Re: [tcpm] draft-ietf-tcpm-accurate-ecn-24 addressing all WGLC comments

[[e-mail]]

> On 5. Jun 2023, at 10:33, Yoshifumi Nishida <[e-mail]> wrote:
> 
> Hi folks,
> 
> I just would like to put my personal views on ack on ack discussions here.
> First, I think ack on ack has already has some precedences. 
> keep-alive logic can sent a pure ack for proving and expect an ack for it. 
> MPTCP uses 4 WHS and the 4th segment is an ack for the third ACK which is a pure ACK.
> So, I'm not sure if we need to define some rules for ack-on-ack in the doc.
> 
> Also, I'm a bit hesitant to define a detailed logic about how to distinguish an ack that carries ECN signals and a dup ack, such as using TS or SACK blocks.
> I think such things are the part of experiments and should be described in other docs such as ECN++ or ackcc, etc. 
> I personally prefer the doc simply describes the possibilities of such mechanisms and provides general principles and guidelines. 
> 
> As far as I think, the Markku's examples that triggers false retransmissions makes sense. I think they are good examples to show how ack-on-ack can be tricky.
> However, these examples are the case where both sides exchanges data simultaneously and I think there're other cases where we don't have to worry about it. 
> For example, I think bulk transfer or request-response type traffic can be these examples. 
> In these cases, the endpoint which receives acks for acks doesn't have outstanding data. Hence, although these acks are duplicate acks, they won't trigger retransmissions.
> 
> So, I am thinking that it would be good to provide a certain guideline about when to enable this feature and potential risks in some docs.
> But, I am also thinking we should do it outside of accecn doc.
I agree that the ACK of ACK scenario is something which relates to the
experimental ECN++ document.

However, I checked again draft-ietf-tcpm-accurate-ecn-24 and found at the
paragraph right before Section 1.1:

It is RECOMMENDED that the AccECN protocol is implemented alongside
SACK [RFC2018] and the experimental ECN++ protocol
[I-D.ietf-tcpm-generalized-ecn], which allows the ECN capability to
be used on TCP control packets.

Would using I-D.ietf-tcpm-generalized-ecn in combination with RECOMMENDED
not required the ID to be a normative reference? I couldn't find clear rules,
but in my view I could not argument against it. Since the intended status
of I-D.draft-ietf-tcpm-accurate-ecn is PS and the intended status of
I-D.ietf-tcpm-generalized-ecn is Experiemental, this would be a downref.

Is this really intended by the authors or just a leftover?

Best regards
Michael
> 
> Thanks,
> --
> Yoshi
> 
> 
> On Fri, May 26, 2023 at 2:56 PM Markku Kojo <[e-mail]> wrote:
> Bob,
> 
> My apologies you had to wait for the scenarios as it took much longer 
> with my limited cycles than I thought. Anyways, please see my reply to 
> Richard, some scenarios are also included there.
> 
> To keep things easier, it might be good to try to keep the discussion on 
> Acks of Acks (mainly) in the thread with my reply to Richard.
> 
> However, see inline tagged [MK].
> 
> On Wed, 24 May 2023, Bob Briscoe wrote:
> 
> > Markku,
> > 
> > Sorry, it's taken a week to build a comprehensive reply to this long email. See inline tagged
> > [BB]...
> > 
> > On 17/05/2023 12:24, Markku Kojo wrote:
> >       Hi Michael, all,
> >
> >       On Sun, 14 May 2023, [e-mail] wrote:
> >
> >                   On 30. Mar 2023, at 16:53, Bob Briscoe <[e-mail]>
> >                   wrote:
> >
> >                   Michael, Yoshi, Ian (as tcpm chairs),
> >
> >                   To close off the WGLC, I have just posted a new rev of
> >                   accurate-ecn. Hyperlinks quoted at the end.
> >                   You will see the diff is rather extensive. I won't give a
> >                   summary of all the diffs like I usually do. Instead I can just
> >                   refer to the summary I gave in the presentation on Monday:
> >                   https://datatracker.ietf.org/meeting/116/materials/slides-116-tcpm-draft-ietf-tcpm-accurate-ecn
> >
> >                   Thank you again to the people who reviewed this during the WGLC:
> >                   Michael Tüxen, Alex Burr, Gorry Fairhurst and Markku Kojo.
> >
> >                   All changes are editorial, apart from removing the para about
> >                   not mistaking certain ACKs of ACKs for DupACKs, which I will add
> >                   to a rev of the ECN++ draft, hopefully later this week.
> >
> >                   On the list, we have seen agreement from all the reviewers to
> >                   these changes, except no response from Markku yet.
> >                   On Monday, I told Markku that I would post the draft in a few
> >                   days, so everyone can see the updates and diff.
> >
> >             Anyone having additional comments? In particular Markku regarding loss
> >             recovery?
> > 
> >
> >       My apologies for being late with my reply to the author's comments on my review (I've
> >       been extremly busy with other issues since the wg mtng in Yokohama, including the rest
> >       of mtng week).
> >
> >       I don't have much new comments but it seems that my major concern regarding the problem
> >       of sending ACKs of ACKs was not fully understood.
> >
> >       The first thing where I think I was not quite clear is that the major problem with ACKs
> >       of ACKs is not that a pure Ack is made ECN-capable. Instead, the problem is in
> >       generating an Ack of an pure Ack and that is what one should prohibit to avoid problems.
> >       I understand that it might be problematic to formulate rules whether generating an Ack
> >       of an Ack is allowed (and when), instead of just disabling sending ECN-capable ACKs.
> >       I don't have a strong opinion which way the problems with ACKs of ACKs is avoided as
> >       long as they are avoided.
> > 
> > 
> > [BB] See later after your similar point (following your 'Why?' heading)...
> > 
> >
> >       I am preparing a few scenarios to illustriate the problems ACKs of ACks raise and will
> >       send them shortly once I have formulated a more thorough reasoning why sending ACKs of
> >       ACKs is not really a good idea and even seems to be unnecessary in most if not all
> >       cases, i.e., it just results in sending unnecessary packets with not much useful effect
> >       but creates a notable number of problems.
> > 
> > 
> > [BB] Having waited this long, it's rather disappointing to still hear you say "I have an argument,
> > but I'll tell you later."
> 
> [MK] I understand. My sincere apologies again.
> 
> >       It also seems not have been carefully enough considered in terms of the very basic
> >       rubustness principle of "be conservative in what you send ..."
> > 
> > 
> > [BB] The WG has been careful to ensure that ACKs of ACKs are unambiguous (cannot be mistaken for a
> > DupACK), which is what the robustness principle requires. It's just that you think we have missed
> > cases where they will be ambiguous. If you think that, we need to hear them all. 
> > 
> > The robustness principle does not advocate sending nothing just in case some unknown factor might
> > make it ambiguous. Especially given /not/ feeding back congestion notifications has potential to
> > cause harm to others. Also, "no feedback" is much more ambiguous.
> 
> [MK] Please see my reply to Richard and let's continue from there. See 
> also what I meant with "be conservative in what you send ...", that is, 
> in context of CC: avoid sending unnecessary packets or be careful in 
> sending packets just because they might be sometimes useful, send the 
> monly when they are useful.
> 
> >       Given that this draft is intended to become a stds track RFC I am concerned of any text
> >       in this document that indicates (or even hints) that TCP could acknowledge pure ACKS
> >       (this holds particularly the rules and text in Sec 3.2.2.5.1 for the
> >       "Increment-Triggered ACKs"). If it is seen necessary that this doc should have such
> >       pieces of rules and text, I am fine if any such text is moved to an appendix as long as
> >       the appendix makes it cristal clear that the text is valid only in case one is
> >       implementing an experiment as per [I-D.ietf-tcpm-generalized-ecn].
> > 
> > 
> > [BB] See point below about "Generic (Mechanistic) Reflector".
> > 
> >
> >       Why?
> >
> >       1) It is well known that TCP does not acknowledge ACKs and Standards track TCP has not
> >       been specified to acknowledge ACKs. This means that a reader/implementer of this doc
> >       cannot correctly understand the rule for "Increment-Triggered ACKs" unless there is a
> >       normative reference to a spec that specifies ACKs of ACKs (or tells that it is even
> >       possible).
> > 
> > 
> > [BB] ACKs of ACKs can indeed be tricky. But there's no need to consider not ACKing ACKs as an
> > architectural principle. Not Acking ACKs on principle certainly avoids some tricky problems.
> > However, we have a new situation here where, in limited circumstances, ACKs of ACKs are necessary.
> > So the WG has already worked through the tricky problems and they have been addressed in the draft
> > (e.g. mistaking ACKs of ACKs for DupACks, infinite ping-pong, etc). We'll discuss below whether
> > you've found some more trickiness.
> 
> (MK] I did not mean to refer to any principle but, as I said, that a 
> reader/implementer cannot correctly understand the rule for 
> "Increment-Triggered ACKs" because it is well-known to her/him that TCP 
> does not Ack ACKs. This fact is that one can ack ACKs is not specified in 
> this doc nor does this doc give a (normative) reference where it is 
> specified, including the details on which TSecr value to add or which 
> SACK info if any to include when acking a pure Ack. It is easy to 
> misinterpret the "Increment-Triggered ACKs", if one doesn't realize that 
> pure Acks may be acked.
> 
> > What is the new situation?
> >  *  Until ECN was introduced, TCP ACKs only acknowledged data. So there was no need to acknowledge
> >     pure ACKs, which contain no data.
> >  *  When ECN was introduced in RFC3168, TCP ACKs also acknowledged ECN markings. However, because
> >     RFC3168 precluded pure ACKs from being ECN-capable, there was still no need to acknowledge pure
> >     ACKs.
> >  *  RFC5690, and now the ECN++ draft introduce the possibility of ECN-capable pure ACKs. So, in the
> >     limited circumstances described in the AccECN draft, ECN-capable pure ACKs now need to be
> >     acknowledged, because they contain new information - their ECN field.
> > Similarly, even though the final ACK of TCP's 3WHS is an ACK of an ACK , it is sent because it is
> > needed (to prove that the SYN wasn't from a spoofed address).
> 
> [MK] All otherwise clear, but I disagree that the final ACK of TCP's 3WHS 
> is an ACK of an ACK. It is required because SYNACK contains control data 
> that eats one sequence number, i.e., it advances RCV.NXT at the client 
> end and when the ACK arrives at the server it is needed to advance 
> SND.UNA. Very different from Acks of Acks in this draft.
> 
> > It is true that not ACKing ACKs is well-known. However, whether it's well-known as a /principle/, or
> > just as a current /feature/ of TCP is not clear. Anyway, the IETF's job is to update RFCs that are
> > "well-known". We don't have to jump through any special procedural hoops to do something different
> > from what is "well-known". Even if it were prohibited in a stds track RFC, we just have to specify
> > what has to be done instead; in another stds track RFC.
> 
> [MK] Again, I didn't mention it as a /principle/ but as a crucial 
> piece of information that the reader needs to be noted, that is, the 
> things are now different from what is well-known.
> 
> Sure IETF's job is to update RFCs, but if one changes what is prohibited 
> in a stds track RFC, one needs to understand the consequences and explain 
> them as well as give the justification why the change can be done (without 
> problems), instead of just specifying the change.
> 
> > If there are any tutorials, course notes or text books out there that say that not ACKing ACKs is a
> > well-known principle, that's not the IETF's problem. It is the job of the tutors, lecturers and text
> > book authors who wrote those materials to update them.
> > 
> >
> >       2) ACKs of ACKs tend to trigger duplicate Acks. There are tons of algorithms that rely
> >       on the packet conservation principle and the fact that TCP never injects a dupAck unless
> >       a *data* packet has arrived and left the network. This is enforced with "MUST NOT" in
> >       RFC 5681, Sec 4.2, because not conforming to this rule makes any algorithm that rely on
> >       the rule to work incorrectly. These algorithms include (triggering) Fast Retransmit,
> >       (controlling packet rate during) Fast Recovery, (detecting spurious RTOs in) F-RTO,
> >       (calculating PipeAck in) RFC 7661, (calculating DeliveredData in) PRR, etc. Furthermore,
> >       it would make imposible to come up with any new algorihms that rely on this important
> >       basic rule. In most cases such extra dupAcks make these algorithms too aggressive
> >       because any extra dupAck is likely to inject extra packet(s) to the network.
> >
> >       So, it should be cristal clear that without SACK (or Timestamps) a TCP *MUST NOT* send
> >       ACKs of ACKs.
> > 
> > 
> > [BB] Constraining the /Data Receiver/ as you propose would create an interop problem.
> > Explanation: Consider host A and B are not using SACK or timestamps. Nonetheless, with your
> > approach, host A can still send ECN-capable pure ACKs to host B. Then, your rule puts host B in an
> > impossible position, where it gets congestion notifications on ECN-capable pure ACKs, but it is not
> > allowed to send any feedback about them.
> >
> > Instead, if neither timestamps nor SACK are in use for the connection, we need to constrain the
> > /Data Sender/ of a half connection from sending ECN-capable ACKs in the first place. This is the
> > approach the WG has adopted in the AccECN and ECN++ specs.
> 
> [MK] I think I said in the beginning that I have no strong opinion which 
> way Acks of Acks are disabled. However, I apologize that I didn't explain 
> why I phrased MUST NOT send ACKs of ACKs. This is because it might be 
> still useful to allow CE-marked pure Acks and take care of Ack CC by some 
> other means than Acks of Acks. Currently the draft mandates Acks of Acks 
> as the only way to report Ack congestion and I think it is too 
> restrictive in a stds track doc, e.g., it rules out reducing Ack rate 
> simply by reducing data send rate which would solve the interop problem in 
> a very simple way. Moreover, When B gets congestion notifications on 
> ECN-capable pure ACKs, not sending Acks of Acks does not prevent sending 
> feedback; such feedback need not to be delivered immediately but by the 
> time needed. Please see more on this in my reply to Richard.
> 
> > Specifically:
> >  *  The WG makes sure that RFCs about the /Data Sender/ of a half connection (e.g. the ECN++
> >     experiment or other future RFCs) specify that sending ECN-capable pure ACKs is conditional on
> >     having another way to distinguish DupACKs, e.g. negotiating SACK or timestamps (and I will
> >     respond to your later points on the details of these).
> >  *  The AccECN spec (which primarily specifies the feedback behaviour of a /Data Receiver/ in a
> >     half-connection) then only needs to define the Increment-triggered ACK rule.
> > The two together lead to the same outcome you want. But without the interop hole of your approach.
> > 
> > This is consistent with the "Generic (Mechanistic) Reflector" approach of the AccECN spec which
> > says:
> > "AccECN is designed to be a generic reflector of whatever ECN markings it sees, whether or not they
> > are compliant with a current standard."
> > 
> > These ACKs of ACKs are generically necessary to feed back congestion notifications from possible
> > incoming packet patterns, not specifically for ECN++ or AckCC [RFC5690], or any other future RFC
> > (forward compatibility). We'll edit the reference to ECN++ to make it clearer that it's one example,
> > not the only case.
> > 
> > Here's another example of the generic reflector approach, already in the draft:
> > "Although RFC 3168 prohibits an ECN-capable SYN, providing feedback of ECN marking on the SYN
> > supports future scenarios in which SYNs might be ECN-enabled (without prejudging whether they ought
> > to be). ... "
> > 
> >
> >       So, it should be cristal clear that without SACK (or Timestamps) a TCP *MUST NOT* send
> >       ACKs of ACKs.
> > 
> > 
> > I understand that you want this but, as just explained, without SACK or timestamps, the correct
> > approach is to prevent the Data Sender putting the Data Receiver in the position where it would have
> > to ACK ACKs in the first place.
> > 
> > In a connection without SACK or timestamps, if the Data Receiver were to get lots of congestion
> > notifications on ECN-capable ACKs, it would face a difficult dilemma. Which would be more important:
> > Signalling congestion by ACKing ACKs? or ensuring the performance improvements like Fast Retransmit,
> > Fast Recovery etc. work well? The former would prevent harm to others, the latter would prevent harm
> > to self.
> 
> [MK] Please see my reply to Richard to understand why Acks of Acks 
> cause Fast Retransmit, Fast Recovery, etc to cause harm to others in the 
> first place. And also why most of the Acks of Acks seem to be just 
> unnecessary load to the network, that is, they harm others without (much) 
> benefits.
> 
> > Nonetheless, I will add some text to the AccECN draft that explains why it is important for other
> > RFCs not to put a Data Receiver in the position where it has to ACK ACKs iff there is no way to
> > distinguish them from DupACKs.
> > 
> >
> >       3) Even with SACK or Timestamps enabled it is not clear what an
> >       implementer should do. With SACK the AckECN authors seem to make an assumption, which
> >       seems obvious but is not, that an ACK of ACK would would never include SACK option and
> >       hence it could be distinguished from a dupAck. However, RFC 2018 specifies: "If sent at
> >       all, SACK options SHOULD be included in all ACKs which do not ACK the highest sequence
> >       number in the data receiver's queue. So, if there is a hole in the receiver's queue, the
> >       assumption is incorrect and it is unclear which SACK info to include into the SACK
> >       option. Whatever one selects to include, it makes DSACK (RFC 2883] void and breaks any
> >       DSACK-based algorithms unless RFC 2018 is updated.
> > 
> > 
> > [BB]
> > Reading the draft, it is very clear that there is no such assumption. The text said solely what it
> > meant:
> >
> >    ... a host in AccECN
> >    mode that is sending ECN-capable pure ACKs SHOULD add one of the
> >    following additional checks when it tests whether an incoming pure
> >    ACK is a duplicate:
> >
> >    o  If SACK has been negotiated for the connection, but there is no
> >       SACK option on the incoming pure ACK, it is not a duplicate;
> > That is, if an incoming ACK were a duplicate, it would have a SACK option on it. This /relies/ on
> > the rule in RFC2018 that you quote. So there is no need (nor intention) to change SACK behaviour in
> > any way.
> 
> [MK] Sorry I don't understand your claim. Assume A sends data pkts 
> 0,1,2,3,..,10 in the current window to B and pkt 1 is dropped. 
> Simultaneously, B sends data to A such that the data pkts arrive at A 
> after A has injected pkt 10 to the network. These pkts trigger pure 
> cumulative Acks from A to B that follow A's data pkts 1..10 and enough of 
> the Acks get CE-marked due to congestion path A to B. When pkts 2..10 
> arrive at B, each of them trigger a valid dupAck with a SACK block 
> included. When the CE-marked Acks that follow data pkts 2..10 arrive at 
> B, B needs to feedback congestion info on them in Acks of Acks. These 
> Acks of Acks cannot cumulatively ACK the highest sequence number in the 
> data receiver's queue (pkt 10) since pkt 0 is the highest pkt 
> arrived insequence, so the Ack of Ack must include a SACK block as per 
> RFC 2018. What is the SACK block info that the implementer, who follows 
> carefully advice in RFC 2018 and there is no other advice, should 
> include in these Acks unless RFC 2018 is not changed? How does the 
> implementer know how to Ack, if it not specified?
> 
> > (Note: to check this text, you'll need to refer to the previous AccECN draft here:
> >     https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-accurate-ecn-23#section-3.2.2.5.1
> > It had been there since Jul 2021 (since -15). But, as explained above, it is longer in the latest
> > AccECN draft (-24), because it has been moved to the editor's copy of the ECN++ draft, which I wrote
> > on 4 Apr, but I've been waiting for your reply before submitting it.)
> > 
> >
> >       With Timestamps some algorithms like Eifel detection breaks. Moreover, there are other
> >       existing or potentially to-be-created heauristics, including various measurement tools,
> >       that rely on the fact that TCP does not echo a later Timestamp in a pure Ack than what
> >       arrived with the latest data packet. Any such mechanisms are subject to break.
> > 
> > 
> > [BB] I don't fully understand what you're saying here. Can you be clearer please?
> > And can you please bear in mind that we are in the WGLC processing stage now. So review comments
> > ought to be suggesting very specific changes to the text under review.
> 
> [MK] Again, the same problem as with SACK. It is unspecified which 
> TSecr value to put in Acks of Acks. That has not been specified 
> anywhere, so how can I or anyone else check what breaks if anything, and 
> how can an implementer know which TSecr value to include in Acks of Acks?
> 
> It is hard to propose specific changes to text that does not exist or to a 
> problem that seems to be a missing piece of design or a design flaw, until 
> the text exists or the intended design is known or the potentiel design 
> flaw is mutually agreed whether there is a flaw or not.
> 
> (Please note also that the Eifel problem seems not to be serious, but 
> instead the timestamps rule you proposed to distinguish dupAcks from Acks 
> of Acks seems suspicious and calls for clarification).
> 
> Please see more details in my reply to Richard.
> 
> > Again, this text about extra DupACK checks has now been removed from AccECN and will shortly appear
> > in the ECN++ draft instead. I shall post the new ECN++ draft shortly.
> > 
> >
> >       It might be good to hold discussing any details on what breaks and how/why and what are
> >       the consequences until I have sent my reply with scenarios to Richard.
> > 
> > 
> > [BB] Please try to prioritize any comments about the text that is now left in the AccECN draft. The
> > WGLC of AccECN is waiting for no-one else at the moment.
> 
> [MK] Unfortunately I was not able to do this, sorry. I got confused 
> already earlier when I was reviewing AccECN and ended up checking ECN++ 
> quickly as I noted that AccECN draft cited it for these issues. When 
> reading ECN++ I found Sec 3.3.3 and read:
> 
>   "The question of whether and how the receiver of pure ACKs is required to
>    feed back any CE marks on them is outside the scope of the present
>    specification because it is a matter for the relevant feedback
>    specification ([RFC3168] or [I-D.ietf-tcpm-accurate-ecn])."
> 
> This got me totally confused together with the later discussion on the mic 
> at Yokohama mtng because that was what I had read, and I was not able to 
> understand what was intended to go where. And I still not quite know 
> but I think I have a better hunch. The above text still reads in 
> ECN++ draft, but hopefully not in editor's copy?
> 
> Anyways, the major problem is not with any certain text phrases but 
> whether DupAck vs. Ack of Ack problem is solvable and whether 
> injecting Acks of Acks really is a needed and mature enough feature that 
> can be part of a stds track protocol.
> 
> > But also bear in mind that the chairs plan to take ECN++ into WGLC once AccECN WGLC has been
> > cleared. So we need to hear your actual argument about the DupACK text that has been moved to ECN++
> > urgently too. We can't work with "I have an argument, but I'll tell you later".
> 
> [MK] Apologies for the delay again. Please see my reply to Richard for my 
> arguments.
> 
> >       One additional comment regarding the "Change-Triggered ACKs" rule is that it would be
> >       useful to make it more clear how this plays with delayed Acks and how it alters
> >       acknowledgement rate.
> >       I am not sure that what the draft currently says is quite correct:
> >
> >        "The approach can lead to some additional ACKs but it feeds back
> >         the timing and the order in which ECN marks are received with minimal
> >         additional complexity. If CE marks are infrequent, as is the case for
> >         most AQMs at the time of writing, or there are multiple marks in a row,
> >         the additional load will be low.
> >
> >       For example, consider a scenario with bidirectional traffic between A and B where B has
> >       a hole in sequence resulting in every data packet in the current RTT to become acked
> >       (pure duplicate Acks). This may result in a packet flow from A to B where every second
> >       packet is a pure (duplicate) Ack. If there is congestion on the path from A to B such
> >       that a significant number of (data) packets get marked, it may result in acking every
> >       data packet from A to B. This does not necessarily result in low additional load?
> > 
> > 
> > [BB] I don't quite understand how every second packet from A to B is a pure duplicate ACK, but I
> > don't think I need to - I'll assume it's somehow possible.
> > 
> > Then I think you've somehow assumed that the data packets get CE-marked, but the interspersed pure
> > ACKs don't (perhaps you're assuming that the pure ACKs in this case are not ECN-capable? Or perhaps
> > you're assuming size-based packet marking?). Whatever, I agree that, if this scenario did occur,
> > then the change-triggered ACK rule would indeed lead to B ACKing every data packet that arrives,
> > with no delayed ACKs. {Note 1}
> > 
> > Nonetheless, the draft is quite open about the implications of the change-triggered ACK rule on ACK
> > rate. In the sentence straight after the ones you quote, it says:
> >     "However, marking patterns with numerous non-contiguous CE marks could increase the load
> > significantly."
> > And a little earlier it starts out by saying:
> >     "...the 'Change-Triggered ACKs' rule could sometimes cause the ACK rate to be problematic for
> > high performance"
> > 
> > I don't think we need to include examples of how non-contiguous CE marking could occur. And if we
> > did, I'd prefer to use one that was less complex to explain, e.g. a high level of probabilistic AQM
> > marking. But thank you for this point anyway.
> 
> [MK] Thanks for pointing these additional sentences. I somehow missed 
> them and/or did not manage to relate them to the text I quoted. I think 
> this is good enough to address the case I raised, particularly "numerous 
> non-contiguous CE marks could increase the load significantly."
> 
> > {Note 1}: It's ironic that the existing behaviour "where B has a hole in sequence" also results "in
> > every data packet in the current RTT to become acked" (by A). I'm not giving this as an excuse for
> > introducing another case with the same bad behaviour. I'm just highlighting the irony.
> 
> [MK] Maybe ironic but the fact that a hole in sequence results in every 
> data packet in RTT to become acked has a very good reason being as it is 
> because those dupAcks directly control the data rate per the packet 
> conservation principle in various important algos at the data sender,
> such as Fast Recovery, and this behaviour is therefore a crucial part of 
> such congestion control algos.
> 
> Best regards,
> 
> /Markku
> 
> > 
> > Regards
> > 
> > 
> > Bob
> > 
> >
> >       Best regards,
> >
> >       /Markku
> > 
> >
> >             Best regards
> >             Michael
> >
> >                   Cheers
> > 
> >
> >                   Bob
> >
> >                   On 30/03/2023 15:15, [e-mail] wrote:
> >                         A New Internet-Draft is available from the on-line
> >                         Internet-Drafts
> >                         directories. This Internet-Draft is a work item of
> >                         the TCP Maintenance and
> >                         Minor Extensions (TCPM) WG of the IETF.
> >
> >                            Title           : More Accurate Explicit
> >                         Congestion Notification (ECN) Feedback in TCP
> >                            Authors         : Bob Briscoe
> >                                              Mirja Kühlewind
> >                                              Richard Scheffenegger
> >                            Filename        :
> >                         draft-ietf-tcpm-accurate-ecn-24.txt
> >                            Pages           : 64
> >                            Date            : 2023-03-30
> >
> >                         Abstract:
> >                            Explicit Congestion Notification (ECN) is a
> >                         mechanism where network
> >                            nodes can mark IP packets instead of dropping
> >                         them to indicate
> >                            incipient congestion to the endpoints.  Receivers
> >                         with an ECN-capable
> >                            transport protocol feed back this information to
> >                         the sender.  ECN was
> >                            originally specified for TCP in such a way that
> >                         only one feedback
> >                            signal can be transmitted per Round-Trip Time
> >                         (RTT).  Recent new TCP
> >                            mechanisms like Congestion Exposure (ConEx), Data
> >                         Center TCP (DCTCP)
> >                            or Low Latency, Low Loss, and Scalable Throughput
> >                         (L4S) need more
> >                            accurate ECN feedback information whenever more
> >                         than one marking is
> >                            received in one RTT.  This document updates the
> >                         original ECN
> >                            specification in RFC 3168 to specify a scheme
> >                         that provides more than
> >                            one feedback signal per RTT in the TCP header. 
> >                         Given TCP header
> >                            space is scarce, it allocates a reserved header
> >                         bit previously
> >                            assigned to the ECN-Nonce.  It also overloads the
> >                         two existing ECN
> >                            flags in the TCP header.  The resulting extra
> >                         space is exploited to
> >                            feed back the IP-ECN field received during the
> >                         3-way handshake as
> >                            well.  Supplementary feedback information can
> >                         optionally be provided
> >                            in two new TCP option alternatives, which are
> >                         never used on the TCP
> >                            SYN.  The document also specifies the treatment
> >                         of this updated TCP
> >                            wire protocol by middleboxes.
> >
> >                         The IETF datatracker status page for this
> >                         Internet-Draft is:
> >                         https://datatracker.ietf.org/doc/draft-ietf-tcpm-accurate-ecn/
> >
> >                         There is also an htmlized version available at:
> >                         https://datatracker.ietf.org/doc/html/draft-ietf-tcpm-accurate-ecn-24
> >
> >                         A diff from the previous version is available at:
> >                         https://author-tools.ietf.org/iddiff?url2=draft-ietf-tcpm-accurate-ecn-24
> >
> >                         Internet-Drafts are also available by rsync at
> >                         rsync.ietf.org::internet-drafts
> > 
> >
> >                         _______________________________________________
> >                         tcpm mailing list
> >                         [e-mail]
> >                         https://www.ietf.org/mailman/listinfo/tcpm
> > 
> >
> >                   --
> >                   ________________________________________________________________
> >                   Bob Briscoe                               http://bobbriscoe.net/
> > 
> > 
> > 
> > 
> > -- 
> > ________________________________________________________________
> > Bob Briscoe                               http://bobbriscoe.net/
> > 
> >
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