We are interested in solving time dependent problems using domain decomposition methods. In the classical approach, one discretizes first the time dimension and then one solves a sequence of steady problems by a domain decomposition method. In this paper, we treat directly the time dependent problem and we study a Schwarz waveform relaxation algorithm for the convection diffusion equation in two dimensions. We introduce the operators on the interfaces which minimize the convergence rate, resulting in an efficient method: numerical results illustrate the performances and show that the corresponding algorithms converge much faster than the classical one.