These being open internet time services though, and especially in the case of time.windows.com, we are talking about millions of clients. You can build a cluste

These being open internet time services though, and especially in the case of time.windows.com, we are talking about millions of clients. You can build a cluster to handle this, as Microsoft has, but it costs money. The issue is especially clear since, at least historically (I'm not sure about now), the popular time.nist.gov was not a cluster but a single machine. It was nearly impossible to carry out a successful NTP session with that machine due to its load, an especially large problem because some well-meaning vendors configured time.nist.gov as a default on embedded devices. The NIST requests that it not be used except as a source for stratum-2 NTP servers but few people would do that today considering the reliability problems.