Dynamic Source Routing in Ad Hoc Wireless Networks

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Dynamic Source Routing in Ad Hoc Wireless Networks
EECS 600 Advanced Network Research, Spring 2005
Shudong Jin February 9, 2005
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Internet Routing EECS 425

• Distance vector algorithm
• Distributed
• Each router broadcasts to each of its neighbor
  routers its view of the distance to all hosts
• Each router computes the shortest path to each
  host based on the information advertised by each
  of its neighbors.
• Convergence problem
• Link state algorithm
• Centralized
• each router instead broadcasts to all other
  routers in the network its view of the status of
  each of its adjacent network links
• each router then computes the shortest distance
  to each host based on the complete picture of the
  network formed from the most recent link
  information from all routers
• Message overhead

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Why Different in Wireless ad hoc Networks?

• Traditional routing DV and LS
• High overhead from periodic routing
  advertisements
• No power conservation
• Low redundancy
• Asymmetric network transmission rare
• No dynamic topology changes, convergence problem
• Well, the basic distance vector algorithm was
  adapted for routing in wireless ad hoc networks

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Dynamic Source Routing

• Source routing is a routing technique in which
  the sender of a packet determines the complete
  sequence of nodes through which to forward the
  packet the sender explicitly lists this route in
  the packets header, identifying each forwarding
  hop by the address of the next node to which to
  transmit the packet on its way to the destination
  host.
• There are no periodic router advertisements in
  the protocol. Instead, a host dynamically
  determines one based on cached information and on
  the results of a route discovery protocol.

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Network Model and Assumptions

• Cooperative nodes All nodes are willing to fully
  participate in the protocol (forwarding packets
  as required)
• Diameter of the network small, but often greater
  than one. Multi-hop ad hoc network. How many hops
  typically?
• Mobility speed moderate with respect to packet
  transmission latency and transmission range, does
  not always need flooding
• Promiscuous receive mode assumed

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Overview of Basic Operation

• Basic idea
• The sender constructs a source route in the
  packets header, giving the address of each host
  in the network through which the packet should be
  forwarded in order to reach the destination host.
• When a host receives a packet, if this host is
  not the final destination of the packet, it
  simply transmits the packet to the next hop
  identified in the source route in the packets
  header.
• Each mobile host participating in the ad hoc
  network maintains a route cache in which it
  caches source routes that it has learned. (Each
  entry has an expiration period)
• Enabling it
• Route discovery When one host sends a packet to
  another host, the sender first checks its route
  cache for a source route to the destination. If a
  route is found, the sender uses this route to
  transmit the packet. If no route is found, the
  sender may attempt to discover one using the
  route discovery protocol.
• Route maintenance While a host is using any
  source route, it monitors the continued correct
  operation of that route. This monitoring of the
  correct operation of a route in use we call route
  maintenance. When route maintenance detects a
  problem with a route in use, route discovery may
  be used again to discover a new, correct route to
  the destination.

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Route discovery

• The initiating host will finally receive a route
  reply packet listing a sequence of network hops
  through which it may reach the target. Basic
  idea flooding.
• A source (SRC) broadcasts a RREQ packet in search
  for a path to a destination (DST)
• (SRC_ID, Request_ID) pair uniquely identifies a
  session request
• For any node, v, receiving the RREQ
• If (SRC_ID, Request_ID) found in vs list of
  recent requests, discard the RREQ otherwise
• Discard RREQ if vs address already listed in the
  route record in the RREQ otherwise
• Return a RREP to SRC if vs address matches the
  DST address in RREQ (i.e., v is the DST)
  otherwise
• Append vs address to the route record of RREQ
  and re-broadcast it
• Nodes forward the RREQ with the same (SRC_ID,
  request_ID) pair only once. Q Which route is
  taken by the reply?

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Route discovery an example
B
RREQ packet
G
Initiator ID
Initiator seq
D
Target ID
A
Partial route
H
E
C
F
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Route Maintenance

• Easy to maintain, if hop-to-hop acknowledgement
  at link layer is available
• A route error packet sent back the original
  sender.
• If the wireless network does not support such
  lower-level acknowledgements, an equivalent
  acknowledgement signal may be available.
• Passive acknowledgement from intermediate nodes
• Route maintenance can also be performed using
  end-to-end acknowledgements if some route links
  are asymmetric but some other route exists
  between SRC and DEST
• Lower-level acknowledgement tells which hop in
  error, but high-level cannot.

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Optimizations

• Full Use of Route Cache
• Piggybacking on Route Discoveries
• Reflecting Shorter Routes
• Handling Errors

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Full Use of Route Cache

• Many opportunities with route cache
• An intermediate (IN) node also has knowledge of
  the entire path when forwarding packets. Keep it!
• If an IN node receives a RREQ for a DST for which
  it caches the route, it can send RREP back to SRC
  without further propagating RREQ. Save it!
• All broadcast transmissions, so what if neighbors
  are forwarding a path to another node? Steal it!
• If the partial route in the RREQ and cached route
  at the IN node contain a loop. Discard it!
• Other problems and solutions
• A slight reply delay is introduced to avoid
  simultaneous replies from multiple nodes
• SRC specifies minimum number of hops over which
  the packet be propagated

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Piggybacking on Route Discoveries

• Allowing some data to be piggybacked in RREQ
  packets
• when a sender does not have a route cached to the
  destination host, it must initiate a separate
  route discovery, either buffering the original
  packet until the route reply is returned, or
  discarding it
• The amount of data piggybacked must be limited
  (so small packets only).
• Care taken in case the intermediate node with
  cached route to DST sends a RREP to SRC without
  further propagating the piggbacked RREQ
• Recovered the piggybacked data and continue to
  send it

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Reflecting Shorter Routes

• Scenarios
• B transmits a packet to C, with D being the next
  hop after C in the route in the packet
• If D receives this packet, it can examine the
  packet header to see that the packet reached it
  from B in one hop rather than two as intended by
  the route in the packet.
• D may infer that route may be shortened to
  exclude the intermediate hop through C.
• D then sends an unsolicited route reply packet to
  the original sender of the packet, informing it
  that it can now reach D in one hop from B.
• Not simulated/tested, often the basic maintenance
  procedure does it, though slower

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Handling Errors

• When the network is partitioned, too many packets
  but no route
• Exponential backoff used in route discoveries
  when network is partitioned
• In promiscuous mode, eavesdrop on route error
  packets being sent to other hosts.

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Simulation Setup

• Hosts randomly placed in the simulation area (4-5
  hops from one end to another)
• Waypoint mobility model (move/pause)
• Up to three sessions per source node supported
• Packets sent at exponentially distributed sending
  times
• Data link layer ACK per hops
• Access contention not modeled
• No obstruction in the simulation environment

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Results

• Main observations
• Higher node density leads to better overhead
  efficiency
• Higher overhead with more dynamic mobility.
• The worst 2.6 times the optimal value
• Discovered route very close to optimal route
  length (if perfect routing info available). Ratio
  close to 1.0

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Dynamic Source Routing Summary

• DSR is a reactive routing protocol for ad hoc
  networks (no periodic route updates)
• Source obtains information of all nodes along a
  path to the destination when needed
• Advantages
• Does not assume bi-directional links
• Adapts quickly to dynamic network topology
• No loop?
• Easy to implement?
• But a little more overhead in data packets

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Discussions