Delay-Tolerant Networks (DTNs) A tutorial http by pbj18978


									Delay-Tolerant Networks (DTNs)
           A tutorial

             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
  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
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!
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
  – 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
  – Router – forwards bundles within a single DTN
    region. Optionally supports custody transfers.
  – Gateway – forwards bundles between tow or
    more DTN regions. Must support custody
  Delay Isolation via Transport-Layer
• 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 time-
  to-ACK expires, the sender retransmits the
• 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
• 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
•   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
• 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
• Forwarding nodes are authenticated as well as
  user identities and the integrity of messages
• Sender information is authenticated by
  forwarding nodes
• 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
• 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

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