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Delay-Tolerant Networks (DTNs)
A tutorial
http://www.ipnsig.org/reports/DTN_Tutorial11.pdf
Eun Kyoung Kim
Today’s Internet
• Successful at interconnecting communication
devices across the globe
• Based on TCP/IP protocol suite and wired links
• Connected in end-to-end, low-delay paths
between sources and destinations
• Low error rates and relatively symmetric
bidirectional data rates
Evolving Wireless Networks Outside
the Internet
• Independent networks, each supporting
specialized communication requirements and
adapted to a particular homogeneous
communication region
• Support long and variable delays, arbitrarily
long periods of link disconnection, high error
rates, and large bidirectional data-rate
asymmetries
Evolving Wireless Networks Outside
the Internet
• Examples
– Terrestrial civilian networks connecting mobile
wireless devices
– Wireless military battlefield networks connecting
troops, aircraft, satellites, and sensors
– Outer-space networks, such as the InterPlaNetary
(IPN) Internet project
• Require the intervention of an agent that can
translate between incompatible networks
characteristics and act as a buffer for mismatched
network delays
The Concept of a Delay-Tolerant
Networks (DTN)
• A network of regional networks supporting
interoperability among them
• An overlay on top of regional networks,
including the Internet
• accommodate long delays between and within
regional networks, and translate between
regional network communication
characteristics
Why a Delay-Tolerant Network (DTN)?
• The Internet’s underlying assumptions
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Continuous, bidirectional end-to-end path
Short round-trips
Symmetric data rates
Low error rates
• The characteristics of evolving and potential networks
–
–
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Intermittent connectivity
Long or variable delay
Asymmetric data rates
High error rates
• New architectural concept is needed!
Store-And-Forward Message Switching
• The problems of DTNs can be overcome by storeand-forward massage switching
• DTN routers need persistent storage for their
queues because
– A communication link may not be available for a long
time
– One node may send or receive data much faster or
more reliably than the other node
– A message, once transmitted, may need to be
retransmitted for some reasons
Intermittent Connectivity
• Assume communicating devices (nodes) in
motion and/or operation with limited power
• When nodes must conserve power or
preserve secrecy, links are shut down ->
intermittent connectivity, network partition
• On the Internet, intermittent connectivity
causes loss of data, while DTNs isolate delay
with a store-and-forward technique
Opportunistic Contacts
• Network nodes may need to communicate
during opportunistic contacts, in which a
sender and receiver make contact at an
unscheduled time
Scheduled Contacts
• If potentially communicating nodes move
along predictable paths, they can predict or
receive time schedules of their future
positions and thereby arrange their future
communication sessions
• Require time-synchronization
The Bundle Layer
• A new protocol layer overlaid on top of
heterogeneous region-specific lower layers,
with which application programs can
communicate across multiple regions
Bundles and Bundle Encapsulation
• Bundles (messages) consist of
– A source-application’s user data
– Control information, provided by the source
application for the destination application
– A bundle header, inserted by the bundle layer
A Non-Conversational Protocol
• DTN bundle layers communicate between
themselves using simple sessions with
minimal or no round-trips
• Any acknowledgement from the receiving
node is optional, depending on the class of
service selected
DTN Nodes
• An entity with a bundle layer
– Host – sends and/or receives bundles, but does
not forward them. Optionally supports custody
transfers.
– Router – forwards bundles within a single DTN
region. Optionally supports custody transfers.
– Gateway – forwards bundles between tow or
more DTN regions. Must support custody
transfers.
Delay Isolation via Transport-Layer
Termination
• DTN routers and gateways terminate transport
protocols at the bundle layer
Custody Transfers
• The bundle layer supports node-to-node
retransmission by means of custody transfers
• If no ACK is returned before the sender’s timeto-ACK expires, the sender retransmits the
bundle
• A bundle custodian must store a bundle until
– Another node accepts custody, or
– Expiration of the bundle’s time-to-live
• Do not guarantee end-to-end reliability
Moving Points of Retransmission
Forward
• The bundle layer uses reliable transport-layer
protocols together with custody transfers to
move points of retransmission progressively
forward toward the destination
Internet vs. DTN Routing
• The protocol stacks of all nodes include both
bundle and transport layers
• DTN gateways can run different lower-layer
protocols (below the bundle layer) on each
side of their double stack, which allows
gateways to span two regions that use
different lower-layer protocols
Classes of Bundle Services
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•
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Custody Transfer
Return Receipt
Custody-Transfer Notification
Bundle-Forwarding Notification
Priority of Delivery
Authentication
DTN Regions
• A region composing a DTN, in which
communication characteristics are
homogeneous
• Has a unique region ID
Names and Addresses
• Each DTN node has a two-part name,
consisting of a region ID and an entity ID
Security
• Forwarding nodes are authenticated as well as
user identities and the integrity of messages
• Sender information is authenticated by
forwarding nodes
Security
• Both users and forwarding nodes have private
and public key-pairs and certificates
• Senders can sign their bundles with their
private key, producing a bundle-specific digital
signature
• Receivers can confirm the authenticity of the
sender, the integrity of message, and the
sender’s CoS rights using the signature and
the sender’s public key