As mentioned earlier, in the case where the user link cost functions are no longer symmetric,
one cannot compute the solution to the U-O, that is, to the network equilibrium,
problem using standard optimization algorithms. We emphasize, again, that such general
cost functions are very important from an application standpoint since they allow for asymmetric
interactions on the network. For example, allowing for asymmetric cost functions
permits one to handle the situation when the flow on a particular link affects the cost on
another link in a different way than the cost on the particular link is affected by the flow on
the other link.