Transport protocols and applications for Internet use in space

Abstract

An Internet datagram delivery service between space systems only provides end-to-end addressability. Building systems and performing actual spacecraft operations requires a variety of services operating over the Internet datagram delivery service. This paper discusses ways to use the capabilities of the upper layer Internet protocols to support the varied communication needs of satellites. It focuses on protocols in the transport layer (layer 4) and application layer (layer 7) which use the basic packet delivery capabilities of the Internet Protocol (IP) and the network layer (layer 3). The transport layer primarily adds data stream multiplexing and reliable data delivery options for use by applications. Data stream multiplexing is provided by the port mechanism in the User Datagram Protocol (UDP) and the Transport Control Protocol (TCP). UDP provides a basic packet delivery service similar to that used in today's spacecraft while TCP provides an automatic retransmission capability for reliable data stream delivery. Data streaming is also supported by the Real Time Protocol (RTP) which operates over UDP. Each of these protocols has benefits and limitations in various space communication environments with a range of link errors, propagation delays, and bit rates. Transport protocol selection and operational usage are discussed with respect to satellite communication requirements. Finally, actual spacecraft operations are performed by using applications running over transport protocols. The use of standard Internet applications such as NTP, FTP, SMTP, and telnet is discussed with respect to satellite operational requirements. The actual use and performance of many of these protocols by the Operating Missions as Nodes on the Internet (OMNI) project at NASA/GSFC on orbit with the UoSAT-12 spacecraft is also described.