Delay Tolerant Networking

New Scientist reports on networking from space. If you grew up with the Gemini and Apollo NASA programs, as I did, you remember that they used to have scheduled communication times when they could contact the astronauts. Without the Star Trek concept of “sub-space radio”, which seemed to work pretty much all the time except when the plot demanded that it not, NASA had to catch communication as catch could.

But in those days, much of the data came back with them physically. That’s no longer true, and craft such as the International Space Station will stay up there for a good, long time. What they give us has to come back in the same way we get most of our data now, the way you’re getting this: the Internet, or, more precisely, a network similar to the Internet.

And the article gets the central points right: the key to the system is an experimental protocol called Delay Tolerant Networking (DTN).

While the Earth-bound internet uses a protocol called TCP/IP to allow distant machines to communicate over cables, the ISS payload uses delay-tolerant networking (DTN), which is being developed to cope with the patchy coverage in space that arises when spacecraft pass behind planets or suffer power outages.

If data passing between computers using TCP/IP goes missing, the two keep communicating until everything has been sent. But in space such to-ing and fro-ing of data is impractical.

DTN circumvents this problem by commanding each node in the network to store information until it can find another node that can receive it. Data is relayed in a chain and should only need to be transmitted once.

DTN got its first live space test last fall, but it’s the culmination of a number of years of research anchored in the IETF — actually in a sub-organization called the Internet Research Task Force (IRTF), technically independent of the IETF.

The work started in a group rather fancifully called the Interplanetary Internet Research Group (ipnrg), and is now in the more down-to-Earth, but no less lofty, Delay-Tolerant Networking Research Group (dtnrg), co-chaired by my DKIM working group co-chair, Stephen Farrell. Meanwhile, the ipnrg continues as the Internet Society’s Interplanetary Internet Special Interest Group (IPN SIG), which Stephen also chairs.

Of course, the “interplanetary” aspect of all this is compelling to those of us fascinated by space exploration and astronomy. But DTN also applies to the Earth-bound Internet in many ways. The dtnrg’s charter talks about some of it:

The Delay-Tolerant Networking Research Group (DTNRG) is chartered to address the architectural and protocol design principles arising from the need to provide interoperable communications with and among extreme and performance-challenged environments where continuous end-to-end connectivity cannot be assumed. Examples of such environments include spacecraft, military/tactical, some forms of disaster response, underwater, and some forms of ad-hoc sensor/actuator networks.

Among the challenges to be addressed are: large delay for transmissions resulting from either physical link properties or extended periods of network partitioning, routing capable of operating efficiently with frequently-disconnected, pre-scheduled, or opportunistic link availability, high per-link error rates making end-to-end reliability difficult, heterogeneous underlying network technologies (including non-IP-based internets), and application structure and security mechanisms capable of limiting network access prior to data transit in an environment where round-trip-times may be very large.

And, indeed, the DTN Architecture document, RFC 4838, addresses issues from transport and routing to network management and security.

If space be the final frontier, we’ll be taking the Internet there with us.