In previous releases of OS X, there was a command called ipfw, that permitted throttling of network bandwidth, which is useful for running some kinds of tests on software. It appears that as of Yosemite that command is gone. So what is the replacement and how can a user to throttle bandwidth?
the replacement is
man pfctl. also, see
man dnctl for traffic shaping purposes.
Apple's Network Link Conditioner utility (only available in their developer downloads section, behind a login) uses these two facilities to do its thing.