Delay-Tolerant Networks (DTNs) A tutorial ipnsig/reports/DTN_Tutorial11.pdf
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Transcript of Delay-Tolerant Networks (DTNs) A tutorial ipnsig/reports/DTN_Tutorial11.pdf
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Delay-Tolerant Networks (DTNs)A tutorial
http://www.ipnsig.org/reports/DTN_Tutorial11.pdf
Eun Kyoung Kim
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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
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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
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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
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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
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Why a Delay-Tolerant Network (DTN)?
• The Internet’s underlying assumptions– Continuous, bidirectional end-to-end path– Short round-trips– Symmetric data rates– Low error rates
• The characteristics of evolving and potential networks– Intermittent connectivity– Long or variable delay– Asymmetric data rates– High error rates
• New architectural concept is needed!
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Store-And-Forward Message Switching
• The problems of DTNs can be overcome by store-and-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
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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
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Opportunistic Contacts
• Network nodes may need to communicate during opportunistic contacts, in which a sender and receiver make contact at an unscheduled time
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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
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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
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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
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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
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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.
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Delay Isolation via Transport-Layer Termination
• DTN routers and gateways terminate transport protocols at the bundle layer
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Custody Transfers
• The bundle layer supports node-to-node retransmission by means of custody transfers
• If no ACK is returned before the sender’s time-to-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
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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
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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
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Classes of Bundle Services
• Custody Transfer• Return Receipt• Custody-Transfer Notification• Bundle-Forwarding Notification• Priority of Delivery• Authentication
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DTN Regions
• A region composing a DTN, in which communication characteristics are homogeneous
• Has a unique region ID
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Names and Addresses
• Each DTN node has a two-part name, consisting of a region ID and an entity ID
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Security
• Forwarding nodes are authenticated as well as user identities and the integrity of messages
• Sender information is authenticated by forwarding nodes
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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