Re: [Ntp] Pool and DNS

[Marcus Dansarie <[e-mail]>]

On 2022-10-19 06:42, Hal Murray wrote:
> 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.

I think you're basing this on a couple of false assumptions. First, I 
don't know where the cookie lifetime of one day comes from. That's not 
specified in RFC 8915. In most cases, we would expect the cookie to be 
valid as long as the cookie encryption parameters shared between NTS-KE 
server and NTP server are not changed. This could potentially be a very 
long time.

A client that uses Khronos could just keep a cookie cache for all 
servers in its pool and not require any larger poll time using 
NTS-enabled NTP servers than for non-NTS NTP servers. The only time 
NTS-KE handshakes should normally be required is the first time the 
client gets time from a particular 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.

We actually thought about pool applications when writing RFC 8915. The 
way to implement it is not very complicated: The pool administrator runs 
one or more NTS-KE servers. The NTS-KE servers have a shared key with 
each of the pool members and indicates which server the cookies are 
valid for using a NTPv4 Server Negotiation NTS-KE record. Clients can 
also request cookies for a particular pool member using the same record. 
(This may also solve the server removal problem, since clients would be 
expected to stop polling a particular server when they run out of 
cookies and the NTS-KE server refuses to assign new ones for that 
particular server.)

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

I think that combining NTS with Khronos could potentially create a 
really robust system, since Khronos detects attacks that NTS cannot 
protects against.

Kind regards,
Marcus