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The Internet is a connection between communication devices across the globe. Wired and wireless networks enable the interconnection of these network devices widely separated in different geographical locations. Mobile Ad-hoc Network is a network where the source and destination nodes are not in the predefined communication environment. The nodes in this network change position unexpectedly and rapidly because the nodes are mobile. The network potential activity is to discover topology and deliver the message to a destination due to the decentralized nature of the network.
Wireless sensor networks are spatially distributed over a geographical area. These networks are popularly used to examine temperature, chemical and motion pressures, and sound etc. This wireless sensor network works in a limited range and the connection between the nodes is not always established. The nodes in this network transmit their findings to a base station for analysis.
The wide disbursement, battery concern and intentional jamming make it impossible for full wireless connection between the nodes. Furthermore data traffic on this network waits for bandwidth for communication while high priority voice traffics are carried out on the same link.
Exotic Media Network:-
Exotic Media Network works near the earth's satellite communication, the audio signal is used in air and optical signal is used in water in free space communication. This network may cause disruptions in communication due to planetary movements or crossing of scheduled ships or may suffer from weather (environment condition). 
Delay Tolerant Network:-
Delay Tolerant Network is a network where connection between the nodes is intermittent; there is no assumption of end to end connection between nodes like the TCP/IP works.
Delay Tolerant Network is an ad-hoc wireless network that works in the nuisance environment of disruptions and delay. 
The current networking technology has a fundamental assumption that there is an end to end connection between source and destination through multiple intermediary devices, these assumptions can easily fail due to unreliable network, power saving and mobility of the nodes.
To enable communication between challenged networks or where there is no pre-existing infrastructure of network environments, Delay Tolerant Network is introduced.
The Delay Tolerant Network is divided in the form of regions. The message communicates in the form of bundles with variable size and follows the store and forward technique rather than in TCP/IP, where the message is in the form of packets and relays from the transport layer. Delay Tolerant Network resolves the session expiry problem of TCP/IP, the bundles in Delay Tolerant Network transmits with longer potential delay.
Problems of DTN:-
Congestion exceeds buffer limits:-
Delay Tolerant Network works on large delay for transferring of data where the pre-existing network infrastructure is not available or the availability of the next hop is not predefined. The message continuously waits in the queue until the next hop is not found; the waiting of message in the queue for delivery could be large from hours to days. Because most routing protocols in DTN work on the principle of "store and forward technique" or replicating the message. However the new devices have large capacity to store the message and work well with higher probability of delivering the message but the situation becomes worse when congestion occurs.  DTN mobile ad-hoc networks may store the audio or video files. But sharing of large files on the network while others have small messages to communicate but use network more frequently; this could cause the network congestion and exceed the buffer limit even though most of the buffer size is already occupied by messages waiting for response from the next hop for communication.
In DTN network, nodes continuously need to discover the neighbourhood node for communication. Searching of neighbour nodes consumes large amount of energy rather than infrequent data transfer. . The 802.11 radio consumes 99.5% of its energy to search for neighbour nodes for communication. 
When DTN network works in the environment of infrastructure lacking power or solar system capability the network equipment turf off the battery and even the charging is available the system run off the battery to limit their duty cycle. Moreover, few systems go to power save mode which would be unexpected break of connection from the network. This kind of disconnection affects the routing protocols even when the data communication is normally ahead of time. In addition to this discovery of new nodes that join the network in idle period of time is a challenge for the network.
DTN Security Threats:-
The medium of communication is space in DTN rather than end to end connection of TCP/IP needs more awareness of security. However, communication between the nodes is temporary to a limited extent.
A defective node can jam the network by flooding the traffic in communication network.
The application can transfer the data frequently or at higher rate for which they lack permission.
A defective node can create additional management message for each bundle.
Confidentiality and integrity:-
In DTN the threat may relates to confidentially and integrity where the attacker alter the address of source and destination and other control field of bundles related for communication. In addition attacker may eavesdrop the content of bundle and altering payload of bundle.
Denial Service Attack:-
This kind of attack occurs in sensor network when defective node tries to broadcast the high energy signal over the network to jam the network. Furthermore more sophisticated attacks also happen when attacker violates the media access control; when the neighbour continuously requests or transmits a signal with a request to send a signal. 
Information gathering attack:-
A DTN network does not include firewalls and has lots of reachable ports. The passive attacker can perform an information gathering attack by entering in the server through the open redundant ports. 
Long term disconnection:-
Low density and mobility attribute of nodes can cause long term disconnection. Handover between the mobile networks causes temporary disconnection of communication signal. Few other causes of disconnection can be identified as the movement of satellite and buses act as data router. Low capability devices also disconnect the network due to low duty cycle operation.
The routing in challenging networks is critical where there is no predefined infrastructure for communication. In interplanetary communication the distance between the earth and moon is 8min/light. Sometimes, the environment can be a cause of disruption between these communication systems. Furthermore the limited resources of communication system could also be the cause of network disconnection such as limited battery, computational speed and transmission rate. In addition to this mobility of communication nodes could also be the cause of interruptions in communication.
Epidemic routing is simple because it does not require knowledge about the network. When a node wishes to start communication, a sent message or bundle stored in local buffer is tagged with unique id. When any new node is encountered, both nodes share a list of unique id bundles stored in the local buffer called summary vector. Using this summary vector the nodes communicate with the bundles which they don't have. After completion of communication, the local buffer still retains the messages that were already communicated and starts exchanging the same message again and again in future whenever the connection establishes.
The epidemic routing is easy to implement and increases the probability of message delivery and decreases the delay of message delivery. However with too many copies of the same message in a network, the network resources such as buffer memory and battery energy of intermediate node are wasted in epidemic routing. Moreover congestion problems could also be encountered with unlimited flooding of messages when the intermediate buffer size exceeds its limit.
Spray and Wait Routing Protocol:-
Spray and Wait routing protocols work similar to epidemic routing protocols with flooding of the same message but in this routing protocols flooding of similar messages in communication channels is limited by L. It works on a two phase mode; spray phase transfers the same message of copies with limited L number to its neighbour node. Whereas in wait phase every node retains its copy without transferring the same copy until the desired destination does not meet the destined bundle or message. Since spray and wait routing protocols limit the flooding of messages so the network resources wastage can be reduced. The binary algorithm of spray and wait routing protocols play a vital role to increase the speed of communication and reduce the delay of delivery of bundles. 
PRoPHET Routing Protocol (Probabilistic Routing Protocol using History of Encounters and Transitivity):-
The static nodes where power saving is scheduled may cause intermittent connection between the nodes. In addition to this the contact between mobile nodes is not completely random or being in contact in the location several times before or its likely the node will visit the location again.
The PRoPHET routing protocol maintains the history of encounters between the nodes and transitivity for probabilistic estimate of nodes meetings. This routing protocol is an alternative to epidemic routing with less requirement of buffer space and bandwidth.
The PRoPHET routing protocol works similar to the epidemic routing; when a new node is encountered they share summary vector, but in this it also includes the delivery predictability information. Thus to establish communication P (a, b) â‚¬ [0, 1] at every node a for each known destination b is established.
PRoPHET routing protocols is advantageous over the epidemic routing where because of replication of same messages misuse of network resources is possible. However the buffer size and bandwidth is limited to resources similar to the epidemic routing but buffer overflow handled by deleting the old messages or the messages whose time out limit expires. This buffer management prevents wastage of buffer memory by unnecessary storage of redundant messages.
Delay Tolerant Transport Protocol:-
Delay tolerant network is used to support challenged networks where heterogeneous network connectivity may not be available for long; congestion and error rate of data transmission could affect more often.
Delay tolerant transport protocol is a dynamic characteristic transport protocol use to increase the efficiency of network. This protocol provides end to end transmission with route adaption.
DTTP data transfer units are packets. The transmission of packets works on the principle of store and forward technique. The persistent storage of packets in the intermediate node during transmission of packets survives the packets after network disconnection. Every packet in the DTTP transfer protocol has its own unique custody id. Custody transfer processes the transfer of packets from one node to the next until the destination is not reached. When any intermediate node accepts the packets and corresponding custody is received then sender deletes the message from its buffer after receiving is acknowledged. Following the deletion of message by sender after receipt of acknowledgement, the buffer overflow can be avoided and the detection of lost packets can be verified for retransmission.
The routing in DTTP can be dynamic or manual path selection. The DTN nodes in this routing technique maintain the matrices for upcoming connections. The matrices may contain information like transmission characteristics, duration and start time of connection etc. The transmission performance record is maintained at the router module of the node of DTTP so the available route is recorded and updated.
Christos V. Samaras determined that the local measurement enhanced the ultimate communication to the end point or to the destination. 
Deep Space Transfer Protocol:-
Deep space transfer protocol is proposed for space networking introduced by proactive transmission and retransmission of messages. It also works on the principle of store and forward principle. It transmits messages on predefined link for full utilization of links and sends selective negative acknowledgment (SNAck) back to the sender when packets are lost during communication. Furthermore, retransmission of message performs when senders encounter any corruption of data packets.
Components of DSTP:-
The advance information of link availability and link capacity helps nodes for full utilization of bandwidth for entire communication. The link can be used efficiently without increase of transmission rate and avoid congestion.
The DSTP sends positive acknowledgement to release buffer space at the sender's side when nothing went wrong at the recipient's side. The SNAck acknowledges the sender for retransmission of message when the corruption of message happens during the transmission. The technique of acknowledgement really makes DTN reliable when the connection between the nodes is intermittent.
Double Automatic Retransmission:-
Double Automatic Retransmission implies retransmission of packets when packet loss happens in communication. This allows immediate and efficient recovery of corrupted messages by sending each packet twice with certain delay between retransmission and original transmission.
However the space link can be fully utilized by using Double automatic transmission performed but error rate estimation deviation between sender and receiver can mislead the Double Automatic Retransmission mechanism. 
CCSDS File Delivery Protocol (CFDP):-
Commutative file delivery Protocol uses space for delivery of the file where connection between the communication nodes is not predefined.  This protocol provides bulk of data transfer by splitting messages into data units; which is mainly required in super craft. The protocol requires limited fundamental components to operate hence minimizing the use of resources because of transfer of PDU among the two file stores.
Features of CFDP:-
CFDP delivers files continuously and efficiently and the files can be recovered if files are corrupted because of disruptions in network.
Third party control of data transfer can be established using proxy.
The protocol employee dynamic network configuration for disconnected channel.
The transmission of data units can executed in acknowledge and unacknowledged mode.
Great approach of Suspend and Resume of link depends upon predictable link outages.
Licklider Transmission Protocols (LTP):-
Licklider Transmission Protocol is a point to point protocol used to transmit data over deep space RF (Radio Frequency) links. The partial reliability is promised by dividing the message into two segments, one is "red part" and the other is "green part".
The red part is requested by the user for reliable transmission and assurance of successful transmission can be understood with the receipt of acknowledgement. And retransmission occurs whenever disruptions occur in the transmission or loss of data is encountered. However, the red part does not have any higher priority semantic compared to green part. Licklider Transmission Protocols can provide User Datagram Protocols (UDP) and Transmission Control Protocol (TCP) concurrently in one session. LTD is designed to work directly over data link layer but it may be implemented over UDP for certain reasons such as private local area network.
Licklider Transmission Protocol fails to maintain security concerns; any attacker can listen to the communication over satellite and radio broadcasts. Denial of service (Dos) attack is more critical in this protocol where the time out values are long.