Re: [Tsv-art] Tsvart last call review of draft-ietf-bmwg-b2b-frame-03

"Black, David" <David.Black@dell.com> Wed, 25 November 2020 21:27 UTC

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From: "Black, David" <David.Black@dell.com>
To: "MORTON, ALFRED C (AL)" <acm@research.att.com>, "tsv-art@ietf.org" <tsv-art@ietf.org>
CC: "bmwg@ietf.org" <bmwg@ietf.org>, "draft-ietf-bmwg-b2b-frame.all@ietf.org" <draft-ietf-bmwg-b2b-frame.all@ietf.org>, "last-call@ietf.org" <last-call@ietf.org>, "Black, David" <David.Black@dell.com>
Thread-Topic: Tsvart last call review of draft-ietf-bmwg-b2b-frame-03
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Archived-At: <https://mailarchive.ietf.org/arch/msg/tsv-art/2ntEC3gPxfpJQEOsDN1pwnB1MaE>
Subject: Re: [Tsv-art] Tsvart last call review of draft-ietf-bmwg-b2b-frame-03
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Al,

> I clarified:
> The duration of the trial MUST include least 2 seconds in addition to the time
> required to send and receive each burst of frames, to ensure that DUT buffers to
> deplete.
> 
> and I'll add:
> The upper search limit for the time to send each burst MUST be configurable as
> high as 30 seconds (buffer time results reported at the configured upper limit are
> likely invalid, and the test MUST be repeated with a higher search limit).

I think this will work - please make sure to address the scenario in which the search algorithm hits the upper search limit without causing any frame loss (e.g., upper search limit configured to 2 seconds for a bizarre DUT that has 10 seconds of buffering).

Thanks, --David

> -----Original Message-----
> From: MORTON, ALFRED C (AL) <acm@research.att.com>
> Sent: Wednesday, November 25, 2020 1:20 PM
> To: Black, David; tsv-art@ietf.org
> Cc: bmwg@ietf.org; draft-ietf-bmwg-b2b-frame.all@ietf.org; last-call@ietf.org
> Subject: RE: Tsvart last call review of draft-ietf-bmwg-b2b-frame-03
> 
> 
> [EXTERNAL EMAIL]
> 
> David,
> 
> Allow me to  points more clearly:
> 
> - I'm relying on the Binary Search aspect of the procedure to measure unexpected
> large buffer times; searches are an integral part of the procedure.
> 
> - These are benchmarking measurements in an isolated test environment, where
> we will control every source of variability possible to foster repeatability.
> 
> - The trial duration has two components: time to conduct sending + receiving, plus
> additional waiting time for buffer depletion. The test equipment orchestrates this
> easily, because it is a single device.
> 
> I clarified:
> The duration of the trial MUST include least 2 seconds in addition to the time
> required to send and receive each burst of frames, to ensure that DUT buffers to
> deplete.
> 
> and I'll add:
> The upper search limit for the time to send each burst MUST be configurable as
> high as 30 seconds (buffer time results reported at the configured upper limit are
> likely invalid, and the test MUST be repeated with a higher search limit).
> 
> Al
> 
> 
> > -----Original Message-----
> > From: Black, David [mailto:David.Black@dell.com]
> > Sent: Wednesday, November 25, 2020 9:35 AM
> > To: MORTON, ALFRED C (AL) <acm@research.att.com>om>; tsv-art@ietf.org
> > Cc: bmwg@ietf.org; draft-ietf-bmwg-b2b-frame.all@ietf.org; last-
> > call@ietf.org; Black, David <David.Black@dell.com>
> > Subject: RE: Tsvart last call review of draft-ietf-bmwg-b2b-frame-03
> >
> > Al,
> >
> > We appear to not be communicating on a key aspect of the concern:
> >
> > > A burst of 1.7 seconds falls well within the coverage of a 2 second
> > > trial to count the frames and determine that loss has occurred. We
> > > don't need a big margin time- wise. Also, the trial duration
> > > increases with the search parameters: IOW, if the tester is still
> > > sending frames after 2 seconds, then obviously we can't allow the trial to
> terminate prematurely.
> >
> > The 1.6 seconds value is *not* an upper bound - it was measured in
> > practice on an operational conference network as part of what appears
> > to have been a just-in-time slide preparation exercise.  Hence that
> > 1.6 seconds of buffering ought to be viewed as being somewhere in the
> > midst of the expected distribution of buffer times.  For that reason,
> > the upper bound on test duration needs to be set with a safety factor
> > to ensure that even if a bizarre DUT has 10 seconds of buffering, the
> > B2B test depletes that buffering so that the B2B test is testing the
> > forwarding path as intended, not the buffering capacity.
> >
> > > I want to avoid over-reacting to the buffer bloat case. We all agree
> > > that bloated buffers are bad, and knowing a DUT has buffering >=1
> > > second may be enough. I certainly do not think that trial durations
> > > for the B2B Frame benchmark need to be as long as you originally
> > > suggested, where the overwhelming number of test conditions can use a <<1
> second burst and 2 second buffer depletion time:
> >
> > I agree with this approach provided that there is a test procedure (in
> > the B2B test and/or its prerequisites) that will discover the behavior
> > of the bizarre DUT with 10 seconds of buffering and adjust the B2B
> > test time to ensure that this excessive buffering is depleted.
> >
> > > I hope the approach of the single sentence above works for you.
> >
> > Based on this discussion, in addition to that sentence (which I think
> > has to contain a "MUST" for situations in which excessive buffering is
> > found), I would like to see a brief explanation of how the procedures
> > in the draft, along with additional test procedures pulled in from RFC
> > 2544 or elsewhere, avoid a 2 second test duration on a DUT with 10
> > seconds of buffering.
> >
> > Thanks, --David
> >
> > > -----Original Message-----
> > > From: MORTON, ALFRED C (AL) <acm@research.att.com>
> > > Sent: Wednesday, November 25, 2020 12:00 AM
> > > To: Black, David; tsv-art@ietf.org
> > > Cc: bmwg@ietf.org; draft-ietf-bmwg-b2b-frame.all@ietf.org; last-
> > call@ietf.org
> > > Subject: RE: Tsvart last call review of draft-ietf-bmwg-b2b-frame-03
> > >
> > >
> > > [EXTERNAL EMAIL]
> > >
> > > Hi David, some more clarity below.
> > > Al
> > >
> > >
> > > > -----Original Message-----
> > > > From: Black, David [mailto:David.Black@dell.com]
> > > > Sent: Tuesday, November 24, 2020 3:02 PM
> > > > To: MORTON, ALFRED C (AL) <acm@research.att.com>om>; tsv-art@ietf.org
> > > > Cc: bmwg@ietf.org; draft-ietf-bmwg-b2b-frame.all@ietf.org; last-
> > > > call@ietf.org; Black, David <David.Black@dell.com>
> > > > Subject: RE: Tsvart last call review of
> > > > draft-ietf-bmwg-b2b-frame-03
> > > >
> > > > Al,
> > > >
> > > > > [acm] I agree with your observation that there are cases where
> > > > > trial
> > > > duration
> > > > > should be increased to accommodate the encountered in the DUT,
> > > > > but not
> > > > as a
> > > > > mandate for all testing. I have four factors in mind:
> > > > > 1. Some of the virtual network DUTs we are testing now have very
> > > > > small
> > > > buffers,
> > > > > and the B2B stream of frames is quite short -- less than 2000
> > > > frames@10GE in
> > > > > some cases -- so 2 seconds fully sufficient.
> > > > > 2. The trial duration is a factor in total test duration, where
> > > > > each
> > > > trial is one step in
> > > > > the Binary Search. We need to manage the tension between the
> > > > > time needed
> > > > to
> > > > > reach a search result and confidence that we have depleted the
> > queues.
> > > > > 3. The RFC 2544 Latency benchmark will tell us if bufferbloat is
> > > > present.
> > > > > 4. The current text says "at least 2 seconds".
> > > > >
> > > > > So I suggest adding the following text:
> > > > >
> > > > >     The duration of the trial MUST be at least 2 seconds, to
> > > > > allow
> > DUT
> > > > >     buffers to deplete. When RFC2544 Latency measurements
> > > > > indicate
> > that
> > > > >     large buffers are present in the DUT, the trial duration
> > > > > SHOULD
> > be
> > > > >     increased to ensure that buffer depletion takes place,
> > > > > without
> > unduly
> > > > >     extending the total test time.
> > > >
> > > > The overall approach of collecting evidence that there is a
> > > > problem before increasing the 2 second minimum duration is fine,
> > > > but the details appear to need more attention:
> > > >
> > > > 1. Obtaining the RFC 2544 Latency measurements would need to be
> > > > added to Section 4 (Prerequisites) of this draft to ensure that
> > > > the buffer size information is available.
> > > [acm]
> > > Section 26.2 Latency is a very primitive procedure by today's standards.
> > It is
> > > common for testers to measure delay on every packet, and report a
> > > true
> > average.
> > > This is enough to alert the operator to the presence of bufferbloat,
> > > but
> > there's no
> > > assurance that the Latency benchmark measures the extent of the
> > > buffer
> > time.
> > >
> > > >
> > > > 2. I did not see any requirements in the RFC 2544 Latency test
> > > > (Section
> > > > 26.2) to deplete buffers.  Did I miss something?
> > > [acm]
> > > It's an overall requirement in Section 23, after running the trial:
> > >
> > >    d) Wait for two seconds for any residual frames to be received.
> > >
> > > >
> > > > 3. I would think that the trial duration MUST be increased, not
> > > > just SHOULD be increased if there is evidence of large buffer
> > > > size, as buffer depletion appears to be a necessary characteristic
> > > > of this B2B measurement.
> > > [acm]
> > >
> > > We need to keep in mind that the sentence(s) in this discussion
> > > pertain
> > to the trials
> > > of the B2B Frame Benchmark testing, in the section that begins:
> > >
> > >   Each trial in the test requires the tester to send a burst of
> > > frames
> > (after idle time)
> > > with the minimum inter-frame gap, and to count the corresponding
> > > frames forwarded by the DUT.
> > >
> > > Let's say we have a DUT that offers 1.6 seconds of buffering. The
> > > Binary
> > Search will
> > > increase the length of the burst until the buffer drops frames, and
> > > then
> > try to find
> > > the longest burst where frame loss is zero. This simple procedure
> > > always
> > waits the
> > > minimum trial duration for frames to exit the DUT.
> > >
> > > A burst of 1.7 seconds falls well within the coverage of a 2 second
> > trial to count the
> > > frames and determine that loss has occurred. We don't need a big
> > > margin
> > time-
> > > wise. Also, the trial duration increases with the search parameters:
> > IOW, if the
> > > tester is still sending frames after 2 seconds, then obviously we
> > > can't
> > allow the trial
> > > to terminate prematurely.
> > >
> > > So, I suggest we use this wording instead:
> > >
> > >        The duration of the trial MUST be at least 2 seconds in
> > > addition
> > to
> > >        the time to send each burst of frames, to allow DUT buffers
> > > to
> > deplete.
> > >
> > > and we establish an adaptive waiting time for extreme cases,
> > > consistent
> > with
> > > RFC2544 section 23 item d). For a DUT with 1.6 second buffers, the
> > > Trial
> > duration
> > > would be 3.6 seconds.
> > >
> > > BTW, one tester issue we are trying to fix here is the case when the
> > frame header
> > > processing rate is sufficiently high that no buffer limit is
> > > measureable
> > at the large
> > > frame sizes, and we implore people not to waste time with the B2B
> > > Frame Benchmark at those frame sizes. One commercial tester sent
> > > bursts up to
> > 30
> > > seconds in length, and then happily reported the impossible result
> > > of 30
> > second
> > > buffers.
> > >
> > > I want to avoid over-reacting to the buffer bloat case. We all agree
> > that bloated
> > > buffers are bad, and knowing a DUT has buffering >=1 second may be
> > enough. I
> > > certainly do not think that trial durations for the B2B Frame
> > > benchmark
> > need to be
> > > as long as you originally suggested, where the overwhelming number
> > > of
> > test
> > > conditions can use a <<1 second burst and 2 second buffer depletion
> > time:
> > >
> > > > > > Hence, the 2 second minimum duration ought to be increased by
> > > > > > at least a factor of 10.  I'd suggest changing it to 30
> > > > > > seconds or 60
> > seconds...
> > >
> > > I hope the approach of the single sentence above works for you.
> > >
> > >
> > >
> > > >
> > > > It also looks like the link to the entire slide deck didn't make
> > > > it into my original review correctly - that slide deck is at:
> > > > https://urldefense.com/v3/__http://www.taht.net/*d/lca_tcp3.odp__;fg!!
> > > > BhdT !0oWabwvUrtVlGQY7ZKnFnMa-
> > > un_e95tfCJcRKIWG7wECiJkVISb4sjd9Salwehw$
> > > > .  In addition to slide 6 from this slide deck (Figure 1 in the
> > > > APNIC blog), slide 14 is also relevant to this discussion.
> > > >
> > > > Thanks, --David
> > > >
> > > > > -----Original Message-----
> > > > > From: MORTON, ALFRED C (AL) <acm@research.att.com>
> > > > > Sent: Tuesday, November 24, 2020 11:11 AM
> > > > > To: Black, David; tsv-art@ietf.org
> > > > > Cc: bmwg@ietf.org; draft-ietf-bmwg-b2b-frame.all@ietf.org; last-
> > > > call@ietf.org
> > > > > Subject: RE: Tsvart last call review of
> > > > > draft-ietf-bmwg-b2b-frame-03
> > > > >
> > > > >
> > > > > [EXTERNAL EMAIL]
> > > > >
> > > > > Hi David, Thanks for your review and comment!
> > > > >
> > > > > Please see a proposed resolution below, [acm] Al
> > > > >
> > > > > > -----Original Message-----
> > > > > > From: David Black via Datatracker [mailto:noreply@ietf.org]
> > > > > > Sent: Tuesday, November 24, 2020 10:27 AM
> > > > > > To: tsv-art@ietf.org
> > > > > > Cc: bmwg@ietf.org; draft-ietf-bmwg-b2b-frame.all@ietf.org;
> > > > > > last- call@ietf.org
> > > > > > Subject: Tsvart last call review of
> > > > > > draft-ietf-bmwg-b2b-frame-03
> > > > > >
> > > > > > Reviewer: David Black
> > > > > > Review result: Ready with Issues
> > > > > >
> > > > > ...
> > > > > >
> > > > > > This draft updates the back-to-back frame testing procedure in
> > > > > > RFC
> > > > > > 2544 to take account of experience.
> > > > > >
> > > > > > The draft is in good shape, with one notable exception in
> > > > > > Section
> > 5.2:
> > > > > >
> > > > > >    The duration of the trial MUST be at least 2 seconds, to
> > > > > > allow
> > DUT
> > > > > >    buffers to deplete.
> > > > > >
> > > > > > That duration of 2 seconds has been carried forward from RFC
> > > > > > 2544 without change.  A 2 second duration may have been
> > > > > > sufficient to deplete buffers in 1999, but that is no longer reliably the case.
> > > > > > For example, on-site measurement of the network for the 2020
> > > > > > Linux Conference in Australia indicated a at least 1.6 seconds
> > > > > > of buffering, as indicated by Figure 1 at
> > > > > >
> > > > https://urldefense.com/v3/__https://blog.apnic.net/2020/01/22/buff
> > > > erbl
> > > > oat-
> > > > may-be-solved-but-its-not-over-
> > > > yet/__;!!BhdT!wOuQE0NajXs4dT7tdIMVQU5FFpb0JiU0-
> > > > yK2DOVVn0ecoYjf7mFEABLmlwDk$ ,
> > > > > > which is slide 6 in from the complete slide deck at:
> > > > > >
> > > > https://urldefense.com/v3/__https://blog.apnic.net/2020/01/22/buff
> > > > erbl
> > > > oat-
> > > > may-be-solved-but-its-not-over-
> > > > yet/__;!!BhdT!wOuQE0NajXs4dT7tdIMVQU5FFpb0JiU0-
> > > > yK2DOVVn0ecoYjf7mFEABLmlwDk$
> > > > > > .  Experience with bufferbloat suggests that one network
> > > > > > device was primarily responsible.  Also, see slide 14 in that slide deck.
> > > > > >
> > > > > > That 1.6 seconds measured on an actual network is entirely too
> > > > > > close to 2 seconds for confidence that buffers will be
> > > > > > depleted in any
> > > > tested device.
> > > > > > Hence, the 2 second minimum duration ought to be increased by
> > > > > > at least a factor of 10.  I'd suggest changing it to 30
> > > > > > seconds or 60 seconds as convenient round numbers, and
> > > > > > providing the rationale that increased buffering in WiFi
> > > > > > devices, e.g., home "routers," as indicated by experience with
> > > > > > bufferbloat measurements, is the reason
> > > > for the
> > > > > increased duration.
> > > > > >
> > > > > [acm]
> > > > > [acm] I agree with your observation that there are cases where
> > > > > trial
> > > > duration
> > > > > should be increased to accommodate the encountered in the DUT,
> > > > > but not
> > > > as a
> > > > > mandate for all testing. I have four factors in mind:
> > > > > 1. Some of the virtual network DUTs we are testing now have very
> > > > > small
> > > > buffers,
> > > > > and the B2B stream of frames is quite short -- less than 2000
> > > > frames@10GE in
> > > > > some cases -- so 2 seconds fully sufficient.
> > > > > 2. The trial duration is a factor in total test duration, where
> > > > > each
> > > > trial is one step in
> > > > > the Binary Search. We need to manage the tension between the
> > > > > time needed
> > > > to
> > > > > reach a search result and confidence that we have depleted the
> > queues.
> > > > > 3. The RFC 2544 Latency benchmark will tell us if bufferbloat is
> > > > present.
> > > > > 4. The current text says "at least 2 seconds".
> > > > >
> > > > > So I suggest adding the following text:
> > > > >
> > > > >     The duration of the trial MUST be at least 2 seconds, to
> > > > > allow
> > DUT
> > > > >     buffers to deplete. When RFC2544 Latency measurements
> > > > > indicate
> > that
> > > > >     large buffers are present in the DUT, the trial duration
> > > > > SHOULD
> > be
> > > > >     increased to ensure that buffer depletion takes place,
> > > > > without
> > > > unduly
> > > > >     extending the total test time.
> > > > >
> > > > > I hope this suggestion resolves your issue; thanks for
> > > > > highlighting
> > it!
> > > > > Al