File sharing applications in mobile ad hoc networks (MANETs) have attracted more and more attention in recent years. The efficiency of file querying suffers from the distinctive properties of such networks including node mobility and limited communication range and resource. An intuitive method to alleviate this problem is to create file replicas in the network. However, despite the efforts on file replication, no research has focused on the global optimal replica creation with minimum average querying delay.

Specifically, current file replication protocols in mobile ad hoc networks have two shortcomings. First, they lack a rule to allocate limited resources to different files in order to minimize the average querying delay. Second, they simply consider storage as available resources for replicas, but neglect the fact that the file holders’ frequency of meeting other nodes also plays an important role in determining file availability. Actually, a node that has a higher meeting frequency with others provides higher availability to its files. This becomes even more evident in sparsely distributed MANETs, in which nodes meet disruptively.

In this paper, we introduce a new concept of resource for file replication, which considers both node storage and node meeting ability. We theoretically study the influence of resource allocation on the average querying delay and derive an optimal file replication rule (OFRR) that allocates resources to each file based on its popularity and size. We then propose a file replication protocol based on the rule, which approximates the minimum global querying delay in a fully distributed manner. Our experiment and simulation results show the superior performance of the proposed protocol in comparison with other representative replication protocols.



With the increasing popularity of mobile devices, e.g., smartphones and laptops, we envision the future of MANETs consisted of these mobile devices. By MANETs, we refer to both normal MANETs and disconnected MANETs, also known as delay tolerant networks (DTNs). The former has a relatively dense node distribution in an area while the latter has sparsely distributed nodes that meet each other opportunistically. On the other side, the emerging of mobile file sharing applications on the peer-to-peer (P2P) file sharing over such MANETs. The local P2P file sharing model provides three advantages. First, it enables file sharing when no base stations are available (e.g., in rural areas). Second, with the P2P architecture, the bottleneck on overloaded servers in current clientserver based file sharing systems can be avoided. Third, it exploits otherwise wasted peer to peer communication opportunities among mobile nodes. As a result, nodes can freely and unobtrusively access and share files in the distributed MANET environment, which can possibly support interesting applications.

For example, mobile nodes can share files based on users’ proximity in the same building or in a local community. Tourists can share their travel experiences or emergency information with other tourists through digital devices directly even when no base station is available in remote areas. Drivers can share road information through the vehicle-to-vehicle communication. However, the distinctive properties of MANETs, i.e., node mobility, limited communication range and resource, have rendered many difficulties in realizing such a P2P file sharing system. For example, file searching turns out to be difficult since nodes in MANETs move around freely and can exchange information only when they are within the communication range. Broadcasting can quickly discover files, but it leads to the broadcast storm problem with high energy consumption.

Probabilistic routing and file discovery protocols avoid broadcasting by forwarding a query to a node with higher probability of meeting the destination. But the opportunistic encountering of nodes in MANETs makes file searching and retrieval non-deterministic. File replication is an effective way to enhance file availability and reduce file querying delay. It creates replicas for a file to improve its probability of being encountered by requests. Unfortunately, it is impractical and inefficient to enable every node to hold the replicas of all files in the system considering limited node resources. Also, file querying delay is always a main concern in a file sharing system. Users often desire to receive their requested files quickly no matter whether the files are popular or not. Thus, a critical issue is raised for further investigation: how to allocate the limited resource in the network to different files for replication so that the overall average file querying delay is minimized? Recently, a number of file replication protocols have been proposed for MANETs. In these protocols, each individual node replicates files it frequently queries or a group of nodes create one replica for each file they frequently query. In the former, redundant replicas are easily created in the system, thereby wasting resources.

In the latter, though redundant replicas are reduced by group based cooperation, neighboring nodes may separate from each other due to node mobility, leading to large query delay. There are also some works addressing content caching in disconnected MANETs/ DTNs for efficient data retrieval or message routing. They basically cache data that are frequently queried on places that are visited frequently by mobile nodes. Both the two categories of replication methods fail to thoroughly consider that a node’s mobility affects the availability of its files. In spite of efforts, current file replication protocols lack a rule to allocate limited resources to files for replica creation in order to achieve the minimum average querying delay, i.e., global search efficiency optimization under limited resources. They simply consider storage as the resource for replicas, but neglect that a node’s frequency to meet other nodes (meeting ability in short) also influences the availability of its files. Files in a node with a higher meeting ability have higher availability.




AUTHOR: X. Zhuo, Q. Li, W. Gao, G. Cao, and Y. Dai

PUBLISH: Proc. IEEE 19th Int’l Conf. Network Protocols (ICNP), 2011.


The recent popularization of hand-held mobile devices, such as smartphones, enables the inter-connectivity among mobile users without the support of Internet infrastructure. When mobile users move and contact each other opportunistically, they form a Delay Tolerant Network (DTN), which can be exploited to share data among them. Data replication is one of the common techniques for such data sharing. However, the unstable network topology and limited contact duration in DTNs make it difficult to directly apply traditional data replication schemes. Although there are a few existing studies on data replication in DTNs, they generally ignore the contact duration limits. In this paper, we recognize the deficiency of existing data replication schemes which treat the complete data item as the replication unit, and propose to replicate data at the packet level. We analytically formulate the contact duration aware data replication problem and give a centralized solution to better utilize the limited storage buffers and the contact opportunities. We further propose a practical contact Duration Aware Replication Algorithm (DARA) which operates in a fully distributed manner and reduces the computational complexity. Extensive simulations on both synthetic and realistic traces show that our distributed scheme achieves close-to-optimal performance, and outperforms other existing replication schemes.



AUTHOR: X. Zhuo, Q. Li, G. Cao, Y. Dai, B.K. Szymanski, and T.L. Porta,

PUBLISH: Proc. IEEE Eighth Int’l Conf. Mobile Adhoc and Sensor Systems (MASS), 2011.


Data access is an important issue in Delay Tolerant Networks (DTNs), and a common technique to improve the performance of data access is cooperative caching. However, due to the unpredictable node mobility in DTNs, traditional caching schemes cannot be directly applied. In this paper, we propose DAC, a novel caching protocol adaptive to the challenging environment of DTNs. Specifically, we exploit the social community structure to combat the unstable network topology in DTNs. We propose a new centrality metric to evaluate the caching capability of each node within a community, and solutions based on this metric are proposed to determine where to cache. More importantly, we consider the impact of the contact duration limitation on cooperative caching, which has been ignored by the existing works. We prove that the marginal caching benefit that a node can provide diminishes when more data is cached. We derive an adaptive caching bound for each mobile node according to its specific contact patterns with others, to limit the amount of data it caches. In this way, both the storage space and the contact opportunities are better utilized. To mitigate the coupon collector’s problem, network coding techniques are used to further improve the caching efficiency. Extensive trace-driven simulations show that our cooperative caching protocol can significantly improve the performance of data access in DTNs.



AUTHOR: Z. Li and H. Shen

PUBLISH: IEEE Trans. Computers, vol. 62, no. 1, pp. 83-97, Jan. 2012.


However, current probabilistic forwarding methods only consider node contact frequency in calculating the utility while neglecting the influence of contact duration on the throughput, though both contact frequency and contact duration reflect the node movement pattern in a social network. In this paper, we theoretically prove that considering both factors leads to higher throughput than considering only contact frequency. To fully exploit a social network for high throughput and low routing delay, we propose a Social network oriented and duration utility-based distributed multicopy routing protocol (SEDUM) for DTNs. SEDUM is distinguished by three features. First, it considers both contact frequency and duration in node movement patterns of social networks. Second, it uses multicopy routing and can discover the minimum number of copies of a message to achieve a desired routing delay. Third, it has an effective buffer management mechanism to increase throughput and decrease routing delay. Theoretical analysis and simulation results show that SEDUM provides high throughput and low routing delay compared to existing routing approaches. The results conform to our expectation that considering both contact frequency and duration for delivery utility in routing can achieve higher throughput than considering only contact frequency, especially in a highly dynamic environment with large routing messages.




This work focuses on Delay Tolerant Networks (DTNs) in a social network environment. DTNs do not have a complete path from a source to a destination most of the time. Previous data routing approaches in DTNs are primarily based on either flooding or single-copy routing. However, these methods incur either high overhead due to excessive transmissions or long delays due to suboptimal choices for relay nodes. Probabilistic forwarding that forwards a message to a node with a higher delivery utility enhances single-copy routing.

Previous file sharing applications in mobile ad hoc networks (MANETs) have attracted more efficiency of file querying suffers from the distinctive properties of MANETs including node mobility and limited communication range and resource. An intuitive method to alleviate this problem is to create file replicas in the network. However, despite the efforts on file replication, no research has focused on the global optimal replica sharing with minimum average querying delay communication links between mobile nodes are transient and network maintenance overhead is a major performance bottleneck for data transmission. Low node density makes it difficult to establish end-to-end connection, thus impeding a continuous end-to-end path between a source and a destination.

DTN networks for communication in outer space, but is now directly accessible from our pockets both the characteristics of MANETs and the requirements of P2P file sharing an application layer overlay network. We port a DTN type solution into an infrastructure-less environment like MANETs and leverage peer mobility to reach data in other disconnected networks. This is done by implementing an asynchronous communication model, store-delegate-and-forward, like DTNs, where a peer can delegate unaccomplished file download or query tasks to special peers. To improve data transmission performance while reducing communication overhead, we select these special peers by the expectation of encountering them again in future and assign them different download starting point on the file.


  • Limited communication range and resource have rendered many difficulties in realizing such a P2P file sharing system. For example, file searching turns out to be difficult since nodes in MANETs move around freely and can exchange information only when they are within the communication range.
  • The disadvantage is that it lacked of transparency. Receiving a URL explicitly points to certain data replica and that the browser will become aware of the switching between the different machines.
  • And for scalability, the necessity of making contact with is always the same, the single service machine can make it bottleneck as the number of clients increase which makes situation worse.


We propose a distributed file replication protocol that can approximately realize the optimal file replication rule with the two mobility models in a distributed manner in the OFRR in the two mobility models (i.e., Equations (22) and (28)) have the same form, we present the protocol in this section without indicating the specific mobility model. We first introduce the challenges to realize the OFRR and our solutions. We then propose a replication protocol to realize OFRR and analyze the effect of the protocol.

We propose the priority competition and split file replication protocol (PCS). We first introduce how a node retrieves the parameters needed in PCS and then present the detail of PCS. we briefly prove the effectiveness of PCS. We refer to the process in which a node tries to copy a file to its neighbors as one round of replica distribution. Recall that when a replica is created for a file with P, the two copies will replicate files with priority P =2 in the next round. This means that the creation of replicas will not increase the overall P of the file. Also, after each round, the priority value of each file or replica is updated based on the received requests for the file.

Then, though some replicas may be deleted in the competition, the total amount of requests for the file remains stable, making the sum of the Ps of all replicas and the original file roughly equal to the overall priority value of the file. Then, we can regard the replicas of a file as an entity that competes for available resource in the system with accumulated priority P in each round. Therefore, in each round of replica distribution, based on our design of PCS, the overall probability of creating a replica for an original file


The community-based mobility model has been used in content dissemination or routing algorithms for disconnected MANETs/DTNs to depict node mobility. In this model, the entire test area is split into different sub-areas, denoted as caves. Each cave holds one community.

RWP model, we can assume that the inter-meeting time among nodes follows exponential distribution. Then, the probability of meeting a node is independent with the previous encountered node. Therefore, we define the meeting ability of a node as the average number of nodes it meets in a unit time and use it to investigate the optimal file replication.

PCS, we used two routing protocols in the experiments. We first used the Static Wait protocol in the GENI experiment, in which each query stays on the source node waiting for the destination. We then used a probabilistic routing protocol (PROPHET) in which a node routes requests to the neighbor with the highest meeting ability.



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