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: