Politecnico di Milano Facoltà di Ingegneria dell Informazione. WI-7 Ad hoc networks. Wireless Internet Prof. Antonio Capone

Transcription

1 Politecnico di Milano Facoltà di Ingegneria dell Informazione WI-7 Ad hoc networks Wireless Internet Prof. Antonio Capone

2 Acknowlegments o This class notes are mostly based on the teaching material of: n Prof. Eylem Ekici (Ohio State University at Columbus) n Prof. Nitin H. Vaidya (University of Illinois at Urbana-Champaign) A. Capone: Wireless Internet 2

3 Introduction o Mobile Ad Hoc Networks (MANET): n Networks of potentially mobile network nodes n Nodes equipped with wireless communication interfaces n No pre-established infrastructure n Communication between peers involve multiple hops o Implications n Nodes act both as hosts as well as routers n Dynamic network topology A. Capone: Wireless Internet 3

4 Ad Hoc Network Abstractions o Every node can communicate directly with a subset of mobile nodes (neighbors) n Communication range of a node varies depending on physical changes n Communication range abstracted as circles A. Capone: Wireless Internet 4

5 Mobile Ad Hoc Networks o Mobility causes topology changes n Topology changes lead to changes in data delivery decisions n Introduces real-time adaptation requirements A. Capone: Wireless Internet 5

6 Mobile Ad Hoc Networks o Advantages of Mobile Ad Hoc Networks n Rapid deployment o Particularly important for emergency response and security applications n Infrastructure independence o No infrastructure needed to kick-start deployment o Attractive for disaster recovery (remember Katrina) n Flexibility o Addition, removal, and relocation of nodes automatically handled o Enables new applications where number of participants is dynamic and unpredictable A. Capone: Wireless Internet 6

7 Example Applications o Disaster recovery, emergency, security applications n Law enforcement n Natural and man-made disaster recovery o Civilian applications n Conference room networks n Networking in large vessels n Personal area networks n Vehicular networks o Military applications n Ground-based battlefield networks n Hybrid platform networks (land, air, and sea based) A. Capone: Wireless Internet 7

8 MANET Properties o Homogeneous MANETs: n All nodes carry same properties o Communication equipment and range o Processing capabilities, memory, energy supplies n All nodes have identical functionalities o All nodes are hosts and routers o Leads to flat organization of the network o Heterogeneous MANETs: n Nodes have different hardware o Communication equipment and range o Variation of node resources o Leads to inherent hierarchical organization n Nodes with diverse functions o Host vs. router, cluster member vs. cluster head A. Capone: Wireless Internet 8

9 Node Mobility Properties o Node mobility descriptors n Speed, Direction, Movement patterns o Movement of groups of nodes n Highly uncorrelated movements o Exhibition halls, festival grounds n Highly correlated movements o Commuters on trains, truck convoys n Coordinated movements o Movement of military units n Hybrid mobility o Movement of personal area networks A. Capone: Wireless Internet 9

10 Data Traffic Properties o Data traffic is generally applicationdependent n Bandwidth requirements n Timeliness constraints n Reliability constraints n Security constraints o Effects on delivery methods n Point-to-point vs. point-to-multi-point n Pure MANET vs. access to infrastructure o Addressing requirements n Host-based, content-based, other A. Capone: Wireless Internet 10

11 Problems to Address o Physical layer n Range, symmetry, power control o MAC layer n Hidden terminal problem, asymmetrical links, error control, energy efficiency, fairness o Network layer n Point-to-point, point-to-multi-point, flat, hierarchical, proactive, reactive, hybrid, mobility-tailored o Transport layer n Packet loss discrimination, intermediate buffering A. Capone: Wireless Internet 11

12 Ultimate Goal o Develop solutions that can n Be used in all ad hoc networks n Satisfy various application-level constraints n Adapt to changing topological properties n Integrate various types of nodes into MANET n Interact with fixed infrastructures o This goal has not been reached so far A. Capone: Wireless Internet 12

13 Routing in Mobile Ad Hoc Networks A. Capone: Wireless Internet 13

14 Introduction o Routing in ad hoc networks should account for host mobility, which leads to dynamic topologies o Routing protocols designed for static (or slowly changing) networks n May not keep up with the rate of change n Waste limited resources n May not cater to specific performance criteria such as energy consumption o As usual, no single protocol is optimal for all ad hoc network types and conditions A. Capone: Wireless Internet 14

15 Protocol Classification Routing Protocols for MANETs Reactive Hybrid Proactive Geographic o Reactive Protocols n Determine the paths on-demand o Proactive Protocols n Maintain paths regardless of traffic conditions o Hybrid Protocols n Generally maintain local paths proactively, and create large scale paths reactively o Geographic Protocols n Based on geographical location of nodes A. Capone: Wireless Internet 15

16 Protocol Classification o Reactive Protocols n Generally involve large delays between the request and first packet delivery n Incur low overhead in low traffic scenarios o Proactive Protocols n Packets are immediately delivered as paths are already established n Results in high path maintenance overhead since the paths are kept regardless of traffic patterns o Hybrid Protocols n Operate midway of delay and overhead performance o Geographic Protocols n Can be used only when location information is available A. Capone: Wireless Internet 16

17 Trade-Off o Latency of route discovery n Proactive protocols may have lower latency since routes are maintained at all times n Reactive protocols may have higher latency because a route from X to Y will be found only when X attempts to send to Y o Overhead of route discovery/maintenance n Reactive protocols may have lower overhead since routes are determined only if needed n Proactive protocols can (but not necessarily) result in higher overhead due to continuous route updating o Which approach achieves a better trade-off depends on the traffic and mobility patterns A. Capone: Wireless Internet 17

18 Flooding for Data Delivery o Sender S broadcasts data packet P to all its neighbors o Each node receiving P forwards P to its neighbors o Sequence numbers used to avoid the possibility of forwarding the same packet more than once o Packet P reaches destination D provided that D is reachable from sender S o Node D does not forward the packet A. Capone: Wireless Internet 18

19 Flooding for Data Delivery Y Z S E A B H C I G F K J D M N L Represents a node that has received packet P Represents that connected nodes are within each other s transmission range A. Capone: Wireless Internet 19

20 Flooding for Data Delivery Broadcast transmission Y Z S E A B H C I G F K J D M N L Represents a node that receives packet P for the first time Represents transmission of packet P A. Capone: Wireless Internet 20

21 Flooding for Data Delivery Y Z S E A B H C I G F K J D M N L Node H receives packet P from two neighbors: potential for collision A. Capone: Wireless Internet 21

22 Flooding for Data Delivery Y Z S E A B H C I G F K J D M N L Node C receives packet P from G and H, but does not forward it again, because node C has already forwarded packet P once A. Capone: Wireless Internet 22

23 Flooding for Data Delivery Y Z S E A B H C I G F K J D M N L Nodes J and K both broadcast packet P to node D Since nodes J and K are hidden from each other, their transmissions may collide => Packet P may not be delivered to node D at all, despite the use of flooding A. Capone: Wireless Internet 23

24 Flooding for Data Delivery Y Z S E A B H C I G F K J D M N L Node D does not forward packet P, because node D is the intended destination of packet P A. Capone: Wireless Internet 24

25 Flooding for Data Delivery Y Z S E B A H Flooding completed C I G F K J D M N L Nodes unreachable from S do not receive packet P (e.g., node Z) Nodes for which all paths from S go through the destination D also do not receive packet P (example: node N) A. Capone: Wireless Internet 25

26 Flooding for Data Delivery Y Z S E A B H C I G F K J D M N L Flooding may deliver packets to too many nodes (in the worst case, all nodes reachable from sender may receive the packet) A. Capone: Wireless Internet 26

27 Flooding for Data Delivery: Advantages o Simplicity o May be more efficient than other protocols when rate of information transmission is low enough that the overhead of explicit route discovery/maintenance incurred by other protocols is relatively higher n this scenario may occur, for instance, when nodes transmit small data packets relatively infrequently, and many topology changes occur between consecutive packet transmissions o Potentially higher reliability of data delivery n Because packets may be delivered to the destination on multiple paths A. Capone: Wireless Internet 27

28 Flooding for Data Delivery: Disadvantages o Potentially, very high overhead n Data packets may be delivered to too many nodes who do not need to receive them o Potentially lower reliability of data delivery n Flooding uses broadcasting -- hard to implement reliable broadcast delivery without significantly increasing overhead n Broadcasting in IEEE MAC is unreliable n In our example, nodes J and K may transmit to node D simultaneously, resulting in loss of the packet n in this case, destination would not receive the packet at all A. Capone: Wireless Internet 28

29 Flooding of Control Packets o Many protocols perform (potentially limited) flooding of control packets, instead of data packets o The control packets are used to discover routes o Discovered routes are subsequently used to send data packet(s) o Overhead of control packet flooding is amortized over data packets transmitted between consecutive control packet floods A. Capone: Wireless Internet 29

30 Reactive Protocols A. Capone: Wireless Internet 30

31 Dynamic Source Routing (DSR) o When node S wants to send a packet to node D, but does not know a route to D, node S initiates a route discovery o Source node S floods Route Request (RREQ) o Each node appends own identifier when forwarding RREQ A. Capone: Wireless Internet 31

32 Route Discovery in DSR Y Z S E A B H C I G F K J D M N L Represents a node that has received RREQ for D from S A. Capone: Wireless Internet 32

33 Route Discovery in DSR Broadcast transmission Y [S] Z S E A B H C I G F K J D M N L [X,Y] Represents transmission of RREQ Represents list of identifiers appended to RREQ A. Capone: Wireless Internet 33

34 Route Discovery in DSR Y S E [S,E] Z A B H C [S,C] I G F K J D M N L Node H receives packet RREQ from two neighbors: potential for collision A. Capone: Wireless Internet 34

35 Route Discovery in DSR Y Z S E A B H C I G F [S,C,G] [S,E,F] K J D M N L Node C receives RREQ from G and H, but does not forward it again, because node C has already forwarded RREQ once A. Capone: Wireless Internet 35

36 Route Discovery in DSR Y Z A B H S C I E G F K [S,E,F,J] M J D [S,C,G,K] N L Nodes J and K both broadcast RREQ to node D Since nodes J and K are hidden from each other, their transmissions may collide A. Capone: Wireless Internet 36

37 Route Discovery in DSR Y Z A B H S C I E G F K J [S,E,F,J,M] M L D N Node D does not forward RREQ, because node D is the intended target of the route discovery A. Capone: Wireless Internet 37

38 Route Discovery in DSR o Destination D on receiving the first RREQ, sends a Route Reply (RREP) o RREP is sent on a route obtained by reversing the route appended to received RREQ o RREP includes the route from S to D on which RREQ was received by node D A. Capone: Wireless Internet 38

39 Route Reply in DSR Y S E RREP [S,E,F,J,D] Z A B H C I G F K J M D N L Represents RREP control message A. Capone: Wireless Internet 39

40 Route Reply in DSR o Route Reply can be sent by reversing the route in Route Request (RREQ) only if links are guaranteed to be bi-directional n To ensure this, RREQ should be forwarded only if it received on a link that is known to be bi-directional o If unidirectional (asymmetric) links are allowed, then RREP may need a route discovery for S from node D n n Unless node D already knows a route to node S If a route discovery is initiated by D for a route to S, then the Route Reply is piggybacked on the Route Request from D. o If IEEE MAC is used to send data, then links have to be bi-directional (since Ack is used) A. Capone: Wireless Internet 40

41 Dynamic Source Routing (DSR) o Node S on receiving RREP, caches the route included in the RREP o When node S sends a data packet to D, the entire route is included in the packet header n hence the name source routing o Intermediate nodes use the source route included in a packet to determine to whom a packet should be forwarded A. Capone: Wireless Internet 41

42 Data Delivery in DSR Y DATA [S,E,F,J,D] Z S E A B H C I G F K J D M N L Packet header size grows with route length A. Capone: Wireless Internet 42

43 When to Perform a Route Discovery o When node S wants to send data to node D, but does not know a valid route node D A. Capone: Wireless Internet 43

44 DSR Optimization: Route Caching o Each node caches a new route it learns by any means o When node S finds route [S,E,F,J,D] to node D, node S also learns route [S,E,F] to node F o When node K receives Route Request [S,C,G] destined for node, node K learns route [K,G,C,S] to node S o When node F forwards Route Reply RREP [S,E,F,J,D], node F learns route [F,J,D] to node D o When node E forwards Data [S,E,F,J,D] it learns route [E,F,J,D] to node D o A node may also learn a route when it overhears Data packets A. Capone: Wireless Internet 44

45 Use of Route Caching o When node S learns that a route to node D is broken, it uses another route from its local cache, if such a route to D exists in its cache. Otherwise, node S initiates route discovery by sending a route request o Node X on receiving a Route Request for some node D can send a Route Reply if node X knows a route to node D o Use of route cache n can speed up route discovery n can reduce propagation of route requests A. Capone: Wireless Internet 45

46 Use of Route Caching [S,E,F,J,D] [E,F,J,D] S E [F,J,D],[F,E,S] A B H C [C,S] I G [G,C,S] F K [J,F,E,S] J D M N L Z [P,Q,R] Represents cached route at a node (DSR maintains the cached routes in a tree format) A. Capone: Wireless Internet 46

47 Route Caching: Can Speed up Route Discovery [S,E,F,J,D] [E,F,J,D] S E [F,J,D],[F,E,S] B C [G,C,S] A [C,S] G H [K,G,C,S] K I RREQ When node Z sends a route request for node C, node K sends back a route reply [Z,K,G,C] to node Z using a locally cached route F [J,F,E,S] J RREP Z D M N L A. Capone: Wireless Internet 47

48 Route Caching: Can Reduce Propagation of Route Requests [S,E,F,J,D] S [E,F,J,D] F B [J,F,E,S] C [G,C,S] M J A [C,S] G H D [K,G,C,S] K I RREP RREQ Z Assume that there is no link between D and Z. Route Reply (RREP) from node K limits flooding of RREQ. In general, the reduction may be less dramatic. E [F,J,D],[F,E,S] A. Capone: Wireless Internet 48 Y N L

49 Route Error (RERR) Y RERR [J-D] Z S E A B H C I G F K J D M N L J sends a route error to S along route J-F-E-S when its attempt to forward the data packet S (with route SEFJD) on J-D fails Nodes hearing RERR update their route cache to remove link J-D A. Capone: Wireless Internet 49

50 Route Caching: Beware! o Stale caches can adversely affect performance o With passage of time and host mobility, cached routes may become invalid o A sender host may try several stale routes (obtained from local cache, or replied from cache by other nodes), before finding a good route A. Capone: Wireless Internet 50

51 Dynamic Source Routing: Advantages o Routes maintained only between nodes who need to communicate n reduces overhead of route maintenance o Route caching can further reduce route discovery overhead o A single route discovery may yield many routes to the destination, due to intermediate nodes replying from local caches A. Capone: Wireless Internet 51

52 Dynamic Source Routing: Disadvantages o Packet header size grows with route length due to source routing o Flood of route requests may potentially reach all nodes in the network o Care must be taken to avoid collisions between route requests propagated by neighboring nodes n insertion of random delays before forwarding RREQ o Increased contention if too many route replies come back due to nodes replying using their local cache n Route Reply Storm problem n Reply storm may be eased by preventing a node from sending RREP if it hears another RREP with a shorter route A. Capone: Wireless Internet 52

53 Dynamic Source Routing: Disadvantages o An intermediate node may send Route Reply using a stale cached route, thus polluting other caches o This problem can be eased if some mechanism to purge (potentially) invalid cached routes is incorporated. A. Capone: Wireless Internet 53

54 Ad Hoc On-Demand Distance Vector Routing (AODV) o DSR includes source routes in packet headers o Resulting large headers can sometimes degrade performance n particularly when data contents of a packet are small o AODV attempts to improve on DSR by maintaining routing tables at the nodes, so that data packets do not have to contain routes o AODV retains the desirable feature of DSR that routes are maintained only between nodes which need to communicate A. Capone: Wireless Internet 54

55 AODV o Route Requests (RREQ) are forwarded in a manner similar to DSR o When a node re-broadcasts a Route Request, it sets up a reverse path pointing towards the source n AODV assumes symmetric (bi-directional) links o When the intended destination receives a Route Request, it replies by sending a Route Reply o Route Reply travels along the reverse path set-up when Route Request is forwarded A. Capone: Wireless Internet 55

56 Route Requests in AODV Y Z S E A B H C I G F K J D M N L Represents a node that has received RREQ for D from S A. Capone: Wireless Internet 56

57 Route Requests in AODV Broadcast transmission Y Z S E A B H C I G F K J D M N L Represents transmission of RREQ A. Capone: Wireless Internet 57

58 Route Requests in AODV Y Z S E A B H C I G F K J D M N L Represents links on Reverse Path A. Capone: Wireless Internet 58

59 Reverse Path Setup in AODV Y Z S E A B H C I G F K J D M N L Node C receives RREQ from G and H, but does not forward it again, because node C has already forwarded RREQ once A. Capone: Wireless Internet 59

60 Reverse Path Setup in AODV Y Z S E A B H C I G F K J D M N L A. Capone: Wireless Internet 60

61 Reverse Path Setup in AODV Y Z S E A B H C I G F K J D M N L Node D does not forward RREQ, because node D is the intended target of the RREQ A. Capone: Wireless Internet 61

62 Route Reply in AODV Y Z S E A B H C I G F K J D M N L Represents links on path taken by RREP A. Capone: Wireless Internet 62

63 Route Reply in AODV o An intermediate node (not the destination) may also send a Route Reply (RREP) provided that it knows a more recent path than the one previously known to sender S o To determine whether the path known to an intermediate node is more recent, destination sequence numbers are used o The likelihood that an intermediate node will send a Route Reply when using AODV not as high as DSR n A new Route Request by node S for a destination is assigned a higher destination sequence number. An intermediate node which knows a route, but with a smaller sequence number, cannot send Route Reply A. Capone: Wireless Internet 63

64 Forward Path Setup in AODV Y Z S E A B H C I G F K J D M N L Forward links are setup when RREP travels along the reverse path Represents a link on the forward path A. Capone: Wireless Internet 64

65 Data Delivery in AODV Y DATA Z S E A B H C I G F K J D M N L Routing table entries used to forward data packet. Route is not included in packet header. A. Capone: Wireless Internet 65

66 Timeouts o A routing table entry maintaining a reverse path is purged after a timeout interval n timeout should be long enough to allow RREP to come back o A routing table entry maintaining a forward path is purged if not used for a active_route_timeout interval n if no data is being sent using a particular routing table entry, that entry will be deleted from the routing table (even if the route may actually still be valid) A. Capone: Wireless Internet 66

67 Link Failure Reporting o A neighbor of node X is considered active for a routing table entry if the neighbor sent a packet within active_route_timeout interval which was forwarded using that entry o When the next hop link in a routing table entry breaks, all active neighbors are informed o Link failures are propagated by means of Route Error messages, which also update destination sequence numbers A. Capone: Wireless Internet 67

68 Route Error o When node X is unable to forward packet P (from node S to node D) on link (X,Y), it generates a RERR message o Node X increments the destination sequence number for D cached at node X o The incremented sequence number N is included in the RERR o When node S receives the RERR, it initiates a new route discovery for D using destination sequence number at least as large as N A. Capone: Wireless Internet 68

69 Destination Sequence Number o Continuing from the previous slide o When node D receives the route request with destination sequence number N, node D will set its sequence number to N, unless it is already larger than N A. Capone: Wireless Internet 69

70 Link Failure Detection o Hello messages: Neighboring nodes periodically exchange hello message o Absence of hello message is used as an indication of link failure o Alternatively, failure to receive several MAC-level acknowledgement may be used as an indication of link failure A. Capone: Wireless Internet 70

71 Why Sequence Numbers in AODV o To avoid using old/broken routes n To determine which route is newer o To prevent formation of loops A B C D E n Assume that A does not know about failure of link C- D because RERR sent by C is lost n Now C performs a route discovery for D. Node A receives the RREQ (say, via path C-E-A) n Node A will reply since A knows a route to D via node B n Results in a loop (for instance, C-E-A-B-C ) A. Capone: Wireless Internet 71

72 Why Sequence Numbers in AODV A B C D E n Loop C-E-A-B-C A. Capone: Wireless Internet 72

73 Optimization: Expanding Ring Search o Route Requests are initially sent with small Time-to-Live (TTL) field, to limit their propagation n DSR also includes a similar optimization o If no Route Reply is received, then larger TTL tried A. Capone: Wireless Internet 73

74 Summary: AODV o Routes need not be included in packet headers o Nodes maintain routing tables containing entries only for routes that are in active use o At most one next-hop per destination maintained at each node n Multi-path extensions can be designed n DSR may maintain several routes for a single destination o Unused routes expire even if topology does not change A. Capone: Wireless Internet 74

75 Link Reversal Algorithm A B F C E G D A. Capone: Wireless Internet 75

76 Link Reversal Algorithm A B F Links are bi-directional C E G But algorithm imposes logical directions on them D Maintain a directed acyclic graph (DAG) for each destination, with the destination being the only sink This DAG is for destination node D A. Capone: Wireless Internet 76

77 Link Reversal Algorithm A B F C E G D Link (G,D) broke Any node, other than the destination, that has no outgoing links reverses all its incoming links. Node G has no outgoing links A. Capone: Wireless Internet 77

78 Link Reversal Algorithm A B F C E G Represents a link that was reversed recently D Now nodes E and F have no outgoing links A. Capone: Wireless Internet 78

79 Link Reversal Algorithm A B F C E G Represents a link that was reversed recently D Now nodes B and G have no outgoing links A. Capone: Wireless Internet 79

80 Link Reversal Algorithm A B F C E G Represents a link that was reversed recently D Now nodes A and F have no outgoing links A. Capone: Wireless Internet 80

81 Link Reversal Algorithm A B F C E G Represents a link that was reversed recently D Now all nodes (other than destination D) have an outgoing link A. Capone: Wireless Internet 81

82 Link Reversal Algorithm A B F C E G D DAG has been restored with only the destination as a sink A. Capone: Wireless Internet 82

83 Link Reversal Algorithm o Attempts to keep link reversals local to where the failure occurred n But this is not guaranteed o When the first packet is sent to a destination, the destination oriented DAG is constructed o The initial construction does result in flooding of control packets A. Capone: Wireless Internet 83

84 Link Reversal Algorithm o The previous algorithm is called a full reversal method since when a node reverses links, it reverses all its incoming links o Partial reversal method: A node reverses incoming links from only those neighbors who have not themselves reversed links previously n If all neighbors have reversed links, then the node reverses all its incoming links n Previously at node X means since the last link reversal done by node X A. Capone: Wireless Internet 84

85 Partial Reversal Method A B F C E G Link (G,D) broke D Node G has no outgoing links A. Capone: Wireless Internet 85

86 Partial Reversal Method A B F C E G D Represents a link that was reversed recently Represents a node that has reversed links Now nodes E and F have no outgoing links A. Capone: Wireless Internet 86

87 Partial Reversal Method A B F C E G Represents a link that was reversed recently D Nodes E and F do not reverse links from node G Now node B has no outgoing links A. Capone: Wireless Internet 87

88 Partial Reversal Method A B F C E G Represents a link that was reversed recently D Now node A has no outgoing links A. Capone: Wireless Internet 88

89 Partial Reversal Method A B F C E G Represents a link that was reversed recently D Now all nodes (except destination D) have outgoing links A. Capone: Wireless Internet 89

90 Partial Reversal Method A B F C E G D DAG has been restored with only the destination as a sink A. Capone: Wireless Internet 90

91 Link Reversal Methods: Advantages o Link reversal methods attempt to limit updates to routing tables at nodes in the vicinity of a broken link o Each node may potentially have multiple routes to a destination A. Capone: Wireless Internet 91

92 Link Reversal Methods: Disadvantage o Need a mechanism to detect link failure n hello messages may be used n but hello messages can add to contention o If network is partitioned, link reversals continue indefinitely A. Capone: Wireless Internet 92

93 Link Reversal in a Partitioned Network A B F C E G D This DAG is for destination node D A. Capone: Wireless Internet 93

94 Full Reversal in a Partitioned Network A B F C E G D A and G do not have outgoing links A. Capone: Wireless Internet 94

95 Full Reversal in a Partitioned Network A B F C E G D E and F do not have outgoing links A. Capone: Wireless Internet 95

96 Full Reversal in a Partitioned Network A B F C E G D B and G do not have outgoing links A. Capone: Wireless Internet 96

97 Full Reversal in a Partitioned Network A B F C E G D E and F do not have outgoing links A. Capone: Wireless Internet 97

98 Full Reversal in a Partitioned Network A B F In the partition disconnected from destination D, link reversals continue, until the partitions merge C E G Need a mechanism to minimize this wasteful activity D Similar scenario can occur with partial reversal method too A. Capone: Wireless Internet 98

99 Temporally-Ordered Routing Algorithm (TORA) o TORA modifies the partial link reversal method to be able to detect partitions o When a partition is detected, all nodes in the partition are informed, and link reversals in that partition cease A. Capone: Wireless Internet 99

100 Partition Detection in TORA E A C B DAG for destination D D F A. Capone: Wireless Internet 100

101 Partition Detection in TORA A B E C D F TORA uses a modified partial reversal method Node A has no outgoing links A. Capone: Wireless Internet 101

102 Partition Detection in TORA A B E C D F TORA uses a modified partial reversal method Node B has no outgoing links A. Capone: Wireless Internet 102

103 Partition Detection in TORA A B E C D F Node B has no outgoing links A. Capone: Wireless Internet 103

104 Partition Detection in TORA A B E C D F Node C has no outgoing links -- all its neighbor have reversed links previously. A. Capone: Wireless Internet 104

105 Partition Detection in TORA A B E C D F Nodes A and B receive the reflection from node C Node B now has no outgoing link A. Capone: Wireless Internet 105

106 Partition Detection in TORA A B E C Node B propagates the reflection to node A D F Node A has received the reflection from all its neighbors. Node A determines that it is partitioned from destination D. A. Capone: Wireless Internet 106

107 Partition Detection in TORA A B E D C On detecting a partition, node A sends a clear (CLR) message that purges all directed links in that partition F A. Capone: Wireless Internet 107

108 TORA o Improves on the partial link reversal method by detecting partitions and stopping non-productive link reversals o Paths may not be shortest o The DAG provides many hosts the ability to send packets to a given destination n Beneficial when many hosts want to communicate with a single destination A. Capone: Wireless Internet 108

109 TORA Design Decision o TORA performs link reversals o However, when a link breaks, it looses its direction o When a link is repaired, it may not be assigned a direction, unless some node has performed a route discovery after the link broke n if no one wants to send packets to D anymore, eventually, the DAG for destination D may disappear o TORA makes effort to maintain the DAG for D only if someone needs route to D n Reactive behavior A. Capone: Wireless Internet 109

110 Proactive Protocols A. Capone: Wireless Internet 110

111 Link State Routing o Each node periodically floods status of its links o Each node re-broadcasts link state information received from its neighbor o Each node keeps track of link state information received from other nodes o Each node uses above information to determine next hop to each destination A. Capone: Wireless Internet 111

112 Optimized Link State Routing (OLSR) o The overhead of flooding link state information is reduced by requiring fewer nodes to forward the information o A broadcast from node X is only forwarded by its multipoint relays o Multipoint relays of node X are its neighbors such that each two-hop neighbor of X is a one-hop neighbor of at least one multipoint relay of X n Each node transmits its neighbor list in periodic beacons, so that all nodes can know their 2-hop neighbors, in order to choose the multipoint relays A. Capone: Wireless Internet 112

113 Optimized Link State Routing (OLSR) o Nodes C and E are multipoint relays of node A B F J G C A D E H K Node that has broadcast state information from A A. Capone: Wireless Internet 113

114 Optimized Link State Routing (OLSR) o Nodes C and E forward information received from A B F J G C A D E H K Node that has broadcast state information from A A. Capone: Wireless Internet 114

115 Optimized Link State Routing (OLSR) o Nodes E and K are multipoint relays for node H o Node K forwards information received from H n E has already forwarded the same information once B F J G C A D E H K Node that has broadcast state information from A A. Capone: Wireless Internet 115

116 OLSR o OLSR floods information through the multipoint relays o The flooded information itself is for links connecting nodes to respective multipoint relays o Routes used by OLSR only include multipoint relays as intermediate nodes A. Capone: Wireless Internet 116

117 Destination-Sequenced Distance-Vector (DSDV) o Each node maintains a routing table which stores n next hop towards each destination n a cost metric for the path to each destination n a destination sequence number that is created by the destination itself n Sequence numbers used to avoid formation of loops o Each node periodically forwards the routing table to its neighbors n Each node increments and appends its sequence number when sending its local routing table n This sequence number will be attached to route entries created for this node A. Capone: Wireless Internet 117

118 Destination-Sequenced Distance-Vector (DSDV) o Assume that node X receives routing information from Y about a route to node Z X Y Z o Let S(X) and S(Y) denote the destination sequence number for node Z as stored at node X, and as sent by node Y with its routing table to node X, respectively A. Capone: Wireless Internet 118

119 Destination-Sequenced Distance-Vector (DSDV) o Node X takes the following steps: X Y Z n If S(X) > S(Y), then X ignores the routing information received from Y n If S(X) = S(Y), and cost of going through Y is smaller than the route known to X, then X sets Y as the next hop to Z n If S(X) < S(Y), then X sets Y as the next hop to Z, and S(X) is updated to equal S(Y) A. Capone: Wireless Internet 119

120 Hybrid Protocols A. Capone: Wireless Internet 120

121 Zone Routing Protocol (ZRP) Zone routing protocol combines o Proactive protocol: which pro-actively updates network state and maintains route regardless of whether any data traffic exists or not o Reactive protocol: which only determines route to a destination if there is some data to be sent to the destination A. Capone: Wireless Internet 121

122 ZRP o All nodes within hop distance at most d from a node X are said to be in the routing zone of node X o All nodes at hop distance exactly d are said to be peripheral nodes of node X s routing zone A. Capone: Wireless Internet 122

123 ZRP o Intra-zone routing: Pro-actively maintain state information for links within a short distance from any given node n Routes to nodes within short distance are thus maintained proactively (using, say, link state or distance vector protocol) o Inter-zone routing: Use a route discovery protocol for determining routes to far away nodes. Route discovery is similar to DSR with the exception that route requests are propagated via peripheral nodes. A. Capone: Wireless Internet 123

124 ZRP: Example with Zone Radius = d = 2 S performs route discovery for D B F A S C E D Denotes route request A. Capone: Wireless Internet 124

125 ZRP: Example with d = 2 S performs route discovery for D B F A S C E D Denotes route reply E knows route from E to D, so route request need not be forwarded to D from E A. Capone: Wireless Internet 125

126 ZRP: Example with d = 2 S performs route discovery for D B F A S C E D Denotes route taken by Data A. Capone: Wireless Internet 126

127 Geographic routing A. Capone: Wireless Internet 127

128 Geographic Distance Routing (GEDIR) o Rather than maintaining routing tables and discovering paths, one can also use the geographic location of nodes n Requires that each node knows it own location (e.g., using GPS) n Requires knowledge of all neighbor locations o It is based on sending the packet to the neighbor that is closest to the destination n Works only if nodes are located densely n Obstacles and low node density may lead to routing failures A. Capone: Wireless Internet 128

129 GEDIR Example S D Regular Operation (not necessarily minimum hop) S D Routing fails because 3 has no neighbors closer to D than itself o To overcome the problem of not finding closer neighbors, expanded local search algorithms are also proposed n When stuck, broadcast a path discovery request with small TTL, use discovered path for forwarding data A. Capone: Wireless Internet 129

130 Greedy Perimeter Stateless Routing (GPSR) o Another geographic routing algorithm o Like GEDIR, it is also based on greedy forwarding n Maintain a list of neighbors with their locations n Send the packet to the node nearest to the destination (Most Forward within Radius MFR) n Avoid routing loops A. Capone: Wireless Internet 130

131 GPSR o Avoiding routing gaps: n Use perimeter routing n Mark the line connecting the intermediate node with destination n Take the hop to its immediate left (counter-clockwise) n Right hand rule! A. Capone: Wireless Internet 131

132 GPSR o Perimeter routing requires that graphs are planar n No edge in the graph crosses another edge o Planarization algorithms Relative Neighbor Graph Gabriel Graph In both cases, eliminate link uv A. Capone: Wireless Internet 132