  Harald Racke [STOC 2008] described a new method to obtain hierarchical
decompositions of networks in a way that minimizes the congestion. Racke's
approach is based on an equivalence that he discovered between minimizing
congestion and minimizing stretch (in a certain setting). Here we present
Racke's equivalence in an abstract setting that is more general than the one
described in Racke's work, and clarifies the power of Racke's result. In
addition, we present a related (but different) equivalence that was developed
by Yuval Emek [ESA 2009] and is only known to apply to planar graphs.
