Problems with real networks:
	They do not stay still, routes change, links are upgraded, pings are blocked, hosts go down, hosts have configurations changed, routers are misconfigured and/or crash
	Must avoid impacting production traffic, need agreement of remote administrators and/or network folks
	Many of the stacks are alpha or beta releases and are in the kernel: kernel crashes, developer wants to add new feature, measurer wants stability or throw out previous measurements. Some stacks were not available in time (Westwood+, GridDT)