# Routing Protocols for Delay Tolerant Networks:
## 2 Reference Architecture
### Implementation Aspects of DTN Routing Protocols
Fault tolerance is proportional to the number of replicas in the network and also takes into consideration the delivery probability. Messages with low fault tolerance are forwarded first in order to increase the number of copies in the network of messages with a low probability of reaching the destination. In this case, forwarding is history based, while queue management is destination dependent, and replication is unlimited. _MaxProp [29]:_ Each node has a routing table, which predicts the likelihood (_i.e.,_ the cost) to reach another node in the future through its current neighbors. Routing tables are updated on the basis of the information obtained by the neighbors, and messages are ordered and forwarded according to their cost to reach the destination. As such, forwarding is history based, and replication is unlimited. The queuingtechnique is a combination of destination dependent (Dijkstra to evaluate the cost to reach the destination) and destination independent (the current number of hops that a message has accumulated). _RAPID [18]:_ Most DTN protocols aim at maximizing the probability of finding a path between the source and the destination, neglecting all of the other relevant metrics, such as delay, energy consumption, _etc._ On the contrary, RAPID tries to optimize a specific routing metric (e.g., worst-case delivery delay). RAPID treats DTN routing as a resource allocation problem. It translates routing metrics into per-message utilities, which determine how messages should be replicated in the system. When there is a need to make room for new messages, messages with lower utility are deleted first. The forwarding technique is history based; messages with higher marginal utility are forwarded first, and replication is unlimited. Queue management is destination dependent; messages are prioritized according to their utility value (_i.e.,_ the estimated delay to reach the destination). _Cluster-Based Routing [22]:_ The basic idea of this protocol is to distributively group mobile nodes with similar delivery probability (_i.e.,_ nodes that are frequently in contact) into a cluster. Nodes in a cluster can interchangeably share their resources (such as buffer space) for overhead reduction and load balancing. This protocol uses single copy replication and FIFO queuing, and its forwarding technique is history based (cluster based). _NETCAR [40]:_ The scheduler of NETCAR uses the neighborhood index (contact history) to select the messages to be forwarded. Message replication is controlled using parameters that come from the context (buffer occupancy, maxTTL, minEpidemicLevel, maxEpidemicLevel, _etc._). Message aging and the number of replicas are used to evict messages. NETCAR uses a history-based forwarding with controlled replication and destination independent queue management. _ORWAR [41]:_ This protocol uses context information, such as speed, direction, radio range and bandwidth, to estimate the contact window size. Only messages that can be transmitted in such an interval are forwarded. A message utility function (defined by the application), the so-called \"utility per bit ratio\", ranks the messages in the queue and also regulates the number of replicas for each message. Forwarding is context based; replication can be considered controlled, and queue management is destination independent. _HiBOp [42]:_ In HiBOp, a mix of context and history information is used to forward messages only to nodes with a higher delivery probability. This knowledge is also used to control the amount of message replicas (a measure of redundancy and fault tolerance for the message is computed). Only the source can replicate a message; the other nodes can only forward it. The number of replicas is calculated such that the probability of losing all of the replicas is below the threshold specified by the application. This implies that replication is controlled, and forwarding is a mix of context and history based. _BubbleRap [7] and SimBet [8]:_ These are both social-based protocols.