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Delay Tolerant Network - Journal
Delay Tolerant Network - Journal
Delay Tolerant Network - Journal
Delay Tolerant Network - Journal
Delay Tolerant Network - Journal
Delay Tolerant Network - Journal
Delay Tolerant Network - Journal
Delay Tolerant Network - Journal
Delay Tolerant Network - Journal
Delay Tolerant Network - Journal
Delay Tolerant Network - Journal
Delay Tolerant Network - Journal
Delay Tolerant Network - Journal
Delay Tolerant Network - Journal
Delay Tolerant Network - Journal
Delay Tolerant Network - Journal
Delay Tolerant Network - Journal
Delay Tolerant Network - Journal
Delay Tolerant Network - Journal
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Delay Tolerant Network - Journal

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These days, the interests in challenged networks are increasing and many researches are performed to seek a reliable end-to-end connectivity under harsh environments, which have a long propagation …

These days, the interests in challenged networks are increasing and many researches are performed to seek a reliable end-to-end connectivity under harsh environments, which have a long propagation delay, high error rates, low data rate, and intermittent connectivity. Delay Tolerant Network was introduced to provide challenged networks with reliable transmission and interoperability with an overlay network concept. In this paper, we present comprehensive overview of Delay Tolerant Network and introduce a study case about the implementation of this network. This paper is designed to encourage the exploration of this field by giving basic concept and also motivate to investigate this area by presenting a study case at the end section.

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  • 1. 1 Delay Tolerant Network Laili Aidi Jung Changsu School of Information and Communication Technology KTH, Stockholm, Sweden {aidi, changsu}@kth.se the network. Abstract—The interest in challenged networks is increasingand many researchers seek reliable end-to-end connectivityunder harsh environments, specifically long propagation delay, II. HISTORYhigh error rates, low data rate, and intermittent connectivity. Theconcept of a Delay Tolerant Network was introduced to provide DTNs were originally conceived to support thechallenged networks based upon reliable transmission and Inter-planetary Internet (IPN) [3]. There wasinteroperability with an overlay network. In this paper, we growing demand for a new network architecture topresent a comprehensive overview of Delay Tolerant Networkand introduce a study case about the implementation of such a support communications in the context of longnetwork. This paper is designed to encourage the exploration of propagation delay, low data rates, and intermittentthis field by presenting the basic concept and a study. connectivity. The Interplanetary Internet initiativeKeywords: Delay Tolerant Networks, Routing, Security, Challenge tried to find a solution and suggested a new networkNetwork architecture to support reliable transmission between a station on the Earth and satellites, with an I. INTRODUCTION overlay network [5]. The Internet Research Task Force (IRTF) DTNM ANY evolving wireless networks such as military, space, and underwater networks have Research Group and the DARPA (Defense Advanced Research Project Agency) Disruptioncharacteristics different from the Internet. For Tolerant Networking program advanced thisexample, the instability of the links, long concept. The IRTF DTN Research Grouppropagation and queuing delays, extremely generalized the concept of an Interplanetary Internetasymmetric data rate, and high link error rates. For into challenged networks. DARPA was interested inthese reasons, new communication protocols are the development of protocols for transmittingrequired to reduce the number of retransmissions bundles to DTN nodes [18].while providing reliable transmissions despite thehigh error rates and long delays [3]. III. DRIVING FORCE A Delay/Disruptive Tolerant Network (DTN) isdesigned to provide interoperable communications A. Challenged Networkbetween a wide range of networks with different Some of the characteristics of a challengedperformance standards, environments, and network are mentioned below [2, 7]:reliability in case of failure of hardware (network) 1. Path and Linkand or software (protocol) [2, 3]. A DTN must • High Error Rate and Asymmetric Dataaccommodate long delays between and/or within Rates. The transmission rates may be low; the latencies may be large, connectivity may be intermittent, high mobility in combination with Manuscript was received on March 12, 2011, and then improved on May15, 2011 based on comment of Prof. G. Q. Maguire Jr. weak signal strength and aggravating This work was done while Authors were master student of Communication circumstances result in high link-error rateSystems, at KTH Royal Institute Of Technology. Laili Aidi contributed forpart II History, part III Driving Force, part IV Protocol and Overlay making end-to-end reliable communicationArchitecture, part VII Routing, part IX Technology and part X difficult. Furthermore, due to the intermittentImplementation. Jung Changsu contributed for part I Introduction, part VNetwork Architecture, part VI Bundle and Encapsulation, part VIII Security, connectivity, data rates may be low or highlypart XI Study Case: KioskNet System, and part XII Conclusion. The work was submitted to Prof. G. Q. Maguire Jr. as assignment of asymmetric; the return channel may beIK2555 - Wireless and Mobile Network Architectures class. The authors are unavailable, and the elapsing time between asolely responsible for the contents of this work.
  • 2. 2 request and response may be hours rather than long usage, due to environmental dangers or milliseconds, and communications that do power exhaustion. It is not feasible to utilize arrive may exhibit a high error rate [19]. conventional end-to-end acknowledgment • Disconnection. The lack of end-to-end schemes to verify delivery, because the connectivity may be more common than network may be disconnected for a long period, connectivity, due to high host or network in fact the round-trip or one-way delivery time mobility (satellite passes, moving vehicle, etc). may exceed the sending node’s lifetime. Motion of other objects or interference, Therefore it is better to delegate the delivery of opportunistic (nodes arrive in communication traffic to another to any other party. range due to random walk), dynamic • Low Duty Cycle Operation. The transmission membership of individual nodes, low-duty- schedule should receive a special consideration cycle, and unpredictable operation are common in the routing decision, as the duty cycles of the attributes (for example in low-capability node may be low, in order to achieve devices such as might be used in sensor reasonable longevity of the entire network. networks). Thus, the communication pattern is often • Long and Variable Delay. The queuing time scheduled in advance due to the limited power. in this network could be extremely large, hard • Limited Resources. There is frequently to estimate, and source-initiated transmission limitation in memory and processing capability might be expensive because it is limited [3]. in the node, hence, if the network is designed Thus, there is a need to store data in a buffer or for reliability, then the end-node should empty queue for a potentially long period at each their retransmission buffers quickly, rather than router, if there is no direct path to the wait for an end-to-end acknowledgment. destination node [19].2. Network • Interoperability Consideration / Intermittent Connectivity. Interoperability on a large scale is rarely designed in challenged networks [3]. This is because these networks tend to be simple and local in scope. Partitions may occur because of geographic distance, lack of radio signal strength or other factors [19]. Because they are deployed on limited memory and power devices, thus cross communication all over that links has not become a primary feature. There is the problem of frequent failure, low reliability, and or congestion. Thus, there is no guarantee of discontinuous end-to- Figure 1. A Challenged Network’s Characteristics end connectivity. [3] • Security. The approach to only secure the endpoints of the network is not sufficient due to B. Interoperability the link capacity limit. Thus, the access to the service should be protected at the earliest point TCPs handshake and slow start mechanism are in the topology. sources of further obstacles in a challenged network3. End System with a long-delay. Although there are other • Limited Longevity. The end node, which improved protocols (SCTP, HSTCP, etc), that can frequently a highly integrated, low-power multiplex units of application data for multiple consumption, low-cost device, may not last for sessions over a single-layer connection (association), multiple round trips are still required
  • 3. 3in order to transmit application data for session case of session oriented protocols), but a store-and-setup [12]. There are several approaches that could forward DTN isolates the delays and can hide thisadapt the Internet to challenged networks unusual intermittent connectivity at the cost of storage,characteristics [2]: potentially duplicated messages, and increased1. Using Link-repair approach. This approach delay. enables Internet protocols to operate over a This type of DTN is based on in-network comparatively well-performing physical storage, retransmission, name based late binding, infrastructure, by maintaining the end-to-end and routing that is tolerant of network partitioning reliability and fate-sharing model of the Internet. [2]. The delivery semantics mechanism,2. Attaching network-specific proxy agent at the asynchronous messaging and postal mail as Class of edge of the Internet. This approach provides Service (CoS) are also widely used in the current access from the Internet to the challenged network applications such as voicemail and email network and vice versa. [14]. Furthermore, the links in such a DTN are expected to be diverse, including Radio Frequency Moreover, in order to achieve interoperability (RF), Ultra-Wide Band (UWB), and Free-Spacebetween large diverse networks, with extreme optical, and/or Acoustic (Sonar or Ultrasonic)environments and network partitioning, these technology [3, 7].approaches above will not be adequate or desirable[2]. Attaching a proxy agent does not provide a IV. PROTOCOL AND OVERLAY ARCHITECTUREsolution when using a challenged network for data A. Bundle Layertransit purposes, such as to access remotely This bundle layer architecture was proposed bydeployed conventional networks (Intranet) via Kevin Fall, before it was developed into RFC 4838challenged networks (as an intermediate network). by the DTN Research Group [3, 14]. As can be seen Thus, there is a need to define a new standard in figure 2, in this DTN architecture, there is athat provides end-to-end communication through bundle layer that ties the Application and Transportmultiple regions in a disconnected network, which Layer and all other low-layers into region specificcan tolerate errors and large variable-delay layers. Each regional layer may be a different typeenvironment using a generalized suite of of network, but the bundle layer enables them toprotocols. Another approach has been suggested, communicate regardless of the network types, bybased on a message-oriented reliable overlay transmitting bundles using store-and-forwardarchitecture, forming an internetwork of challenged message switching mechanism across or on top ofInternets [2]. The design is based on the the various regional layers [1, 3, 16].interoperability properties of the classical Internetdesign, the robust non-interactive deliverysemantics of electronic mail, and a subset of theclasses of service of a postal system. C. Store-and-Forward message switching A store-and-forward based DTN was originallydesigned to support an Interplanetary Internet(IPN). It is designed to operate above the existingprotocols in various network architectures and touse store-and-forward message switching, where thedata is transferred from a node, held until this nodehas a scheduled transfer, and then forwarded it toanother potentially dissimilar network [2].Intermittent connectivity in the Internet can lead topacket loss and even termination of a session (in the Figure 2. The Bundle Layer [3]
  • 4. 4 B. Licklider Transmission Protocol This approach enables to communication across Licklider Transmission Protocol (LTP) is amultiple regions without any given effect on the retransmission-based reliability protocol that runsupper-layer application. This Bundle Service over a link with extremely long message round-tripLayering can provide general-purpose delay- times and/or frequent interruptions in connectivitytolerant protocol services: custody transfer, [12]. This protocol is designed as a reliablesegmentation and reassembly, end-to-end reliability, "convergence layer" protocol, underlying the DTNend-to-end security, and end-to-end routing. Below Bundle protocol [22]. LTP is point-to-pointare the different CoSs, which are provided by DTN oriented, while the Bundle protocol moves bundlesBundle Layer [3]: end to end.1. Custody Transfer is the acknowledgement of a custodial-acceptance from a node to its previous V. NETWORK ARCHITECTURE custodian. This is necessary to implement of A. Region retransmission responsibility delegation, so the As the objective is to interconnect different sender node can transfer its retransmission networks asynchronously, a DTN can utilize the responsibilities to another node, this freeing regional networks, where each network is seen as a resources to be used for another bundle. region with its specific communication protocol [3,2. Return Receipt is the confirmation to the source 7]. Region is used to interconnect boundaries or reply-to node that the destination node has between nodes in different network protocols, received the bundle. addressing standard. It is identified by a region ID,3. Custody-Transfer Notification is the which is knowable by the other regions of that DTN notification to the source or reply-to node [3]. whenever a node accepts a bundle’s custody transfer.4. Bundle-Forwarding Notification is the B. Node notification to the source or reply-to node Each node in a DTN might be a host, router, or whenever a bundle is forwarded to the next node. gateway. These entities act as source, destination, or5. Priority of Delivery with a value of: Bulk, forwarder [3]. Normal, or Expedited. 1. Host. A host sends or receives bundles (i.e., it is6. Authentication is a procedure to verify the the source and or destination of bundle transfer), identity of the sender and message’s integrity. and requires storage to queue bundles. It needs optional custody transfer capacity for The figure below illustrates some of CoSs retransmission [3].described above: 2. Router. A router forwards each bundle to another node in the same DTN region, and may optionally support custody transfer. A router requires storage to store incoming packets before forwarding these to outgoing links because [2]: • There is no guarantee of that next hop link is currently available. • Asymmetric data rate between sender and receiver • Retransmission due to the high error rate link 3. Gateway. A gateway is an interconnection point that forwards bundles to other DTN regions with different protocol stacks by supporting Figure 3. DTN CoS [3] interoperability. This gateway must have storage
  • 5. 5 for reliable delivery and perform mapping VII. ROUTING between different transport layers. It also A. Name and Address performs authentication and check arriving data DTN adopts name tuples, which consist of two before forwarding it [2, 3]. variable length strings as addresses for delivering VI. BUNDLES AND ENCAPSULATION messages to its destination. Name tuples have the form of: {Region Name, Entry Name}. The region A. Bundles name is globally unique and translated in DTN Bundles are messages, which have a bundle gateways to route messages to the specified region.header, control information, and a source- Using its hierarchical region structures can reduceapplication’s user data. Control information the size of DTN forwarding tables and supportspecifies the ways of handling, storing, and additional flexibility due to the variable lengthdisposing of user data. The Bundle layer inserts the strings [2].bundle header. This header is intended for The entity name is an identifier and can befragmentation of the message [3]. resolved within the specified region, thus it does not need to be globally unique [16]. When messages B. Fragmentation traverse heterogeneous regions, only its region The bundle in the Bundle layer may be divided name is used for identifying its destination region.into several bundle fragments (smaller routable The and then entity name is only translated withinunits so as increase improve the possibility of the destination region. This late binding has twodelivery and increase performance [22]. These advantages in DTN source nodes [5]:fragments will be reassembled at the final 1. The source node can generate and deliverdestination. The message fragmentation in DTN is messages without any knowledge of eachbased on dynamics mechanisms, which could be different regional identifier systems. As a[14, 20]: consequence, various regions can add new1. Proactive fragmentation. This approach is used naming and addressing systems without where the contact volume can be predicted to changing their regions. optimize that contact’s usage. The application 2. There is no delay for mapping the destination to data may be separated into smaller blocks and a globally unique address when generating the transmitted as independent bundles. Only the message. final destination is responsible for reassembling these bundles. B. Knowledge Oracles2. Reactive fragmentation. This approach is used There are 2 important terms when talking about when there is a disconnection while transmitting DTN routing [14]: fragmented bundles. When a node receives a 1. Contact means a period of time (interval) during partially fragmented bundle, this node marks the which network connectivity is strictly positive, bundle as a fragment and carries out normal and the delay and capacity can be considered to forwarding. The previous-hop node can be constant. There are several types of contacts: recognize the delivery of abnormal • Scheduled Contact. A scheduled contact may fragmentation via a convergence layer protocol exist between a base station somewhere on and it creates a reactive fragment to send the earth and a low earth orbiting relay satellite, as remaining bundles to eliminate waste of partially it can be predicted when the link between them fragmented bundles [14, 22]. will be available and for how long it will be available. • Opportunistic Contact. Opportunistic contact occurs when two entities are present the same
  • 6. 6 place, where there is neither scheduled nor distributed system, because the queuing oracle is predicted contact available. affected by both new arriving messages and the • Predicted Contact. A predicted contact is not routing algorithm choices [33]. scheduled, but predictions of its existence can 5. Traffic Demand Oracle can answer any be made by analyzing previous observations. questions regarding the present or future traffic2. Contact’s volume means the product of the demand. It provides the set of messages injected capacity and the duration of the contact (i.e., the into the system at any time [33]. volume of data that can be transferred during the contact). Moreover, Group Membership Oracles to encapsulate particular knowledge about the group There are also sets of abstract knowledge dynamic, as described below:oracles, which encapsulate particular knowledge 1. Local Membership Oracle can only answerabout the network [33, 34]. These oracles are questions regarding group membership of thedefined in order to understand the fundamental node itself [26].trade-off between performance and knowledge, 2. Delayed Membership Oracle can answer anywhich is required by different routing algorithms questions regarding membership of an endpoint[33]. If contacts and its contact’s volumes are at a specific time [26, 34].known ahead of time, intelligent routing and 3. Complete Membership Oracle can answer anyforwarding decisions can be made (optimally for question regarding membership of all nodes atsmall networks). Below are the set of abstract any time [26, 34].Contact Knowledge Oracles:1. Null Contact Oracle, this happen when no Based on those oracles, we can classify the question about contact opportunity can be routing algorithms in DTN into several classes [33]: answered, it represents no knowledge about 1. Zero Knowledge is a class of algorithms that do network topology [26]. not utilize any oracle, thus they may perform2. Contacts Summary Oracle can provide long- poorly. term network topology or contacts aggregate 2. Complete Knowledge is a class of algorithms statistics, thus enabling calculation of the that utilizes all the oracles (contacts summary, average waiting time until the next contact. This complete contact, queuing and traffic demand). racle only has partial knowledge; hence it can 3. Partial Knowledge is a class of algorithms that respond with time-invariant or summary uses one or more of the other oracles characteristics about contacts (for example, the (congestion, queuing). The message is routed expected average time between contact independently based on the future traffic occurrences and average contact duration) [26, demand. 33, 34].3. Complete Contacts Oracle can answer any C. Strategy questions regarding network topology or contacts The traditional routing objective is to select a between two nodes at any time. Thus it can path that minimizes some simple metric (e.g. the specify the exact time when a contact will occur, number of hops). However, the most desirable the duration, capacity and delay of the contact, objective of routing in DTN is not immediately etc. A contacts summary oracle can be obvious, although the natural objective is to constructed using the Contacts Oracle, but not maximize the probability of message delivery vice versa [26, 33, 34]. because of its challenges [33].4. Queuing Oracle can give information about There is a need to define a new routing protocol instantaneous buffer occupancies (queuing) at for the DTN architecture, because the assumptions any node at any time and route around congested (continuous connectivity, low delay, and low packet nodes. It is the most difficult oracle to realize loss), which are used in traditional routing protocols
  • 7. 7(distance vector or link state) are no longer valid in total traffic generated (both data and controlDTN network [16]. The DTN routing topology is a packets) in the networks;time-varying multi-graph where there is no 4. Average message delay, defines the average ofguarantee of the availability of the next hops link the end-to-end bundle delivery latencies for eachfor a certain or long time period [20]. In this algorithm.architecture, the data rate between sender andreceiver could be asymmetric, as one node might be The routing strategy in DTN itself is classifiedmuch faster than another and the characteristic of into 2 categories, which is implemented in differenthigh error rate link might also cause retransmission. DTN, where each of them has different DTN Nodes route the bundle along the path. As characteristic and numerous approaches for itsillustrated in figure 4 below, the DTN Gateway has routing protocol, as shown below [16]:the same double-stack as a DTN Router. However, 1. Deterministic Routing. This strategy is buildthe gateway utilize different lower-layer protocols based on the assumption that the next nodes andallowing them to bridge between regions that use the connection between them are known. Thus,different lower-layer protocols and taking inter- protocols that use this strategy are implementedregion routing responsibility, while the DTN Router in deterministic or predictable topologies.supports intra-region routing [3]. DTN routers need 2. Stochastic Routing. This strategy is built basedstorage for queuing because of the fundamental on assumption that the network behavior islimitations discussed above. unpredictable. The protocols that use this strategy depend on decisions regarding where and when to forward messages and implemented in stochastic and time-evolving topologies. A simple approach could be just forwarding the message any node that is reachable, or based on history data, mobility patterns, etc. More specific Unicast routing techniques in DTN and comparisons can be found in [27] and [33]. Figure 4. DTN Protocol stacks and routing [3] Routing computation in DTN becomes D. Anycast and Multicastchallenging because the delivery paths through the The goal of multicast routing is to reach allgraph are lossy, and contact intervals and volumes nodes in the group, while Anycast routing is toare unknown precisely ahead of time [14]. These reach at least one node from a particular group.issues still become an active area in the (emerging) Thus, both of them need mechanism to guideresearch of DTN. The performance of different replication, forwarding, and buffer managementrouting algorithms can be compared using decisions [31]. Anycast and Multicast in a DTN areperformance metrics, as shown below [17]: challenging due to unpredictability of network1. Message delivery ratio, defines number of connectivity, long delivery delay, and limited unique multicast bundles that successfully arrive storage capacity characteristic [26, 30]. They also completely at all the receivers over the total have to deal with dynamic group membership, number of bundles which are expected to be because the group membership may change during received. the bundle delivery; introducing ambiguity in2. Data efficiency, defines ratio between the unique bundles received by the receivers and the Anycast and Multicast semantics. There are total data traffic. several semantic models in DTN for Anycast and3. Overall efficiency, defines ratio between the Multicast [26, 30, 34]: unique bundles received by the receivers and 1. Temporal Interval Membership (TIM) / Temporal Membership (TM) model:
  • 8. 8 membership is determined by the time interval network resources, because only a single copy of (membership interval), thus this model is a time- the message can exist in network storage [33]. based definition of group membership. The message can be held until the destination is2. Temporal Point Membership (TPM) / found or be forwarded through intermediate node Temporal Delivery (TD) model: the via a utility metric [31]. However, in general, membership is determined by both the this technique also limits the message delivery membership interval and delivery interval rates in many DTN. There are several that indicates the time period during the approaches based on this technique: message should be delivered to the intended • Expected Multi-Destination Delay for receivers. Anycast (EMDDA) that utilizes the3. Current Membership (CM) / Current-Model uncontrolled random movement of the node. Delivery (CMD) model: the message should be The Anycast routing is determined by delivered to the node that is a current member of evaluating different routing metrics (Practical the group when the message arrives to it. This is Expected Delay / PED) for selecting what makes this semantics different from the forwarding nodes [30]. This metric other two above, where receivers are not characterizes the expected delay of taking required to be group members at the time of different paths with corresponding probability message delivery. of connectivity between the nodes. However, this approach does not consider network traffic The figure below illustrates semantic models during its routing selection and assumes alldescribed above: nodes are stationary, except for a few mobile nodes that act as message carriers, thus providing a very constrained environment for evaluation. • The Anycast genetic algorithms to make route decisions [32]. This approach assumes that all mobility is deterministic and known ahead of time, which is not always true for DTN [31]. 2. Multi-copy techniques / Replication based. This technique increases the message delivery rates, since multiple copies of a message exist in Figure 5. DTN Semantic Models [26, 34] the network. However, it uses more network resources and inherently is not scalable. There The important point in order to achieve Anycast are several approaches based on this techniquein DTN is to expose knowledge to the routing [31]:protocol about the groups in the network so it can • Flooding-based protocols, appropriate in non-directly act based on that knowledge resource-constrained environments, thus[31]. Different from Unicast DTN, where the approach does not place a limit on the numberbundle destination is determined when it is of times a message can be replicated, andgenerated; in Anycast DTN, the destination can be focuses on smart buffer management andany one of a group of nodes and both the path to a transmission ordering techniques to handlegroup member and the destination can change potentially large numbers of replicates.dynamically during routing the process, depending Example protocols include as Epidemic,on the current device movement situation. There ProPHET, MaxProp, and RAPID [31].are several Anycast routing strategies that have • Quota-based protocols, suitable for resource-been proposed [31]: constrained environments, thus there are a hard1. Single-copy technique / Forwarding-based. limit on the number of times a message is This technique is generally much less wasteful of
  • 9. 9 allowed to be replicated. This is done by transmitted to the next hop. The destination node attaching a quota flag to every message, so the will de-capsulate the message and forwards the total number of replicas from a message never original multicast message to the intended exceeds its original quota. Example protocols receiver according to the messages delivery include Spray and Wait, Spray and Focus, and constraints. This approach has the worst delivery EBR [31]. ratio and routing efficiency because it sends a separate copy of the messages to each receiver, Multicast DTN has a semantic model where the which significantly increases contention for nodebundle needs to be buffered in the nodes storage storage and transmission opportunities, anduntil it is deleted because of buffer overhead or time results in message drops.expiration [26, 34]. This approach reduces delay 2. Broadcast-Based Routing (BBR) / epidemicand improves bundle availability. Thus, other nodes routing, here the message will be flooded(except the source) can handle the join request and a throughout all the nodes in the network in ordernode can send its buffered bundle to the new to reach the intended receivers [26, 34]. Thismembers when contact is available. This is approach achieves the highest delivery ratio thatnecessary to overcome network partitions and does not require any knowledge about contact orhandle delay of join requests. The node also membership [26]. It also has the lowest delaymaintains the local forwarding state for each of its because messages are flooded to all nodes, hencebuffer bundles to determine the route or next hop always following the shortest path [34].[34]. This Forwarding State is computed and However, it has the lowest routing efficiencyupdated based on the nodes Contact and Group because of message redundancy.membership knowledge. This forwarding state is 3. Tree-Based Routing (TBR), here the message isnecessary because the data is forwarded in the units forwarded along a tree that is rooted at the sourceof a bundle and intended receiver depends on the and reaches all receivers [26, 34]. The messagesemantic model that is used. is only duplicated at nodes that have more than The traditional Multicast methods for Internet one outgoing path.and mobile ad hoc networks are not suitable in 4. Group-Based Routing (GBR) uses theDTN, because of the frequent network partitions forwarding group concept, in order to increaseand sparse connectivity among nodes making the of the chance of delivery. Thus the message iscomplexity to maintaining a source-rooted multicast flooded within the forwarding group [26, 34].tree during a multicast session too high. Along with BBR, this approach also achieves theAdditionally, the application data would suffer from highest delivery ratio, because the message maylarge end-to end delivery latencies [17]. Moreover, be forwarded to receivers via multiple paths,the traditional approaches may fail to deliver a which better exploits available contactmessage when the link is highly unavailable. There opportunities (contact summary oracle).are several existing routing approaches forsupporting multicast communications in a DTN: The figure below illustrates routing approaches1. Unicast-based routing (UBR) / Unicast- described above: Multicast (U-Multicast), here the Unicast transfer mechanism is used to realize a multicast service, for example the source sends a copy of the message to every intended receiver, this message encapsulates the original multicast message [17, 26, 34]. The source node buffers the multicast message and sends new Unicast Figure 6. DTN Routing Approach: (a) UBR (b) messages when it learns of new intended BBR (c) TBR (d) GBR [26, 34] receivers, then removes this message after it is
  • 10. 105. Static tree-based routing (STBR) constructs the static shortest path tree from the source to the estimated intended receivers of a message, starting at the message generation time [26, 34]. Thus, if a message misses a contact with a node then it needs to wait for the next opportunity to connect to this node, which may significantly increase the message delay and disallows nodes from utilizing more accurate local information to the forward message using a better path. Figure 7. DTBR and OS-Multicast [17, 24]6. Dynamic Tree-Based Multicasting Algorithm enables the node to dynamically determine the 7. Context Aware Multicast Routing (CAMR) next-hops of a message based on its view of [17] is a density based adaptive multicast routing network conditions (local queuing information or scheme, which uses opportunistic connectivity newly available contact information) [34]. Since and additional information, for example, node this algorithm can adapt to network conditions, it location and node velocity. This scheme performs slightly better than STBR. increases average message delivery delay. • DTBR (Dynamic Tree-Based Routing), here However, it provides efficiency and high the upstream node assigns the receiver list for delivery ratio with reasonable data efficiency, its downstream nodes based on its network especially when the network becomes sparser. condition view [26]. The downstream nodes can only forward bundles to the receivers in the list. However, this technique assumes each node has complete knowledge or summary of the link states in the network. Unfortunately, this is hard to achieve in practical scenarios. • On-demand Situation-aware multicast (OS- multicast), here a unique multicast tree is constructed for each bundle and the tree is adjusted at each intermediate DTN node Figure 8. Conceptual performance of DTN muticast according to the current network conditions routing approaches in different levels of knowledge [17]. The node dynamically adjusts the initially [17] constructed tree when it receives a bundle, based on its view of network condition views. Thus it has a smaller delay, better message VIII. SECURITY delivery ratios, any newly discovered path will be quickly utilized, and achieves higher A. Issues efficiency when the probability of link Typically a DTN has very limited resources such unavailability is high and the duration of link as transmission bandwidth, storage, and processing downtime is large. However, the downside of cycles. Therefore, some restrictions should be this approach is that, the receiver may receive placed on accessing this network and delivering multiple copies of the same bundle and relies messages without authorization and authentication. on opportunistic connectivity among nodes for Moreover, even authorized applications should be delivery. restricted when they attempt to access services that they are not allowed to use. In DTN, there are two different security aspects that should be considered, these are LTP security and Bundle protocol security [16].
  • 11. 111. LTP Security. LTP is a point-to-point protocol bundles and create bundle-specific signature. After thus the upper or lower layer will handle most verifying the sender’s identity and CoS in the security concerns. For this reason, LTP only forwarding node, the forwarding node replaces the supports data integrity with LTP authentication sender’s signature with its own and forwards the and LTP cookie mechanism, which uses a data [3]. random number to make a DoS attack more difficult [16].2. Bundle Layer Security. The Bundle Layer is vulnerable to attack because it is an overlay network. Thus the bundle layer encounters many threats and should consider lower layer’s security issues [23]. Figure 9. Security steps [3].The bundle layer protocols can be attacked bymultiple underlying network components because C. Bundle Security Protocol Specificationbundles may travel across multiple networks. In the “Bundle Security Protocol Specification”During this journey, bundles are modified [29], the functionality of data integrity andintentionally and new bundles are inserted by confidentiality are provided as the bundle securityunderlying networks. To prevent unauthorized protocol. There are four security blocks in thisinsertion of bundles, DTN nodes should have the specification [29]:ability to detect and delete these unexpected 1. Bundle Authentication Block (BAB), supportsbundles [23]. Network accesses and resource a function for assurance of the authenticity andconsumptions from unauthorized objects can cause integrity of the bundle travelling along a singleserious threats because of the nodes limited hop from forwarder to intermediate receiver. Theresources, particularly storage and battery power operation of this block is to protect a bundle on a[23]. The bundle layer can be exploited by hop-by-hop basis unlike other security blocks.increasing resource consumption, for example by a Currently BAB only defines a shared-key HashDoS attack. Due to the long latency in DTN, the Message Authentication Code (HMAC) and thedamage may be worse than traditional networks. To key does not need to be unique but is onlyavoid unexpected resource consumption, we should required to be shared between nodes.only accept authenticated messages and drop others 2. Payload Integrity Block (PIB), the authenticity[16, 23]. and integrity of the payload from the PIB security-source to the PIB security-destination. Any node, which is located in between the B. Bundle Security security-source and the security-destination, can Authentication is carried out in forwarding nodes examine the authentication information.(routers and gateways) and these nodes also verify 3. Payload Confidentiality Block (PCB) specifiesthe authenticity of sender’s information to protect that the encryption of the payload be donenetwork resources from unauthorized traffic as early wholly or partially by the PCB security-sourceas possible. This also differentiates DTN networks for the purpose of protecting the content whilefrom other networks, which mutually authenticate being transported to the PCB security-the user identities and message integrity, but the destination.router that forwards the traffic itself is not 4. Extension Security Blcok (ESB) providesauthenticated [3]. security not for payload blocks, but rather for The DTN also has a unique mechanism when non-payload blocks in a bundle so ESB is notusing public-key cryptography, where both users applicable to PIB and PCB that are related toand forwarders have their own key-pairs and payload security blocks. The ESB is located incertificates. A sender uses its private key to sign
  • 12. 12 the same position where it should be protected in implementation is intended to reduce complex the bundle [29]. operations by the user, enabling them to perform data communication operations seamlessly and D. Open Issues in Bundle Security more effectively in terms of delay, intermittent environments, etc. This is not possible using the The bundle security protocol is still under TCP/IP based architectures [7].development and some critical issues still remain. Currently, the DTN architecture research isWe will present some issues for the bundle security carried on by several research groups, such as [3,protocol below [23]: 15]: The Internet Research Task Force’s Delay-1. The level of flexibility. The bundle security Tolerant Networking Research Group (DTNRG), protocol can combine applications of the The InterPlaNetary (IPN) Internet Project, NASA confidentiality and integrity services flexibly but Jet Propulsion Laboratory (International Space it prevents insecure combinations of application Station Research), Google Laboratory, Intel such as including plain-text signatures. Research Corporation, SPARTA, The MITRE Therefore, the level of flexibility is an open issue Corporation, Distributed Systems Group - Trinity in the bundle security protocol. This flexibility College Dublin, ISTRAC - ISRO, University of may support a Virtual Private Network (VPN), California - Berkeley (UCB), University of but the complexity can cause high costs for California - Santa Barbara (UCSB) , University of implementation and be insecure [23]. Southern California (USC), Helsinki University of2. Key Management. No key management Technology, Luleå University of Technology, schemes exist in DTN deployments. In fact, University of Massachusetts Amherst, etc. existing schemes need hard coding. Additionally, The wireless DTN technology also may be replacement of trusted keys in all nodes makes it diverse and implemented using several difficult to adopt this scheme. One possible technologies, such as [3]: solution is to adopt the resurrecting duckling 1. Radio Frequency (RF) scheme that is suggested for ad-hoc networks. If 2. Ultra-Wide Band (UWB) a node has a “close encounter” with another 3. Acoustic (Sonar, Ultrasonic) node, the node can exchange a key through a 4. Free-space Optical Communications (FSOC) is trustworthy channel and store it for later use. If an extreme example of the directional antenna DTN nodes have enough space in their storage, mobile ad hoc network (MANET) [25]. The these nodes may flood public keys whenever networking design issues in FSOC come from nodes encounter each other [23]. the challenges in pointing, acquisition, and3. Canonicalization of bundles. The definition of tracking. It becomes extremely difficult because a canonical bundle form is an open issue for data of long ranges and mobility on rugged terrain, integrity. The sender and receiver require the and there is a resultant resource allocation same bytes of bundle of signature for integrity, problems precision of pointing requirement, but the bundles may be changed during traversal need to be done with an optical laser head in one between nodes [23]. connection service. Thus, there is a need in IX. TECHNOLOGY FSOC for topology control. Since the DTN approaches are designed to overcome the A. Model and Feasibility network with intermittent connectivity, thus it The trend of DTN seems to be more of an can be fundamental to solve the FSOCanalytical research, mobility model, etc [11]. The networking problem.ongoing work is to extend the DTN architecture to B. Relevant Standardsmart mobile phone-based mobile ad hoc networks(MANETs) so that a node can effectively use 1. Homing-pigeon-based DTN (HoP-DTN), anmultiple communication links and networks. This experimental method in RFC 1149: Standard for the Transmission of IP Datagrams on Avian
  • 13. 13 Carriers [8] and RFC 2549: IP over Avian DTN is used to increases the robustness of the Carriers with Quality of Service [9] communication network and timeliness of data2. Licklider Transmission Protocol - Security returned from operating space assets, so it can Extensions, IETF RFC 5327, experimental [10] reduce risk and cost, increase safety and science3. Licklider Transmission Protocol - Specification, return, and improve operational awareness [15]. IETF RFC 5326, experimental [11] Additionally, DTN can reduce human labor costs4. Licklider Transmission Protocol - Motivation, through automation of communications operations. IETF RFC 5325, informational [12]5. Bundle Protocol Specification, IETF RFC 5050, experimental [13]6. Delay-Tolerant Network Architecture, IETF RFC 4838, informational [14] X. IMPLEMENTATION The development of DTN networks has beenmore sophisticated and detailed in terms ofprotocols and approaches, analysis of stateless Figure 10. The Challenged Network Examples [3]routing algorithms and has thus far found nocommercial use [11]. DTN is currently being 2. Terrestrial Civilian Network. Even though thestudied in space networks and other research Outer-Space implementation is the primaryenvironments (such as Earth Applications) in order beneficiary of the DTN research, many terrestrialto exploit stressed, disconnected, disrupted network implementations are used andnetworks, preventing failures, enhance safety and contributed to DTN research as well:security, and to learn new knowledge [15]: • Drive-by Vehicular and Ferry based1. The Outer Space / Deep-Space Networks: Networks [21] include DakNet, Message ferry Inter Planetary Networks. (hybrid between MANET and DTN), Village The Interplanetary Internet (IPN) is a computer network.network in space, and an example of wireless • Mule Networks / Node in a box: Hagle,network outside of the Internet [3]. The Zebranet (tracking wild animal in wildlife,communication outside of the Internet is done by manage their habitat effectively by attachingusing independent networks, that each might be run wireless sensor node, collecting location dataon power-limitation, specialized communication and opportunistically reporting their historythat is not be able to exchange information. They when they are in radio range of base station [2,also have different link delay and connectivity, 19, 22]), Sámi Network Connectivity (Reindeerdata-rate asymmetry, error rates, addressing and herd tracking by the Saami tribesmen in Arcticreliability mechanism, QoS and trust boundary, etc Circle [22]), SWIM, Mobile Ad Hoc Network,[3]. AUDTWMN (Water monitoring application The IPN is defined as a network of regions, such Test bed for DTN research [22]), Carrieras a terrestrial Internet region, a surface of the Pigeons (Implemented by Bergen Linux usersplanet region, or ground-to-orbit region, etc. Each group: RFC 1149, RFC 2549).region has its own communication in terms of • Challenge Link / Flakynet, for example in thesecurity, resource maintenance, etc [6]. It is a store- Remote / Developing region: Tier, Seismicand-forward network that runs over interplanetary monitoring (Early warning system against earthdistances, might be disconnected, and run over a quakes, volcano and landslides [22]), SenDTwireless backbone with error-prone links and delays (implemented by Trinity College Dublinranging to minutes or hours, when a connection Ireland to monitor lakes in Ireland [22]), UUCPeven exists [4]. [21].
  • 14. 143. Mobile Sensor Networks (Acoustic KioskNet challenged to make a robust system underwater networks). This network is with two key concepts. The first one is the adoption designed to exist in a large-scale area and nodes of a single-board-computer, low-cost and low- per network [5]. It has the characteristic of an power kiosk controller for wireless communication extremely limited end-node power, memory, using a vehicle. A vehicle delivers data to a gateway CPU capability and scheduled communication or receives data from a gateway. This ‘mechanical between nodes. There is also an implementation backhaul’ can allow Internet access without the cost of “proxy” nodes in the network in order to of equipments such as satellite dishes and towers in translate the data in the Internet protocols to its remote areas. Second, KioskNet uses refurbished native protocols. PCs, which use boot images from the kiosk4. Wireless Military Tactical / Battlefield controller that can offer a very secure boot images networks. The network may run in environment virus-free. In addition, the refurbished PC does not with several causes for disconnection, such as have a hard disk to avoid a hard disk failure and mobility, environmental factors, intentional viruses. Aside from two key concepts, KioskNet has jamming, etc [5]. There is a strong requirement a few characteristics like low-cost for infrastructure protection and the sensor or (70$/kiosk/month), low power (6~8W), a LiveCD field agent may be deployed over limited and free software [24]. wireless coverage. The challenge also increases because the data traffic may be delayed as it may B. Component have to compete for a limited bandwidth with Below are the components of KioskNet network: other high priority services, and there is a 1. Kiosks. concern in the battery and wide physical Every kiosk has its own kiosk controller. A kiosk dispersant of agent. Thus, wireless connectivity controller utilizes recycled PCs to provide some could not be continuously maintained among all functions, those are, a network boot function, a agents [15, 19]. network file system, user management, and network connectivity through dial-up, GSM/GPRS, Very XI. STUDY CASE: KIOSKNET SYSTEM Small Aperture Terminal (VSAT), or mechanical backhaul. A kiosk controller is constantly possesses A. Overview a wireless network interface or other connectivity, The KioskNet system was developed by the which are mentioned above [24].University of Waterloo for providing low cost This system considered two types of users whoInternet services to the poorest villages of can access a kiosk controller for their connection.developing countries using the DTN concept. This First, most users are expected to use cheap recycledsystem uses vehicles to deliver data from villages to PCs (terminals) to connect the system. In this case,Internet gateways and provides various kinds of a kiosk controller provides these diskless PCs with aservices to rural residents, for example, birth, network boot image and applications by means ofmarriage, and death certificates; medical Network File System (NFS). Second, some otherconsultation, and agricultural problems. This system users who are government officials, NGO membersshould have some essential requirements for the or wealthier residents access kiosks with their ownreliable connections and the low cost of mobile devices. Unfortunately, this system’smaintenance. Especially, the cost is a very critical software does not support connectivity to theseissue for sustaining this system in those remote users because of some technical issues.regions. Moreover, deploying this system had great If kiosks are located in the same geographicalchallenges because of many obstacles such as area, these kiosks consist of a KioskNet region forlimited electrical power, dust, mechanical damages, routing and certification [24]. Figure 11 representscomputer viruses, frequent failure of kiosk that a single server entity in KioskNet administratescomputers and network connections [24]. two regions in this system.
  • 15. 152. Ferries. This system defines ferries as many types ofvehicles like a car, buses, motorcycles or trains bymeans of supporting various connectivity options. Aferry has a cheap computer powered by a vehicle’sown battery and the computer has a 20-40GB harddisk and a WiFi network interface. Ferries contactwith kiosk controllers and gateways usingopportunistic ways for the time duration of 20seconds to 5 minutes. During this communication Figure 11. KioskNet overview [36]period, ferries can transfer 10-150MB of bundlesusing a store-and-forward mechanism [24]. C. Security Architecture The ultimate security goal of this system is to3. Gateways. offer the best possible security service to the whole A gateway is always connected to the Internet components of KioskNet such as the infrastructures,through DSL or broadband Internet access. This users and terminals. To meet security requirementsconnection of a gateway is possible with its WiFi and reliable operation, this system needs fournetwork interface. Moreover, it has storage to distinct entities, which are KioskNet Franchisers,receive data from a ferry and to upload the data to KioskNet Franchisees, KioskNet Users andthe proxy through the Internet [24]. Application service providers [24, 28]. 1. Entities. We present Security Entities of this4. Proxy. system as below: This system is expected to support • KioskNet Franchisers. Franchisers are publiccommunication between a kiosk user and the or private organizations such as non-Internet but legacy servers cannot provide governmental organizations (NGOs). Theconnectivity when the network has long delays and franchisers own and check the integrity of theirdisconnections like this situation. Therefore, a KioskNet infrastructure components such asspecial proxy is needed for supporting gateways, ferries, Kiosk controllers andcommunication with a legacy server. A proxy proxies. The basic function of franchisers is toshould be divided by two halves so that one half set detect the improper usage of infrastructures byup a session for disconnection-tolerant connection any entities [24, 28].with applications within a kiosk controller or • KioskNet Franchisees. Franchisees are privatemobile devices. The other half establishes a organizations or licensed individuals. Theconnection with legacy servers instead of responsibilities of franchisees are to operateintermittently connected users. For data forwarding their kiosk terminals and protect terminals fromfrom a half to the other half within proxy, malicious software [28].application plug-ins are required, for example, a • KioskNet Users. Users can access KioskNetSMTP plug-in for sending mail content to a legacy services and applications that franchisees ownmail server [24]. and support [28]. • Application Service Providers (ASPs). These5. Legacy Server. entities are licensed by franchisers for The legacy servers are typical servers supporting providing their applications to a KioskNet as anapplications such as IMAP, SMTP and HTTP with example of banking services to local residentsTCP/IP protocol [24]. [28]. 2. Certificate. All entities have unique credentials containing a 2048-bit RSA key and a Public Key
  • 16. 16 Certificate. The public key of a franchiser is use public key encryption for ensuing end-to-end certified by a secure root CA server at the secure communication like SSL but this University of Waterloo using its own private key approach is very difficult to be applied to this and this signature is stored as the form of an delay-tolerant environment due to handshake for X.509 certificate. In turn, franchisers issue the generating a shared key. Therefore, the KioskNet certificates of franchisees and ASPs. When users generates random 256 bit keys using AES-CBC register and create their identification at their (Advanced Encryption Standard Cipher kiosks, franchisees certify the users in an Algorithm in Cipher Block Chaining automatic way. In addition, franchisers also issue Mode). The recipients encrypt this key using its unique credentials to KioskNet’s gateways and public key and decrypt the data after decrypting ferries. A public key database, preserved at the the AES key by using their own private keys proxy is used to periodically broadcast public [24]. key certificates for users, franchisees and ASPs all over a franchiser’s region and these XII. CONCLUSION certificates are also copied to all kiosk The main goal of DTN architecture is to provide controllers [24]. interoperability between different kinds of networks3. Infrastructure integrity. Digital signatures are in wide-ranging regions even though this network used on all remote commands and software has many limitations such as long delay, updates from franchiser for ensuring the security intermittent connectivity, limited power, and high of this infrastructure. In this system, kiosk error rates. This architecture originated from the controllers are very vulnerable to attacks so Interplanetary Internet architecture then it is spread franchisers do not give root access authorities to to the challenged networks more generally. kiosk controllers to prevent them from In this paper, we have summarized an overview modifying the software and accessing private of Delay Tolerant Networking, especially about information [24]. routing and security. In addition, we give an4. Protecting recycled PCs. For protecting recycled example of the design and implementation of the PCs from viruses and malicious software, these KioskNet for better understanding of DTN. PCs can only boot using read-only disk images Moreover, we think that the implementation of stored in kiosk controllers through NFS protocol. DTN is a good solution to supply people in remote This boot images can be modified and updated areas with the Internet service for their by franchiser administrative staffs [24]. communication to the world even though DTN still5. User data protection. User data is only stored in has many open issues. kiosk controllers and this system offers encrypted virtual space for each user’s home REFERENCES directory. This file system is encrypted with the [1] Jimmy Ray. “Delay Tolerant Networking: How to use Twitter and Hulu on long Space Missions to Mars”. [Online]. user’s password and mounted when users login Available: http://www.networkworld.com/community/node/43557 at a terminal with their password. Moreover, [2] [Feb. 16, 2011] Kevin Fall. “A Delay-Tolerant Network Architecture for Challenged users can access their encrypted home directories Internets”. [Online]. Available : http://www.dtnrg.org/docs/papers/IRB- using the Linux DM-Crypt disk encryption TR-03-003.pdf [Feb. 16, 2011] [3] Forrest Warthman. “Delay Tolerant Networks (DTNs): A Tutorial module. With this mechanism, attackers who Version 1.1”. [Online]. Available: have a root authority cannot modify other users’ http://www.dtnrg.org/docs/tutorials/warthman-1.1.pdf [Feb. 16, 2011] [4] Scott Burleigh, Vinton Cerf, Robert Durst, Kevin Fall, Adrian Hooke, data [24]. Keith Scott, Howard Weiss. (2003, Oct). “The Interplanetary Internet: A6. Communication privacy and integrity. Before Communications Infrastructure for Mars Exploration” Journal title. [Online]. IAC-02-Q.3.3.03. 2. Available: delivering user data to the kiosk controller, the http://www.ipnsig.org/reports/IAF-Oct-2002.pdf [Feb. 16, 2011] encryption and signing of user data is performed [5] Burleigh, S., Hooke, A., Torgerson, L., Fall, K., Cerf, V., Durst, B., Scott, K., and Weiss, H. Delay-tolerant networking: an approach to at kiosk terminal for supporting privacy and interplanetary internet. IEEE Communications Magazine 41, 6 (June authenticity. In traditional systems’ case, they 2003), 128–136.
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  • 18. 18Peer review from Guojun Wang (guojun@kth.se):The authors have given an excellent description to the delay tolerant network, including the protocols, theoverall architecture, the technologies, implementations and so on. The levels between each part are veryclear and all the formats are qualified. However, it will be better to give an overview of what you havepresented at the beginning of this paper instead of putting that in the conclusion part. You also give onesentence to the origin of DTN, which is followed with tag [7] in the conclusion paragraph. Actually, itreduces the role of conclusion. In addition, you give a new opinion at the end, "Moreover, we think that theimplementation of DTN is a good solution to supply people in remote areas... ". New idea should beavoided when you are giving an end to this paper.Author’s Comment:We thanks for deep review from reviewer. The overview of this work appears on the Abstract section,where it was written, "we summarize the overview of Delay Tolerant Network and introduce a case studyabout the implementation of Delay Tolerant Network." We used "summarize" word instead of "explain",because we know that there are still so much detail of the concept that we cannot cover in this paper, due tothe page and word limit. Later we paraphrase again the sentence so it can be more clearly by become, “Inthis paper, we present comprehensive overview of Delay Tolerant Network and introduce a study caseabout the implementation of this network”. At the conclusion, we think that, it is important to sum up anddeliver the global idea that is appear in the entire paper in simple sentence, which is "the implementation ofDTN is a good solution to supply people in remote areas with the Internet service for their communicationto the world even though DTN still has many open issues." Thus we think, this last sentence is important inorder to conclude the core idea of the technology that has been talked about. However, after carefully thinkabout the point that reviewer mentioned in Conclusion, we decided to remove the citation in the reference,because it will lead reader to think that we still present new idea in this conclusion, however that point isalready presented at the main section of the paper including its citation.Peer review from Merabi Kechkhoshvili (merabi@kth.se)First I will make my point about overall feeling from this report. It is obvious that authors had wide rangeof references and they did a deep research in order to present this document. It is written in reallyprofessional manner and is appealing to person who is familiar with particular topic. I would also add thatreport is done in appropriate style: • Well-divided paragraph structure. • Good knowledge of language and broad vocabulary. • Although the topic is huge, it is written without redundant information.However there are some minor details that are worth to pay more attention: • Firstly, I want to say about introduction part: it is not describing how the document is organized. I would like you to put more information about chapters and what are you going to describe in following sections. • Secondly, while reading the document, several figures are too difficult to understand. I think you need to put more information and some explanation about figures. • Finally, from my perspective this paper will be difficult to read be people, who are not really familiar with the topic.But once again I must stress exceptional quality of presented report.Author’s Comment:We thanks for deep review from reviewer. After reading the points of your review carefully, we consider tomake some changes in the Abstract part to make the document organization more clearly, by puttingsentences about what the reader can expect from this work. We agree that the organization of the documentnor should be mentioned in the Introduction part, because that part should be as introductory for the topic
  • 19. 19itself, not to the document organization or chapter explanation. Moreover, the figures that is presented inthis paper have been explained by paragraph above them, and due to limited page and word, we decide notto explain more about that. We’ve also realized this work digs little bit deeper and detail of DTN concept,thus it will not just give basic general overview of this technology.

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