A Delay-Tolerant Network Architecture for Challenged Internets

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A Delay-Tolerant Network Architecture for
Challenged Internets

• Kevin Fall

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Challenged Networks

• Terrestrial mobile networks
• Unexpected partitions due to node mobility or RF
  interference
• Periodic, predictable partitions
• e.g. Commuter bus acting as store and forward
  switch

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Challenged Networks (cont.)

• Exotic Media Networks
• Near-Earth satellites, very long-distance radio
  (deep space) etc.
• High latencies with predictable interruption
• Outage due to environmental conditions
• Predictably available store and forward network
  service e.g. low-earth orbiting satellites

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Challenged Networks (cont.)

• Military Ad-Hoc Networks
• Operate in hostile environments
• mobile nodes, environmental factors or
  intentional jamming cause disconnections
• Data traffic may be pre-empted by higher priority
  voice traffic
• Strong infrastructure protection requirements

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Challenged Networks (cont.)

• Sensor networks
• Limited end-node power, memory and CPU capability
• Thousands or millions of nodes per network
• Communication scheduled to conserve power
• Interfaced to other networks using proxy nodes

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Current Solutions

• Link-repair approach
• Engineer problem links to appear similar to
  regular links
• Use proxy agents
• Attach challenged networks at edges using proxy
  agents
• Does not provide a general way to use these
  networks for data transit

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Characteristics of Challenged Networks

• Path and Link characteristics
• Network architectures
• End System characteristics

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Path and Link characteristics

• High latency, low data rate
• e.g. 10 kbps, 1-2 second latencies
• Asymmetric data rates
• e.g. remote instruments large return channel,
  small uplink for device control
• Protocols should be terse and dynamic control
  functions performed open-loop or hop-by-hop

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Path and Link characteristics

• Disconnection
• Non-faulty disconnections
• Motion
• Predictable satellite passes, bus acts as router
• Random motion of nodes/routers, interference
• Low-duty-cycle operation
• Routing subsystem should not treat predictable
  disconnections as faults and can use this
  information to pre-schedule messages

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Path and Link characteristics

• Long queueing times
• Conventional networks rarely greater than a
  second
• Challenged network could be hours or days due to
  disconnection

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Network Architectures

• Interoperability considerations
• Networks may use application-specific framing
  formats, data packet size restrictions, limited
  node addressing and naming etc.
• Security
• End-to-end approach not attractive
• Require end-to-end exchanges of keys
• Undesirable to carry traffic to destination
  before authentication/access control check

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End System Characteristics

• Limited longevity
• Round-trip time may exceed nodes lifetime making
  ACK-based policies useless
• Low duty cycle operation
• Disconnection affects routing protocols
• Limited resources
• Affects ability to store and retransmit data due
  to limited memory

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Can we use TCP/IP?

• Transport layer (TCP)
• High latency and moderate to high loss rates
  severely limit TCPs performance
• Network layer (IP)
• Performance affected by loss of fragments
• Routing
• High latency will cause current routing protocols
  to incorrectly label links as non-operational

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Proxies and Protocol Boosters

• Proxies and protocol boosters are inherently
  fragile
• Increase system complexity if mobility is
  frequent
• May require both directions to flow through the
  proxy fail for asymmetric routing
• Application proxies have limited re-use abilities
  and may fail to take advantage of special
  resources of the proxy node

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Delay Tolerant Message-Oriented Overlay
Architecture
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Abstraction

• Message switching
• Use message aggregates or bundles
• Allows networks path selection and scheduling
  functions a-priori knowledge of the size and
  performance requirements of data transfers
• Overlay architecture
• DTN will operate over existing protocol stacks
  and provide a gateway when a node touches two or
  more dissimilar networks

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Regions and DTN Gateways

• DTN gateways are interconnection points between
  dissimilar network protocol and addressing
  families called regions
• e.g. Internet-like, Ad-hoc, Mobile etc.
• DTN gateways
• Perform reliable message routing
• Perform security checks
• Store messages for reliable delivery
• Resolve globally-significant name tuples to
  locally-resolvable names for internal destined
  traffic

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Name Tuples

• Two variable length portions
• Region name
• Globally-unique hierarchically structured region
  name
• Used by DTN gateways for forwarding messages
• Entity name
• Resolvable within the specified region, need not
  be unique outside it
• E.g. internet.icann.int, http//www.ietf.org/

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Class of Service

• Similar to the Postal service
• Delivery priority low, ordinary, high
• Notifications of mailing, delivery to receiver
  and route taken
• Reliable delivery using custody transfer at each
  routing hop

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Path Selection and Scheduling

• End-to-end path routing path cannot be assumed to
  exist
• Can solve a multicommodity flow optimization
  problem using approximate algorithms, since the
  protocol is message based

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Custody Transfer

• Two types of message nodes
• Persistent (P) and non-persistant (NP)
• P nodes assumed to contain persistent memory
  storage and participate in custody transfer
• Custody Transfer
• Acknowledged delivery of message from one DTN hop
  to the next and passing of reliability delivery
  responsibility

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Custody Transfer (cont.)

• Advantages
• Relieves potentially resource-poor end nodes from
  maintaining end-to-end connection states
• Useful for overcoming high loss rates along the
  delivery path
• As reliable as typical end-to-end reliability

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Protocol Translation and Convergence Layers

• Bundle forwarding function assumes underlying
  reliable delivery capability with message
  boundaries
• Convergence layer augments underlying network
  protocols appropriately

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Time Synchronization

• Need for time synchronization
• Provide a mechanism to deliver pre-programmed
  control instructions to be executed at future
  points in time
• Use for scheduling, path selection and to remove
  expired pending messages
• Propose time synchronization on the order of 1 ms

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Security

• Each message contains
• Identity of sender
• Requested class of service (CoS)
• Use public key cryptography
• First DTN router verifies user and validates CoS
  request
• Re-signs message using its key
• Core routers need only cache keys of their
  neighbours

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Congestion and Flow Control

• Flow control is hop-by-hop
• Uses underlying protocols mechanisms if they
  exist
• Congestion control
• Refers to contention of persistent storage at a
  DTN forwarder
• Current approach uses a priority queue
• Priority inversion and head-of-line blocking can
  occur

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Application Interface

• Applications must be able to operate in a regime
  where request/response time may exceed the
  longevity of the client and server processes
• Application interface is non-blocking
• Also has registration and callback functions
  between bundle-based applications and the local
  forwarding agent

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Implementation
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Implementation (cont.)

• Prototype DTN system under Linux
• Application interface
• Rudimentary bundle forwarding across scheduled
  and always on connections
• Detection of new and lost contacts
• Two convergence layers
• TCP/IP
• Bundle-based proxy to the Berkeley mote network

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Conclusion

• DTN architecture attempts to provide
  interoperable communications between and among
  challenged networks
• Design uses message switching with in-network
  retransmission, late-binding of names and routing
  tolerant of network partitioning