Re: [Ntp] I-D Action: draft-ietf-ntp-using-nts-for-ntp-27.txt

[Miroslav Lichvar <[e-mail]>]

On Tue, Mar 24, 2020 at 11:02:28AM -0700, [e-mail] wrote:
> A diff from the previous version is available at:
> https://www.ietf.org/rfcdiff?url2=draft-ietf-ntp-using-nts-for-ntp-27

One of the changes in the latest version is a new section on
recommended NTS-KE retry intervals. There was some discussion in the
github issue #153 and there was a suggestion to continue here.

Does anyone else think that the minimum retry interval of 10 seconds
is way too short?

I think it should be at least 1024 seconds (corresponding to the
default NTP maxpoll), with an exception for retrying a TCP connection
when the server doesn't accept the connection, or it's closed before
the TLS handshake to implement a rate limiting. Is there anything else
that would be likely to change on the server for the NTS-KE to succeed
just after 10 seconds?

In my tests NTS-KE seems to be about 200x more expensive than an
NTS-NTP (a single core handling about 500 NTS-KE requests per second
or 100000 NTS-NTP requests per second). That's with the AES-NI
support.

A widely used polling interval of NTP clients is between 64 and 1024
seconds. That means a single NTS client retrying NTS-KE after only 10
seconds wasted the same amount of resources as about 10000 clients
using only NTS-NTP. That's crazy.

Yes, the server can limit the number of threads available to NTS-KE or
limit the connection rate, but that will have a disproportionate
impact on clients using more reasonable retry intervals.

I suggest to modify the second paragraph of the section to the
following:

  Clients SHOULD use an exponential backoff with a base of 2. The
  initial retry interval SHOULD be at least 16 seconds if the previous
  NTS-KE connection failed, or the server closed it before the TLS
  handshake, and 1024 seconds in other cases. The maximum interval
  SHOULD be at least 524288 seconds (~6 days). The minimum interval in
  seconds, t, for the nth retry can be calculated as

		t = min(R * 2^(n-1), 2^19)
		where R is 16 or 1024 depending on the previous error

I suggest powers of 2 to make it compatibille with NTP polling
intervals and avoid floating point operations.

-- 
Miroslav Lichvar