Consistency control for distributed interactive media

Abstract

In this paper we present a generic consistency control service for distributed interactive media, i.e. media which allow a distributed group of users to interact with the medium itself. Consistency control is vital to these media since they typically require that a local copy of the medium's state be maintained by each user's application. Our service helps the applications to keep the local state copies consistent. The main characteristics of this service are as follows: a significant number of inconsistencies are prevented by using a mechanism called local lag. Inconsistencies that cannot be prevented are repaired by an improved timewarp algorithm that can be executed locally without burdening the network or the applications of other users. Exceptional situations and consistency during late-join situations are supported by a consistent state request mechanism. Moreover, the service also supports the application in detecting intention conflicts between the actions of distinct users. The major part of this functionality is based on a media model and the application level protocol for distributed interactive media (RTP/I) and can thus be reused by arbitrary RTP/I-based applications. In order to demonstrate the feasibility of our approach and to evaluate its performance we have integrated the generic consistency service into a shared whiteboard system.