# DTN Protocols

### Forwarding Based

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

- 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

- 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 get stuck in a loop.

### Replication Based

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.

#### Epidemic

- 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 similarly to an epidemic.
- This method achieves minimal latency and high delivery probabilities, but suffers from limited resources.

#### MaxProp

- Like Epidemic, MaxProp floods the network, but each message has a priority.
    - Messages are stored in an **ordered queue** in the **message buffer**.
    - Messages with a higher probability of being delivered have a higher priority of being forwarded first.
- To determine the probability, it looks at the **history of encounters**, maintaining a vector that **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 its destination
    - Once this ACK signal is received, all local copies of redundant messages are dropped.

#### PROPHET

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, but the routing tables **rapidly grow** as a result of the amount of information about the nodes required to calculate the probability predictions.