Mobile Ad Hoc Networks

1、1mobile ad hoc networksnitin h. vaidyatexas a&m u/faculty/vaidya/ 2000 nitin vaidya2notesgfor a larger set of tutorial slides, please go to/faculty/vaidyaand follow the link to seminars3notesgnames in brackets, as in xyz00, refer to

2、 a document in the list of referencesgthe handout may not be as readable as the original slides, since the slides contain colored text and figuresinote that different colors in the colored slides may look identically black in the black-and-white handout4tutorial outlinegintroductiongunicast routingg

3、medium access control gperformance of udp and tcpgsecurity issuesgimplementation issuesgstandards activitiesgopen problems5notesgonly most important features of various schemes are typically discussedgmost schemes include many more details, and optimizationsinot possible to cover all details in this

4、 tutorialgbe aware that some protocol specs have changed several timesgjargon used to discuss a scheme may occasionally differ from that used by the proposers6mobile ad hoc networks (manet)introduction and generalities7mobile ad hoc networksgformed by wireless hosts which may be mobilegwithout (nece

5、ssarily) using a pre-existing infrastructuregroutes between nodes may potentially contain multiple hops8mobile ad hoc networksgmay need to traverse multiple links to reach a destination9mobile ad hoc networks (manet)gmobility causes route changes10why ad hoc networks ?gease of deploymentgspeed of de

6、ploymentgdecreased dependence on infrastructure11many applicationsgpersonal area networkingicell phone, laptop, ear phone, wrist watchgmilitary environmentsisoldiers, tanks, planesgcivilian environmentsitaxi cab networkimeeting roomsisports stadiumsiboats, small aircraftgemergency operationsisearch-

7、and-rescueipolicing and fire fighting12many variationsgfully symmetric environmentiall nodes have identical capabilities and responsibilitiesgasymmetric capabilitiesitransmission ranges and radios may differ ibattery life at different nodes may differiprocessing capacity may be different at differen

8、t nodesispeed of movementgasymmetric responsibilitiesionly some nodes may route packets isome nodes may act as leaders of nearby nodes (e.g., cluster head)13many variationsgtraffic characteristics may differ in different ad hoc networksibit rateitimeliness constraintsireliability requirementsiunicas

9、t / multicast / geocastihost-based addressing / content-based addressing / capability-based addressinggmay co-exist (and co-operate) with an infrastructure-based network14many variationsgmobility patterns may be differentipeople sitting at an airport loungeinew york taxi cabsikids playingimilitary m

10、ovementsipersonal area networkgmobility characteristicsispeedipredictabilitydirection of movementpattern of movementiuniformity (or lack thereof) of mobility characteristics among different nodes15challengesglimited wireless transmission rangegbroadcast nature of the wireless mediumihidden terminal

11、problem (see next slide)gpacket losses due to transmission errorsgmobility-induced route changesgmobility-induced packet lossesgbattery constraintsgpotentially frequent network partitionsgease of snooping on wireless transmissions (security hazard)16hidden terminal problembcanodes a and c cannot hea

12、r each othertransmissions by nodes a and c can collide at node bnodes a and c are hidden from each other17research on mobile ad hoc networksvariations in capabilities & responsibilities xvariations in traffic characteristics, mobility models, etc. xperformance criteria (e.g., optimize throughput, re

13、duce energy consumption) + increased research funding =significant research activity18the holy grailga one-size-fits-all solutioniperhaps using an adaptive/hybrid approach that can adapt to situation at handgdifficult problemgmany solutions proposed trying to address asub-space of the problem domain

14、19assumptiongunless stated otherwise, fully symmetric environment is assumed implicitlyiall nodes have identical capabilities and responsibilities20unicast routinginmobile ad hoc networks21why is routing in manet different ?ghost mobilityilink failure/repair due to mobility may have different charac

15、teristics than those due to other causesgrate of link failure/repair may be high when nodes move fastgnew performance criteria may be usediroute stability despite mobilityienergy consumption22unicast routing protocolsgmany protocols have been proposedgsome have been invented specifically for manetgo

16、thers are adapted from previously proposed protocols for wired networksgno single protocol works well in all environmentsisome attempts made to develop adaptive protocols23routing protocolsgproactive protocolsidetermine routes independent of traffic patternitraditional link-state and distance-vector

17、 routing protocols are proactivegreactive protocolsimaintain routes only if neededghybrid protocols24trade-offglatency of route discoveryiproactive protocols may have lower latency since routes are maintained at all timesireactive protocols may have higher latency because a route from x to y will be

18、 found only when x attempts to send to ygoverhead of route discovery/maintenanceireactive protocols may have lower overhead since routes are determined only if needediproactive protocols can (but not necessarily) result in higher overhead due to continuous route updatinggwhich approach achieves a be

19、tter trade-off depends on the traffic and mobility patterns25overview of unicast routing protocols26flooding for data deliverygsender s broadcasts data packet p to all its neighborsgeach node receiving p forwards p to its neighborsgsequence numbers used to avoid the possibility of forwarding the sam

20、e packet more than oncegpacket p reaches destination d provided that d is reachable from sender sgnode d does not forward the packet27flooding for data deliverybasefhjdcgikrepresents that connected nodes are within each others transmission rangezyrepresents a node that has received packet pmnl28floo

21、ding for data deliverybasefhjdcgikrepresents transmission of packet prepresents a node that receives packet p forthe first timezybroadcast transmissionmnl29flooding for data deliverybasefhjdcgik node h receives packet p from two neighbors: potential for collisionzymnl30flooding for data deliverybase

22、fhjdcgik node c receives packet p from g and h, but does not forward it again, because node c has already forwarded packet p oncezymnl31flooding for data deliverybasefhjdcgikzym nodes j and k both broadcast packet p to node d since nodes j and k are hidden from each other, their transmissions may co

23、llide = = packet p may not be delivered to node d at all, despite the use of floodingnl32flooding for data deliverybasefhjdcgikzy node d does not forward packet p, because node d is the intended destination of packet pmnl33flooding for data deliverybasefhjdcgik flooding completed nodes unreachable f

24、rom 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)zymnl34flooding for data deliverybasefhjdcgik flooding may deliver packets to too many nodes (in the worst case, all nodes reachable from sender ma

25、y receive the packet)zymnl35flooding for data delivery: advantagesgsimplicitygmay 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 higherithis scenario may oc

26、cur, for instance, when nodes transmit small data packets relatively infrequently, and many topology changes occur between consecutive packet transmissionsgpotentially higher reliability of data deliveryibecause packets may be delivered to the destination on multiple paths36flooding for data deliver

27、y: disadvantagesgpotentially, very high overheadidata packets may be delivered to too many nodes who do not need to receive themgpotentially lower reliability of data deliveryiflooding uses broadcasting - hard to implement reliable broadcast delivery without significantly increasing overhead broadca

28、sting in ieee 802.11 mac is unreliableiin our example, nodes j and k may transmit to node d simultaneously, resulting in loss of the packet in this case, destination would not receive the packet at all 37flooding of control packetsgmany protocols perform (potentially limited) flooding of control pac

29、kets, instead of data packetsgthe control packets are used to discover routesgdiscovered routes are subsequently used to send data packet(s)goverhead of control packet flooding is amortized over data packets transmitted between consecutive control packet floods38dynamic source routing (dsr) johnson9

30、6gwhen node s wants to send a packet to node d, but does not know a route to d, node s initiates a route discoverygsource node s floods route request (rreq) geach node appends own identifier when forwarding rreq39route discovery in dsrbasefhjdcgikzyrepresents a node that has received rreq for d from

31、 smnl40route discovery in dsrbasefhjdcgikrepresents transmission of rreqzybroadcast transmissionmnlsx,y represents list of identifiers appended to rreq41route discovery in dsrbasefhjdcgik node h receives packet rreq from two neighbors: potential for collisionzymnls,es,c42route discovery in dsrbasefh

32、jdcgik node c receives rreq from g and h, but does not forward it again, because node c has already forwarded rreq oncezymnls,c,gs,e,f43route discovery in dsrbasefhjdcgikzym nodes j and k both broadcast rreq to node d since nodes j and k are hidden from each other, their transmissions may collide nl

33、s,c,g,ks,e,f,j44route discovery in dsrbasefhjdcgikzy node d does not forward rreq, because node d is the intended target of the route discoverymnls,e,f,j,m45route discovery in dsrgdestination d on receiving the first rreq, sends a route reply (rrep)grrep is sent on a route obtained by reversing the

34、route appended to received rreqgrrep includes the route from s to d on which rreq was received by node d46route reply in dsrbasefhjdcgikzymnlrrep s,e,f,j,drepresents rrep control message47route reply in dsrgroute reply can be sent by reversing the route in route request (rreq) only if links are guar

35、anteed to be bi-directionalito ensure this, rreq should be forwarded only if it received on a link that is known to be bi-directionalgif unidirectional (asymmetric) links are allowed, then rrep may need a route discovery for s from node d iunless node d already knows a route to node siif a route dis

36、covery is initiated by d for a route to s, then the route reply is piggybacked on the route request from d.gif ieee 802.11 mac is used to send data, then links have to be bi-directional (since ack is used)48dynamic source routing (dsr)gnode s on receiving rrep, caches the route included in the rrepg

37、when node s sends a data packet to d, the entire route is included in the packet headerihence the name source routinggintermediate nodes use the source route included in a packet to determine to whom a packet should be forwarded49data delivery in dsrbasefhjdcgikzymnldata s,e,f,j,dpacket header size

38、grows with route length50when to perform a route discoverygwhen node s wants to send data to node d, but does not know a valid route node d51dsr optimization: route cachinggeach node caches a new route it learns by any meansgwhen node s finds route s,e,f,j,d to node d, node s also learns route s,e,f

39、 to node fgwhen node k receives route request s,c,g destined for node, node k learns route k,g,c,s to node sgwhen node f forwards route reply rrep s,e,f,j,d, node f learns route f,j,d to node dgwhen node e forwards data s,e,f,j,d it learns route e,f,j,d to node dga node may also learn a route when i

40、t overhears data packets52use of route cachinggwhen 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 requestgnode x on receiving a route request for som

41、e node d can send a route reply if node x knows a route to node dguse of route cache ican speed up route discoveryican reduce propagation of route requests53use of route cachingbasefhjdcgikp,q,r represents cached route at a node (dsr maintains the cached routes in a tree format)mnls,e,f,j,de,f,j,dc,

42、sg,c,sf,j,d,f,e,sj,f,e,sz54use of route caching:can speed up route discoverybasefhjdcgikzmnls,e,f,j,de,f,j,dc,sg,c,sf,j,d,f,e,sj,f,e,srreqwhen node z sends a route requestfor node c, node k sends back a routereply z,k,g,c to node z using a locallycached routek,g,c,srrep55use of route caching:can red

43、uce propagation of route requestsbasefhjdcgikzymnls,e,f,j,de,f,j,dc,sg,c,sf,j,d,f,e,sj,f,e,srreqassume 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.k,g,c,srrep56route error (rerr)basefhjdcgikzymnlrerr j-dj

44、 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 failsnodes hearing rerr update their route cache to remove link j-d57route caching: beware!gstale caches can adversely affect performancegwith passage of time and host mobility, cach

45、ed routes may become invalidga sender host may try several stale routes (obtained from local cache, or replied from cache by other nodes), before finding a good routegan illustration of the adverse impact on tcp will be discussed later in the tutorial holland9958dynamic source routing: advantagesgro

46、utes maintained only between nodes who need to communicateireduces overhead of route maintenancegroute caching can further reduce route discovery overheadga single route discovery may yield many routes to the destination, due to intermediate nodes replying from local caches59dynamic source routing:

47、disadvantagesgpacket header size grows with route length due to source routinggflood of route requests may potentially reach all nodes in the networkgcare must be taken to avoid collisions between route requests propagated by neighboring nodesiinsertion of random delays before forwarding rreqgincrea

48、sed contention if too many route replies come back due to nodes replying using their local cacheiroute reply storm problemireply storm may be eased by preventing a node from sending rrep if it hears another rrep with a shorter route60dynamic source routing: disadvantagesgan intermediate node may sen

49、d route reply using a stale cached route, thus polluting other cachesgthis problem can be eased if some mechanism to purge (potentially) invalid cached routes is incorporated. gfor some proposals for cache invalidation, see hu00mobicom61flooding of control packetsghow to reduce the scope of the rout

50、e request flood ?ilar ko98mobicomiquery localization castaneda99mobicomghow to reduce redundant broadcasts ?ithe broadcast storm problem ni99mobicom62location-aided routing (lar) ko98mobicomgexploits location information to limit scope of route request floodilocation information may be obtained usin

51、g gpsgexpected zone is determined as a region that is expected to hold the current location of the destinationiexpected region determined based on potentially old location information, and knowledge of the destinations speedgroute requests limited to a request zone that contains the expected zone an

52、d location of the sender node63expected zone in larxyrx = last known location of node d, at time t0y = location of node d at current time t1, unknown to node sr = (t1 - t0) * estimate of ds speedexpected zone64request zone in larxyrsrequest zonenetwork spaceba65largonly nodes within the request zone

53、 forward route requestsinode a does not forward rreq, but node b does (see previous slide)grequest zone explicitly specified in the route requestgeach node must know its physical location to determine whether it is within the request zone66largonly nodes within the request zone forward route request

54、sgif route discovery using the smaller request zone fails to find a route, the sender initiates another route discovery (after a timeout) using a larger request zoneithe larger request zone may be the entire networkgrest of route discovery protocol similar to dsr67ad hoc on-demand distance vector ro

55、uting (aodv) perkins99wmcsagdsr includes source routes in packet headersgresulting large headers can sometimes degrade performanceiparticularly when data contents of a packet are smallgaodv attempts to improve on dsr by maintaining routing tables at the nodes, so that data packets do not have to con

56、tain routesgaodv retains the desirable feature of dsr that routes are maintained only between nodes which need to communicate68aodvgroute requests (rreq) are forwarded in a manner similar to dsrgwhen a node re-broadcasts a route request, it sets up a reverse path pointing towards the sourceiaodv ass

57、umes symmetric (bi-directional) linksgwhen the intended destination receives a route request, it replies by sending a route replygroute reply travels along the reverse path set-up when route request is forwarded69route requests in aodvbasefhjdcgikzyrepresents a node that has received rreq for d from

58、 smnl70route requests in aodvbasefhjdcgikrepresents transmission of rreqzybroadcast transmissionmnl71route requests in aodvbasefhjdcgik represents links on reverse pathzymnl72reverse path setup in aodvbasefhjdcgik node c receives rreq from g and h, but does not forward it again, because node c has a

59、lready forwarded rreq oncezymnl73reverse path setup in aodvbasefhjdcgikzymnl74reverse path setup in aodvbasefhjdcgikzy node d does not forward rreq, because node d is the intended target of the rreqmnl75route reply in aodvbasefhjdcgikzyrepresents links on path taken by rrep mnl76forward path setup i

60、n aodvbasefhjdcgikzymnlforward links are setup when rrep travels alongthe reverse pathrepresents a link on the forward path77data delivery in aodvbasefhjdcgikzymnlrouting table entries used to forward data packet.route is not included in packet header.data78summary: aodvgroutes need not be included