Re: [Ntp] NTPv5 Loop Detection without Stratum - Why do we want this?

[Miroslav Lichvar <[e-mail]>]

On Thu, Sep 08, 2022 at 06:18:24PM -0400, Danny Mayer wrote:
> On 9/8/22 2:24 AM, Miroslav Lichvar wrote:
> > An NTP client can select multiple sources at the same time for its
> > synchronization. Look for "combine" in the NTP RFCs. With the "ntpq
> > peers" command you would see sources marked with the * and + symbols.
> Only is chosen (marked with a *). The + ones are possible candidates.

Only one is chosen to be the system peer (*), which determines stratum
and other values of the client, but other sources can pass the
clustering algorithm and be combined with the system peer for
synchronization. They have the + symbol.

Sources that don't pass the clustering algorithm have the - symbol.
They are not used for synchronization.

> > > > In orphan mode stratum is interpreted differently. Symmetric mode or
> > > > client/server mode makes no difference.
> > > There is no symmetric mode for client/server. A client is by definition one
> > > stratum value larger than the server. Peers can have the same stratum.
> > This is about the onwire protocol, not about source selection or
> > synchronization. A client polling a server can have a smaller, equal,
> > or higher stratum than the server.
> That's irrelevant. You are polling a server for time, what you do with it
> depends on the analysis done on the packets.

Right. So can you explain where measurements made in symmetric mode
are handled differently than in client/server mode? A pointer to the
RFC or code is sufficient.

> > Two client/server associations in opposite directions provide the same
> > functionality as a symmetric association, except the number of
> > exchanged messages is doubled, it's much easier to implement and
> > understand, and there are no replay attacks.
> That's a naive assumption. You can send any kind of replay no matter what
> the mode of the packet. Why do you think that you are sending twice as many
> packets in a symmetric association?

In symmetric mode each packet is a request and response at the same
time, i.e. the number of messages is halved when compared to two
client/server associations.

As the responses follow the polling interval and are not send
immediately when the request is received, this makes the symmetric
mode susceptible to replay attacks. In client/server mode each request
has its own response, independently from the polling of the other
client/server association.

> > > > If a local stratum-1 server had a poor reference clock (e.g. a GPS
> > > > receiver without PPS), it would be better to use a distant server with
> > > > higher stratum, i.e. stratum doesn't say much about accuracy.
> > > Then it wouldn't be a stratum-1 server any more.
> > What it would be?
> 
> I can only assume you have not read Section 3 of RFC5905 which tells you the
> answer.

Which part exactly?

In Section 3 I see: "Primary servers are assigned stratum one"

That is, a server that uses a poor reference clock is still stratum
one. A client which polls multiple servers would likely prefer a
different server, possibly with a higher stratum.

-- 
Miroslav Lichvar