  In this paper, we consider networks with multiple unicast sessions.
Generally, non-linear network coding is needed to achieve the whole rate region
of network coding. Yet, there exist networks for which routing is sufficient to
achieve the whole rate region, and we refer to them as routing-optimal
networks. We identify a class of routing-optimal networks, which we refer to as
information-distributive networks, defined by three topological features. Due
to these features, for each rate vector achieved by network coding, there is
always a routing scheme such that it achieves the same rate vector, and the
traffic transmitted through the network is exactly the information transmitted
over the cut-sets between the sources and the sinks in the corresponding
network coding scheme. We present more examples of information-distributive
networks, including some examples from index coding and single unicast with
hard deadline constraint.
