1. A system for converting an incoming message of a first message type into an outgoing message of a second message type comprising: a repository for containing a plurality of Protocol Conversion and Coordination Language (PCCL) specifications comprising: a specification of peers; a set of session correlation rules; a set of matching rules; and an action specification; a PCCL application compiled from a PCCL specification selected from the plurality of PCCL specifications; a message receiver processor, which uses the set of specification of peers in a first domain, to identify one or more peers communicating via an incoming message of a first type; a session correlator, which applies the set of session correlation rules to the identified peers and information extracted from content of the incoming message to map the incoming message to a PCCL session among a plurality of PCCL sessions if an appropriate PCCL session is already active or to a newly created PCCL session if an appropriate PCCL session does not exist; a condition matcher, which uses the set of matching rules and session data of the PCCL session to which the incoming message is mapped, to determine a set of actions to be performed to convert the incoming message into an outgoing message of a second type; an action executor which uses the action specification to execute the set of actions; and a message sender processor which sends said outgoing message to one or more peers in a second domain.