The DTN architecture
implements store and forward message switching by overlaying the bundle layer
on top of heterogeneous regional networks. Bundles also called messages
(message switched). The bundle layer stores and forward bundle fragments
between nodes. A single bundle layer protocol is used in all regional networks
and make up a DTN.
DTN designed to
use storage within the network to support the store and forward operation over
a networking path as shown in Figure 2.8. IP end to end communication in
between the source and destination must wait for complete path while DTN store
and forward delivery did not need to wait for a complete path. DTN store and
forward delivery will perform incremental progress without end to end path.
Application
Data Units (ADUs) are a DTN-enabled application that sends messages of
arbitrary length. The bundle layer
transformed the ADUs into "bundles" which is a one or more protocol
data unit. Bundles are forwarded by DTN nodes and have two or more
"blocks" of data as shown in Figure 2.9.
Application
data or other information in each block used to deliver the containing bundle
to its destinations. Blocks used to hold
information in the header or payload portion of protocol data units in other
protocol architectures. It is called
"block" instead of "header" because blocks may not appear
at the beginning of a bundle due to particular processing requirements [8] .
Bundle layer as
shown in Figure 2.10 support end to end transfer across heterogeneous network
protocol stacks. An essential element of the bundle-based style of forwarding
is that bundles have a place to wait in a queue until a communication contact
is available. Size of storage used in
this operation is depending on the system requirement. The assumptions for this
operation as follows:
i.
Storage is available
and well-distributed throughout the network.
ii.
Storage is sufficiently
persistent and robust to store bundles until forwarding can occur.
iii.
This
"store-and-forward" model is a better choice than attempting to
effect continuous connectivity or other alternatives.
Bundle Protocol
nodes and endpoints are identified by name. Bundle Protocol names are used Uniform
Resource Identifier (URI). Each URI begins with a scheme name followed by a
series of characters where the syntax is defined by the scheme. This is examples
of Bundle Protocol name:
i.
dtn://thismachine/ping
ii.
dtn:pop:mailto:myemail@gmail.com
The bundle protocol
is designed with a Primary Bundle Block, payload block which contains the ADU data
itself and a set of other blocks. The following fields are all present in the
primary bundle block [3] :
i.
Creation Timestamp - a
concatenation of the bundle is creation time and a monotonically increasing
sequence number such that the creation timestamp is guaranteed to be unique for
each ADU originating from the same source.
The creation timestamp is based on the time-of-day an application
requested an ADU to be sent (not when the corresponding bundle(s) are sent into
the network). DTN nodes are assumed to
have a basic time synchronization capability.
ii. Lifespan
- the time-of-day at which the message is no longer useful. If a bundle is stored in the network (including
the source is DTN node) when its lifespan is reached, it may be discarded. The lifespan of a bundle is expressed as an
offset relative to its creation time.
iii. Class
of Service Flags - indicates the delivery options and priority class for the
bundle.
iv. Source
EID - EID of the source.
v. Destination
EID - EID of the destination.
vi. Report-To
Endpoint ID - an EID identifying where should be sent. This may or may not identify the same
endpoint as the Source EID.
vii. Custodian
EID - EID of the current custodian of a bundle.
The lifespan field is
very useful in an attempt to keep the email data in this offline email system
for a longer period of time until it transferred to another device. Another
block such as metadata block is used to identify the content. It could be used
to implement ‘network as a database’ and can be encrypted separately from the
payload. The payload block indicates information about the contained payload
and the payload itself.
Binding interprets
an endpoint identifier (EID) to select the next hop or next region specific
layer the bundle can be forwarded toward its destination. Figure 2.11 shows the
bundle layer through every region specific layer. Late binding means that the
binding of a bundle is a destination to a particular set of destination
identifiers or addresses does not necessarily happen at the bundle source.
Binding might occur at the source, during transit or at the destination. DTN
nodes use some form of persistence storage and stored bundles can maintain even
if the system restarts. Persistence assumes that storage is available,
distributed well through the network and store bundles sufficiently until
forwarding can occur [9] .
DTN can store
bundles at intermediate nodes. It can still route even there is a waiting time.
Primarily the route will be forwarded to the nodes that will have connectivity
later on. In Figure 2.12, data can be transferred directly between user
computer A (PC A) and user computer B (PC B) when both are connected. When the
PC B is disconnected, DTN routes data to Temporary Data Storage and deliver it
later to the PC B. When the PC B is reconnected, data stored in the Temporary
Data Storage is delivered even if PC A is disconnected.
Security also can be
provided to protect against unauthorized use. Bundle Authentication Block (BAB)
as shown in Figure 2.13 [7] , provides hop-by-hop
authentication and integrity protection for the bundle between adjacent bundle
nodes. Unauthorized resource utilization can be prevented by enabling modified
bundles to be detected and discarded at the first node at the destination
receiver.
