Timestamps

The initial timestamp is to be chosen randomly (just as the initial sequence number is selected randomly):

to avoid replays
to increase security (this assumes that the intruder does not have access to all the packets flowing to the destination)

The timestamp granularity (i.e., the units) are determined by the payload type {often based on the sampling rate}

Transcript

[slide95] The timestamps, we said, were there, but initially we should, of course, choose them randomly, just like the initial sequence numbers should be chosen randomly, both to avoid replays and to increase security. And the granularity is determined by the payload type, as we said, based on the sample rate.