OPC-XML defines a standard method for accessing automation data in the industry field. This protocol relies on web service communications allowing a great compatibility and a software/hardware independent definition. Nevertheless, this underlying protocol does not take into account temporal Quality of Service. Therefore, when a contract is defined between the client and the server, its execution has to be monitored to check if expected temporal properties are respected. In this paper, we propose to enrich the contract definition in order to define mandatory and optional quality of service. We propose an observation mechanism for the network congestion detection. Thanks to this information, it is possible to dynamically adapt the QoS to preserve the mandatory properties.