This attribute specifies when bumping occurs in the route selection
process. A connection with higher (numerically less) setupPriority
may bump a connection with lower (numerically higher)
setupPriority in order to acquire its bandwidth and be established. If
bumpPreference is bumpWhenNecessary, bumping occurs only
when it is necessary. That is, a connection will bump another
connection only if there is no other way for the connection to be
established. If bumpPreference is bumpToObtainBestRoute,
bumping occurs in order to use the best available route it may have.