Re: [tcpm] [EXTERNAL] Re: Last Call: <draft-ietf-tcpm-rack-13.txt>(TheRACK-TLPlossdetectionalgorithmforTCP) to Proposed Standard

[Yuchung Cheng <[e-mail]>]

On Thu, Dec 17, 2020 at 1:59 PM Martin Duke <[e-mail]> wrote:

> This is good, thanks.
>
> I might also add the sentence "In the absence of PRR, when TCP RACK
> detects a lost retransmission it MUST trigger a second congestion response"
> or something to that effect. What do you think?
>
Sure I'd revise a bit to be even more detailed and consistent with the
principle in RFC5681

"In the absence of PRR, when RACK-TLP detects a lost retransmission the
congestion control MUST trigger an additional congestion response per the
aforementioned principle in RFC5681.
If multiple retransmissions were lost in a window, the congestion control
specified in RFC5681 only reacts once per window, but some other congestion
control may want to respond multiple times differently. The congestion
control implementer is advised to carefully consider this subtle situation."



> On Thu, Dec 17, 2020 at 11:36 AM Yuchung Cheng <[e-mail]> wrote:
>
>> How about
>>
>> "9.3.  Interaction with congestion control
>>
>> RACK-TLP intentionally decouples loss detection ...
>> As mentioned in Figure 1 caption, RFC5681 mandates a principle that
>> Loss in two successive windows of data, or the loss of a
>> retransmission, should be taken as two indications of congestion, and
>> therefore reacted separately. However implementation of RFC6675 pipe
>> algorithm may not directly account for this newly detected congestion
>> events properly. PRR [RFCxxxx] is RECOMMENDED for the specific
>> congestion control actions taken upon the losses detected by RACK-TLP"
>>
>>
>> To Makku's request for "what's the justification to enter fast recovery".
>> Consider this example w/o RACK-TLP
>>
>> T0: Send 100 segments but application-limited. All are lost.
>> T-2RTT: app writes so another 3 segments are sent. Made to the
>> destination and triggered 3 DUPACKs
>> T-3RTT: 3 DUPACK arrives. start fast recovery and subsequent cc reactions
>> to burst ~50 packets with Reno
>>
>> In this case any ACK occured before RTO is (generally) considered
>> clock-acked, and how I understand Van's initial design.  This behavior
>> existed decades before RACK-TLP. RACK-TLP essentially changes the
>> "3-segments by app" to "1-segment by tcp".
>>
>> On Thu, Dec 17, 2020 at 10:52 AM Praveen Balasubramanian <
>> [e-mail]> wrote:
>>
>>> I agree that we should have a note in this RFC about congestion control
>>> action upon detecting lost retransmission(s).
>>>
>>>
>>>
>>> *From:* tcpm <[e-mail]> *On Behalf Of * Martin Duke
>>> *Sent:* Thursday, December 17, 2020 7:30 AM
>>> *To:* Markku Kojo <[e-mail]>
>>> *Cc:* [e-mail] Extensions <[e-mail]>;
>>> [e-mail]; Michael Tuexen <[e-mail]>;
>>> [e-mail]; Last Call <[e-mail]>;
>>> tcpm-chairs <[e-mail]>
>>> *Subject:* [EXTERNAL] Re: [tcpm] Last Call:
>>> <draft-ietf-tcpm-rack-13.txt>(TheRACK-TLPlossdetectionalgorithmforTCP) to
>>> Proposed Standard
>>>
>>>
>>>
>>> Hi Markku,
>>>
>>>
>>>
>>> Thanks, now I understand your objections.
>>>
>>>
>>>
>>> Martin
>>>
>>>
>>>
>>> On Thu, Dec 17, 2020 at 12:46 AM Markku Kojo <[e-mail]>
>>> wrote:
>>>
>>> Hi,
>>>
>>> On Wed, 16 Dec 2020, Martin Duke wrote:
>>>
>>> > 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.
>>>
>>> Correct. Just normal the rule that allows sending new data during fast
>>> recovery.
>>>
>>> > 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.
>>>
>>> As I have said earlier, RFC 6675 does not repeat all congestion control
>>> principles from RFC 5681. It definitely honors the CC principle that
>>> requires to treat a loss of a retransmission as a new congestion
>>> indication and another MD. I believe I am obligated to know this as a
>>> co-author of RFC 6675. ;)
>>>
>>> RFC 6675 explicitly indicates that it follows RFC 5681 by stating in the
>>> abstract:
>>>
>>> " ... conforms to the spirit of the current congestion control
>>>   specification (RFC 5681 ..."
>>>
>>> And in the intro:
>>>
>>>    "The algorithm specified in this document is a straightforward
>>>     SACK-based loss recovery strategy that follows the  guidelines
>>>     set in [RFC5681] ..."
>>>
>>> I don't think there is anything unclear in this.
>>>
>>> RFC 6675 and all other standard congestion controls (RFC 5581 and RFC
>>> 6582) handle a loss of a retransmission by "enforcing" RTO to detect it.
>>> And RTO guarantees MD. RACK-TLP changes the loss detection in this case
>>> and therefore the standard congestion control algorithms do not have
>>> actions to handle it corrrectly. That is the point.
>>>
>>> BR,
>>>
>>> /Markku
>>>
>>> > 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 <[e-mail]>
>>> 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.
>>> >
>>> > Regards
>>> > Martin
>>> >
>>> >
>>> > On Wed, Dec 16, 2020 at 4:11 PM Markku Kojo <[e-mail]>
>>> 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
>>> >       <[e-mail]>
>>> >       > 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>
>>> >       >
>>> >       >
>>> >
>>> >
>>> >
>>>
>>>