Subject: [ntpwg] Questions on draft-ietf-ntp-using-nts-for-ntp-01

From: Miroslav Lichvar <[email-masked]>
Date: Fri, 28 Aug 2015 13:58:05 +0200

I have some questions and comments on draft-ietf-ntp-using-nts-for-ntp-01.

I think Florian has commented on this issue in the previous version of
the draft, but I'm not sure if/how it was resolved. How does the
client verify the identity of the server after receiving the
server_assoc message? For persistent associations it would probably be
a check of the hostname from the certificate, but what about ephemeral
associations? Would it be acceptable for the client to be configured
with a public key that the server certificate has to be signed by?

There doesn't seem to be anything specific to symmetric associations.
I guess the simplest approach is to treat them in NTS as two
independent client/server associations, created separately and then
each packet having both time_request and time_response messages.
However, a special care must be taken when creating the association.
To succeed even when an attacker is sending spoofed packets to the
peers, I think there has to be a server_assoc/cook reply for each
client_assoc/cook request, i.e. the initialization has to be
independent from the normal peer polling process. Should be the
assoc/cook messages sent in the NTP client/server mode packets or keep
them in the symmetric mode? My preference would be the former.

Will there be any recommendations on the minimum interval between
client requests when creating an NTS association? I think it should
follow the NTP rules on polling interval. With the 2s (i)burst
interval and server_assoc message that didn't require more than one
packet, the client could have the first secure timestamps after 4
seconds.

Should be the client_assoc message (or other NTS messages) required to
include padding to prevent traffic amplification attacks?

What exactly is preventing NTS to be usable with NTP pools? Is that
meant for the pool.ntp.org pool where the servers are not controlled
by a single entity or is there a requirement that one certificate
cannot be used on multiple servers?

-- 
Miroslav Lichvar
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