[rmcat] Benjamin Kaduk's No Objection on draft-ietf-rmcat-nada-12: (with COMMENT)

Benjamin Kaduk via Datatracker <noreply@ietf.org> Thu, 05 September 2019 17:23 UTC

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Subject: [rmcat] Benjamin Kaduk's No Objection on draft-ietf-rmcat-nada-12: (with COMMENT)
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Benjamin Kaduk has entered the following ballot position for
draft-ietf-rmcat-nada-12: No Objection

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COMMENT:
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Thanks for this document; it provides a clear picture of an interesting
scheme, and it will be interesting to see further reports of its
efficacy.  I just have some essentially editorial comments, that may not
even be worth responding to, so I will try to sneak them in after the
telechat.

Do we need to say anything further about aborting the flow when the
level of congestion does not support even the minimum bitrate supported
by the encoder?  Mirja thinks this is sufficiently well-known that we
probably don't need to, which seems reasonable to me.

Section 4

   o  Accelerated ramp-up: when the bottleneck is deemed to be
      underutilized, the rate increases multiplicatively with respect to
      the rate of previously successful transmissions.  The rate

"multiplicatively" is synonymous with "exponentially" here?
(Thank you for specifying it this way, though, to make clear what the
multiplier value is.)

Section 4.3

                                   QBOUND
       gamma = min(GAMMA_MAX, ------------------)     (3)
                               rtt+DELTA+DFILT

It seems impossible for the minimum to be GAMMA_MAX with the defaults
for QBOUND, DELTA, and DFILT, since the second argument to min() is less
than 50/(120+100) = 0.23, which is much less than the default GAMMA_MAX
of 0.5.

Section 5.1.1

   one-way delay and base delay.  By re-estimating the base delay
   periodically, one can avoid the potential issue of base delay
   expiration, whereby an earlier measured base delay value is no longer
   valid due to underlying route changes.  All delay estimations are

I think that clock *rate* skew between peers could also cause issues
with base delay values growing less useful over time.  Clock rate skew
is much less prominent than absolute clock skew, though.

   based on sender timestamps with a recommended granularity of 100
   microseconds or higher.

Is this a normative requirement?
Also, pedantically, I think "granularity of 100 microseconds or higher"
has "higher" apply to the numerical value 100, so it would
include a granularity of 1 second.  I see that later on we recommend
measuring the delay as an integer multiple of 100 microseconds, so this
seems like the intended sense, but does the scheme become less useful if
the granularity of the measurment becomes too coarse-grained (e.g., the
second granularity I mentioned above)?  If so, we may want to put a
bound on the other direction as well.

Section 5.3

   o  Aggregated congestion signal (x_curr): the most recently updated
      value, calculated by the receiver according to Section 4.2.  This
      information is expressed with a unit of 100 microsecond (i.e.,
      1/10 of a millisecond) in 15 bits.  This allows a maximum value of
      x_curr at approximately 3.27 second.

There's perhaps a minor editorial mismatch between the "information is
required" intro and declarative "is expressed in [specific format]".
(Similarly for r_recv.)