A **flow network** G = (V, E) is a directed graph where each edge (u,v) E has capacity c(u,v) 0.

If (u,v) E, then c(u,v) = 0.

Distinguish source (s V) and sink (t V) as the vertices between which the flow is to be maximized.

- in-degree(s) = 0
- out-degree(t) = 0

For every v V, v is on a path from s to t. , therefore the graph is connected.