To verify satisfaction of communication requirements imposed by unique missions, as early as 2000, the Communications Networking Group at the Jet Propulsion Laboratory (JPL) saw the need for an environment to support interplanetary communication protocol design, validation, and characterization. JPL’s Multi-mission Advanced Communications Hybrid Environment for Test and Evaluation (MACHETE), described in ”Simulator of Space Communication Networks” (NPO-41373) NASA Tech Briefs, Vol. 29, No. 8 (August 2005), p. 44, combines various commercial, non-commercial, and in-house custom tools for simulation and performance analysis of space networks. The MACHETE environment supports orbital analysis, link budget analysis, communications network simulations, and hardware-in-the-loop testing. As NASA is expanding its Space Communications and Navigation (SCaN) capabilities to support planned and future missions, building infrastructure to maintain services and developing enabling technologies, an important and broader role is seen for MACHETE in design-phase evaluation of future SCaN architectures.
To support evaluation of the developing Delay Tolerant Networking (DTN) field and its applicability for space networks, JPL developed MACHETE models for DTN – Bundle Protocol (BP) and Licklider/Long-haul Transmission Protocol (LTP). DTN is an Internet Research Task Force (IRTF) architecture providing communication in and/or through highly stressed networking environments such as space exploration and battlefield networks. Stressed networking environments include those with intermittent (predictable and unknown) connectivity, large and/or variable delays, and high bit error rates. To provide its services over existing domain specific protocols, the DTN protocols reside at the application layer of the TCP/IP stack, forming a store-and-forward overlay network. The key capabilities of the Bundle Protocol include custody-based reliability, the ability to cope with intermittent connectivity, the ability to take advantage of scheduled and opportunistic connectivity, and late binding of names to addresses.
Internet standards are published in Request For Comments (RFCs), and the Bundle Protocol and LTP are described in RFC 5050 and RFC 5326, respectively. BP provides the store-carry-forward, custody transfer and naming capabilities of the DTN, while LTP was specifically developed for longdelay links. LTP allows for “red” and “green” data portions in a single session, where the red data portion uses retransmission and the green data portion does not. Unlike common Internet retransmission protocols, LTP adds the ability to suspend and resume timers when the link’s status changes. On occasion, the models are extended to include non-standard experimental features for validating project-specific performance or behavioral requirements. For instance, unlike standard simulation models, the BP model supports external traffic injection, which was used to verify correct behavior of the SharedNet middleware over DTN protocols and described at the SMC-IT 2006 conference (Second International Con ference On Space Mission Challenges For Information Tech nology). The MACHETE LTP model supports all standard functions of LTP along with an optional priority-aware queuing system to prevent lower priority from blocking higher-priority traffic arriving later.
Furthermore, MACHETE contains Consultative Committee for Space Data Systems (CCSDS) protocol standards, such as Proximity-1, Advanced Orbiting Systems (AOS), Packet Telemetry/Telecommand, Space Communications Protocol Specification (SCPS), and the CCSDS File Delivery Protocol (CFDP). So, with the addition of DTN protocol libraries interplanetary network, engineers at JPL can characterize future space network performance trade-offs.