Disruption Tolerant Networks (DTNs) are employed in applications where the network is likely to be disrupted due to environmental conditions or where the network topology makes it impossible to find a direct route from the sender to the receiver. Underwater networks typically use acoustic waves for transmitting data. However, these waves are susceptible to interference from sources of noise such as the wake from ships, sounds from snapping shrimp, and collisions from acoustic waves generated by other nodes.
DTNs are good candidates for situations where successfully delivering the message is more important than low delivery times and high network throughput. This is true for certain applications of underwater networks. DTNs can also create new options for network topologies, such as opening up the possibility of using data muling nodes if the network is resilient to delays.
The Acoustic Research Laboratory (ARL) at NUS has developed their own Groovy-based underwater network simulator called UnetStack, in which network protocols can be designed and tested in a simulator. These protocols can later be directly deployed on physical hardware, such as Subnero's underwater modems. Hence, this project revolves around creating a new UnetStack protocol called DtnLink for enabling disruption tolerant networking in various use cases of the ARL.
Unlike conventional network protocols which rely on end-to-end connectivity at a given instant of time, DTNs do not require a complete path from the source to the destination when transmitting the message.
To accomplish this, all types of DTN protocols employ a type of Store-Carry-And-Forward (SCAF) mechanism to store the message until it can be sent to the destination. The message can either be sent directly to the destination or via another node in multi-hop routing.
Some DTN routing algorithms use packet replication to send datagrams to the destination. This approach might be sub-optimal for underwater networks which are constrained by transmission power limitations and suffer from packet collisions when the network is flooded with messages. Therefore, an implementation of DTNs for UnetStack MAY NOT use packet replication.
The DTN protocol developed in this project is referred to as DtnLink.
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Data Muling: UnetStack is used on sensor nodes for collecting sensor measurements from parts of the ocean. The sensor stores the data until a diver can retrieve the sensor. This is a labour intensive procedure. To supplant this,
DtnLinkcan be used for sending the sensor’s data to an AUV when it comes in range of the sensor. -
Time Varying Links: A concern in underwater networks is that certain links are only available under certain conditions. For example, high bandwidth optical links are short-ranged and require a Line of Sight to the destination for communication. Ideally,
DtnLinkshould be able to choose the most optimal link depending on the link’s availability and bitrate. -
USB Link:
DtnLinkwill maintain a list of pending messages in the node’s non-volatile storage. As a potential alternative to sending these messages wirelessly, a USB Link agent could work in conjunction withDtnLinkfor automatically copying these messages to an external storage device. -
NUSwan: The NUSwan is a water surface dwelling robot which autonomously collects data about the water quality in Singapore’s reservoirs with its sensors. This data is relayed to the cloud using an LTE connection. However, in large reservoirs, the LTE connection may be temporarily unavailable due to lack of coverage.
DtnLinkcan store pending messages and then send these messages when the LTE link is available.
DtnLink supports the following features:
- Fragmentation of large messages (payloads)
- Detection of duplicate messages caused due to dropped
DatagramDeliveryNtfs - Stop-And-Wait sending to reduce channel congestion
- Short-circuiting to save header space
After cloning this repo, run any of the simulation scripts using the UnetStack3 JARs. This project is not compatible with older versions of UnetStack.
DtnLink can be dropped into any Unet container to enable disruption tolerant communication. On startup DtnLink will probe for Link agents supporting the RELIABILITY parameter (e.g. ReliableLink). All DatagramReq sent to DtnLink must have a set TTL value or the DatagramReq will be refused at the outset.
To use DtnLink for multi-hop paths, populate the Routing table of Router with RouteDiscoveryNtf messages with the link set to DtnLink for each hop.
The following parameters of DtnLink can be configured by the user at runtime:
- Link Priorities - the order in which
DtnLinkshould attempt sending messages - Order of sending messages (
ARRIVAL,EXPIRY,RANDOM) - Time periods for some functions
- Short-circuiting - send messages to destination without DTN headers
Using the IntelliJ IDEA IDE is the easiest way to setup your environment for running simulations in the sim/ directory of this repository. More information about downloading and setting up IDEA with the requisite Unet libs can be found in this blog post.
DtnLink uses a JUnit test suite for running regression tests. This uses the methodology of "Black Box" unit testing where we check if DtnLink is giving us the expected output for known inputs. To test if DtnLink is behaving as expected, run DtnTest in the test/ sub-directory to make sure everything is working correctly.
The Request For Comments (RFC) document has more technical details about the project and offers a high-level idea of the design decisions which went into writing DtnLink.
My final thesis presentation, presented to the Acoustic Research Lab on 25 April 2019, has useful diagrams and simulation results which can help in understanding how DtnLink works in detail.
The source files are distributed under the MIT License (http://opensource.org/licenses/MIT), so feel free to fork and modify DtnLink for your projects!