TRILL (TRansparent Interconnection of Lots of Links): ECN (Explicit Congestion Notification) Support
draft-ietf-trill-ecn-support-07

[Ballot comment]
I'm balloting "no objection" based on explanations from the author about
all three points raised in my original discuss. I have requested that the
authors include a summary of these explanations in the document to
aid implementors in understanding why Table 3 is defined the way it is,
so they don't erroneously conclude that the table is incorrect.

My original discuss text and original comments appear below for posterity.

---------------------------------------------------------------------------

Thanks to the authors, chairs, shepherd, and working group for the effort that
has been put into this document.

I have concerns about some ambiguity and/or self-contradiction in this
specification, but I suspect these should be easy to fix. In particular, the
behavior defined in Table 3 doesn't seem to be consistent with the behavior
described in the prose.

For easy reference, I've copied Table 3 here:

>      +---------+----------------------------------------------+
>      | Inner  |  Arriving TRILL 3-bit ECN Codepoint Name    |
>      | Native  +---------+------------+------------+----------+
>      | Header  | Not-ECT | ECT(0)    | ECT(1)    |    CE  |
>      +---------+---------+------------+------------+----------+
>      | Not-ECT | Not-ECT | Not-ECT(*) | Not-ECT(*) |    |
>      |  ECT(0) |  ECT(0) |  ECT(0)    |  ECT(1)    |    CE  |
>      |  ECT(1) |  ECT(1) |  ECT(1)(*) |  ECT(1)    |    CE  |
>      |    CE  |      CE |      CE    |      CE(*) |    CE  |
>      +---------+---------+------------+------------+----------+
>
>                      Table 3. Egress ECN Behavior
>
>  An asterisk in the above table indicates a currently unused
>  combination that SHOULD be logged. In contrast to [RFC6040], in TRILL
>  the drop condition is the result of a valid combination of events and
>  need not be logged.

The prose in this document indicates:

1. Ingress gateway either copies the native header value to the TRILL ECN
    codepoint (resulting in any of the four values above) or doesn't insert
    any ECN information in the TRILL header.

2. Intermediate gateways can set the CCE flag, resulting in "CE" in the
    table above.

Based on the above, a packet arriving at an egress gateway can only be in one of
the following states:

A. TRILL header is Not-ECT because no TRILL node inserted ECN information.

B. TRILL header value == Native header value because the ingress gateway
    copied it from native to TRILL.

C. TRILL header is "CE" because an intermediate node indicated congestion.

If that's correct, I would think that any state other than those three needs
to be marked with an (*). In particular, these two states fall into that
classification, and seem to require an asterisk:

  - Native==CE && TRILL==ECT(0)

  - Native==ECT(0) && TRILL==ECT(1)

In addition, the behavior this table defines for Native==ECT(0) && TRILL==ECT(1)
is somewhat perplexing: for this case, the value in the TRILL header takes
precedence; however, when Native==ECT(1) && TRILL==ECT(0) the Native header
takes precedence. Or, put another way, this table defines ECT(1) to always
override ECT(0). I don't find any prose in here to indicate why this needs to be
treated differentially, so I'm left to conclude that this is a typographical
error. If that's not the case, please add motivating text to Table 3 explaining
why ECT(1) is treated differently than ECT(0) for baseline ECN behavior.

---------------------------------------------------------------------------

I also have a small handful of editorial suggestions and nits to propose.

Please expand "TRILL" upon first use and in the title; see
https://www.rfc-editor.org/materials/abbrev.expansion.txt for guidance.

---------------------------------------------------------------------------
§1:

>  In [RFC3168] it was recognized that tunnels and lower layer protocols

"In [RFC3168], it was..."
(insert comma)

---------------------------------------------------------------------------

§2:

>  These fields are show in Figure 2 as "ECN" and "CCE". The TRILL-ECN

"...are shown..."


>  The CRItE bit is the critical Ingress-to-Egress summary
>  bit and will be one if and only if any of the bits in the CItE range
>  (21-26) is one or there is a critical feature invoked in some further

"...if any of the bits... are one or..."
(replace "is" with "are")



>  The first three have the same meaning as the corresponding ECN field
>  codepoints in the IPv4 or IPv6 header as defined in [RFC3168].

Section 1.1 defines "IP" to mean both IPv4 and IPv6. It would seem cleaner and
easier to read if the document were to leverage that definition here.


>  However codepoint 0b11 is called Non-Critical Congestion Experienced

"However, codepoint..."
(insert comma)

---------------------------------------------------------------------------

§3.3.2:

>  If an RBridge supports ECN, for the two cases of an IP and a non-IPR

"...non-IP"

---------------------------------------------------------------------------

§4:

>  Section 3 specifies interworking between TRILL and the original
>  standardized form of ECN in IP [RFC3168].

Please indicate this at the top of Section 3. When I was puzzling over Table 3,
I spent some time trying to figure out whether the behavior I describe in my
DISCUSS above was due to behavior described in RFC 8311 or the experiments it
contemplates.

---------------------------------------------------------------------------

Appendix A:

>  o  the meaning of CE markings applied by an L4S queue is not the same
>    as the meaning of a drop by a "Classic" queue (contrary to the
>    original requirement for ECN [RFC3168]).

I think, when citing this exception, it makes much more sense to point to RFC
8311 (where the exception to RFC 3168's requirement is defined) than to RFC 3168
in a vacuum.

>    Instead the likelihood

Insert a comma after "Instead".

>    that the Classic queue drops packets is defined as the square of
>    the likelihood that the L4S queue marks packets (e.g. when there

Insert a comma after "e.g.,"

[Ballot discuss]
Thanks to the authors, chairs, shepherd, and working group for the effort that
has been put into this document.

I have concerns about some ambiguity and/or self-contradiction in this
specification, but I suspect these should be easy to fix. In particular, the
behavior defined in Table 3 doesn't seem to be consistent with the behavior
described in the prose.

For easy reference, I've copied Table 3 here:

>      +---------+----------------------------------------------+
>      | Inner  |  Arriving TRILL 3-bit ECN Codepoint Name    |
>      | Native  +---------+------------+------------+----------+
>      | Header  | Not-ECT | ECT(0)    | ECT(1)    |    CE  |
>      +---------+---------+------------+------------+----------+
>      | Not-ECT | Not-ECT | Not-ECT(*) | Not-ECT(*) |    |
>      |  ECT(0) |  ECT(0) |  ECT(0)    |  ECT(1)    |    CE  |
>      |  ECT(1) |  ECT(1) |  ECT(1)(*) |  ECT(1)    |    CE  |
>      |    CE  |      CE |      CE    |      CE(*) |    CE  |
>      +---------+---------+------------+------------+----------+
>
>                      Table 3. Egress ECN Behavior
>
>  An asterisk in the above table indicates a currently unused
>  combination that SHOULD be logged. In contrast to [RFC6040], in TRILL
>  the drop condition is the result of a valid combination of events and
>  need not be logged.

The prose in this document indicates:

1. Ingress gateway either copies the native header value to the TRILL ECN
    codepoint (resulting in any of the four values above) or doesn't insert
    any ECN information in the TRILL header.

2. Intermediate gateways can set the CCE flag, resulting in "CE" in the
    table above.

Based on the above, a packet arriving at an egress gateway can only be in one of
the following states:

A. TRILL header is Not-ECT because no TRILL node inserted ECN information.

B. TRILL header value == Native header value because the ingress gateway
    copied it from native to TRILL.

C. TRILL header is "CE" because an intermediate node indicated congestion.

If that's correct, I would think that any state other than those three needs
to be marked with an (*). In particular, these two states fall into that
classification, and seem to require an asterisk:

  - Native==CE && TRILL==ECT(0)

  - Native==ECT(0) && TRILL==ECT(1)

In addition, the behavior this table defines for Native==ECT(0) && TRILL==ECT(1)
is somewhat perplexing: for this case, the value in the TRILL header takes
precedence; however, when Native==ECT(1) && TRILL==ECT(0) the Native header
takes precedence. Or, put another way, this table defines ECT(1) to always
override ECT(0). I don't find any prose in here to indicate why this needs to be
treated differentially, so I'm left to conclude that this is a typographical
error. If that's not the case, please add motivating text to Table 3 explaining
why ECT(1) is treated differently than ECT(0) for baseline ECN behavior.

[Ballot comment]
I also have a small handful of editorial suggestions and nits to propose.

Please expand "TRILL" upon first use and in the title; see
https://www.rfc-editor.org/materials/abbrev.expansion.txt for guidance.

---------------------------------------------------------------------------
§1:

>  In [RFC3168] it was recognized that tunnels and lower layer protocols

"In [RFC3168], it was..."
(insert comma)

---------------------------------------------------------------------------

§2:

>  These fields are show in Figure 2 as "ECN" and "CCE". The TRILL-ECN

"...are shown..."


>  The CRItE bit is the critical Ingress-to-Egress summary
>  bit and will be one if and only if any of the bits in the CItE range
>  (21-26) is one or there is a critical feature invoked in some further

"...if any of the bits... are one or..."
(replace "is" with "are")



>  The first three have the same meaning as the corresponding ECN field
>  codepoints in the IPv4 or IPv6 header as defined in [RFC3168].

Section 1.1 defines "IP" to mean both IPv4 and IPv6. It would seem cleaner and
easier to read if the document were to leverage that definition here.


>  However codepoint 0b11 is called Non-Critical Congestion Experienced

"However, codepoint..."
(insert comma)

---------------------------------------------------------------------------

§3.3.2:

>  If an RBridge supports ECN, for the two cases of an IP and a non-IPR

"...non-IP"

---------------------------------------------------------------------------

§4:

>  Section 3 specifies interworking between TRILL and the original
>  standardized form of ECN in IP [RFC3168].

Please indicate this at the top of Section 3. When I was puzzling over Table 3,
I spent some time trying to figure out whether the behavior I describe in my
DISCUSS above was due to behavior described in RFC 8311 or the experiments it
contemplates.

---------------------------------------------------------------------------

Appendix A:

>  o  the meaning of CE markings applied by an L4S queue is not the same
>    as the meaning of a drop by a "Classic" queue (contrary to the
>    original requirement for ECN [RFC3168]).

I think, when citing this exception, it makes much more sense to point to RFC
8311 (where the exception to RFC 3168's requirement is defined) than to RFC 3168
in a vacuum.

>    Instead the likelihood

Insert a comma after "Instead".

>    that the Classic queue drops packets is defined as the square of
>    the likelihood that the L4S queue marks packets (e.g. when there

Insert a comma after "e.g.,"

[Ballot comment]
I agree with Mirja about the status of L4S, but would go even farther - L4S is only one of the ECN experiments that https://datatracker.ietf.org/doc/rfc8311/ was intended to accommodate, so you might want to capture that in the appendix (basically saying "L4S is one example of the ways TRILL ECN handling may evolve", or something like that).

Is

  If an RBridge supports ECN, for the two cases of an IP and a non-IPR
  inner packet, the egress behavior is as follows:

really "non-IPR"? I'm guessing it should be "non-IP".

s/significnat/significant/

[Ballot comment]
Given L4S is not published yet and in any case experimental, I would recommend to remove section 4.2 entirely and just keep the appendix as an informational documentation of the proposed alogrithm.

(Via drafts-lastcall@iana.org): IESG/Authors/WG Chairs:

The IANA Services Operator has completed its review of draft-ietf-trill-ecn-support-04. If any part of this review is inaccurate, please let us know.

The IANA Services Operator understands that, upon approval of this document, there is a single action which we must complete.

In the TRILL Extended Header Flags registry on the Transparent Interconnection of Lots of Links (TRILL) Parameters registry page located at:

https://www.iana.org/assignments/trill-parameters/

The existing entry for bits 9-13 is currently:

Bits Purpose Reference
-------+---------------------------------------------+--------------
9-13 available non-critical hop-by-hop flags [RFC7179]

It will be changed to the following two lines:

Bits Purpose Reference
-------+---------------------------------------------+--------------
9-11 available non-critical hop-by-hop flags [RFC7179]
12-13 TRILL-ECN (Explicit Congestion Notification) [ RFC-to-be ]

The existing entry for bits 21-26 is currently:

Bits Purpose Reference
-------+---------------------------------------------+--------------
21-26 available critical ingress-to-egress flag [RFC7179]

This will be changed to:

Bits Purpose Reference
-------+---------------------------------------------+--------------
21-25 available critical ingress-to-egress flag [RFC7179]
26 Critical Congestion Experienced (CCE) [ RFC-to-be ]

The IANA Services Operator understands that this is the only action required to be completed upon approval of this document.

Note:  The actions requested in this document will not be completed until the document has been approved for publication as an RFC. This message is only to confirm the list of actions that will be performed.


Thank you,

Sabrina Tanamal
Senior IANA Services Specialist

The following Last Call announcement was sent out (ends 2018-02-05):

From: The IESG
To: IETF-Announce
CC: Susan Hares , akatlas@gmail.com, trill-chairs@ietf.org, trill@ietf.org, draft-ietf-trill-ecn-support@ietf.org, shares@ndzh.com
Reply-To: ietf@ietf.org
Sender:
Subject: Last Call:  (TRILL: ECN (Explicit Congestion Notification) Support) to Proposed Standard


The IESG has received a request from the Transparent Interconnection of Lots
of Links WG (trill) to consider the following document: - 'TRILL: ECN
(Explicit Congestion Notification) Support'
  as Proposed Standard

The IESG plans to make a decision in the next few weeks, and solicits final
comments on this action. Please send substantive comments to the
ietf@ietf.org mailing lists by 2018-02-05. Exceptionally, comments may be
sent to iesg@ietf.org instead. In either case, please retain the beginning of
the Subject line to allow automated sorting.

Abstract


  Explicit congestion notification (ECN) allows a forwarding element to
  notify downstream devices, including the destination, of the onset of
  congestion without having to drop packets. This can improve network
  efficiency through better flow control without packet drops. This
  document extends ECN to TRILL switches, including integration with IP
  ECN, and provides for ECN marking in the TRILL Header Extension Flags
  Word (see RFC 7179).





The file can be obtained via
https://datatracker.ietf.org/doc/draft-ietf-trill-ecn-support/

IESG discussion can be tracked via
https://datatracker.ietf.org/doc/draft-ietf-trill-ecn-support/ballot/


No IPR declarations have been submitted directly on this I-D.


The document contains these normative downward references.
See RFC 3967 for additional information:
    draft-ietf-tsvwg-ecn-encap-guidelines: Guidelines for Adding Congestion Notification to Protocols that Encapsulate IP (None - IETF stream)

(1) What type of RFC is being requested (BCP, Proposed Standard,
    Internet Standard, Informational, Experimental, or Historic)? Why
    is this the proper type of RFC? Is this type of RFC indicated in
    the title page header?

Proposed Standard status is requested as indicated on the title
page. This document modifies the TRILL protocol by specifying, with
backward compatibility, the use of previously reserved TRILL Header
bits.

(2) The IESG approval announcement includes a Document Announcement
    Write-Up. Please provide such a Document Announcement Write-Up.
    Recent examples can be found in the "Action" announcements for
    approved documents. The approval announcement contains the
    following sections:

  Technical Summary:

This specification provides for any Explicit Congestion Notification
(ECN) marking in traffic ingressed by TRILL to be copied into the
TRILL Extension Header Flags Word [RFC7179]. It also enables
congestion marking by a congested transit TRILL switch in the TRILL
Header Extension Flags Word.  At TRILL egress, it specifies how any
ECN markings in the TRILL Header Flags Word and in the encapsulated
traffic are combined so that subsequent forwarding elements can see if
congestion was experienced at any previous point in the path from the
Source.

  Working Group Summary:

There was no particular controversy about this document. Although some
minor technical refinements were made during the WG process, the WG
appears to have been favorably disposed towards this work from the
beginning.

  Document Quality:

No implementations of draft-ietf-trill-ecn-support-03.
Document has been reviewed by RTG-DIR (QA review).

  Personnel:
    Document Shepherd: Susan Hares
    Responsible Area Director: Alia Atlas
      RTG-DIR review: Loa Anderson
Review 1:
https://datatracker.ietf.org/doc/review-ietf-trill-ecn-support-01-rtgdir-early-andersson-2017-01-21/
Review 2:
https://datatracker.ietf.org/doc/review-ietf-trill-ecn-support-03-rtgdir-early-andersson-2017-06-19/

Shepherd's report
https://mailarchive.ietf.org/arch/msg/trill/8L-hgWSUw4zrWyAq3MbSOlg8iI8

(3) Briefly describe the review of this document that was performed
    by the Document Shepherd. If this version of the document is not
    ready for publication, please explain why the document is being
    forwarded to the IESG.

  Document has been reviewed for technical content against normative references.
  (see shepherd's report email
https://mailarchive.ietf.org/arch/msg/trill/8L-hgWSUw4zrWyAq3MbSOlg8iI8).

  ID-NITS have been, and the only nits is that the copyright year is 2015.
  The shepherd has been backlogged with other work, and did not
  get this reviewed promptly in December.  The first editorial revision from the
  AD or the IESG will fix it.

(4) Does the document Shepherd have any concerns about the depth or
    breadth of the reviews that have been performed?

No concerns about the reviews of this document.

(5) Do portions of the document need review from a particular or from
    broader perspective, e.g., security, operational complexity, AAA,
    DNS, DHCP, XML, or internationalization? If so, describe the
    review that took place.

Normal reviews for routing draft (rtg-dir, ops-dir, sec-dir)
and any reviews AD feels is necessary for INT ECN.

(6) Describe any specific concerns or issues that the Document
    Shepherd has with this document that the Responsible Area
    Director and/or the IESG should be aware of? For example, perhaps
    he or she is uncomfortable with certain parts of the document, or
    has concerns whether there really is a need for it. In any event,
    if the WG has discussed those issues and has indicated that it
    still wishes to advance the document, detail those concerns here.

No particular concernts.

(7) Has each author confirmed that any and all appropriate IPR
    disclosures required for full conformance with the provisions of
    BCP 78 and BCP 79 have already been filed. If not, explain why?

Yes, see
  Bob Biscoe
https://www.ietf.org/mail-archive/web/trill/current/msg07790.html
  Donald Eastlake
https://www.ietf.org/mail-archive/web/trill/current/msg07789.html

(8) Has an IPR disclosure been filed that references this document?
    If so, summarize any WG discussion and conclusion regarding the
    IPR disclosures.

No IPR disclosure has been filed against this document.

(9) How solid is the WG consensus behind this document? Does it
    represent the strong concurrence of a few individuals, with
    others being silent, or does the WG as a whole understand and
    agree with it?

WG had good initial discussions, and strong concurrent on technology.

(10) Has anyone threatened an appeal or otherwise indicated extreme
    discontent?

No.

(11) Identify any ID nits the Document Shepherd has found in this
    document. (See http://www.ietf.org/tools/idnits/ and the
    Internet-Drafts Checklist). Boilerplate checks are not enough;
    this check needs to be thorough.

Only ID nit complaint is that it noticed some pseudo-code and
suggested it might be bracketed with and
lines but this seems inappropriate for psuedo-code that is not
intended to be compiled.    will also
trigger YANG automatic review - which would be a mistake.

(12) Describe how the document meets any required formal review
    criteria, such as the MIB Doctor, media type, and URI type
    reviews.

No such formal review required.

(13) Have all references within this document been identified as
    either normative or informative?

Yes.

(14) Are there normative references to documents that are not ready
    for advancement or are otherwise in an unclear state? If such
    normative references exist, what is the plan for their
    completion?

There is a normative reference to
draft-ietf-tsvwg-ecn-encap-guidelines which is intended to be a Best
Current Practice.

(15) Are there downward normative references references (see RFC
    3967)? If so, list these downward references to support the Area
    Director in the Last Call procedure.

There are no downward references.

(16) Will publication of this document change the status of any
    existing RFCs? Are those RFCs listed on the title page header,
    listed in the abstract, and discussed in the introduction? If the
    RFCs are not listed in the Abstract and Introduction, explain
    why, and point to the part of the document where the relationship
    of this document to the other RFCs is discussed. If this
    information is not in the document, explain why the WG considers
    it unnecessary.

This document does not change the status of any existing RFC.

(17) Describe the Document Shepherd's review of the IANA
    considerations section, especially with regard to its consistency
    with the body of the document. Confirm that all protocol
    extensions that the document makes are associated with the
    appropriate reservations in IANA registries. Confirm that any
    referenced IANA registries have been clearly identified.

The only IANA action required by this document is the allocation of
three bits in the TRILL Extended Header Flags for which a registry
already exists. This is clearly indicated in the IANA Considerations
section.

(18) List any new IANA registries that require Expert Review for
    future allocations. Provide any public guidance that the IESG
    would find useful in selecting the IANA Experts for these new
    registries.

No new registries created.

(19) Describe reviews and automated checks performed by the Document
    Shepherd to validate sections of the document written in a formal
    language, such as XML code, BNF rules, MIB definitions, etc.

Although there is a little pseudo-code, there is no mormal language
requiring validation in this document.