This work extends asynchronous multiparty session types
(MPST) with
explicit connection actions
to support protocols with op-
tional and dynamic participants. The actions by which endpoints are
connected and disconnected are a key element of real-world protocols
that is not treated in existing MPST works. In addition, the use cases
motivating explicit connections often require a more relaxed form of mul-
tiparty choice: these extensions do not satisfy the conservative restric-
tions used to ensure safety in standard syntactic MPST. Instead, we de-
velop a modelling-based approach to validate MPST safety and progress
for these enriched protocols. We present a toolchain implementation, for
distributed programming based on our extended MPST in Java, and a
core formalism, demonstrating the soundness of our approach. We discuss
key implementation issues related to the proposed extensions: a practi-
cal treatment of choice subtyping for MPST progress, and multiparty
correlation of dynamic binary connections.