CEDAR: a Core-Extraction Distributed Ad Hoc Routing Algorithm Prasun Sinha Raghupathy Sivakumar Vaduvur Bharghavan
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CEDAR: a Core-Extraction Distributed Ad hoc Routing algorithm Prasun Sinha Raghupathy Sivakumar Vaduvur Bharghavan University of Illinois at Urbana-Champaign g Email:fprasun, sivakumr, bharghav @timely.crhc.uiuc.edu Abstract—CEDAR is an algorithm for QoS routing in ad hoc network en- by propagating the bandwidth availability information of vironments. It has three key components: (a) the establishment and main- stable links in the core graph. The basic idea is that the in- tenance of a self-organizing routing infrastructure called the core for per- forming route computations, (b) the propagation of the link-state of stable formation about stable high-bandwidth links can be made high-bandwidth links in the core through increase/decrease waves, and (c) a known to nodes far away in the network, while information QoS route computation algorithm that is executed at the core nodes using about dynamic links or low bandwidth links should remain only locally available state. local. Slow-moving increase waves and fast moving de- Our preliminary performance evaluation shows that CEDAR is a robust and adaptive QoS routing algorithm that reacts effectively to the dynamics crease waves which denote corresponding changes in avail- of the network while still approximating link-state performance for stable able bandwidths on links, are used to propogate non-local networks. information over core nodes. Keywords—Ad hoc routing, QoS routing inftro.tex Route computation: Route computation first establishes a core path from the dominator (See Section II) of the source I. INTRODUCTION to that of the destination. The core path provides the direc- An ad hoc network is a dynamic multi-hop wireless network tionality of the route from the source to the destination. Us- that is established by a group of mobile hosts on a shared wire- ing this directional information, CEDAR iteratively tries to less channel by virtue of their proximity to each other. Ad hoc find a partial route from the source to the domain of the fur- core networks find applicability in military environments, wherein a thest possible node in the path (which then becomes platoon of soldiers or a fleet of ships may establish an ad hoc net- the source for the next iteration) satisfying the requested work in the region of their deployment. Military network envi- bandwidth, using only local information. Effectively, the ronments typically require quality of service for their mission- computed route is a shortest-widest furthest path using the core critical applications. Hence, the focus of this paper is to provide path as the guideline. quality of service routing in ad hoc networks. The rest of this paper is organized as follows. Section II de- Ad hoc networks are dynamic in nature, and transmissions scribes the network model and terminology used in the paper. are susceptible to fades, interference, and collisions from hid- Section III describes the computation and dynamic management den/exposed stations. These characteristics make it a challeng- of the core of the network. Section IV describes link state propa- ing task to design a QoS routing algorithm for ad hoc networks. gation through the core using increase and decrease waves. Sec- Following are the main design goals for such an algorithm: tion V describes the route computation algorithm of CEDAR, 1. The algorithm should be highly robust and should degrade and puts together the algorithms described in the previous sec- gracefully with increasing mobility. tions. Section VI analyzes the performance of CEDAR through 2. Route computation should not require maintenance of simulations and Section VII summarizes the paper. global information. 3. The computed route should be highly likely to sustain the II. NETWORK MODEL AND TERMINOLOGY requested bandwidth for the flow. We assume that all the hosts communicate on the same shared 4. The route computation should involve only a few hosts as wireless channel. Neighborhoodis assumed to be a commutative broadcast in the whole network is expensive. B B A property (i.e. if A can hear , then can hear ). Because of 5. Hosts should have quick access to routes when connections the local nature of transmissions, hidden and exposed stations need to be established. abound in an ad hoc network. We assume the use of a CSMA/CA We propose CEDAR as a QoS routing algorithm, which like algorithm such as MACAW [1] for reliable unicast commu- achieves the above design goals for small to medium size ad hoc nication, and for solving the problem of hidden/exposed stations. networks consisting of tens to hundreds of nodes. The following Essentially, data transmission is preceded by a control packet is a brief description of the three key components of CEDAR. handoff, and the sequence of packets exchanged in a commu- Core extraction: A set of hosts is distributedly and dynam- nication is the following: RTS (from sender to receiver) - CTS ically elected to form the core of the network by approx- (from receiver to sender) - Data (from sender to receiver) - ACK imating a minimum dominating set of the ad hoc network using only local computation and local state. Each core host A shortest widest path is the maximum bandwidth path. If there are several maintains the local topology of the hosts in its domain, and such paths, it is the one with the least number of hops. A hidden station is a host that is within the range of the receiver but not the also performs route computation on behalf of these hosts. transmitter, while an exposed station is within the range of the transmitter but not Link state propagation: QoS routing in CEDAR is achieved the receiver. 1 (from receiver to sender). Local data broadcasts are not assumed 2. Local broadcasts are highly unreliable in ad hoc networks to be reliable. due to the abundance of hidden and exposed stations. We assume that each host can estimate the available band- Topology information propagation [4] and route probes [2], width using some link-level mechanisms. We also assume a [3] are inevitable in order to establish routes and will, of ne- close coordination between the MAC and routing layers is as- cessity, need to be broadcast if every node performs route sumed. In particular, reception of RTS and CTS control mes- computation. While the adverse effects of unreliable broad- sages at the MAC layer is used to improve the behavior of the casts are typically not considered in most of the related routing layer, as explained in Section III. work on ad hoc routing, we have observed that flooding in We represent the ad hoc network by means of an undirected ad hoc networks is highly lossy [5]. On the other hand, if V E V graph G , where is the set of nodes in the graph (hosts only a core subset of nodes in the ad hoc network perform in the network), and E is the set of edges in the graph (links in route computations, it is possible to set up reliable unicast th i N x x the network). The i open neighborhood, of node is the channels between nearby core nodes and accomplish both i set of nodes whose distance from x is not greater than , except the topology updates and route probes much more effec- th i i N x x node x itself. The closed neighborhood of node is tively. i fxg N . The issues with having only a core subset of nodes performing V V A dominating set S is a set such that every node in route computations are threefold. First, nodes in the ad hoc net- S is either in S or is a neighbor of a node in . A dominating set work that do not perform route computation must have easy ac- with minimum cardinality is called a minimum dominating set cess to a nearby core node so that they can quickly request routes (MDS). Also, every node not in S chooses one of its neighbors to be set up. Second, the establishment of the core must be a u v who is in S as its dominator.Avirtual link between two purely local computation. In particular, no core node must need G u v nodes in the dominating set S is a path in from to . We use to know the topology of the entire core graph. Third, a change the term tunnel interchangeably with virtual link in our discus- in the network topology may cause a recomputation of the core sions. graph. Recomputation of the core graph must only occur in the locality of the topology change, and must not involve a global re- V G Given an MDS C of graph , we define a core of the graph computation of the core graph. On the other hand, the locally re- C V E E fu v j u V v V u C C C C C , where computed core graph must still only comprise of a small number V N v g . Thus, the core graph consists of the MDS nodes C , of core nodes - otherwise the benefit of restricting route compu- V and a set of virtual links between every two nodes in C that are tation to a small core graph is lost. Our core computation algo- u v within a distance 3 of each other in G. Two nodes and which rithm satisfies the above requirements. u v have a virtual link in the core are said to be nearby nodes. Thus, from the definition of the core,ifG is connected, then a B. Generation and Maintenance of the Core in CEDAR G core C of must also be connected (via virtual links). Ideally, the core comprises of the nodes in a minimum domi- G V E RCHITECTURE AND THE ORE V III.