  Stochastic (Markovian) process algebra extend classical process algebra with
probabilistic exponentially distributed time durations denoted by rates (the
parameter of the exponential distribution). Defining a semantics for such an
algebra, so to derive Continuous Time Markov Chains from system specifications,
requires dealing with transitions labeled by rates. With respect to standard
process algebra semantics this poses a problem: we have to take into account
the multiplicity of several identical transitions (with the same rate). Several
techniques addressing this problem have been introduced in the literature, but
they can only be used for semantic definitions that do not exploit a structural
congruence relation on terms while inferring transitions. On the other hand,
using a structural congruence relation is a useful mechanism that is commonly
adopted, for instance, in order to define semantics in reduction style for
non-basic process algebras such as the pi-calculus or richer ones. In this
paper we show how to define semantics for Markovian process algebra when
structural congruence is used while inferring transitions and, as an
application example, we define the reduction semantics for a stochastic version
of the pi-calculus. Moreover we show such semantics to be correct with respect
to the standard one (defined in labeled operational semantics style).
