Efficient Causally Ordered Communications for Multimedia Real-time Applications

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.