[tor-dev] Revisiting prop224 overlap descriptor logic and descriptor lifetimes

[George Kadianakis]

Hello people,

I invite you to check out another round of time period-related prop224 spec
changes, based on our discussions in Montreal. These new changes simplify the
overlap descriptor publishing logic, and improve the caching lifetime of
descriptors in HSDirs.

You can find them in my branch `prop224-montreal-timeperiods` or here:
    https://gitweb.torproject.org/user/asn/torspec.git/log/?h=prop224-montreal-timeperiods

The main issue for me right now is that I can't recall how this helps with
clock skewed clients, even though that was a big part of our discussion in Montreal.

Specifically, I think that clients (and HSes) should determine the set of
responsible HSDirs (i.e. the current time period) based on the "valid-after" of
their latest consensus, instead of using their local clock. This way, as long
as the client's skewed clock is good enough to verify the latest consensus, the
client will have a consistent view of the network and SRV (assuming an
honest/updated dirguard). I tried to clarify this a bit in commit 465156d, so
please let me know if it's not a good idea.

Am I missing something wrt clock skewed clients here? If yes, can someone
demonstrate the effects of these changes with an example, so that I can clarify
the proposal further?

Feedback is welcome!
If I receive positive feedback, I will merge this in torspec.git ASAP.

Thanks!