On optimality of routing for multi-source multicast communication scenarios with node uplink constraints

We consider multi-source multicast communication scenarios in which each node has an aggregate outbound traffic capacity and can directly communicate with any other node. This is motivated by peer-to-peer (P2P) information dissemination applications on the Internet in which the uplink capacity of nodes is usually the bottleneck, being several times smaller than the downlink capacity. We also allow the communication in a group to be helped by non-receiver nodes (with respect to that group) as relays. Extending an earlier result for the single source case, we show that when coding is not allowed across sources, routing is optimal. Also, as a rather surprising discovery, we show that when all groups have pairwise identical or disjoint receivers, routing is optimal even when coding across sources is allowed. Moreover, routing along a linear number of trees per source is sufficient to achieve this. The latter scenario is common in multiparty conferencing systems, hence our results have interesting practical applications in the design of infrastructure-less P2P multiparty conferencing systems.