  Multiparty sessions are systems of concurrent processes, which allow several
participants to communicate by sending and receiving messages. Their overall
behaviour can be described by means of global types. Typable multiparty session
enjoy lock-freedom. We look at multiparty sessions as open systems by a
suitable definition of connection transforming compatible processes into
gateways (forwarders). A relation resembling the standard subtyping relation
for session types is used to formalise compatibility. We show that the session
obtained by connection can be typed by manipulating the global types of the
starting sessions. This allows us to prove that lock-freedom is preserved by
connection.
