I spent a little longer looking at the specs more carefully, and I explained (1) incorrectly in my last two messages. P21..29 are not Limited Transmit packets.
However, unless I'm missing something else, 6675 is clear that the recovery period does not end until the cumulative ack advances, meaning that detecting the lost retransmission of P1 does not trigger another MD directly.
Thanks for this exercise! It's refreshed my memory of these details after working on slightly different QUIC algorithms a long time.
On Wed, Dec 16, 2020, 18:55 Martin Duke <martin.h.duke@xxxxxxxxx> wrote:
(1) Flightsize: in RFC 6675. Section 5, Step 4.2:(4.2) ssthresh = cwnd = (FlightSize / 2) The congestion window (cwnd) and slow start threshold (ssthresh) are reduced to half of FlightSize per [RFC5681]. Additionally, note that [RFC5681] requires that any segments sent as part of the Limited Transmit mechanism not be counted in FlightSize for the purpose of the above equation.IIUC the segments P21..P29 in your example were sent because of Limited Transmit, and so don't count. The flightsize for the purposes of (4.2) is therefore 20 after both losses, and the cwnd does not go up on the second loss.(2)" Even a single shot burst every time there is significant loss
event is not acceptable, not to mention continuous aggressiveness, and
this is exactly what RFC 2914 and RFC 5033 explicitly address and warn
about.""Significant loss event" is the key phrase here. The intent of TLP/PTO is to equalize the treatment of a small packet loss whether it happened in the middle of a burst or the end. Why should an isolated loss be treated differently based on its position in the burst? This is just a logical extension of fast retransmit, which also modified the RTO paradigm. The working group consensus is that this is a feature, not a bug; you're welcome to feel otherwise but I suspect you're in the rough here.RegardsMartinOn Wed, Dec 16, 2020 at 4:11 PM Markku Kojo <kojo@xxxxxxxxxxxxxx> wrote:Hi Martin,
See inline.
On Wed, 16 Dec 2020, Martin Duke wrote:
> Hi Markku,
>
> There is a ton here, but I'll try to address the top points. Hopefully
> they obviate the rest.
Sorry for being verbose. I tried to be clear but you actually removed my
key issues/questions ;)
> 1.
> [Markku]
> "Hmm, not sure what you mean by "this is a new loss detection after
> acknowledgment of new data"?
> But anyway, RFC 5681 gives the general principle to reduce cwnd and
> ssthresh twice if a retransmission is lost but IMHO (and I believe many
> who have designed new loss recovery and CC algorithms or implemented
> them
> agree) that it is hard to get things right if only congestion control
> principles are available and no algorithm."
>
> [Martin]
> So 6675 Sec 5 is quite explicit that there is only one cwnd reduction
> per fast recovery episode, which ends once new data has been
> acknowledged.
To be more precise: fast recovery ends when the current window becomes
cumulatively acknowledged, that is,
(4.1) RecoveryPoint (= HighData at the beginning) becomes acknowledged
I believe we agree and you meant this although new data below
RecoveryPoint may become cumulatively acknowledged already earlier
during the fast recovery. Reno loss recovery in RFC 5681 ends, when
(any) new data has been acknowledged.
> By definition, if a retransmission is lost it is because
> newer data has been acknowledged, so it's a new recovery episode.
Not sure where you have this definition? Newer than what are you
referring to?
But, yes, if a retransmission is lost with RFC 6675 algorithm,
it requires RTO to be detected and definitely starts a new recovery
episode. That is, a new recovery episode is enforced by step (1.a) of
NextSeg () which prevents retransmission if a segment that has already
been retransmitted. If RACK-TLP is used for detecting loss with RFC 6675
things get different in many ways, because it may detect loss of a
retransmission. It would pretty much require an entire redesign
of the algorith. For example, calculation of pipe does not consider
segments that have been retransmitted more than once.
> Meanwhile, during the Fast Recovery period the incoming acks implicitly
> remove data from the network and therefore keep flightsize low.
Incorrect. FlightSize != pipe. Only cumulative acks remove data from
FlightSize and new data transmitted during fast recovery inflate
FlightSize. How FlightSize evolves depends on loss pattern as I said.
It is also possible that FlightSize is low, it may err in both
directions. A simple example can be used as a proof for the case where
cwnd increases if a loss of retransmission is detected and repaired:
RFC 6675 recovery with RACK-TLP loss detection:
(contains some inaccuracies because it has not been defined how
lost rexmits are calculated into pipe)
cwnd=20; packets P1,...,P20 in flight = current window of data
[P1 dropped and rexmit of P1 will also be dropped]
DupAck w/SACK for P2 arrives
[loss of P1 detected after one RTT from original xmit of P1]
[cwnd=ssthresh=10]
P1 is rexmitted (and it logically starts next window of data)
DupAcks w/ SACK for original P3..11 arrive
DupAck w/ SACK for original P12 arrives
[cwnd-pipe = 10-9 >=1]
send P21
DupAck w/SACK for P13 arrives
send P22
...
DupAck w/SACK for P20 arrives
send P29
[FlightSize=29]
(Ack for rexmit of P1 would arrive here unless it got dropped)
DupAck w/SACK for P21 arrives
[loss of rexmit P1 detected after one RTT from rexmit of P1]
SET cwnd = ssthresh = FlightSize/2= 29/2 = 14,5
CWND INCREASES when it should be at most 5 after halving it twice!!!
> We can continue to go around on our interpretation of these documents,
> but fundamentally if there is ambiguity in 5681/6675 we should bis
> those RFCs rather than expand the scope of RACK.
As I said earlier, I am not opposing bis, though 5681bis wuold not
be needed, I think.
But let me repeat: if we publish RACK-TLP now without necessary warnings
or with a correct congesion control algorithm someone will try to
implement RACK-TLP with RFC 6675 and it will be a total mesh. The
behavior will be unpredictable and quite likely unsafe congestion
control behavior.
> 2.
> [Markku]
> " In short:
> When with a non-RACK-TLP implementation timer (RTO) expires: cwnd=1
> MSS,
> and slow start is entered.
> When with a RACK_TLP implementation timer (PTO) expires,
> normal fast recovery is entered (unless implementing
> also PRR). So no RTO recovery as explicitly stated in Sec. 7.4.1."
>
> [Martin]
> There may be a misunderstanding here. PTO is not the same as RTO, and
> both mechanisms exist! The loss response to a PTO is to send a probe;
> the RTO response is as with conventional TCP. In Section 7.3:
No, I don't think I misunderstood. If you call timeout with
another name, it is still timeout. And congestion control does not
consider which segments to send (SND.UNA vs. probe w/ higher sequence
number), only how much is sent.
You ignored my major point where I decoupled congestion control from loss
detection and loss recovery and compared RFC 5681 behavior to RACK-TLP
behavior in exactly the same scenario where an entire flight is lost and
timer expires.
Please comment why congestion control behavior is allowed to be radically
different in these two implementations?
RFC 5681 & RFC 6298 timeout:
RTO=SRTT+4*RTTVAR (RTO used for arming the timer)
1. RTO timer expires
2. cwnd=1 MSS; ssthresh=FlightSize/2; rexmit one segment
3. Ack of rexmit sent in step 2 arrives
4. cwnd = cwnd+1 MSS; send two segments
...
RACK-TLP timeout:
PTO=min(2*SRTT,RTO) (PTO used for arming the timer)
1. PTO times expires
2. (cwnd=1 MSS); (re)xmit one segment
3. Ack of (re)xmit sent in srep 2 arrives
4. cwnd = ssthresh = FlightSize/2; send N=cwnd segments
If FlightSize is 100 segments when timer expires, congestion control is
the same in steps 1-3, but in step 4 the standard congestion control
allows transmitting 2 segments, while RACK-TLP would allow
blasting 50 segments.
> After attempting to send a loss probe, regardless of whether a loss
> probe was sent, the sender MUST re-arm the RTO timer, not the PTO
> timer, if FlightSize is not zero. This ensures RTO recovery remains
> the last resort if TLP fails.
> "
This does not prevent the above RACK-TLP behavior from getting realized.
> So a pure RTO response exists in the case of persistent congestion that
> causes losses of probes or their ACKs.
Yes, RTO response exists BUT only after RACK-TLP at least once blasts the
network. It may well be that with smaller windows RACK-TLP is successful
during its TLP initiated overly aggressive "fast recovery" and never
enters RTO recovery because it may detect and repair also loss of
rexmits. That is, it continues at too high rate even if lost rexmits
indicate that congestion persists in successive windows of data. And
worse, it is successful because it pushes away other compatible TCP
flows by being too aggressive and unfair.
Even a single shot burst every time there is significant loss
event is not acceptable, not to mention continuous aggressiveness, and
this is exactly what RFC 2914 and RFC 5033 explicitly address and warn
about.
Are we ignoring these BCPs that have IETF consensus?
And the other important question I'd like to have an answer:
What is the justification to modify standard TCP congestion control to
use fast recovery instead of slow start for a case where timeout is
needed to detect the packet losses because there is no feedback and ack
clock is lost? RACK-TLP explicitly instructs to do so in Sec. 7.4.1.
As I noted: based on what is written in the draft it does not intend to
change congestion control but effectively it does.
/Markku
> Martin
>
>
> On Wed, Dec 16, 2020 at 11:39 AM Markku Kojo <kojo@xxxxxxxxxxxxxx>
> wrote:
> Hi Martin,
>
> On Tue, 15 Dec 2020, Martin Duke wrote:
>
> > Hi Markku,
> >
> > Thanks for the comments. The authors will incorporate
> many of your
> > suggestions after the IESG review.
> >
> > There's one thing I don't understand in your comments:
> >
> > " That is,
> > where can an implementer find advice for correct
> congestion control
> > actions with RACK-TLP, when:
> >
> > (1) a loss of rexmitted segment is detected
> > (2) an entire flight of data gets dropped (and detected),
> > that is, when there is no feedback available and a
> timeout
> > is needed to detect the loss "
> >
> > Section 9.3 is the discussion about CC, and is clear that
> the
> > implementer should use either 5681 or 6937.
>
> Just a cite nit: RFC 5681 provides basic CC concepts and
> some useful CC
> guidelines but given that RACK-TLP MUST implement SACK the
> algorithm in
> RFC 5681 is not that useful and an implementer quite likely
> follows
> mainly the algorithm in RFC 6675 (and not RFC 6937 at all
> if not
> implementing PRR).
> And RFC 6675 is not mentioned in Sec 9.3, though it is
> listed in the
> Sec. 4 (Requirements).
>
> > You went through the 6937 case in detail.
>
> Yes, but without correct CC actions.
>
> > If 5681, it's pretty clear to me that in (1) this is a
> new loss
> > detection after acknowledgment of new data, and therefore
> requires a
> > second halving of cwnd.
>
> Hmm, not sure what you mean by "this is a new loss
> detection after
> acknowledgment of new data"?
> But anyway, RFC 5681 gives the general principle to reduce
> cwnd and
> ssthresh twice if a retransmission is lost but IMHO (and I
> believe many
> who have designed new loss recovery and CC algorithms or
> implemented them
> agree) that it is hard to get things right if only
> congestion control
> principles are available and no algorithm.
> That's why ALL mechanisms that we have include a quite
> detailed algorithm
> with all necessary variables and actions for loss recovery
> and/or CC
> purposes (and often also pseudocode). Like this document
> does for loss
> detection.
>
> So the problem is that we do not have a detailed enough
> algorithm or
> rule that tells exactly what to do when a loss of rexmit is
> detected.
> Even worse, the algorithms in RFC 5681 and RFC 6675 refer
> to
> equation (4) of RFC 5681 to reduce ssthresh and cwnd when a
> loss
> requiring a congestion control action is detected:
>
> (cwnd =) ssthresh = FlightSize / 2)
>
> And RFC 5681 gives a warning not to halve cwnd in the
> equation but
> FlightSize.
>
> That is, this equation is what an implementer intuitively
> would use
> when reading the relevant RFCs but it gives a wrong result
> for
> outstanding data when in fast recovery (when the sender is
> in
> congestion avoidance and the equation (4) is used to halve
> cwnd, it
> gives a correct result).
> More precisely, during fast recovery FlightSize is inflated
> when new
> data is sent and reduced when segments are cumulatively
> Acked.
> What the outcome is depends on the loss pattern. In the
> worst case,
> FlightSize is signficantly larger than in the beginning of
> the fast
> recovery when FlightSize was (correctly) used to determine
> the halved
> value for cwnd and ssthresh, i.e., equation (4) may result
> in
> *increasing* cwnd upon detecting a loss of a rexmitted
> segment, instead
> of further halving it.
>
> A clever implementer might have no problem to have it right
> with some
> thinking but I am afraid that there will be incorrect
> implementations
> with what is currently specified. Not all implementers have
> spent
> signicicant fraction of their career in solving TCP
> peculiarities.
>
> > For (2), the RTO timer is still operative so
> > the RTO recovery rules would still follow.
>
> In short:
> When with a non-RACK-TLP implementation timer (RTO)
> expires: cwnd=1 MSS,
> and slow start is entered.
> When with a RACK_TLP implementation timer (PTO) expires,
> normal fast recovery is entered (unless implementing
> also PRR). So no RTO recovery as explicitly stated in Sec.
> 7.4.1.
>
> This means that this document explicitly modifies standard
> TCP congestion
> control when there are no acks coming and the
> retransmission timer
> expires
>
> from: RTO=SRTT+4*RTTVAR (RTO used for arming the timer)
> 1. RTO timer expires
> 2. cwnd=1 MSS; ssthresh=FlightSize/2; rexmit one
> segment
> 3. Ack of rexmit sent in step 2 arrives
> 4. cwnd = cwnd+1 MSS; send two segments
> ...
>
> to: PTO=min(2*SRTT,RTO) (PRO used for arming the timer)
> 1. PTO times expires
> 2. (cwnd=1 MSS); (re)xmit one segment
> 3. Ack of (re)xmit sent in srep 2 arrives
> 4. cwnd = ssthresh = FlightSize/2; send N=cwnd
> segments
>
> For example, if FlightSize is 100 segments when timer
> expires,
> congestion control is the same in steps 1-3, but in step 4
> the
> current standard congestion control allows transmitting 2
> segments,
> while RACK-TLP would allow blasting 50 segments.
>
> Question is: what is the justification to modify standard
> TCP
> congestion control to use fast recovery instead of slow
> start for a
> case where timeout is needed to detect loss because there
> is no
> feedback and ack clock is lost? The draft does not give any
> justification. This clearly is in conflict with items (0)
> and (1)
> in BCP 133 (RFC 5033).
>
> Furthermore, there is no implementation nor experimental
> experience
> evaluating this change. The implementation with
> experimental experience
> uses PRR (RFC 6937) which is an Experimental specification
> including a
> novel "trick" that directs PRR fast recovery to effectively
> use slow
> start in this case at hand.
>
>
> > In other words, I am not seeing a case that requires new
> congestion
> > control concepts except as discussed in 9.3.
>
> See above. The change in standard congestion control for
> (2).
> The draft intends not to change congestion control but
> effectively it
> does without any operational evidence.
>
> What's also is missing and would be very useful:
>
> - For (1), a hint for an implementer saying that because
> RACK-TLP is
> able to detect a loss of a rexmit unlike any other loss
> detection
> algorithm, the sender MUST react twice to congestion
> (and cite
> RFC 5681). And cite a document where necessary correct
> actions
> are described.
>
> - For (1), advise that an implementer needs to keep track
> when it
> detects a loss of a retransmitted segment. Current
> algorithms
> in the draft detect a loss of retransmitted segment
> exactly in
> the same way as loss of any other segment. There seems
> to be
> nothing to track when a retransmission of a
> retransmitted segment
> takes place. Therefore, the algorithms should have
> additional
> actions to correctly track when such a loss is detected.
>
> - For (1), discussion on how many times a loss of a
> retransmission
> of the same segment may occur and be detected. Seems
> that it
> may be possible to drop a rexmitted segment more than
> once and
> detect it also several times? What are the
> implications?
>
> - If previous is possible, then the algorithm possibly also
> may detect a loss of a new segment that was sent during
> fast
> recovery? This is also loss in two successive windows of
> data,
> and cwnd MUST be lowered twice. This discussion and
> necessary
> actions to track it are missing, if such scenario is
> possible.
>
> > What am I missing?
>
> Hope the above helps.
>
> /Markku
>
>
> <snipping the rest>
>
>
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