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UML models consist of several diagrams of different types describing different views of a software system ranging from specifications of the static system structure to descriptions of system snapshots and dynamic behaviour. In this paper a heterogeneous approach to the semantics of UML is proposed where each diagram type can be described in its "natural" semantics, and the relations between diagram types are expressed by appropriate translations. More formally, the UML family of diagram types is represented as a "heterogeneous institution environment": each diagram type is described as an appropriate institution where typically the data structures occurring in a diagram are represented by signature elements whereas the relationships between data and the dynamic behaviour of objects are captured by sentences; in several cases, the diagrams are themselves the sentences. The relationship between two diagram types is described by a socalled institution comorphism, and in case no institution comorphism exists, by a co-span of such comorphisms. Consistency conditions between different diagrams are derived from the comorphism translations. This heterogeneous semantic approach to UML is illustrated by several example diagram types including class diagrams, OCL, and interaction diagrams.

Abstract. Availability is an important security property for Internet services and a key ingredient of most service level agreements. It can be compromised by distributed Denial of Service (DoS) attacks. In this work we propose a formal pattern-based approach to study defense mechanisms against DoS attacks. We enhance pattern descriptions with formal models that allow the designer to give guarantees on the behavior of the proposed solution. The underlying executable specification formalism we use is the rewriting logic language Maude and its real-time and probabilistic extensions. We introduce the notion of stable availability, which means that with very high probability service quality remains very close to a threshold, regardless of how bad the DoS attack can get. Then we present two formal patterns which can serve as defenses against DoS attacks: the Adaptive Selective Verification (ASV) pattern, which enhances a communication protocol with a defense mechanism, and the Server Replicator (SR) pattern, which provisions additional resources on demand. However, ASV achieves availability without stability, and SR cannot achieve stable availability at a reasonable cost. As a main result we show, by statistical model checking with the PVeStA tool, that the composition of both patterns yields a new improved pattern which guarantees stable availability at a reasonable cost.

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