Geoffrey G. Xie

Delay guarantee of virtual clock server

In a packet switching network, each communication channel is statistically shared among many traffic flows that belong to different end-to-end sessions. We present and prove a delay guarantee for the virtual clock service discipline (inspired by time division multiplexing). The guarantee has several desirable properties, including the following firewall property: the guarantee to a flow is unaffected by the behavior of other flows sharing the same server. There is no assumption that sources are flow controlled or well behaved. We first introduce and define the concept of an active flow. The delay guarantee is then formally stated as a theorem. We show how to obtain delay bounds from the delay guarantee of a single server for different specifications.

Network policy languages: a survey and a new approach

A survey of current network policy languages is presented. Next, a summary of the techniques for detecting policy conflicts is given. Finally, a new language, path-based policy language, which offers improvements to these is introduced. Previous network policy languages vary from the very specific, using packet filters at the bit level, to the more abstract where concepts are represented, with implementation details left up to individual network devices. As background information a policy framework model and policy-based routing protocols are discussed. The PPLs path-based approach for representing network policies is advantageous in that quality of service and security policies can be associated with an explicit path through the network. This assignment of policies to network flows aids in new initiatives such as integrated services. The more stringent requirement of supporting path-based policies can easily be relaxed with the use of wild card characters to also support differentiated services and best-effort service, which is provided by the Internet today.

Tight Performance Bounds of Multihop Fair Access for MAC Protocols in Wireless Sensor Networks and Underwater Sensor Networks

This paper investigates the fundamental performance limits of medium access control (MAC) protocols for particular multihop, RF-based wireless sensor networks and underwater sensor networks. A key aspect of this study is the modeling of a fair-access criterion that requires sensors to have an equal rate of underwater frame delivery to the base station. Tight upper bounds on network utilization and tight lower bounds on the minimum time between samples are derived for fixed linear and grid topologies. The significance of these bounds is two-fold: First, they hold for any MAC protocol under both single-channel and half-duplex radios; second, they are provably tight. For underwater sensor networks, under certain conditions, we derive a tight upper bound on network utilization and demonstrate a significant fact that the utilization in networks with propagation delay is larger than that in networks with no propagation delay. The challenge of this work about underwater sensor networks lies in the fact that the propagation delay impact on underwater sensor networks is difficult to model. Finally, we explore bounds in networks with more complex topologies.

A network layer protocol for UANs to address propagation delay induced performance limitations

This paper provides a description of a novel network layer protocol for underwater acoustic networking (UAN) that provides a mechanism for network control and management enabling the implementation of responsive, self-configuring, adaptable, and scalable networks whose performance are predictable. The protocol draws from the demonstrated efficiencies of multi-protocol labeled switching, dynamic source routing, and multi-constraint based resource allocation schemes. The paper describes the expected benefits of establishing full duplex functionality between network nodes and presents some of the preliminary simulation findings regarding the viability of autonomously determining the network topology utilizing the full duplex node connections.

Shedding light on the glue logic of the internet routing architecture

Recent studies reveal that the routing structures of operational networks are much more complex than a simple BGP/IGP hierarchy, highlighted by the presence of many distinct instances of routing protocols. However, the glue (how routing protocol instances interact and exchange routes among themselves) is still little understood or studied. For example, although Route Redistribution (RR), the implementation of the glue in router software, has been used in the Internet for more than a decade, it was only recently shown that RR is extremely vulnerable to anomalies similar to the permanent route oscillations in BGP. This paper takes an important step toward understanding how RR is used and how fundamental the role RR plays in practice. We developed a complete model and associated tools for characterizing interconnections between routing instances based on analysis of router configuration data. We analyzed and characterized the RR usage in more than 1600 operational networks. The findings are: (i) RR is indeed widely used; (ii) operators use RR to achieve important design objectives not realizable with existing routing protocols alone; (iii) RR configurations can be very diverse and complex. These empirical discoveries not only confirm that the RR glue constitutes a critical component of the current Internet routing architecture, but also emphasize the urgent need for more research to improve its safety and flexibility to support important design objectives.

Incorporating Realistic Acoustic Propagation Models in Simulation of Underwater Acoustic Networks: A Statistical Approach

The development of protocols to advance the state of the art in underwater acoustic networks (UANs) relies on the use of computer simulations to analyze protocol performance. It is typical for designers to abstract away much of the detail of the physical environment in order to simplify the development of the simulation and ensure the simulation runtime performance is reasonable. The validity of the simulation results becomes questionable. There are, though, very high fidelity models developed by acoustic engineers and physicists for predicting acoustic propagation characteristics. In addition to these models, empirical data collections have been generated for many geographic regions of interest to UAN planners. However, incorporating these engineering and physics models or data collections into a network simulation is problematic, as the models are computationally complex and the data sets are not directly usable for acoustic signal propagation characterization. This paper presents a statistical method for developing a computationally efficient and simulation friendly approximation of a physics model of path loss. This method may also be used to adapt empirical data sets for use in network simulation in the same manner. The method was applied to the output of the Monterey-Miami Parabolic Equation model to assess its impact on the runtime performance of an OPNET-based simulation. Results of that simulation are compared to results from a previous OPNET simulation that simply used distance to determine reception. The simulation results confirm the incorporation of the MMPE approximation does not noticeably impact the runtime performance of the simulation. Anecdotally, the simulation confirms earlier results that suggest contention-based access controls without collision avoidance techniques may outperform the typical access technique adapted from wireless radio networks that employs collision avoidance, contrary to conventional wisdom

Burst scheduling: architecture and algorithm for switching packet video

The authors observed that variable bit rate (VBR) video, which is a sequence of encoded pictures, has very large rate fluctuations from picture to picture. In designing a new traffic model, the authors retain the basic notion of a flow but allow the flow rate to fluctuate. In particular, they introduce the concept of a burst which, in a video flow, is a sequence of packets that carry the bits of an encoded picture. They present the architecture of a class of packet switching networks, called burst scheduling networks, for carrying video, audio, and data traffic. The class is characterized by (i) use of virtual clock value as priority in scheduling, (ii) end-to-end delay and delay jitter guarantees provided to flows conforming to the new traffic model, and (iii) traffic flows (in particular, video flows) scheduled efficiently in bursts. Some experimental results are presented from a discrete-event simulation in which traces from several MPEG video sequences were used as video sources.

Analyzing the Performance of Multi-hop Underwater Acoustic Sensor Networks

Multi-hop underwater acoustic sensor networks constrain the performance of medium access control protocols. The efficiency of the well-known RTS-CTS scheme is degraded due to long propagation delays of such networks. Recently, interest in Aloha variants has surfaced; however, the performance of such protocols within the context of multi-hop networks is not well studied. In this paper, we identify the challenges of modeling contention-based medium access control protocols and present a model for analyzing Aloha variants for a simple string topology as a first step toward analyzing the performance of contention-based proposals in multi-hop underwater acoustic sensor networks. An application of the model suggests that Aloha variants are vary sensitive to traffic loads and network size.

Towards systematic design of enterprise networks

Enterprise networks are important, with size and complexity even surpassing carrier networks. Yet, the design of enterprise networks remains ad hoc and poorly understood. In this paper, we show how a systematic design approach can handle two key areas of enterprise design: virtual local area networks (VLANs) and reachability control. We focus on these tasks given their complexity, prevalence, and time-consuming nature. Our contributions are threefold. First, we show how these design tasks may be formulated in terms of network-wide performance, security, and resilience requirements. Our formulations capture the correctness and feasibility constraints on the design, and they model each task as one of optimizing desired criteria subject to the constraints. The optimization criteria may further be customized to meet operator-preferred design strategies. Second, we develop a set of algorithms to solve the problems that we formulate. Third, we demonstrate the feasibility and value of our systematic design approach through validation on a large-scale campus network with hundreds of routers and VLANs.

Understanding Route Redistribution

Route redistribution (RR) has become an integral part of IP network design as the result of a growing need for disseminating certain routes across routing protocol boundaries. While RR is widely used and resembles BGP in several nontrivial aspects, surprisingly, the safety of RR has not been systematically studied by the networking community. This paper presents the first analytical model for understanding the effect of RR on network wide routing dynamics and evaluating the safety of a specific RR configuration. We first illustrate how easily inaccurate configurations of RR may cause severe routing instabilities, including route oscillations and persistent routing loops. At the same time, general observations regarding the root causes of these instabilities are provided. We then introduce a formal model based on the general observations to represent and study the safety of route redistribution. Using the model, we prove that given a RR configuration, determining whether the redistributions result in a cycle is NP-hard. Given this complexity, we present a sufficient condition, which can be checked in polynomial time with the proposed analytical model, for ensuring the safety of a RR configuration. Finally, the paper proposes potential changes to the current RR protocol to guarantee safety.