Ad hoc Routing: Issues and Algorithms Why we study this paper identify the issues of routing support in mobile ad hoc networks understand some design options for ad hoc routing support schemes algorithm overviews of a few algorithms New trends in ad hoc routing

Mobile Ad Hoc Networking (MANET) No backbone infrastructure, typically operate over a single wireless channel dynamic topologies: nodes are free to move arbitrarily bandwidth-constrained, variable capacity links energy-constrained operation limited physical security may have both unicast and multicast/broadcast traffic typically assume reliable broadcast at the MAC/link layer in the near term, MANET functions as stub networks (RFC2501): all traffic carried by MANET nodes will either be sourced or sinked within the MANET NOT as transit networks carrying traffic that enters and then leaves MANET

Goals for Ad Hoc Routing Near-term goal (RFC2501): provides for effective operation over a wide range of mobile networking “contexts” (i.e. a set of MANET characteristics) supports connectionless IP service: IP layer mobile routing reacts efficiently to topological changes and traffic demands while maintaining effective routing in a MANET context Qualitative properties of MANET routing protocols: distributed operation: no centralized solution loop-freedom: avoid problems such as a small fraction of packets spinning around in the net for arbitrary time periods demand-based operation: routing algorithms adapt to the traffic pattern in the net on a demand or need basis proactive operation: flip-side of demand-based operation security, sleep period operation, unidirectional link support.

Goals for Ad Hoc Routing (contd) Network context network size (# of nodes), network connectivity (avg degree of a node), topological rate of change, link capacity, fraction of unidirectional links, traffic patterns, mobility (more on topological correlation), fraction & frequency of sleeping nodes Quantitative metrics to assess ad hoc routing protocols: end-to-end data throughput and delay route acquisition time: of particular concern with on demand routing algorithms percentage of out-of-order delivery efficiency: internal measure of its effectiveness average number of data bits transmitted/data bit delivered average number of control bits transmitted/data bit delivered average number of control and data packets transmitted/data packet delivered

Some Architectural Considerations Link level operation ability to detect link appearances & failures in the presence of changing topology: periodic probing using overhead packets (faster link status sensing) lack of link-level ACKs of transmitted message packets (slower sensing, no overhead) shortest path routing: is it worth the cost ? changing topology: slow versus fast congested network: light versus heavy how to handle the gray area ? adaptive hard state versus soft state

Brief Review on Shortest-path Routing Internet supports shortest-path routing Properties for shortest path: subpaths of shortest paths are shortest paths the weight of a shortest path is the sum of the weights of its subpaths Dijkstra’s algorithm: builds up a shortest path tree by computing the weighted breadth first tree from the source Bellman Ford algorithm: builds a shortest-path tree by incrementally expanding the permissible set of nodes that are allowed to be intermediate hops Distributed Bellman Ford Algorithm distance vector and link state routing distance vector: exchange vectors of distance estimates to destinations (e.g. RIP) link state: maintain the entire network topology at all routers (OSPF)

Design Options in Ad Hoc Routing Who serve as routers: every mobile host is a router a subset of the nodes are routers (e.g., due to energy concerns) every host is a router but only a subset of routers are used to generate routes (virtual backbone concept) what is the neighborhood info to propagate link state, distance vectors, known routes metrics for route computation weighted shortest path, interference metrics, congestion metrics, power/energy metrics, throughput metrics how many routes are maintained single route for a source-destination pair multiple routes for a src-dest pair when to generate routes static versus on demand

Destination-Sequenced Distance-Vector Routing (DSDV) Based on the Bellman Ford routing algorithm with improvements such as freedom from loops in routing table each node maintains a routing table recording all of the possible destinations and the distance to each destination each entry is marked with a sequence number assigned by the destination node routing tables are periodically transmitted throughout the network route updates via two types of packets: full dump packets carrying all available routing info incremental packets carrying the change only since last full dump

Destination-Sequenced Distance-Vector Routing (DSDV) Operations using sequence number associate monotonically increasing sequence number with the neighborhood update information. When info about a destination is being passed, the latest sequence number generated by that destination is used. This enables source nodes to distinguish fresh info from stale info use sequence numbers to make routing loop-free each node generates an even sequence number every time it propagates its neighborhood update whenever a node detects a loss of connectivity with a neighbor, it creates a neighborhood update, with an odd sequence number that is generated by incrementing the known seq number for the neighborhood by 1 if the destination generates a new neighborhood update, it receives precedence over the loss of connectivity update reduce the frequency of updates by using some stability heuristics: damping route update propagation by an amount of time it takes for a route to settle once a change has been advertised.

Temporally-Ordered Routing Algorithm (TORA) Basic ideas: source initiated routing based on link reversal decouples the generation of potentially far-reaching control message propagation from the rate of topological changes: localizing messaging to a small set of nodes near the change its operation can be biased towards high reactivity and bandwidth conservation rather than routing optimization routes established only when necessary by constructing a directed acyclic graph rooted at the destination using a “query/reply” process reaction to link failure only when necessary (i.e., when a node loses its last downstream link) scope of failure reactions minimized (i.e., the # of nodes that must participate) no reaction to link activation

Temporally-Ordered Routing Algorithm (TORA) Detailed operations each router is assigned a “height”, links are directed from a higher router to a lower router, always set a UPSTREAM router with height higher than the DOWNSTREAM routers link reversal: each node i other than the destination keeps a list of its neighboring nodes j that have reversed the direction of the corresponding links (i, j). At each iteration, each node i that has no outgoing links reverses the directions of the links (i,j) for all j that do not appear on the list, and empties the link when a failure happens (a node loses its last downstream link & becomes a local minimum), the node selects a new height such that it becomes a global maximum. Then, the failure propagates outward from the point of the original failure.

Ad Hoc On-Demand Distance Vector (AODV) Key features builds upon DSDV, thus a distance vector algorithm using destination sequencing for route updates improves DSDV by generating routes on demand (instead of maintaining a complete list of routes), thus reducing the number of required broadcasts authors classify it as a pure on-demand route acquisition system, since nodes that are not on a selected path do not maintain routing info or participate in routing table exchanges

Ad Hoc On-Demand Distance Vector (AODV) Path discovery process -- when a src does not have a route to the desired destination: limited range broadcast used for neighborhood discovery: source broadcasts a route request (RREQ) packet to its neighbors, which then forward RREQ to its neighbors, until either the destination or a “fresh enough” route to the destination is located routes are cached by intermediate nodes for some time since last use reply to RREQ only if they have a route to the destination whose corresponding destination sequence number is greater or equal to that contained in the RREQ When forwarding RREQ, record in their route tables the address from which the 1st copy of the broadcast packet is received, thereby establishing a reverse path

AODV When RREQ reaches destination or an intermediate node with fresh enough route: responds with a route reply packet back to the neighbor from which it first receives RREQ When RREQ is routed back along the reverse path, nodes along this path set up forward route entries in their route tables which point to the node from which RREQ came. A route timer is associated with each route entry: if it is not used in its lifetime, delete the entry AODV only supports symmetric link (RREP and RREQ follow the same route, but in reverse directions) link failure management: infinite metric assigned to broken links if a node along the route moves, its upstream neighbor detects it and forwards a notification message (RREQ with infinite metric) link breakage triggers notification back to uses of formerly active links (further upstream nodes) until the src is reached. The src may re-initiate route discovery

Dynamic Source Routing (DSR) An on-demand routing protocol based on source routing Designed for the scenario where traffic flows only from a few source nodes to a few destination nodes source and destination (as well as participating nodes) gather routing info into caches which is used to compute source routes to the desired dest., through the exchange of flooded query and reply packets containing full path information once discovered, routes are used until they fail as determined by the failure of attempted message transmissions appropriate for communication with infrequently accessed destinations and makes sense when maintaining continuous routing from many sources is not necessary

Dynamic Source Routing (DSR) Route discovery: the source checks whether it has a valid route entry in its cache if no, broadcast a route request packet each intermediate node checks whether it knows of a route to the destination. If NO, adds its own address to the route record of the packet and forwards the packet along its outgoing links “route request” packet contains a route record yielding the sequence of hops taken. “route reply” is generated when the request reaches either the destination or an intermediate node that has a valid route to the destination; route record is placed in the reply message. once discovered, routes are used until they fail as determined by the failure of attempted message transmissions

Further Discussions Goals revisited other approaches ? route optimality, fast route reactivity, scalability topological changes: slow versus fast flooding, multi-path forwarding, single shortest path other approaches ? “virtual backbone”, hierarchical design, landmarks, geographic routing...

New Trends in Ad Hoc Routing New requirements for routing protocols Energy efficiency Security Quality of service Multicast or anycast Changes in technology Location information No mobility (sensor network)