[Ntp] Follow-up to yesterday's mic comment about PTP security

[Daniel Franke <[e-mail]>]

My comments yesterday about PTP security shifted context a few times
so it may have been hard to follow what I was claiming. My assertions
were:

1. If you need 50ms precision, pick some good public NTP servers and use NTS.

2. If you need 100µs precision, colocate a time source in the same
datacenter as the client systems. Use NTP and NTS; you don't need PTP
for this.

3. If you need 1µs precision, use PTP and physically secure the link
between the time source and the clients so that cryptographic
authentication is unnecessary.

4. If you need 1µs precision over an adversarial network, good luck!
This is simply not achievable and no amount of cryptographic pixie
dust is ever going to save you.