This document discusses delay tolerant networks (DTNs), which are designed to provide reliable communication under harsh conditions characterized by high error rates, long propagation delays, and intermittent connectivity. It presents a comprehensive overview of DTN architecture, protocols, and their implementations, highlighting their necessity in challenging environments such as military and space applications. The paper encourages further exploration in the field of DTNs, introducing concepts like store-and-forward message switching, interoperability, and various routing algorithms.

1. 1
Delay Tolerant Network
Laili Aidi Jung Changsu
School of Information and Communication Technology KTH, Stockholm, Sweden
{aidi, changsu}@kth.se
the network.
Abstract—The interest in challenged networks is increasing
and many researchers seek reliable end-to-end connectivity
under harsh environments, specifically long propagation delay, II. HISTORY
high error rates, low data rate, and intermittent connectivity. The
concept of a Delay Tolerant Network was introduced to provide
DTNs were originally conceived to support the
challenged networks based upon reliable transmission and Inter-planetary Internet (IPN) [3]. There was
interoperability with an overlay network. In this paper, we growing demand for a new network architecture to
present a comprehensive overview of Delay Tolerant Network
and introduce a study case about the implementation of such a support communications in the context of long
network. This paper is designed to encourage the exploration of propagation delay, low data rates, and intermittent
this field by presenting the basic concept and a study.
connectivity. The Interplanetary Internet initiative
Keywords: Delay Tolerant Networks, Routing, Security, Challenge tried to find a solution and suggested a new network
Network architecture to support reliable transmission
between a station on the Earth and satellites, with an
I. INTRODUCTION overlay network [5].
The Internet Research Task Force (IRTF) DTN
M ANY evolving wireless networks such as
military, space, and underwater networks have
Research Group and the DARPA (Defense
Advanced Research Project Agency) Disruption
characteristics different from the Internet. For
Tolerant Networking program advanced this
example, the instability of the links, long
concept. The IRTF DTN Research Group
propagation and queuing delays, extremely
generalized the concept of an Interplanetary Internet
asymmetric data rate, and high link error rates. For
into challenged networks. DARPA was interested in
these reasons, new communication protocols are
the development of protocols for transmitting
required to reduce the number of retransmissions
bundles to DTN nodes [18].
while providing reliable transmissions despite the
high error rates and long delays [3]. III. DRIVING FORCE
A Delay/Disruptive Tolerant Network (DTN) is
designed to provide interoperable communications A. Challenged Network
between a wide range of networks with different Some of the characteristics of a challenged
performance standards, environments, and network are mentioned below [2, 7]:
reliability in case of failure of hardware (network) 1. Path and Link
and or software (protocol) [2, 3]. A DTN must • High Error Rate and Asymmetric Data
accommodate long delays between and/or within Rates. The transmission rates may be low; the
latencies may be large, connectivity may be
intermittent, high mobility in combination with
Manuscript was received on March 12, 2011, and then improved on May
15, 2011 based on comment of Prof. G. Q. Maguire Jr.
weak signal strength and aggravating
This work was done while Authors were master student of Communication circumstances result in high link-error rate
Systems, at KTH Royal Institute Of Technology. Laili Aidi contributed for
part II History, part III Driving Force, part IV Protocol and Overlay making end-to-end reliable communication
Architecture, part VII Routing, part IX Technology and part X difficult. Furthermore, due to the intermittent
Implementation. Jung Changsu contributed for part I Introduction, part V
Network Architecture, part VI Bundle and Encapsulation, part VIII Security, connectivity, data rates may be low or highly
part XI Study Case: KioskNet System, and part XII Conclusion.
The work was submitted to Prof. G. Q. Maguire Jr. as assignment of
asymmetric; the return channel may be
IK2555 - Wireless and Mobile Network Architectures class. The authors are unavailable, and the elapsing time between a
solely responsible for the contents of this work.

2. 2
request and response may be hours rather than long usage, due to environmental dangers or
milliseconds, and communications that do power exhaustion. It is not feasible to utilize
arrive may exhibit a high error rate [19]. conventional end-to-end acknowledgment
• Disconnection. The lack of end-to-end schemes to verify delivery, because the
connectivity may be more common than network may be disconnected for a long period,
connectivity, due to high host or network in fact the round-trip or one-way delivery time
mobility (satellite passes, moving vehicle, etc). may exceed the sending node’s lifetime.
Motion of other objects or interference, Therefore it is better to delegate the delivery of
opportunistic (nodes arrive in communication traffic to another to any other party.
range due to random walk), dynamic • Low Duty Cycle Operation. The transmission
membership of individual nodes, low-duty- schedule should receive a special consideration
cycle, and unpredictable operation are common in the routing decision, as the duty cycles of the
attributes (for example in low-capability node may be low, in order to achieve
devices such as might be used in sensor reasonable longevity of the entire network.
networks). Thus, the communication pattern is often
• Long and Variable Delay. The queuing time scheduled in advance due to the limited power.
in this network could be extremely large, hard • Limited Resources. There is frequently
to estimate, and source-initiated transmission limitation in memory and processing capability
might be expensive because it is limited [3]. in the node, hence, if the network is designed
Thus, there is a need to store data in a buffer or for reliability, then the end-node should empty
queue for a potentially long period at each their retransmission buffers quickly, rather than
router, if there is no direct path to the wait for an end-to-end acknowledgment.
destination node [19].
2. Network
• Interoperability Consideration / Intermittent
Connectivity. Interoperability on a large scale
is rarely designed in challenged networks [3].
This is because these networks tend to be
simple and local in scope. Partitions may occur
because of geographic distance, lack of radio
signal strength or other factors [19]. Because
they are deployed on limited memory and
power devices, thus cross communication all
over that links has not become a primary
feature. There is the problem of frequent
failure, low reliability, and or congestion. Thus,
there is no guarantee of discontinuous end-to- Figure 1. A Challenged Network’s Characteristics
end connectivity. [3]
• Security. The approach to only secure the
endpoints of the network is not sufficient due to
B. Interoperability
the link capacity limit. Thus, the access to the
service should be protected at the earliest point TCP's handshake and slow start mechanism are
in the topology. sources of further obstacles in a challenged network
3. End System with a long-delay. Although there are other
• Limited Longevity. The end node, which improved protocols (SCTP, HSTCP, etc), that can
frequently a highly integrated, low-power multiplex units of application data for multiple
consumption, low-cost device, may not last for sessions over a single-layer connection
(association), multiple round trips are still required

3. 3
in order to transmit application data for session case of session oriented protocols), but a store-and-
setup [12]. There are several approaches that could forward DTN isolates the delays and can hide this
adapt the Internet to challenged networks unusual intermittent connectivity at the cost of storage,
characteristics [2]: potentially duplicated messages, and increased
1. Using Link-repair approach. This approach delay.
enables Internet protocols to operate over a This type of DTN is based on in-network
comparatively well-performing physical storage, retransmission, name based late binding,
infrastructure, by maintaining the end-to-end and routing that is tolerant of network partitioning
reliability and fate-sharing model of the Internet. [2]. The delivery semantics mechanism,
2. Attaching network-specific proxy agent at the asynchronous messaging and postal mail as Class of
edge of the Internet. This approach provides Service (CoS) are also widely used in the current
access from the Internet to the challenged network applications such as voicemail and email
network and vice versa. [14]. Furthermore, the links in such a DTN are
expected to be diverse, including Radio Frequency
Moreover, in order to achieve interoperability (RF), Ultra-Wide Band (UWB), and Free-Space
between large diverse networks, with extreme optical, and/or Acoustic (Sonar or Ultrasonic)
environments and network partitioning, these technology [3, 7].
approaches above will not be adequate or desirable
[2]. Attaching a proxy agent does not provide a IV. PROTOCOL AND OVERLAY ARCHITECTURE
solution when using a challenged network for data A. Bundle Layer
transit purposes, such as to access remotely This bundle layer architecture was proposed by
deployed conventional networks (Intranet) via Kevin Fall, before it was developed into RFC 4838
challenged networks (as an intermediate network). by the DTN Research Group [3, 14]. As can be seen
Thus, there is a need to define a new standard in figure 2, in this DTN architecture, there is a
that provides end-to-end communication through bundle layer that ties the Application and Transport
multiple regions in a disconnected network, which Layer and all other low-layers into region specific
can tolerate errors and large variable-delay layers. Each regional layer may be a different type
environment using a generalized suite of of network, but the bundle layer enables them to
protocols. Another approach has been suggested, communicate regardless of the network types, by
based on a message-oriented reliable overlay transmitting bundles using store-and-forward
architecture, forming an internetwork of challenged message switching mechanism across or on top of
Internets [2]. The design is based on the the various regional layers [1, 3, 16].
interoperability properties of the classical Internet
design, the robust non-interactive delivery
semantics of electronic mail, and a subset of the
classes of service of a postal system.
C. Store-and-Forward message switching
A store-and-forward based DTN was originally
designed to support an Interplanetary Internet
(IPN). It is designed to operate above the existing
protocols in various network architectures and to
use store-and-forward message switching, where the
data is transferred from a node, held until this node
has a scheduled transfer, and then forwarded it to
another potentially dissimilar network [2].
Intermittent connectivity in the Internet can lead to
packet loss and even termination of a session (in the Figure 2. The Bundle Layer [3]

4. 4
B. Licklider Transmission Protocol
This approach enables to communication across Licklider Transmission Protocol (LTP) is a
multiple regions without any given effect on the retransmission-based reliability protocol that runs
upper-layer application. This Bundle Service over a link with extremely long message round-trip
Layering can provide general-purpose delay- times and/or frequent interruptions in connectivity
tolerant protocol services: custody transfer, [12]. This protocol is designed as a reliable
segmentation and reassembly, end-to-end reliability, "convergence layer" protocol, underlying the DTN
end-to-end security, and end-to-end routing. Below Bundle protocol [22]. LTP is point-to-point
are the different CoSs, which are provided by DTN oriented, while the Bundle protocol moves bundles
Bundle Layer [3]: end to end.
1. Custody Transfer is the acknowledgement of a
custodial-acceptance from a node to its previous V. NETWORK ARCHITECTURE
custodian. This is necessary to implement of A. Region
retransmission responsibility delegation, so the
As the objective is to interconnect different
sender node can transfer its retransmission
networks asynchronously, a DTN can utilize the
responsibilities to another node, this freeing
regional networks, where each network is seen as a
resources to be used for another bundle.
region with its specific communication protocol [3,
2. Return Receipt is the confirmation to the source
7]. Region is used to interconnect boundaries
or reply-to node that the destination node has
between nodes in different network protocols,
received the bundle.
addressing standard. It is identified by a region ID,
3. Custody-Transfer Notification is the
which is knowable by the other regions of that DTN
notification to the source or reply-to node
[3].
whenever a node accepts a bundle’s custody
transfer.
4. Bundle-Forwarding Notification is the B. Node
notification to the source or reply-to node Each node in a DTN might be a host, router, or
whenever a bundle is forwarded to the next node. gateway. These entities act as source, destination, or
5. Priority of Delivery with a value of: Bulk, forwarder [3].
Normal, or Expedited. 1. Host. A host sends or receives bundles (i.e., it is
6. Authentication is a procedure to verify the the source and or destination of bundle transfer),
identity of the sender and message’s integrity. and requires storage to queue bundles. It needs
optional custody transfer capacity for
The figure below illustrates some of CoSs retransmission [3].
described above: 2. Router. A router forwards each bundle to
another node in the same DTN region, and may
optionally support custody transfer. A router
requires storage to store incoming packets before
forwarding these to outgoing links because [2]:
• There is no guarantee of that next hop link is
currently available.
• Asymmetric data rate between sender and
receiver
• Retransmission due to the high error rate link
3. Gateway. A gateway is an interconnection point
that forwards bundles to other DTN regions with
different protocol stacks by supporting
Figure 3. DTN CoS [3]
interoperability. This gateway must have storage

5. 5
for reliable delivery and perform mapping VII. ROUTING
between different transport layers. It also A. Name and Address
performs authentication and check arriving data
DTN adopts name tuples, which consist of two
before forwarding it [2, 3].
variable length strings as addresses for delivering
VI. BUNDLES AND ENCAPSULATION messages to its destination. Name tuples have the
form of: {Region Name, Entry Name}. The region
A. Bundles name is globally unique and translated in DTN
Bundles are messages, which have a bundle gateways to route messages to the specified region.
header, control information, and a source- Using its hierarchical region structures can reduce
application’s user data. Control information the size of DTN forwarding tables and support
specifies the ways of handling, storing, and additional flexibility due to the variable length
disposing of user data. The Bundle layer inserts the strings [2].
bundle header. This header is intended for The entity name is an identifier and can be
fragmentation of the message [3]. resolved within the specified region, thus it does not
need to be globally unique [16]. When messages
B. Fragmentation traverse heterogeneous regions, only its region
The bundle in the Bundle layer may be divided name is used for identifying its destination region.
into several bundle fragments (smaller routable The and then entity name is only translated within
units so as increase improve the possibility of the destination region. This late binding has two
delivery and increase performance [22]. These advantages in DTN source nodes [5]:
fragments will be reassembled at the final 1. The source node can generate and deliver
destination. The message fragmentation in DTN is messages without any knowledge of each
based on dynamics mechanisms, which could be different regional identifier systems. As a
[14, 20]: consequence, various regions can add new
1. Proactive fragmentation. This approach is used naming and addressing systems without
where the contact volume can be predicted to changing their regions.
optimize that contact’s usage. The application 2. There is no delay for mapping the destination to
data may be separated into smaller blocks and a globally unique address when generating the
transmitted as independent bundles. Only the message.
final destination is responsible for reassembling
these bundles. B. Knowledge Oracles
2. Reactive fragmentation. This approach is used There are 2 important terms when talking about
when there is a disconnection while transmitting DTN routing [14]:
fragmented bundles. When a node receives a 1. Contact means a period of time (interval) during
partially fragmented bundle, this node marks the which network connectivity is strictly positive,
bundle as a fragment and carries out normal and the delay and capacity can be considered to
forwarding. The previous-hop node can be constant. There are several types of contacts:
recognize the delivery of abnormal • Scheduled Contact. A scheduled contact may
fragmentation via a convergence layer protocol exist between a base station somewhere on
and it creates a reactive fragment to send the earth and a low earth orbiting relay satellite, as
remaining bundles to eliminate waste of partially it can be predicted when the link between them
fragmented bundles [14, 22]. will be available and for how long it will be
available.
• Opportunistic Contact. Opportunistic contact
occurs when two entities are present the same

6. 6
place, where there is neither scheduled nor distributed system, because the queuing oracle is
predicted contact available. affected by both new arriving messages and the
• Predicted Contact. A predicted contact is not routing algorithm choices [33].
scheduled, but predictions of its existence can 5. Traffic Demand Oracle can answer any
be made by analyzing previous observations. questions regarding the present or future traffic
2. Contact’s volume means the product of the demand. It provides the set of messages injected
capacity and the duration of the contact (i.e., the into the system at any time [33].
volume of data that can be transferred during the
contact). Moreover, Group Membership Oracles to
encapsulate particular knowledge about the group
There are also sets of abstract knowledge dynamic, as described below:
oracles, which encapsulate particular knowledge 1. Local Membership Oracle can only answer
about the network [33, 34]. These oracles are questions regarding group membership of the
defined in order to understand the fundamental node itself [26].
trade-off between performance and knowledge, 2. Delayed Membership Oracle can answer any
which is required by different routing algorithms questions regarding membership of an endpoint
[33]. If contacts and its contact’s volumes are at a specific time [26, 34].
known ahead of time, intelligent routing and 3. Complete Membership Oracle can answer any
forwarding decisions can be made (optimally for question regarding membership of all nodes at
small networks). Below are the set of abstract any time [26, 34].
Contact Knowledge Oracles:
1. Null Contact Oracle, this happen when no Based on those oracles, we can classify the
question about contact opportunity can be routing algorithms in DTN into several classes [33]:
answered, it represents no knowledge about 1. Zero Knowledge is a class of algorithms that do
network topology [26]. not utilize any oracle, thus they may perform
2. Contacts Summary Oracle can provide long- poorly.
term network topology or contacts aggregate 2. Complete Knowledge is a class of algorithms
statistics, thus enabling calculation of the that utilizes all the oracles (contacts summary,
average waiting time until the next contact. This complete contact, queuing and traffic demand).
racle only has partial knowledge; hence it can 3. Partial Knowledge is a class of algorithms that
respond with time-invariant or summary uses one or more of the other oracles
characteristics about contacts (for example, the (congestion, queuing). The message is routed
expected average time between contact independently based on the future traffic
occurrences and average contact duration) [26, demand.
33, 34].
3. Complete Contacts Oracle can answer any C. Strategy
questions regarding network topology or contacts
The traditional routing objective is to select a
between two nodes at any time. Thus it can
path that minimizes some simple metric (e.g. the
specify the exact time when a contact will occur,
number of hops). However, the most desirable
the duration, capacity and delay of the contact,
objective of routing in DTN is not immediately
etc. A contacts summary oracle can be obvious, although the natural objective is to
constructed using the Contacts Oracle, but not maximize the probability of message delivery
vice versa [26, 33, 34]. because of its challenges [33].
4. Queuing Oracle can give information about There is a need to define a new routing protocol
instantaneous buffer occupancies (queuing) at for the DTN architecture, because the assumptions
any node at any time and route around congested (continuous connectivity, low delay, and low packet
nodes. It is the most difficult oracle to realize loss), which are used in traditional routing protocols

7. 7
(distance vector or link state) are no longer valid in total traffic generated (both data and control
DTN network [16]. The DTN routing topology is a packets) in the networks;
time-varying multi-graph where there is no 4. Average message delay, defines the average of
guarantee of the availability of the next hop's link the end-to-end bundle delivery latencies for each
for a certain or long time period [20]. In this algorithm.
architecture, the data rate between sender and
receiver could be asymmetric, as one node might be The routing strategy in DTN itself is classified
much faster than another and the characteristic of into 2 categories, which is implemented in different
high error rate link might also cause retransmission. DTN, where each of them has different
DTN Nodes route the bundle along the path. As characteristic and numerous approaches for its
illustrated in figure 4 below, the DTN Gateway has routing protocol, as shown below [16]:
the same double-stack as a DTN Router. However, 1. Deterministic Routing. This strategy is build
the gateway utilize different lower-layer protocols based on the assumption that the next nodes and
allowing them to bridge between regions that use the connection between them are known. Thus,
different lower-layer protocols and taking inter- protocols that use this strategy are implemented
region routing responsibility, while the DTN Router in deterministic or predictable topologies.
supports intra-region routing [3]. DTN routers need 2. Stochastic Routing. This strategy is built based
storage for queuing because of the fundamental on assumption that the network behavior is
limitations discussed above. unpredictable. The protocols that use this
strategy depend on decisions regarding where
and when to forward messages and implemented
in stochastic and time-evolving topologies. A
simple approach could be just forwarding the
message any node that is reachable, or based on
history data, mobility patterns, etc.
More specific Unicast routing techniques in
DTN and comparisons can be found in [27] and
[33].
Figure 4. DTN Protocol stacks and routing [3]
Routing computation in DTN becomes D. Anycast and Multicast
challenging because the delivery paths through the The goal of multicast routing is to reach all
graph are lossy, and contact intervals and volumes nodes in the group, while Anycast routing is to
are unknown precisely ahead of time [14]. These reach at least one node from a particular group.
issues still become an active area in the (emerging) Thus, both of them need mechanism to guide
research of DTN. The performance of different replication, forwarding, and buffer management
routing algorithms can be compared using decisions [31]. Anycast and Multicast in a DTN are
performance metrics, as shown below [17]: challenging due to unpredictability of network
1. Message delivery ratio, defines number of connectivity, long delivery delay, and limited
unique multicast bundles that successfully arrive storage capacity characteristic [26, 30]. They also
completely at all the receivers over the total have to deal with dynamic group membership,
number of bundles which are expected to be
because the group membership may change during
received.
the bundle delivery; introducing ambiguity in
2. Data efficiency, defines ratio between the
unique bundles received by the receivers and the Anycast and Multicast semantics. There are
total data traffic. several semantic models in DTN for Anycast and
3. Overall efficiency, defines ratio between the Multicast [26, 30, 34]:
unique bundles received by the receivers and 1. Temporal Interval Membership (TIM) /
Temporal Membership (TM) model:

8. 8
membership is determined by the time interval network resources, because only a single copy of
(membership interval), thus this model is a time- the message can exist in network storage [33].
based definition of group membership. The message can be held until the destination is
2. Temporal Point Membership (TPM) / found or be forwarded through intermediate node
Temporal Delivery (TD) model: the via a utility metric [31]. However, in general,
membership is determined by both the this technique also limits the message delivery
membership interval and delivery interval rates in many DTN. There are several
that indicates the time period during the approaches based on this technique:
message should be delivered to the intended • Expected Multi-Destination Delay for
receivers. Anycast (EMDDA) that utilizes the
3. Current Membership (CM) / Current-Model uncontrolled random movement of the node.
Delivery (CMD) model: the message should be The Anycast routing is determined by
delivered to the node that is a current member of evaluating different routing metrics (Practical
the group when the message arrives to it. This is Expected Delay / PED) for selecting
what makes this semantics different from the forwarding nodes [30]. This metric
other two above, where receivers are not characterizes the expected delay of taking
required to be group members at the time of different paths with corresponding probability
message delivery. of connectivity between the nodes. However,
this approach does not consider network traffic
The figure below illustrates semantic models during its routing selection and assumes all
described above: nodes are stationary, except for a few mobile
nodes that act as message carriers, thus
providing a very constrained environment for
evaluation.
• The Anycast genetic algorithms to make route
decisions [32]. This approach assumes that all
mobility is deterministic and known ahead of
time, which is not always true for DTN [31].
2. Multi-copy techniques / Replication based.
This technique increases the message delivery
rates, since multiple copies of a message exist in
Figure 5. DTN Semantic Models [26, 34]
the network. However, it uses more network
resources and inherently is not scalable. There
The important point in order to achieve Anycast
are several approaches based on this technique
in DTN is to expose knowledge to the routing
[31]:
protocol about the groups in the network so it can
• Flooding-based protocols, appropriate in non-
directly act based on that knowledge
resource-constrained environments, thus
[31]. Different from Unicast DTN, where the
approach does not place a limit on the number
bundle destination is determined when it is
of times a message can be replicated, and
generated; in Anycast DTN, the destination can be
focuses on smart buffer management and
any one of a group of nodes and both the path to a
transmission ordering techniques to handle
group member and the destination can change
potentially large numbers of replicates.
dynamically during routing the process, depending
Example protocols include as Epidemic,
on the current device movement situation. There
ProPHET, MaxProp, and RAPID [31].
are several Anycast routing strategies that have
• Quota-based protocols, suitable for resource-
been proposed [31]:
constrained environments, thus there are a hard
1. Single-copy technique / Forwarding-based.
limit on the number of times a message is
This technique is generally much less wasteful of

9. 9
allowed to be replicated. This is done by transmitted to the next hop. The destination node
attaching a quota flag to every message, so the will de-capsulate the message and forwards the
total number of replicas from a message never original multicast message to the intended
exceeds its original quota. Example protocols receiver according to the message's delivery
include Spray and Wait, Spray and Focus, and constraints. This approach has the worst delivery
EBR [31]. ratio and routing efficiency because it sends a
separate copy of the messages to each receiver,
Multicast DTN has a semantic model where the which significantly increases contention for node
bundle needs to be buffered in the node's storage storage and transmission opportunities, and
until it is deleted because of buffer overhead or time results in message drops.
expiration [26, 34]. This approach reduces delay 2. Broadcast-Based Routing (BBR) / epidemic
and improves bundle availability. Thus, other nodes routing, here the message will be flooded
(except the source) can handle the join request and a throughout all the nodes in the network in order
node can send its buffered bundle to the new to reach the intended receivers [26, 34]. This
members when contact is available. This is approach achieves the highest delivery ratio that
necessary to overcome network partitions and does not require any knowledge about contact or
handle delay of join requests. The node also membership [26]. It also has the lowest delay
maintains the local forwarding state for each of its because messages are flooded to all nodes, hence
buffer bundles to determine the route or next hop always following the shortest path [34].
[34]. This Forwarding State is computed and However, it has the lowest routing efficiency
updated based on the node's Contact and Group because of message redundancy.
membership knowledge. This forwarding state is 3. Tree-Based Routing (TBR), here the message is
necessary because the data is forwarded in the units forwarded along a tree that is rooted at the source
of a bundle and intended receiver depends on the and reaches all receivers [26, 34]. The message
semantic model that is used. is only duplicated at nodes that have more than
The traditional Multicast methods for Internet one outgoing path.
and mobile ad hoc networks are not suitable in 4. Group-Based Routing (GBR) uses the
DTN, because of the frequent network partitions forwarding group concept, in order to increase
and sparse connectivity among nodes making the of the chance of delivery. Thus the message is
complexity to maintaining a source-rooted multicast flooded within the forwarding group [26, 34].
tree during a multicast session too high. Along with BBR, this approach also achieves the
Additionally, the application data would suffer from highest delivery ratio, because the message may
large end-to end delivery latencies [17]. Moreover, be forwarded to receivers via multiple paths,
the traditional approaches may fail to deliver a which better exploits available contact
message when the link is highly unavailable. There opportunities (contact summary oracle).
are several existing routing approaches for
supporting multicast communications in a DTN: The figure below illustrates routing approaches
1. Unicast-based routing (UBR) / Unicast- described above:
Multicast (U-Multicast), here the Unicast
transfer mechanism is used to realize a multicast
service, for example the source sends a copy of
the message to every intended receiver, this
message encapsulates the original multicast
message [17, 26, 34]. The source node buffers
the multicast message and sends new Unicast Figure 6. DTN Routing Approach: (a) UBR (b)
messages when it learns of new intended BBR (c) TBR (d) GBR [26, 34]
receivers, then removes this message after it is

10. 10
5. Static tree-based routing (STBR) constructs
the static shortest path tree from the source to the
estimated intended receivers of a message,
starting at the message generation time [26, 34].
Thus, if a message misses a contact with a node
then it needs to wait for the next opportunity to
connect to this node, which may significantly
increase the message delay and disallows nodes
from utilizing more accurate local information to
the forward message using a better path.
Figure 7. DTBR and OS-Multicast [17, 24]
6. Dynamic Tree-Based Multicasting Algorithm
enables the node to dynamically determine the
7. Context Aware Multicast Routing (CAMR)
next-hops of a message based on its view of
[17] is a density based adaptive multicast routing
network conditions (local queuing information or
scheme, which uses opportunistic connectivity
newly available contact information) [34]. Since
and additional information, for example, node
this algorithm can adapt to network conditions, it
location and node velocity. This scheme
performs slightly better than STBR.
increases average message delivery delay.
• DTBR (Dynamic Tree-Based Routing), here
However, it provides efficiency and high
the upstream node assigns the receiver list for
delivery ratio with reasonable data efficiency,
its downstream nodes based on its network
especially when the network becomes sparser.
condition view [26]. The downstream nodes
can only forward bundles to the receivers in the
list. However, this technique assumes each
node has complete knowledge or summary of
the link states in the network. Unfortunately,
this is hard to achieve in practical scenarios.
• On-demand Situation-aware multicast (OS-
multicast), here a unique multicast tree is
constructed for each bundle and the tree is
adjusted at each intermediate DTN node
Figure 8. Conceptual performance of DTN muticast
according to the current network conditions
routing approaches in different levels of knowledge
[17]. The node dynamically adjusts the initially
[17]
constructed tree when it receives a bundle,
based on its view of network condition views.
Thus it has a smaller delay, better message VIII. SECURITY
delivery ratios, any newly discovered path will
be quickly utilized, and achieves higher A. Issues
efficiency when the probability of link Typically a DTN has very limited resources such
unavailability is high and the duration of link as transmission bandwidth, storage, and processing
downtime is large. However, the downside of cycles. Therefore, some restrictions should be
this approach is that, the receiver may receive placed on accessing this network and delivering
multiple copies of the same bundle and relies messages without authorization and authentication.
on opportunistic connectivity among nodes for Moreover, even authorized applications should be
delivery. restricted when they attempt to access services that
they are not allowed to use. In DTN, there are two
different security aspects that should be considered,
these are LTP security and Bundle protocol security
[16].

11. 11
1. LTP Security. LTP is a point-to-point protocol bundles and create bundle-specific signature. After
thus the upper or lower layer will handle most verifying the sender’s identity and CoS in the
security concerns. For this reason, LTP only forwarding node, the forwarding node replaces the
supports data integrity with LTP authentication sender’s signature with its own and forwards the
and LTP cookie mechanism, which uses a data [3].
random number to make a DoS attack more
difficult [16].
2. Bundle Layer Security. The Bundle Layer is
vulnerable to attack because it is an overlay
network. Thus the bundle layer encounters many
threats and should consider lower layer’s
security issues [23]. Figure 9. Security steps [3].
The bundle layer protocols can be attacked by
multiple underlying network components because C. Bundle Security Protocol Specification
bundles may travel across multiple networks. In the “Bundle Security Protocol Specification”
During this journey, bundles are modified [29], the functionality of data integrity and
intentionally and new bundles are inserted by confidentiality are provided as the bundle security
underlying networks. To prevent unauthorized protocol. There are four security blocks in this
insertion of bundles, DTN nodes should have the specification [29]:
ability to detect and delete these unexpected 1. Bundle Authentication Block (BAB), supports
bundles [23]. Network accesses and resource a function for assurance of the authenticity and
consumptions from unauthorized objects can cause integrity of the bundle travelling along a single
serious threats because of the node's limited hop from forwarder to intermediate receiver. The
resources, particularly storage and battery power operation of this block is to protect a bundle on a
[23]. The bundle layer can be exploited by hop-by-hop basis unlike other security blocks.
increasing resource consumption, for example by a Currently BAB only defines a shared-key Hash
DoS attack. Due to the long latency in DTN, the Message Authentication Code (HMAC) and the
damage may be worse than traditional networks. To key does not need to be unique but is only
avoid unexpected resource consumption, we should required to be shared between nodes.
only accept authenticated messages and drop others 2. Payload Integrity Block (PIB), the authenticity
[16, 23]. and integrity of the payload from the PIB
security-source to the PIB security-destination.
Any node, which is located in between the
B. Bundle Security
security-source and the security-destination, can
Authentication is carried out in forwarding nodes examine the authentication information.
(routers and gateways) and these nodes also verify 3. Payload Confidentiality Block (PCB) specifies
the authenticity of sender’s information to protect that the encryption of the payload be done
network resources from unauthorized traffic as early wholly or partially by the PCB security-source
as possible. This also differentiates DTN networks for the purpose of protecting the content while
from other networks, which mutually authenticate being transported to the PCB security-
the user identities and message integrity, but the destination.
router that forwards the traffic itself is not 4. Extension Security Blcok (ESB) provides
authenticated [3]. security not for payload blocks, but rather for
The DTN also has a unique mechanism when non-payload blocks in a bundle so ESB is not
using public-key cryptography, where both users applicable to PIB and PCB that are related to
and forwarders have their own key-pairs and payload security blocks. The ESB is located in
certificates. A sender uses its private key to sign

12. 12
the same position where it should be protected in implementation is intended to reduce complex
the bundle [29]. operations by the user, enabling them to perform
data communication operations seamlessly and
D. Open Issues in Bundle Security more effectively in terms of delay, intermittent
environments, etc. This is not possible using the
The bundle security protocol is still under
TCP/IP based architectures [7].
development and some critical issues still remain.
Currently, the DTN architecture research is
We will present some issues for the bundle security
carried on by several research groups, such as [3,
protocol below [23]:
15]: The Internet Research Task Force’s Delay-
1. The level of flexibility. The bundle security Tolerant Networking Research Group (DTNRG),
protocol can combine applications of the
The InterPlaNetary (IPN) Internet Project, NASA
confidentiality and integrity services flexibly but
Jet Propulsion Laboratory (International Space
it prevents insecure combinations of application Station Research), Google Laboratory, Intel
such as including plain-text signatures.
Research Corporation, SPARTA, The MITRE
Therefore, the level of flexibility is an open issue
Corporation, Distributed Systems Group - Trinity
in the bundle security protocol. This flexibility
College Dublin, ISTRAC - ISRO, University of
may support a Virtual Private Network (VPN),
California - Berkeley (UCB), University of
but the complexity can cause high costs for California - Santa Barbara (UCSB) , University of
implementation and be insecure [23].
Southern California (USC), Helsinki University of
2. Key Management. No key management
Technology, Luleå University of Technology,
schemes exist in DTN deployments. In fact, University of Massachusetts Amherst, etc.
existing schemes need hard coding. Additionally,
The wireless DTN technology also may be
replacement of trusted keys in all nodes makes it
diverse and implemented using several
difficult to adopt this scheme. One possible
technologies, such as [3]:
solution is to adopt the resurrecting duckling
1. Radio Frequency (RF)
scheme that is suggested for ad-hoc networks. If 2. Ultra-Wide Band (UWB)
a node has a “close encounter” with another
3. Acoustic (Sonar, Ultrasonic)
node, the node can exchange a key through a
4. Free-space Optical Communications (FSOC) is
trustworthy channel and store it for later use. If an extreme example of the directional antenna
DTN nodes have enough space in their storage,
mobile ad hoc network (MANET) [25]. The
these nodes may flood public keys whenever
networking design issues in FSOC come from
nodes encounter each other [23]. the challenges in pointing, acquisition, and
3. Canonicalization of bundles. The definition of
tracking. It becomes extremely difficult because
a canonical bundle form is an open issue for data
of long ranges and mobility on rugged terrain,
integrity. The sender and receiver require the
and there is a resultant resource allocation
same bytes of bundle of signature for integrity,
problem's precision of pointing requirement,
but the bundles may be changed during traversal need to be done with an optical laser head in one
between nodes [23].
connection service. Thus, there is a need in
IX. TECHNOLOGY FSOC for topology control. Since the DTN
approaches are designed to overcome the
A. Model and Feasibility network with intermittent connectivity, thus it
The trend of DTN seems to be more of an can be fundamental to solve the FSOC
analytical research, mobility model, etc [11]. The networking problem.
ongoing work is to extend the DTN architecture to
B. Relevant Standard
smart mobile phone-based mobile ad hoc networks
(MANETs) so that a node can effectively use 1. Homing-pigeon-based DTN (HoP-DTN), an
multiple communication links and networks. This experimental method in RFC 1149: Standard for
the Transmission of IP Datagrams on Avian

13. 13
Carriers [8] and RFC 2549: IP over Avian DTN is used to increases the robustness of the
Carriers with Quality of Service [9] communication network and timeliness of data
2. Licklider Transmission Protocol - Security returned from operating space assets, so it can
Extensions, IETF RFC 5327, experimental [10] reduce risk and cost, increase safety and science
3. Licklider Transmission Protocol - Specification, return, and improve operational awareness [15].
IETF RFC 5326, experimental [11] Additionally, DTN can reduce human labor costs
4. Licklider Transmission Protocol - Motivation, through automation of communications operations.
IETF RFC 5325, informational [12]
5. Bundle Protocol Specification, IETF RFC 5050,
experimental [13]
6. Delay-Tolerant Network Architecture, IETF
RFC 4838, informational [14]
X. IMPLEMENTATION
The development of DTN networks has been
more sophisticated and detailed in terms of
protocols and approaches, analysis of stateless Figure 10. The Challenged Network Examples [3]
routing algorithms and has thus far found no
commercial use [11]. DTN is currently being 2. Terrestrial Civilian Network. Even though the
studied in space networks and other research Outer-Space implementation is the primary
environments (such as Earth Applications) in order beneficiary of the DTN research, many terrestrial
to exploit stressed, disconnected, disrupted network implementations are used and
networks, preventing failures, enhance safety and contributed to DTN research as well:
security, and to learn new knowledge [15]: • Drive-by Vehicular and Ferry based
1. The Outer Space / Deep-Space Networks: Networks [21] include DakNet, Message ferry
Inter Planetary Networks. (hybrid between MANET and DTN), Village
The Interplanetary Internet (IPN) is a computer network.
network in space, and an example of wireless • Mule Networks / Node in a box: Hagle,
network outside of the Internet [3]. The Zebranet (tracking wild animal in wildlife,
communication outside of the Internet is done by manage their habitat effectively by attaching
using independent networks, that each might be run wireless sensor node, collecting location data
on power-limitation, specialized communication and opportunistically reporting their history
that is not be able to exchange information. They when they are in radio range of base station [2,
also have different link delay and connectivity, 19, 22]), Sámi Network Connectivity (Reindeer
data-rate asymmetry, error rates, addressing and herd tracking by the Saami tribesmen in Arctic
reliability mechanism, QoS and trust boundary, etc Circle [22]), SWIM, Mobile Ad Hoc Network,
[3]. AUDTWMN (Water monitoring application
The IPN is defined as a network of regions, such Test bed for DTN research [22]), Carrier
as a terrestrial Internet region, a surface of the Pigeons (Implemented by Bergen Linux users
planet region, or ground-to-orbit region, etc. Each group: RFC 1149, RFC 2549).
region has its own communication in terms of • Challenge Link / Flakynet, for example in the
security, resource maintenance, etc [6]. It is a store- Remote / Developing region: Tier, Seismic
and-forward network that runs over interplanetary monitoring (Early warning system against earth
distances, might be disconnected, and run over a quakes, volcano and landslides [22]), SenDT
wireless backbone with error-prone links and delays (implemented by Trinity College Dublin
ranging to minutes or hours, when a connection Ireland to monitor lakes in Ireland [22]), UUCP
even exists [4]. [21].

14. 14
3. Mobile Sensor Networks (Acoustic KioskNet challenged to make a robust system
underwater networks). This network is with two key concepts. The first one is the adoption
designed to exist in a large-scale area and nodes of a single-board-computer, low-cost and low-
per network [5]. It has the characteristic of an power kiosk controller for wireless communication
extremely limited end-node power, memory, using a vehicle. A vehicle delivers data to a gateway
CPU capability and scheduled communication or receives data from a gateway. This ‘mechanical
between nodes. There is also an implementation backhaul’ can allow Internet access without the cost
of “proxy” nodes in the network in order to of equipments such as satellite dishes and towers in
translate the data in the Internet protocols to its remote areas. Second, KioskNet uses refurbished
native protocols. PCs, which use boot images from the kiosk
4. Wireless Military Tactical / Battlefield controller that can offer a very secure boot images
networks. The network may run in environment virus-free. In addition, the refurbished PC does not
with several causes for disconnection, such as have a hard disk to avoid a hard disk failure and
mobility, environmental factors, intentional viruses. Aside from two key concepts, KioskNet has
jamming, etc [5]. There is a strong requirement a few characteristics like low-cost
for infrastructure protection and the sensor or (70$/kiosk/month), low power (6~8W), a LiveCD
field agent may be deployed over limited and free software [24].
wireless coverage. The challenge also increases
because the data traffic may be delayed as it may B. Component
have to compete for a limited bandwidth with
Below are the components of KioskNet network:
other high priority services, and there is a
1. Kiosks.
concern in the battery and wide physical
Every kiosk has its own kiosk controller. A kiosk
dispersant of agent. Thus, wireless connectivity
controller utilizes recycled PCs to provide some
could not be continuously maintained among all
functions, those are, a network boot function, a
agents [15, 19].
network file system, user management, and network
connectivity through dial-up, GSM/GPRS, Very
XI. STUDY CASE: KIOSKNET SYSTEM Small Aperture Terminal (VSAT), or mechanical
backhaul. A kiosk controller is constantly possesses
A. Overview a wireless network interface or other connectivity,
The KioskNet system was developed by the which are mentioned above [24].
University of Waterloo for providing low cost This system considered two types of users who
Internet services to the poorest villages of can access a kiosk controller for their connection.
developing countries using the DTN concept. This First, most users are expected to use cheap recycled
system uses vehicles to deliver data from villages to PCs (terminals) to connect the system. In this case,
Internet gateways and provides various kinds of a kiosk controller provides these diskless PCs with a
services to rural residents, for example, birth, network boot image and applications by means of
marriage, and death certificates; medical Network File System (NFS). Second, some other
consultation, and agricultural problems. This system users who are government officials, NGO members
should have some essential requirements for the or wealthier residents access kiosks with their own
reliable connections and the low cost of mobile devices. Unfortunately, this system’s
maintenance. Especially, the cost is a very critical software does not support connectivity to these
issue for sustaining this system in those remote users because of some technical issues.
regions. Moreover, deploying this system had great If kiosks are located in the same geographical
challenges because of many obstacles such as area, these kiosks consist of a KioskNet region for
limited electrical power, dust, mechanical damages, routing and certification [24]. Figure 11 represents
computer viruses, frequent failure of kiosk that a single server entity in KioskNet administrates
computers and network connections [24]. two regions in this system.

15. 15
2. Ferries.
This system defines ferries as many types of
vehicles like a car, buses, motorcycles or trains by
means of supporting various connectivity options. A
ferry has a cheap computer powered by a vehicle’s
own battery and the computer has a 20-40GB hard
disk and a WiFi network interface. Ferries contact
with kiosk controllers and gateways using
opportunistic ways for the time duration of 20
seconds to 5 minutes. During this communication Figure 11. KioskNet overview [36]
period, ferries can transfer 10-150MB of bundles
using a store-and-forward mechanism [24]. C. Security Architecture
The ultimate security goal of this system is to
3. Gateways.
offer the best possible security service to the whole
A gateway is always connected to the Internet
components of KioskNet such as the infrastructures,
through DSL or broadband Internet access. This
users and terminals. To meet security requirements
connection of a gateway is possible with its WiFi
and reliable operation, this system needs four
network interface. Moreover, it has storage to
distinct entities, which are KioskNet Franchisers,
receive data from a ferry and to upload the data to
KioskNet Franchisees, KioskNet Users and
the proxy through the Internet [24].
Application service providers [24, 28].
1. Entities. We present Security Entities of this
4. Proxy.
system as below:
This system is expected to support
• KioskNet Franchisers. Franchisers are public
communication between a kiosk user and the
or private organizations such as non-
Internet but legacy servers cannot provide
governmental organizations (NGOs). The
connectivity when the network has long delays and
franchisers own and check the integrity of their
disconnections like this situation. Therefore, a
KioskNet infrastructure components such as
special proxy is needed for supporting
gateways, ferries, Kiosk controllers and
communication with a legacy server. A proxy
proxies. The basic function of franchisers is to
should be divided by two halves so that one half set
detect the improper usage of infrastructures by
up a session for disconnection-tolerant connection
any entities [24, 28].
with applications within a kiosk controller or
• KioskNet Franchisees. Franchisees are private
mobile devices. The other half establishes a
organizations or licensed individuals. The
connection with legacy servers instead of
responsibilities of franchisees are to operate
intermittently connected users. For data forwarding
their kiosk terminals and protect terminals from
from a half to the other half within proxy,
malicious software [28].
application plug-ins are required, for example, a
• KioskNet Users. Users can access KioskNet
SMTP plug-in for sending mail content to a legacy
services and applications that franchisees own
mail server [24].
and support [28].
• Application Service Providers (ASPs). These
5. Legacy Server.
entities are licensed by franchisers for
The legacy servers are typical servers supporting
providing their applications to a KioskNet as an
applications such as IMAP, SMTP and HTTP with
example of banking services to local residents
TCP/IP protocol [24].
[28].
2. Certificate. All entities have unique credentials
containing a 2048-bit RSA key and a Public Key

16. 16
Certificate. The public key of a franchiser is use public key encryption for ensuing end-to-end
certified by a secure root CA server at the secure communication like SSL but this
University of Waterloo using its own private key approach is very difficult to be applied to this
and this signature is stored as the form of an delay-tolerant environment due to handshake for
X.509 certificate. In turn, franchisers issue the generating a shared key. Therefore, the KioskNet
certificates of franchisees and ASPs. When users generates random 256 bit keys using AES-CBC
register and create their identification at their (Advanced Encryption Standard Cipher
kiosks, franchisees certify the users in an Algorithm in Cipher Block Chaining
automatic way. In addition, franchisers also issue Mode). The recipients encrypt this key using its
unique credentials to KioskNet’s gateways and public key and decrypt the data after decrypting
ferries. A public key database, preserved at the the AES key by using their own private keys
proxy is used to periodically broadcast public [24].
key certificates for users, franchisees and ASPs
all over a franchiser’s region and these XII. CONCLUSION
certificates are also copied to all kiosk The main goal of DTN architecture is to provide
controllers [24]. interoperability between different kinds of networks
3. Infrastructure integrity. Digital signatures are in wide-ranging regions even though this network
used on all remote commands and software has many limitations such as long delay,
updates from franchiser for ensuring the security intermittent connectivity, limited power, and high
of this infrastructure. In this system, kiosk error rates. This architecture originated from the
controllers are very vulnerable to attacks so Interplanetary Internet architecture then it is spread
franchisers do not give root access authorities to to the challenged networks more generally.
kiosk controllers to prevent them from In this paper, we have summarized an overview
modifying the software and accessing private of Delay Tolerant Networking, especially about
information [24]. routing and security. In addition, we give an
4. Protecting recycled PCs. For protecting recycled example of the design and implementation of the
PCs from viruses and malicious software, these KioskNet for better understanding of DTN.
PCs can only boot using read-only disk images Moreover, we think that the implementation of
stored in kiosk controllers through NFS protocol. DTN is a good solution to supply people in remote
This boot images can be modified and updated areas with the Internet service for their
by franchiser administrative staffs [24]. communication to the world even though DTN still
5. User data protection. User data is only stored in has many open issues.
kiosk controllers and this system offers
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18. 18
Peer review from Guojun Wang (guojun@kth.se):
The authors have given an excellent description to the delay tolerant network, including the protocols, the
overall architecture, the technologies, implementations and so on. The levels between each part are very
clear and all the formats are qualified. However, it will be better to give an overview of what you have
presented at the beginning of this paper instead of putting that in the conclusion part. You also give one
sentence to the origin of DTN, which is followed with tag [7] in the conclusion paragraph. Actually, it
reduces the role of conclusion. In addition, you give a new opinion at the end, "Moreover, we think that the
implementation of DTN is a good solution to supply people in remote areas... ". New idea should be
avoided when you are giving an end to this paper.
Author’s Comment:
We thanks for deep review from reviewer. The overview of this work appears on the Abstract section,
where it was written, "we summarize the overview of Delay Tolerant Network and introduce a case study
about the implementation of Delay Tolerant Network." We used "summarize" word instead of "explain",
because we know that there are still so much detail of the concept that we cannot cover in this paper, due to
the page and word limit. Later we paraphrase again the sentence so it can be more clearly by become, “In
this paper, we present comprehensive overview of Delay Tolerant Network and introduce a study case
about the implementation of this network”. At the conclusion, we think that, it is important to sum up and
deliver the global idea that is appear in the entire paper in simple sentence, which is "the implementation of
DTN is a good solution to supply people in remote areas with the Internet service for their communication
to the world even though DTN still has many open issues." Thus we think, this last sentence is important in
order to conclude the core idea of the technology that has been talked about. However, after carefully think
about the point that reviewer mentioned in Conclusion, we decided to remove the citation in the reference,
because it will lead reader to think that we still present new idea in this conclusion, however that point is
already presented at the main section of the paper including its citation.
Peer review from Merabi Kechkhoshvili (merabi@kth.se)
First I will make my point about overall feeling from this report. It is obvious that authors had wide range
of references and they did a deep research in order to present this document. It is written in really
professional manner and is appealing to person who is familiar with particular topic. I would also add that
report is done in appropriate style:
• Well-divided paragraph structure.
• Good knowledge of language and broad vocabulary.
• Although the topic is huge, it is written without redundant information.
However there are some minor details that are worth to pay more attention:
• Firstly, I want to say about introduction part: it is not describing how the document is organized. I
would like you to put more information about chapters and what are you going to describe in following
sections.
• Secondly, while reading the document, several figures are too difficult to understand. I think you need
to put more information and some explanation about figures.
• Finally, from my perspective this paper will be difficult to read be people, who are not really familiar
with the topic.
But once again I must stress exceptional quality of presented report.
Author’s Comment:
We thanks for deep review from reviewer. After reading the points of your review carefully, we consider to
make some changes in the Abstract part to make the document organization more clearly, by putting
sentences about what the reader can expect from this work. We agree that the organization of the document
nor should be mentioned in the Introduction part, because that part should be as introductory for the topic

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itself, not to the document organization or chapter explanation. Moreover, the figures that is presented in
this paper have been explained by paragraph above them, and due to limited page and word, we decide not
to explain more about that. We’ve also realized this work digs little bit deeper and detail of DTN concept,
thus it will not just give basic general overview of this technology.