AD HOC NETWORKS RAJINI M 2 ND SEM M.Tech DECS PESIT
OUTLINE Introduction Features of Ad hoc network Types of ad hoc network MANETs Network architecture MAC protocols Routing protocols Denial of service attacks Solution to Denial of service attack
INTRODUCTION An Ad-hoc network is a local area network or some other small network, especially one with wireless (or temporary plug in connections), in which some of the network devices are the part of the network only for the duration of a communications session. Allows new network devices to be quickly added. Each user has a unique network address that is recognized as the part of the network. Collection of nodes that do not rely on a predefined infrastructure Auto-configurable network and Self organizing Nodes are mobile and hence have dynamic network topology. Nodes in ad-hoc networks play both the roles of routers and terminals. Routing protocol required
EXAMPLES Classroom Ad hoc network between student PDAs and workstation of the instructor Large IT campus Employees of a company moving within a large campus with PDAs, laptops, and cell phones Moving soldiers with wearable computers Eavesdropping, denial-of-service and impersonation attacks can be launched Shopping mall, restaurant, coffee shops Customers spend part of the day in a networked mall of specialty shops, coffee shops, and restaurants
AD HOC NETWORK
FEATURES There are certain features that can determine the efficiency and effectiveness of wireless ad hoc network. Network settling time. Network join time Network depart time Network recovery time Frequency of updates(overhead) Memory byte requirement Network scalability
Knowledge of nodal location Effect to topology changes Adaptation to radio communication environment Power consciousness Single or multi channel Bidirectional or unidirectional links Preservation of network security QoS :Routing and handling of priority messages. real time voice and video services
DIFFERENCE BETWEEN CELLULAR AND AD-HOC NETWORKS Cellular Networks Ad-hoc Networks Fixed, pre-located cell sites and base stations. Static backbone network topology Relatively favorable environment and stable connectivity. Detailed planning before base stations can be installed. No fixed base stations, very rapid deployment. Highly dynamic network topologies, with multi-hop. Hostile environment (losses, noise) and irregular connectivity. Ad-hoc network automatically forms and conforms to change.
TYPES OF AD HOC NETWORKS MANET WSN WMN VANETs
MANET A Mobile Ad-hoc Network (MANET) is a collection of autonomous nodes or terminals which communicate with each other by forming a multi-hop radio network and maintaining connectivity in a decentralized manner over relativelybandwidth constrained wireless links.. Each device in a MANET is free to move independently in any direction, and will therefore change its links to other devices frequently. The topology is highly dynamic and frequent changes in the topology may be hard to predict.
CRITERIA NODE TO BE PART OF A NETWORK To be connected to a network, a node should must be within the area of influence of at least one node on the network. A node with no remaining power, or one that is off, is not currently a part of the network. Even if the source and the destination nodes are not within each other s communication range, data packets are forwarded to the destination by relaying transmission through other nodes that exist between the two nodes.
MANET CONSTRAINTS AND ISSUES Lack of a centralized entity Network topology changes frequently and unpredictably Routing and Mobility Management Channel access/bandwidth availability Hidden/Exposed station problem Lack of symmetrical links Physical security is limited due to the wireless transmission. Affected by higher loss rates, and can experience higher delays and jitter than fixed networks due to the wireless transmission. As nodes are battery operated (power constraint), energy savings are an important system design criterion.
NETWORK ARCHITECTURE MANETs is formed by set of mobile nodes such as laptops, mobile phones etc. Mobile ad hoc networks are based on wireless links(air). It can use single hop or multi hop communication. In single hop communication, all hosts are in one coverage area and hence communication is direct from host to host In multi hop communication, host communicate using intermediate hosts as many coverage area intersects with each other.
CELLULAR WIRELESS Single hop wireless connectivity to the wired world Space divided into cells A base station is responsible to communicate with hosts in its cell Mobile hosts can change cells while communicating Hand-off occurs when a mobile host starts communicating via a new base station
MULTI HOP COMMUNICATION May need to traverse multiple links to reach destination Mobility causes route changes
Single hop architecture multi hop architecture
EFFECT OF MOBILITY ON THE PROTOCOL STACK Application new applications and adaptations Transport congestion and flow control Network addressing and routing Link media access and handoff Physical transmission errors and interference
MAC PROTOCOLS The topology is highly dynamic and frequent changes in the topology may be hard to predict. MAC is responsible for resolving the conflicts among different nodes for channel access. There are two problems Hidden terminal problem Exposed terminal problem
HIDDEN AND EXPOSED TERMINALS Hidden terminals A sends to B, C cannot receive A C wants to send to B, C senses a free medium (CS fails) collision at B, A cannot receive the collision (CD fails) A is hidden for C Exposed terminals A B C B sends to A, C wants to send to another terminal (not A or B) C senses carrier, finds medium in use and has to wait A is outside the radio range of C, therefore waiting is not necessary C is exposed to B
CLASSIFICATIONS OF MAC PROTOCOLS Ad hoc network MAC protocols can be classified into three types: Contention-based protocols Contention-based protocols with reservation mechanisms Contention-based protocols with scheduling mechanisms Other MAC protocols MAC Protocols for Ad Hoc Wireless Networks Sender-Initiated Protocols Single-Channel Protocols MACAW FAMA Contention-Based Protocols Receiver-Initiated Protocols Multichannel Protocols BTMA DBTMA ICSMA Contention-based protocols with reservation mechanisms Synchronous Protocols RI-BTM A MACA-BI MARCH D-PRM A CATA HRMA SRM A/PA FPRP Asynchronous Protocols Contention-based protocols with scheduling mechanisms MACA/PR RTMAC RI-BTM A MACA-BI MARCH Other MAC Protocols Directional Antennas MMAC MCSMA PCM RBAR
CLASSIFICATIONS OF MAC PROTOCOLS Contention-based protocols Sender-initiated protocols: Packet transmissions are initiated by the sender node. Single-channel sender-initiated protocols: A node that wins the contention to the channel can make use of the entire bandwidth. Multichannel sender-initiated protocols: The available bandwidth is divided into multiple channels. Receiver-initiated protocols: The receiver node initiates the contention resolution protocol. Contention-based protocols with reservation mechanisms Synchronous protocols: All nodes need to be synchronized. Global time synchronization is difficult to achieve. Asynchronous protocols: These protocols use relative time information for effecting reservations. 21
CLASSIFICATIONS OF MAC PROTOCOLS Contention-based protocols with scheduling mechanisms Node scheduling is done in a manner so that all nodes are treated fairly and no node is starved of bandwidth. Scheduling-based schemes are also used for enforcing priorities among flows whose packets are queued at nodes. Some scheduling schemes also consider battery characteristics. Other protocols are those MAC protocols that do not strictly fall under the above categories.
MULTIPLE ACCESS WITH COLLISION AVOIDANCE(MACA) MACA uses signaling packets for collisionavoidance RTS (request to send) :sender request the right to send from a receiver with a short RTS packet before it sends a data packet CTS (clear to send) :receiver grants the right to send as soon as it is ready to receive Signaling packets contain sender address receiver address packet size The neighbor node that overhears an RTS packet has to defer its own transmission until the associated CTS packet is transmitted.
Then any node overhearing a CTS packet would defer for the length of expected data transmission When a node wants to transmit a data packet, it first transmit a RTS (Request To Send) frame. The receiver node, on receiving the RTS packet, if it is ready to receive the data packet, transmits a CTS (Clear to Send) packet. Once the sender receives the CTS packet without any error, it starts transmitting the data packet. If a packet transmitted by a node is lost, the node uses the binary exponential back-off (BEB) algorithm to back off a random interval of time before retrying. The binary exponential back-off mechanism used in MACA might starves flows sometimes.
MACA EXAMPLES MACA avoids the problem of hidden terminals A and C want to send to B A sends RTS first C waits after receiving CTS from B RTS CTS CTS A B C MACA avoids the problem of exposed terminals B wants to send to A, C to another terminal now C does not have to wait for it cannot receive CTS from A RTS RTS CTS A B C 25
Limitations MACA does not provide ACK RTS-CTS approach does not always solve the hidden node problem Example A sends RTS to B B sends CTS to A; At the same time, D sends RTS to C The CTS & RTS packets collide at C A transmits data to B; D resends RTS to C; C sends CTS to D The data & CTS packets collide at B
MACAW MACAW (MACA for Wireless) is a revision of MACA(without ACK). The sender senses the carrier to see and transmits a RTS (Request To Send) frame if no nearby station transmits a RTS. The receiver replies with a CTS (Clear To Send) frame. Neighbors see CTS, then keep quiet. see RTS but not CTS, then keep quiet until the CTS is back to the sender. The receiver sends an ACK when receiving an frame. Neighbors keep silent until see ACK. Collisions There is no collision detection. The senders know collision when they don t receive CTS. They each wait for the exponential backoff time. 27
MACAW (MACA FOR WIRELESS) RTS-CTS-DS-DATA-ACK RTS from A to B CTS from B to A Data Sending (DS) from A to B Data from A to B ACK from B to A Random wait after any successful/unsuccessful transmission Significantly higher throughput than MACA Does not completely solve hidden & exposed node problems
POWER AWARE MAC PROTOCOLS Minimize expensive retransmissions due to collisions Transceivers should be kept in standby mode as much as possible Switch to low power mode sufficient for the destination to receive the packet Two categories Alternate between sleep and awake cycles Vary transmission power
PAMAS (POWER AWARE MEDIUM ACCESS CONTROL WITH SIGNALING) RTS-CTS exchanges over a signaling channeling Data transmission over a separate data channel Receiver sends out a busy tone, while receiving a data packet over the signaling channel Nodes listen to the signaling channel to determine when it is optimal to power down transceivers A node powers itself off if it has nothing to transmit and its neighbor is transmitting A node powers off if at least one neighbor is transmitting and another is receiving Use of ACK and transmission of multiple packets can enhance performance Radio transceiver turnaround time was not considered
PCM: POWER CONTROL MEDIUM ACCESS CONTROL Send RTS & CTS packets using max available power Send DATA & ACK with the min power required to communicate between the sender and receiver Based on the received signal strength of the RTS/CTS packet, adjust the power level for DATA transmission Drawbacks Requires rather accurate estimation of the received signal strength, which is hard in wireless communication Difficult to implement frequent changes in the transmission power level
PCMA: POWER CONTROLLED MULTIPLE ACCESS Control transmit power of the sender The receiver is just able to receive the packet Avoid interfering other neighboring nodes not involved in the packet exchange Two channels: one for busy tone & another for data Request Power To Send (RTPS) & Accept Power To Send (APTS) on the data channel Every receiver periodically sends out a busy tone Sender does carrier sensing
Busy Tone Multiple Access Protocols (BTMA) The transmission channel is split into two: a data channel for data packet transmissions a control channel used to transmit the busy tone signal When a node is ready for transmission, it senses the channel to check whether the busy tone is active. If not, it turns on the busy tone signal and starts data transmissions Otherwise, it reschedules the packet for transmission after some random rescheduling delay. Any other node which senses the carrier on the incoming data channel also transmits the busy tone signal on the control channel, thus, prevent two neighboring nodes from transmitting at the same time. Dual Busy Tone Multiple Access Protocol (DBTMAP) is an extension of the BTMA scheme. a data channel for data packet transmissions a control channel used for control packet transmissions (RTS and CTS packets) and also for transmitting the busy tones.
ROUTING PROTOCOLS
CONVENTIONAL ROUTING PROTOCOLS? Not designed for highly dynamic, low bandwidth networks Count-to-infinity problem and slow convergence Loop formation during temporary node failures and network partitions Protocols that use flooding techniques create excessive traffic and control overhead
MANET PROTOCOLS Proactive Protocols Table driven Continuously evaluate routes No latency in route discovery Large capacity to keep network information current A lot of routing information may never be used! Reactive Protocols On Demand Route discovery by some global search Bottleneck due to latency of route discovery May not be appropriate for realtime communication
Table Driven Routing Protocol Send periodic updates of the routes. Each node uses routing information to store the location information of other nodes in the network and this information is then used to move data among different nodes in the network. Have lower latency since routes are maintained at all times 37
DESTINATION SEQUENCED DISTANCE VECTOR (DSDV) Each Route is tagged with a sequence number originated by destination Hosts perform periodic & triggered updates, issuing a new sequence number Sequence number indicates the freshness of a route Routes with more recent sequence numbers are preferred for packet forwarding If same sequence number, one having smallest metric used
TOPOLOGY CHANGES Broken links assigned a metric of Any route through a hop with a broken link is also assigned a metric of routes are assigned new sequence numbers by any host and immediately broadcast via a triggered update If a node has an equal/later sequence number with a finite metric for an route, a route update is triggered
DSDV OPERATION
DAMPING FLUCTUATIONS Routes preferred if later sequence numbers, or smaller metric for same sequence numbers Problem : Table fluctuations if worse metrics are received first, causing a ripple of triggered updates Solution : Use average settling time as a parameter before advertising routes Tantamount to using two tables, one for forwarding packets and another for advertising routes
DYNAMIC SOURCE ROUTING (DSR) Each packet header contains a route, which is represented as a complete sequence of nodes between a source-destination pair Protocol consists of two phases route discovery route maintenance Optimizations for efficiency Route cache Piggybacking Error handling
DSR ROUTE DISCOVERY Source broadcasts route request (id, target) Intermediate node action Discard if id is in <initiator, request id> or node is in route record If node is the target, route record contains the full route to the target; return a route reply Else append address in route record; rebroadcast Use existing routes to source to send route reply; else piggyback
DSR ROUTE MAINTENANCE Use acknowledgements or a layer-2 scheme to detect broken links; inform sender via route error packet If no route to the source exists Use piggybacking Send out a route request and buffer route error Sender truncates all routes which use nodes mentioned in route error Initiate route discovery
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
ROUTE DISCOVERY IN DSR Broadcast transmission Y [S] Z S E A B H C I G F K J D M N L Represents transmission of RREQ [X,Y] Represents list of identifiers appended to RREQ
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
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
ROUTE DISCOVERY IN DSR 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
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
ROUTE DISCOVERY IN DSR Destination D on receiving the first RREQ, sends a Route Reply (RREP) RREP is sent on a route obtained by reversing the route appended to received RREQ RREP includes the route from S to D on which RREQ was received by node D
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
DYNAMIC SOURCE ROUTING (DSR) Node S on receiving RREP, caches the route included in the RREP When node 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 a packet should be forwarded
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
DSR OPTIMIZATION: ROUTE CACHING Each node caches a new route it learns by any means When node S finds route [S,E,F,J,D] to node D, node S also learns route [S,E,F] to node F When node K receives Route Request [S,C,G] destined for node, node K learns route [K,G,C,S] to node S When node F forwards Route Reply RREP [S,E,F,J,D], node F learns route [F,J,D] to node 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 Problem: Stale caches may increase overheads
OPTIMIZATIONS FOR EFFICIENCY Route Cache Use cached entries for during route discovery Promiscuous mode to add more routes Use hop based delays for local congestion Must be careful to avoid loop formation Non propagating RREQs
OPTIMIZATIONS Piggybacking Data piggybacked on route request Packet Problem : route caching can cause piggybacked route replies to be discarded Improved Error Handling when network becomes partitioned, buffer packets and use exponential back-off for route discovery Listen to route replies promiscuously to remove entries Use negative information to ignore corrupt replies
DYNAMIC SOURCE ROUTING: ADVANTAGES Routes maintained only between nodes who need to communicate reduces overhead of route maintenance 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
DSR: DISADVANTAGES Packet header size grows with route length due to source routing Flood of route requests may potentially reach all nodes in the network Potential collisions between route requests propagated by neighboring nodes insertion of random delays before forwarding RREQ Increased contention if too many route replies come back due to nodes replying using their local cache Route Reply Storm problem Stale caches will lead to increased overhead
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 AODV assumes symmetric (bi-directional) links When the intended destination receives a Route Request, it replies by sending a Route Reply (RREP) Route Reply travels along the reverse path set-up when Route Request is forwarded
AODV FORWARD PATH SETUP RREQ arrives at a node that has current route to the destination ( larger/same sequence number) unicast request reply (RREP)<source_addr, dest_addr, dest_sequence_#, hop_cnt,lifetime> to neighbor RREP travels back to the source along reverse path each upstream node updates dest_sequence_#, sets up a forward pointer to the neighbor who transmit the RREP
AODV REVERSE PATH SETUP Counters : Sequence number, Broadcast id Reverse Path Broadcast route request (RREQ) < source_addr, source_sequence-#, broadcast_id, dest_addr, dest_sequence_#, hop_cnt > RREQ uniquely identified by <source_addr, broadcast_id> Route reply (RREP) if neighbor is the target, or knows a higher dest_sequence_# Otherwise setup a pointer to the neighbor from whom RREQ was received Maintain reverse path entries based on timeouts
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
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
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
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
REVERSE PATH SETUP IN AODV Y Z S E A B H C I G F K J D M N L
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
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
ROUTE REQUEST AND ROUTE REPLY Route Request (RREQ) includes the last known sequence number for the destination 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 Intermediate nodes that forward the RREP, also record the next hop to destination A routing table entry maintaining a reverse path is purged after a timeout interval A routing table entry maintaining a forward path is purged if not used for a active_route_timeout interval
LINK FAILURE 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 Neighboring nodes periodically exchange hello message When the next hop link in a routing table entry breaks, all active neighbors are informed Link failures are propagated by means of Route Error (RERR) messages, which also update destination sequence numbers
ROUTE ERROR 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 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
AODV: SUMMARY Routes need not be included in packet headers Nodes maintain routing tables containing entries only for routes that are in active use At most one next-hop per destination maintained at each node DSR may maintain several routes for a single destination Sequence numbers are used to avoid old/broken routes Sequence numbers prevent formation of routing loops Unused routes expire even if topology does not change
DSR VS. AODV DSR AODV routing table format full path next hop route checking passive acks hello pings rate of propogation of fast slower topology changes ability to handle frequent good fair topology change CPU / memory usage high low scalability poor excellent 74
FISHEYE STATE ROUTING PROTOCOL The basic principle is that it can capture pixel information with greater accuracy near its eye s focal point. This accuracy decreases with increase in the distance from the centre of the focal point. Each node exchanges topology information with its neighbor only instead of flooding. This is done periodically rather than being driven by an event. It defines routing scopes, which is the set of nodes that are reachable in specific number of hops. The frequency of exchange decreases with the increase in scope. This process reduces the overhead in routing. Advantages: the reduces the BW consumed by the link state update packets. So used for large and highly mobile ad hoc networks.
DENIAL OF SERVICE ATTACKS A Denial of service attack is an explicit attempt to prevent the legitimate user of a service or data. The common method of attack involves overloading the target system with requests, such that it cannot respond to legitimate traffic. it makes the system or service unavailable for the user. The basic types of attack are: consumption of bandwidth or consumption of processor time, obstructing the communication between two machines, disruption of service to a specific system or person, disruption of routing information, disruption of physical components etc. If the sensor network encounters DoS attacks, the attack gradually reduces the functionality as well as the overall performance of the wireless sensor network.
MODES OF ATTACK The three basic types of attack are: a. Consumption of limited or scares resources(network bandwidth, memory) b. Alteration or destruction of configuration information. c. Physical destruction of network components.
the hidden terminal problem reduces the capacity of a network due to increasing the number of collisions, while the exposed terminal problem reduces the network capacity due to the unnecessarily deferring nodes from transmitting.
KEY BENEFITS OF MOBILE AD-HOC NETWORKS No expensive infrastructure must be installed Use of unlicensed frequency spectrum Quick distribution of information around sender Use of ad-hoc networks can increase mobility and flexibility, as ad-hoc networks can be brought up and torn down in a very short time. Ad-hoc networks can be more economical in some cases, as they eliminate fixed infrastructure costs and reduce power consumption at mobile nodes. Because of multi-hop support in ad-hoc networks, communication beyond the Line of Sight (LOS) is possible at high frequencies.
KEY BENEFITS OF MOBILE AD-HOC NETWORKS Multi-hop ad-hoc networks can reduce the power consumption of wireless devices. More transmission power is required for sending a signal over any distance in one long hop than in multiple shorter hops Reduced interference levels, increases spectrum reuse efficiency, and makes it possible to use unlicensed unregulated frequency bands(because of short communication links radio emission levels can be kept low).