# Routing Protocols for Delay Tolerant Networks:
## 4 Technical Recommendations  
In the previous section, we examined 10 well-established DTN routing protocols and comparatively evaluated their performance. We looked into their mechanisms and how they are related to the three basic components used by the reference architecture proposed. In this section, we summarize the discussion by establishing a connection across the three core components, the approaches used to implement them in the group of protocols studied and the most important characteristics of DTNs. Our objective is to provide technical recommendations and design considerations for protocol engineers and network administrators that can take into account when selecting or designing new routing protocols, based on the given set of application characteristics and requirements. Table 4 provides a synopsis of the technical recommendations.  
The main properties of DTNs that should be considered when designing a DTN routing protocol are: (1) contact time and node mobility patterns; (2) node resources; and (3) network traffic imposed by the application.  
The FW technique is important when nodes' movement patterns are highly correlated. Forwarding messages to nodes that will likely meet the final destination more frequently improves the delivery ratio and reduces the delivery delay, and in particular, when contact time is critical (_i.e._, it is short), the selection of appropriate messages to be forwarded is crucial. On the other hand, as movement patterns become less correlated, the importance of FW is reduced; essentially, the acquired knowledge does not help predict the future. An important element to consider in the set-up of forwarding filters,  
Figure 13: ICT distribution for Infocom traces and Levy Walk in our scenario.  
rules and policies is the availability of memory to buffer the messages. Excessive forwarding of messages will generate message buffer overflows, thus eventually hindering the overall performance.  
In all practical cases in which the buffer size is limited, irrespective of the mobility patterns, QM should be well designed taking into consideration the expected traffic and the implications of the adopted FW and R techniques. Clearly, more traffic is generated, either by the application itself or by aggressive FW and R techniques, and more load is handled by the QM.  
When designing a QM component, it is important to provide rules and mechanisms for prioritizing messages according to their destination. This can be exploited if nodes have predictable movements. It is equally important to also prioritize messages in terms of age, hops or number or replicas in the network. Another very important parameter for tuning the performance of the QM component is message TTL. Infinite TTL should be avoided, unless the application is generating a low volume of messages.  
QM highly affects the performance of the protocols. For example, we have seen that thanks to an effective QM policy, MaxProp shows good performance in terms of delivery ratio and delay, even if a high number of messages is dropped, primarily due to the unlimited replicas generated by the protocol.  
Finally, the replication technique is necessary to control the number of message copies. The larger the number of replicas, the better the chance to reach the destination, but the more network resource bandwidth and buffer space are needed. For example, when contact opportunities and mobility are low, but the nodes have enough resources in terms of bandwidth and buffering, more replication will increase reliability. On the other hand, if the nodes are highly mobile and nodes' resources are limited, less replication would better utilize the bandwidth.