The construction of reliable concurrent and distributed systems is an extremely difficult endeavour. For complex systems, it requires modular development strategies based on precise interface specifications that allow the various modules to interact properly. In this extended abstract we are concerned with message passing systems where partners engage in long and complex interactions, as opposed to, say, remote procedure calls composed of a pair of simple interactions. Session types allow for the description of continuous series of interactions between several partners. In the simpler case, they detail protocols between two partners [Honda et al. 1998]; recently the original setting was widened to encompass multiple partners [Honda et al. 2008]. In this paper we deal with binary sessions only. Through a running example we visit session types and a functional concurrent language equipped with buffered semantics. Apart from the traditional "well typed programs do not go wrong", the semantics proposed allows for two extra interesting results: the ability to predict the required buffer size, and that of anticipating an output with respect to an input operation.
