Delay-Tolerant Network (DTN)
An introduction to DTN
In 1998, the idea of an interplanetary backbone was born that would support man and robotic space exploration, explained Mr Vint Cerf (Vice President, Chief Internet Evangelist for Google). It was then noted that the internet protocol suite (TCP/IP) was working well on Earth and could be used on Mars. However, different architectures were needed to accommodate a range of parameters that differed largely from those that made TCP/IP work well on Earth. Therefore, researchers from NASA, European, Japanese, and Korean state agencies, as well as the Consultative Committee for Space Data Systems (CCSDS) created the bundle protocol, an experimental delay-tolerant networking (DTN) protocol designed for unstable communications networks.
Two important characteristics of the bundle protocol are delaying name resolution by doing a two-step Domain Name System (DNS) lookup and storing data in the network in case the link is broken. Core parts of the bundle protocol design are strong authentication and encryption to protect against abuse.
In terms of routing traffic through the network, since we can compute the orbits of different spacecraft or at least the trajectories of spacecraft, it is possible to calculate when devices will be able to communicate with each other. A contact graph mapping is then created to indicate when devices can communicate with each other.
When it comes to the standardisation of the bundle protocol, an IETF Working Group was set up in 2014 to standardise bundle protocols and transmission protocols of Licklider. The CCSDS has been working in parallel with the IETF on the standardisation of bundle and JSON protocols that are part of that protocol suite.
The use of the bundle protocol
Some of the examples given about the use of the bundle protocol were: supporting International Space Station crew interactions with Earth, implementation on Android phones, implementation by Technische Universität Braunschweig.
Mr Oscar Garcia (Pilot Projects Working Group Lead, InterPlanetary Networking Special Interest Group (IPNSIG)) gave some examples of IPNSIG’s membership use of DTN. In November 2020, the IPNSIG developed the first DTN communication in cloud servers. The IPNSIG implemented DTN to medical records for space exploration. A DTN management system was developed for Spatiam Corporation in February 2021. A test bed of 100 DTN servers is in development at Utica College, NY. The IPNSIG is also implementing DTN in imagery analysis and processing with AI using Cloud services. IPNSIG PWG members have participated in an experiment by D3TN GmbH, transmitting data to and from the European Space Agency’s (ESA) OPS-SAT satellite, allowing the connection between different implementations of the DTN protocol.
The IPNSIG has also published ‘Strategy Toward a Solar System Internet For Humanity’ report, which is the first attempt to lay out a strategy toward the realisation of a Solar System Internet (SSI).
The use of DTN to enable a safer work environment for reindeer herders in the nomadic Sámi community was explained by Mr Samo Grasic (Dalvvadis Economic Association, Sweden). The goal of the project is to build a reliable reindeer tracking service, provide short messaging community service to reindeer herders, and extend the current network coverage with a drone that can hover and collect the data.
The platform is a tailor-made solution, made to withstand harsh winter conditions and designed for a highly power-constrained environment. It is solar and battery powered. It employs the LoRa-DTN protocol, which is a lightweight store-and-forward protocol (wherein data is sent to an intermediate station where it is kept and sent at a later time, either to the next intermediate station or the final destination). The entire system was built on the DTN architecture with the support of the IPNSIG, which is important as it allows later GPS tracking service to be extended to other services such as short messages and any payload can be carried over this network. The network is synchronised to save bandwidth and the 433MHz LoRA radios in use are in sleep mode most of the time. They ‘wake up’ four times an hour on average to exchange data.
Grasic noted that it is quite important that the reindeer herders own and maintain this network, in order to keep it alive. This is particularly important because it is rather costly to send an external actor to access one of the remote base stations. Some of the challenges identified are connecting LoRa-DTN with the rest of the DTN IPNSIG operational network, routing of traffic as the LoRa-DTN network is less partitioned than expected, radio interference with commercial drones due to the highly powered video transmitters on the DTN drones.
The use of DTN technology for environmental monitoring in Postojna Cave was explained by Mr Boštjan Grašič (Research & Development, MEIS environmental consulting d.o.o). DTN technology is used to transfer data from two meteorological stations in Postojna Cave to a station connected to the internet, via data mules mounted on the regular tourist train that runs in the cave.
The usual delivery delay time for meteorological data is 5-10 minutes. To analyse the delivery delay time, researchers working on the project produced the radial frequency diagram (sunflower).
DTN technology is very useful for cases with existing schedules and known routes, and where legacy internet infrastructure is not available, Grašič noted. He then shared a paper explaining in detail the project, named ‘Karst show caves – how DTN technology as used in space assists automatic environmental monitoring and tourist protection – experiment in Postojna Cave.’