Process Algebra for Event-Driven Runtime Verification: A Case Study of Wireless Network Management

Runtime verification is analysis based on information extracted from a running system. Traditionally this involves reasoning about system states, for example using trace predicates. We have been investigating runtime verification for event-driven systems and in that context we propose a higher level of abstraction can be useful, namely reasoning at the level of user-perceived system events. And when considering events, then the natural formalism for verification is a form of

process algebra

.

We employ a universal process algebra that encapsulates both dynamic and spatial behaviour, based on Robin Milner’s

bigraphs

[1]. Our models are an extension of his bigraphical reactive systems. These consist of a set of bigraphs that describe spatial and communication relationships, and a set of bigraphical reaction rules that define how bigraphs can evolve over time. We have extended the basic formalism to bigraphical reactive systems

with sharing

[2], to allow for spatial locations that can overlap.

In this talk we present a case study involving wireless home network management and the automatic generation of bigraphical models, and their analysis, in real-time. Wireless home networking is chosen as our case study because it is notoriously difficult to install and manage, especially for non-expert users. The Homework network management system [4] has been designed to provide user-oriented support in home wireless local area network (WLAN) environments. The Homework user interface includes drag and drop, comic-strip style interaction for users, and the information plane uses a stream database to record (raw and derived) events. Events include network behaviours such as detecting that a new machine has joined the network, resulting in new links and granting a DCHP lease, and user-intiated behaviours such as enforcing or dropping a policy. Policies forbid or allow access control; for example, a policy might block UDP and TCP traffic from a given site. All network and policy events (simple and derived) are recorded as a stream of tuples in the stream database. This part of the management system is illustrated in the left hand side of Figure 1.