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

["Bob Briscoe (IC)" <[e-mail]>]

Michael,

The rule in RFC2026 is "Standards track specifications normally must not depend on other standards track specifications which are at a lower maturity level or on non standards track specifications". My thinking for making SACK and ECN++ informative refs was that recommendation in itself does not create a dependency. The more clear-cut test is that the AccECN spec is defined for the case without ECN++ and without SACK, so I think that means it doesn’t depend on them.

Nonetheless, following a promise to Markku (also on the list), I have added the following to the local copy of AccECN after this ref to ECN++:
    "...or any equivalent future protocol that allows the ECN capability to be used on TCP control packets"
And I've similarly generalized all the refs to ECN++.

Does that resolve this concern?


Bob

> On 18 Jun 2023, at 22:03, [e-mail] wrote:
> 
>> 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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