# Routing Protocols for Delay Tolerant Networks:
## 3 Performance Evaluation
### Evaluation Using Synthetic Mobility Patterns
#### 3.2.3 Advanced Protocol  
We proceed by examining advanced protocols that combine more than one technique. The effectiveness of these approaches are shown in Figure 4, where the delivery ratio is always remarkably high. Furthermore, the overhead grows almost linearly (except for FAD), and it is in any case an order of magnitude smaller than that shown by single technique protocols. To our understanding, this experimental evidence indicates that the combination of the proposed techniques can significantly increase the scalability of the protocols.  
FAD [6] implements both queue management and the forwarding techniques. Both depend on the estimation of the message destination meeting probability; however, the accuracy of this value is highly  
Figure 4: Synthetic traces. Random walk (**a,c,e**); Levy Walk (**b,d,f**). Advanced protocols.  
coupled to the movement patterns among the nodes. Thus, the combination of these two techniques results in a good performance using levywalkmobility (or real traces, as we will see later), while in a very high overhead using random walk mobility (see Figure 4e,f). Although the designers of FAD attempted to limit the forwarding, as the main thing responsible for overhead, examining its behavior in a broader set of protocols, but under a dedicated community-based movement, we understand that their design goal does not achieve high success under random movements of nodes. Another possible explanation of this behavior is that (1) in the considered scenario, TTL is set to infinity and (2) FAD keeps in queue the most important messages, namely the messages with lower fault tolerance values. Those messages are never evicted (TTL is infinity) and are often retransmitted, increasing the overhead. To confirm this fact, we conducted some other experiments where the TTL is set to a limited value. In this additional set of experiments, FAD experiences a low overhead.  
MaxProp [29] and RAPID [18] are two advanced protocols that implement queue management and forwarding techniques. In contrast to FAD, their forwarding policy introduces message reordering rules, but without filtering out any message. Despite the different forwarding approach, the performance achieved is very similar. When a filter is applied, messages can be forwarded or not, while re-ordering implies that all messages can be possibly forwarded according to the new ordering, if there is a sufficient amount of time. The QM technique guarantees a high delivery ratio (even with a small buffer size) and a low delay, as well as limited overhead, even if unlimited replication is employed.  
SCAR [26] is the last protocol considered in this section and is the only protocol implementing all of the proposed techniques. However, the SCAR queue management is very simple (it only uses master-backup copies), and in the considered scenario, the context information (_i.e._, battery level, mobility pattern, _etc._) is similar for all nodes, limiting the effectiveness of the forwarding. As a consequence, SCAR shows high delay and a relatively small delivery ratio, while the overhead is always small thanks to the effectiveness of the replication technique.