The Office of Space Communications and Navigation (SCaN) at NASA
Headquarters leads the Delay Tolerant Networking (DTN) investigation
with the goal of advancing the maturity and heritage (space
flight use) of the DTN communication protocols. Delay tolerant
networks make use of store-and-forward techniques within the
network in order to compensate for intermittent link connectivity.
In the DTN the fundamental concept is an architecture based
on Internet-independent middleware where protocols at all layers
are used that best suit the operation within each environment,
with a new overlay network protocol (bundle protocol) inserted
between the applications and the locally optimized communications
stacks. Many applications can benefit from the reliable delivery
of messages in a disconnected network.?? The internet, in contrast,
is a connected network where internet protocols, most notably
transmission control protocol/internet protocol (TCP/IP), are
dependent upon (low) latencies of approximately milliseconds.
This low latency, coupled with low bit error rates (BER), allows
TCP to reliably transmit and receive acknowledgements for messages
traversing the terrestrial Internet. One of the best examples
of high latency, high BER links, with intermittent connectivity
is that of space communications. One-way trip times, at the
speed of light, from the Earth to the moon incurs a delay of
1.7 seconds; while one-way trip times to Mars incur a minimum
delay of 8 minutes. The problem of latency for interplanetary
links is exasperated with increased BER due to solar radiation.
In addition, the celestial bodies are in constant motion, which
can block the required line-of-sight between transmit and receive
antennas, resulting in links that at best are only intermittently
connected. Intermittent link connectivity is commonplace terrestrially
as well. One example is the plethora of battery-powered mobile
communications devices that go in and out of communication
range to wired service interface points and are turned on and
off at the users discretion.
Military applications in the DTN arena are substantial, allowing
the retrieval of critical information in mobile battlefield scenarios
using only intermittently connected network communications. For
these types of applications, the delay tolerant protocol should
transmit data segments across multiple-hop networks that consist
of differing regional networks based on environmental network
parameters (latency, loss, BER). This essentially implies that
data from low-latency networks for which TCP may be suitable
must also be forward across the long-haul interplanetary link.
DTN achieves message reliability via employing custody transfer.
The concept of custody transfer, where responsibility of some
data segment (bundle or bundle fragment), migrates with the data
segment as it progresses across a series of network hops is a
fundamental strategy such that reliable delivery is accomplished
on a hop-by-hop basis instead of an end-to-end basis which is
impractical over high latency links.?? DTN is a set of protocols
that act together to enable a standardized method of performing
store and forward communications. DTN operates in two basic environments:
low-propagation delay and high-propagation delay. In a low-propagation
environment such as may occur in near-planetary or planetary
surface environments, DTN bundle agents can utilize underlying
Internet protocols that negotiate connectivity in real-time.
In high-propagation delay environments such as deep space, DTN
bundle agents must use other methods, such as some form of scheduling,
to enable connectivity between the two agents.?? The convergence
layer protocols provide the standard methods for transferring
the bundles over various communications paths. The bundle agent
discovery protocols are the equivalent to dynamic routing protocols
in IP networks. To date the location of bundle agents, DTN agents,
has been managed, analogous to static routing in internet protocol
(IP) networks.?? The security protocols for DTN are important
for the bundle protocol. The stressed environment of the underlying
networks over which the bundle protocol will operate makes it
important that the DTN be protected from unauthorized use, and
this stressed environment poses unique challenges on the mechanisms
needed to secure the bundle protocol. DTNs are likely to be deployed
in organizationally heterogeneous environments where one does
not control the entire network infrastructure. Furthermore, DTNs
may very likely be deployed in environments where a portion of
the network might become compromised, posing the usual security
challenges related to confidentiality, integrity and availability.
The DTN protocol suite is still under active development. In addition to network security, research goals for the DTN activity will focus on testing and evolving important network services including naming and addressing, time synchronization, routing, network management and class of service.?? The DTN experiments on ISS consist of software which is to be placed on both Commercial Generic Bioprocessing Apparatus, CGBA-4 and CGBA-5, and then tested from a ground operations center. This software is not in any critical path of the CGBA operations and may be turned off at anytime. This software does not preclude the use of the CGBA units for other purposes or research support.
As NASA extends its reach to the Moon and beyond, a networked architecture such as DTN will be required to successfully complete these missions. The experiments that will be performed are designed to test the DTN protocol suite in an actual space environment, and to determine how well the protocols perform and what improvements may need to be made. The impact of the results of the research will help to advance the technical maturity of the DTN communications technology so that it is available for NASA use in both human and robotic Exploration missions.