Re: [tcpm] Comments on draft-ietf-tcpm-accurate-ecn

Yuchung Cheng <> Wed, 11 July 2018 18:36 UTC

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From: Yuchung Cheng <>
Date: Wed, 11 Jul 2018 11:35:58 -0700
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To: Bob Briscoe <>
Cc: "Scharf, Michael (Nokia - DE/Stuttgart)" <>, "" <>, "" <>
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Subject: Re: [tcpm] Comments on draft-ietf-tcpm-accurate-ecn
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Hi Bob,

Neal and I evaluated the earlier draft. It is well-thought out but
we're concerned about the options. Option is not mandatory but the
lack of it also reduces accuracy. Option runs into space issues w/
SACK and offload issues w/ TSO/GRO. They can be addressed for sure but
aren't easy.

We're curious how much more "accuracy" it buys over current
DCTCP-style ECN. Is there any study to show trade-offs of
full-ACE-w-options vs ACE-wo-options vs current DCTCP-ECN?

On Wed, Jul 11, 2018 at 11:00 AM, Bob Briscoe <> wrote:
> Michael, tcpm list,
> As well as addressing your points, as Mirja has already mentioned below, we
> added a whole new appendix giving the rationale for the bits and codepoints
> that AccECN has proposed to use on 1) the SYN and 2) SYN/ACK. A 3rd
> subsection also identifies space for future evolution. It also points to
> where rationale was already given in the body of the draft.
> The appendix is in the draft submitted last week, available here:
> We'd be interested to hear whether this allays your concerns.
> We have asked to present this in Montreal as well.
> Cheers
> Bob
> On 02/07/18 16:54, Mirja Kühlewind wrote:
>> Hi Micheal,
>> I addressed a couple of your comments below.
>> For the other, bigger comments regarding extensibility, that I did not yet
>> address below, we plan to add a new section to the appendix to explain
>> extensibility options as previously discussed by mail. We will probably send
>> a separate email on that part.
>> Mirja
>>> Am 12.03.2018 um 01:59 schrieb Scharf, Michael (Nokia - DE/Stuttgart)
>>> <>om>:
>>> Hi Mirja,
>>> Thanks a lot for the explanation. I won't follow-up on some of the
>>> editorial suggestions.
>>> Yet, I continue to believe that some formal wording in the document needs
>>> to change, as explained below.
>>> Thanks
>>> Michael (with no hat on)
>>>> -----Original Message-----
>>>> From: Mirja Kühlewind []
>>>> Sent: Monday, March 05, 2018 1:54 PM
>>>> To: Scharf, Michael (Nokia - DE/Stuttgart) <>
>>>> Cc:;
>>>> Subject: Re: Comments on draft-ietf-tcpm-accurate-ecn
>>>> Hi Micheal,
>>>> thanks for your feedback and sorry for my late reply.
>>>> Please see inline.
>>>>> Am 03.12.2017 um 20:17 schrieb Scharf, Michael (Nokia - DE/Stuttgart)
>>>> <>om>:
>>>>> Hi all,
>>>>> I have read draft-ietf-tcpm-accurate-ecn-05 (without the appendix). I
>>>> believe this document needs further work before moving forward.
>>>>> Please find below my comments marked as [ms]. I have read the
>>>> document independent of the review from Gorry. I apologize if there is
>>>> duplication.
>>>>> Thanks
>>>>> Michael (with no hat on)
>>>>> ******************************
>>>>> * Abstract:
>>>>>    Recently, new TCP mechanisms like Congestion Exposure (ConEx) or
>>>>> Data
>>>> Center TCP
>>>>>    (DCTCP) need more accurate ECN feedback information whenever more
>>>>>    than one marking is received in one RTT.
>>>>> [ms] I don't think this statement is fully backed by RFC 8257. I
>>>>> suggest to
>>>> remove this, or replace it by a more generic statement that more
>>>> accurate
>>>> information can be useful for several TCP extensions.
>>>> I disagree. Both ConEx and DCTCP need more accurate information. They do
>>>> not need the mechanism that is specified in this draft, however, this is
>>>> not
>>>> what the sentences is saying.
>>> In my understanding (as a non-native speaker), the use of the word "need"
>>> is not correct here. DCTCP as specified in RFC 8257 can be implemented
>>> without any such mechanism.
>>> What would work for me is something of the form "... Data Center TCP
>>> cannot get precise ECN feedback whenever more than one marking is received
>>> in one RTT“.
>> This is not correct. DCTP need more than one feedback signal per RTT and
>> therefore cannot use RFC3168; instead it implement it’s own feedback
>> mechanism. However, to avoid confusion such that people could assume DCTP
>> would not work without the accECN scheme as specified in this doc, I
>> rephrased to:
>> "Recently, proposed
>>        mechanisms like Congestion Exposure (ConEx <xref
>> target="RFC7713"/>),
>>        DCTCP <xref target="RFC8257"/> or L4S <xref
>>        target="I-D.ietf-tsvwg-l4s-arch"/> need to know when more than one
>>        marking is received in one RTT which is
>>        information that cannot be provided by the feedback scheme as
>> specified in
>>        <xref target="RFC3168"/>."
>>>>>    This document specifies an
>>>>>    experimental scheme to provide more than one feedback signal per RTT
>>>>>    in the TCP header.  Given TCP header space is scarce, it overloads
>>>>>    the three existing ECN-related flags in the TCP header and provides
>>>>>    additional information in a new TCP option.
>>>>> [ms] This statement needs to be rewritten to correctly reflect what is
>>>> requested from IANA. My understanding is that this experimental document
>>>> asks for allocation of a reserved TCP header flag. This needs to be
>>>> called out
>>>> prominently, IMHO. In addition, since this is not a standard, the
>>>> suggested
>>>> experimentation with the main TCP header must IMHO be explicitly
>>>> mentioned. I also suggest to have later in a document a section that
>>>> explicitly
>>>> explains why it is appropriate to modify the main TCP header in an
>>>> experiment.
>>>> I don’t know if any requirement that IANA assignment need to be called
>>>> out
>>>> in the abstract but we can do that. However, I believe the question if
>>>> this
>>>> document should or should not assign the bit is still not completely
>>>> solved, or
>>>> is it?
>>> I believe this question will have to be reviewed during WGLC and, more
>>> importantly, IETF last call. For the moment, my concern is that the document
>>> correctly describes the IANA allocation.
>>> I would like to see here a statement such as : "Given TCP header space is
>>> scarce, this specification allocates a reserved header bit and overloads the
>>> two ECN flags in the TCP header ...“.
>> A bit lengthy but now:
>> "Given TCP header space is
>>        scarce, it allocates a reserved header bit, that was previously
>> used for
>>        ECN-Nonce which was recently declared historic, and overloads the
>>        two existing ECN flags in the TCP header. Further, additional
>>        information can be provided in a new TCP option that however is not
>> used
>>        on the TCP SYN."
>>>>> * 1.  Introduction
>>>>>    Recently, proposed mechanisms like Congestion Exposure (ConEx
>>>>>    [RFC7713]), DCTCP [RFC8257] or L4S [I-D.ietf-tsvwg-l4s-arch] need
>>>>>    more accurate ECN feedback information whenever more than one
>>>> marking
>>>>>    is received in one RTT.
>>>>> [ms] At least for RFC 8257 seems to be implementable withoit this.
>>>>> Instead
>>>> of stating a "need", it would IMHO make more sense to discuss the
>>>> benefits
>>>> of the suggested mechanism in this document of its own, independent of
>>>> other proposals. To me, this document should be independent of other
>>>> documents and specifically other experiments. We have to think about
>>>> cases
>>>> where not all experiments are successful. Then independent documents
>>>> will
>>>> be more future-proof in future.
>>>> This is a naming collision… The sentence was meant to say that these
>>>> mechanisms new more accurate ECN feedback than provided today by
>>>> RFC3168 but it was not meant to say that these mechanism have to use the
>>>> scheme as specified in this document.
>>>> I added the following part sentence:
>>>> „Recently, proposed mechanisms like Congestion Exposure (ConEx
>>>> [RFC7713]), DCTCP [RFC8257] or L4S [I-D.ietf-tsvwg-l4s-arch] need more
>>>> accurate ECN feedback information than provided by the feedback scheme
>>>> as specified in [RFC3168] whenever more than one marking is received in
>>>> one
>>>> RTT. This document specifies an alternative feedback scheme that
>>>> provides
>>>> more accurate information and could be used by these new TCP
>>>> extensions.“
>>>> Does this help?
>>> See my proposal for the abstract. I continue to disagree with the term
>>> "need" but I think this can be sorted out by another term.
>>>>>    If AccECN progresses from experimental to the standards
>>>>>    track, it is intended to be a complete replacement for classic TCP/
>>>>>    ECN feedback, not a fork in the design of TCP.
>>>>> [ms] This sentence should be removed, as this is speculation.
>>>> Why? It states an intent… and that’s the intent that we have.
>>>>>    Until the AccECN experiment succeeds, [RFC3168] will remain as the
>>>>>    standards track specification for adding ECN to TCP.
>>>>> [ms] This sentence should be removed (or reworded)
>>>> Why? Does it help to add an only here:
>>>> "Until the AccECN experiment succeeds, [RFC3168] will remain as the only
>>>> standards track specification for adding ECN to TCP.“
>>> This wording is better.
>>>>>    AccECN feedback overloads flags and fields in the main TCP header
>>>>>    with new definitions, so both ends have to support the new wire
>>>>>    protocol before it can be used.
>>>>> [ms] In my reading this experimental document asks for *new* allocation
>>>> of a reserved TCP header flag.
>>>> Is this better?
>>>> "AccECN feedback overloads the two existing ECN flags as well as the
>>>>       currently reserved and previously called NS flag in the main TCP
>>>> header
>>>>       with new definitions, so both ends have to support the new wire
>>>> protocol
>>>>       before it can be used.“
>>>> I understand that you are not happy with the word „overload“ here but
>>>> the
>>>> point of this sentence really is that the flags can/could be used
>>>> differently
>>>> and therefore we need a new negotiation before we can use them.
>>> For me the following would work: "AccECN feedback overloads the two
>>> existing ECN flags and
>>> allocates the currently reserved and previously called NS flag in the
>>> main TCP header.
>>> Given the new definitions, both ends have to support the new wire
>>> protocol
>>> before it can be used."
>>> I believe the wording has to be crystal clear on the reservation of bit 7
>>> when it is discussed the first time in the text. In follow-up sections,
>>> maybe shorter terms could be used.
>> Okay, now:
>> "AccECN feedback overloads the two existing ECN flags and
>>      allocates the currently reserved and previously called NS flag in the
>>      TCP header, to be used as one field indicating the number of
>> congestion
>>      experienced marked packets. Given the new definitions of these three
>> bits,
>>      both ends     have to support the new wire protocol before it can be
>> used.
>>      Therefore during the TCP handshake the two ends use these three bit
>> in
>>      the TCP header to negotiate the most advanced feedback protocol
>>      that they can both support in a backward compatible way to
>>      <xref target="RFC3168"/>."
>>>> If you prefer, we can also remove the NS flag in this list, as ECN Nonce
>>>> was
>>>> anyway never deployed.
>>>>>    For that we refer to [RFC3168] or any RFC that
>>>>>    specifies a different response to TCP ECN feedback, for example:
>>>>>    [RFC8257]; or the ECN experiments referred to in
>>>>>    [I-D.ietf-tsvwg-ecn-experimentation], namely: a TCP-based Low
>>>>> Latency
>>>>>    Low Loss Scalable (L4S) congestion control
>>>>> [I-D.ietf-tsvwg-l4s-arch];
>>>>>    ECN-capable TCP control packets [I-D.ietf-tcpm-generalized-ecn], or
>>>>>    Alternative Backoff with ECN (ABE)
>>>>>    [I-D.ietf-tcpm-alternativebackoff-ecn].
>>>>> [ms] At least ABE seems orthogonal. Anyway, I think this paragraph can
>>>>> just
>>>> be deleted. If other experiments need more accurate feedback, it is up
>>>> to
>>>> them to explain how they would use this mechanism. This document should
>>>> focus on how to signal the feedback, not how to use that.
>>>> Yes, that is what the paragraph says. Isn’t it better to be explicit
>>>> about this?
>>>>>    It is likely (but not required) that the AccECN protocol will be
>>>>>    implemented along with the following experimental additions to the
>>>>>    TCP-ECN protocol: ECN-capable TCP control packets and
>>>>> retransmissions
>>>>>    [I-D.ietf-tcpm-generalized-ecn], which includes the ECN-capable SYN/
>>>>>    ACK experiment [RFC5562]; and testing receiver non-compliance
>>>>>    [I-D.moncaster-tcpm-rcv-cheat].
>>>>> [ms] I am a big fan of simple, standalone documents. In my view, the
>>>>> TCPM
>>>> working group should publish draft-ietf-tcpm-accurate-ecn and
>>>> draft-ietf-
>>>> tcpm-generalized-ecn independent documents, which probably implies that
>>>> draft-ietf-tcpm-generalized-ecn does not use AccECN. If experimentation
>>>> with ECT in SYN requires a combination, this could be done in a new,
>>>> third
>>>> document. Apart from having simpler focused documents, this could
>>>> significantly help later with moving forward documents to standards
>>>> track.
>>>> I disagree, however, this is a discussion to have on draft-ietf-tcpm-
>>>> generalized-ecn. I don’t see a problem in  providing a reference here
>>>> that
>>>> says „it is likely…“ and nothing more.
>>>>> * 1.1.  Document Roadmap
>>>>> [ms] A macroscopic comment is that this document has a lot of
>>>>> introduction
>>>> and tutorial text with lot's of redundancy towards other documents. I
>>>> think
>>>> the document can be made much easier to read by shorten it. In many
>>>> cases
>>>> this is just an editorial change as there is redundancy. As one such
>>>> example,
>>>> just remove this section.
>>>> I guess this a matter of taste. As an AD, I’m a big fan of short and
>>>> concise
>>>> documents, however, some redundancy can also help understanding,
>>>> especially if you explain things multiple times but with a different
>>>> level of
>>>> detail. I personally would not need the roadmap but I know many people
>>>> who find these things helpful and to be honest I don’t see how removing
>>>> this
>>>> part makes the doc any better. If you don’t want it, don’t read it.
>>>>> * 1.2.  Goals
>>>>> [ms] I think this section can also just be removed.
>>>> I have to say I also don’t see the point of removing this part. Given
>>>> we’ve
>>>> done the work on requirements, I think we should also link to this doc
>>>> somewhere.
>>>>> * 1.3.  Experiment Goals
>>>>>    TCP is critical to the robust functioning of the Internet, therefore
>>>>>    any proposed modifications to TCP need to be thoroughly tested. The
>>>>>    present specification describes an experimental protocol that adds
>>>>>    more accurate ECN feedback to the TCP protocol.  The intention is to
>>>>>    specify the protocol sufficiently so that more than one
>>>>>    implementation can be built in order to test its function,
>>>>> robustness
>>>>>    and interoperability (with itself and with previous version of ECN
>>>>>    and TCP).
>>>>> [ms] I think all what is written in this paragraph is obvious, no?
>>>>> Can't we just
>>>> delete this?
>>>> Sure, however, I don’t think it hurts to spell it out. For me both is
>>>> fine, keep it
>>>> or remove it.
>>>>>    The experimental protocol will be considered successful if it is
>>>>>    deployed and if it satisfies the requirements of [RFC7560] in the
>>>>>    consensus opinion of the IETF tcpm working group.  In short, this
>>>>>    requires that it improves the accuracy and timeliness of TCP's ECN
>>>>>    feedback, as claimed in Section 5, while striking a balance between
>>>>>    the conflicting requirements of resilience, integrity and
>>>>>    minimisation of overhead.  It also requires that it is not unduly
>>>>>    complex, and that it is compatible with prevalent equipment
>>>>>    behaviours in the current Internet (e.g. hardware offloading and
>>>>>    middleboxes), whether or not they comply with standards.
>>>>>    Testing will mostly focus on fall-back strategies in case of
>>>>>    middlebox interference.  Current recommended strategies are
>>>>> specified
>>>>>    in Sections 3.1.2, 3.2.3, 3.2.4 and 3.2.7.  The effectiveness of
>>>>>    these strategies depends on the actual deployment situation of
>>>>>    middleboxes.  Therefore experimental verification to confirm large-
>>>>>    scale path traversal in the Internet is needed before finalizing
>>>>> this
>>>>>    specification on the Standards Track.
>>>>> [ms] These two paragraphs must be entirely rewritten. As I have
>>>> mentioned before, I don't think an RFC should speculate about TCPM and
>>>> its
>>>> consensus opinion. I would suggest a wording along the lines of:
>>>>> <ms>
>>>>>    The experimental protocol will be considered successful if
>>>>>    testing confirms that the proposed mechanism can be deployed at
>>>>> large
>>>> scale.
>>>>>    Testing will mostly focus on fall-back strategies in case of
>>>>>    middlebox interference.  Current recommended strategies are
>>>>> specified
>>>>>    in Sections 3.1.2, 3.2.3, 3.2.4 and 3.2.7.  The effectiveness of
>>>>>    these strategies depends on the actual deployment situation of
>>>>>    middleboxes.  Therefore experimental verification to confirm large-
>>>>>    scale path traversal in the Internet is needed, e.g., by support in
>>>>>    major TCP stacks.
>>>>> </ms>
>>>> I don’t understand your point here. I don’t think that the paraphrase
>>>> speculates about the consensus of tcpm, in contrast it say tcpm has to
>>>> decided if the requirements previously specified by tcpm are
>>>> sufficiently
>>>> fulfilled. I don’t see a reason to not mention the requirement draft as
>>>> this
>>>> draft as tcpm consensus and was written for this purpose.
>>> My suggested wording uses the expression "can be deployed at large scale"
>>> and I believe this is relevant.
>>> The document already describes in Section 5 how the protocol satisfies
>>> the agreed requirements for a more accurate ECN feedback protocol [RFC7560].
>>> So, if the TCPM working group publishes this document with the content of
>>> Section 5, I believe the TCPM working group already has reached consensus
>>> that the protocol meets requirements. In addition, it is possible that new
>>> requirements would be identified in future, e.g., as an outcome of the
>>> experiment, and that would obviously have to be considered by TCPM. In that
>>> case, for the success of the experiment not only RFC 7560 would matter, but
>>> also further requirements. My proposed wording does not have all these
>>> problems.
>>> In a nutshell, I continue to believe that this section has to change.
>> Okay, used your proposed wording. You have a point about the requirement
>> and I mis-read you proposal earlier as „has to be deployed large-scale“.
>>>>> * 1.5.  Recap of Existing ECN feedback in IP/TCP
>>>>> [ms] This section could probably be shortened as well.
>>>>>    The last bit in byte 13 of the TCP header was defined as the Nonce
>>>>>    Sum (NS) for the ECN Nonce [RFC3540].  RFC 3540 was never deployed
>>>>> so
>>>>>    it is being reclassified as historic, making this TCP flag available
>>>>>    for use by the AccECN experiment instead.
>>>>> [ms] This wording, as well as Figure 1, needs to take into account the
>>>>> IANA
>>>> status when draft-ietf-tsvwg-ecn-experimentation is published.
>>>> Is does. However, I can explicitly say that is has be re-clssified as
>>>> reserved.
>>>> "RFC 3540 was never deployed so it is being reclassified as historic
>>>> [I-D.ietf-
>>>> tsvwg-ecn-experimentation] and the respective flag has been marked as
>>>> „reserved“ in the IANA TCP Header Flags registry, making this TCP flag
>>>> available for use by the AccECN experiment instead.“
>>>> Better?
>>>>> In my understanding, this experimental document asks for new assignment
>>>> of a reserved TCP header flag.
>>>> As I said I’m not sure if we have fully concluded this discussion yet.
>>>> However,
>>>> what we really would want to is mention somewhere that this experiment
>>>> with this flags is running. I guess there are three options:
>>>> 1) keep it in the registry as reserved and conserve the knowledge in
>>>> tcpm
>>>> that this experiment is running and no other experimental RFC such use
>>>> this
>>>> flags as long as this experiment is running.
>>>> 2) Keep is marked as reserved but add a note about this experiment in
>>>> the
>>>> IANA registry
>>>> 3) Or assign it right away with IESG approval. I guess in this case tcpm
>>>> could
>>>> also consider to change the registration policy to „IETF Review“.
>>> The current registration policy for the TCP header flags is "standards
>>> action". I understand that the IESG could approve exceptions. But given the
>>> policy, I believe the document has to be very precise on the request
>>> regarding bit 7.
>> Okay, it now says:
>> "[TO BE REMOVED: IANA is requested to update the existing entry in the
>> Transmission Control Protocol (TCP) Header Flags registration
>> (
>> for Bit 7 to "AE (Accurate ECN), previously used by Historic as NS (Nonce
>> Sum) [RFC3540, RFC8311]" and change the reference to this RFC-to-be instead
>> of RFC8311.]“
>> I guess we could also ask IANA to add an additional comment column instead
>> (but not sure if we then have to update RFC3168, which I think we really
>> don’t want. Should be fine now.
>>>>> * 2.  AccECN Protocol Overview and Rationale
>>>>>    o  an essential part that re-uses ECN TCP header bits to feed back
>>>>>       the number of arriving CE marked packets.  This provides more
>>>>>       accuracy than classic ECN feedback, but limited resilience
>>>>> against
>>>>>       ACK loss;
>>>>> [ms] The word "re-use" is IMHO not correct.
>>>> I think this is nit picking. Using a different phrasing here makes the
>>>> sentence
>>>> unnecessary complicated. We don’t try to some how get a round the fact
>>>> that we need to handle the flag registration correctly. However, here
>>>> the
>>>> point really is to explain how the protocol word. The main point of
>>>> using the
>>>> work „re-use“ here is really that we say that these flags are or have
>>>> been
>>>> used different by other TCP extension (and we therefore need a proper
>>>> negotiation scheme).
>>> If the allocation of a reserved flag is correctly explained in the
>>> abstract and introduction, I think these sentences can use a bit relaxed
>>> terminology.
>>>>>    The two part design was necessary, given limitations on the space
>>>>>    available for TCP options and given the possibility that certain
>>>>>    incorrectly designed middleboxes prevent TCP using any new options.
>>>>> [ms] IMHO it would make sense to more explicitly mention the downsides
>>>> of only specifying an option and not allocating a TCP header flag, in
>>>> this
>>>> experimental document.
>>>> We need to use the flags (all three of them) for the negotiation.
>>> I think that an explanation why negotiation by a TCP option would not
>>> solve all use cases (or requirements) would help here.
>>>>> The obvious  alternative would be to postpone the header flag
>>>>> allocation to
>>>> a follow-up standards track document and just keep it reserved.
>>>> We can still do that. We should discuss and make a final decision.
>>>>>    The essential part overloads the previous definition of the three
>>>>>    flags in the TCP header that had been assigned for use by ECN.  This
>>>>>    design choice deliberately replaces the classic ECN feedback
>>>>>    protocol, rather than leaving classic ECN feedback intact and adding
>>>>>    more accurate feedback separately because:
>>>>> [ms] Similar like previous comments, in my reading there are only _two_
>>>> ECN header flags.
>>>> I think there are three flags that "had been assigned for use by ECN“ as
>>>> ECN
>>>> Nonce is also an ECN mechanism. The fact that one of the flags is now
>>>> marked as reserved instead, it not that important for me here.
>>>>> And, in addition, I think care is needed with wording such "replaces
>>>>> the
>>>> classic ECN feedback". I don't think this experiment replaces the ECN
>>>> standards. That would be up to a follow-up PS.
>>>> This sentence is not meant to say that RFC3168 is replaced. Actually we
>>>> don’t.
>>>> You can still use RFC3168 even if AccECN is implemented and deploy
>>>> (however, we do intent that AccECN will be used as the default scheme in
>>>> future and RFC3168 is hopefully simply not needed anymore at some point,
>>>> even though you probably still need to have it implemented as the
>>>> negotiation specified in this draft covers that as well, anyway...). The
>>>> sentence says that if AccECN is negotiation, the header flags as used by
>>>> RFC3168 and previously ECN Nonce are used differently (aka re-used).
>>>> That’s
>>>> all.
>>>>> 2.1.  Capability Negotiation
>>>>>    AccECN is a change to the wire protocol of the main TCP header,
>>>>>    therefore it can only be used if both endpoints have been upgraded
>>>>> to
>>>>>    understand it.  The TCP client signals support for AccECN on the
>>>>>    initial SYN of a connection and the TCP server signals whether it
>>>>>    supports AccECN on the SYN/ACK.  The TCP flags on the SYN that the
>>>>>    client uses to signal AccECN support have been carefully chosen so
>>>>>    that a TCP server will interpret them as a request to support the
>>>>>    most recent variant of ECN feedback that it supports.  Then the
>>>>>    client falls back to the same variant of ECN feedback.
>>>>> [ms] As this is an experimental specification, I would really like to
>>>>> see a
>>>> discussion how a future standards track version of more accurate ECN
>>>> could
>>>> be negotiated.
>>>> As described in this draft. There will be no different. AccECN IS and
>>>> will
>>>> always be backward compatible with RFC3168.
>>> That is not the problem I think about. I wonder about a PS version of
>>> accurate ECN feedback that would possibly include changes as compared to
>>> this experiment, e.g., because the experiment may have some lessons learnt.
>>> What options would we have to negotiate the PS version, and how could a
>>> stack implementing the PS version figure out whether the remote end uses the
>>> experimental or the PS version of the protocol?
>>> The reason why I ask is because I don't see any easy solution to this but
>>> I may miss something. Maybe this would not be a concern if there were some
>>> codepoints left.
>>>>> How could both endpoints detect whether the other one implements the
>>>> future standards track version?
>>>> If the initiator implements AccECN it will request it’s use. If the
>>>> receiver also
>>>> implement it, it will/can negotiate it, if not it will look like an
>>>> RFC3168 request
>>>> for the receiver (as the NS flags will be ignored in the SYN) and it
>>>> will
>>>> negotiate RFC3168 ECN feedback if implemented. There is no additional
>>>> detection needed.
>>> My concern is the migration strategy for a future version, given that we
>>> only experiment right now. For instance, if the initiator implements the
>>> experimental version but the recipient implements the PS version of AccECN,
>>> how would that work? Under the assumption that there are changes, would
>>> there be a way to know? Maybe the answer to this question is no, given the
>>> small number of bits we have. But not having any room for extensions is not
>>> necessarily good protocol design.
>>>>> For instance, would the only safe variant be that we allocate yet
>>>>> another
>>>> reserved TCP header flag in a proposed standard to negotiate the
>>>> standards
>>>> track version, thus investing another reserved bit in the TCP header?
>>>> No, that’s exactly what we use the NS flags for in the handshake.
>>> If the PS version of AccECN was different to the experimental version of
>>> AccECN, and if there was a deployed base of both, I still believe we could
>>> end up in a situation in which we had to allocate yet another TCP header
>>> flag to distinguish the different versions of AccECN. The NS flag would not
>>> work for that if it is used by the experimental version. The risk of having
>>> to spend further TCP header flags somehow concerns me. Of course, that
>>> problem would only matter if the AccECN experiment succeeds somehow, but
>>> lessons learnt would require protocol changes, which is speculation.
>>>>> I may be wrong, but to me it is too early to speculate how the PS
>>>>> version
>>>> would look like, and whether it would have to be different to the
>>>> experimental version, due to lessons learnt.
>>>> Of course you can always be wrong. However, the handshake negotiation is
>>>> not the part we need experimentation for. That part is straight forward
>>>> and
>>>> works. If we really happen to detect a problem in that part, we would
>>>> need
>>>> to end the experiment declare failure and start over new.
>>> My concern is ending the experiment when the experiment got (partly)
>>> deployed in the Internet. In that case neither a new RFC nor a change of the
>>> IANA registry will solve the migration issue.
>>>>> I believe in the IETF we typically design protocols that allow future
>>>> extension, and it is not exactly clear to be how AccECN could be
>>>> extended
>>>> later.
>>>> This is an TCP extension. If we want future extension we use the usually
>>>> TCP
>>>> mechanism (by defining a new TCP option I guess).
>>> The root cause of my concern is that this proposal does *not* use the
>>> usual way to experiment with TCP by options. It experiments with a header
>>> flag, including in the SYN, and it seems to consume all codepoints. So, I
>>> see the risk of a protocol design "not ready for future improvements".
>>> I cannot easily propose text. Maybe "lack of extensibility" is just one
>>> of the short-comings of the protocol design that cannot be avoided but that
>>> short-coming would have to be noted.
>>>>>    An AccECN TCP client does not send the new AccECN Option on the SYN
>>>>>    as SYN option space is limited and successful negotiation using the
>>>>>    flags in the main header is taken as sufficient evidence that both
>>>>>    ends also support the AccECN Option.  The TCP server sends the
>>>>> AccECN
>>>>>    Option on the SYN/ACK and the client sends it on the first ACK to
>>>>>    test whether the network path forwards the option correctly.
>>>>> [ms] For what it is worth, I would personally be quite fine with
>>>>> allowing (or
>>>> even mandating) an option in the SYN in the experimental version of this
>>>> protocol. For instance, saving the SYN option space would then an
>>>> excellent
>>>> reason for moving towards the PS specification. I am also fine with
>>>> being in
>>>> the rough part of the consensus here.
>>>> The point is that we really don’t need the option in the SYN as we don’t
>>>> use it
>>>> for negotiation purposes as we use the header bits instead. So why
>>>> should
>>>> we waste the space?
>>> For instance, mandating the option in the SYN would be away for the
>>> receiver to distinguish the experiment from a follow-up PS version of the
>>> spec, as the PS version may not mandate the option, to save header space.
>>> Maybe that proposal does not make any sense, and it may only have
>>> downsides. But the document already speculates about a PS-follow-up. So it
>>> seems a valid question to ask if the EXP and the PS version of the spec have
>>> to be identical. This all comes down to the SYN negotiation.
>>>>> * 2.3.  Delayed ACKs and Resilience Against ACK Loss
>>>>>    If the AccECN Option is not available, e.g. it is being stripped by
>>>>> a
>>>>>    middlebox, the AccECN protocol will only feed back information on CE
>>>>>    markings (using the ACE field).  Although not ideal, this will be
>>>>>    sufficient, because it is envisaged that neither ECT(0) nor ECT(1)
>>>>>    will ever indicate more severe congestion than CE, even though
>>>>> future
>>>>>    uses for ECT(0) or ECT(1) are still unclear
>>>>>    [I-D.ietf-tsvwg-ecn-experimentation].
>>>>> [ms] This needs to be reworded
>>>> Why?
>>>>> * 2.4.  Feedback Metrics
>>>>>    The CE packet counter in the ACE field and the CE byte counter in
>>>>> the
>>>>>    AccECN Option both provide feedback on received CE-marks.  The CE
>>>>>    packet counter includes control packets that do not have payload
>>>>>    data, while the CE byte counter solely includes marked payload
>>>>> bytes.
>>>>>    If both are present, the byte counter in the option will provide the
>>>>>    more accurate information needed for modern congestion control and
>>>>>    policing schemes, such as DCTCP or ConEx.
>>>>> [ms] I suggest to write in the last sentence only "... the option will
>>>>> provide
>>>> the more accurate information needed for congestion control". In
>>>> general, I
>>>> would prefer to have references to other mechanisms at only few (ideally
>>>> a
>>>> *single*) places in the document, instead of mixing them together.
>>>> Sorry, I don’t see your point here. ConEx has been mentioned previously,
>>>> so
>>>> why not also mention it here.
>>> As written earlier, in my understanding DCTCP does not "need" this. I
>>> would suggest to have one place where to define exactly how this mechanism
>>> can be used by other TCP extensions. Having said this, in this specific
>>> paragraph I am less concerned about that than elsewhere.
>>>>>    Feedback in bytes is recommended in order to protect against the
>>>>>    receiver using attacks similar to 'ACK-Division' to artificially
>>>>>    inflate the congestion window, which is why [RFC5681] now recommends
>>>>>    that TCP counts acknowledged bytes not packets.
>>>>> [ms] At least the last part and the reference to RFC 5681 is IMHO not
>>>> needed here.
>>>> Why? RFC5681 explains/refers the ACK division attack, so I think it is a
>>>> very
>>>> good reference to have here.
>>>>> * 2.5.  Generic (Dumb) Reflector
>>>>>    The ACE field provides information about CE markings on both data
>>>>> and
>>>>>    control packets.  According to [RFC3168] the Data Sender is meant to
>>>>>    set control packets to Not-ECT.  However, mechanisms in certain
>>>>>    private networks (e.g. data centres) set control packets to be ECN
>>>>>    capable because they are precisely the packets that performance
>>>>>    depends on most.
>>>>>    For this reason, AccECN is designed to be a generic reflector of
>>>>>    whatever ECN markings it sees, whether or not they are compliant
>>>>> with
>>>>>    a current standard.  Then as standards evolve, Data Senders can
>>>>>    upgrade unilaterally without any need for receivers to upgrade too.
>>>>>    It is also useful to be able to rely on generic reflection behaviour
>>>>>    when senders need to test for unexpected interference with markings
>>>>>    (for instance [I-D.kuehlewind-tcpm-ecn-fallback] and
>>>>>    [I-D.moncaster-tcpm-rcv-cheat]).
>>>>>    The initial SYN is the most critical control packet, so AccECN
>>>>>    provides feedback on whether it is CE marked.  Although RFC 3168
>>>>>    prohibits an ECN-capable SYN, providing feedback of CE marking on
>>>>> the
>>>>>    SYN supports future scenarios in which SYNs might be ECN-enabled
>>>>>    (without prejudging whether they ought to be).  For instance,
>>>>>    [I-D.ietf-tsvwg-ecn-experimentation] updates this aspect of RFC 3168
>>>>>    to allow experimentation with ECN-capable TCP control packets.
>>>>> [ms] To me, the only thing that matters in this document that AccECN
>>>>> can
>>>> provide feedback on whether the SYN is CE marked. The discussion on how
>>>> to experiment with ECT e.g. in SYNs IMHO does not belong into this
>>>> document. So it seems sufficient here to note that one of the benefits
>>>> of
>>>> AccECN is that CE marks in SYNs can be fed back.
>>>> I disagree. Explicitly saying that AccECN is only an feedback scheme and
>>>> DOES
>>>> NOT define how the information is used is VERY important because people
>>>> come back to me over and over again and mix these things up.
>>>>> * 3.1.1.  Negotiation during the TCP handshake
>>>>>    Given the ECN Nonce [RFC3540] is being reclassified as historic, the
>>>>>    present specification renames the TCP flag at bit 7 of the TCP
>>>>> header
>>>>>    flags from NS (Nonce Sum) to AE (Accurate ECN) (see IANA
>>>>>    Considerations in Section 6).
>>>>> [ms] As mentioned before, this needs to be rewritten to ask for new
>>>>> IANA
>>>> allocation of bit 7 in the TCP header flags.
>>>> I really don’t understand this comment. That is what the IANA section
>>>> does
>>>> as referred here correctly.
>>> In my reading of
>>> this flag currently has no name, i.e., it does not have the name NS. So this
>>> statement on "renaming" is formally incorrect IMHO.
>>> I'd suggest something like: "This specification assigns the name AE
>>> (Accurate ECN) to the TCP flag at bit 7; this flag has previously been known
>>> as NS (Nonce Sum) …".
>> Now:
>> "Given the ECN Nonce <xref target="RFC3540"/> has been
>>            reclassified as historic <xref target="RFC8311"/>, the present
>> specification re-allocates the TCP
>>            flag at bit 7of the TCP header flags, which was previously
>> called NS
>>            (Nonce Sum), as the AE
>>            (Accurate ECN) flag (see IANA Considerations in <xref
>>            target="accecn_IANA_Considerations"/>)."
>>>> Again, yes, we can discuss if this document should do this, but if
>>>> published as
>>>> it is, section 6 says everything that is needs to say. (I does not put
>>>> any
>>>> request on IANA, instead it is written as it would show up
>>>> post-publication,
>>>> implicitly providing a request to IANA at approval time. That is fine.)
>>>>>    Table 2: ECN capability negotiation between Client (A) and Server
>>>>> (B)
>>>>> [ms] As far as I can see, in -05 this table allocates all existing
>>>>> codepoints,
>>>> while -03 had two currently unused codepoints. Not having any codepoints
>>>> left seems to me not really future proof, e.g., regarding future
>>>> proposed
>>>> standards in this space (and I personally believe that TCP header flags
>>>> must
>>>> be allocated in a PS). And I don't fully see a need of feeding back ECT0
>>>> and
>>>> specifically ECT1 in the TCP header flags as part of the experiment. Do
>>>> we
>>>> know for sure that this is the only possible use case of these two
>>>> unallocated
>>>> header bits? And why can't e.g. this be done in a TCP option instead? Or
>>>> do I
>>>> miss something?
>>>> The point is really, if we don’t assign them now and start deployment we
>>>> effetely we not be able to every assign them again because don’t have a
>>>> different negotiation mechanisms. Realizing this, it is just the right
>>>> think to
>>>> define the space completely that is negotiated as use for AccECN in the
>>>> handshake.
>>> I disagree that consuming all codepoints and not having room for future
>>> extensions is the "right thing" to do. It is in fact the wrong thing. But
>>> possibly there is no alternative with the few bits, so maybe we end up doing
>>> it.
>>> Yet, at minimum, using all codepoints needs to be reasoned in the
>>> document. Specifically since in -03 a different protocol design seemed
>>> possible, which looked to me more future-proof.
>>>>> * 3.1.2.  Retransmission of the SYN
>>>>>    However,
>>>>>    current measurements imply that a drop is less likely to be due to
>>>>>    middlebox interference than other intermittent causes of loss, e.g.
>>>>>    congestion, wireless interference, etc.
>>>>> [ms] Such wording IMHO doesn't belong into normative text. This may
>>>> actually also apply to other heuristics discussed in this section, which
>>>> are not
>>>> really important for interoperability.
>>>> I don’t really understand your point. This sentence is solely meant to
>>>> reason
>>>> the design decision to not say that a sender SHOULD attempt to re-
>>>> negotiation after a loss.
>>> One could split the reasoning from the normative design. The normative
>>> specification may still be relevant in 20 years from now. Middleboxes will
>>> almost likely be different in 20 years. Having said this, this is more of an
>>> editorial remark.
>>>>> 3.2.7.  Path Traversal of the AccECN Option
>>>>>  Testing the AccECN Option during the Handshake
>>>>>    The TCP client MUST NOT include the AccECN TCP Option on the SYN.
>>>>> [ms] I am not sure if I really understand the motivation for not
>>>>> allowing a
>>>> option in the SYN. If the sender has space in the SYN left, what is the
>>>> harm in
>>>> an experimental version of the protocol? And I may miss something, but
>>>> what would prevent the use of 2-byte option to negotiate the use of
>>>> AccECN, e.g., to avoid experimental allocation of bit 7 in the initial
>>>> SYN?
>>>> I did have a draft on that as a proposal for an alternative design.
>>>> However,
>>>> the gourd was more supportive of this design as it is proof of middlebox
>>>> SYN
>>>> option mangling which is a know problem.
>>>> Therefore we simply don’t need option in the SYN and there is no reason
>>>> to
>>>> waste the space.
>>>>> While I think many tutorial text in this document could be shortened, I
>>>> believe the use of a reserved TCP header flag should be reasoned.
>>>> I’m actually uncertain what you expect here.
>>> For instance, this reply with the explanation of SYN option mangling
>>> seems useful explanation.
>>>>> * 3.2.8.  Usage of the AccECN TCP Option
>>>>>    The following rules determine when a Data Receiver in AccECN mode
>>>>>    sends the AccECN TCP Option, and which fields to include:
>>>>>    Change-Triggered ACKs:  If an arriving packet increments a different
>>>>>       byte counter to that incremented by the previous packet, the Data
>>>>>       Receiver MUST immediately send an ACK with an AccECN Option,
>>>>>       without waiting for the next delayed ACK (this is in addition to
>>>>>       the safety recommendation in Section 3.2.5 against ambiguity of
>>>>>       the ACE field).
>>>>>       This is stated as a "MUST" so that the data sender can rely on
>>>>>       change-triggered ACKs to detect transitions right from the very
>>>>>       start of a flow, without first having to detect whether the
>>>>>       receiver complies.  A concern has been raised that certain
>>>>> offload
>>>>>       hardware needed for high performance might not be able to support
>>>>>       change-triggered ACKs, although high performance protocols such
>>>>> as
>>>>>       DCTCP successfully use change-triggered ACKs.
>>>>> [ms] To me this sounds like a perfect example for a SHOULD with
>>>>> additional
>>>> guidance why implementing this SHOULD is really important.
>>>> This is one of the most discussed point from the author and we really
>>>> tried to
>>>> get additional guidance here of what to do also from outside the IETF
>>>> but did
>>>> no clear feedback.
>>>> As explained this MUST enables additional functionality. However, this
>>>> is an
>>>> experiment document. If we detect that this MUST does actually hinder
>>>> implementation or has just never been implemented, we should reconsider
>>>> this in the final PS RFC.
>>>> I was on the SHOULD side of the discussion, but can say that the
>>>> implementation in Linux was way more simple then expected. Offloading
>>>> might be a different topic but that is where I could not get a clear
>>>> feedback
>>>> and offloading could probably be anyway optimized for use with AccECN
>>>> (if it
>>>> gets deploy widely).
>>> If a MUST requires changes in hardware, I think there must be a clear
>>> reason.
>>> As individual contributor, with the current explanation in the text, I
>>> believe this has to be a SHOULD.
>>>> I though we added this as something to mention in the exp goals section,
>>>> but
>>>> obviously we didn’t. I added the following text now:
>>>> "Another experimentation focus is the implementation feasibiliy of
>>>> change-
>>>> triggered ACKs as described in section 3.2.8. While on average this
>>>> should not
>>>> lead to a higher ACK rate, it changes the ACK patter which especially
>>>> can have
>>>> an impact on hardware offload. Further experimentation is needed to
>>>> advise
>>>> if this should a hard requirement or just prefer behavior.“
>>> If it is unclear if a MUST can actually be implemented, having a MUST is
>>> in my opinion the wrong approach.
>>> One could equally state here that further experimentation is needed to
>>> determine whether the SHOULD can be upgraded to a MUST.
>> I’m still open for everything here, but I believe this was discussed and
>> agreed in the working group…?
>>>>>    For the avoidance of doubt, the change-triggered ACK mechanism is
>>>>>    deliberately worded to ignore the arrival of a control packet with
>>>>> no
>>>>>    payload, which therefore does not alter any byte counters, because
>>>>> it
>>>>>    is important that TCP does not acknowledge pure ACKs.  The change-
>>>>>    triggered ACK approach will lead to some additional ACKs but it
>>>>> feeds
>>>>>    back the timing and the order in which ECN marks are received with
>>>>>    minimal additional complexity.
>>>>> [ms] The additional acks create network load. I think some wording is
>>>> needed on the tradeoff between information accuracy and network load.
>>>> There are network environments in which any additional packet is very
>>>> expensive (e.g., energy) and it is not clear to me how the protocol
>>>> design
>>>> takes into account the potential overhead of additional ACKs. Maybe this
>>>> could be another reason for a SHOULD.
>>>> The above. However, this is not really an additional ACK because you do
>>>> delay the next one. Further experimentation needed.
>>> The document states "lead to some additional ACKs". If that does not
>>> increase network load, I think it has to be explicitly explained why the ACK
>>> load is at most equal to a current TCP stack, in all potential cases. If it
>>> can increases network load, it has to be reasoned why increasing load (and
>>> risk of reverse congestion and the like) is worth the effort.
>>> I agree that this may be an area of experimentation, but I believe then
>>> it has to be explained to implementers what the tradeoffs are.
>> Added:
>> "Especially, if only few
>>            CE marks occurred or multiple marks in a row, the additional
>> load will
>>            be low. Other, unexpected marking pattern could increase the
>> load
>>            significantly, however, investigating the additional load it
>> part
>>            of the proposed experimentation."
>>>>> * 4.2.  Compatibility with Other TCP Options and Experiments
>>>>>    AccECN is compatible (at least on paper) with the most commonly used
>>>>>    TCP options: MSS, time-stamp, window scaling, SACK and TCP-AO.  It
>>>>> is
>>>>>    also compatible with the recent promising experimental TCP options
>>>>>    TCP Fast Open (TFO [RFC7413]) and Multipath TCP (MPTCP [RFC6824]).
>>>>> [ms] I would suggest the wording "... compatible with the experimental
>>>> TCP options ..." or even "... compatible with the TCP options …".
>>>> These option are to experimental..?
>>> "It is also compatible with the experimental TCP options TCP Fast Open
>>> (TFO [RFC7413]) and Multipath TCP (MPTCP [RFC6824])." would have same
>>> technical meaning. Having said this, this is editorial only.
>>>> The point of using „commonly used“ was to say that we checked on those
>>>> as
>>>> they seem important. Just because your favorite experiment option is not
>>>> listed here it doesn’t means it incompatible, we just didn’t check. I’m
>>>> okay to
>>>> removed „commonly used“ but I don’t think it makes anything better.
>>>>> * 4.3.  Compatibility with Feedback Integrity Mechanisms
>>>>> [ms] Quite a bit in this section is experimental work, which IMHO
>>>>> should be clearly emphasized. The one exception is…
>>>> I would really like to keep this section because integrity is usually
>>>> the fist
>>>> question that come up when I present AccECN. Effectively these are two
>>>> independent topics, however, I really think it help people to understand
>>>> the
>>>> whole picture if this is also discussed in this document.
>>>>>       However, TCP-AO is often too brittle to use on many end-to-end
>>>>>       paths, where middleboxes can make verification fail in their
>>>>>       attempts to improve performance or security, e.g. by
>>>>>       resegmentation or shifting the sequence space.
>>>>> [ms] I am not sure if deployment challenges of other options need to be
>>>> discussed in this document.
>>>> If we keep the discussion, I guess we should mention this as well. As
>>>> the doc
>>>> clearly stated section 4 is not meant to be normative.
>>> As far as I can see, there are use cases for TCP-AO where middleboxes are
>>> simply not a problem, but it is exactly this sort of discussion that may not
>>> be needed in this document. But I won't rat-hole on this comment here.
>>>>>    Originally the ECN Nonce [RFC3540] was proposed to ensure integrity
>>>>>    of congestion feedback.  With minor changes AccECN could be
>>>>> optimised
>>>>>    for the possibility that the ECT(1) codepoint might be used as an
>>>>> ECN
>>>>>    Nonce . However, given RFC 3540 is being reclassified as historic,
>>>>>    the AccECN design has been generalised so that it ought to be able
>>>>> to
>>>>>    support other possible uses of the ECT(1) codepoint, such as a lower
>>>>>    severity or a more instant congestion signal than CE.
>>>>> [ms] The discussion of RFC 3540 can probably be removed to a large
>>>>> extent.
>>>> Unfortunately, please still think that ECN Nonce, even though is was
>>>> never
>>>> deployed and doesn’t really work, is the only was to provide integrity
>>>> protection and we need it as a prerequisite to deploy ECN at all… i
>>>> would
>>>> really prefer to keep this in this non-normative part of the doc.
>>>>> * 6.  IANA Considerations
>>>>> [ms] I think this section needs to be rewritten to request a new
>>>>> allocation
>>>> of bit 7 of the TCP header flags. At least for the process I think it
>>>> would make
>>>> sense to have somewhere in the document a comprehensive explanation of
>>>> why an experimental document requests a change of the main TCP header,
>>>> and why this cannot be avoided (most notably in the initial SYN) by an
>>>> alternative protocol design.
>>>> As I said previously this section is written as it would look like after
>>>> the
>>>> allocation has happened with publication approval of the IESG. This is
>>>> fine.
>>>> Having a discussion about an experiment doc assigning a flag (or not) is
>>>> a
>>>> question for tcpm as a whole and not specifically this document. How do
>>>> we
>>>> envision to every use any further flags? We go to PS right away? Or
>>>> should
>>>> we change the registration policy? For me the latter makes actually more
>>>> sense. However, if we don’t want/can to decide this now, we also could
>>>> go
>>>> forward as it is with IESG approval. However, is this case it is also
>>>> not needed
>>>> to explain this in the document. The responsible AD has to explain this
>>>> to the
>>>> other IESG probably in the ballot or even better the shepherd could
>>>> provide
>>>> these information in the write-up.
>>>>> * 9.  Comments Solicited
>>>>>    Comments and questions are encouraged and very welcome.  They can
>>>> be
>>>>>    addressed to the IETF TCP maintenance and minor modifications
>>>>> working
>>>>>    group mailing list <>rg>, and/or to the authors.
>>>>> [ms] This section is not needed IMHO
>>>> Yes, it will be removed before publication.
>>>>> 10.  References
>>>>>    [I-D.ietf-tsvwg-ecn-experimentation]
>>>>>               Black, D., "Relaxing Restrictions on Explicit Congestion
>>>>>               Notification (ECN) Experimentation",
>>>>> draft-ietf-tsvwg-ecn-
>>>>>               experimentation-07 (work in progress), October 2017.
>>>>> [ms] Normative reference?
>>>> Don’t see why. No need to read ietf-tsvwg-ecn-experimentation to
>>>> understand the spec in this doc.
>>>> Thanks!
>>>> Mirja
>> _______________________________________________
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> ________________________________________________________________
> Bob Briscoe                     
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