Re: [auth48] AUTH48: RFC-to-be 9523 <draft-ietf-ntp-chronos-25> for your review

[[e-mail]]

Authors,

While reviewing this document during AUTH48, please resolve (as necessary) the following questions, which are also in the XML file.

1) <!--[rfced] Please review our updates to Section 2.1 "Terms and
     Abbreviations".  In the original, the expansions of abbreviations
     and document titles were intermixed.  If the current text does
     not suit, please let us know any objections.    -->


2) <!--[rfced] Should "rate" be added to the second sentence to match the
     first?

Original:
     |    B     | An upper bound on the client's clock error rate    |
     |          | (ms/sec).                                          |

     |   ERR    | An upper bound on the client's clock error between |
     |          | Khronos polls (ms).   


Perhaps:
     |    B     | An upper bound on the client's clock error rate    |
     |          | (ms/sec).                                          |

     |   ERR    | An upper bound on the client's clock error rate    |
     |          | between Khronos polls (ms).   

-->


3) <!--[rfced] Does the following suggested text correctly capture your
     intent?  if not, please let us know how we may rephrase.

Original:
   Calibration is performed at the first time the Khronos is executed,
   and also periodically, once in a long time (every two weeks).

Perhaps:
   Calibration is performed the first time Khronos is executed
   and periodically thereafter (once every two weeks). -->


4) <!--[rfced] Is there a way to rephrase this sentence for clarity?  Is
     the meaning that Khronos forces the DNS queries that are sent to
     addresses of NTP pools to do the collecting of a group of all
     received IP addresses?

Original:
To this end, Khronos makes DNS queries to addresses of NTP pools
collect the union of all received IP addresses.

-->


5) <!--[rfced] Should "selected to" read as "selected by the..."?  Or is
     the meaning "selected to be part of the Khronos pool"?  Please
     also review the capitalization of "Internet" here:

Original:
   In addition, servers can be selected to Khronos pool manually or by
   using other NTP pools (such as NIST internet time servers).

Perhaps:
   In addition, servers can be selected by the Khronos pool manually or by
   using other NTP pools (such as NIST Internet time servers).

or

Perhaps:
   In addition, servers can be selected to be part of the Khronos pool
   manually or by using other NTP pools (such as NIST Internet time
   servers).

-->


6) <!--[rfced] Would it be helpful for the readers to move the following
     text to appear above the list in Section 3.2 (assuming it applies
     to both bullet points)?

Original:
   (where w and ERR are as described in Table 1).

Perhaps:
   Khronos checks that the following two conditions hold for the
   remaining sampled offsets (where w and ERR are as described in Table 1): -->


7) <!--[rfced] In the following, may we clarify what "to arrive to" was
     communicating?

Original:
...and the chances to arrive to repeated resampling
are low (see Section 5 for more details).

Perhaps:
...and the chances of repeated resampling
are low (see Section 5 for more details).

or

Perhaps:
...and the chances of ending up with repeated resampling
are low (see Section 5 for more details).

-->


8) <!--[rfced] For the ease/interest of the reader, should a citation be
     included for more information on the "Blufferboat attack"?  If
     so, please let us know what you'd like to cite.  (We will assume
     this reference entry would be informative unless we heard
     otherwise.)  

Original:
The threat model encompasses a broad spectrum of attackers, ranging
from fairly weak (yet dangerous) MitM attackers only capable of
delaying and dropping packets (for example using the Bufferbloat
attack) to extremely powerful attackers who are in control of (even
authenticated) NTP servers (see detailed security requirements
discussion in [RFC7384]).

-->


9) <!--[rfced] Is this a singular/plural change that should be made?  And
     should this mention of a specific attack (MitM) be removed as it
     is mentioned later in the same paragraph?

Original:
The following powerful attacker, including MitM is considered:

[and then later]

Original:
The threat model encompasses a broad spectrum of attackers, ranging
from fairly weak (yet dangerous) MitM attackers...


Perhaps:
The following powerful attackers, including MITM, are considered.

Or

Perhaps:
The following powerful attackers are considered.

-->


10) <!--[rfced] Section 5.3: This section suffers a bit from the fact that
     two scenarios with two sub-scenarios are discussed.  This leads
     to a decent amount of repeating text (and possible confusion for
     the reader).

We have updated the sub-cases to appear in ordered list form
(indented) already, but we believe further updates to this section
would make it easier for the reader to understand in a single read
(i.e., naming the two scenarios, referring to the sub-cases by
numbers, and breaking up the paragraph describing the sub-cases).

Please see a further question below the suggested text.

Please let us know if the following updates are agreeable:

Perhaps:
   Time samples that are at most w away from UTC are considered "good",
   whereas other samples are considered "malicious".  Two scenarios are
   considered:

   *  Scenario A: Less than two-thirds of the queried servers are under the
      attacker's control.

   *  Scenario B: The attacker controls more than two-thirds of the queried servers.

   Scenario A consists of two sub-cases:

   1.  there is at least one good sample in the set of samples not
       eliminated by Khronos (in the middle third of samples), and

   2.  there are no good samples in the remaining set of samples.

   In sub-case 1, the other remaining samples, including those
   provided by the attacker, must be close to a good sample
   (otherwise, the first condition of Khronos's system process in
   Section 3.2 is violated and a new set of servers is chosen).  This
   implies that the average of the remaining samples must be close to
   UTC.

   In sub-case 2, since more than a third of the initial samples were
   good, both the (discarded) third-lowest-value samples and the
   (discarded) third-highest-value samples must each contain a good
   sample.  Hence, all the remaining samples are bounded from both
   above and below by good samples, and so is their average value,
   implying that this value is close to UTC [RFC5905].

   In Scenario B, the worst possibility for the client is that all
   remaining samples are malicious (i.e., more than w away from UTC).
   However, as proved in [Khronos], the probability of this scenario is
   extremely low, even if the attacker controls a large fraction (e.g.,
   one-fourth) of the n servers in the local Khronos pool.  Therefore, the
   probability that the attacker repeatedly reaches this scenario
   decreases exponentially, rendering the probability of a significant
   time shift negligible.  We can express the improvement ratio of
   Khronos over NTPv4 by the ratios of their single-shift probabilities.
   Such ratios are provided in Table 2, where higher values indicate
   higher improvement of Khronos over NTPv4 and are also proportional to
   the expected time until a time-shift attack succeeds once.



-->


11)  <!--[rfced] We had the following questions about the pseudocode in
      Section 6:

a) Please review the "type" attribute of each sourcecode element in
the XML file to ensure correctness.

We have updated to use "pseudocode" per the text introducing it.  If
this is incorrect, please see below for further guidance:

If the current list of preferred values for "type"
(https://www.rfc-editor.org/materials/sourcecode-types.txt) does not
contain an applicable type, then feel free to let us know. Also, it is
acceptable to leave the "type" attribute not set.

b) Please review the capitalization of "Then" and the use of a comma
in the following portion of pseudocode:

Original:
      if (max(T) - min(T) <= 2w) and (|avg(T) - tk| < ERR + 2w) Then
          return avg(T) // Normal case

Perhaps:
      if (max(T) - min(T) <= 2w) and (|avg(T) - tk| < ERR + 2w), then
      return avg(T) // Normal case

c) Please review the following line as it exceeds our character limit.
Please let us know how we can update.

Original:
S = sample(m) //gather samples from (tens of) randomly chosen servers

Perhaps:
S = sample(m) //get samples from (tens of) randomly chosen servers
-->


12) <!--[rfced] May we remove the "Implementation Status" section prior to
     publication as an RFC?  Please see RFC 7942 for:

"We recommend that the Implementation Status section should be removed
from Internet-Drafts before they are published as RFCs."

-->


13) <!--[rfced] We note that the title of the reference below is a
     duplicate of [Khronos]. We have updated the reference as it
     appears on the URL provided. Please let us know if any additional
     changes are needed.

Original:
[Ananke_paper]
           Perry, Y., Rozen-Schiff, N., and M. Schapira, "Preventing
           (Network) Time Travel with Chronos", 2021,
           <https://www.ndss-symposium.org/wp-content/uploads/
           ndss2021_1A-2_24302_paper.pdf>.

Current:
[Ananke]
           Perry, Y., Rozen-Schiff, N., and M. Schapira, "A Devil of
           a Time: How Vulnerable is NTP to Malicious Timeservers?",
           Network and Distributed Systems Security (NDSS) Symposium,
           Virtual, DOI 10.14722/ndss.2021.24302, February 2021,
           <https://www.ndss-symposium.org/wp-content/uploads/
           ndss2021_1A-2_24302_paper.pdf>. -->


14) <!--[rfced] Please review the use of the following terms throughout
     the document and let us know how you would like to proceed.

Should the following be made uniform?

time offset and Khnronos time offset

watchdog vs. watchdog mode vs. watchdog mechanism

-->


15) <!--[rfced] Please review the "Inclusive Language" portion of the
     online Style Guide
     <https://www.rfc-editor.org/styleguide/part2/#inclusive_language>
     and let us know if any changes are needed. For example, please
     consider whether "man-in-the-middle" should be updated. We note
     that "machine-in-the-middle" is used in the document. In the
     following sentence, may we replace "man-in-the-middle" with
     "MITM" (the abbreviation defined in this document for
     "machine-in-the-middle")? Or would a different term be
     appropriate here?

Original:
   We note that to accomplish
   this, the attacker must have man-in-the-middle capabilities with
   respect to the communication between each and every client in a large
   group of clients and a large fraction of all NTP servers in the
   queried pool.

Perhaps:
   We note that to accomplish
   this, the attacker must have MITM capabilities with
   respect to the communication between each and every client in a large
   group of clients and a large fraction of all NTP servers in the
   queried pool. -->


16) <!--[rfced] FYI - We have added expansions for abbreviations upon
     first use per Section 3.6 of RFC 7322 ("RFC Style Guide"). Please
     review each expansion in the document carefully to ensure
     correctness. -->


Thank you.

RFC Editor/mc/mf

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--------------------------------------
RFC9523 (draft-ietf-ntp-chronos-25)

Title            : A Secure Selection and Filtering Mechanism for the Network Time Protocol with Khronos
Author(s)        : N. Rozen-Schiff, D. Dolev, T. Mizrahi, M. Schapira
WG Chair(s)      : Dieter Sibold, Karen O'Donoghue

Area Director(s) : Erik Kline, Éric Vyncke