Re: [tcpm] I-D Action: draft-ietf-tcpm-accurate-ecn-16.txt

"Scheffenegger, Richard" <> Sun, 06 February 2022 23:01 UTC

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To: Bob Briscoe <>, Vidhi Goel <>
Cc: Richard Scheffenegger <>, " Extensions" <>, Mirja Kuehlewind <>
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From: "Scheffenegger, Richard" <>
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Subject: Re: [tcpm] I-D Action: draft-ietf-tcpm-accurate-ecn-16.txt
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Hi Bob,

could it be, that the first "data" row and column in this table are
parially transposed?

In the case of sending a Non-ECT SYN, but getting an indication of a
received CE on the SYN-ACK, why would you respond to that indication of
an incorrect CE marking? (With the response being, that the cwnd would
probably never grow beyond 1 or 2 packets per RTT, or possibly diminish
to n RTTs per packet, one a stack supports such low rates).

I am thinking of the most common probable cause - a middlebox
unconditionally overriding the full TOS byte, and simply setting both
rightmost bits continously.

Such a path would be rendered utterly unusable for an AccECN session, not?

So at least that case would be a ND in your nomenclature.

In the column - if the IP ECN is bleached, why disable response to CE
(if that is unlike to come back, as long as the session traverses the
bleaching path)? A "NR" seems more logical here in case of path changes?

But again, all these ins-and-outs should be discussed in a different draft.

I fully agree, that only the normative response of the AccECN feedback
mechanism has to be fully defined - the two bullet point is exactly what
I expect to be stated.



Am 06.02.2022 um 23:27 schrieb Bob Briscoe:
> Richard,
> Indeed, I recently discovered I had missed an item that I added to my
> ToDo list in Mar 2021, based on a conversation between Gorry and Mirja
> where they both made the same point - that fall-back behaviour after
> discovering ECN mangling ought to be non-normative in this draft,
> because it's beyond the scope of the new feedback mechanism:
> We do need to say the following normatively in "Testing for Mangling of
> the IP/ECN Field" because they are relevant to feedback, the Data Sender:
> * remains in AccECN mode
> * MUST continue to feed back any ECN markings on arriving packets.
> BTW, Vidhi, the text you quoted was from §2 on "Protocol Overview and
> Rationale". That was meant to be an example of why it is useful for the
> handshake to give feedback on exactly which IP-ECN codepoint arrived. It
> wasn't meant to be a full spec. So I'll make it clearer that it's only
> an example, rather than try to make it comprehensive, which isn't
> appropriate at that point in the doc.
> Nonetheless, your points are correct, but if they are dealt with anyway,
> they should be dealt with in § "Testing for Mangling of the
> IP/ECN Field" where these scenarios are already covered (except for the
> congestion response, which was silently left out of scope):
> We could just say that the questions of whether to set ECT for
> subsequent packets, and whether to respond to feedback of CE markings
> are beyond the scope of the draft. I think we should, but I think we
> should also give some non-normative advice on what to do. It can only be
> interim advice, not normative requirements. Both because it's out of
> scope, and because the best behaviour and the alternatives will depend
> on deployment experience. So they shouldn't be written normatively in a
> Proposed Standard yet.
> We will do what the WG wants, but assuming it agrees to including
> non-normative advice, following from your (Richard's) list of cases,
> here's a full matrix. I don't propose this matrix should go in the
> AccECN draft - it's just for this email, to check every case is covered.
> 	Feedback in SYN/ACK of SYN's IP-ECN
> IP-ECN on SYN 	Not-ECT 	ECT(0) 	ECT(1) 	CE
> Not ECT 	√ 	NR 	NR 	NR
> ECT(0) 	ND 	√ 	√ 	√
> ECT(1) 	ND 	CU
> 	√ 	√
> CE 	n/a 	n/a 	n/a 	n/a
> Key:
> √       Valid response
> NR    Non-normative advice: Set *N*ot-ECT for subsequent packets, but
> continue to *R*espond to CE feedback
> ND    Non-normative advice: Set *N*ot-ECT for subsequent packets, and
> *D*isable response to CE feedback
> CU    Currently unused transition (not listed as invalid in the draft)
> n/a    not applicable in the AccECN spec, in which there is never a case
> where the sender sets CE when sending a packet.
> In the draft we don't need to mention any cases where CE is sent (but a
> draft on fall-back probably would, because the sender might set CE to
> test the path). We can cover the other cases with two groups of invalid
> transitions: those where the SYN (or SYN-ACK) arriving at the receiver
> appears to have been
> i) ECN-capable
> ii) Not-ECN-capable.
> I propose to also say that details (like what 'subsequent packets'
> implies about how long this behaviour continues) are for future study.
> I'll try to write some text and propose it to this list. Perhaps it
> should be in a new appendix?
> Bob
> On 05/02/2022 12:01, Scheffenegger, Richard wrote:
>> Vidhi,
>> Thanks for reading the draft very closely!
>> Well, in the non-ECT SYN case, where the SYNACK indicates an ECT (ect0,
>> ect1, ce) is the obvious - and fully semantically correct - use case of
>> that possible use.
>> For the set of three codepoints (ECT0, ECT1, CE) the only other possible
>> valid detection scenario would be if a SYN+CE is sent (deliberately,
>> althought one can argue that a sender should not originate CE-marked
>> packets), but a SYNACK without CE is returned (full or partial
>> bleaching). At least under the current RFC3168 definition of IP ECN
>> codepoints.
>> allowed transitions:
>> nonECT -> nonECT
>> ECT0 -> ECT0
>> ECT0 -> ECT1
>> ECT0 -> CE
>> ECT1 -> ECT1
>> ECT1 -> ECT0 (possibly)
>> ECT1 -> CE
>> CE -> CE
>> Thinking about this aspect more, I am now unsure, if the higher level
>> functions, such as detection of IP ECN codepoint mangling, or validating
>> across multiple RTTs, should be in this document - or not rather be in a
>> separate draft, in order to facilitate independent implementation and
>> improvement of mechanisms in that space.
>> However, the above table of allowed IP ECN transistions may be updated
>> independently of AccECN - and performing extensive checks as part of a
>> signal protocol ossifies this IMHO.
>> As a middle ground, perhaps having these descriptions in a non-normative
>> section, clearly stating these to be examples of what an impemented
>> could choose to do?
>> Richard
>> Am 05.02.2022 um 02:14 schrieb Vidhi Goel:
>>> I like your response for all the points. I can review the diff for the
>>> proposed changes, if you’d like before committing them to the draft.
>>> Sorry, I want to add a new comment for this text,
>>> If a TCP client has set the SYN to Not-ECT, but
>>>     receives feedback that the IP-ECN field on the SYN arrived with a
>>>     different codepoint, it can detect such middlebox interference and
>>>     send Not-ECT for the rest of the connection.
>>> This statement holds good for Not-ECT but doesn’t if lets say the SYN
>>> was ECT0 or ECT1 as those could be marked CE and still be valid. Should
>>> we add a statement for ECT marked SYN as well?
>>> Thanks,
>>> Vidhi
>>>> On Feb 4, 2022, at 5:44 AM, Bob Briscoe <
>>>> <>> wrote:
>>>> Vidhi,
>>>> I've increased the table of contents to tocdepth = "4" which might
>>>> help find the relevant sections better, because a lot of the meat in
>>>> this draft is under sections 3.2.2 (ACE) and 3.2.3 (Option).
>>>> I've responded below, but I'd prefer it if you would say these things
>>>> on the list, so we have a better justification for changing the draft.
>>>> Reviewing a draft that has just be re-posted shouldn't imply anything
>>>> about whether Apple is implementing the draft or not.
>>>> (BTW, regarding the proposed change to the initial value of r.e1b, I
>>>> was going to send that  to the list once co-authors agreed to it. But
>>>> it would be nice if some of the edits were not self-generated.)
>>>> See [BB] inline for responses (which I'll repeat if you send to the
>>>> list).
>>>> On 04/02/2022 07:37, Vidhi Goel wrote:
>>>>> Hello Bob and authors,
>>>>> I have read the draft a few times and will send my extensive review
>>>>> at a later point. But right now, I want to ask some critical things
>>>>> that an implementation could benefit from.
>>>>> The draft says,
>>>>> /If a TCP client has set the SYN to Not-ECT, but/
>>>>> /   receives feedback that the IP-ECN field on the SYN arrived with a/
>>>>> /   different codepoint, it can detect such middlebox interference
>>>>> and/
>>>>> /   send Not-ECT for the rest of the connection/
>>>>> 1. On the forward path from client to server, the client will revert
>>>>> to Not-ECT when it sees for example, that it sent a Non-ECT SYN but
>>>>> received ACE encoding other than 0 1 0. What does the client do in
>>>>> the last ACK of the 3WHS -
>>>>>  a. does it stay in AccECN mode and still send AccECN encoding based
>>>>> on the IP code point of SYN-ACK (Table 4)? This would mean that
>>>>> client won’t participate in ECN on the sender half of the connection
>>>>> and only provide AccECN feedback to the server as a receiver.
>>>>>  b.  does it disable AccECN mode and set ACE= 0 0 0 so that a server
>>>>> in AccECN mode can disable ECN based on Table4?
>>>>> While writing this, I realized that the intention is probably a.
>>>>> Could you confirm? Also, when the sender sets Not-ECT in its data
>>>>> packets, it should also disable acting upon any ACE feedback as we
>>>>> could still receive false ACE feedback from the server if the
>>>>> network, lets say, changed 00 (not ECT) to 01(ECT1). If you agree, we
>>>>> should add some text around this. Based on the current text, the
>>>>> sender will always respond to the ACE feedback even if it sends
>>>>> Not-ECT.
>>>>> TBH, this is a complicated scenario, where sender said to the network
>>>>> - I don’t trust you so I can’t use ECN. Feel free to drop my packets.
>>>>> And the network mangles the IP to ECT1 and then set CE (when
>>>>> congested) which would be feedback’ed from the receiver. Now, the
>>>>> packet wasn’t dropped which it should have been. So, is it better to
>>>>> just ignore this feedback because sender doesn’t trust the network or
>>>>> just act on it and reduce cwnd in order to reduce congestion in the
>>>>> network somewhere.
>>>> [BB] Your points are all good ones.
>>>> I'll address this last complex scenario first, because it has
>>>> implications for the earlier questions. First I'll define some
>>>> terminology:
>>>> Simple mangling scenario:
>>>>   * some network function, e.g. broken Diffserv ToS-byte remarking,
>>>>     illegally remarks a Not-ECT  SYN to CE.
>>>> Your complex mangling scenario, repeated here:
>>>>   * some network function, e.g. broken Diffserv ToS-byte remarking,
>>>>     illegally remarks a Not-ECT SYN to ECT0 or ECT1,
>>>>   * then congestion at a subsequent bottleneck is marking some
>>>> packets CE
>>>> Then potentially the client could tailor its behaviour after sending a
>>>> Not-ECT SYN, If the SYN/ACK feedback is
>>>>  1. ECT: disable sending ECT, but continue responding to ECN feedback
>>>>  2. CE, disable sending ECT, and disable response to ECN feedback
>>>> However, this is uncertain, because CE feedback on the SYN/ACK could
>>>> indicate either the simple or the complex mangling scenario.
>>>> A simpler alternative would be to always continue responding to ECN
>>>> feedback. Rationale:
>>>>   * Whether case #1 or #2, assume that mangling of the SYN might have
>>>>     been Not-ECT to ECT, even if the feedback off the SYN is CE.
>>>>   * Then as the connection progresses, if /all/ feedback is CE,
>>>>     there's already a recommendation to fall-back to disabling
>>>>     congestion response.
>>>> If we do this, I think we ought to say "SHOULD continue to respond to
>>>> ECN feedback", not "MUST".
>>>> And we'll need to put this all to the WG.
>>>> 1.a) Now back to the beginning of your point. The text you quote is
>>>> from §2.5 which is in the non-normative "Overview and Rationale"
>>>> section (§2).
>>>> You really need the normative text from:
>>>> <>Testing
>>>>           for Mangling of the IP/ECN Field
>>>> <>
>>>> where it says (end of 1st para):
>>>>     " ...for the remainder of the connection the client MUST NOT send
>>>>     ECN-capable packets, but it MUST continue to feed back any ECN
>>>>     markings on arriving packets."
>>>> So, you're right, the answer is (a).
>>>> It can't be (b), because, once in AccECN mode, 000 on the ACE field
>>>> just becomes a counter value, and no longer negotiates the feedback
>>>> mode.
>>>> 1.b) As per the above, let's conservatively assume complex mangling.
>>>> So, at the end of the first para quoted above I suggest we add:
>>>>     " ...for the remainder of the connection the client MUST NOT send
>>>>     ECN-capable packets *but**it**SHOULD continue to respond to ECN
>>>>     feedback even though **it **is no longer sending ECN-capable
>>>>     packets (see reasoning below)**. T**he client**MUST remain in the
>>>>     same feedback mode and***it MUST continue to feed back any ECN
>>>>     markings on arriving packets."
>>>> I'll do the same for next para about the server.
>>>> I'll work out some text for the reasoning, but I won't give it here.
>>>> There are two other cases where it disables sending ECT, which don't
>>>> say whether it continues to respond to congestion:
>>>>   * "Testing for Mangling" section, penultimate para,  where it's
>>>>     receiving solid CE:
>>>>     "Once a Data Sender has entered AccECN mode it SHOULD check
>>>>     whether all feedback received for the first three or four rounds
>>>>     indicated that every packet it sent was CE-marked. If so, for the
>>>>     remainder of the connection, the Data Sender SHOULD NOT send
>>>>     ECN-capable packets*and it SHOULD NOT respond to ECN feedback*,
>>>>     but *it MUST remain in the same feedback mode and *it MUST
>>>>     continue to feed back any ECN markings on arriving packets*(in its
>>>>     role as Data Receiver)*."
>>>>   * Next section "Zeroing of the ACE Field", 2nd para:
>>>>     "If the value of this ACE field is zero (0b000), the Data Sender
>>>>     disables sending ECN-capable packets for the remainder of the
>>>>     half-connection by setting the IP/ECN field in all subsequent
>>>>     packets to Not-ECT, *but**it**SHOULD continue to respond to ECN
>>>>     feedback even though **it **is no longer sending ECN-capable
>>>>     packets.* *It MUST also remain in the same feedback mode and it
>>>>     MUST continue to feed back any ECN markings on arriving packets
>>>>     (in its role as Data Receiver).*"
>>>> I'll also add reasoning (in the zeroing section, it already says that
>>>> ACE=0b000 is not an unambiguous indication of mangling).
>>>> And this prompts me to edit the bullet in an earlier section about the
>>>> obligation of a sender to respond to congestion feedback:
>>>>         3.1.5.
>>>> <>Implications
>>>>         of AccECN Mode
>>>> <>
>>>>     "It is still obliged to respond appropriately to AccECN feedback
>>>>     that indicates there were ECN marks on *ECN-capable *packets it
>>>>     had previously sent."
>>>> And I'll add a bullet:
>>>>     "*If the sender chooses not to send ECN-capable packets (e.g.
>>>>     because path traversal of the ECN field is suspect), it can ignore
>>>>     any **ECN **feedback about those packets if it is certain that it
>>>>     cannot be valid (see Section 3.2.2, which gives normative
>>>>     requirements for certain specific cases).*"
>>>> I've said 'can' rather than 'MUST' because it's hard to cover all
>>>> cases, e.g. single packets without ECT when it's not clear whether the
>>>> feedback covered another packet that was ECT.
>>>> How does all this sound?
>>>>> 2. On the reverse path from server to client, if a server sends a
>>>>> Not-ECT SYN-ACK and receives ACE handshake encoding on last ACK other
>>>>> than 0 1 0, there is no text like above that says server should send
>>>>> Not-ECT for the rest of the connection (or at least I didn’t find
>>>>> it). I think the server should also do same as client as the two
>>>>> paths could be different. One could make it more complicated by
>>>>> saying, if both client and server decide to not use ECN on their
>>>>> corresponding sender half, then ECN should be disabled but let’s talk
>>>>> about that later.
>>>> [BB] It's in the second para of:
>>>> <>Testing
>>>>           for Mangling of the IP/ECN Field
>>>> <>
>>>>> 3. In general, not setting the ECT (ECT0 or ECT1) code point on an
>>>>> outgoing packet is different from supporting AccECN right as in the
>>>>> host can still provide AccECN feedback on the receive path.
>>>> [BB] Yup.
>>>> Do you think any further explanation is needed in the above sections?
>>>> Bob
>>>>> ….
>>>>> To be continued if more questions come to my mind.
>>>>> Thanks,
>>>>> Vidhi
>>>>>> On Feb 3, 2022, at 7:24 AM, Bob Briscoe <> wrote:
>>>>>> tcpm folks,
>>>>>> This rev to accurate-ecn is the first of two. The second will
>>>>>> hopefully follow on its heels in the next couple of days.
>>>>>> Diffs between this first rev (-16) and -15:
>>>>>> 1.  switches round two fairly large sections, so I've deferred other
>>>>>> changes to a second rev so the diffs won't be masked by the switch
>>>>>> round of sections.
>>>>>> Suggested by Ilpo to match the order in which the tests in these
>>>>>> sections will be executed:
>>>>>> * Test for mangling the IP-ECN field (now,
>>>>>> * Then test for zeroing the ACE field (now
>>>>>> 2. Ilpo suggested some clarifications in " Consistency
>>>>>> between AccECN Feedback Fields", which is about the receiver of
>>>>>> feedback ensuring consistency between the mandatory 3-bit ACE field
>>>>>> and the optional 24-bit counters. In brief (paraphrasing) it
>>>>>> previously only said "MUST consider both fields", when it is now
>>>>>> clearer that it actually meant "MUST reconcile both fields", so that
>>>>>> there is always a consistent baseline for subsequent ACKs.
>>>>>> 3. A minor point is added in an appendix about the details that the
>>>>>> pseudocode doesn't cover.
>>>>>> Bob
>>>>>> PS. #2 & #3 were added to the XML ages ago (Jul '21), so you will
>>>>>> have seen them in the HTML. However, prob due to my clumsiness, the
>>>>>> posted TXT didn't include them whereas the posted XML did (ironic
>>>>>> for a section about consistency). In turn, inconsistency was only
>>>>>> possible because I am having to manually post the TXT for this
>>>>>> draft, due to an unresolved issue with v3 RFC XML tables.
>>>>>> On 03/02/2022 14:43, wrote:
>>>>>>> A New Internet-Draft is available from the on-line Internet-Drafts
>>>>>>> directories.
>>>>>>> This draft is a work item of the TCP Maintenance and Minor
>>>>>>> Extensions WG of the IETF.
>>>>>>>         Title           : More Accurate ECN Feedback in TCP
>>>>>>>         Authors         : Bob Briscoe
>>>>>>>                           Mirja Kühlewind
>>>>>>>                           Richard Scheffenegger
>>>>>>> Filename        : draft-ietf-tcpm-accurate-ecn-16.txt
>>>>>>> Pages           : 60
>>>>>>> Date            : 2022-02-03
>>>>>>> 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 end-points.  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 Scalable Throughput (L4S) need
>>>>>>> more
>>>>>>>    accurate ECN feedback information whenever more than one
>>>>>>> marking is
>>>>>>>    received in one RTT.  This document specifies a scheme to provide
>>>>>>>    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 a new TCP option, which is 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 draft is:
>>>>>>> There is also an htmlized version available at:
>>>>>>> A diff from the previous version is available at:
>>>>>>> Internet-Drafts are also available by rsync at
>>>>>>> <>::internet-drafts
>>>>>>> _______________________________________________
>>>>>>> tcpm mailing list
>>>>>> --
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>>>>>> Bob Briscoe
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>>>> ________________________________________________________________
>>>> Bob Briscoe
>>> _______________________________________________
>>> tcpm mailing list
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> ________________________________________________________________
> Bob Briscoe