Thomas L. Rodeheffer

Autonet: a high-speed, self-configuring local area network using point-to-point links

A sizing gage for use with an LVDT probe for producing an output signal representing the deviation of part dimension from a nominally ideal dimension, and a columnar display means comprising vertically arranged light emitting diodes for indicating the degree and sense; i.e., oversize or undersize, of part size deviation according to which of the emitting diodes is lighted. The probe output is an ac signal of polarity representing the degree of deviation. The probe output signal is converted to a dc voltage the amplitude and polarity of which is representative of part size deviation. The dc signal is used as a comparison base against a precision triangle wave signal to generate a squarewave the transitions of which occur at points in time related to the amplitude and polarity of the dc signal. This squarewave is compared to a reference squarewave of fixed transition time to produce a window pulse the width of which is representative of the degree of part size deviation. The window pulse gates clock pulses from a precision oscillator to a pair of decade counters to address a diode excitation matrix. The tens signal is gated to either the oversize light bank or the undersize light bank according to whether the time variable signal leads or lags the fixed time signal. Nulling and range setting circuit details are disclosed.

Automatic reconfiguration in Autonet

Autonet is a switch-based local area network using 100 Mbit/s full-duplex point-to-point links. Crossbar switches are interconnected to other switches and to host controllers in an arbitrary pattern. Switch hardware uses the destination address in each packet to determine the proper outgoing link for the next step in the path from source to destination. Autonet automatically recalculates these forwarding paths in response to failures and additions of network components. This automatic reconfiguration allows the network to continue normal operation without need of human intervention. Reconfiguration occurs quickly enough that higher-level protocols are not disrupted. This paper describes the fault monitoring and topology acquisition mechanisms that are central to automatic reconfiguration in Autonet.

Policy-based access control for weakly consistent replication

Combining access control with weakly consistent replication presents a challenge if the resulting system is to support eventual consistency. If authorization policy can be temporarily inconsistent, any given operation may be permitted at one node and yet denied at another. This is especially troublesome when the operation in question involves a change in policy. Without a careful design, permanently divergent state can result. We describe and evaluate the design and implementation of an access control system for weakly consistent replication where peers are not uniformly trusted. Our system allows for the specification of fine-grained access control policy over a collection of replicated items. Policies are expressed using a logical assertion framework and access control decisions are logical proofs. Policy can grow to encompass new nodes through fine-grain delegation of authority. Eventual consistency of the replicated data is preserved despite the fact that access control policy can be temporarily inconsistent.

Effective and efficient compromise recovery for weakly consistent replication

Weakly consistent replication of data has become increasingly important both for loosely-coupled collections of personal devices and for large-scale infrastructure services. Unfortunately, automatic replication mechanisms are agnostic about the quality of the data they replicate. Inappropriate updates, whether malicious or simply the result of misuse, propagate automatically and quickly. The consequences may not be noticed until days later, when the corrupted data has been fully replicated, thereby deleting or overwriting all traces of the valid data. In this sort of situation, it can be hard or impossible to restore an entire distributed system to a clean state without losing data and disrupting users. Polygraph is a software layer that extends the functionality of weakly consistent replication systems to support compromise recovery. Its goal is to undo the direct and indirect effects of updates due to a source known after the fact to have been compromised. In restoring a clean replicated state, Polygraph expunges all data due to a compromise or derived from such data, retains as much uncompromised data as possible, and revives valid versions of subsequently compromised data. Our evaluation demonstrates that Polygraph is both effective, retaining uncompromised data, and efficient, re-replicating data only when necessary.

Fidelity-aware replication for mobile devices

Mobile devices often store data in reduced resolutions or custom formats in order to accommodate resource constraints and tailor-made software. The Polyjuz framework enables sharing and synchronization of data across a collection of personal devices that use formats of different fidelity. Layered transparently between the application and an off-the-shelf replication platform, Polyjuz bridges the isolated worlds of different data formats. With Polyjuz, data items created or updated on high-fidelity devices-such as laptops and desktops-are automatically replicated onto low-fidelity, mobile devices. Similarly, data items updated on low-fidelity devices are reintegrated with their high-fidelity counterparts when possible. Polyjuz performs these fidelity reductions and reintegrations as devices exchange data in a peer-to-peer manner, ultimately extending the eventual-consistency guarantee of the underlying replication platform to the multifidelity universe. In this paper, we present the design and implementation of Polyjuz and demonstrate its benefits for fidelity-aware contacts management and picture sharing applications.

Fidelity-Aware Replication for Mobile Devices

Mobile devices often store data in reduced resolutions or custom formats in order to accommodate resource constraints and tailor-made software. The Polyjuz framework enables sharing and synchronization of data across a collection of personal devices that use formats of different fidelity. Layered transparently between the application and an off-the-shelf replication platform, Polyjuz bridges the isolated worlds of different data formats. With Polyjuz, data items created or updated on high-fidelity devices-such as laptops and desktops-are automatically replicated onto low-fidelity, mobile devices. Similarly, data items updated on low-fidelity devices are reintegrated with their high-fidelity counterparts when possible. Polyjuz performs these fidelity reductions and reintegrations as devices exchange data in a peer-to-peer manner, ultimately extending the eventual-consistency guarantee of the underlying replication platform to the multifidelity universe. In this paper, we present the design and implementation of Polyjuz and demonstrate its benefits for fidelity-aware contacts management and picture sharing applications.

Formal Analysis of a Distributed Algorithm for Tracking Progress

Tracking the progress of computations can be both important and delicate in distributed systems. In a recent distributed algorithm for this purpose, each processor maintains a delayed view of the pending work, which is represented in terms of points in virtual time. This paper presents a formal specification of that algorithm in the temporal logic TLA, and describes a mechanically verified correctness proof of its main properties.

Fast encounter-based synchronization for mobile devices

The large and growing number of computing devices used by individuals has caused the challenges of distributed storage to take on increased importance. In addition to desktops and laptops, portable devices, such as cell phones, digital cameras, iPods, and PDAs are capable of storing and sharing data. These devices’ mobility coupled with wireless networking capabilities allows them to opportunistically propagate data to other devices they might encounter, even if connectivity is unplanned and transient. To take advantage of such ad hoc connectivity, devices must have an efficient method for determining which files they hold in common and which versions must be propagated to achieve consistency. This paper presents new techniques for reducing the cost of this data synchronization operation by over an order of magnitude in many situations.