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-