CS176C Spring 2006 Wireless Routing: Reactive Algorithms

1
CS176C Spring 2006Wireless Routing Reactive
Algorithms

• Administrivia
• Midterm exam on Thursday (5/18)
• Includes all material up to and including Today
• Project midterm report due (5/21)
• 1 page description on current progress
• Turn in via email to krishnap_at_cs
• Today
• Reactive algorithms AODV, DSR
• Review Q A for Midterm

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Ad hoc On-demand Distance Vector

• Reactive approach to routing
• Route discovery cycle used for route finding
• Maintenance of active routes
• Sequence numbers used for loop prevention and as
  route freshness criteria
• Descendant of DSDV

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AODV Route Discovery
B
RREQ
S
A
RREQ
C
D

• Node S needs route to D
• S Creates a Route Request (RREQ)
• Enters Ds IP addr, seq , Ss IP addr, seq ,
  hopcount (0)
• Node S broadcasts RREQ to neighbors
• Node A receives RREQ
• Makes reverse route entry for SdestS,
  nexthopS, hopcount1
• It has no route to D, so rebroadcasts RREQ

4
AODV Route Discovery
B
RREQ
S
A
RREQ
RREP
C
D

• Node C receives RREQ
• Makes reverse route entry for SdestS,
  nexthopA, hopcount2
• It has a route to D, and the seq for route to D
  isgt Ds seq in RREQ
• C creates a Route Reply (RREP)Enter Ds IPAddr,
  seq, Ss IPAddr, Hopcount to D (1)
• Unicasts RREP to A

5
AODV Route Discovery
B
RREP
S
A
RREP
C
D

• Node A receives RREP
• Makes forward route entry to DdestD, nexthopC,
  hopcount2
• Unicasts RREP to S
• Node S receives RREP
• Makes forward route entry to Ddest D, nexthop
  A, hopcount3
• Sends data packets on route to D

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AODV Route Maintenance
B
RERR
S
A
RERR
C
D

• Link between C and D breaks
• Node C invalidates route to D in route table
• Node C creates Route Error (RERR) message
• Lists all destinations which are now unreachable
• Sends to upstream neighbors
• Node A receives RERR
• Checks whether C is its next hop on route to D
• Deletes route to D
• Forwards RERR to S

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AODV Route Maintenance
B
RERR
S
A
C
D

• Node S receives RERR
• Checks whether A is its next hop on route to D
• Deletes route to D
• Rediscovers route if still needed

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AODV Optimizations

• Expanding Ring Search
• Prevents flooding of network during route
  discovery
• Control Time To Live (TTL) of RREQ to search
  incrementally larger areas of network
• Advantage Less overhead when successful
• Disadvantage Longer delay if route not found
  immediately
• Local Repair
• Repair breaks in active routes locally instead of
  notifying source
• Use small TTL because destination probably hasnt
  moved far
• If first repair attempt is unsuccessful, send
  RERR to source
• Advantage repair links with less overhead, delay
  and packet loss
• Disadvantage longer delay and greater packet
  loss when unsuccessful

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Dynamic Source Routing (DSR)

• The Other reactive protocol
• Route discovery cycle used for route finding
• Source driven
• AODV relied on intervening hops to maintain state
• In DSR, all active routes maintained by source
  nodes
• Utilizes source routing

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DSR Route Discovery
B
RREQS,A
RREQS
S
A
RREQS,A
RREQS,A
C
D

• Node S needs a route to D
• Broadcasts RREQ packet
• Node A receives packet, has no route to D
• Rebroadcasts packet after adding its address to
  source route

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DSR Route Discovery
B
RREPSACD
S
A
RREQS,A
RREPSACD
RREPSACD
C
D
RREQS,A,C
RREQS,A,C

• Node C receives RREQ, has no route to D
• Rebroadcasts packet after adding its address to
  source route
• Node D receives RREQ, unicasts RREP to C
• Puts source route accumulated in RREQ into RREP
• RREP propagates back to S
• Node C receives RREP, Unicasts to A
• Node A receives RREP, Unicasts to S
• Node S receives RREP
• Uses route for data packet transmissions

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AODV vs. DSR

• DSR uses source routing / AODV uses next hop
  entry
• Implications
• DSR nodes learn more routes during route
  discovery
• Higher bandwidth overhead per packet
• DSR uses route cache / AODV uses route table
• Implications
• DSR has alternate route available when one breaks
• Higher storage requirements

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AODV vs. DSR

• DSR route cache entries do not have
  lifetimesAODV route table entries do have
  lifetimes
• AODV may prematurely delete routes that are still
  valid
• DSR is more likely to use invalid routes
• Wastes bandwidth, higher packet drops, pollution
  of other route caches because of promiscuous
  listening
• DSR nodes respond to each RREQ duplicateAODV
  nodes only respond to first RREQ, unless one
  arrives along a better path
• DSR nodes learn more routes, can populate route
  cache with alternate routes
• DSR suffers from RREP floods, RREPs more
  expensive at MAC layer

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Overall Results

• Overall Results
• DSR performs best in low mobility or less
  stressful situations (lower traffic load)
• Why? cached routes helpful in low mobility, but
  less helpful in high mobility (cache pollution)
• AODV performs better than DSR in higher mobility
  or more stressful situations (higher traffic
  load)
• AODV traffic dominated by RREQs (gt greater
  routing control overhead)
• DSR traffic dominated by RREPS (gt greater MAC
  overhead)

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Another Approach

• Location aided routing
• Nodes use geographical information (i.e. GPS)
• Geographic routing
• Maintain only local neighbors
• Use coordinates to forward packets
• Advantage minimal routing state per node(more
  scalable than AODV/DSR?)
• Challenges
• Sparse networks big hole in network topology
• General solution use geometric planarization to
  compute path around the void

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Midterm Review

• Topics to be covered
• PlanetLab
• Unstructured routing protocols
• Protocols (Gnutella, Kazaa, Bittorrent, Freenet,
  Skype, )
• Their measurements and analysis
• Structured overlay protocols
• Architecture and APIs (DHT, DOLR)
• Protocols Chord, Tapestry/Pastry, CAN
• Optimizations PNS, virtual network coordinates
• Applications distributed storage (OceanStore),
  multicast (Scribe)
• Wireless
• Media Access Control protocols (how and why)
• Routing protocols (proactive and reactive)