Re: [tcpm] CUBIC rfc8312bis / WGLC Issue 2

[Neal Cardwell <[e-mail]>]

Hi Markku and TCPMers,

My understanding of Markku's concern here is that in slow start the cwnd
can continue to grow in response to ACKs after the lost packet was sent, so
that the cwnd is often twice the level of in-flight data at which the loss
happened, by the time the loss is detected. So the cwnd ends up at 2 * 0.7
= 1.4x the level at which losses happened, which causes an unnecessary
follow-on round with losses, in order to again cut the cwnd, this time
to 1.4 * 0.7 = 0.98x of the level that causes losses, which is likely to
finally fit in the network path.

However, there are two technical issues with this concern, as expressed in
the proposed draft text in this thread:

(1) The analysis for slow-start is not correct for the very common case
where the flow is application-limited in slow-start, in which case the cwnd
would not grow at all between the packet loss and the time the loss is
detected. So the text is needlessly strict in this case.

(2) For CUBIC the problematic dynamic (of cwnd growth between loss and loss
detection exceeding the multiplicative decrease) can also occur outside of
slow-start, in congestion avoidance. The CUBIC cwnd growth in congestion
avoidance can be up to 1.5x per round trip. So after a packet loss the cwnd
could grow by 1.5x before loss detection and then be cut in response to
loss by 0.7, causing the ultimate cwnd to be 1.5 * 0.7 = 1.05x the volume
of in-flight data at the time of the packet loss. This would likely cause
an unnecessary follow-on round of packet loss due to failing to cut cwnd
below the level that caused loss. So the problem is actually wider than
slow-start.

AFAICT a complete/general fix for this issue is best solved by recording
the volume of inflight data at the point of each packet transmission, and
then using that metric as the baseline for the multiplicative decrease when
packet loss is detected, rather than using the current cwnd as the
baseline. This is the approach that BBRv2 uses. Perhaps there are other,
simpler approaches as well.

I also agree with Vidhi's concern, that a change to the multiplicative
decrease changes the algorithm substantially. To ensure that the draft/RFC
is not recommending something that has unforeseen significant negative
consequences, we shouldn't make such a significant change to the text until
we get experience w/ the new variation.

best regards,
neal


On Tue, Jul 12, 2022 at 6:08 PM Vidhi Goel <vidhi_goel=
[e-mail]> wrote:

> Hi Markku,
>
> I emailed about this to other co-authors and we think that this change is
> completely untested for Cubic and we think that this could be considered of
> a future version of Cubic, not the current rfc8312bis.
> To change Beta from 0.7 to 0.5 during slow-start, we would at least need
> some experience either from lab testing or deployment since all current
> deployments of Cubic for both TCP and QUIC use 0.7 as Beta during slow
> start. Since a lot of implementations currently use hystart(++) along with
> Cubic, we don’t see any high risk of overaggressive sending rate and that
> is what the current rfc8312bis suggests as well. In fact, changing Beta
> from 0.7 to 0.5 can still be aggressive without using hystart.
>
> Thanks,
> Vidhi
>
> > On Jul 11, 2022, at 5:55 PM, Markku Kojo <kojo=
> [e-mail]> wrote:
> >
> > Hi all,
> >
> > below please find proposed text to solve the Issue 2 a). I will propose
> text to solve 2 b) once we have come to conclusion with 2 a). For
> description and arguments for issues 2 a) and 2 b), please see the original
> issue descriptions below.
> >
> > Sec 4.6. Multiplicative Decrease
> >
> > Old:
> >   The parameter Beta__cubic_ SHOULD be set to 0.7, which is different
> >   from the multiplicative decrease factor used in [RFC5681] (and
> >   [RFC6675]) during fast recovery.
> >
> >
> > New:
> >   If the sender is not in slow start when the congestion event is
> >   detected, the parameter Beta__cubic_ SHOULD be set to 0.7, which
> >   is different from the multiplicative decrease factor used in
> >   [RFC5681] (and [RFC6675].
> >   This change is justified in the Reno-friendly region during
> >   congestion avoidance because a CUBIC sender compensates the higher
> >   multiplicative decrease factor than that of Reno by applying
> >   a lower additive increase factor during congestion avoidance.
> >
> >   However, if the sender is in slow start when the congestion event is
> >   detected, the parameter Beta__cubic_ MUST be set to 0.5 [Jacob88].
> >   This results in the sender continuing to transmit data at the maximum
> >   rate that the slow start determined to be available for the flow.
> >   Using Beta__cubic_ with a value larger than 0.5 when the congestion
> >   event is detected in slow start would result in an overagressive send
> >   rate where the sender injects excess packets into the network and
> >   each such packet is guaranteed to be dropped or force a packet from
> >   a competing flow to be dropped at a tail-drop bottleneck router.
> >   Furthermore, injecting such undelivered packets creates a danger of
> >   congestion collapse (of some degree) "by delivering packets through
> >   the network that are dropped before reaching their ultimate
> >   destination." [RFC 2914]
> >
> >
> >   [Jacob88] V. Jacobson, Congestion avoidance and control, SIGCOMM '88.
> >
> > Thanks,
> >
> > /Markku
> >
> > On Tue, 14 Jun 2022, Markku Kojo wrote:
> >
> >> Hi all,
> >>
> >> this thread starts the discussion on the issue 2: CUBIC is specified to
> use incorrect multiplicative-decrease factor for a congestion event that
> occurs when operating in slow start. And, applying HyStart++ does not
> remove the problem, it only mitigates it in some percentage of cases.
> >>
> >> I think it is useful to discuss this in two phases: 2 a) and 2 b) below.
> >> For anyone commenting/arguing on the part 2 b), it is important to first
> >> acknowledge whether (s)he thinks the original design and logic by Van
> Jacobson is correct. If not, one should explain why Van's design logic is
> incorrect.
> >>
> >> Issue 2 a)
> >> ----------
> >>
> >> To begin with, let's but aside a potential use of HyStart++ (also
> assume tail drop router unless otherwise mentioned).
> >>
> >> The use of an MD factor larger than 0.5 is against the theory and
> original design by Van Jacobson as explained in the congavoid paper
> [Jacob88]. Any MD factor value larger then 0.5 will result sending extra
> packets during Fast Recovery following the congestion event (drop). All
> extra packets will become dropped at a tail-drop bottleneck (if a lonely
> flow).
> >>
> >> Note that at the time when the drop becomes signalled at the TCP
> sender, the size of the cwnd is double the available network capacity that
> slow start determined for the flow. That is, using MD=0.5 is already as
> aggressive as possible, leaving no slack. Therefore, if MD=0.7 is used, the
> TCP sender enters fast recovery with cwnd that is 40% larger that the
> determined network capacity and all excess packets are guaranteed to become
> dropped, or even worse, the excess packets are likely to force packets for
> any competing flows to become unfairly be dropped.
> >>
> >> Moreover, if NewReno loss recovery is in use, a CUBIC sender will
> >> operate overagressively for a very long time. For example, if the
> >> available network capacity for the flow is 100 packets, cwnd will have
> >> value 200 when the congestion is signalled and the CUBIC sender enters
> >> fast recovery with cwnd=140 and injects 40 excess packets for each of
> >> the subsequent 100 RTTs it stays in fast recovery, forcing 4000 packets
> to become inevitably and totally unnecessarily dropped.
> >>
> >> Even worse, this behaviour of sending 'undelivered packets' is against
> >> the congestion control principles as it creates a danger of congestion
> >> collapse (of some degree) "by delivering packets through the network
> >> that are dropped before reaching their ultimate destination." [RFC 2914]
> >>
> >> Such undelivered packets unnecessarily eat capacity from other flows
> >> sharing the path before the bottleneck.
> >>
> >> RFC 2914 emphasises:
> >>
> >> "This is probably the largest unresolved danger with respect to
> >> congestion collapse in the Internet today."
> >>
> >> It is very easy to envision a realistic network setup where this
> creates a degree of congestion collapse where a notable portion of useful
> network capacity is wasted due to the undelivered packets.
> >>
> >>
> >> [Jacob88] V. Jacobson, Congestion avoidance and control, SIGCOMM '88.
> >>
> >>
> >> Issue 2 b)
> >> ----------
> >>
> >> The CUBIC draft suggests that HyStart++ should be used *everywhere*
> instead of the traditional Slow Start (see section 4.10).
> >>
> >> Although the draft does not say it, seemingly the authors suggest using
> HyStart++ instead of traditional Slow Start in order to avoid the problem
> of over-aggressive behaviour discussed above. This, however, has several
> issues.
> >>
> >> First. it is directly in conflict with HyStart++ specification which
> says that HyStart++ should be used only for the initial Slow Start.
> However, the overaggressive behaviour after slow start is also a potential
> problem with slow start during an RTO recovery; in case of sudden
> congestion that reduces available capacity for a flow down to a fraction of
> the currently available capacity, it is very likely that an RTO occurs. In
> such a case the RTO recovery in slow start inevitably overshoots and it is
> crucial for all flows not to be overaggressive.
> >>
> >> Second, the experimental results for initial slow start in HyStart++
> draft suggest that while HyStart++ achieves good results HyStart++ is
> unable to exit slow start early and avoid overshoot in a significant
> percentage of cases.
> >>
> >> Given the above issues, the CUBIC draft must require that MD of 0.5 is
> used when the congestion event occurs while the sender is (still) in slow
> start. The use of MD=0.5 is an obvious stumble in the original CUBIC and
> the original CUBIC authors have already acknowledged this. It seems also
> obvious that instead of correcting the actual problem (use of MD other than
> 0.5), HyStart and HyStart++ have been proposed to address the design
> mistake. While HyStart++ is a useful method also when used with MD=0.5,
> when used alone it only mitigates the impact of the actual problem rather
> than solves the problem.
> >>
> >> What should be done for the cases where HyStart++ exits slow start but
> >> is not able to avoid (some level of) overshoot and dropped packets is
> IMO an open issue. Resolving it requires additional experiments and it
> should be resolved separately when we have more data. For now when we do
> not have enough data and understanding of the behaviour we should IMO
> follow the general IETF guideline "be conservative in what you send" and
> specify that MD = 0.5 should be used for a congestion event that occurs for
> a packet sent in slow start.
> >>
> >> Thanks,
> >>
> >> /Markku
> >>
> >
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