Re: [ntpwg] NTS: DTLS and symmetric mode

[Miroslav Lichvar <[e-mail]>]

On Mon, Jul 31, 2017 at 02:59:30PM -0400, Daniel Franke wrote:
> On 7/31/17, Harlan Stenn <[e-mail]> wrote:
> > Is DTLS well-suited for symmetric associations, which require mutual
> > authentication?
> 
> Yes. DTLS supports mutual authentication through the use of client
> certificates or pre-shared keys.

That will work nicely for an active-passive association, but I'm still
not sure about active-active associations. You said the source port of
a DTLS client is supposed to be random. How will an active peer know
that an incoming connection corresponds to a peer it has already
connected to or it's trying to connect to? With symmetric keys/autokey
it was possible to have multiple associations with the same IP address
(e.g. multiple machines behind NAT).

I think a bigger problem with NTP over DTLS might be that timing
messages are sent on a different port than 123 and are encrypted. This
makes it incompatible with existing HW/configuration and future NTP
extensions.

Here is the list of issues I posted previously (with no response) [1]:

- no stateless passive mode
- problematic pairing of DTLS sessions
- requires timestamping of messages on a new port
  - won't work with HW which can timestamp only packets on port 123
    (or 319)
  - will require changes in QoS classification on routers/switches
- won't work with future NTP extensions for delay corrections in
  routers/switches

[1] http://lists.ntp.org/pipermail/ntpwg/2017-June/003307.html

-- 
Miroslav Lichvar
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