Communication is an essential aspect of services. Services do not only realize simple request-response scenarios, but increasingly implement complex and stateful communication protocols. Such a protocol specifies the order in which messages are sent and received by a service and is an essential part of a service description. Services are usually not executed in isolation, but as a collaboration which is composed of several services. The behavior of such a collaboration is not only determined by the communication protocol of each participating service, but also by the way messages are exchanged. A communication model specifies the properties of the message channels between the services and defines the way how messages are sent and received. ...