EpSoc: Social-Based Epidemic-Based Routing Protocol in Opportunistic Mobile Social Network

In opportunistic networks, the nature of intermittent and disruptive connections degrades the efficiency of routing. Epidemic routing protocol is used as a benchmark for most of routing protocols in opportunistic mobile social networks (OMSNs) due to its high message delivery and latency. However, Epidemic incurs high cost in terms of overhead and hop count. In this paper, we propose a hybrid routing protocol called EpSoc which utilizes the Epidemic routing forwarding strategy and exploits an important social feature, that is, degree centrality. Two techniques are used in EpSoc.Messages’ TTL is adjusted based on the degree centrality of nodes, and the message blocking mechanism is used to control replication. Simulation results show that EpSoc increases the delivery ratio and decreases the overhead ratio, the average latency, and the hop counts as compared to Epidemic and Bubble Rap.


Introduction
Opportunistic mobile social network (OMSN) [1][2][3][4] is a promising networking model for data dissemination.In the OMSN, mobile nodes grab the opportunity of encountering the peer (they are in the communication range of each other) to forward the data.e OMSN incurs intermittent and disruptive connectivity due to node mobility.To tackle with this complex environment, the store-carry-forward scheme is applied in the OMSN.If no connection is available at a particular time, a mobile node stores data in its buffer and carries them until it encounters other mobile nodes to forward the data [5][6][7][8].Various approaches have been proposed to address the information delivery problem in the OMSN such as in [9][10][11][12].
e main concerns of OMSN routing approaches are yielding high delivery ratio, low delay, and low overhead or cost on networks and nodes.
Flooding is one of the dominant schemes to disseminate data in the OMSN [13].Each message will be flooded to every node in the network.Multiple copies of each message are generated and spread in the network.Epidemic [9] routing protocol is the flooding-based routing protocol.When two nodes encounter, they exchange all of their messages.is results in messages spread over the whole network by pairwise contacts between two nodes.If no buffer constraints are applied, Epidemic represents the upper bound in message delivery and latency.Epidemic routing is used as a benchmark and a reference for the most of other routing protocols in the opportunistic network.e main drawback of the Epidemic scheme is its high overhead.Many schemes are proposed to decrease the overhead in Epidemic-based approaches by limiting the number of message replicas [14][15][16].An effective scheme to control replication spread is the vaccine [17].It applies the antipacket mechanism to control replica distribution in Epidemic-based routing.In [18], a new scheme is proposed to control the replication of epidemically distributed information.Based on the vaccine scheme, signal distribution is early controlled by the fully immunized vaccine.In addition, a partially immunized vaccine is initiated when there is a local-forwarding opportunity to vaccine more packets.
In the OMSN, mobile devices are portable by humans so that social features of people can be exploited for networking purposes [19][20][21].Social-based protocols utilize social properties of mobile users such as similarity, centrality, and friendship to improve routing efficiency in the opportunistic mobile social network.is is because social features are more stable and less changeable than other features like mobility patterns.HiBOp [22] and CiPRO [23] exploit the similarity social feature and the user's context information to forward data.LASS [24] takes into account the difference of members' activity within the node's community for data dissemination.In ML-SOR [25], node centrality (different types of centralities), the similarity between communities, and social ties are all exploited to effectively select the forwarding node.MCAR [26] exploits the preferred communities of people during their daily lives for effective information delivery.Direct (inside one community) and indirect (via different communities) contacts are considered in MCAR.In SPRINT-SELF [27], network and node overhead is decreased by exploiting social information of mobile users.e authors consider the social community of nodes and utilize it to predict future behavior based on contact history.In addition, they proposed a new mechanism to avoid selfish nodes for more improvements.
Social features are utilized widely for buffer management.Liu et al. [28] utilized social features and the congestion level to develop the forwarding strategy that drops the message with the minimum social link rather than random dropping.
In SRAMSW [29], the buffer management mechanism is combined with social features to enhance the spray-based routing.Expired messages are deleted, successfully delivered messages are acknowledged, and messages are prioritized according to their spray times and residence time.In addition, three social features: centrality, similarity, and friendship, are adopted for better forwarding decision and to avoid the dead-end problem.
We hypothesize that combining social features with the Epidemic-based forwarding scheme improves the efficiency of routing in the OMSN.In this paper, we present an Epidemic-based Social-based routing protocol (EpSoc) that combines the advantages of the forwarding strategy used in the Epidemic routing protocol with the positive impact of exploiting social features.EpSoc exploits the degree centrality social feature to adapt the time to live (TTL) of the routed message.If a message is forwarded to a node which has higher degree centrality (socially active node), its TTL value will be decreased.If these messages are dropped in the active node when TTL is zero, the blocking mechanism is used to reject receiving replications of these messages.e rest of this paper is organized as follows: the next section reviews the related work.We describe in detail our proposed algorithm EpSoc in Section 3. In Section 4, we introduce the performance evaluation and the discussion results.Finally, Section 5 concludes the paper.

Related Works
Delivering data to the destination at the right time with minimum resources is an optimal condition for any given routing protocols.Epidemic has optimal performance in terms of delivery ratio and latency.However, it suffers from high overhead cost.One of the solutions is to exploit social information to improve Epidemic-based routing protocols in the OMSN.Degree centrality is a social feature that is exploited widely in the literature to improve routing in the mobile social networks.For example, Bubble Rap [11] utilizes the mobile node's degree centrality to provide cost-effective routing as compared to Epidemic routing.Bubble Rap exploits two social and structural metrics, namely, centrality and community.It selects high centrality nodes and community members of destination as relays.In the Bubble Rap algorithm, nodes belong to different sizes of communities and have different levels of popularity (i.e., rank).Each node is assumed to have two rankings: global denotes the popularity (i.e., connectivity) of the node in the entire society and local denotes the popularity within its community.Messages are forwarded to nodes that have higher global ranking until a node in the destination's community is found.
en, the messages are forwarded to nodes having a higher local ranking within the destination's community.
Similar to Bubble Rap, CAOR [30] exploits degree centrality and similarity social features to improve routing.However, CAOR constructed autonomous communities based on common interest locations between mobile nodes.Members of a community with high centralities act as the home of this community.CAOR also has a mechanism to turn the routing between lots of nodes to the routing between a few community homes.It also applies the reverse Dijkstra algorithm to determine the optimal relays and compute the minimum expected delivery delay.In [31], cultural algorithm (CA), ant colony optimization (ACO), and social connectivity between users are combined to address the routing problem.Social metrics of nodes including degree and betweenness centralities are analyzed to support forwarding decision in the opportunistic network environment.
Besides exploited in the routing problem, social features were also applied to solve different type of problems.For example, works in [32] exploit social features (degree and betweenness centralities) to solve the throwbox placement problem based on the given graph.A user's degree is equal to the total number of its neighbors, and betweenness is the total number of shortest paths passing through the node.e work also introduced the concept of location degree to measure how many mobile users have the location as one of their top visited locations and the location betweenness to show how important the location is for the entire social graph.Social metrics such as degree centrality, social activeness, and community acquaintance are also applied to enhance data delivery in VSNs in [33].e social interactions between nodes where nodes of similar interests or nodes belonging to the same community have greater probability to encounter each other.Social centrality is also utilized for congestion control in the postdisaster environment [34].
Unlike the aforementioned works, instead of considering community or homing structuring, our proposed protocol is based on the flooding-based forwarding strategy of Epidemic.is is because we intended to design our forwarding protocol to have high delivery ratio and low delay as the Epidemic-based forwarding strategy.Moreover, we also aimed to avoid extra time required to form and maintain the community structure.us, to decrease the overhead, we utilized degree centrality and adapted the message's TTL (Time to live) to control the forwarding nodes' activity.e 2 Mobile Information Systems message's TTL is considered in literature when developing efficient routing protocols.Miao et al. [33] proposed the adaptive multistep routing protocol for mobile delay-tolerant networks (MDTNs).eir aim is to get the balance between the delay and cost of message delivery.e time to live of the message is used in order to allocate the minimum number of copies necessary to achieve a given delivery probability.In [34], the authors considered the contact information of nodes and the time to live message's property to make route decision and improve performance.ey built a replica distribution criterion between two encountered nodes based on residual messages' TTL.In our proposed solution, we do not limit the number of replicas but allow messages to be spread in the network and then we utilize degree centrality to decrease the TTL and consequently decrease the overhead.Also, the blocking mechanism is adopted in the active node to cancel receiving replications of the same message which is seen previously.

EpSoc Routing Protocol
EpSoc is the routing algorithm designed to decrease overhead in the Epidemic protocol by embedding social features in routing the message in the opportunistic network.To achieve this objective, we propose two mechanisms.First, the message's TTL is adapted based on degree centrality of the nodes in the OMSN.e second one is the message blocking mechanism that is used to prevent receiving the replications of runout TTL messages in active nodes.

Degree Centrality.
Node centrality indicates the popularity of the node in the network, whereby node centrality in a social network is the reflection of its social relative importance [35].A higher node degree centrality means the node connects with many numbers of nodes in the network.A degree centrality for a given node i can be calculated as follows: where N is the number of nodes in the network and a(i, k) � 1 if a direct link exists between node i and node k and i ≠ k.
We adopt the CWindow [11] calculation algorithm to calculate degree centrality.CWindow divides the day into time windows and calculates the average of nodes' degree over these windows to estimate the node's centrality.To decrease the processing overhead results from processing any changes of degree centrality, CWindow calculates the node's degree centrality at regular intervals instead of every time the centrality changes.
We pick the CWindow algorithm to calculate degree centrality because it takes into account the changes in node centrality over time and averages the node's centralities for few window intervals which is suitable for people's behavior in the OMSN.It is also used by other social-based protocols that consider the node's degree centrality such as Bubble Rap [11], Dlife [36], and SCORP [37].

EpSoc Forwarding Strategy.
Figure 1 depicts the forwarding process in EpSoc, where two mechanisms are applied.
e TTL value of the forwarded messages to N2 is decreased by dividing it by the DC value of N2.We call these messages socially infected messages.Node N2 registers the ID of these messages as seen messages in its blocking register.Node N2 will drop the socially infected message when it expires.In Figure 1(b), the Epidemic forwarding strategy which we adopted in our algorithm causes replications of socially infected messages to be sent to node N2 from other nodes such as N4.In this case, node N2 will reject receiving the message with the reason that it is a seen message (its ID is stored in the block register).
Based on (1), the TTL value is adapted using the following equation: e two combined mechanisms adopted in EpSoc enhance routing performance.If a node is socially active, it meets more nodes in the network and has a higher potential to deliver more messages.Decreasing messages' TTL value in active nodes results in releasing space in their buffers and increases the ability to deliver more messages.is will increase the delivery ratio.e blocking mechanism controls the replications by preventing decreased TTL messages from hitting again the previously traversed actives nodes.e result is decreasing network overhead.Regarding average latency, the messages that are delivered by active nodes have a shorter end-to-end delay compared to other messages delivered by lower activity nodes, so average latency will be decreased too.We conclude that decreasing message life socially by combining the message blocking scheme affects positively the performance of the Epidemic routing scheme in the OMSN.

Pseudocode of the EpSoc Message
Forwarding.We used CWindow to calculate the centrality of the node.Each node N i records the encountered nodes in the network.When node N i encounters N j , the degree centrality values are calculated using CWindow.en, centrality values are exchanged between both nodes.N i compares its centrality value DC i with the N j centrality value DC j .If DC j is greater than DC i , which means that N j is socially more active than N i , then for each message m i in the N i buffer Buf N i , the TTL value m i TTL is decreased by dividing it by the node N j centrality value DC j .
e ID of each socially infected message is registered in the N j blocking register Block N j .e time complexity of the EpSoc forwarding algorithm is Ο(M), where M is the number of messages carried in the node's buffer and needed to be sent or forwarded.
Algorithm 1 shows the complete pseudocode of EpSoc.

Performance Evaluations
4.1.Data Set.We adopt the Cambridge experimental data set of haggle [38]. is data set includes traces of Bluetooth Mobile Information Systems sightings by groups of users carrying small devices (iMotes) for a number of days in campus environments.e experiments are conducted in the computer laboratory that includes the undergraduate rst-year and second-year students and also some Ph.D. and postgraduate students which lasted for 11 days.

Simulation Setup.
We use the opportunistic network environment (ONE) [39] simulator to evaluate our algorithm.Also, comparison with Epidemic and Bubble Rap routing protocols is included to measure the performance of our proposed algorithm EpSoc.We want to justify that the Epidemic algorithm performs better when considering social features in forwarding messages.e simulator settings are tabulated in Table 1.
In each experiment, we compare the performance of the protocols EpSoc, Epidemic, and Bubble Rap based on the following metrics.

Successful Delivery Ratio.
It is the ratio between the number of delivered messages and the total number of created messages.e ideal value of the successful delivery ratio is 1.0 when all created messages are delivered to their destinations.

Overhead Ratio.
It is the additional bytes that are sent for successfully delivering a message to a destination.

Average Latency.
It is the average of the time elapsed between message creation and delivery.

Average Hop Count.
It is the average of the number of hops that messages must take in order to reach the destination.

Experiments and Discussion.
To evaluate our work, we will consider two features: bu er size and message TTL value.
ese two features have a high impact on routing performance in the OMSN.In our work, we a ect these two features.We adjust the TTL value socially and use the blocking mechanism to manage bu er storing.Consequently, our experiments are to measure the performance of EpSoc when varying the TTL and bu er size.e comparison will be with Epidemic and Bubble Rap protocols.

Varying Bu er Size. For varying the bu er size, we
xed the value of TTL to 2.5 d.Figures 2-5 show the performance comparisons between EpSoc, Epidemic, and Bubble Rap in terms of delivery ratio, overhead ratio, average latency, and average hop count, respectively.
Figure 2 shows the delivery ratio with bu er size.Generally, for Epidemic, Bubble Rap, and EpSoc, increasing the ( Buf N j ← m i forward message to N j (10) Block N j ← m i ID (11) End if (12) End if (13) End for (14) End if (15) End for ALGORITHM 1: Pseudocode of the forwarding strategy in EpSoc.bu er size will increase the delivery ratio.is is because more messages can be carried by intermediate nodes which consequently deliver more messages to destinations.Changing the number of delivered messages a ects the delivery ratio, overhead ratio, average latency, and average hop count.Regarding the delivery ratio, it is clear that delivering more messages results in a higher value, while the decrease in delivered messages results in a lower value.In the lower bu er size scenario (1-25 MB), the delivery ratio of Epidemic is the lowest because of the redundancy.Bubble Rap and EpSoc outperform Epidemic due to utilizing social features.In higher bu er size scenarios (35-55 MB), Epidemic achieves higher delivery ratio than Bubble Rap.Larger bu er size alleviates the negative impact of dropping messages because of replication and enables Epidemic to deliver more messages.Our protocol EpSoc outperforms both Epidemic and Bubble Rap.Blocking runout TTL messages from being received by active nodes results in more space in their bu er and therefore can carry more di erent messages when encountering other nodes and later deliver them to destinations.In addition, the decrease of TTL of the messages that are forwarded to the more active nodes results in better utilization of the bu er's space.Decreased TTL message copies are dropped earlier enabling carrying more other messages.Active nodes deliver the message quickly.erefore, the number of delivered messages is increased, and consequently, the delivery ratio is increased.e relation of overhead ratio with bu er size is shown in Figure 3.When increasing the bu er size, the overhead is decreased for all algorithms.Regarding Bubble Rap, overhead is decreased with bu er increase because no replication exists.We observe from Figure 3 that both protocols Bubble Rap and EpSoc outperform Epidemic signi cantly and EpSoc outperforms Epidemic.We mentioned formerly that applying the strategy of blocking messages and decreasing TTL increase the number of delivered messages.In addition, the blocking message to be resent to active nodes decreases the number of replications in the network and hence decreases the forwardings.As a result, decreasing the forwardings and increasing the delivered messages decrease the overhead ratio in the network.Bubble Rap achieves lower overhead than EpSoc for lower bu er size scenarios (1, 2, 5, and 15 MB). is is because of two reasons: rst, the replication and Epidemic strategy adopted in EpSoc, and second, low bu er size causes dropping relayed messages early due to bu er over ow which in turn decreases the e ciency of our proposed algorithm compared to Bubble Rap in terms of overhead.However, for larger bu er size scenarios (25,35,45, and 55 MB), EpSoc is more e cient and its overhead ratio is very close to that of Bubble Rap (for 35, 45, and 55 MB, it is slightly better than Bubble Rap).
Figure 4 shows that average latency for all the three protocols, which increases when the bu er size is increased.A low-sized bu er only relays low-latency messages which will reach their destinations quickly.On the other hand, a highersized bu er allows messages to be carried for a longer time which contribute to a higher average latency.EpSoc decreases the average latency signi cantly.EpSoc optimizes the bu er usage where messages forwarded to active nodes have low TTL.With the low TTL, a relay node has more free space for new messages in its bu er as the bu er quickly dropped the old messages.
In Figure 5, the average hop count is recorded.e average hop count increases when the bu er size is increased.Epidemic has more hop count which indicates more nodes experiencing the duplicated messages, whereas Bubble Rap has lower hop count because it prevents replication of messages.For EpSoc, the number of hop count is contributed by allowing replication as in Epidemic.
e worst achievement is because of redundancy.Bubble Rap does not apply replication so that it outperforms EpSoc.EpSoc achieves better average hop count than Epidemic.
is is because of the message blocking strategy which decreases the number of replications in the network and hence decreases the forwardings.In Figure 6, the relation of delivery ratio with TTL is depicted.For lower TTL (10 m-3 h), the delivery ratio of Epidemic is slightly higher than that of Bubble Rap and EpSoc.e reason is that, for short TTL messages, exploiting social features will not be very e ective due to quickly messages dropping.In addition, the higher replication occurred in Epidemic increases the number of delivered messages.When TTL is increased, Bubble Rap and EpSoc outperform Epidemic due to utilizing the social features.In high TTL scenarios (1.5 d-1 w), EpSoc outperforms Bubble Rap. is indicates an e ciency of EpSoc in its social feature selection.
Shortening the life of messages in the active nodes and blocking runout TTL messages from hitting again the active nodes cause an increment in the delivered messages and decrement in forwardings.erefore, delivery ratio grows up.
Figure 7 compares the performance of the protocols with di erent values of TTL in terms of overhead ratio.When TTL is very low (10 m-1 h), Epidemic, EpSoc, and Bubble Rap achieve low overhead.e reason is that messages are dropped quickly.For high TTL values, Bubble Rap achieves the best and Epidemic the worst.
Our protocol EpSoc manages to decrease the overhead better than Epidemic.is is because EpSoc is a combination of the ooding-based forward strategy with social features.
From Figure 8, in terms of average latency, EpSoc appears to be outperforming others especially with high TTL  is is because our algorithm always enables the active node to carry messages with lower TTL.
From Figure 9, we observed that if TTL is very low (10 m-1 h), all routing protocols in the experiment have low average hop count. is is due to the low number of forwardings between nodes as message life exhausted quickly.Bubble Rap and EpSoc have almost stable performance when TTL is increased.Exploited social features in Bubble Rap and EpSoc and applying the blocking mechanism in EpSoc decrease the e ect of changing the value of TTL on the number of traversed nodes to the destination.On the contrary, when the TTL is increased, the average hop count of Epidemic increases signi cantly because of the ooding-based forwarding strategy.For EpSoc, the average hop count is decreased signi cantly compared to Epidemic because of the social e ect of active nodes.

Conclusion
In this paper, we investigate the ooding-based forwarding strategy, that is, Epidemic with social features to improve the routing performance in the opportunistic mobile social network (OMSN).Inspired by the advantages of Epidemic routing protocols in terms of delivery ratio and delivery delay and by exploiting the social activity of nodes, we formulated a oodingbased social-based routing protocol named as EpSoc.Simulation experiments using real data sets are conducted to evaluate our protocol performance.From the presented results, our approach increases the delivery ratio and decreases the delivery overhead, average latency, and average hop count as compared to the Epidemic protocol.As for benchmark social-based routing protocol performance (Bubble Rap), our protocol decreases the average latency signi cantly with a better delivery ratio in high bu er size and low TTL scenarios.Generally, we manage to exploit the advantage of Epidemic and Bubble Rap to improve the data dissemination in the OMSN.

Additional Points
Signi cance.Social features of a node can be utilized to have an e ective role in the routing protocol in the OMSN network.
is paper presents the routing protocol that exploits degree centrality to increase the delivery ratio and decrease the overhead and latency.Moreover, exploiting other social features such as similarity and community may lead to being more e cient routing protocol.erefore, combining social features with other forwarding techniques such as the Epidemic-based strategy is signi cant to have an e cient forwarding strategy in the OMSN to support green technologies.Mobile Information Systems