be made for implementing IP routing directly within the constellation network, in order to transport IP traffic well and to provide good support for IP multicast and for emerging IP-based Quality of Service (QoS) guarantees. This paper examines strategies for implementing and operating IP routing effectively within satellite constellation networks, given known constraints on the constellation resulting from satellite mobility, global visibility, routing and addressing.

Saratoga: a Delay-Tolerant Networking convergence layer with efficient link utilization

Saratoga is a lightweight transport protocol based on the user datagram protocol (UDP/IP). Saratoga was developed by Surrey Satellite Technology Ltd (SSTL) for file transfers of imaging data recorded onboard the Internet-Protocol-Based Disaster Monitoring Constellation (DMC) satellites, and has been in operational use from low Earth orbit since 2004. Saratoga focuses only on efficient communication to the next hop when link connectivity is available, by filling the link with packets sent at line rate. This ensures that as much data as possible is transferred to the peering node during a twelve-minutes-or-less pass over a satellite ground station. Saratoga uses a minimal bandwidth-efficient negative acknowledgement mechanism to ensure reliable data transfer. We examine how Saratoga can be adapted to serve as an efficient convergence layer for delay-tolerant networking (DTN), by transferring DTN bundles as well as files. This will allow DTN networks to increase efficiency of communication across briefly-available disrupted links -for long-distance deep space links as well as for short-distance terrestrial mobile ad-hoc networks.

A bundle of problems

Delay-Tolerant Networking” (DTN) is a neologism used for a new store-and-forward architecture and protocol suite intended for disrupted networks where there is intermittent or ad-hoc connectivity. This has been proposed as one approach to supporting delay-tolerant networks. Work in the late 1990s on the “Interplanetary Internet” forms the basis for current DTN protocols and architecture. That early work considered transport protocols robust to the hours-long propagation delays of deep-space communications. DTN is also known, primarily in military circles, as Disruption-Tolerant Networking, due to the dynamic links and outages in the military tactical environment, rather than long-delay links. In both cases, DTN technologies are well-suited to applications that are mostly asynchronous and insensitive to large variations in delivery conditions. DTN networks differ sufficiently from traditional terrestrial networks in their characteristics and connectivity that link, network and transport protocols must be carefully considered and chosen to cope with these different characteristics, or new protocols can be designed that are suited for the problems that these DTN network conditions impose. The “Bundle Protocol” exists within the DTN architecture, which sends bundles over subnet-specific transport protocols, called “convergence layers.” “Bundling” has undergone a large amount of shared development and design over a period of years as a research effort. We examine the Bundle Protocol and its related architecture closely, and discuss areas where we have found that the current Bundle approach is not well-suited to many of the operational concepts that it was intended to support. Problems with the Bundle Protocol and its convergence layers exist in mechanisms for error detection and overall reliability. This weakens the Bundle Protocols suitability to disrupted and error-prone networks. We show that these reliability issues can lead to performance problems in DTN networks, requiring mitigation. Open research and development areas also exist with design choices in handling timing information, in determining necessary and sufficient security mechanisms, in its Quality of Service capabilities, and in other aspects of application or content identification. We show that the existing DTN bundling architecture has a number of open real-world deployment issues that can be addressed. We suggest possible remediation strategies for these weak areas of the bundle protocol that we have been working on. We also look at alternate approaches to DTN networking. Rather than only providing criticism, this paper identifies open issues, where work on modifying the Bundle Protocol is encouraged and approaches to address its various problems are suggested.

Experience with Delay-Tolerant Networking from orbit†

The disaster monitoring constellation (DMC), constructed by Surrey Satellite Technology Ltd (SSTL), is a multi-satellite Earth-imaging low-Earth-orbit sensor network where captured image swaths are stored onboard each satellite and later downloaded from each satellite payload to a ground station. The DMC is currently unique in its adoption of the Internet Protocol (IP) for its imaging payloads and for satellite command and control, based around reuse of commercial networking and link protocols. Earth images are downloaded from the satellites using a custom IP-based high-speed transfer protocol developed by SSTL, Saratoga, which works well in unusual link environments. Store-and-forward of images with capture and later download during passes over ground stations gives each satellite the characteristics of a node in a Delay/Disruption Tolerant Network (DTN). DTNs are under investigation in an Internet Research Task Force (IRTF) DTN research group (RG), which has developed a bundle architecture and protocol. We experiment with use of this DTNRG bundle concepts onboard the UK-DMC satellite, by examining how Saratoga can be used as a convergence layer to carry the DTN Bundle Protocol, so that sensor images can be delivered to ground stations and beyond as bundles. This is the first use of the Bundle Protocol from orbit. We use our practical experience to examine the strengths and weaknesses of the Bundle architecture for DTN use, paying attention to fragmentation, custody transfer, and reliability issues. We similarly examine and discuss an alternative network stack, based around proposed use of the Hypertext Transfer Protocol (HTTP) that we have been architecting, which we believe has potential applications across a range of DTN networks. We use our practical experience to make suggestions about DTN use and adoption of IP in sensor networks.

Effects on TCP of routing strategies in satellite constellations

A broadband satellite network uses a constellation of a number of similar satellites to provide wireless networking services to the Earth. A number of these constellation networks are under development. This article introduces the types of satellite constellation networks, and examines how overall performance of TCP communications carried across such a network can be affected by the choice of routing strategies used within the network. Constellations utilizing direct intersatellite links are capable of using multiple paths between satellites simultaneously as a strategy to spread network load. This allows more general routing strategies than shortest-path routing, but we show these strategies to be detrimental to the performance of individual TCP connections.

A fair traffic conditioner for the assured service in a differentiated services Internet

A research issue under investigation in the context of differentiated services (DiffServ) is the fair distribution of bandwidth between aggregates sharing the same assured forwarding (AF) class. Multiplexing both responsive and unresponsive flows, e.g. TCP and UDP respectively, leads to unfair sharing of the available bandwidth in over-provisioned networks. To date, much effort has concentrated on experiments using different methods for mapping TCP and UDP flows of the same AF class to the three possible drop precedences of the AF specification. Although this approach may protect responsive from unresponsive flows, it has not been shown to provide adequate fairness. We present a traffic conditioner able to provide fairness between responsive and unresponsive flows originating from the same customer network, using a fair two-rate three-color marker. Its capability for fairness is based on the use of the FRED fair active buffer algorithm to control the token allocation of the token buckets residing in the traffic conditioner. We also show that by employing fair multiple RED (FMRED) at the DiffServ domain ingress node, the overall fairness of the customer network aggregates is improved when compared to the case where the vanilla MRED algorithm is used.

Experience with Delay-Tolerant Networking from Orbit

The disaster monitoring constellation (DMC), constructed by Surrey Satellite Technology Ltd (SSTL), is a multi-satellite Earth-imaging low-Earth-orbit sensor network where captured image swaths are stored onboard each satellite and later downloaded from each satellite payload to a ground station. The DMC is currently unique in its adoption of the Internet Protocol (IP) for its imaging payloads and for satellite command and control, based around reuse of commercial networking and link protocols. Earth images are downloaded from the satellites using a custom IP-based high-speed transfer protocol developed by SSTL, Saratoga, which works well in unusual link environments. Store-and-forward of images with capture and later download during passes over ground stations gives each satellite the characteristics of a node in a Delay/Disruption Tolerant Network (DTN). DTNs are under investigation in an Internet Research Task Force (IRTF) DTN research group (RG), which has developed a bundle architecture and protocol. We experiment with use of this DTNRG bundle concepts onboard the UK-DMC satellite, by examining how Saratoga can be used as a convergence layer to carry the DTN Bundle Protocol, so that sensor images can be delivered to ground stations and beyond as bundles. This is the first use of the Bundle Protocol from orbit. We use our practical experience to examine the strengths and weaknesses of the Bundle architecture for DTN use, paying attention to fragmentation, custody transfer, and reliability issues. We similarly examine and discuss an alternative network stack, based around proposed use of the Hypertext Transfer Protocol (HTTP) that we have been architecting, which we believe has potential applications across a range of DTN networks. We use our practical experience to make suggestions about DTN use and adoption of IP in sensor networks.

Using Internet nodes and routers onboard satellites

An Internet router was integrated into the UK-DMC remote-sensing satellite as a secondary experimental payload. This commercial product has been orbiting in space for over three years. We describe the integration of the router and satellite and the successful on-orbit testing of the router, which took place using the Virtual Mission Operations Center (VMOC) application as part of a larger systems internetworking exercise. Placing this Cisco router in Low Earth Orbit (CLEO) onboard a small satellite is one step towards extending the terrestrial networking model to the near-Earth space environment as part of a merged space-ground architecture.

Large File Transfers from Space Using Multiple Ground Terminals and Delay-Tolerant Networking

Abstract-We use Delay-Tolerant Networking (DTN) to break control loops between space-ground communication links and ground-ground communication links to increase overall file delivery efficiency, as well as to enable large files to be proactively fragmented and received across multiple ground stations. DTN proactive fragmentation and reactive fragmentation were demonstrated from the UK-DMC satellite using two independent ground stations. The files were reassembled at a bundle agent, located at Glenn Research Center in Cleveland Ohio. The first space-based demonstration of this occurred on September 30 and October 1, 2009. This paper details those experiments.

Satellite Constellation Networks

Satellite constellations are introduced. The effects of their orbital geometry on network topology and the resulting effects of path delay and handover on network traffic are described. The design of the resulting satellite network as an autonomous system is then discussed.