DHTbased unicast for mobile ad hoc networks

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DHT-based unicast for mobile ad hoc networks

• Thomas Zahn, Jochen Schiller
• Institute of Computer Science
• Freie Universitat Berlin
• ?????

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Outline

• Introduction
• MADPastry
• MADPastrys Unicast
• Simulation Results
• Conclusion

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Introduction (1/4)

• Both MANETs and peer-to-peer (P2P) overlay
  networks share a number of key characteristics.
• the lack of a central infrastructure
• highly dynamic network topologies
• the need for self-organization

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Introduction (2/4)

• Distributed Hash Tables (DHTs) have been proposed
  for large-scale network applications.
• Route a packet based on a key (rather than a
  fixed destination address) to the (unknown) node
  in the network that is currently responsible for
  the given key within a bounded number of hops.
• This overlay routing is also referred to as
  indirect routing.

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Introduction (3/4)

• DHTs do not closely concern themselves with the
  physical (routing) aspects of the underlying
  network
• designed to form overlay networks in
  Internet-based networks where physical routing is
  practically taken for granted.

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Introduction (4/4)

• In MANETs, DHT can provide
• indirect routing
• direct physical routing (unicasting)
• MADPastry
• a DHT substrate explicitly designed for MANETs
• DHT-based unicast can achieve
• better packet delivery rates
• lower network traffic

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MADPastry (1/8)

• MADPastry combines AODV ad hoc routing and
  Pastry overlay routing at the network layer
• provide an efficient primitive for key-based
  routing in MANETs.

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MADPastry (2/8)

• Each node in a MADPastry network assigns itself a
  unique overlay id
• defines its logical position on the virtual
  overlay id ring.
• A message's packet header contains a message key.
• MADPastry routes the message to that node in the
  network that is currently responsible for the
  message key
• the node whose overlay id is currently the
  numerically closest to the message key among all
  MADPastry nodes in the network.
• To avoid message broadcasts (e.g. for route
  discovery)
• MADPastry considers physical locality in the
  construction of its routing tables.

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MADPastry (3/8) - Clusters

• Random Landmark
• No fixed landmark nodes, landmark keys instead
• These keys divide the logical overlay id space
  into equal sections
• 0800..00, 1800..00, ......., F800..00
• Node currently closest to a landmark key
• Become temporary landmark node
• Periodically issue beacon messages
• Nodes overhear these beacon messages and
  periodically determine the physically closest
  temporary landmark node.

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MADPastry (4/8) - Clusters

• Node associates itself with closest temporary
  landmark
• assumes same overlay ID prefix
• If need be, a node assigns itself a new overlay
  id sharing the same prefix with the new closest
  temporary landmark node.
• physically close nodes forming overlay clusters
  with common id prefixes
• Physically close nodes are also likely to be
  close in the overlay

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MADPastry (5/8)- Spatial Topology
Spatial Topology
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MADPastry (6/8) Routing Tables

• MADPastry maintains three different routing
  tables
• AODV-style routing table for physical routes from
  a node to specific target nodes
• stripped down Pastry routing table that only
  contain landmark keys
• standard Pastry leaf set for indirect routing

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MADPastry (7/8) Routing Tables
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MADPastry (8/8) - Routing

• When a node wants to send a packet to a specific
  key
• Consults its Pastry routing table and/or leaf set
  to determine the closest prefix match, as
  stipulated by standard Pastry.
• Consults its AODV routing table for the physical
  route to execute this overlay hop.
• Intermediate nodes on the physical path of an
  overlay hop consult their AODV table for the
  corresponding next physical hop.
• When a packet reaches the destination of an
  overlay hop, that node again consults its Pastry
  routing table and/or leaf set to determine the
  next overlay hop.
• This process continues until the packet reaches
  the eventual target node that is responsible for
  the packet key ( whose overlay id is the
  numerically closest to the packet key).

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MADPastrys Unicast (1/7) Address Publication

• Each nodes has exactly one temporary address
  server
• Address server stores its client's current
  overlay ID
• Node A hashes its node ID
• address server key (ASK).
• Node A publishes its current overlay ID towards
  ASK
• Node currently responsible for node A's hash key
  becomes node A's address server

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MADPastrys Unicast (2/7) Address Publication
ASKh(17) ?B7A9CC
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MADPastrys Unicast (3/7) Address Resolution

• Node A wants to communicate with node B
• Node A does not know node B's current overlay ID
• Node A hashes node B's net ID to get ASK
• Node A sends request towards ASK
• Node B's address server replies with node B's
  current overlay ID

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MADPastrys Unicast (4/7) Address Resolution
ASK h(17) ?B7A9CC
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MADPastrys Unicast (5/7) Unicast

• Node A uses overlay ID from reply to send message
  to node B
• MADPastry delivers message using indirect routing

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MADPastrys Unicast (6/7) Unicast
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MADPastrys Unicast (7/7)

• Shortest Routing Paths
• A direct, straight path from the source node to
  the destination node
• When using AODV and it doesnt know the path to
  that destination, it needs to engage in a costly
  route discovery process
• MADPastrys Routing Paths
• Probably travel multiple overlay hops
• The entries in its routing table are likely
  up-to-date and valid
• Usually wont have to discover a route because it
  can use at each overlay routing step any existing
  route that would bring the packet numerically
  closer to its key

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Simulation Results (1/6)

• Compare MADPastry's unicast against a popular ad
  hoc routing protocol
• AODV (reactive)
• OLSR (proactive)
• Simulations in ns2
• 1 random request every 10s per node

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Simulation Results (2/6)

• Constant node velocity1.4 m/s
• Varying network sizes (50,100,150,200,250)

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Simulation Results (3/6)
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Simulation Results (4/6)
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Simulation Results (5/6)

• Constant network size250 nodes
• Varying node velocities (0.1,1.4, 2.5,5.0 m/s)

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Simulation Results (6/6)
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Conclusion (2/2)

• MADPastry already present in the MANET to provide
  key-based, indirect routing
• MADPastry can also provide point-to-point
  unicasting
• No need to maintain ad hoc routing protocol in
  parallel for DHT applications that use MADPastry
  handle their point-to-point routing

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Conclusion (2/2)

• MADPastry's unicast can outperform popular
  reactive and proactive ad hoc routing protocols
• In MANETs it can be advantageous to travel
  numerous short up-to-date routes instead of one
  long direct route