The authors are interested in optimizing the network cost ofmulticast routing. To that end, they analyze the problem of constructingSteiner trees in graphs where each edge is assigned a cost and a timedelay. The trees should satisfy certain time-delay constraints, and theauthors consider three: bounded longest delay, bounded delay variation(any two recipients should receive a message within a specified amountof time), and minimum average delay. The three problems are solved withdynamic programming algorithms that run in pseudo-polynomial time andare NP-hard. The authors also investigate the spanning forest withbandwidth constraint problem and give a polynomial-time algorithm forit. This latter problem is important in the context of group multicastrouting.