Re: [ippm] Benjamin Kaduk's No Objection on draft-ietf-ippm-twamp-yang-11: (with COMMENT)

[Mahesh Jethanandani <[e-mail]>]

Hi Ben,

> On Jun 18, 2018, at 9:18 AM, Benjamin Kaduk <[e-mail]> wrote:
> 
> Benjamin Kaduk has entered the following ballot position for
> draft-ietf-ippm-twamp-yang-11: No Objection
> 
> When responding, please keep the subject line intact and reply to all
> email addresses included in the To and CC lines. (Feel free to cut this
> introductory paragraph, however.)
> 
> 
> Please refer to https://www.ietf.org/iesg/statement/discuss-criteria.html
> for more information about IESG DISCUSS and COMMENT positions.
> 
> 
> The document, along with other ballot positions, can be found here:
> https://datatracker.ietf.org/doc/draft-ietf-ippm-twamp-yang/
> 
> 
> 
> ----------------------------------------------------------------------
> COMMENT:
> ----------------------------------------------------------------------
> 
> Perhaps I am confused and/or misreading things, but the descriptions
> of the control-client and session-reflector include discussion of
> 'sid' session identifiers as if they were always used, but the mode
> bitmap includes a separate bit for negotiation of
> 'individual-session-control' for session identifier usage.  Is there
> some conflict between this mandatory/negotiable distinction, or are
> they actually talking about different things?

I believe Al has responded on this issue already.

> 
> Comments below in document order, but please pay special note to the
> (potential) need for global uniqueness of key-ids, the PBKDF2
> iteration count, and the list of sensitive nodes to call out in the
> security considerations.
> 
> 
> Section 3.1
> 
>   o  Authentication and encryption attributes such as KeyID, Token and
>      the Client Initialization Vector (Client-IV); see also the last
>      paragraph of Section 6 in OWAMP [RFC4656] and Randomness
>      Requirements for Security [RFC4086].
> 
> I'm confused about what the RFC4656 reference is intended to call
> out -- the reliance on AES to be resistant to chosen plaintext, or
> the randomly generated challenge from the server, or the existential
> forgeries?

The reference should have been to Section 3.1 of RFC4656.

> 
>   o  Information pertaining to the test packet stream, such as the test
>      starting time, which performance metric is to be used Registry for
>      Performance Metrics [I-D.ietf-ippm-metric-registry], or whether
>      the test should be repeated.
> 
> Is there something missing before or around "Registry for
> Performance Metrics"?  The current text is hard to read.

Agree. How about this:

OLD
      Information pertaining to the test packet stream, such as the test
      starting time, which performance metric is to be used Registry for
      Performance Metrics [I-D.ietf-ippm-metric-registry <https://tools.ietf.org/html/draft-ietf-ippm-twamp-yang-11#ref-I-D.ietf-ippm-metric-registry>], or whether
      the test should be repeated.


NEW
      Information pertaining to the test packet stream, such as the test
      starting time, which performance metric is to be used, as defined in Registry for
      Performance Metrics [I-D.ietf-ippm-metric-registry <https://tools.ietf.org/html/draft-ietf-ippm-twamp-yang-11#ref-I-D.ietf-ippm-metric-registry>], or whether
      the test should be repeated.


> 
> Section 3.4
> 
>   Each Session-Reflector is associated with zero or more TWAMP-Test
>   sessions.  For each test session, the REFWAIT timeout parameter which
>   determines whether to discontinue the session if no packets have been
>   received (TWAMP [RFC5357], Section 4.2) can be configured.
> 
> nit: I think this would be easier to read if "which
> determines...received" was offset by commas or parentheses.

How about this:

OLD
   For each test session, the REFWAIT timeout parameter which
   determines whether to discontinue the session if no packets have been
   received (TWAMP [RFC5357], Section 4.2 <https://tools.ietf.org/html/rfc5357#section-4.2>) can be configured.


NEW
   For each test session, the REFWAIT timeout parameter can be configured, which
   determines whether to discontinue the session, if no packets have been
   received (TWAMP [RFC5357], Section 4.2 <https://tools.ietf.org/html/rfc5357#section-4.2>).



> 
>   Read-only access to other data model parameters, such as the Sender
>   IP address is foreseen.  Each test session can be uniquely identified
>   by the 4-tuple mentioned in Section 3.2.
> 
> Nit: comma after "Sender IP address”.

Ok.

> 
> Section 4.1
> 
>   [...] Specifically, mode-preference-chain lists the
>   mode and its corresponding priority, expressed as a 16-bit unsigned
>   integer, where zero is the highest priority and subsequent integers
>   increase by one.
> 
> I thought I remembered some discussion about this text being unclear
> and removing "and subsequent integers increase by one" being
> proposed.  But I don't see that discussion in an obvious place, so
> maybe it was on a different document.

You do have a good memory. It was for this document. See the discussion on the other thread with Pete, where this has come up and edits proposed.

> 
>   Note that the list of preferred Modes may set bit position
>   combinations when necessary, such as when referring to the extended
>   [...]
> 
> Maybe "may set multiple bits independently" would be more clear?
> But it seems that some bit combinations don't make any sense, like
> unauthenticated+authenticated -- is there need for more expository
> text here?

How about:

OLD:
   Note that the list of preferred Modes may set bit position
   combinations when necessary, such as when referring to the extended
   TWAMP features in [RFC5618 <https://tools.ietf.org/html/rfc5618>], [RFC5938 <https://tools.ietf.org/html/rfc5938>], [RFC6038 <https://tools.ietf.org/html/rfc6038>], and [RFC7717 <https://tools.ietf.org/html/rfc7717>].  If
   the Control-Client cannot determine an acceptable Mode, it MUST
   respond with zero Mode bits set in the Set-up Response message,
   indicating it will not continue with the control connection.



NEW:
   Note that the list of preferred Modes may set multiple bit position
   independently, such as when referring to the extended
   TWAMP features in [RFC5618 <https://tools.ietf.org/html/rfc5618>], [RFC5938 <https://tools.ietf.org/html/rfc5938>], [RFC6038 <https://tools.ietf.org/html/rfc6038>], and [RFC7717 <https://tools.ietf.org/html/rfc7717>].  If
   the Control-Client cannot determine an acceptable Mode, or when the 
   bit combinations do not make sense, e.g. both authenticated and 
   unauthenticated bit set, it MUST respond with zero Mode bits set 
   in the Set-up Response message, indicating it will not continue with 
   the control connection.

> 
>   [...] The secret-key is the shared secret, a sequence
>   of octets of arbitrary length whose interpretation is unspecified.
>   The key-id and secret-key encoding SHOULD follow Section 9.4 of YANG
>   [RFC7950]. [...]
> 
> Section 9.4 of YANG is for (printable) strings, but the secret-key
> is binary -- should this get a Section 9.8 reference as well?
> I'm also not sure that leaving it as "arbitrary length"
> is great -- if we're using it to derive 16-byte AES keys and 32-byte
> HMAC-SHA1 keys, we could at least say "SHOULD contain at least 128
> bits of entropy”.

This was my bad. I changed the text for the secret-key from string to binary in the YANG model, at the behest of Tom Petch, but forgot to update this text. I can update the reference to Section 9.8 of RFC7950. Will add text around the 128 bits of entropy.

> 
> Section 4.2
> 
>   [...] The Server, being prepared to conduct
>   sessions with more than one Control-Client, uses KeyIDs to choose the
>   appropriate secret-key; a Control-Client would typically have
>   different secret keys for different Servers. key-id tells the Server
>   which shared-secret the Control-Client wishes to use for
>   authentication or encryption.
> 
> Does this imply a global uniqueness requirement for key-ids?  If so,
> that should be called out more clearly.

Section 4.1 says. Do you think this is clear enough?

   Both the Server and
   the Control-Client use the same mappings from KeyIDs to shared
   secrets (key-id and secret-key in Figure 3, respectively)

> 
> Section 4.3
> 
>                            | name                      |
>                            | ctrl-connection-name {ro} |
>                            | fill-mode                 |
>                            | number-of-packets         |
>                            | state {ro}                |
>                            | sent-packets         {ro} |
>                            | rcv-packets          {ro} |
>                            | last-sent-seq        {ro} |
>                            | last-rcv-seq         {ro} |
>                            +---------------------------+
> 
> nit: should the "{ro}" on "state" be right-aligned with the others?

Ok. 

> 
> Is there any privacy concern about exposing the parent-connection
> 4-tuple?

As discussed in the other thread, the test is run between the four logical entities within the same provider network. That combined with the security of the session, and use of access control (NACM) can limit how much is exposed and what they can do with that information.

> 
> Section 5.2
> 
> In the 'count' leaf, a default value of 10 (corresponding to an
> iteration count of 2^10 == 1024 for PBKDF2) is described.  This
> seems quite low for a PBKDF2 iteration count, by modern standards.
> In "normal" cryptographic protocols we would generally be using a
> default closer to 32768 == 2^15 (which I see is the default *max*
> count value, and there is additional discussion of the issue in the
> leaf description for that leaf).  Perhaps one could make an argument
> that this is just for test setups and the keys and data exchanged
> are "not very valuable", but there is always risk of key sharing
> across protocols, and my preference is to present the strong
> defaults and give users the option to reduce where appropriate.
> What are the authors' thoughts here?

We can bump the default for ‘count to be 15, and the default for ‘max-count-exponent’ to 20.

> 
> Section 7
> 
> There are probably more nodes that can get called out as
> particularly vulnerable, such as the count and max-count nodes that
> can cause a long time to be spent on PBKDF2 iterations, the dscp
> markings, the mode bitmask, etc.

The question of more nodes being identified from a vulnerability perspective has been called out on the other thread. We can use the above discussion, and the point you make below to call out ‘count’ and ‘max-count-exponent’. What do you see as issue with different DSCP markings or with mode bit mask?

> 
> Appendix A
> 
> The <secret-key> elements appear to be using base64-encoded values.
> Where is it specified that such encoding is used for the binary
> values?  (I assume this is just my ignorance of a generic standard,
> so please enlighten me!)

RFC 7950 says in section 9.8.2 that:

   Binary values are encoded with the base64 encoding scheme (see
   Section 4 in [RFC4648] <https://tools.ietf.org/html/rfc4648#section-4>).


> 
> Am I reading it right that the <count>30</count> means 2^30 (one
> billion) PBKDF2 iterations?  Has this actually been run in practice?
> It seems like it would be painfully slow.

It was setup as an example to show how a test session could be configured. We can reduce the value of count to 15.

Thanks.

> 
> 

Mahesh Jethanandani
[e-mail]