On the introduction of quality of service awareness in legacy distributed applications

A number of distributed applications require communication services with Quality of Service (QoS) guarantees. Work undertaken within the Internet Engineering Task Force (IETF) has led to the definition of novel architectural models for the Internet with QoS support. According to these models, the network has to be appropriately configured in order to provide applications with the needed performance guarantees. In a first proposal, called Integrated Services, applications need to explicitly interact with network routers by means of a signaling protocol (such as RSVP), in order to enforce QoS on a per-flow basis. The Differentiated Services architecture, on the other hand, looks after scalability, thus providing performance guarantees to aggregates of flows. In the case of real-time applications, a hybrid model capable of putting together micro-flow guarantees in the access network and aggregate management in the backbone seems to represent the ideal tradeoff between strict performance and scalability. In this scenario, giving applications a means to interact with the underlying QoS services is of primary importance. Hence, several special-purpose APIs have been defined to let applications negotiate QoS parameters across QoS-capable networks. However, so far, none of these APIs is available for the use of programmers in different operating environments. We believe that such features should be embedded in programming environments for distributed applications. In this work we present how we included QoS control features in a programming language that since years has been adopted for the development of network-based applications: Tcl. We present QTcl, an extension of Tcl, which provides program- mers with a new set of primitives fully compliant with the standard SCRAPI programming interface for the RSVP protocol. We gave QTcl a high portability, in that it enables standard QoS negotiation to be performed in a seamless fashion on the most common operating systems.