Wireless Protocol Validation Under Uncertainty

Runtime validation of wireless protocol implementations cannot always employ direct instrumentation of the tested device. It may not implement the required instrumentation, or the instrumentation may alter the its behavior. Wireless sniffers can monitor wireless traffic without instrumentation but introduce both new validation challenges and opportunities. Losses caused by wireless propagation mean that sniffers cannot perfectly reconstruct the actual packet trace. As a result, accurate validation requires distinguishing between specification deviations that represent errors from those caused by sniffer uncertainty. At the same time, the sniffer provides new opportunities to manipulate the wireless traffic received by the device and perform more rigorous testing. In collaboration with researchers at Microsoft Research we are exploring ways to exploit sniffers to validate wireless protocols.

We have begun by addressing the challenge of validation given the uncertainty and loss caused by the sniffer’s incomplete view of the packet trace received by the device. Sniffers introduce two kinds of errors. They may miss packets that the device received, or receive packets that the device missed. While cooperation between multiple sniffers can reduce overall error rates, even many sniffers working together cannot reproduce the device’s packet trace with perfect certainty.

Beginning with a state machine provided by the protocol designer that represents correct protocol behavior, we address sniffer uncertainty by producing an augmented state machine. The augmented state machine adds transitions reflecting sniffer uncertainty. Producing the augmented state machine is an automated process, provided the original state machine in a suitable format.

Validating a wireless trace then reduces to determining whether it is accepted by the augmented state machine. Because the sniffer introduces probabilistic transitions, the search space will expand exponentially and the search problem can be shown to be NP-complete. However, in practice simple heuristics can limit the number of states that need to be considered and keep the search tractable.

Our approach has proved successful in detecting both artificial errors introduced in existing protocol implementations and previously unknown errors in new protocol implementations. As a next step we are working to use the sniffer to manipulate the live packet trace to help stress the tested device by introducing loss and packet errors.

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Created 7/10/2016
Updated 8/1/2016
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Built 8/17/2017 @ 19:29 EDT