Overall, this draft looks very good.  It uses 'should' non-normatively more often than I'd like.  In some cases, I wonder if they should be normative SHOULDs.

Section 1.2(Scope)

Timestamps are also used in RTP(RFC3550).  Based on your document, I think these are out of scope, because they're 'application' timestamps, not network timestamps?  I was unclear on this until I read the draft, so if they are out of scope, it would have helped me if you said that application timestamps, such as RTP are out of scope.

Section 3, under "Epoch:"

Since on power up is an example of a relative timestamp and not the only possible relative timestamp, I'd suggest ", in which the epoch could be the time at which..."  The current reading implies that all relative timestamps are due to the start being the power up time, and I don't think that's true.

Section 4

I'd change "Specifically, if the network..." to "For example, if the network..."

Section 9, Security Considerations

The sentence beginning with "Timestamps can be spoofed or modified..." implies that all network timestamps have this property, but you rightly point out later in the paragraph that one can add a MAC to prevent modification.  I'd suggest something like "In some cases, timestamps can be..." to avoid the implication that all of them can be spoofed or modified.



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