Dear all, i saw several works related to Network coding and i am still not sure about  the difference between multicast and multiple unicast network communication paradigm.

As far as i understood in multicast one node sends to its successors the same information (which is of course a combination of its incoming flows).

In multiple unicast there are several sources (s_i) and each source needs to communicate to its relative sink (t_i) and each node can send different combination to its neighbours. Is it correct?

When i have a mixed case where different sources whose information needs to reach a subset of the sinks nodes...can this Still be considered multiple unicast? 

Moreover, is the Optimal Network coding (in sense of troughput) solved for a general alphabet and multiple unicast with some polynomial algorithm?

Thanks 

Gianni