Link and Routing Issues for Internet Protocols in Space' Keith Hogie, Ed Criscuolo, Ron Parise Computer Sciences Corp 7700 Hubble Dr. Lanham-Seabrook MD 20706 (301) 794-2999 [email protected] universal, end-to-end data communication architecture for Absfract-The first step in using Intemet Protocols in space space. This low-level datagram delivery approach using off- is to establish the basic Internet datagram delivery service the-shelf, low-cost, commodity-level standards will become over satellite RF links. This paper discusses the low-level increasingly significant in the years to come. Both Earth and data link and data routing issues related to using Internet space science missions plan to fly more and more sensors protocols to support spacecraft communications. It covers and have them interact to form a "SensorWeb"[2]. The issues related to layer 1 (physical), layer 2 (data link), and present labor-intensive, mission-specific techniques for layer 3 (network). It does NOT cover layer 4 (transport) and processing and routing data do not scale well and will above. become prohibitively expensive. This work is about defining At the physical layer, the paper presents various applications an architecture that allows science missions to be deployed of forward-error-correction (FEC) coding techniques, such "faster, better, and cheaper" by using the technologies that as convolutional coding and Reed-Solomon. It describes have been extremely successful in today's Intemet. approaches for using these techniques in ways that are independent of the protocols used at the data link layer and 2. OVERVIEW OF LOWERLAYER PROTOCOLS IN above. SPACE At the data link layer, common, commercially available The goal of the Operating Missions as Nodes on the Internet framing schemes are discussed along with how they can be (OMNI) project at NASA's Goddard Space Flight Center easily deployed. A rationale is provided for the selection of (GSFC) is to define and demonstrate an end-to-end HDLC/frame relay ftaming along with IETF multi-protocol communication architecture for future space missions. The encapsulation. authors have combined their knowledge and experience in At the network layer, the Internet Protocol end-to-end Internet technologies and space communication, addressability and routing is discussed in the context of command/control, and data processing systems in space-based applications. Standard solutions for dealing developing the following end-to-end data communication with the intermittent and mobile links of satellites are also concept. discussed. These include a discussion of Mobile IP and mobile routing protocols. End-to-End Network Concept Finally, deployment of these protocols in both spacecraft The data communication requirements of many advanced and ground systems are discussed. Details of current space missions involve seamless, transparent connectivity implementations by the Operating Missions as Nodes on the between space-based instruments, investigators, ground- Intemet (OMNI) project at NASNGSFC using operational based instruments and other spacecraft. The key to an space and ground systems such as UoSAT-12 and TDRSS architecture that can satisfy these requirements is the use of are also provided. the Internet Protocol. IP is the technology that drives the public Internet and therefore draws billions of dollars annually in research and development funds. Most private TABLEOF CONTENTS networks, including NASA's operational communication 1. INTRODUCTION network, utilize IP as their underlying protocol. IP provides 2. OVERVIEW OF LOWERLAYER PROTOCOLS IN SPACE a basic standardized mechanism for end-to-end 3. PROPOSED ARCHITECTURE communication between applications across a network. The 4. PHYSICALLAYER ISSUES protocol provides for automated routing of data through any number of intermediate network nodes without affecting the 5. LINK LAYER ISSUES endpoints. 6. NETWORKLAYER ISSUES 7. GROUND-BASEDDEMONSTRATIONS Thirty years ago, spacecraft communication was a very special business with organizations such as NASA breaking 8. SPACE BASED DEMONSTRATION new ground and designing new protocols for the "special" 9. CONCLUSIONS space environment. However, communication technology 10. ACKNOWLEDGEMENTS has undergone huge changes over the last thirty years. Communication applications such as cell phones, Internet 1. INTRODUCTION telephone calls anywhere in the world, and worldwide network access from a handheld computer, that once would This paper discusses issues related to the use of standard have seemed impossible, are now in common use. Internet protocols and link layers for satellite Spacecraft environments still pose numerous challenges but communication. It focuses on the lower layer protocols that most of these have direct analogs and solutions in the deliver data over the space link and how those protocols can ground-based mobile IP and wireless networking industries, operate with the Internet Protocol (IP)[l] to provide a such as: U.S. Government work not protected by U.S. copyright 2-963 0 intermittent communication links The major change from today‘s satellite communication systems is to change the format of the data on the space-to- 0 highly asymmetric or unidirectional communication ground link. Changing the spacecraft data format to match links formats used by standard Internet devices allows the 0 bit error rates higher than most hardwired links extensive use of COTS networking devices in ground stations. This greatly reduces equipment cost and allows 0 multiple mobile nodes forming a dynamic network much easier upgrades to accommodate future mission needs. topology The ground station may also be either a passthrough site or 0 maintaining a single address for a spacecraft as it uses a store-and-forward site. The passthrough operation is different ground stations implemented using standard Internet routing. Store-and- forward operations can be built using standard computers The increasing popularity of laptop computers, handheld and distributed computing technology. digital assistants, and Internet cell phones has driven the development of protocols to handle mobile nodes, such as The figure shows two different types of spacecraft. One Mobile IP (MIP) [3] and mobile routing. They are also spacecraft only has a single Ip address and is very similar to driving the development of new protocols such as Cellular current spacecraft. Instruments still interface with the IP[4], Dynamic Source Routing (DSR) [5], and other ad- command and data handling (C&DH) system and the change hoc-networking protocols. is in the data format from the C&DH to the radio frequency (RF) system. This is similar to a mobile laptop using This paper will examine selected lower layer protocols standard Mobile IP solutions. The other type of spacecraft specified by the Intemet Engineering Task Force (ETF), shown is one with an onboard local area network (LAN) and and map them to spacecraft applications. It will also multiple IP addresses. The onboard LAN requires Mobile describe how standard, commercially available routing protocols and an onboard router to hide the mobility communication hardware and sohare were used to test issues fkom each node on the LAN. some of these concepts with an orbiting spacecraft. Of course, as spacecraft become more easily accessible 3. PROPOSEDARCHITECTURE network nodes, communication security becomes more of an issue. But as the Intemet becomes more critical to everyday Recognizing the clear benefits of IP as an end-to-end business operations, extensive security solutions such as networking protocol, the OMNI project developed a Virtual Private Networking[6] (VPN) are being developed reference system architecture for the space and ground which can also be applied to satellite missions. segments of future IP missions. The goals were to maximize the use of commercial-off-the-shelf (COTS) hardware and The key to this whole architecture is that it is built upon the protocols while avoiding creating any new “space-specific” protocol layering concepts of the IS0 OS1 network reference solutions. A high-level view of this architecture appears in model. figure 1. TDRSS Figure 1 - End-to-End Space Internet 2 - 964 A key component of the diagram is the single network layer protocol (IP) that provides a common denominator that ties The IS0 OS1 model defines a protocol stack with seven together all the other protocols. It provides global layers for any network. Those seven layers and their addressing and datagram delivery while also isolating the associated areas of standardization are: upper and lower layers. This global addressibility has 1. physical - bit stream details, signal voltage, cable allowed the Internet to grow to hundreds of millions of users specifications, modulation techniques while still supporting data delivery between all users. The layer isolation has been critical in allowing evolution in both 2. data link - supports transfer of data across the physical the lower and upper layers with minimal change and link in frames reconfiguration of existing users. 3. network - hides data transmission details from upper This paper is focused on the lower layer protocol issues layers, end-to-end addressing related to extending IP to the spacecraft environment. Some 4. transport - data stream multiplexing, reliable and of the primary protocols identified for space related use are unreliable transparent data transfer between end points shown in the bottom three layers of figure 2. However, getting IP to the spacecraft