Mobile Ad Hoc Networks. Ad Hoc Networks - A New Communication Paradigm 1. Mobile Ad Hoc Networks (MANET) Mobile Ad Hoc Networks. Why Ad Hoc Networks?

Transcription

1 obile d oc etworks d oc etworks - ew ommunication Paradigm 1 ormed by wireless hosts which may be mobile Without (necessarily) using a pre-existing infrastructure Routes between nodes may potentially contain multiple hops (due to limited transmission range) 1 These slides were adapted from a tutorial given by. Vaidya at 2001 obiom. ou may view/download the full presentation at the following address: Wireless omputing and etwork ystems Page 1 Wireless omputing and etwork ystems Page 2 obile d oc etworks ay need to traverse multiple links to reach a destination obile d oc etworks (T) obility causes route changes Wireless omputing and etwork ystems Page 3 Wireless omputing and etwork ystems Page 4 Why d oc etworks? ase of deployment peed of deployment ecreased dependence on infrastructure any pplications Personal area networking cell phone, laptop, ear phone, wrist watch ilitary environments soldiers, tanks, planes ivilian environments taxi cab network meeting rooms sports stadiums boats, small aircraft mergency operations search-and-rescue policing and fire fighting Unknown? Wireless omputing and etwork ystems Page 5 Wireless omputing and etwork ystems Page 6 1

2 any Variations ully ymmetric nvironment all nodes have identical capabilities and responsibilities symmetric apabilities transmission ranges and radios may differ battery life at different nodes may differ processing capacity may be different at different nodes speed of movement symmetric Responsibilities only some nodes may route packets some nodes may act as leaders of nearby nodes (e.g., cluster head) any Variations Traffic characteristics may differ in different ad hoc networks bit rate timeliness constraints reliability requirements unicast / multicast / geocast host-based addressing / content-based addressing / capability-based addressing ay co-exist (and co-operate) with an infrastructure-based network Wireless omputing and etwork ystems Page 7 Wireless omputing and etwork ystems Page 8 any Variations obility patterns may be different people sitting at an airport lounge ew ork taxi cabs kids playing military movements personal area network obility characteristics speed predictability direction of movement pattern of movement uniformity (or lack thereof) of mobility characteristics among different nodes Wireless omputing and etwork ystems Page 9 hallenges imited wireless transmission range roadcast nature of the wireless medium idden terminal problem (see next slide) Packet losses due to transmission errors obility-induced route changes and packet loss connectivity and propagation patterns traffic and mobility patterns obility induced packet loss attery constraints Potentially frequent network partitions ase of snooping on wireless transmissions (security hazard) Wireless omputing and etwork ystems Page 10 hallenges in d-oc etworks The challenges in the design of d-oc networks stem from the following facts: the lack of centralized entity self-organizing and distributed protocols the possibility of rapid platforms movement (highly versatile topology) efficient and robust protocols all communication is carried over the wireless medium power and spectrum efficient communications ompare this with the fixed (cellular) networks Research on obile d oc etworks Variations in capabilities & responsibilities X Variations in traffic characteristics, mobility models, etc. X Performance criteria (e.g., optimize throughput, reduce energy consumption) + ncreased research funding = ignificant research activity stablishment of the T T Working roup to study Pbased routing in ad-hoc networks Wireless omputing and etwork ystems Page 11 Wireless omputing and etwork ystems Page 12 2

3 The oly rail d oc rchitectural esign hoices (1) one-size-fits-all solution Perhaps using an adaptive/hybrid approach that can adapt to situation at hand ifficult problem any solutions proposed trying to address a sub-space of the problem domain Routing/obility anagement: Proactive vs. Reactive Routing Protocol edium ccess ontrol: To ense or ot to ense, this is a question! ultimedia Traffic: ow to handle Qo for real-time traffic? s it possible at all? ecurity: uthentication, uthorization, Privacy, in the ad hoc networking environment. Wireless omputing and etwork ystems Page 13 Wireless omputing and etwork ystems Page 14 d oc rchitectural esign hoices (2) rchitectural esign hoices (1) elected examples of ad-hoc architectural choices: flat vs. hierarchical (multi-tiered) network proactive vs. reactive routing protocols degree of globalization in topological changes frequency of communication of topological changes criteria for route selection / max route determination sizing of nodal transmission radius synchronized vs. unsynchronized operation vs. dialog-based The lat etwork rchitecture lat d-oc etwork Wireless omputing and etwork ystems Page 15 Wireless omputing and etwork ystems Page 16 rchitectural esign hoices (2) ierarchical d-oc etwork rchitecture tier-2 network cluster head Overview of Unicast Routing Protocols tier-1 network tier-1 network cluster tier-1 network tier-1 network Two-Tier d-oc etwork Wireless omputing and etwork ystems Page 17 Wireless omputing and etwork ystems Page 18 3

4 Why is Routing in T different? Proactive Vs. Reactive Routing (1) ost mobility link failure/repair due to mobility may have different characteristics than those due to other causes Rate of link failure/repair may be high when nodes move fast ew performance criteria may be used route stability despite mobility energy consumption Proactive Protocols continuously update the reachability information at all the network nodes when a route is requested, it is immediately available examples of proactive protocols are: istance-vector (V), hortest Path irst (P), Open hortest Path irst (OP) Proactive protocols may be inappropriate for the ad hoc networking environment as they waste too much wireless resources, especially for large, highly mobile network. Wireless omputing and etwork ystems Page 19 Wireless omputing and etwork ystems Page 20 Proactive Vs. Reactive Routing (2) Reactive Protocols discover routes only upon demand involve some sort of global search there may be a significant delay associated with the route discovery examples of reactive protocols are the family of flooding protocols Reactive protocols may be inappropriate for the adhoc networking environment as they are too slow and consume too much network resources, especially for large, highly mobile network. Trade-Off atency of route discovery Proactive protocols may have lower latency since routes are maintained at all times Reactive protocols may have higher latency because a route from X to will be found only when X attempts to send to Overhead of route discovery/maintenance Reactive protocols may have lower overhead since routes are determined only if needed Proactive protocols can (but not necessarily) result in higher overhead due to continuous route updating Which approach achieves a better trade-off depends on the traffic and mobility patterns Wireless omputing and etwork ystems Page 21 Wireless omputing and etwork ystems Page 22 Routing Protocols for d-oc etworks looding for ata elivery The following are few examples of routing protocols for ad-hoc networks: ORP = Optimized inks tate Routing Protocol draft-ietf-manet-olsrp-04.txt R = ynamic ource Routing draft-ietf-manet-dsr-05.txt OV = d-hoc On-emand istance Vector draft-ietf-manet-aodv-08.txt TOR = Temporary-Ordered Routing lgorithm draft-ietf-manet-tora-spec-03.txt RP = one Routing Protocol draft-ietf-manet-zone-ierp-01.txt draft-ietf-manet-zone-iarp-01.txt draft-ietf-manet-zone-brp-01.txt ender broadcasts data packet P to all its neighbors ach node receiving P forwards P to its neighbors equence numbers used to avoid the possibility of forwarding the same packet more than once Packet P reaches destination provided that is reachable from sender ode does not forward the packet Wireless omputing and etwork ystems Page 23 Wireless omputing and etwork ystems Page 24 4

5 looding for ata elivery looding for ata elivery roadcast transmission Represents a node that has received packet P Represents that connected nodes are within each other s transmission range Wireless omputing and etwork ystems Page 25 Represents a node that receives packet P for the first time Represents transmission of packet P Wireless omputing and etwork ystems Page 26 looding for ata elivery looding for ata elivery ode receives packet P from two neighbors: potential for collision ode receives packet P from and, but does not forward it again, because node has already forwarded packet P once Wireless omputing and etwork ystems Page 27 Wireless omputing and etwork ystems Page 28 looding for ata elivery odes and both broadcast packet P to node ince nodes and are hidden from each other, their transmissions may collide => Packet P may not be delivered to node at all, despite the use of flooding Wireless omputing and etwork ystems Page 29 looding for ata elivery ode does not forward packet P, because node is the intended destination of packet P Wireless omputing and etwork ystems Page 30 5

6 looding for ata elivery looding for ata elivery looding completed odes unreachable from do not receive packet P (e.g., node ) odes for which all paths from go through the destination also do not receive packet P (example: node ) Wireless omputing and etwork ystems Page 31 looding may deliver packets to too many nodes (in the worst case, all nodes reachable from sender may receive the packet) Wireless omputing and etwork ystems Page 32 looding for ata elivery: dvantages looding for ata elivery: isadvantages implicity ay 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 this scenario may occur, for instance, when nodes transmit small data packets relatively infrequently, and many topology changes occur between consecutive packet transmissions Potentially higher reliability of data delivery ecause packets may be delivered to the destination on multiple paths Potentially, very high overhead ata packets may be delivered to too many nodes who do not need to receive them Potentially lower reliability of data delivery looding uses broadcasting -- hard to implement reliable broadcast delivery without significantly increasing overhead roadcasting in is unreliable n our example, nodes and may transmit to node simultaneously, resulting in loss of the packet in this case, destination would not receive the packet at all Wireless omputing and etwork ystems Page 33 Wireless omputing and etwork ystems Page 34 looding of ontrol Packets any protocols perform (potentially limited) flooding of control packets, instead of data packets The control packets are used to discover routes iscovered routes are subsequently used to send data packet(s) Overhead of control packet flooding is amortized over data packets transmitted between consecutive control packet floods ommon eatures of Reactive Routing Protocols (i.e. OV, R, TOR) Route discovery process is initiated only when a route is needed. Route discovery is based on the exchange of queries and replies. Route queries are broadcast. Route discovery process may produce multiple routes. Protocol design ensures loop freedom of routes. route maintenance mechanism is provided to inform nodes of invalid routes. Wireless omputing and etwork ystems Page 35 Wireless omputing and etwork ystems Page 36 6

7 ynamic ource Routing (R) [ohnson96] Route iscovery in R When node wants to send a packet to node, but does not know a route to, node initiates a route discovery ource node floods Route Request (RRQ) ach node appends own identifier when forwarding RRQ Represents a node that has received RRQ for from Wireless omputing and etwork ystems Page 37 Wireless omputing and etwork ystems Page 38 Route iscovery in R Route iscovery in R roadcast transmission [] [,] [,] [X,] Represents transmission of RRQ Represents list of identifiers appended to RRQ Wireless omputing and etwork ystems Page 39 ode receives packet RRQ from two neighbors: potential for collision Wireless omputing and etwork ystems Page 40 Route iscovery in R Route iscovery in R [,,] [,,] [,,,] [,,,] ode receives RRQ from and, but does not forward it again, because node has already forwarded RRQ once Wireless omputing and etwork ystems Page 41 odes and both broadcast RRQ to node ince nodes and are hidden from each other, their transmissions may collide Wireless omputing and etwork ystems Page 42 7

8 Route iscovery in R Route iscovery in R ode does not forward RRQ, because node is the intended target of the route discovery Wireless omputing and etwork ystems Page 43 [,,,,] estination on receiving the first RRQ, sends a Route Reply (RRP) RRP is sent on a route obtained by reversing the route appended to received RRQ RRP includes the route from to on which RRQ was received by node Wireless omputing and etwork ystems Page 44 Route Reply in R Route Reply in R RRP [,,,,] Route Reply can be sent by reversing the route in Route Request (RRQ) only if links are guaranteed to be bidirectional To ensure this, RRQ should be forwarded only if it received on a link that is known to be bi-directional f unidirectional (asymmetric) links are allowed, then RRP may need a route discovery for from node Unless node already knows a route to node f a route discovery is initiated by for a route to, then the Route Reply is piggybacked on the Route Request from. Represents RRP control message Wireless omputing and etwork ystems Page 45 f is used to send data, then links have to be bi-directional (since ck is used) Wireless omputing and etwork ystems Page 46 ynamic ource Routing (R) ata elivery in R ode on receiving RRP, caches the route included in the RRP When node sends a data packet to, the entire route is included in the packet header hence the name source routing ntermediate nodes use the source route included in a packet to determine to whom a packet should be forwarded T [,,,,] Packet header size grows with route length Wireless omputing and etwork ystems Page 47 Wireless omputing and etwork ystems Page 48 8

9 When to Perform a Route iscovery When node wants to send data to node, but does not know a valid route node R Optimization: Route aching ach node caches a new route it learns by any means When node finds route [,,,,] to node, node also learns route [,,] to node When node receives Route Request [,,] destined for node, node learns route [,,,] to node When node forwards Route Reply RRP [,,,,], node learns route [,,] to node When node forwards ata [,,,,] it learns route [,,,] to node node may also learn a route when it overhears ata packets Wireless omputing and etwork ystems Page 49 Wireless omputing and etwork ystems Page 50 Use of Route aching Use of Route aching When node learns that a route to node is broken, it uses another route from its local cache, if such a route to exists in its cache. Otherwise, node initiates route discovery by sending a route request ode X on receiving a Route Request for some node can send a Route Reply if node X knows a route to node Use of route cache can speed up route discovery can reduce propagation of route requests Wireless omputing and etwork ystems Page 51 [,,,,] [,] [,,,] [P,Q,R] Represents cached route at a node (R maintains the cached routes in a tree format) [,,] [,,],[,,] [,,,] Wireless omputing and etwork ystems Page 52 Use of Route aching: an peed up Route iscovery Use of Route aching: an Reduce Propagation of Route Requests [,,,,] [,] [,,,] [,,] [,,],[,,] [,,,] [,,,] RRP RRQ When node sends a route request for node, node sends back a route reply [,,,] to node using a locally cached route Wireless omputing and etwork ystems Page 53 [,,,,] [,] [,,,] [,,] [,,,] [,,],[,,] RRQ [,,,] Wireless omputing and etwork ystems Page 54 RRP ssume that there is no link between and. Route Reply (RRP) from node limits flooding of RRQ. n general, the reduction may be less dramatic. 9

10 Route rror (RRR) Route aching: eware! RRR [-] sends a route error to along route --- when its attempt to forward the data packet (with route ) on - fails tale caches can adversely affect performance With passage of time and host mobility, cached routes may become invalid sender host may try several stale routes (obtained from local cache, or replied from cache by other nodes), before finding a good route odes hearing RRR update their route cache to remove link - Wireless omputing and etwork ystems Page 55 Wireless omputing and etwork ystems Page 56 ynamic ource Routing: dvantages Routes maintained only between nodes who need to communicate reduces overhead of route maintenance Route caching can further reduce route discovery overhead single route discovery may yield many routes to the destination, due to intermediate nodes replying from local caches Wireless omputing and etwork ystems Page 57 ynamic ource Routing: isadvantages Packet header size grows with route length due to source routing lood of route requests may potentially reach all nodes in the network are must be taken to avoid collisions between route requests propagated by neighboring nodes insertion of random delays before forwarding RRQ ncreased contention if too many route replies come back due to nodes replying using their local cache Route Reply torm problem Reply storm may be eased by preventing a node from sending RRP if it hears another RRP with a shorter route Wireless omputing and etwork ystems Page 58 ynamic ource Routing: isadvantages looding of ontrol Packets n intermediate node may send Route Reply using a stale cached route, thus polluting other caches This problem can be eased if some mechanism to purge (potentially) invalid cached routes is incorporated. some proposals for cache invalidation tatic timeouts daptive timeouts based on link stability ow to reduce the scope of the route request flood? R [o98obicom] Query localization [astaneda99obicom] ow to reduce redundant broadcasts? The roadcast torm Problem [i99obicom] Wireless omputing and etwork ystems Page 59 Wireless omputing and etwork ystems Page 60 10

11 d oc On-emand istance Vector Routing (OV) [Perkins99Wmcsa] R includes source routes in packet headers Resulting large headers can sometimes degrade performance particularly when data contents of a packet are small OV attempts to improve on R by maintaining routing tables at the nodes, so that data packets do not have to contain routes OV retains the desirable feature of R that routes are maintained only between nodes which need to communicate OV Route Requests (RRQ) are forwarded in a manner similar to R When a node re-broadcasts a Route Request, it sets up a reverse path pointing towards the source OV assumes symmetric (bi-directional) links When the intended destination receives a Route Request, it replies by sending a Route Reply Route Reply travels along the reverse path set-up when Route Request is forwarded Wireless omputing and etwork ystems Page 61 Wireless omputing and etwork ystems Page 62 istance Vector Routing Route Requests in OV (a) subnet. (b) nput from,,,, and the new routing table for. Represents a node that has received RRQ for from Wireless omputing and etwork ystems Page 63 Wireless omputing and etwork ystems Page 64 Route Requests in OV Route Requests in OV roadcast transmission Represents transmission of RRQ Represents links on Reverse Path Wireless omputing and etwork ystems Page 65 Wireless omputing and etwork ystems Page 66 11

12 Reverse Path etup in OV Reverse Path etup in OV ode receives RRQ from and, but does not forward it again, because node has already forwarded RRQ once Wireless omputing and etwork ystems Page 67 Wireless omputing and etwork ystems Page 68 Reverse Path etup in OV Route Reply in OV ode does not forward RRQ, because node is the intended target of the RRQ Wireless omputing and etwork ystems Page 69 Represents links on path taken by RRP Wireless omputing and etwork ystems Page 70 Route Reply in OV n intermediate node (not the destination) may also send a Route Reply (RRP) provided that it knows a more recent path than the one previously known to sender To determine whether the path known to an intermediate node is more recent, destination sequence numbers are used The likelihood that an intermediate node will send a Route Reply when using OV is not as high as R new Route Request by node for a destination is assigned a higher destination sequence number. n intermediate node which knows a route, but with a smaller sequence number, cannot send Route Reply Wireless omputing and etwork ystems Page 71 orward Path etup in OV orward links are setup when RRP travels along the reverse path Represents a link on the forward path Wireless omputing and etwork ystems Page 72 12

13 ata elivery in OV Timeouts T routing table entry maintaining a reverse path is purged after a timeout interval timeout should be long enough to allow RRP to come back routing table entry maintaining a forward path is purged if not used for a active_route_timeout interval 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) Routing table entries used to forward data packet. Route is not included in packet header. Wireless omputing and etwork ystems Page 73 Wireless omputing and etwork ystems Page 74 ink ailure Reporting Route rror 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 When the next hop link in a routing table entry breaks, all active neighbors are informed ink failures are propagated by means of Route rror messages, which also update destination sequence numbers When node X is unable to forward packet P (from node to node ) on link (X,), it generates a RRR message ode X increments the destination sequence number for cached at node X The incremented sequence number is included in the RRR When node receives the RRR, it initiates a new route discovery for using destination sequence number at least as large as Wireless omputing and etwork ystems Page 75 Wireless omputing and etwork ystems Page 76 estination equence umber When node receives the route request with destination sequence number, node will set its sequence number to. ink ailure etection ello messages: eighboring nodes periodically exchange hello message bsence of hello message is used as an indication of link failure lternatively, failure to receive several level acknowledgement may be used as an indication of link failure Wireless omputing and etwork ystems Page 77 Wireless omputing and etwork ystems Page 78 13

14 Why equence umbers in OV Why equence umbers in OV To avoid using old/broken routes To determine which route is newer To prevent formation of loops ssume that does not know about failure of link - because RRR sent by is lost ow performs a route discovery for. ode receives the RRQ (say, via path --) ode will reply since knows a route to via node Results in a loop (for instance, ---- ) oop ---- Wireless omputing and etwork ystems Page 79 Wireless omputing and etwork ystems Page 80 Optimization: xpanding Ring earch ummary: OV Route Requests are initially sent with small Timeto-ive (TT) field, to limit their propagation R also includes a similar optimization f no Route Reply is received, then larger TT tried Routes need not be included in packet headers odes maintain routing tables containing entries only for routes that are in active use t most one next-hop per destination maintained at each node R may maintain several routes for a single destination Unused routes expire even if topology does not change Wireless omputing and etwork ystems Page 81 Wireless omputing and etwork ystems Page 82 14