Delay Tolerant Networks Bulk Bulk bundles are shipped on a"least effort"basis.No bundles of this class will be shipped until all bundles of other classes bound for the same destination and originating from the same source have been shipped. Normal Normal-class bundles are shipped prior to any bulk-class bundles and are otherwise the same as bulk bundles. Expedited Expedited bundles,in general,are shipped prior to bundles of other classes and are otherwise the same. Applications specify their requested priority class and data life time for each ADU they send.This information,coupled with policy applied at DTN nodes that select how messages are forwarded and which routing algorithms are in use,affects the overall likelihood and timeliness of ADU delivery.The priority class of a bundle is only required to relate to other bundles from the same source.This means that a high priority bundle from one source may not be delivered faster(or with some other superior quality of service)than a medium priority bundle from a different source.It does mean that a high priority bundle from one source will be handled preferentially to a lower priority bundle sent from the same source. Congestion Control The Delay Tolerant Networking architecture (DTN)(Fall,2003)supports a custody transfer concept implemented by an acknowledged transfer of data to persistent,reliable storage.A node"taking custody" of a message makes a commitment to deliver the message to its destination or another custodian node, effectively migrating one or both of the ends described in the end-to-end argument(Saltzer and Clark, 1984)to new locations.The goal of custody transfer is to use hop-by-hop (custodian-to-custodian)reli- ability to improve end-to-end reliability and to free retransmission buffers at a sender as soon as pos- sible.To implement this facility,the node taking custody("custodian")must generally reserve storage for messages it takes custody of,resulting in a reduced amount of storage remaining for either taking custody of subsequent messages or for merely doing its ordinary task of switching messages.When faced with persistent demand,a custodian unable to release or otherwise transfer custody of its messages will ultimately exhaust its storage resources-a form of DTN congestion.This type of congestion can easily result in head-of-line blocking,preventing further traffic from flowing even when some outgoing connections are available (Fall et al.,2003).Easing congestion at a custodian is a nontrivial task.The options include discarding messages,moving them toward their ultimate destination(typically the most desirable case),or moving them to some other place.The potential of long delays and interruptions of custody transfer operations between custodians makes the management of message migration to combat congestion especially difficult(Seligman et al.,2006). 458
458 Delay Tolerant Networks Bulk Bulk bundles are shipped on a “least effort” basis. No bundles of this class will be shipped until all bundles of other classes bound for the same destination and originating from the same source have been shipped. Normal Normal-class bundles are shipped prior to any bulk-class bundles and are otherwise the same as bulk bundles. Expedited Expedited bundles, in general, are shipped prior to bundles of other classes and are otherwise the same. Applications specify their requested priority class and data life time for each ADU they send. This information, coupled with policy applied at DTN nodes that select how messages are forwarded and which routing algorithms are in use, affects the overall likelihood and timeliness of ADU delivery. The priority class of a bundle is only required to relate to other bundles from the same source. This means that a high priority bundle from one source may not be delivered faster (or with some other superior quality of service) than a medium priority bundle from a different source. It does mean that a high priority bundle from one source will be handled preferentially to a lower priority bundle sent from the same source. Congestion Control The Delay Tolerant Networking architecture (DTN) (Fall, 2003) supports a custody transfer concept implemented by an acknowledged transfer of data to persistent, reliable storage. A node “taking custody” of a message makes a commitment to deliver the message to its destination or another custodian node, effectively migrating one or both of the ends described in the end-to-end argument (Saltzer and Clark, 1984) to new locations. The goal of custody transfer is to use hop-by-hop (custodian-to-custodian) reliability to improve end-to-end reliability and to free retransmission buffers at a sender as soon as possible. To implement this facility, the node taking custody (“custodian”) must generally reserve storage for messages it takes custody of, resulting in a reduced amount of storage remaining for either taking custody of subsequent messages or for merely doing its ordinary task of switching messages. When faced with persistent demand, a custodian unable to release or otherwise transfer custody of its messages will ultimately exhaust its storage resources– a form of DTN congestion. This type of congestion can easily result in head-of-line blocking, preventing further traffic from flowing even when some outgoing connections are available (Fall et al., 2003). Easing congestion at a custodian is a nontrivial task. The options include discarding messages, moving them toward their ultimate destination (typically the most desirable case), or moving them to some other place. The potential of long delays and interruptions of custody transfer operations between custodians makes the management of message migration to combat congestion especially difficult (Seligman et al., 2006)
Delay Tolerant Networks ROUTING AND BUFFER MANAGEMENT IN DTN DTN uses store-carry-and-forward protocols:there,a node may store a message in its buffer and carry it along for long periods of time,until an appropriate forwarding opportunity arises.Additionally,multiple message replicas are often propagated to increase delivery probability.This combination of long-term storage and replication imposes a high storage overhead on unbounded nodes (e.g.handhelds).Thus, efficient buffer management policies are necessary to decide which messages should be discarded, when node buffers are operated close to their capacity("Recommendations on a",2008).This section highlights the issues and challenges in buffer management in DTN.This section will also cover some efficient approaches for buffer management in DTN. Delay Tolerant Networks are wireless networks where disconnections may occur frequently due to propagation phenomena,node mobility,and power outages.Propagation delays may also be long due to the operational environment (e.g.deep space,underwater).In order to achieve data delivery in such challenging networking environments,researchers have proposed the use of store-carry-and-forward protocols:there,a node may store a message in its buffer and carry it along for long periods of time, until an appropriate forwarding opportunity arises.Additionally,multiple message replicas are often propagated to increase delivery probability.This combination of long-term storage and replication im- poses a high storage overhead on untethered nodes (e.g.handhelds).Thus,efficient buffer management policies are necessary to decide which messages should be discarded,when node buffers are operated close to their capacity. In DTN,the"store-carry-forward"mechanism is used for message transmission.These messages are delivered to their final destinations in a hop-by-hop manner.As a result,many problems arise such as how to drop and how to schedule the messages,in the buffer due to the impulsive nature of the nodes.Many changeable situations may occur like limited storage node capacity,short contact duration between the two nodes,and so on (Ahmed et al.,2016).Buffer Management technology is a fundamental approach that manages the various resources among different situations as per the technique used.An efficient buffer management technique decides at each step which of the messages is to be dropped first,when the buffer is full as well as which messages are to be transmitted,when bandwidth is limited(Fathima and Wahidabanu,2011). The nodes in the DTN require proper buffer management approach to get low delay and high data delivery.The buffer management,in this case,refers to the proper use of scheduling and dropping poli- cies used by the nodes at the time of the buffer overflow and congestion (Mansuri,2013). Buffer Management Policies The popular dropping policies techniques for buffer management used in DTNs are described. Drop Least Recently Received(DLR) In the DLR buffer management technique,as the name implies,the packet which is stays for a long time in the buffer will be dropped first.This is due to the fact that it has less probability of being conceded to the other nodes (Mansuri,2013) 459
459 Delay Tolerant Networks ROUTING AND BUFFER MANAGEMENT IN DTN DTN uses store-carry-and-forward protocols: there, a node may store a message in its buffer and carry it along for long periods of time, until an appropriate forwarding opportunity arises. Additionally, multiple message replicas are often propagated to increase delivery probability. This combination of long-term storage and replication imposes a high storage overhead on unbounded nodes (e.g. handhelds). Thus, efficient buffer management policies are necessary to decide which messages should be discarded, when node buffers are operated close to their capacity (“Recommendations on a”, 2008). This section highlights the issues and challenges in buffer management in DTN. This section will also cover some efficient approaches for buffer management in DTN. Delay Tolerant Networks are wireless networks where disconnections may occur frequently due to propagation phenomena, node mobility, and power outages. Propagation delays may also be long due to the operational environment (e.g. deep space, underwater). In order to achieve data delivery in such challenging networking environments, researchers have proposed the use of store-carry-and-forward protocols: there, a node may store a message in its buffer and carry it along for long periods of time, until an appropriate forwarding opportunity arises. Additionally, multiple message replicas are often propagated to increase delivery probability. This combination of long-term storage and replication imposes a high storage overhead on untethered nodes (e.g. handhelds). Thus, efficient buffer management policies are necessary to decide which messages should be discarded, when node buffers are operated close to their capacity. In DTN, the “store-carry-forward” mechanism is used for message transmission. These messages are delivered to their final destinations in a hop-by-hop manner. As a result, many problems arise such as how to drop and how to schedule the messages, in the buffer due to the impulsive nature of the nodes. Many changeable situations may occur like limited storage node capacity, short contact duration between the two nodes, and so on (Ahmed et al., 2016). Buffer Management technology is a fundamental approach that manages the various resources among different situations as per the technique used. An efficient buffer management technique decides at each step which of the messages is to be dropped first, when the buffer is full as well as which messages are to be transmitted, when bandwidth is limited (Fathima and Wahidabanu, 2011). The nodes in the DTN require proper buffer management approach to get low delay and high data delivery. The buffer management, in this case, refers to the proper use of scheduling and dropping policies used by the nodes at the time of the buffer overflow and congestion (Mansuri, 2013). Buffer Management Policies The popular dropping policies techniques for buffer management used in DTNs are described. Drop Least Recently Received (DLR) In the DLR buffer management technique, as the name implies, the packet which is stays for a long time in the buffer will be dropped first. This is due to the fact that it has less probability of being conceded to the other nodes (Mansuri, 2013)