Paolo Narvaez

New dynamic SPT algorithm based on a ball-and-string model

A key functionality in todays widely used interior gateway routing protocols such as OSPF and IS-IS involves the computation of a shortest path tree (SPT). In many existing commercial routers, the computation of an SPT is done from scratch following changes in the link states of the network. As there may coexist multiple SPTs in a network with a set of given link states, such recomputation of an entire SPT not only is inefficient but also causes frequent unnecessary changes in the topology of an existing SPT and creates routing instability.This paper presents a new dynamic SPT algorithm that makes use of the structure of the previously computed SPT. Our algorithm is derived by recasting the SPT problem into an optimization problem in a dual linear programming framework, which can also be interpreted using a ball-and-string model. In this model, the increase (or decrease) of an edge weight in the tree corresponds to the lengthening (or shortening) of a string. By stretching the strings until each node is attached to a tight string, the resulting topology of the model defines an (or multiple) SPT(s). By emulating the dynamics of the ball-and-string model, we can derive an efficient algorithm that propagates changes in distances to all affected nodes in a natural order and in a most economical way. Compared with existing results, our algorithm has the best-known performance in terms of computational complexity as well as minimum changes made to the topology of an SPT. Rigorous proofs for correctness of our algorithm and simulation results illustrating its complexity are also presented.

An acknowledgment bucket scheme for regulating TCP flow over ATM

The available bit rate (ABR) service has been developed to support data traffic over ATM networks and provide end-to-end congestion control at the ATM layer. However, as most data applications today are only connected to ATM via legacy networks, simulation results in ATM Forum contributions suggest that ABR may still cause congestion at network interfaces and cannot provide adequate flow control on an end-to-end basis between data sources. We propose a new efficient scheme for regulating TCP traffic over ATM networks with the goal of minimizing the network interface buffer requirement. The key idea underlying our scheme is to match the TCP source rate to the ABR explicit rate by controlling the flow of TCP acknowledgments at the network interfaces. We present analytical and simulation results to show that our scheme has minimum buffer requirement, yet offers the same throughput performance as that with an infinite buffer. Moreover, our scheme is transparent to the TCP layer and requires no modification in the ATM network except at the network interface.

Local restoration algorithm for link-state routing protocols

Link-state protocols such as OSPF are the dominant routing technology in todays Internet. Despite their many advantages, these protocols require the flooding of new information across the entire routing area after changes in any link state (e.g., link failures). As the routing area grows or the frequency of link-state changes increases, the overhead (in terms of bandwidth and processing cost) of flooding becomes prohibitive. Furthermore, such flooding over a large area will cause temporary inconsistency of link states among many routers, potentially creating many transient routing loops that can last for a long time. This limits the scalability of the routing protocols to large routing areas. To overcome such problems, we present in this paper a novel algorithm that minimizes the amount of information distributed by link-state routing protocols. Upon a link failure, our algorithm will distribute the link-state changes to the minimum number of routers that are needed to ensure loop-free routing. Moreover, implementing our algorithm requires only a simple extension to any existing link-state protocol.

Optimal feedback control for ABR service in ATM

The efficient support of data traffic over ATM networks requires congestion control, whose objectives include maximizing throughput, minimizing switch buffer requirement, and attaining a fair bandwidth allocation. With available bit rate (ABR) service in ATM, congestion control is achieved by requiring data sources to adjust their rates based on the feedback from the network. The difficulties in providing effective ABR service are caused by the burstiness of data traffic, the dynamic nature of the available bandwidth, as well as the feedback delay. Using a new design methodology described in our previous paper, we present here a congestion control algorithm that is provably stable and is optimal in the sense that it has the shortest possible transient response time. Moreover, our algorithm achieves fair bandwidth allocation among contending connections and maximizes network throughput. It also delivers good performance for switches that use a FIFO queuing discipline. Essentially, the algorithm implicitly measures the round-trip delay using resource enhancement cells, and establishes an observer (in the control theoretic sense) to control the flow of data in the network. Rigorous analysis as well as simulation results are presented to substantiate our claims.

Fault-Tolerant Routing in the Internet without Flooding

Link-state protocols such as OSPF are the dominant routing technology in today’s Internet. Despite their many advantages, these protocols require the flooding of new information across the entire routing area after changes in any link state (e.g. link failures). As the routing area grows or the frequency of link-state changes increases, the overhead (in terms of bandwidth and processing cost) of flooding becomes prohibitive. Furthermore, such flooding over a large area will cause temporary inconsistency of link states among many routers, potentially creating many transient routing loops that can last for a long time. This limits the scalability of the routing protocols to large routing areas.

Acknowledgment bucket scheme for regulating TCP flow over ATM

The available bit rate (ABR) service has been developed to support data traffic over ATM networks and provide end-to-end congestion control at the ATM layer. However, as most data applications today are only connected to ATM via legacy networks, simulation results in past ATM Forum contributions suggest that ABR may still cause congestion at network edges and cannot provide adequate flow control on an end-to-end basis. In this paper, we propose a new efficient scheme for regulating TCP traffic over ATM networks with the goal of minimizing the network-edge buffer requirement while maximizing TCP goodput. The key idea underlying our scheme is to match the TCP source rate to the ABR explicit rate by controlling the flow of TCP acknowledgments at network edges. We present analytical and simulation results to show that our scheme has negligible buffer requirement, yet offers similar goodput performance as that with infinite buffer. Moreover, our scheme is transparent to the TCP layer and requires no modification in the ATM network except at the network edge.

New techniques for regulating TCP flow over heterogeneous networks

We study a typical network environment where TCP traffic is generated from a source connected to a LAN (e.g. Ethernet) aggregated through an IP edge router to an access network (e.g. ATM, frame relay). Congestion at the edge router occurs when the bandwidth available in the access network cannot support the aggregated traffic generated from the LAN. This paper presents a novel approach to reduce router congestion and improve TCP performance. In particular, we propose new techniques that regulate TCP acknowledgments at the edge router without changing existing TCP implementations at the end systems. Our techniques minimize buffer requirement at network edges while maximizing the throughput. Analytical and simulation results are presented in thes paper to substantiate our claims.

TCP performance improvement over high-speed flow switching networks

Future broadband networks may provide very high bandwidth on demand for certain applications e.g. downloading of a gigabyte file to a high-performance workstation. Motivated by such applications, we study the performance of TCP under the scenario where there is long network latency and rapidly changing bandwidth. We show that existing TCP cannot utilize the bandwidth effectively under such network dynamics. We propose a modification of TCP that would separate the flow control from the loss recovery mechanisms, allowing the efficient use of the available bandwidth over a network that has a high bandwidth-delay product and in which the bandwidth can vary by orders of magnitude over a short period of time. We present simulation results demonstrating the significant performance gain of our new approach.

New transfer methods for client-server traffic in ATM networks

This paper presents new transfer schemes for client-server traffic over ATM networks, with the objective of achieving a bounded transmission time of about one second for as many connection requests as possible. We evaluate via simulations the performance of these new schemes in terms of network utilization, cell loss ratio, and transmission time. The simulation results show that these schemes provide effective means of supporting client-server traffic with quasi-real time requirements.

QoS support over ultrafast TDM optical networks

HLAN is a promising architecture to realize Tb/s access networks based on ultra-fast optical TDM technologies. This paper presents new research results on efficient algorithms for the support of quality of service over the HLAN network architecture. In particular, we propose a new scheduling algorithm that emulates fair queuing in a distributed manner for bandwidth allocation purpose. The proposed scheduler collects information on the queue of each host on the network and then instructs each host how much data to send. Our new scheduling algorithm ensures full bandwidth utilization, while guaranteeing fairness among all hosts.