The flow abstraction is comparable to the communication substrate
of FARA [3]. However, important and deliberate differences
exist. Flows are constrained to single network technology domains,
whereas the communication substrate provides end-to-end
connectivity across technology domains. The specific definition of
a flow allows the internetworking functions to control the mapping
at technology boundaries. The flow abstraction is not an interface
– it is an abstraction of the connectivity provided by the
underlying technology. In contrast, the communication substrate is
an end-to-end communication primitive in its own right.