[Ntp] Pool and DNS

[Hal Murray <[e-mail]>]

> Around 250 DNS queries are required by Khronos to obtain a pool of 1000 NTP
> servers, which is equal to the number of queries performed by NTPv4 in a
> duration of 10 days (assuming a query per hour).

Where did that "a query per hour" come from?

It's not wildly wrong and I don't have a better number.  But I worry that 
somebody will assume it is a solid data point and use it in some future line 
of reasoning.

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My reading is that pool traffic is bimodal.  ntpd makes a few DNS requests on 
startup then uses those servers for a long time.  sntp makes a DNS request 
every time it is run.

You can see that by watching the traffic on a server in the pool when you 
disable it by setting the traffic rate to monitor-only.  The traffic drops 
abruptly by X%, then slowly decays.  "Slowly" is ballpark of 10% per day.  X 
is ballpark of 60%.  (I didn't get decent data because I only figured out what 
was going on after I had already messed things up.  Next time...)

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The main problem with DNS and the pool is that there is currently no clean way to remove a server from the pool.  If you remove it from the pool, the sntp traffic will vanish since it will get removed from future DNS answers.  But the ntpd traffic will continue to use existing servers as long as ntpd keeps running and the servers keep responding.

There is no way to ask if a server is still in the pool.  If there was, ntpd should check every day or week or ...  That would add more DNS traffic from the long running ntpd servers.  Khronos would add much more.

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The more interesting question is NTS.  The NTS-KE step is fairly heavyweight, much more work than a DNS lookup.

The current parameters are 8 cookies with a lifetime of a day.  A day is 86400 seconds.  That means we need to poll each server every 10800 seconds.  With 1000 servers and M=15 (from the draft) that will take a Khronos poll every (10800/1000)*15 => 162 seconds.  If Khronos runs at 10x the poll time of ntpd, that's not going to work.

We could gain a factor of 2 or 3 by dropping cookies older than a day.  (and asking for several more to fill up again when we do poll a server)

Of course, NTS doesn't work with the pool and I don't see how to fix that, so this is all a wild goose chase.


Maybe we should be comparing Khronos with 1 or 2 trusted servers using NTS.


-- 
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