[Ntp] Benjamin Kaduk's Abstain on draft-ietf-ntp-interleaved-modes-06: (with COMMENT)

Benjamin Kaduk via Datatracker <noreply@ietf.org> Wed, 14 July 2021 19:28 UTC

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Subject: [Ntp] Benjamin Kaduk's Abstain on draft-ietf-ntp-interleaved-modes-06: (with COMMENT)
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Benjamin Kaduk has entered the following ballot position for
draft-ietf-ntp-interleaved-modes-06: Abstain

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COMMENT:
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While this is an interesting approach, and a very clever technique to
add new functionality in the face of perceived constraints, it's not
clear to me that the perceived constraints are indeed constraints in
reality -- this possible mismatch between perceived constraints and
actual constraints seems to be at the core of Warren and Rob's discuss
points, so I will not belabor it further here.  Furthermore, the overall
quality of the document in terms of describing the protocol does not
seem to be at the level that I have come to expect from proposed
standards. In particular, if I tried to implement this protocol, I think
I would find myself needing to make guesses or choose among multiple
possible interpretations of the text in many places.  Accordingly, I
can't support publishing this document as a proposed standard in its
current form, so I'm balloting Abstain.  It would certainly be possible
to improve the document into a form that I can support, but since I
don't have a specific list of actionable changes that would be needed to
do so, I have to ballot Abstain rather than Discuss.

The discussion of "user space"/"kernel", "system call", and the like
seems to implicitly assume a certain architecture and may not be
applicable to fully all devices speaking NTP.  But these are certainly
common architectures and abstractions, and I don't expect that using
this framing for the discussion will hamper readability.

There seem to be some scenarios where packets can be sent in both basic
mode and interleaved mode within the same association, and it cannot
always be predicted whether a response or "next message" will be in
interleaved mode or not.  It seems like this could present significant
implementation complexity in terms of deciding whether or not to process
a packet "now" as a basic-mode packet or wait for a follow-up that
includes the more-accurate interleaved-mode timestamp, or some more
complicated combination of the two.  Is there any implementation
guidance that we can give about how to process packets when there may or
may not be a later follow-up with a correction to it?

Section 1

   Requests and responses cannot always be formed in interleaved mode.
   Servers, clients, and peers are required to support both interleaved
   and basic modes.

I'm not sure I understand the intended statement of requirement here.
Are we proposing (by virtue of Updates: 5905) that all NTP
implementations are required to support interleaved mode?  Or just that,
in order to use interleaved mode, all parties are required to have
support for it?

Section 2

Breaking this up into subsections might help readability.

   A client request in the basic mode has an origin timestamp equal to
   the transmit timestamp from the previous server response, or is zero.
   A server response in the basic mode has an origin timestamp equal to
   the transmit timestamp from the client's request.  The transmit
   timestamps correspond to the packets in which they are included.

Since we use the receive timestamp from the client's request in some of
our later discussion, I would strongly suggest including the receive
timestamp in the description of basic mode here (and interleaved mode in
the subsequent paragraph).

   A client request in the interleaved mode has an origin timestamp
   equal to the receive timestamp from the previous server response.  A
   server response in the interleaved mode has an origin timestamp equal
   to the receive timestamp from the client's request.  The transmit
   timestamps correspond to the previous packets that were sent to the
   server or client.

Please provide a lot more precision about what "transmit timestamps
correspond to the previous packets" means -- I might guess that it's
just (client, server) using the same transmit timestamp used by the
(client, server) to generate the initial exchange that preceded
interleaved mode, but I also might guess a lot of other things.

   The transmit and receive timestamps in server responses need to be
   unique to prevent two different clients from sending requests with
   the same origin timestamp and the server responding in the
   interleaved mode with an incorrect transmit timestamp.  If the

IIUC (having read the rest of the document), the requirement here is
that if we take the union of all transmit timestamps and all receive
timestamps generated by the server, there must be no duplicates.  This
text as written did not lead me to that conclusion; I had initially
assumed that it only meant "for each response packet generated, do not
generate x.rec == x.xmt", which is a much weaker condition (and very
hard to achieve accidentally!).  I strongly suggest rewording to improve
clarity.

   timestamps are not guaranteed to be monotonically increasing, the
   server SHOULD check that the transmit and receive timestamp is not
   already saved as a receive timestamp of a previous request (from the
   same IP address if the server separates timestamps by addresses), and
   generate a new timestamp if necessary.

It would be nice to have a little more exposition on whether it's
only monotonicity of transmit timestamps that's important for being able
to skip this check, since "the timestamps" as written could also refer
to the receive timestamp given the previous discussion.  Additionally,
the mention of "transmit and receive timestamp is not already saved"
implies that this check is only needed when the two timestamps have the
same value, and I'm not entirely sure whether that's the case.

   A response in the interleaved mode MUST contain the transmit
   timestamp of the response which contained the receive timestamp
   matching the origin timestamp from the request.  [...]

"contain" doesn't say much about containing *where*; I suggest being
very clear that it contains in the transmit timestamp field the same
transmit timestamp used in the previous response.

   The first request from a client is always in the basic mode and so is
   the server response.  It has a zero origin timestamp and zero receive
   timestamp.  Only when the client receives a valid response from the
   server, it will be able to send a request in the interleaved mode.

While this is true of the first request ever, it's not quite true for
the first request of a given exchange, since RFC 5905 allows the client
to send the transmit timestamp from a previous server response as the
origin timestamp.  We do mention at the end of the section that
draft-ietf-ntp-data-minimization recommends sending zero as the origin
timestamp, but that's of course not something that can be strongly
relied upon.  I'd actually suggest just noting here that the first
request has to be detected to be in basic mode, and forward-reference a
dedicated subsection at the end of the section that talks about how to
identify such requests.

   When the client receives a response from the server, it performs the
   tests described in RFC 5905.  Two of the tests are modified for the
   interleaved mode:

Indicating more precisely exactly which tests are modified (e.g., that
these are the numbered tests from figure 22 of RFC 5905) could be
useful.

   1.  The check for duplicate packets SHOULD compare both receive and
       transmit timestamps in order to not drop a valid response in the
       interleaved mode if it follows a response in the basic mode and
       they contain the same transmit timestamp.

   2.  The check for bogus packets SHOULD compare the origin timestamp
       with both transmit and receive timestamps from the request.  If
       the origin timestamp is equal to the transmit timestamp, the
       response is in the basic mode.  If the origin timestamp is equal
       to the receive timestamp, the response is in the interleaved
       mode.

If I understand correctly, ignoring either of these SHOULDs would result in
completely failing to use interleaved mode.  Does that mean that they're
MUSTs?

   A check for a non-zero origin timestamp works with clients that
   implement NTP data minimization [I-D.ietf-ntp-data-minimization].  To
   detect requests in the basic mode from clients that do not implement
   the data minimization, the server can encode in low-order bits of the
   receive and transmit timestamps below precision of the clock a bit
   indicating whether the timestamp is a receive timestamp.  If the
   server receives a request with a non-zero origin timestamp which does
   not indicate it is a receive timestamp of the server, the request is
   in the basic mode and it is not necessary to save the new receive and
   transmit timestamp.

In the vein of my earlier comment, I'd suggest rewording this
significantly and making it a dedicated subsection on detecting if a
request is in basic or interleaved mode.  That might look something like
this:

% As part of request processing, the server must determine whether a
% given request is in basic or interleaved mode (and thus whether or not
% to respond in basic or interleaved mode).  A request with a zero
% origin timestamp is unambiguously in basic mode, but RFC 5905 allows a
% client to send a request with a non-zero origin timestamp copied from
% the transmit timestamp of a previous response from that server.  While
% [I-D.ietf-ntp-data-minimization] recommends always using zero for the
% request's origin timestamp, that is not a behavior that the server can
% rely upon, and so an additional mechanism is needed in order to detect
% whether a request with non-zero origin timestamp is in basic mode.
% While having the server store all the transmit timestamps it has
% previously used (potentially scoped to just this client), combined
% with a requirement to maintain uniqueness across all of the transmit
% and receive timestamps the server generates, would perform this
% function, it would require an unfeasible amount of resources and
% degrade the quality of time synchronization.  An alternate approach is
% to use a low-order bit in the timestamp field, below the precision of
% the server's clock, to indicate whether a timestamp is sent as a
% transmit timestamp or a receive timestamp.  When processing requests
% with non-zero origin timestamps, that bit can then be used to
% determine whether the request was in basic mode (transmit timestamp)
% or interleaved mode (receive timestamp).

Section 3

   The interleaved symmetric mode uses the same principles as the
   interleaved client/server mode.  A packet in the interleaved
   symmetric mode has a transmit timestamp which corresponds to the
   previous packet sent to the peer [...]

Please apply the same change regarding "corresponds to" that was made in
§2.

   In order to prevent the peer from matching the transmit timestamp
   with an incorrect packet when the peers' transmissions do not
   alternate (e.g. they use different polling intervals) and a previous
   packet was lost, the use of the interleaved mode in symmetric
   associations requires additional restrictions.

Though the specific "restrictions" are enumerated as specific conditions
below (scare quotes since they are only "SHOULD"-level requirements),
the subsequent discussion suggests that in practice there are also
implications on the relationship between the polling intervals of the
two peers.  Should we say something here about how the polling intervals
should be within a factor of (1.5? 2?) of each other in order for
interleaved mode to be feasible?

   A peer A SHOULD send a peer B a packet in the interleaved mode only
   when all of the following conditions are met:

What happens if I violate this SHOULD?

   third packet sent by the peer A is in the interleaved mode.  The
   second packet sent by the peer B is in the interleaved mode, but the
   following packets sent by the peer are in the basic mode, because
   multiple responses are sent per request.

Please clarify whether the final "the peer" is peer A or peer B (or is
meant to be "the peers" plural).

   The peers SHOULD compute the offset and delay using one of the two
   sets of timestamps specified in the client/server section.  They MAY
   switch between them to minimize the interval between T1 and T4 in
   order to reduce the error in the measured delay.

One of the sets of timestamps doesn't use T1, so I'm not sure what is
supposed to be minimized here.

Section 4

   If the difference is larger than a specified maximum (e.g. 1 second),
   the packet SHOULD NOT be used for synchronization.

Not used at all, or used only in basic mode?

Section 7

   Clients using the interleaved mode SHOULD randomize all bits of both
   receive and transmit timestamps, as recommended for the transmit
   timestamp in the NTP client data minimization
   [I-D.ietf-ntp-data-minimization], to make it more difficult for off-
   path attackers to guess the origin timestamp.  [...]

I would suggest going into a little more detail on the scope of
consequences if an off-path attacker does successfully guess the origin
timestamp.

   Protecting symmetric associations in the interleaved mode against
   replay attacks is even more difficult than in the basic mode.  The
   NTP state needs to be protected not only between the reception and
   transmission in order to send the peer a packet with a valid origin
   timestamp, but all the time to not lose the timestamps which will be
   needed to complete a measurement when the following packet in the
   interleaved mode is received.

Is there some existing guidance on how to protect symmetric associations
that we can refer to (and would be effective under these constraints)?

NITS

Section 1

   This document describes an interleaved client/server, interleaved
   symmetric, and interleaved broadcast mode.  In these modes, the
   server sends a single packet, which contains a transmit timestamp
   corresponding to the previous packet that was sent to the client or
   peer.  [...]

The "sends a single packet" phrasing is a little weird, since we have a
four-packet architecture and the comparison we're making is that any
given request gets only a single response, not that there is only one
packet needed overall.

Section 2

   The client SHOULD NOT update its NTP state when an invalid response
   is received to not lose the timestamps which will be needed to
   complete a measurement when the subsequent response in the
   interleaved mode is received.

I suggest a comma after "invalid response is received".