Re: [tsvwg] David Black's WGLC review of draft-ietf-tsvwg-ecn-encap-guidelines-12

[Bob Briscoe <[e-mail]>]

David,

See inline. No response means accepted...

On 16/05/2019 18:13, Black, David wrote:
>
> Bob,
>
> Thanks for the comprehensive response, most of which I agree with.   
> Please also take note of Joe Touch’s comments on fragmentation and the 
> relationship to draft-intarea-tunnels.
>
> I have a few more comments – anything that isn’t mentioned here is 
> fine with me.
>
> i)
>
> > Expanded sentence at the end of Intro to:
> > "This document updates and completely replaces the brief paragraph 
> of advice to subnetwork designers about ECN in Section 13 of [RFC3819] 
> (BCP 89)."
>
> That’ll suffice for now – I still expect to get dinged on this by an 
> AD during IESG evaluation, but we can wait until then to determine 
> what else is required.
>
> ii)
>
> > [D] Page 5, first complete paragraph
>
> [… snip …]
>
> > Done; added:
> > "Similarly, if the DSCP is wiped at the boundary between Diffserv 
> domains, the special ECN semantics would also be lost."
>
> wiped -> changed (e.g., zeroed)
>
> iii)
>
> > ECN-PDU:  A PDU that is part of a feedback loop within which all the
>
> >    nodes that need to propagate *the*explicit congestion notification
>
> >   *signal in the PDU*back to the Load Regulator are ECN-capable *of 
> doing so*.
>
> I think this improved definition needs an additional hair-splitting 
> clarification:
>
> A PDU that is -> A PDU whose congestion indication (if any) is
>
> The rationale for this clarification is to exclude signals back to the 
> Load Regulator, e.g., TCP ECE.
>
[BB] OK. Actually, rather than trying to be subtle, I'm going to just 
say this much more straightforwardly:

     ECN-PDU:  A PDU at the IP layer or below carrying an explicit 
congestion notification signal that is part of a feedback loop within 
which all the nodes that need to propagate the signal back to the Load 
Regulator are capable of doing so.

I've also used 'signal' rather than 'indication' so that '(if any)' is 
not needed (because indication implies the positive aspect of the 
signal, whereas signal implies both congestion indication and 
non-indication).

> > I've also extended the above correction into the next definition:
>
> > Not-ECN-PDU:  A PDU that is part of a feedback-loop within which some  *at least one*
> >    node necessary to propagate *the*  explicit congestion notification*signal in the PDU *  
> >    back to the load regulator are  *is*  notECN-capable*of doing so*.
>
> And apply the same change to this definition also.
>
[BB] That's not possible 'cos, by definition, an ECN-PDU doesn't have a 
congestion indication, so we can't use that trick to exclude feedback 
packets. I'll just resort to adding "...at the IP layer or below..." 
here too:

     Not-ECN-PDU:  A PDU at the IP layer or below that is part of a 
feedback-loop within which at least one node necessary to propagate any 
explicit congestion notification signals back to the Load Regulator is 
not capable of doing so.


Bob

> Thanks, --David
>
> *From:*Bob Briscoe <[e-mail]>
> *Sent:* Wednesday, May 15, 2019 7:28 PM
> *To:* Black, David; [e-mail]
> *Cc:* John Kaippallimalil
> *Subject:* Re: [tsvwg] David Black's WGLC review of 
> draft-ietf-tsvwg-ecn-encap-guidelines-12
>
> [EXTERNAL EMAIL]
>
> David,
>
> I've addressed all your points inline (and RS's and AMcG's points in 
> recent emails).
>
> I've edited my local copy of the draft as I describe. I'll submit it 
> in a few hours time. I'm off to bed now, and in the morning I just 
> want to double-check I haven't missed anyone's comments before posting 
> it. That might also allow you, my co-author or anyone else enough time 
> to come back on any of my responses below before I post...
>
> On 09/05/2019 01:10, Black, David wrote:
>
>     First of all, this is a well-written and comprehensive draft on
>     ECN encapsulation design guidelines.
>
>     I found no major issues, and all of the minor issues are truly
>     minor – some border on editorial,
>
>     and all can be dealt with via relatively minor edits..
>
>     --- Minor ---
>
>     [A] Need an explicit statement of exactly what the update to RFC
>     3819 is,
>
>     preferably in a section whose name uses "RFC 3819" and appears in the
>
>     table of contents.  Do this before some AD tells you to ;-). This is
>
>     also the only thing that idnits found that needs attention.
>
>
> RFC 3819 contained a paragraph of brief musings on how subnetworks 
> might handle ECN. So ecn-encap completely replaces that para with a 
> whole document. I hope the new text quoted below is sufficient to 
> explain that there is no specific section that updates RFC 3819. 
> Rather the whole document does:
>
> Copied sentence below from end of intro to abstract:
> "This document updates the advice to subnetwork designers about ECN in 
> RFC 3819."
>
> Expanded sentence at the end of Intro to:
> "This document updates and completely replaces the brief paragraph of 
> advice to subnetwork designers about ECN in Section 13 of [RFC3819] 
> (BCP 89)."
>
>
>     [B] 1. Introduction, first paragraph, last sentence
>
>     The phrase "egress at any layer" is too general, hence incorrect
>     (e.g.,
>
>     layer 7 was not intended to be included) - "lower layer" needs to be
>
>     used for clarity.
>
>     OLD
>
>        If an egress at any layer is not ECN-aware, or if the
>
>        ultimate receiver or sender is not ECN-aware, congestion needs
>     to be
>
>        indicated by dropping a packet, not marking it.
>
>     NEW
>
>        If a lower layer header that may contain ECN congestion
>     indications is
>
>        removed by an egress that is not ECN-aware, or if the ultimate
>     receiver or
>
>        sender is not ECN-aware, then lower layer congestion needs to
>     be indicated
>
>        by dropping a packet, not marking it.
>
>     END
>
> Thx. Used verbatim (except s/an egress/a subnet egress/).
>
>
>     [C] 1.1 Scope
>
>     Second paragraph and the two-bullet list imply that RFC 4774
>     states that use
>
>     of ECT(1) to signal alternative ECN semantics is a best current
>     practice.
>
>     That's not correct, as RFC 4774 covers only the first bullet on
>     use of DSCP,
>
>     not the second bullet on use of ECT(1), as that second bullet is
>     based solely
>
>     on RFC 8311. Some rewriting is in order to disconnect the second
>     bullet from
>
>     RFC 4774.
>
> Done:
> "There are two main examples for how alternative ECN semantics have 
> been defined in practice:
>     o RFC 4774 suggests...
>     o RFC 8311 suggests..."
>
>
>     Based on this, all other places where RFC 4774 is mentioned or
>     cited should
>
>     be checked to determine whether RFC 8311 also ought to be
>     mentioned or cited.
>
> Done.
>
> The one other occurrence of RFC4774 says:
>     "The guidelines are also designed to ensure compliance with the 
> more general best current practice for the design of alternate ECN 
> schemes given in [RFC4774]..."
>
> Added: "...and extended by [RFC8311]"
>
>
>
>     [D] Page 5, first complete paragraph
>
>     Alternative semantics for the ECN field can be defined to depend on
>
>        the traffic class indicated by the DSCP.  Therefore correct
>
>     propagation of congestion signals could depend on correct propagation
>
>        of the DSCP between the layers.
>
>     This also depends on correct propagation across diffserv
>     boundaries - that
>
>     and the potential damage wrought by bleaching DSCPs to zero at
>     such boundaries
>
>     both deserve mention here.
>
> Done; added:
> "Similarly, if the DSCP is wiped at the boundary between Diffserv 
> domains, the special ECN semantics would also be lost."
>
>
>
>     [E] 1.1 Scope, end of section
>
>        In the feed-backward mode, propagation of
>
>     congestion signals for multicast and anycast packets is out-of-scope
>
>        (because it would be so complicated that it is hoped no-one would
>
>        attempt such an abomination).
>
>     Much as I may agree with it :-), the parenthetical remark is
>     inappropriate
>
>     for an archival RFC, and hence needs to be removed.
>
> When doing transport area reviews, I have criticised other authors for 
> just ruling stuff out of scope without justification or explanation of 
> the implications. So rather than delete, I've toned this down:
>
> "... (because the complexity would make it unlikely to be attempted)."
>
> BTW, for archival documents IMO light-heartedness is no less 
> appropriate than unrelenting blandness.
>
>
>     [F] 2. Terminology
>
>
>
>        ECN-PDU: A PDU that is part of a feedback loop within which all the
>
>           nodes that need to propagate explicit congestion notifications
>
>           back to the Load Regulator are ECN-capable.
>
>     That first sentence is too broad as it includes both the forward and
>
>     reverse directions of the feedback loop, whereas only the forward
>     direction is intended (FWIW, the definition of PDU does not suffice to
>
>     narrow this to the forward direction only).  For example, a TCP packet
>
>     with the ECE flag set in the TCP header and Not-ECT in the ECN
>     field in
>
>     the IP header would be an ECN-PDU under this definition, which is not
>
>     what was intended.  The immediately following definition of
>     Not-ECN-PDU
>
>     has the same problem.
>
> The point of building on the OSI term 'PDU' was because a PDU is 
> defined by the layer that is stated as relevant. By the OSI 
> definition, a PDU at a certain layer does not include lower layer 
> headers that encapsulate it.
>
> So, in the example you give of a TCP packet with ECE set, that is a 
> TCP PDU. By OSI definition, a TCP PDU does not include the IP header 
> that encapsulates it. The IP header is only relevant when considering 
> it as an IP PDU. And then the TCP PDU within it is just payload (the 
> SDU that was passed to IP) and not part of the protocol of the IP PDU.
>
>
>     The narrowing concept that needs to be added is that if the PDU
>     carries
>
>     a congestion indication, then that congestion indication
>     participates in
>
>     such a feedback loop,
>
> Yes, valid point. Thank you.
>
>    ECN-PDU:  A PDU that is part of a feedback loop within which all the
>
>       nodes that need to propagate *the*explicit congestion notification
>
> *signal in the PDU*back to the Load Regulator are ECN-capable *of 
> doing so*.
>
>
>
>     and even that is subtle when there are multiple
>
>     possible congestion indication locations.  An example of the
>     subtlety is
>
>     that the same PDU may be an ECN-PDU in feed-up-and-forward mode
>     because
>
>     the layer 3 and 4 protocols support ECN but may be a Not-ECN-PDU for
>
>     feed-forward-and-up mode because the L2 decapsulator ignores and
>     discards
>
>     L2 congestion indications.
>
> I think the bold words in the phrase "nodes that *need to* propagate" 
> deals with that subtlety.
>
> In your example, if the subnet is propagating the PDU's ECN signal in 
> feed-up-and-forward mode and the L2 decapsulator doesn't propagate 
> ECN, it's not relevant because it also doesn't *need to* propagate 
> ECN. Whereas, if the subnet propagates ECN in feed-forward-and-up 
> mode, the decap does *need to*.
>
> I've also extended the above correction into the next definition:
>
>     Not-ECN-PDU:  A PDU that is part of a feedback-loop within whichsome  *at least one*
>        node necessary to propagate*the*  explicit congestion notification*signal in the PDU *  
>        back to the load regulatorare  *is*  notECN-capable*of doing so*.
>
>
>
>     [G] 2. Terminology
>
>     Congestion Baseline:  The location of the function on the path that
>
>     initialised the values of all congestion notification fields in a
>
>     sequence of packets, before any are set to the congestion
>
>     experienced (CE) codepoint if they experience congestion further
>
>     downstream.  Typically the original data source at layer-4.
>
>
>     That's counter-intuitive - a baseline is a level, not a location. I've
>
>     tagged this as a minor issue as opposed to editorial because it causes
>
>     severe confusion in the only place that this term is used, namely
>     item 3
>
>     in Section 4.3.  That item 3 is about a Monitoring Baseline which is
>
>     a level not a location.
>
>     The Congestion Baseline term should be removed, and item 3 in Section
>
>     4.3 revised accordingly.  The term "Congestion Baseline" is used twice
>
>     there, but only the latter of those two uses needs to be retained and
>
>     expanded after deletion of this definition.
>
> You're right. I've done as you suggest.
>
> I've also nearly completely re-written the second half of the section, 
> because I had temporarily forgotten the rationale, and even I couldn't 
> understand what the rationale was after reading my own words.
>
>
>     [H] 3. Modes of Operation
>
>     Feed-Up-and-Forward:  A lower layer switch feeds-up congestion
>
>     notification directly into the ECN field in the higher layer
>
>     (e.g.  IP) header, irrespective of whether the node is at the
>
>     egress of a subnet.
>
>     Remove "the ECN field in" as that's too restrictive, even though
>     that's
>
>     a common case, and move it into the example – “(e.g., ECN field in the
>
>     IP header)" where "header" moved inside the parentheses.
>
> Done.
>
>
>
>     [I] 3.3. Feed-Backward Mode
>
>     Please make it clear that the critique of the ATM ABR relative rate
>
>     control mechanism also applies to Ethernet QCN, as that's a more
>
>     current example.  This should be connected to the QCN discussion at
>
>     the end of Section 4.2.
>
> I didn't want to imply any criticism of QCN, which always made it 
> clear it was only applicable to a single subnet.
>
> However, I can now see the need to have this written down, given 
> certain companies are giving incentives to churn out Frankenpatents 
> (any random combination of existing ideas, no matter whether useful or 
> correct). So I've kept ATM as the example, but added at the end:
>
>     QCN [IEEE802.1Qau] would suffer from similar problems if extended to
>     multiple subnets.  However, from the start QCN was clearly stated as
>     solely applicable to a single subnet (see Section 6).
>
>
>
>     [J] 4.3 Encapsulation Guidelines
>
>     Resolving minor issue [G] requires some edits to item 3 in this
>     section:
>
>     - Remove "(the Congestion Baseline)" from the third paragraph.
>
>     - Rewrite start of fourth paragraph:
>
>     OLD
>
>            Most information can be extracted if the Congestion Baseline is
>
>     standardised at the node that is regulating the load (the Load
>
>     Regulator--typically the data source).
>
>     NEW
>
>            More information can be extracted if the protocol specification
>
>     requires that congestion fields be initialized to indicate no
>
>     congestion only by the node that is regulating the load (the
>
>            Load Regulator--typically the data source).
>
>     END
>
> See minor issue [G] above - already completely rewritten this text.
>
>
>
>     [K] 12.2 Informative References
>
>     Please check the references to 802.1Qah and 802.1Qau with IEEE.  The
>
>     correct references to use now may be the latest version of 802.1Q with
>
>     specific functionality and/or clauses identified.
>
> Done.
> 802.1Qau -> 802.1Q Sections 30--33
> 802.1ah -> 802.1Q (as a whole - too scattered to call out particular 
> sections).
>
> Pat Thaler having retired, I no longer have a co-author who can advise 
> on IEEE citation correctness.
> But the above is good enough.
>
> [I knew someone would point that out - I should have dealt with it 
> but, even tho I purportedly have free access to the full rolled-up 
> text of the current standard (IEEE802.1Q-2018), I've always had 
> trouble viewing it. The IEEE pay-wall insists on displaying the free 
> copy in my browser in a way that it can't be downloaded, but in 
> Firefox the activity bar spins but nothing ever appears. After much 
> gnashing of teeth, I discovered it works OK in Chrome.]
>
>
>     --- Editorial/Nits ---
>
>     Abstract, last sentence
>
>        Following these guidelines should assure
>
>     interworking between new lower layer congestion notification
>
>     mechanisms, whether specified by the IETF or other standards bodies.
>
>     between new -> among IP layer and
>
> Yup.
>
>     1. Introduction, first paragraph
>
>     Suggest removing use of the word "buffer" - "lower layer" suffices in
>
>     all of the places where this word is used in this paragraph.
>
> Not as simple as that. I've guessed that the idea of a buffer doing 
> marking jars with you, so I've addressed the two occurrences of 
> 'buffer' as follows:
>
> 1.   When a lower layer buffer drops a packet
>
> This was intended to introduce the context as congestion loss, not 
> transmission loss. So left unchanged.
>
> 2. In
>     contrast, when a lower layer marks a packet with ECN, the marking
>     needs to be explicitly propagated up the layers.  The same is true if
>     a buffer marks the outer header of a packet that encapsulates inner
>     tunnelled headers.
>
> The second sentence is about tunnels, not lower layers.
> So, I've introduced AQM into both sentences:
>
>     In
>     contrast, when active queue management (AQM) at a lower layer marks a
>     packet with ECN, the marking needs to be explicitly propagated up the
>     layers.  The same is true if AQM marks the outer header of a packet
>     that encapsulates inner tunnelled headers.
>
>
>
>     1. Introduction, second paragraph
>
>        The purpose of this document is to guide the addition of congestion
>
>     notification to any subnet technology or tunnelling protocol, so that
>
>        lower layer equipment can signal congestion explicitly and it will
>
>        propagate consistently into encapsulated (higher layer) headers,
>
>        otherwise the signals will not reach their ultimate destination.
>
>     Suggest: "equipment" -> "functionality"
>
> Sorry, the word 'functionality' gets to me like finger-nails on a 
> blackboard.
> https://www.dailywritingtips.com/is-that-even-a-word/
>
> I've used s/equipment/AQM algorithms/
>
>
>
>     Page 5, first full paragraph:
>
>        If a particular protocol design chooses to contradict a
>
>        'SHOULD (NOT)' given in the advice below, it MUST include a sound
>
>     justification.
>
>     chooses to contradict -> chooses not to follow
>
> Yup
>
>     1.1. Scope, first paragraph
>
>        This document only concerns wire protocol processing of explicit
>
>     notification of congestion.
>
>     Remove "wire"
>
> I think that loses the intended sense in too much ambiguity.
> 'Protocol processing' could include the AQM algorithms, which this 
> sentence is trying to exclude.
>
>
>     Section 3.1, second paragraph
>
>        In these cases, ECN may best be supported by standardising explicit
>
>     notification of congestion into the lower layer protocol that carries
>
>        the data forwards.  It will then also be necessary to define
>     how the
>
>        egress of the lower layer subnet propagates this explicit
>     signal into
>
>        the forwarded upper layer (IP) header.  It can then continue
>     forwards
>
>        until it finally reaches the destination transport (at L4).  Then
>
>        typically the destination will feed this congestion
>     notification back
>
>        to the source transport using an end-to-end protocol (e.g. TCP).
>
>        This is the arrangement that has already been used to add ECN
>     to IP-
>
>        in-IP tunnels [RFC6040], IP-in-MPLS and MPLS-in-MPLS [RFC5129].
>
>     The referents of "It" at the start of the second and third
>     sentences are
>
>     unclear and different.  Suggested rephrasings:
>
>     It will then also be necessary to define ->
>
>          This necessitates also specifying
>
>     It can then continue -> This signal continues
>
> Good.
>
>     Section 3.1, first paragraph after Figure 1
>
>     Cite one or more of the 3GPP references for GTP tunnels, as this
>     is the
>
>     first occurrence of that concept in this draft.
>
> Yup
>
>     Section 3.1, second paragraph after Figure 1
>
>     Cite a Frame Relay reference for the FECN bit..  Moving the [Buck00]
>
>     citation up to immediately follow "Frame Relay" in the first line
>     suffices.
>
> Good.
>
>     Section 3.2 first paragraph
>
>        These are Ethernet switches that bury into the Ethernet payload ...
>
>     bury -> burrow
>
>     [Spelling checker is missing “read user’s mind” feature :-)]
>
> I've used dig.
>
> Hard to read user's mind. Easier to tell user what to think ;)
>
>
>     Section 3.4
>
>     The second paragraph could be improved by citing references for
>     fat-tree
>
>     and Clos network topologies.
>
> Done.
>
>
>     Section 4.1, second paragraph
>
>        Therefore section 4 of
>
>     [I-D.ietf-tsvwg-rfc6040update-shim] requires network operators to
>
>        configure the ingress of a non-ECN tunnel so that it zeros the ECN
>
>        field in the outer IP header.
>
>     non-ECN tunnel -> tunnel that does not support ECN
>
> OK
>
>     Section 4.1, third paragraph
>
>        Even
>
>        if the shim(s) encapsulate a L2 header, it is often possible to
>     find
>
>        an inner IP header within the L2 header
>
>     Latter instance of "L2 header" should be "L2 PDU"
>
> Good.
>
>     Section 4.1, last paragraph
>
>        Instead a companion specification
>
>     [I-D.ietf-tsvwg-rfc6040update-shim] has been prepared that has
>
>     sufficient standards track status to update standards track
>
>     protocols.
>
>
>     sufficient -> the appropriate
>
> Done.
>
>     Authors Addresses
>
>     Please check these - Pat Thaler's contact info probably needs to
>     be updated.
>
> In progress.
>
>
>
> Bob
>
>     Thanks, --David
>
>     ----------------------------------------------------------------
>
>     David L. Black, Senior Distinguished Engineer
>
>     Dell EMC, 176 South St., Hopkinton, MA 01748
>
>     [phone] *New * Mobile: [phone]
>
>     [e-mail] <mailto:[e-mail]>
>
>     ----------------------------------------------------------------
>
>
>
> -- 
> ________________________________________________________________
> Bob Briscoehttp://bobbriscoe.net/

-- 
________________________________________________________________
Bob Briscoe                               http://bobbriscoe.net/