Ad Hoc Networking

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Ad Hoc Networking

• Tom Roeder
• CS415 2005sp

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Part IV questions?
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What is an ad hoc network?

• Nodes discover and maintain routes
• no use of infrastructure
• Every host is also a router
• (thus not all routers are trusted)
• Can be done over the infrastructure

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Features of an ad hoc network

• Change in reachability
• over time as nodes die and come back
• over space as nodes move
• Often power constrained
• The screen is the constraint on your laptops, but
  on many smaller machines, it is the network
• Oft cited 1 packet for 3000 instructions
• It adapts!
• Must not rely on static configurations

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Applications

• Sensor networks
• little, power-constrained motes
• attached to animals in a park
• scattered on the ground from the air (military)
• Rescue workers in a disaster area
• Educational apps (www.silicon-chalk.com)
• Operating systems
• MagnetOS (Mobisys 05) (www.cs.cornell.edu/people/
  egs/magnetos)

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Overlays

• leads in to P2P systems
• Why bother?
• route around problems
• build multicast trees
• illegally share files
• gain anonymity
• build trust networks
• optimize RSS feeds
• If this interests you, check out Copano

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Types of ad hoc routing

• Proactive
• DSDV, Link-state variants
• Reactive
• DSR, AODV
• Hybrid
• ZRP, HARP, SHARP
• Overlay
• Were not going to discuss this more

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Costs and benefits proactive

• Pushes information
• low message latency
• high state overhead
• high message overhead
• OK when network is small
• Full link state grows as n2
• Can seriously impact throughput
• Not good for high mobility

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Costs and benefits reactive

• Generates route at send time
• high initial latency
• caching helps tremendously
• no wasted route information
• can lead to broadcast storms
• brings the network down even faster at the end
• Good for reasonably high mobility
• too fast and theres nothing we can do
• Widely used

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802.11b MAC layer

• To send a packet, must reserve the medium
• Uses a CSMA protocol
• Additional optional protocol for 802.11b is
• RTS (Ready To Send)
• CTS (Clear To Send)
• Data
• ACK
• Hidden terminal problem
• may get lower throughput than expected

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Distance Vector protocols

• Key Distance Vector idea
• Instead of storing the full path, just keep
  direction
• If I want to get to A, my next hop is B
• Trade DV information with neighbors via flooding
• Based on distributed Bellman-Ford algorithm
• Can suffer from loops and counting-to-infinity
• AODV finds distance vectors reactively
• Based on DSDV, which does it proactively
• Uses a sequence number to try to avoid problems

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AODV information per node

• A table (cache) of known distance vectors
• refresh rate will controll the message overhead
• ltseqnum, dest, hop, hopcountgt
• seqnum incremented on new information
• used to avoid counting to infinity.
• Remember the last known seqnum for each cache elt
• dest the identifier of the destination
• note that identifiers are arbitrary
• hop the identifier of next hop to get to dest
• hopcount how many hops on this route

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AODV route requests
A
B

• Node A wants to send a packet to B
• broadcasts a RREQ (with some TTL)
• heard by B, B sends a reply
• A sends directly to B
• Node A wants to send a packet to C
• broadcasts a RREQ
• heard by B, but B just heard from C
• Sends reply lt1, C, B, 1gt
• A sends packet to B, who forwards it to C

C
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AODV route replies

• A node receiving a RREQ sends a route reply
• from its cache if it has this route
• else it forwards the RREQ
• It also updates its path to the requestor with
  the RREQ and TTL, if it is better
• If a node hears a better route reply
• it doesnt send its own
• it records the better route
• this helps avoid broadcast storms in flooding

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AODV route caches

• Think of the route cache as an optimization
• We could always choose to flood
• This would just have high latency and broadcast
  storms, but would still be correct (dont do it!)
• Timeout is critical
• When a link goes down, the cache is wrong
• We dont do explicit invalidation
• real AODV does
• uses MAC link error info to guess at disconnection

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AODV example
RREQ
RREQ
A
RREQ
RREQ
RREQ
RREQ
RREQ
B
RREQ
RREQ
RREQ
RREP
RREQ
RREQ
RREQ
RREQ
RREP
!
RREP
C
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AODV details to ignore

• Counting to infinity is possible but hard
• see http//www.cse.ucsc.edu/research/
  ccrg/publications/hari.icc.2005.pdf
• Dont worry about security
• We are not managing the errors explicitly
• This is clearly suboptimal, but easier
• See the AODV and DSR papers if youre interested
• Dont worry if you get low throughput

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AODV header spec

• type (2 bits)
• RREP, RREQ, DATA
• seqnum (4 bytes)
• incremented on new routes
• from node (4 bytes)
• destination node (4 bytes)
• Time To Live (1 byte)
• set to MAX_PATH_LENGTH

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Layering and Abstraction

• AODV is a layer below miniports
• it acts like IP for us
• it should encapsulate the miniports code
• Other than AODV control packets,
• all packets should be miniports or minisockets
• we still are delivering to miniports on remote
  node
• We have simply taken away the reliance on the IP
  routing infrastructure

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Testing over the infrastructure

• We provide a broadcast layer for your code
• file format
• saranac
• heineken
• dosequis
• kingfisher
• tecate
• .xx.x
• x.xx.
• xx...
• .x..x
• x..x.

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Testing over the infrastructure

• Use the network_broadcast_pkt
• To send to all reachable nodes
• When you dont know the direction RREQ
• For returning cached RREPs (optimization)
• Use the network_send_pkt
• For returning RREPs
• For data packets
• You implement miniroute_send_pkt
• Does AODV, then unicasts the packet

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Testing over the wireless

• We do not have enough tablets to give you
• You would need a large network to test this
• We will do it in section
• We will schedule a few other times
• Can also use any 802.11b Windows device
• laptop
• desktop with wireless card

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Broadcast storm

• Issues with flooding wireless networks
• May have already heard an answer, but unicast
• May have a better answer than one you hear
• n2 flooding is expensive to discover linear
  paths
• What can we do?
• Damping
• Promiscuous unicast listening

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Implementations and help

• For real implementations, see
• AODV moment.cs.ucsb.edu/AODV/aodv.html
• DSR www.monarch.cs.cmu.edu/dsr-impl.html
• To try out AODV without the hassle, see
• sns www.cs.cornell.edu/People/egs/sns/
• simulated implementation of AODV
• Papers
• See the above sites for references or just google
  it