DTN Protocols

Where each message may only be under the custody of a single node.

Upon forwarding the message, the receiving node also takes on the responsibility of custody.
This means there will exist only one copy of the message within the network at any period of time.

Direct transmission is the simplest single-copy forwarding protocol possible.
Once the source has generated a message, it will retain custody and carry it until it encounters the destination.
Once a connection with the destination is established, the message is forwarded directly
- This uses minimal resources
- Has unbounded amounts of latency
- Probability of a message being delivered is only as likely as the probability of the node encountering the destination node

First contact is a single-copy based forwarding protocol - it randomly chooses a node out of all possible nodes and forwards as many messages as possible to that node.
If no connections are available, the first encountered node will be used.
Once the message(s) are sent, the messages on the original node are deleted, relinquishing custody to the new node.
This protocol routes messages throughout the network via a random walk pattern.
- This can lead to packets being routed to dead ends.
- Packets can make negative progress or getting stuck in a loop.

Replication-based protocols disseminate messages throughout the network via replication of the messages.

When one node encounters another, it will forward the message while retaining the local copy it has.
- The existence of multiple copies increases the probability of message delivery and reduces latency.
- The more nodes carrying the message, the more chance one node encounters the destination.
However this also means there are many redundant messages on the network - therefore more resources are needed.

Utilising the flooding concept, Epidemic aims to achieve message delivery by flooding the network with message copies.
When any two nodes meet, they compare messages.
- They then exchange messages they do not have in common
- This is repeated allowing the messages to spread similar to an epidemic.
This method achieves minimal latency & high delivery probabilities however suffers from limited resources.

Like epidemic, maxprop floods the network, however each message has a priority.
- Messages stored in a ordered-queue in the message buffer.
- Messages with a higher probability of being delivered have a higher priory of being forwarded first.
To determine the probability, it looks at history of encounters, maintaining a vector with tracks the likelihood of the node encountering any other node in the network.
- When two nodes meet, they exchange messages and vectors, updating their own local copy.
- These vectors are then used to compute the shortest path for each message, messages are then ordered within the buffer by destination cost.
MaxProp uses overhead messages to acknowledge when a message has reached it destination
- Once this ACK signal is received, all local copies of redundant messages are dropped.

Probabilistic Routing Protocol using History of Encounters and Transitivity (PRoPHET)

PROPHET maintains a vector that keeps track of a history of the encountered nodes.
It uses this vector to calculate the probability of a message copy reaching its destination by being forwarded to a particular node.
- When a source node forwards a message copy, it selects a subset of nodes that it can possibly send to.
- The algorithm then ranks these nodes based on the calculated probabilities, with the copy being forwarded to the highest ranked nodes first.
This is effective however the routing tables rapidly grow as a result of the amount of information on the nodes required to calculate the probability predictions.