Wireless Networking & Mobile Computing

Wireless Networking & Mobile Computing CS 752/852 - Spring 2012 Network Layer: Ad Hoc Routing Tamer Nadeem Dept. of Computer Science

The OSI Communication Model Page 2 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

The OSI Communication Model Page 3 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

MANET Mobile Ad Hoc Networks Collection of mobile nodes connected by wireless links, forming an autonomous network Nodes run on batteries power consumption is an issue Thus routing is multi-hop each node also acts as a router Scalability is an issue A B H S C I E G F K J D M Page 4 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

MANET vs Cellular MANET Cellular Communications No infrastructure, lack of centralized control Fixed, pre-located base stations Frequent topology changes due to node mobility All communications over wireless medium Static backbone network topology Reliable backbone links; only last link is wireless Page 5 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Applications Civilian environments meeting rooms smart homes multimedia classrooms Emergency operations disaster relief search and rescue law enforcement Military applications Page 6 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Challenges Limited wireless transmission range Broadcast nature of the wireless medium Packet losses due to transmission errors Mobility-induced route changes Mobility-induced packet losses Battery constraints Potentially frequent network partitions Ease of snooping on wireless transmissions (security hazard) 7 Page 7 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Topology Challenges 1 3 2 Network connectivity defined by node proximity and wireless medium characteristics 1 2 3 4 4 1 2 3 4 Due to mobility the network topology undergoes rapid changes 1 2 3 4 Page 8 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

MAC Challenges Hidden terminal problem Unreliable physical layer multi-path fading high Bit Error Rate (BER) other impairments Mobility Page 9 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Routing Schemes Broadcast Unicast Geocast Multicast Anycast Page 10 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Unicast Routing in Mobile Ad Hoc Networks Page 11 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Routing in MANET Why is routing in MANET challenging? Without (necessarily) using a pre-existing infrastructure Main culprit: mobility network topology is changing rapidly (link failure/repair) Rate of link failure/repair may be high when nodes move fast Main challenge: maintain and distribute up-to-date routing information without saturating the network Page 12 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Routing in MANET Numerous protocols have been proposed Flavors: adapted from protocols previously proposed for wired networks invented specifically for MANET Major approaches Proactive: routing information is maintained proactively regardless of communication requests e.g. traditional link-state and distance-vector routing protocols are proactive Reactive: a route to a destination is found and maintained only when the route is needed Page 13 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Proactive vs Reactive Latency 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 Y will be found only when X attempts to send to Y 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 is better depends on traffic and mobility patterns Page 14 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Sample Routing Protocols Pure flooding DSR AODV LAR Page 15 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Flooding (1/5) Sender S broadcasts data packet P to all its neighbors Each node receiving P forwards P to its neighbors Sequence numbers are used to avoid forwarding the same packet more than once The destination D does not forward the packet Page 16 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Flooding (2/5) Broadcast transmission Y Z A B H S C I E G F K J D M N L Represents a node that receives packet P for the first time Represents transmission of packet P Page 17 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Flooding (3/5) Y Z A B H S C I E G F K J D M N L Node H receives packet P from two neighbors: potential for collision Page 18 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Flooding (4/5) Y Z A B H S C I E 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 Page 19 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Flooding (5/5) Y Z A B H S C I E G F K J D M N L Flooding completed 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) Page 20 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Flooding: Summary Advantage: simplicity Potentially, very high overhead data packets may be delivered to too many nodes who do not need to receive them Potentially lower reliability of data delivery flooding uses MAC broadcasting -- hard to implement reliable broadcast delivery without significantly increasing overhead Broadcasting in IEEE 802.11 MAC is unreliable Need end-to-end reliability Page 21 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Flooding Control Packets Many protocols perform limited flooding of control packets, instead of data packets Control packets are used to discover routes or to send linkstate updates Discovered 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 Page 22 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Dynamic Source Routing (DSR) [Johnson96] When source S wants to send a packet to node D, but does not know a route to D, it initiates a route discovery Source node S floods a Route Request (RREQ) Each node appends own identifier when forwarding the RREQ Page 23 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

DSR: Route Discovery (1/3) Broadcast transmission Y A B H S [S] C I E G F K J D M Z N L Represents transmission of RREQ [X,Y] Represents list of identifiers appended to RREQ Page 24 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

DSR: Route Discovery (2/3) Y A B H S E C [S,C] I [S,E] G F K J D M Z N L Page 25 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

DSR: Route Discovery (3/3) Y Z A B H S C I E G F [S,E,F,J] M J D K [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 Page 26 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

DSR: Route Reply (1/2) Destination D upon receiving the first RREQ, sends back to S a Route Reply (RREP) RREP is sent on the route obtained by reversing the route appended to the received RREQ RREP includes the route from S to D on which RREQ was received by node D Page 27 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

DSR: Route Reply (2/2) Y A B H S C I E RREP [S,E,F,J,D] F J G K D M Z N L Represents RREP control message Page 28 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

DSR: Data Delivery (1/2) S upon receiving RREP, caches the route included in the RREP When S sends a data packet to D, the entire route is included in the packet header hence the name source routing Intermediate nodes use the source route included in a packet to determine to whom the packet should be forwarded Page 29 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

DSR: Data Delivery (2/2) Y A B H S DATA [S,E,F,J,D] E F C G K I J D M Z N L Packet header size grows with route length Page 30 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

DSR Optimization: Route Caching Each node caches a new route it learns by some means When S finds route [S,E,F,J,D] to node D, S also learns route [S,E,F] to node F When node K receives the Route Request [S,C,G], K learns route [K,G,C,S] to S When node F forwards Route Reply RREP [S,E,F,J,D], F learns route [F,J,D] to D When node E forwards Data [S,E,F,J,D], it learns route [E,F,J,D] to node D A node may also learn a route when it overhears data packets Page 31 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

DSR: Route Caching (1/3) Advantages: can speed up route discovery can reduce propagation of route requests (RREQ) Page 32 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

DSR: Route Caching (2/3) [S,E,F,J,D] [E,F,J,D] Y A B H S C [C,S] I E [G,C,S] G F [K,G,C,S] [F,J,D],[F,E,S] RREQ [J,F,E,S] M J D K Z RREP N L Route Reply (RREP) from node K limits flooding of RREQ In general, the reduction may be less dramatic Page 33 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

DSR: Route Caching (3/3) Stale caches can adversely affect performance With passage of time and host mobility, cached routes may become invalid In some cases, a sender may try several stale routes (obtained from its local cache, or from cache of some other node), before finding a good route Page 34 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

DSR: Route Maintenance - Route Error (RERR) Y A B H S C I E RERR [J-D] F G K J D M Z N L When J s attempt to forward a data packet (with route S-E-F-J-D) on link (J-D) fails, J sends a route error (RERR) to S along route J-F-E-S Nodes hearing RERR update their route cache to remove link (J-D) When node S receives the RERR, it initiates a new RREQ Page 35 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Summary of DSR Routes maintained only between nodes who need to communicate Route caching can further reduce route discovery overhead A single route discovery may yield many routes to the destination, due to intermediate nodes replying from local caches Complicated Cache Management mechanisms Packet header size grows with route length due to source routing Page 36 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Ad-hoc On-Demand Distance Vector Routing (AODV) Route Requests (RREQ) are forwarded in a manner similar to DSR When a node re-broadcasts a Route Request, it sets up a reverse path pointing towards the source 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 Page 37 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

AODV: Route Request (1/5) Broadcast transmission Y Z A B H S C I E G F K J D M N L Represents transmission of RREQ Page 38 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

AODV: Route Request (2/5) Y Z A B H S C I E G F K J D M N L Represents links on Reverse Path Page 39 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

AODV: Route Request (3/5) Y Z A B H S C I E G F K J D M N L Represents links on Reverse Path F sets up a reverse path entry in its routing table: reverse path entry Destination S Next Hop E Page 40 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

AODV: Route Request (4/5) Y Z A B H S C I E G F K J D M N L Page 41 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

AODV: Route Request (5/5) Y Z A B H S C I E G F K J D M N L D does not forward RREQ, because it is the intended target of the RREQ Page 42 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

AODV: Route Reply (1/2) Y Z A B H S C I E G F K J D M N L Represents links on path taken by RREP Page 43 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

AODV: Route Reply (1/2) An intermediate node may also send a Route Reply (RREP) provided that it knows a more recent path than the one previously known to sender S To determine whether the path known to an intermediate node is more recent, destination sequence numbers are used Forward links are set up when RREP travels along the reverse path Page 44 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

AODV: Forward Path Setup Y Z A B H S C I E G F K J D M N L F adds a forward path entry in its routing table: forward path entry Destination Next Hop S E Page 45 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing D J

AODV: Data Delivery Y A B H S DATA E C I G F K J D M Z N L Represents a link on the forward path Routing table entries used to forward data packet Route is not included in packet header Page 46 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

AODV: Route Maintenance - Route Error (RERR) When node X is unable to forward packet P (from node S to node D) on link (X,Y), it generates a RERR message Node X increments the destination sequence number for D cached at node X The incremented sequence number N is included in the RERR When node S receives the RERR, it initiates a new route discovery for D using destination sequence number at least as large as N Page 47 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Information Freshness Assured Each originating node maintains a monotonically increasing sequence number. Used by other nodes to determine the freshness of the information. Every nodes routing table contains the latest information available about the sequence number for the IP address of the destination node for which the routing information is maintained. Updated whenever a node receives new information about the sequence number from RREQ, RREP, or RERR messages received related to that destination. Page 48 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Information Freshness Assured AODV depends on each node in the network to own and maintain its destination sequence number. A destination node increments its own sequence number immediately before it originates a route discovery A destination node increments its own sequence number immediately before it originates a RREP in response to a RREQ The node treats its sequence number as an unsigned number when incrementing accomplishing sequence number rollover. Destination information is assured by comparing the sequence number of the incoming AODV message with its sequence number for that destination. Page 49 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

ADOV: Summary Nodes maintain routing tables containing entries only for routes that are in active use Routes are not included in packet headers At most one next-hop per destination is maintained at each node DSR may maintain several routes for a single destination Page 50 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Flooding of Control Packets How to reduce the scope of the route request flood? LAR [Ko98Mobicom] Query localization [Castaneda99Mobicom] How to reduce redundant broadcasts? The Broadcast Storm Problem [Ni99Mobicom] Page 51 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Location-Aided Routing (LAR) [Ko98Mobicom] Exploits location information to limit scope of route request flood Location information may be obtained using GPS Expected Zone is determined as a region that is expected to hold the current location of the destination Expected region determined based on potentially old location information, and knowledge of the destination s speed Route requests limited to a Request Zone that contains the Expected Zone and location of the sender node Page 52 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Expected Zone in LAR X = last known location of node D, at time t0 Y = location of node D at current time t1, unknown to node S r = (t1 - t0) * estimate of D s speed r X Y Expected Zone Page 53 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Request Zone in LAR Network Space Request Zone A B r X Y S Page 54 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

LAR Only nodes within the request zone forward route requests Node A does not forward RREQ, but node B does (see previous slide) Request zone explicitly specified in the route request Each node must know its physical location to determine whether it is within the request zone If 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 zone the larger request zone may be the entire network Page 55 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

LAR Variations: Adaptive Request Zone Each node may modify the request zone included in the forwarded request Modified request zone may be determined using more recent/accurate information, and may be smaller than the original request zone B S Request zone adapted by B Request zone defined by sender S Page 56 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Location Aided Routing (LAR) Advantages reduces the scope of route request flood reduces overhead of route discovery Disadvantages Nodes need to know their physical locations Does not take into account possible existence of obstructions for radio transmissions 57 Page 57 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Other Routing Protocols Plenty of other routing protocols Geographic Distance Routing (GEDIR) Energy-aware routing QoS routing Routing with Guaranteed Delivery Page 58 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

MANET Routing: Issues How to compare these protocols fairly? Message overhead Routing delays Throughput Energy Consumption Comparisons are based mainly on empirical results Lack of analytical results Interaction with MAC protocols Scalability: >200 nodes is already a problem A possible solution for scalability: hierarchical organization of ad hoc networks Page 59 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

MANET Routing: Many Variations Fully Symmetric Environment all nodes have identical capabilities and responsibilities Asymmetric Capabilities transmission ranges and radios may differ battery life at different nodes may differ processing capacity may be different at different nodes speed of movement Asymmetric Responsibilities only some nodes may route packets some nodes may act as leaders of nearby nodes (e.g., cluster head) Page 60 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

MANET Routing: Many 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 May co-exist (and co-operate) with an infrastructure-based network Page 61 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

MANET Routing: Many Variations Mobility patterns may be different people sitting at an airport lounge New York taxi cabs kids playing military movements personal area network Mobility characteristics speed predictability direction of movement pattern of movement uniformity (or lack thereof) of mobility characteristics among different nodes Page 62 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Research on Mobile Ad Hoc Networks Variations in capabilities & responsibilities X Variations in traffic characteristics, mobility models, etc. X Performance criteria (e.g., optimize throughput, reduce energy consumption) = Significant research activity Page 63 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Questions Page 64 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

More on IEEE 802.11 DCF Collision Avoidance Mechanism CSMA + RTS/CTS (physical/virtual carrier sensing) Collision Resolution Mechanism Binary Exponential Backoff (BEB) Does IEEE802.11 DCF work well in multi-hop networks? Answer: No! (has known fairness problems) Page 65 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

What is Fairness? What does fairness mean in a multi-hop environment? Max-min fairness an allocation is max-min fair if there is no way to give more bandwidth to a flow without decreasing the allocation of a flow of lesser or equal bandwidth Practical approach ensure that each flow gets a minimum share of the bandwidth allow those flows that are not interfering with the current transmitting flows to send simultaneously Page 66 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Unfairness in IEEE 802.11 DCF f0 f1 f2 f3 f4 A B C D E F Throughput of each flow: f0: 1298 f1: 684 f2: 47 f3: 1173 f4: 1913 Page 67 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

The Flow Contention Graph f0 f1 f2 f3 f4 A B C D E F Nodes represent flows Two flows connected by an edge means they conflict with each other f1 f3 f0 f2 f4 Page 68 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

An example of scheduling f1 f3 f0 f2 f4 Flow Contention Graph Scheduling: Slot 1: f0 + f3/f4 Slot 2: f1 + f4 Slot 3: f2 Slot 4: f3 + f0 Slot 5: f4 +f0/f1 Slot 6: f0 + f3/f4 Slot 7: f1 + f4 Slot 8: f2 Slot 9: f3 + f0 Slot 10: f4 +f0/f1 Each flow is guaranteed to be scheduled once in every 5 slots! Page 69 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

One can do better f1 f3 f0 f2 f4 Flow Contention Graph Improved schedule: Slot 1: f0 + f3 Slot 2: f1 + f4 Slot 3: f2 Slot 4: f0+f3 Slot 5: f1+f4 Slot 6: f2 Each flow is guaranteed to be scheduled once in every 3 slots! Page 70 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing

Challenges Which scheduling is better? Trade-off between fairness and overall throughput How to implement a fair scheduler in a fully distributed way? Page 71 Spring 2012 CS 752/852 - Wireless Networking and Mobile Computing