In SIP-based networks, there is a need to provide basic network media services. Such services include network announcements, user interaction, and conferencing services. These services are basic building blocks, from which one can construct interesting applications. In order to have interoperability between servers offering these building blocks (also known as Media Servers) and application developers, one needs to be able to locate and invoke such services in a well-defined manner. This document describes a mechanism for providing an interoperable interface between Application Servers, which provide application services to SIP-based networks, and Media Servers, which provide the basic media processing building blocks.