Delay-Tolerant Networking for CisLunar Operations 14 May 2004

DTN Overview Objective: enable effective communication despite lengthy round-trip delays – whether due to long signal propagation latencies, interrupted connectivity on the end-to-end path, or both. Features: Bundling protocol builds on and includes all of the concepts built into CFDP relaying: deferred transmission, store-and-forward operation, underlying point-to-point retransmission for end-to-end reliability. Adds automatic dynamic route computation, adapted from routing experience in the Internet. Adds automatic reactive fragmentation to deal with truncated contacts. Also adds built-in support for security and congestion avoidance. Unlike CFDP relaying, delay-tolerant networking architecture is designed to scale up indefinitely.

DTN Architecture Overlay network operational objectives: Run over Internet protocols wherever possible. Run over domain-specific (e.g., CCSDS) protocols as necessary. Insulate applications from having to know the difference. DTN design principles: A postal model of communications. Telephonic, conversational communication is a special case that only works under favorable conditions. Epistolary communication is the more general and more robust model. Forego dialogue and negotiation; instead, “bundle” with each message the answers to questions that might be asked about it. Tiered functionality. Use overlay network protocol to do whatever the underlying transmission systems cannot, but no more.

An Application Example relay orbiter 1 TCP/IP over Proximity-1 R/F link AOS deep space R/F link, with link-layer ARQ lander rover #2 workstation rover #1 Earth TCP/IP over wireless LAN tracking station Mars Internet relay orbiter 2

Why DTN? Cislunar architecture proposals seem to be aimed at continuous bent-pipe relaying of IP through a constellation of orbiters to a large number of ground antennae at the three DSN stations, concentrating on continuous coverage of the lunar south pole. Isn’t bent-pipe relaying of IP enough? Cislunar signal propagation delays are small. End-to-end connectivity should be continuous between any point on Earth and: Any spacecraft in transit to the moon at any time. Any point on the unobstructed surface of the near side of the moon at any time (DTE). Any point on the unobstructed surface of the lunar southern hemisphere, much of the time.

When DTN can be helpful DTN will be useful for cislunar communications under conditions similar to those in which it will be useful in terrestrial communications: wherever connectivity is not continuous. Exploration rover is within LRO high-availability footprint but LRO is not currently overhead. Rover is within LRO high-availability footprint and LRO is overhead but rover is not in line of sight. It’s in a crater or a canyon. Rover or lander is not within any high-availability footprint at all: Far side of the moon. Northern hemisphere, especially the north pole. (LROs are frequently overhead but opportunities are very brief because orbits are elliptical.)

Future-proofing Cislunar operations are envisioned as precursor to more extensive Mars operations. Mars signal propagation delays are much greater and relay orbiter coverage will likely be less – DTN will be increasingly important for Mars network performance and scalability. Experience with DTN in cislunar communications will help us prepare for Mars network operations.