by Roberto Baldoni, Achour Mostefaoui and Michel Raynal
Multimedia real-time collaborative applications or groupware real-time applications requires participants to exchange real-time audio and video informations over a communication network. This flow of informations must preserve the causal dependency even though part of the informations can be lost or can be discarded if it violates the timing constraints imposed by a real-time interaction. In this paper we propose a communication abstraction to cope with unreliable communication networks with real-time delivery constraints: messages have a lifetime, Delta, after which their contents can no longer be used, moreover some of them can be lost. This new abstraction, called Delta- causal order, requires to deliver as much messages as possible within their lifetime in such a way that these deliveries respect causal order. An efficient protocol is proposed in the case of one-to-one communications. Variations of this protocol well suited to particular cases (e.g. broadcast communications) are also shown.