A Semantic Approach to Protocold using Process Algebra

We study of the operational behaviour of the speciication language Protocold. We examine whether BPA(+) (Basic Process Algebra with backtracking) forms a useful basis for a clear representation of the execution of certain language constructs from Protocold. Our primary focus is on those constructs that contain some form of backtracking. Backtracking in Protocold contains the undoing of bindings of object names (logical variables). We rst provide a short introduction to both formalisms, and then make a rough comparison of them both. Some elementary Protocold features, such as object names and the binding of object names, are then deened relative to a special data environment for BPA(+). Finally, a small language PCL is deened for the description of the operational behaviour of Protocold-like expressions, as well as a mapping from (PCL) expressions to (BPA(+)) terms. Some operational equivalences that hold in Protocold are deened as rules in PCL, such that algebraic manipulation of (PCL) expressions is possible. Normal forms over (PCL) are deened, and it turns out that a mapping of these normal forms to (BPA(+)) terms preserves some typical properties of backtracking in PCL. A canonical mapping does not exist, however, because backtracking in a Protocold (and (PCL)) expression is`global': it is not limited to the scope of certain operators. We think that, although the language Protocold is too rich to be fully expressed in terms of BPA(+) (particularly where the speciication of abstract data types is at stake), the operational behaviour of Protocold speciications can be speciied adequately in BPA(+).