cs 268: computer networking l-12 ad hoc networks

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  • CS 268: Computer NetworkingL-12 Ad Hoc Networks

    Srinivasan Seshan, 2004

  • Mobile RoutingMobile IPAd-hoc network routingAssigned readingPerformance Comparison of Multi-Hop Wireless Ad Hoc Routing ProtocolsA High Throughput Path Metric for MultiHop Wireless Routing


  • *Overview

    Internet routing

    Ad hoc routing

    Ad hoc routing metrics

  • *How to Handle Mobile Nodes?Dynamic Host Configuration (DHCP)Host gets new IP address in new locationsProblemsHost does not have constant name/address how do others contact hostWhat happens to active transport connections?NamingUse DHCP and update name-address mapping whenever host changes addressFixes contact problem but not broken transport connections

  • *Handling Mobile Nodes (Transport) TCP currently uses 4 tuple to describe connection Modify TCP to allow peers address to be changed during connectionSecurity issuesCan someone easily hijack connection?Difficult deployment both ends must support mobility

  • *Handling Mobile NodeLink layer mobilityLearning bridges can handle mobility this is how it is handled in most campus networksEncapsulated PPP (PPTP) Have mobile host act like he is connected to original LANWorks for IP AND other network protocolsMulticastSolves similar problem how to route packets to different sets of hosts at different timesCant we just reuse same solutions?Dont really have solution for multicast either!

  • *Handling Mobile Nodes (Routing)Allow mobile node to keep same address and nameHow do we deliver IP packets when the endpoint moves?Why cant we just have nodes advertise route to their address?What about packets from the mobile host?Routing not a problemWhat source address on packet?Key design considerationsScaleIncremental deployment

  • *Basic Solution to Mobile Routing Same as other problems in Computer ScienceAdd a level of indirectionKeep some part of the network informed about current locationNeed technique to route packets through this location (interception)Need to forward packets from this location to mobile host (delivery)

  • *InterceptionSomewhere along normal forwarding pathAt sourceAny router along pathRouter to home networkMachine on home network (masquerading as mobile host)Clever tricks to force packet to particular destinationMobile subnet assign mobiles a special address range and have special node advertise route

  • *DeliveryNeed to get packet to mobiles current locationTunnelsTunnel endpoint = current locationTunnel contents = original packetsSource routingLoose source route through mobile current locationNetwork address translation (NAT)What about packets from the mobile host?

  • *Mobile IP (RFC 2290)InterceptionTypically home agent hosts on home networkDeliveryTypically IP-in-IP tunnelingEndpoint either temporary mobile address or foreign agentTerminologyMobile host (MH), correspondent host (CH), home agent (HA), foreign agent (FA)Care-of-address, home address

  • *Mobile IP (MH at Home)Mobile Host (MH)Visiting LocationHomeInternetCorrespondent Host (CH)Packet

  • *Mobile IP (MH Moving)Visiting LocationHomeInternetCorrespondent Host (CH)PacketHome Agent (HA)Mobile Host (MH)I am here

  • *Mobile IP (MH Away Foreign Agent)Visiting LocationHomeInternetCorrespondent Host (CH)PacketHome Agent (HA)Foreign Agent (FA)EncapsulatedMobile Host (MH)

  • *Mobile IP (MH Away - Collocated)Visiting LocationHomeInternetCorrespondent Host (CH)PacketHome Agent (HA)Mobile Host (MH)Encapsulated

  • *Other Mobile IP IssuesRoute optimalityTriangle routing Can be improved with route optimizationUnsolicited binding cache update to senderAuthentication Registration messages Binding cache updatesMust send updates across networkHandoffs can be slowProblems with basic solutionReverse path check for securityDo we really need it

  • *Overview

    Internet routing

    Ad hoc routing

    Ad hoc routing metrics

  • Ad Hoc RoutingGoal: Communication between wireless nodesNo external setup (self-configuring)Often need multiple hops to reach dst*

  • *Ad Hoc RoutingCreate multi-hop connectivity among set of wireless, possibly moving, nodesMobile, wireless hosts act as forwarding nodes as well as end systemsNeed routing protocol to find multi-hop pathsNeeds to be dynamic to adapt to new routes, movementInteresting challenges related to interference and power limitationsLow consumption of memory, bandwidth, powerScalable with numbers of nodesLocalized effects of link failure

  • Challenges and VariantsPoorly-defined linksProbabilistic delivery, etc. Kind of n2 linksTime-varying link characteristicsNo oracle for configuration (no ground truth configuration file of connectivity)Low bandwidth (relative to wired)Possibly mobilePossibly power-constrained*

  • *Problems Using DV or LSDV protocols may form loopsVery wasteful in wireless: bandwidth, powerLoop avoidance sometimes complexLS protocols: high storage and communication overheadMore links in wireless (e.g., clusters) - may be redundant higher protocol overhead

  • *Problems Using DV or LSPeriodic updates waste powerTx sends portion of battery power into airReception requires less power, but periodic updates prevent mobile from sleepingConvergence may be slower in conventional networks but must be fast in ad-hoc networks and be done without frequent updates

  • *Proposed ProtocolsDestination-Sequenced Distance Vector (DSDV)DV protocol, destinations advertise sequence number to avoid loops, not on demandTemporally-Ordered Routing Algorithm (TORA)On demand creation of hbh routes based on link-reversalDynamic Source Routing (DSR)On demand source route discoveryAd Hoc On-Demand Distance Vector (AODV)Combination of DSR and DSDV: on demand route discovery with hbh routing

  • *DSR ConceptsSource routingNo need to maintain up-to-date info at intermediate nodesOn-demand route discoveryNo need for periodic route advertisements

  • *DSR ComponentsRoute discoveryThe mechanism by which a sending node obtains a route to destinationRoute maintenanceThe mechanism by which a sending node detects that the network topology has changed and its route to destination is no longer valid

  • *DSR Route DiscoveryRoute discovery - basic ideaSource broadcasts route-request to DestinationEach node forwards request by adding own address and re-broadcastingRequests propagate outward until:Target is found, orA node that has a route to Destination is found

  • *C Broadcasts Route Request to FASourceCGHDestinationFEDBRoute Request

  • *C Broadcasts Route Request to FASourceCGHDestinationFEDBRoute Request

  • *H Responds to Route RequestASourceCGHDestinationFEDBG,H,F

  • *C Transmits a Packet to FASourceCGHDestinationFEDBFH,FG,H,F

  • *Forwarding Route RequestsA request is forwarded if:Node is not the destinationNode not already listed in recorded source routeNode has not seen request with same sequence numberIP TTL field may be used to limit scopeDestination copies route into a Route-reply packet and sends it back to Source

  • *Route CacheAll source routes learned by a node are kept in Route CacheReduces cost of route discoveryIf intermediate node receives RR for destination and has entry for destination in route cache, it responds to RR and does not propagate RR furtherNodes overhearing RR/RP may insert routes in cache

  • *Sending DataCheck cache for route to destinationIf route exists thenIf reachable in one hopSend packetElse insert routing header to destination and sendIf route does not exist, buffer packet and initiate route discovery

  • *DiscussionSource routing is good for on demand routes instead of a priori distributionRoute discovery protocol used to obtain routes on demandCaching used to minimize use of discoveryPeriodic messages avoidedBut need to buffer packets

  • *Overview

    Internet routing

    Ad hoc routing

    Ad hoc routing metrics

  • ETX measurement resultsDelivery is probabilisticA 1/r^2 model wouldnt really predict this!Sharp cutoff (by spec) of good vs no reception. Intermediate loss range band is just a few dB wide!Why?Biggest factor: Multi-path interference802.11 receivers can suppress reflections < 250nsOutdoor reflections delay often > 1 \mu secDelay offsets == symbol time look like valid symbols (large interferece)Offsets != symbol time look like random noiseSmall changes in delay == big changes in loss rate*

  • Deciding Between LinksMost early protocols: Hop CountLink-layer retransmission can mask some lossBut: a 50% loss rate means your link is only 50% as fast!Threshold?Can sacrifice connectivity. Isnt a 90% path better than an 80% path?Real life goal: Find highest throughput paths*

  • Is there a better metric?Cut-off thresholdDisconnected networkProduct of link delivery ratio along pathDoes not account for inter-hop interference Bottleneck link (highest-loss-ratio link)Same as aboveEnd-to-end delayDepends on interface queue lengths*

  • ETX Metric Design GoalsFind high throughput paths

    Account for lossy links

    Account for asymmetric links

    Account for inter-link interference

    Independent of network load (dont incorporate congestion)*

  • Forwarding Packets is ExpensiveThroughput of 802.11b =~ 11Mbits/sIn reality, you can get about 5.What is throughput of a chain?A B C ?A B C D ?Assume minimum power for radios.

    Routing metric should take this into account! Affects throughput*

  • ETXMeasure each links delivery probability with broadcast probes (& measure reverse)P(delivery) = ( df * dr ) (ACK must be delivered too)Link ETX = 1