The new communication environment has brought new requirements on the qualities of service for which the present "best effort" semantics is inadequate. The response to this evolution goes through the definition of a new model of QoS for the lower layers. In the Òbest effortÓ model, when a service provider accepts a transmission with a given QoS, it does not commit itself to any duty about the way it will take account of this QoS. The "guaranteed" QoS requires resources reservation mechanisms which are not always available. We present in this paper a new semantics for the QoS. It allows a service user to express more accurately its requirements, and although it does not include yet a concept of guarantee of the result, the fact is that it assures the users that the provider will monitor the selected parameters and return some defined feedback about the way it succeeds in meeting their requirements. New negotiation rules have also been specified, which are consistent with the new semantics. An example illustrates the practical use of the notions we have introduced which are at the origin of the OSI95 transport service.