Chandra N. Sekharan

DST-A routing protocol for ad hoc networks using distributed spanning trees

A dynamic ad hoc network consists of a collection of mobile hosts with frequently changing network topology. We propose a distributed algorithm that adapts to the topology by utilizing spanning trees in the regions where the topology is stable, and resorting to an intelligent flooding-like approach in highly dynamic regions of the network. Routing is performed using the spanning trees based on a hold-and-forward or shuttling method. We introduce the notion of connectivity-through-time and holding time to quantify the performance of the routing algorithms for various network connectivity scenarios. Using simulation, we study the throughput, reachability and message-reachability ratio of the proposed network under various connection/reconnection rates and holding times.

Solving the all-pair shortest path query problem on interval and circular-arc graphs

In this paper, we study the following all-pair shortest path query problem: Given the interval model of an unweighted interval graph of n vertices, build a data structure such that each query on the shortest path (or its length) between any pair of vertices of the graph can be processed efficiently (both sequentially and in parallel). We show that, after sorting the input intervals by their endpoints, a data structure can be constructed sequentially in O(n) time and O(n) space; using this data structure, each query on the length of the shortest path between any two intervals can be answered in O(1) time, and each query on the actual shortest path can be answered in O(k) time, where k is the number of intervals on that path. Furthermore, this data structure can be constructed optimally in parallel, in O(log n) time using O(n/log n) CREW PRAM processors; each query on the actual shortest path can be answered in O(1) time using k processors. Our techniques can be extended to solving the all-pair shortest path query problem on circular-arc graphs, both sequentially and in parallel, in the same complexity bounds. As an immediate consequence of our results, we improve by a factor of n the space complexity of the previously best-known sequential all-pair shortest path algorithm for unweighted interval graphs.

Multicast Routing with Delay and Delay Variation Constraints for Collaborative Applications on Overlay Networks

Computer supported collaborative applications on overlay networks are gaining popularity among users who are geographically dispersed. Examples of these kinds of applications include video-conferencing, distributed database replication, and online games. This type of application requires a multicasting subnetwork, using which messages should arrive at the destinations within a specified delay bound. These applications also require that destinations receive the message from the source at approximately the same time. The problem of finding a multicasting subnetwork with delay and delay-variation bound has been proved to be an NP complete problem in the literature and heuristics have been proposed for this problem. In this paper, we provide an efficient heuristic to obtain a multicast subnetwork on an overlay network, given a source and a set of destinations that is within a specified maximum delay and a specified maximum variation in the delays from a source to the destinations. The time-complexity of our algorithm is O(|E|+nk log(|E|/n)+m2k), where n and |E| are the number of nodes and edges in the network, respectively, k is the number of shortest paths determined, and m is the number of destinations. We have shown that our algorithm is significantly better in terms of time-complexity than existing algorithms for the same problem. Our extensive empirical studies indicate that our heuristic uses significantly less runtime in comparison with the best-known heuristics while achieving the tightest delay variation for a given end-to-end delay bound

Protocol for dynamic ad-hoc networks using distributed spanning trees

A dynamic ad-hoc network consists of a collection of mobile hosts with frequently changing network topology. We propose a distributed algorithm that adapts to the topology by utilizing spanning trees in the regions where the topology is stable, and resorting to an intelligent flooding-like approach in highly dynamic regions of the network. Routing is performed using the spanning trees based a hold-and-forward or shuttling mechanisms. We introduce the notion of connectivity-through-time and the parameter holding-time as new fundamental concepts that can be used by ad-hoc routing algorithms. For various network connectivity scenarios we evaluate the impact of these concepts on the performance of ad-hoc routing algorithms. Using simulation, we study the throughput, reachability and message–reachability ratio of the proposed schemes under various connection/disconnection rates and holding times.

Distributed floor control protocols for computer collaborative applications on overlay networks

Computer supported collaborative applications on overlay networks are gaining popularity among users who are geographically dispersed. Examples of these kinds of applications include video-conferencing, collaborative design and simulation, distance learning, and online games. One of the important issues in collaborative applications is floor control wherein the end-users coordinate among themselves to gain exclusive access to the communication channel. An end-user who wins the floor, sends message to all other participating end-users. In this paper, to solve the floor control problem we present an implementation and evaluation of ALOHA and distributed queue dual bus (DQDB) distributed MAC (medium access control) protocols on overlay networks. As an initial step in the implementation of these MAC protocols, we propose an algorithm to construct an efficient communication channel among the Network Service Nodes (NSNs) in the overlay network. We also show that our implementation scheme (first one among decentralized floor control protocols) preserves the causal ordering of messages. We compare the efficiencies of the proposed implementation of floor control protocols using an analytical model that is verified using extensive simulation experiments

Unified all-pairs shortest path algorithms in the chordal hierarchy

Abstract The objective of this paper is to advance the view that solving the all-pairs shortest path (APSP) problem for a chordal graph G is a two-step process: the first step is determining vertex pairs at distance two (i.e., computing G 2 ) and the second step is finding the vertex pairs at distance three or more. The main technical result here is that the APSP problem for a chordal graph can be solved in O ( n 2 ) time (optimally), if G 2 is already known. It can be shown that computing G 2 for chordal graphs is as hard as for general graphs. We then show certain subclasses of chordal graphs for which G 2 can be computed more efficiently. This leads to optimal APSP algorithms for these classes of graphs in a more natural way than previously known results. Finally, we present an optimal parallel algorithm for the APSP problem on chordal graphs by exploiting new structural properties of shortest paths. Our parallel algorithm uses O ( M ( n )) operations where M ( n ) is the time needed for the fastest known algorithm for multiplying two n × n matrices over a ring.

Load balancing methods for ray tracing and binary tree computing using PVM

We propose efficient load balancing methods for two computational problems namely ray tracing and bottom-up binary tree computing in a distributed environment. In the context of ray tracing, we propose a variant of a static load balancing technique presented in [15] where the sampling is based on partitioning the object space. Our approach partitions the image instead and uses an efficient scheduling technique for load balancing. Computations carried out on a binary tree arise naturally in image processing and network optimization problems. Many of these problems are solved efficiently in parallel by the popular tree contraction technique [1]. In this paper, we explore the tree-contraction technique in a distributed setting using the grain packing method [9]. Implementations of our algorithms on a cluster of workstations using Parallel Virtual Machine (PVM) [6] demonstrate nearperfect load balancing.

Multicast Routing with Delay and Delay Variation Constraints for Multimedia Applications

We provide an efficient heuristic algorithm to obtain a multicast network for multimedia applications, given a source and a set of destinations, that is within a specified maximum delay and a specified maximum variation in the delays from a source to the destinations. The time-complexity of our algorithm is O(|E| + nk log (|E|/n) + m 2 k), where n and |E| are the number of nodes and edges in the network, respectively and k is the number of shortest paths computed and m is the number of destinations. We have shown that our algorithm has significant improvements in term of time-complexity compared with the existing algorithms. Our extensive empirical studies indicate that our heuristic uses significantly less run-time in comparison with the best-known heuristics and yet achieves the tightest delay variation for a given end-to-end delay bound.

A distributed rerouting algorithm for mobile-mobile connections in connection-oriented networks

We present a distributed algorithm for rerouting to accommodate unlimited movement of mobile hosts in a mobile-mobile connection. The algorithm is source initiated-that is the source base station is responsible for the rerouting. The algorithm ensures that the establishment of non-optimal and incorrect paths is avoided and that no packets in the session are lost as there is always a path between the base stations in charge of the source and the destination. The design of this distributed algorithm ensures that the processing at the mobile hosts is kept to a minimum. In addition, the proposed framework for rerouting in mobile-mobile communications is independent of the type of rerouting scheme.

Performance evaluation of wireless TCP schemes under different rerouting schemes in mobile networks

We study the performance of the two wireless TCP schemes-the split-connection and the snoop TCP under four rerouting schemes (full rerouting, partial rerouting, cell forwarding and virtual tree rerouting). The wireless TCP schemes are compared based on the source throughput, average round-trip delay and cumulative source disruption time. We analyse the performance of the wireless TCP schemes and their interaction with the rerouting schemes.