In recent years, several RDF Stream Processing (RSP) systems have emerged, which allow querying RDF streams using extensions of SPARQL that include operators to take into account the velocity of this data. These systems are heterogeneous in terms of syntax, capabilities and evaluation semantics. Recently, the W3C RSP Group started to work on a common model for representing and querying RDF streams. The emergence of such a model and its accompanying query language is expected to take the most representative, significant and important features of previous efforts, but will also require a careful design and definition of its semantics. In this work, we present a proposal for the query semantics of the W3C RSP query language, and we discuss how it can capture the semantics of existing engines (CQELS, C-SPARQL, SPARQLstream), explaining and motivating their differences. Then, we use RSP-QL to analyze the current version of the W3C RSP Query Language proposal.