Richard G. Ogier

A reliable, efficient topology broadcast protocol for dynamic networks

We present, prove correctness for, and evaluate a protocol for the reliable broadcast of topology and link-state information in a multihop communication network with a dynamic topology, such as a wireless network with mobile nodes. The protocol is called topology broadcast based on reverse path forwarding (TBRPF), and uses the concept of reverse-path forwarding (RPF) to broadcast link-state updates in the reverse direction along the spanning tree formed by the minimum-hop paths from all nodes to the source of the update TBRPF uses the topology information received along the broadcast trees to compute the minimum-hop paths that form the trees themselves, and is the first topology broadcast protocol based on RPF with this property. The use of minimum-hop trees instead of shortest-path trees (based on link costs) results in less frequent changes to the broadcast trees and therefore less communication cost to maintain the trees. Simulations show that TBRPF achieves up to a 98% reduction in communication cost compared to flooding in a 20-node network.

Distributed algorithms for computing shortest pairs of disjoint paths

Distributed algorithms for finding two disjoint paths of minimum total length from each node to a destination are presented. The algorithms have both node-disjoint and link-disjoint versions and provide each node with information sufficient to forward data on the disjoint paths. It is shown that this problem can be reduced to the problem of finding a minimal shortest path from each node to the destination in a modified network, and a distributed algorithm on the original network that simulates a shortest-paths algorithm running on the modified network is presented. The algorithm has a smaller space complexity than any previous distributed algorithm for the same problem, and a method for forwarding packets is presented that does not require any additional space complexity. A synchronous implementation of the algorithm is also presented and studied. >

A distributed algorithm for finding shortest pairs of disjoint paths

A distributed asynchronous algorithm is presented for finding two disjoint paths of minimum total length from each possible source node to a destination. The algorithm has both node-disjoint and link-disjoint versions, and provides each node with information sufficient to make incremental routing decisions for forwarding packets over the disjoint paths. For a network in which all links have unit length, it is shown that a synchronous implementation of the algorithm has communication and time complexities O( mod E mod +D mod N mod ) and O(D/sub 2/), respectively, where D is the networks diameter and D/sub 2/ is the maximum, over all nodes i, of the total number of links in the shortest pair of disjoint paths from i to the destination.<<ETX>>

Minimum-delay routing in continuous-time dynamic networks with Piecewise-constant capacities

A single-source single-sink dynamic network is considered where the link flows are real-valued measurable functions defined on a time interval and where storage is allowed at the nodes. Piecewise-constant link and storage capacities are given. A τ-maximum flow is a dynamic flow assignment that maximizes the total amount of commodity reaching the sink before time τ. The problem considered is that of computing a flow which is simultaneously τ-maximum for all τ. Such a flow solves a minimum-delay dynamic routing problem. An algorithm is presented which computes an optimal flow in O( | N | 4T4) time, where | N | is the number of nodes and T is the number of times that the capacities change. Previous polynomial-time algorithms have been given only for the case of constant capacities.

Minimum-expected-delay alternate routing

The authors consider the problem of finding a routing strategy that minimizes the expected delay from every source to a single destination in a network in which each link fails and recovers according to a Markov chain. It is assumed that each node knows the current state of its own outgoing links and the state-transition probabilities for every link of the network. It is shown that the general problem is #P-complete, and two special cases are considered: case 1 assumes the network is a directed acyclic graph oriented toward the destination, and case 2 assumes that the link states are independent from slot to slot. For each case, it is proved that the optimal routing strategy has a simple state-independent representation. An efficient algorithm is presented for finding the optimal strategy.

Network Control and Data Transport for C 3 I Applications

In this paper, communication networks that serve military C3I applications are considered. Such networks must operate under highly dynamic conditions, both in terms of network topology and in the volume and distribution of their traffic. The nodes in these networks have to adapt, therfore, to frequent changes while using their scarce resources efficiently and delivering reliable data transport at all times. To help with this, algorithms that provide some of the required functionality, namely, adaptive routing and network reconfiguration schemes are presented. Specifically, two distributed routing algorithms are considered: (1) a shortest-path algorithm that converges after a number of cycles in the order of the networks diameter, and that is almost loop free, and (2) a quasi-static algorithm that confines the message exchange following a link failure to a small set of nodes in order to yield a routing that is close to optimal. We next describe a random node interconnection procedure that enables the nodes to control the neighborhood they communicate directly with. This procedure yields a connected network with high probability if the nodal degree is above some threshold. Efficient scheduling of transmissions over radio channels is essential for high network throughput. An algorithm for optimum scheduling is described and its complexity, which is the best known so far, is discussed. Finally, we discuss a side effect of the requirement for efficient, reliable packet delivery, that is, packets that arrive out of order, and present performance models of the resequencing required at the receiver.

Neural network methods with traffic descriptor compression for call admission control

We present and evaluate new techniques for call admission control (in ATM networks) based on neural networks. The methods are applicable to very general models that allow heterogeneous traffic sources and finite buffers. A feedforward neural network (NN) is used to predict whether or not accepting a requested new call would result in a feasible aggregate scream, i.e., one that satisfies the QOS requirements. The NN input vector is a traffic descriptor for the aggregate stream that has the following beneficial properties: its dimension is independent of the number of traffic classes; and it is additive, allowing it to be updated efficiently by simply adding the traffic descriptor of the new call. A novel asymmetric error function for the NN helps achieve our asymmetric objective in which rejecting an infeasible stream is more important than accepting a feasible one. We present a NN design that provides an optimal linear compression of the NN inputs to a smaller number of traffic parameters. The special case of one compressed parameter corresponds to an NN version of the equivalent bandwidth. Experiments show our methods to be better than methods based on the equivalent bandwidth, with respect to call blocking probability and the percentage of feasible streams that an correctly classified.

Fair charging policies and minimum-expected-cost routing in internets with packet loss

An economics-based definition of fair pricing is proposed, and fair, simple, and mathematically sound charging policies are derived that properly handle costs and risks in lossy internetworks. Also described are simple and efficient source-based algorithms (having both link-state and distance-vector versions) for computing the optimal routing tables with respect to the new charging measures. These charging measures and algorithms have the property that, by minimizing the individual domains expenses, they also minimize the global resource consumption. In addition, a distributed destination-based algorithm is presented for computing optimal paths that allows for faster response time in highly dynamic and unreliable internets. Finally, preliminary accounting procedures are presented for implementing the proposed charging policies.<<ETX>>

Robust routing for minimum worst-case expected delay in unreliable networks

The authors consider the problem of finding a routing strategy that minimizes the worst-case expected delay from every source to a single destination in an unreliable network, given a constraint on the number of outgoing links at each node that can be inoperational at any point in time. Subject to this constraint, links can fail and recover arbitrarily often. A node having a packet to forward must choose a single link on which to transmit the packet, but does not know in advance which links will be inoperational during the transmission. If a transmission fails, the packet is retransmitted (not necessarily on the same link) after some fixed amount of time. It is shown that the optimal routing strategy is a stationary randomized strategy in which each node selects the forwarding link according to a fixed probability distribution. An efficient algorithm that computes an epsilon -optimal solution in O( mod E mod log( mod V mod / epsilon )) time, for any positive number epsilon , is presented.<<ETX>>