This Lori MacVitte article explains the differences between SDN flows and messages. Before it gets to that, it looks at the reasons why networks are naturally stratified. It has to do with the fact that flows are not messages.
What's been missing, however, is a discussion on why SDN needs to address higher order services differently in the first place. The reason is fairly simple: the network is naturally stratified.
-- Lori MacVittie
There's still some difficulty in SDN related to bringing message and packet-based services.