This paper discusses issues relating to the provision of end-to-end quality of service for applications which traverse heterogeneous networks. These issues relate to the user-centric versus the network-centric approach to QoS specification. Firstly, we examine the orthogonal nature of the current method of QoS specification (i.e. no functional relationship between the parameters), and the failure of such an approach to meet the applications' requirements. Secondly, we examine the mismatch between the discrete offerings of LAN/MAN protocols and the continuum of application requirements. We propose a method of specifying QoS requirements which captures the functional dependencies between QoS parameters. The issues discussed in this paper constitute a framework for our research on building an abstract model which provides cost-effective QoS to applications within a heterogeneous network environment.