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Ad Hoc Networking

Ad Hoc Networking. Tom Roeder CS415 2005sp. Part IV questions?. 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. Features of an ad hoc network.

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Ad Hoc Networking

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  1. Ad Hoc Networking Tom Roeder CS415 2005sp

  2. Part IV questions?

  3. 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

  4. 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

  5. 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)

  6. 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

  7. Types of ad hoc routing • Proactive • DSDV, Link-state variants • Reactive • DSR, AODV • Hybrid • ZRP, HARP, SHARP • Overlay • We’re not going to discuss this more

  8. 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

  9. 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 there’s nothing we can do • Widely used

  10. 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

  11. 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

  12. AODV information per node • A table (cache) of known distance vectors • refresh rate will controll the message overhead • <seqnum, dest, hop, hopcount> • 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

  13. 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 <1, C, B, 1> • A sends packet to B, who forwards it to C C

  14. 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 doesn’t send its own • it records the better route • this helps avoid broadcast storms in flooding

  15. 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 (don’t do it!) • Timeout is critical • When a link goes down, the cache is wrong • We don’t do explicit invalidation • real AODV does • uses MAC link error info to guess at disconnection

  16. AODV example RREQ RREQ A RREQ RREQ RREQ RREQ RREQ B RREQ RREQ RREQ RREP RREQ RREQ RREQ RREQ RREP RREP ! C

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

  18. 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

  19. 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

  20. 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.

  21. Testing: over the infrastructure • Use the network_broadcast_pkt • To send to all reachable nodes • When you don’t 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

  22. 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

  23. 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

  24. 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

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