We characterize what is feasible concerning clock synchronization in wireline or wireless networks. We consider a net work of n nodes, equipped with affine clocks relative to a designated clock that exchange packets subject to link delays. Determining all unknown parameters, i.e., skews and offsets of all the clocks as well as the delays of all the communication links, is impossible. All nodal skews, as well as all round-trip delays between every pair of nodes, can be determined correctly. Also, every transmitting node can predict precisely the time indicated by the receiver's clock at which it receives the packet. However, the vector of unknown link delays and clock offsets can only be determined up to an (n - 1)-dimensional subspace, with each degree of freedom corresponding to the offset of one of the (n - 1) clocks. Invoking causality, that packets cannot be received before they are transmitted, the uncertainty set can be reduced to a polyhedron. We also investigate structured models for link delays as the sum of a transmitter-dependent delay, a receiver-dependent delay, and a known propagation delay, and identify conditions which permit a unique solution, and conditions under which the number of the residual degrees of freedom is independent of the network size. For receiver-receiver synchronization, where only receipt times are available, but no time-stamping is done by the sender, all nodal skews can still be determined, but delay differences between neighboring communication links with a common sender can only be characterized up to an affine transformation of the (n - 1) un known offsets. Moreover, causality does not help reduce the uncertainty set.

We focus on the mechanism half of the policy-mechanism divide for networked control systems, and address the issue of what are the appropriate abstractions and architecture to facilitate their development and deployment. We propose an abstraction of ldquovirtual collocationrdquo and its realization by the software infrastructure of middleware. Control applications are to be developed as a collection of software components that communicate with each other through the middleware, called Etherware. The middleware handles the complexities of network operation, such as addressing, start-up, configuration and interfaces, by encapsulating application components in ldquoShellsrdquo which mediate component interactions with the rest of the system. The middleware also provides mechanisms to alleviate the effects of uncertain delays and packet losses over wireless channels, component failures, and distributed clocks. This is done through externalization of component state, with primitives to capture and reuse it for component restarts, upgrades, and migration, and through services such as clock synchronization. We further propose an accompanying use of local temporal autonomy for reliability, and describe the implementation as well as some experimental results over a traffic control testbed.

This paper discusses the use of a communications network security device, called a trust system, to enhance supervisory control and data-acquisition (SCADA) security. The major goal of the trust system is to increase security with minimal impact on existing utility communication systems. A previous paper focused on the technical operation of the trust system by augmenting routers to protect User Datagram Protocol (UDP)-based traffic. This paper concentrates on placing the trust system into a broader context, creates new trust system implementations to increase its flexibility, and demonstrates the trust system using TCP traffic. Specifically, the article expands on previous work in the following ways: 1) the article summarizes major threats against SCADA systems; 2) it discusses new trust system implementations, which allow the trust system to be used with a wider array of network-enabled equipment; 3) it discusses key SCADA security issues in the literature and shows how the trust system responds to such issues; 4) the paper shows the impact of the trust system when widely prevalent TCP/IP network communication is used; and 5) finally, the paper discusses a new hypothetical scenario to illustrate the protection that a trust system provides against insider threats.

No two clocks generally agree. Distributed control applications, however, require accurate timing information. This necessitates some form of message passing for interpretation of distributed clocks. We exhibit a fundamental indeterminacy in estimating time when delays are unequal in the two directions, necessitating an assumption of symmetry. We present the Control Time Protocol as an architecturally clean algorithm for providing timing information in the domain of distributed control. The protocol introduces no additional dependencies beyond those already present in the very control loops, thus enhancing the reliability of the systems. It has been implemented on the testbed in the Convergence Laboratory at the University of Illinois. Experimental results demonstrating how time stamping can be used to enhance stability and performance, as well as measurements of network latencies, are provided.

We anticipate that a possible next phase in the information technology revolution could be the convergence of control, i.e., sensing and actuation, with communication and computing. We address the broad set of issues that we believe to be important to the design, implementation, and proliferation of such systems. In particular, we expound on the topics of the architecture of such systems, methodologies for design, distributed time, services, and middleware. We describe our efforts in the Convergence Lab at the University of Illinois with respect to each of these topics.

This paper investigates network policies and mechanisms to enhance security in SCADA networks using a mix of TCP and UDP transport protocols over IP. It recommends creating a trust system that can be added in strategic locations to protect existing legacy architectures and to accommodate a transition to IP through the introduction of equipment based on modern standards such as IEC 61850. The trust system is based on a best-of-breed application of standard information technology (IT) network security mechanisms and IP protocols. The trust system provides seamless, automated command and control for the suppression of network attacks and other suspicious events. It also supplies access control, format validation, event analysis, alerting, blocking, and event logging at any network-level and can do so on behalf of any system that does not have the resources to perform these functions itself. Latency calculations are used to estimate limits of applicability within a company and between geographically separated company and area control centers, scalable to hierarchical regional implementations.