448 Chapter 20 Delay Tolerant Networks: Architecture,Routing,Congestion, and Security Issues Vandana Kushwaha Banaras Hindu University,India Ratneshwer Gupta Jawaharlal Nehru University,India ABSTRACT Opportunistic networks are one of the emerging evolutions of the network system.In opportunistic net- works,nodes are able to communicate with each other even if the route between source to destination does not already exist.Opportunistic networks have to be delay tolerant in nature (i.e.,able to tolerate larger delays).Delay tolerant network (DTNs)uses the concept of"store-carry-forward"ofdata packets. DTNs are able to transfer data or establish communication in remote area or crisis environment where there is no network established.DTNs have many applications like to provide low-cost internet provi- sion in remote areas,in vehicular networks,noise monitoring,extreme terrestrial environments,etc.It is therefore very promising to identify aspects for integration and inculcation of opportunistic network methodologies and technologies into delay tolerant networking.In this chapter,the authors emphasize delay tolerant networks by considering its architectural,routing,congestion,and security issues. INTRODUCTION Delay Tolerant Networking (DTNs)is a new way of communication that facilitates the data transfer between source and destination even if a fully connected path may not exist between two end nodes.The Delay Tolerant Network(DTN)(Cerf et al.,2007)is an emerging area that has attracted keen research efforts from both academia and industry.DTNs consider an extreme network condition that is different from the traditional communication networks.There may not exist a complete end-to-end path between the data source and destination,and thus network is subject to dynamic node connections and unstable topologies.The communication in DTN is done by exploiting the characteristic of nodes i.e.mobility, D0L:10.4018/978-1-5225-8407-0.ch020 Copyright2019,IGI Global.Copying or distributing in print or electronic forms without written permission of IGI Global is prohibited
448 Copyright © 2019, IGI Global. Copying or distributing in print or electronic forms without written permission of IGI Global is prohibited. Chapter 20 DOI: 10.4018/978-1-5225-8407-0.ch020 ABSTRACT Opportunistic networks are one of the emerging evolutions of the network system. In opportunistic networks, nodes are able to communicate with each other even if the route between source to destination does not already exist. Opportunistic networks have to be delay tolerant in nature (i.e., able to tolerate larger delays). Delay tolerant network (DTNs) uses the concept of “store-carry-forward” of data packets. DTNs are able to transfer data or establish communication in remote area or crisis environment where there is no network established. DTNs have many applications like to provide low-cost internet provision in remote areas, in vehicular networks, noise monitoring, extreme terrestrial environments, etc. It is therefore very promising to identify aspects for integration and inculcation of opportunistic network methodologies and technologies into delay tolerant networking. In this chapter, the authors emphasize delay tolerant networks by considering its architectural, routing, congestion, and security issues. INTRODUCTION Delay Tolerant Networking (DTNs) is a new way of communication that facilitates the data transfer between source and destination even if a fully connected path may not exist between two end nodes. The Delay Tolerant Network (DTN)(Cerf et al., 2007) is an emerging area that has attracted keen research efforts from both academia and industry. DTNs consider an extreme network condition that is different from the traditional communication networks. There may not exist a complete end-to-end path between the data source and destination, and thus network is subject to dynamic node connections and unstable topologies. The communication in DTN is done by exploiting the characteristic of nodes i.e. mobility, Delay Tolerant Networks: Architecture, Routing, Congestion, and Security Issues Vandana Kushwaha Banaras Hindu University, India Ratneshwer Gupta Jawaharlal Nehru University, India
Delay Tolerant Networks available connections,and provided buffer space etc.DTNs find broad applications in the situations where legacy networks cannot work effectively,such as data communications in rural areas where stable communications infrastructure is not available or is costly.DTN is useful for extreme environments like battlefields,volcanic regions,deep oceans,deep space,developing regions etc.,where they suffer challenging conditions as military wars and conflicts,terrorist attacks,earthquakes,volcanic eruptions, floods,storms,hurricanes,severe electromagnetic interferences,congested usage,etc.These challenging conditions result in excessive delays,severe bandwidth restrictions,remarkable node mobility,frequent power outages and recurring communication obstructions(Khabbaz et al.,2011).Vehicular networking is a wide and growing field of DTNs,where many applications are being explored(Benamar et al.,2014). One of these applications is to provide Internet access to vehicles by connecting to roadside wireless base stations(Ott and Kutscher,2004).Non-commercial applications include monitoring and tracking wildlife animals (Juang et al.,2002),and environmental monitoring,such as lake water quality monitoring and roadside noise monitoring.DTNs can be applied in a variety of other fields ranging from healthcare to education to economic efficiency (Abdelkader et al.,2016). The idea of Delay Tolerant Network (DTN)(Warthman,2012)was taken from Inter Planetary Net- works (IPN)(Burleigh et al.,2003),this was started in 1970s.The IPN was invented to communicate between earth and mars.The DTN is a type of wireless ad-hoc network which tolerates the intermittent connectivity.The intermittent connectivity can be defined as the sudden change of state (up/down)of any communication link between the nodes.The DTN can also be defined as intermittently connected wireless ad-hoc network ("Mobile Ad-Hoc and",n.d.)that can tolerate longer delays,intermittent connectivity and prevent data from being lost by using store-carry-forward approach.The Store-carry- forward approach enables the nodes to take the message,store it in the buffer provided at each node and forward the same whenever new node comes in its communication range.DTN technology has become a new research focus in many fields including deep space communications,military tactical communica- tions,and disaster rescue and internet access in remote areas.Internet Research Task Force (IRTF)has organized Delay-Tolerant Research Group(DTNRG)to research OTN technology,and as an important research theme,DTN technology has been accepted by the guidelines in MobiCom 2008 and Milcom 2009 Lu et al.,2010). With the advent of the Internet of Things(IoT)a number of new devices will become part of our day today life.Constrained Application Protocol (CoAP),and its extensions,are specially designed to address the integration of these constrained devices in IoT environment.However,due to their limited resources,they are often unable to be fully connected and instead form intermittently connected and sparse networks in which Delay Tolerant Networking(DTN)is more appropriate,in particular through the Bundle Protocol (BP). The chapter is organized as follows.In next section,the characteristics of DTNs,types of DTNs and applications of DTNs are mentioned in different sub-sections.The architectural structure of DTNs is described in further section.Then Routing and buffer management of DTNs are explained.Security aspects of DTNs are mentioned further.Some case studies of DTNs are given in last section. Characteristics of Delay Tolerant Networks A DTN have the following basic characteristics(Fall et al.,2008): 449
449 Delay Tolerant Networks available connections, and provided buffer space etc. DTNs find broad applications in the situations where legacy networks cannot work effectively, such as data communications in rural areas where stable communications infrastructure is not available or is costly. DTN is useful for extreme environments like battlefields, volcanic regions, deep oceans, deep space, developing regions etc., where they suffer challenging conditions as military wars and conflicts, terrorist attacks, earthquakes, volcanic eruptions, floods, storms, hurricanes, severe electromagnetic interferences, congested usage, etc. These challenging conditions result in excessive delays, severe bandwidth restrictions, remarkable node mobility, frequent power outages and recurring communication obstructions (Khabbaz et al., 2011). Vehicular networking is a wide and growing field of DTNs, where many applications are being explored (Benamar et al., 2014). One of these applications is to provide Internet access to vehicles by connecting to roadside wireless base stations (Ott and Kutscher, 2004). Non-commercial applications include monitoring and tracking wildlife animals (Juang et al., 2002), and environmental monitoring, such as lake water quality monitoring and roadside noise monitoring. DTNs can be applied in a variety of other fields ranging from healthcare to education to economic efficiency (Abdelkader et al., 2016). The idea of Delay Tolerant Network (DTN) (Warthman, 2012) was taken from Inter Planetary Networks (IPN) (Burleigh et al., 2003), this was started in 1970s. The IPN was invented to communicate between earth and mars. The DTN is a type of wireless ad-hoc network which tolerates the intermittent connectivity. The intermittent connectivity can be defined as the sudden change of state (up/down) of any communication link between the nodes. The DTN can also be defined as intermittently connected wireless ad-hoc network (“Mobile Ad-Hoc and”, n. d.) that can tolerate longer delays, intermittent connectivity and prevent data from being lost by using store-carry-forward approach. The Store-carryforward approach enables the nodes to take the message, store it in the buffer provided at each node and forward the same whenever new node comes in its communication range. DTN technology has become a new research focus in many fields including deep space communications, military tactical communications, and disaster rescue and internet access in remote areas. Internet Research Task Force (IRTF) has organized Delay-Tolerant Research Group (DTNRG) to research OTN technology, and as an important research theme, DTN technology has been accepted by the guidelines in MobiCom 2008 and Milcom 2009(Lu et al., 2010). With the advent of the Internet of Things (IoT) a number of new devices will become part of our day today life. Constrained Application Protocol (CoAP), and its extensions, are specially designed to address the integration of these constrained devices in IoT environment. However, due to their limited resources, they are often unable to be fully connected and instead form intermittently connected and sparse networks in which Delay Tolerant Networking (DTN) is more appropriate, in particular through the Bundle Protocol (BP). The chapter is organized as follows. In next section, the characteristics of DTNs, types of DTNs and applications of DTNs are mentioned in different sub-sections. The architectural structure of DTNs is described in further section. Then Routing and buffer management of DTNs are explained. Security aspects of DTNs are mentioned further. Some case studies of DTNs are given in last section. Characteristics of Delay Tolerant Networks A DTN have the following basic characteristics (Fall et al., 2008):
Delay Tolerant Networks Intermittent Connection As the node's mobility and energy are limited,DTN frequently disconnects,thus resulting in continue change in DTN topology.That is to say,the network keeps the status of intermittent connection and partial connection so that there is no guarantee to achieve end-to-end route. High Delay,Low Efficiency,and High Queue Delay End-to-end delay specifies the sum of the total delay of each hop on the specified route.The end-to-end delay involves queuing time,waiting time and transmission time(Cerf et al.,2007).Each hop delay might be very high due to the fact that DTN intermittent connection keeps unreachable in a very long time and thus further leading to a lower data rate and showing the asymmetric features in up-down link data rate.In addition,queuing delay plays a main role in end-to-end delay and frequent fragmentations in DTN make queuing delay increasing. Limited Resource Node's computing and processing ability,communication ability and storage space is weaker than the function of an ordinary computer due to the constraints of price,volume and power.In addition,the limited storage space resulted in higher packet loss rate. Limited Life Time of Node In some special circumstances of the restricted network,the node is common to use the battery power on the state of hostile environment or in harsh conditions,which will cut the life time of node.When the power is off,then the node cannot guarantee normal work.That is to say,it is very possible the power is off when the message is being transmitted. Dynamic Topology Note that the DTN topology is dynamic changing for some reasons such as environmental changes,en- ergy depletion or other failures,which results in dropping out of network.The requirements of entering DTN also make topology change. Poor Security Due to the lack of specialized services and maintenance in real world DTN is vulnerable to threats like eavesdropping,message modification,routing spoofing and Denial of Service(DoS)etc. Heterogeneous Interconnection The architecture of DTN is based on asynchronous message forward and operates as an overlay above the transport layer.DTN can run on different heterogeneous network protocol stacks and DTN gateway ensures the reliable transmission of interconnection message. 450
450 Delay Tolerant Networks Intermittent Connection As the node’s mobility and energy are limited, DTN frequently disconnects, thus resulting in continue change in DTN topology. That is to say, the network keeps the status of intermittent connection and partial connection so that there is no guarantee to achieve end-to-end route. High Delay, Low Efficiency, and High Queue Delay End-to-end delay specifies the sum of the total delay of each hop on the specified route. The end-to-end delay involves queuing time, waiting time and transmission time (Cerf et al., 2007). Each hop delay might be very high due to the fact that DTN intermittent connection keeps unreachable in a very long time and thus further leading to a lower data rate and showing the asymmetric features in up-down link data rate. In addition, queuing delay plays a main role in end-to-end delay and frequent fragmentations in DTN make queuing delay increasing. Limited Resource Node’s computing and processing ability, communication ability and storage space is weaker than the function of an ordinary computer due to the constraints of price, volume and power. In addition, the limited storage space resulted in higher packet loss rate. Limited Life Time of Node In some special circumstances of the restricted network, the node is common to use the battery power on the state of hostile environment or in harsh conditions, which will cut the life time of node. When the power is off, then the node cannot guarantee normal work. That is to say, it is very possible the power is off when the message is being transmitted. Dynamic Topology Note that the DTN topology is dynamic changing for some reasons such as environmental changes, energy depletion or other failures, which results in dropping out of network. The requirements of entering DTN also make topology change. Poor Security Due to the lack of specialized services and maintenance in real world DTN is vulnerable to threats like eavesdropping, message modification, routing spoofing and Denial of Service (DoS) etc. Heterogeneous Interconnection The architecture of DTN is based on asynchronous message forward and operates as an overlay above the transport layer. DTN can run on different heterogeneous network protocol stacks and DTN gateway ensures the reliable transmission of interconnection message
Delay Tolerant Networks The above mentioned characteristics make DTNs different from traditional wired networks and mobile ad-hoc networks. Types of DTNs According to the application domains,DTNs can be categorized as follow: DTN for Satellite Communications Space communication can be generally characterized by long link delay and frequent link disruptions Despite of these characteristics of space communication DTN has been developed to enable automated network communications.DTN was originated from a generalization of requirements identified for in- terplanetary networking(IPN).Ordinary TCP/IP architectures fail to provide satisfactory performance because of the presence ofone or more ofthe following impairments:long delays,disruptions,intermittent links,network partitioning etc.Satellite network is one among the challenged network.It is the network that includes one or more satellite links.LEO (low earth orbit)satellite networks were immediately recognized as a perfect candidate for DTN applications,because of the satellite link intermittency(Cerf et al.,2007).In deep space and LEO satellite networks the communication opportunities or contacts are known in advance i.e.are fully deterministic as they are related to the orbital characteristics of planets and space assets.This kind of connectivity is addressed by specific DTN solutions such as "scheduled contacts"where transport protocol connections start and stop at the beginning and at the end of contacts. In such networks routing must be designed to cope with scheduled contacts and not with opportunistic connectivity as in other challenged networks,therefore specific routing algorithms as contact graph routing designed by NASA.On the other hand GEO satellite networks are not pure challenged networks because they can offer a continuous connectivity at least for fixed terminals.However they are classified as challenged networks because of its long propagation delay of order 600 ms. DTN for Deep Space Communications Delay/disruption tolerant networking(DTN)technology offers a novel way to significantly stressed com- munications in space environments,especially those with long link delay and frequent link disruptions in deep space missions(Burleigh et al.,2003,Fall,2003).DTN was considered as the most suitable technology to be employed in space internetworking by NASA and hopes to fly with it on space missions soon("Recommendations on a",2008).There are numerous research work has been done related with DTN for space communications in the past several years.The Space Internetworking Strategy Group (SISG),which is composed of technical experts appointed by the Inter-agency Operations Advisory Group(IOAG)agencies,considers DTN to be the only mature candidate protocol available to handle long propagation delays,frequent and lengthy network disruption inherent in space missions involving multiple spacecraft ("Recommendations on a",2008). Vehicular DTN (VDTN) Vehicular Delay-Tolerant Networks(VDTNs)are DTNs where vehicles communicate with each other and with fixed nodes placed along the roads in order to disseminate messages.Some of the potential ap- 451
451 Delay Tolerant Networks The above mentioned characteristics make DTNs different from traditional wired networks and mobile ad-hoc networks. Types of DTNs According to the application domains, DTNs can be categorized as follow: DTN for Satellite Communications Space communication can be generally characterized by long link delay and frequent link disruptions. Despite of these characteristics of space communication DTN has been developed to enable automated network communications. DTN was originated from a generalization of requirements identified for interplanetary networking (IPN). Ordinary TCP/IP architectures fail to provide satisfactory performance because of the presence of one or more of the following impairments: long delays, disruptions, intermittent links, network partitioning etc. Satellite network is one among the challenged network. It is the network that includes one or more satellite links. LEO (low earth orbit) satellite networks were immediately recognized as a perfect candidate for DTN applications, because of the satellite link intermittency (Cerf et al., 2007). In deep space and LEO satellite networks the communication opportunities or contacts are known in advance i.e. are fully deterministic as they are related to the orbital characteristics of planets and space assets. This kind of connectivity is addressed by specific DTN solutions such as “scheduled contacts” where transport protocol connections start and stop at the beginning and at the end of contacts. In such networks routing must be designed to cope with scheduled contacts and not with opportunistic connectivity as in other challenged networks, therefore specific routing algorithms as contact graph routing designed by NASA. On the other hand GEO satellite networks are not pure challenged networks because they can offer a continuous connectivity at least for fixed terminals. However they are classified as challenged networks because of its long propagation delay of order 600 ms. DTN for Deep Space Communications Delay/disruption tolerant networking (DTN) technology offers a novel way to significantly stressed communications in space environments, especially those with long link delay and frequent link disruptions in deep space missions (Burleigh et al., 2003, Fall, 2003). DTN was considered as the most suitable technology to be employed in space internetworking by NASA and hopes to fly with it on space missions soon (“Recommendations on a”, 2008). There are numerous research work has been done related with DTN for space communications in the past several years. The Space Internetworking Strategy Group (SISG), which is composed of technical experts appointed by the Inter-agency Operations Advisory Group (IOAG) agencies, considers DTN to be the only mature candidate protocol available to handle long propagation delays, frequent and lengthy network disruption inherent in space missions involving multiple spacecraft (“Recommendations on a”, 2008). Vehicular DTN (VDTN) Vehicular Delay-Tolerant Networks (VDTNs) are DTNs where vehicles communicate with each other and with fixed nodes placed along the roads in order to disseminate messages. Some of the potential ap-
Delay Tolerant Networks plications for these networks are the following:notification of traffic conditions(unexpected jams),road accident warnings,weather reports(ice,snow,fog,and wind),advertisements(free parking spots,nearby fuel prices,etc.),cooperative vehicle collision avoidance,web or email access,or even the gathering of information collected by vehicles such as road pavement defects.Vehicular networks have also been proposed to implement transient networks to benefit developing communities and disaster recovery networks (Isento et al.,2013). DTN for Underwater Communications Underwater networks(UWNs)have the potential to find applications in a wide range of aquatic activi- ties,such as oceanographic data collection,pollution monitoring,offshore exploration,seismic moni- toring,assisted navigation and tactical surveillance.In most cases,these networks will operate in harsh and constrained environments where communication disruption (and,hence,delay)is frequent.In this respect,an underwater network can be viewed as a delay/disruption-tolerant network (DTN)requiring specialized communication protocols. DTN for Emergency Communications Delay-tolerant networks can be used to improve situational awareness during the response to a large- scale disaster.Delay/Disruption Tolerant Networks(DTNs)can be used in man-made or natural disaster stricken areas with communication infrastructure breakdown or power outages.DTN has been developed as a solution to wireless networks experiencing frequent disruptions.DTNs can provide communica- tion support in disaster relief and rescue operations.An evaluation carried out by (Trono et al.,2015) using DTN MapEx a disaster map generator that operates over a DTN with responders and volunteers, carrying mobile devices shows that DTN can improve information availability in disaster stricken areas. Applications of DTNs Some of the major applications of DTNs are summarized below: Deep Space Exploration In the next few decades,NASA and other agencies will plan a series of projects of lunar exploration, Mars exploration and others.In September,2003,Cisco router(CL EO)was launched by satellite to monitor disaster in UK.Till to December 2008,CL EO has done a lot of routing tests in space environ- ment including using Saratoga protocol of bundle layer instead of pervious protocol making full use of the link source to overcome serious asymmetry link conditions.The experiment shows it is feasible to use Bundle Protocol (Wood et al.2008)in space. Studies of Wild Zebra The Zebranet project (Zhang et al.,2004)has installed a global positioning system(GPS)in a zebra collar to study the habits of zebra activities,which is one of the early DTN projects and was started in 2004.Collars start every few minutes to record GPS location information,and every 2 h open radio func- 452
452 Delay Tolerant Networks plications for these networks are the following: notification of traffic conditions (unexpected jams),road accident warnings, weather reports (ice, snow, fog, and wind),advertisements (free parking spots, nearby fuel prices, etc.),cooperative vehicle collision avoidance, web or email access, or even the gathering of information collected by vehicles such as road pavement defects. Vehicular networks have also been proposed to implement transient networks to benefit developing communities and disaster recovery networks (Isento et al., 2013). DTN for Underwater Communications Underwater networks (UWNs) have the potential to find applications in a wide range of aquatic activities, such as oceanographic data collection, pollution monitoring, offshore exploration, seismic monitoring, assisted navigation and tactical surveillance. In most cases, these networks will operate in harsh and constrained environments where communication disruption (and, hence, delay) is frequent. In this respect, an underwater network can be viewed as a delay/disruption-tolerant network (DTN) requiring specialized communication protocols. DTN for Emergency Communications Delay-tolerant networks can be used to improve situational awareness during the response to a largescale disaster. Delay/Disruption Tolerant Networks (DTNs) can be used in man-made or natural disaster stricken areas with communication infrastructure breakdown or power outages. DTN has been developed as a solution to wireless networks experiencing frequent disruptions. DTNs can provide communication support in disaster relief and rescue operations. An evaluation carried out by (Trono et al., 2015) using DTN MapEx a disaster map generator that operates over a DTN with responders and volunteers, carrying mobile devices shows that DTN can improve information availability in disaster stricken areas. Applications of DTNs Some of the major applications of DTNs are summarized below: Deep Space Exploration In the next few decades, NASA and other agencies will plan a series of projects of lunar exploration, Mars exploration and others. In September, 2003, Cisco router (CL EO) was launched by satellite to monitor disaster in UK. Till to December 2008, CL EO has done a lot of routing tests in space environment including using Saratoga protocol of bundle layer instead of pervious protocol making full use of the link source to overcome serious asymmetry link conditions. The experiment shows it is feasible to use Bundle Protocol (Wood et al. 2008) in space. Studies of Wild Zebra The Zebranet project (Zhang et al., 2004) has installed a global positioning system (GPS) in a zebra collar to study the habits of zebra activities, which is one of the early DTN projects and was started in 2004. Collars start every few minutes to record GPS location information, and every 2 h open radio func-
Delay Tolerant Networks tion,when two collars'distance is in communication range they would exchange information(adopted Epidemic routing algorithms).After a period of time,every horse collar stores the position information of others activities.In this experiment,the researcher can know the exact location of zebra only with little information.The further experiment of this project is to resolve the issues of equipment energy, adaptability and data compression. Rural Communication There are many rural communication projects in remote villages to provide the access to Internet.Some of which is try to reduce the cost of communications using the way of asynchronous information trans- mission.For example,Wizzy digital courier service provides Internet access for some village schools in South Africa.This project adopted a simple one-hop delay network,letting couriers drive a motorcycle with USB storage device to come and go between rural schools and cities with permanent Internet con- nection (such a round-trip may take several hours of time),so as to realize the connection between the school and the Internet. Lake Quality Monitoring European Union advises state and local government to launch protect water quality activities(Farrell and Cahill,2006),in this project,the researchers didn't choose end-to-end communication mode,but using special node(data mule)in the lake to cruise,realizing DTN storage and forwarding mechanism. When the ship (data mule)back to dock,mule can exchange information with the gathering nodes ac- cessing Internet.In this project,using data mules--besides low overhead--still can be independent with infrastructures and set flexibly in various carries.Other one such as Ad hoc being used for collecting battlefield information or collecting data in depopulated area is actually one application of DTN.That's to say,DTN has come into people's lives.Notice that with further development of DTN research,its range of applications will be larger,and more fields will be benefited. Military Applications Military communication network is a multi-hop wireless network,and is an ad hoc network.As a re- sult of the impact of the battlefield special circumstances,such as,mobile nodes,enemy interference, geographical environment,etc,the connection between network nodes is intermittent,uncertainties and non-periodic.Therefore military communication network is a typical DTN network.DTN technology can be fully applied in military communication networks. Public Transportation System There are several promising applications of DTN in public transport. Data Dissemination Application For high volume and non-urgent data,it is not wise to use expensive network transmission techniques. Instead,DTN technique could be used as a low cost data dissemination method in Fog computing,par- 453
453 Delay Tolerant Networks tion, when two collars’ distance is in communication range they would exchange information (adopted Epidemic routing algorithms). After a period of time, every horse collar stores the position information of others activities. In this experiment, the researcher can know the exact location of zebra only with little information. The further experiment of this project is to resolve the issues of equipment energy, adaptability and data compression. Rural Communication There are many rural communication projects in remote villages to provide the access to Internet. Some of which is try to reduce the cost of communications using the way of asynchronous information transmission. For example, Wizzy digital courier service provides Internet access for some village schools in South Africa. This project adopted a simple one-hop delay network, letting couriers drive a motorcycle with USB storage device to come and go between rural schools and cities with permanent Internet connection (such a round-trip may take several hours of time), so as to realize the connection between the school and the Internet. Lake Quality Monitoring European Union advises state and local government to launch protect water quality activities (Farrell and Cahill, 2006), in this project, the researchers didn’t choose end-to-end communication mode, but using special node (data mule) in the lake to cruise, realizing DTN storage and forwarding mechanism. When the ship (data mule) back to dock, mule can exchange information with the gathering nodes accessing Internet. In this project, using data mules--besides low overhead--still can be independent with infrastructures and set flexibly in various carries. Other one such as Ad hoc being used for collecting battlefield information or collecting data in depopulated area is actually one application of DTN. That’s to say, DTN has come into people’s lives. Notice that with further development of DTN research, its range of applications will be larger, and more fields will be benefited. Military Applications Military communication network is a multi-hop wireless network, and is an ad hoc network. As a result of the impact of the battlefield special circumstances, such as, mobile nodes, enemy interference, geographical environment, etc, the connection between network nodes is intermittent, uncertainties and non-periodic. Therefore military communication network is a typical DTN network. DTN technology can be fully applied in military communication networks. Public Transportation System There are several promising applications of DTN in public transport. Data Dissemination Application For high volume and non-urgent data, it is not wise to use expensive network transmission techniques. Instead, DTN technique could be used as a low cost data dissemination method in Fog computing, par-
Delay Tolerant Networks ticular for datadissemination among Fog servers and mobile devices(Gao et al.,2017.).DakNet(Pentland et al.,2004)is a DTN technique based application and developed by researchers from the MIT Media Lab.It has been deployed in remote parts of Cambodia and India at a cost two orders of magnitude less compared to traditional landlines networks. DTN ARCHITECTURE The existing TCP/IP-based internet,while fabulously successful in many environments,does not suit all environments.The ability of the"TCP/IP suite"to provide service depends on a number of important assumptions:(i)existence of end-to-end path between source and destination during communication session;(ii)(for reliable communication)that the maximum round-trip time over that path is not exces- sive and not highly variable from packet to packet;and (iii)that the end-to-end loss is relatively small. Delay Tolerant Networks may not satisfy some of the assumptions due to their different characteristics such as long or variable delays,frequent partitioning,data rate asymmetry and interoperating among differently-challenged networks.The DTN architecture should provide the means for dissimilar networks to interoperate(Cerf et al.,2002).The network architecture used for the conventional networks may not be used as it is for DTNs. The DTN architecture provides a common solution for interconnecting heterogeneous gateways or proxies that employ store-and-forward message routing to overcome communication disruptions (Cerf et al.,2007).At its inception,the concepts behind the DTN architecture were primarily targeted at tolerating long delays and predictably-interrupted communications over long distances (i.e.,in deep space).At this point in time,the work was architecture for the Interplanetary Internet(IPN).By March 2003,when the first draft of the eventual RFC 4838 was published,one of the authors had coined the term Delay Tolerant Networking suggesting the intention to extend the IPN concept to other types of networks,specifically including terrestrial wireless networks.Terrestrial wireless networks also suffer disruptions and delay,and the DTN architectural emphasis grew from scheduled connectivity in the IPN case to include other types of networks and patterns of connectivity (e.g.,opportunistic mobile ad-hoc networks with nodes that remain off for significant periods of time)(Fall et al.,2008). The DTN architecture creates a"network of Internets"by providing an end-to-end layer above the transport layer.We call this the "bundle layer."(Cerf et al.,2002).The name "bundle"derives from considering protocols that attempt to minimize the number of round-trip exchanges required to complete a protocol transaction,and dates back to the original IPN work.By "bundling"together all information required completing a transaction (e.g.,protocol options and authentication data),the number of ex- changes can be reduced,which is of considerable interest if the round trip time is hours,days or weeks (Fall et al.,2008).Bundles comprise a collection of typed blocks.Each block contains meta-data;some also contain application data.(Fall et al.,2008).The first or primary block of each bundle,illustrated in Figure 1,contains the DTNequivalents of the data typically found in an IP header on the Internet:version, source and destination EIDs,length,processing flags,and (optional)fragmentation information.It also contains some additional fields,more specific to the bundle protocol:report-to EID,current custodian EID,creation timestamp and sequence number,lifetime and a dictionary.Most fields are variable in length,and use a relatively compact notation called self-delimiting numerical values (SDNVs).Early designs for the primary bundle block used more fixed-length fields,but the relative merit of choosing a fixed-length field for simplicity was ultimately found to be less compelling than the flexibility offered 454
454 Delay Tolerant Networks ticular for data dissemination among Fog servers and mobile devices (Gao et al., 2017.). DakNet (Pentland et al., 2004) is a DTN technique based application and developed by researchers from the MIT Media Lab. It has been deployed in remote parts of Cambodia and India at a cost two orders of magnitude less compared to traditional landlines networks. DTN ARCHITECTURE The existing TCP/IP-based internet, while fabulously successful in many environments, does not suit all environments. The ability of the “TCP/IP suite” to provide service depends on a number of important assumptions: (i) existence of end-to-end path between source and destination during communication session; (ii) (for reliable communication) that the maximum round-trip time over that path is not excessive and not highly variable from packet to packet; and (iii) that the end-to-end loss is relatively small. Delay Tolerant Networks may not satisfy some of the assumptions due to their different characteristics such as long or variable delays, frequent partitioning, data rate asymmetry and interoperating among differently-challenged networks. The DTN architecture should provide the means for dissimilar networks to interoperate (Cerf et al., 2002).The network architecture used for the conventional networks may not be used as it is for DTNs. The DTN architecture provides a common solution for interconnecting heterogeneous gateways or proxies that employ store-and-forward message routing to overcome communication disruptions (Cerf et al., 2007). At its inception, the concepts behind the DTN architecture were primarily targeted at tolerating long delays and predictably-interrupted communications over long distances (i.e., in deep space). At this point in time, the work was architecture for the Interplanetary Internet (IPN). By March 2003, when the first draft of the eventual RFC 4838 was published, one of the authors had coined the term Delay Tolerant Networking suggesting the intention to extend the IPN concept to other types of networks, specifically including terrestrial wireless networks. Terrestrial wireless networks also suffer disruptions and delay, and the DTN architectural emphasis grew from scheduled connectivity in the IPN case to include other types of networks and patterns of connectivity (e.g., opportunistic mobile ad-hoc networks with nodes that remain off for significant periods of time) (Fall et al., 2008). The DTN architecture creates a “network of Internets” by providing an end-to-end layer above the transport layer. We call this the “bundle layer.” (Cerf et al., 2002). The name “bundle” derives from considering protocols that attempt to minimize the number of round-trip exchanges required to complete a protocol transaction, and dates back to the original IPN work. By “bundling” together all information required completing a transaction (e.g., protocol options and authentication data), the number of exchanges can be reduced, which is of considerable interest if the round trip time is hours, days or weeks (Fall et al., 2008). Bundles comprise a collection of typed blocks. Each block contains meta-data; some also contain application data. (Fall et al., 2008). The first or primary block of each bundle, illustrated in Figure 1, contains the DTN equivalents of the data typically found in an IP header on the Internet: version, source and destination EIDs, length, processing flags, and (optional) fragmentation information. It also contains some additional fields, more specific to the bundle protocol: report-to EID, current custodian EID, creation timestamp and sequence number, lifetime and a dictionary. Most fields are variable in length, and use a relatively compact notation called self-delimiting numerical values (SDNVs). Early designs for the primary bundle block used more fixed-length fields, but the relative merit of choosing a fixed-length field for simplicity was ultimately found to be less compelling than the flexibility offered
Delay Tolerant Networks Figure 1.The structure of the primary block of a bundle(Scott and Burleigh,2007) Version(1 byte) Bundle Processing Control Flags (SDNV) Block Length (SDNV) Destination Scheme Offset(SDNV) Destination SSP Offset(SDNV) Source Scheme Offset(SDNV) Source SSP Offset(SDNV) Report-to Scheme Offset(SDNV) Report-to SSP Offset(SDNV) Custodian Scheme Offset(SDNV) Custodian SSP Offset(SDNV) Creation Timestamp(SDNV) Creation Timestamp Sequence Number(SDNV) Lifetime (SDNV) Dictionary Length (SDNV) Dictionary(byte array) Fragment Offset(SDNV,optional) Application data unit length (SDNV,optional) by SDNVs.By setting various bits in the bundle processing control flags,the sender can request a re- port for any of the following events:receipt at destination node,custody acceptance at a node,bundle forwarded/deleted/delivered route,and receipt by destination application.(Scott and Burleigh,2007). The DTN architecture uses store-and-forward message switching technique by overlaying a new transmission protocol,called the bundle protocol on top of the lower-layer protocols such as Internet protocols.The bundle protocol ties together the lower-layer protocols so that application programs can communicate across the same or different sets of lower-layer protocols under conditions that involve long network delays or disruptions.The bundle-protocol agent stores and forwards entire bundles (or bundle fragments)between nodes.A single bundle protocol is used throughout a DTN.On the other hand,the lower-lower protocols below the bundle protocol are chosen depending on the characteristics of each communication environment.The figure below (top)illustrates the bundle-protocol overlay and (bottom)compares the Internet protocol stack (left)with a DTN protocol stack(right). Some important characteristics of DTN architectures are as follows. Store-and-Forward Message Switching (Warthman,2012) DTNs use store-and forward message switching technique to resolve the problems associated with intermittent connectivity,long or variable delay,asymmetric data rates,and high error rates.A DTN- enabled application sends messages of arbitrary length,also called Application Data Units or ADUs. Whole messages(ADUs)or pieces(fragments)of such messages are forwarded from a storage place on one node (switch intersection)to a storage place on another node,along a path that eventually reaches the destination. 455
455 Delay Tolerant Networks by SDNVs. By setting various bits in the bundle processing control flags, the sender can request a report for any of the following events: receipt at destination node, custody acceptance at a node, bundle forwarded/deleted/delivered route, and receipt by destination application. (Scott and Burleigh, 2007). The DTN architecture uses store-and-forward message switching technique by overlaying a new transmission protocol, called the bundle protocol on top of the lower-layer protocols such as Internet protocols. The bundle protocol ties together the lower-layer protocols so that application programs can communicate across the same or different sets of lower-layer protocols under conditions that involve long network delays or disruptions. The bundle-protocol agent stores and forwards entire bundles (or bundle fragments) between nodes. A single bundle protocol is used throughout a DTN. On the other hand, the lower-lower protocols below the bundle protocol are chosen depending on the characteristics of each communication environment. The figure below (top) illustrates the bundle-protocol overlay and (bottom) compares the Internet protocol stack (left) with a DTN protocol stack (right). Some important characteristics of DTN architectures are as follows. Store-and-Forward Message Switching (Warthman, 2012) DTNs use store-and forward message switching technique to resolve the problems associated with intermittent connectivity, long or variable delay, asymmetric data rates, and high error rates. A DTNenabled application sends messages of arbitrary length, also called Application Data Units or ADUs. Whole messages (ADUs) or pieces (fragments) of such messages are forwarded from a storage place on one node (switch intersection) to a storage place on another node, along a path that eventually reaches the destination. Figure 1. The structure of the primary block of a bundle (Scott and Burleigh, 2007)
Delay Tolerant Networks Figure 2.Bundle-protocol overlay with DTN protocol stack (Warthman,2012) Apps Apps Bundle Layer Region- Region- Region- Region- Region- Specific Specific Specific Specific Specific Layers Layers Layers Layers Layers Application Application Bundle common across all DTN regions Transport(TCP) Transport Network (IP) Network specificto each Link Link DTN region Physical Physical Internet Layers DTN Layers Figure 3.Store-And-Forward Message Switching(Warthman,2012) Store Store Store Node Node Node Node store A Fonvard Forward Forward Store-and-forwarding methods are also used here are not node-to-node relays(as shown above)but rather star relays where both the source and destination independently contact a central storage device at the centre of the links. DTN routers need persistent storage for their queues for one or more of the following reasons: ● A communication link to the next hop may not be available for a long time. One node in a communicating pair may send or receive data much faster or more reliably than the other node. ● A message,once transmitted,may need to be retransmitted if an error occurs at an upstream (to- ward the destination)node,or if an upstream node declines acceptance of a forwarded message. By moving whole messages(or fragments thereof)in a single transfer,the message-switching tech- nique provides network nodes with immediate knowledge of the size of messages,and therefore the requirements for intermediate storage space and retransmission bandwidth. 456
456 Delay Tolerant Networks Store-and-forwarding methods are also used here are not node-to-node relays (as shown above) but rather star relays where both the source and destination independently contact a central storage device at the centre of the links. DTN routers need persistent storage for their queues for one or more of the following reasons: • A communication link to the next hop may not be available for a long time. • One node in a communicating pair may send or receive data much faster or more reliably than the other node. • A message, once transmitted, may need to be retransmitted if an error occurs at an upstream (toward the destination) node, or if an upstream node declines acceptance of a forwarded message. By moving whole messages (or fragments thereof) in a single transfer, the message-switching technique provides network nodes with immediate knowledge of the size of messages, and therefore the requirements for intermediate storage space and retransmission bandwidth. Figure 2. Bundle-protocol overlay with DTN protocol stack (Warthman, 2012) Figure 3. Store-And-Forward Message Switching (Warthman, 2012)
Delay Tolerant Networks Nodes and Endpoints (Warthman,2012) A node is an entity with a bundle-protocol agent overlaid on lower-layer communication protocols in DTN.At any moment,a given node may act as a source,destination,or forwarder of bundles: Source or Destination Function As a source or destination,a node sends or receives bundles to or from another node,but it does not for- ward bundles received from other nodes.If the node operates over long-delay links,its bundle protocol requires persistent storage in which to queue bundles until outbound links are available.The node may optionally support custody transfers. Forwarding Function A DTN node can forward bundles between two or more other nodes in one of two situations: Routing-Equivalent Forwarding. The node forwards bundles between two or more other nodes,each of which implement the same lower- layer protocols as the forwarding node.If a forwarding node operates over long-delay links,its bundle protocol requires persistent storage in which to queue bundles until outbound links are available.The node may optionally support custody transfers. Gateway-Equivalent Forwarding: The node forwards bundles between two or more other nodes,each of which implement different lower- layer protocols while the forwarding node implements all such protocols.The node must have persistent storage;support for custody transfers is optional but typically advisable. A bundle endpoint is a set of zero or more nodes that all identify themselves by the same endpoint ID.The common case in which only one node has a given endpoint ID is called a singleton endpoint Every node is uniquely identified by at least one singleton endpoint.Source nodes are always singleton endpoints or null (anonymous source)endpoints,and destination nodes may or may not be singleton endpoints.Endpoints may also be multicast(multiple destination nodes with the same endpoint ID)or null(no nodes).Endpoints may contain multiple nodes,and nodes may be members of multiple endpoints. Priority Classes(Cerf et al.,2002) The DTN architecture offers relative measures of priority (low,medium,high)for delivering ADUs. These priorities differentiate traffic based upon an application's desire to affect the delivery urgency for ADUs,and are carried in bundle blocks generated by the bundle layer based on information specified by the application. Three relative priority classes are defined to date.These priority classes typically imply some relative scheduling prioritization among bundles in queue at a sender: 457
457 Delay Tolerant Networks Nodes and Endpoints (Warthman, 2012) A node is an entity with a bundle-protocol agent overlaid on lower-layer communication protocols in DTN. At any moment, a given node may act as a source, destination, or forwarder of bundles: Source or Destination Function As a source or destination, a node sends or receives bundles to or from another node, but it does not forward bundles received from other nodes. If the node operates over long-delay links, its bundle protocol requires persistent storage in which to queue bundles until outbound links are available. The node may optionally support custody transfers. Forwarding Function A DTN node can forward bundles between two or more other nodes in one of two situations: Routing-Equivalent Forwarding. The node forwards bundles between two or more other nodes, each of which implement the same lowerlayer protocols as the forwarding node. If a forwarding node operates over long-delay links, its bundle protocol requires persistent storage in which to queue bundles until outbound links are available. The node may optionally support custody transfers. Gateway-Equivalent Forwarding: The node forwards bundles between two or more other nodes, each of which implement different lowerlayer protocols while the forwarding node implements all such protocols. The node must have persistent storage; support for custody transfers is optional but typically advisable. A bundle endpoint is a set of zero or more nodes that all identify themselves by the same endpoint ID. The common case in which only one node has a given endpoint ID is called a singleton endpoint. Every node is uniquely identified by at least one singleton endpoint. Source nodes are always singleton endpoints or null (anonymous source) endpoints, and destination nodes may or may not be singleton endpoints. Endpoints may also be multicast (multiple destination nodes with the same endpoint ID) or null (no nodes). Endpoints may contain multiple nodes, and nodes may be members of multiple endpoints. Priority Classes (Cerf et al., 2002) The DTN architecture offers relative measures of priority (low, medium, high) for delivering ADUs. These priorities differentiate traffic based upon an application’s desire to affect the delivery urgency for ADUs, and are carried in bundle blocks generated by the bundle layer based on information specified by the application. Three relative priority classes are defined to date. These priority classes typically imply some relative scheduling prioritization among bundles in queue at a sender: