Re: [trill] QA review of draft-ietf-trill-ecn-support

[Donald Eastlake <[e-mail]>]

Hi Loa,

Thanks for the update.

Donald
===============================
 Donald E. Eastlake 3rd   [phone] (cell)
 155 Beaver Street, Milford, MA 01757 USA
 [e-mail]

On Mon, Jun 19, 2017 at 6:00 AM, Loa Andersson <[e-mail]> wrote:

> Authors, Working Group,
>
> Sorry for the late reply.
>
> I've been asked to re-review the document since it is (was) in working
> group last call.
>
> I've reviewed again
>
> I find that
> - my earlier comments has been satisfactorily addressed
> - I have no further comments on this document
> - I believe we are ready to ask for publication.
>
> /Loa
>
>
> On 2017-01-21 06:02, Loa Andersson wrote:
>
>> Authors,
>>
>> I have been asked to do a Routing Area Directorate QA review of
>> draft-ietf-trill-ecn-support.
>>
>> Caveat - I'm not a congestion control expert, and this will show up in
>> my comments. The place where I ask for clarifications might be perfectly
>> clear for a reader that is up to speed in the area.
>>
>> Summary:
>>
>> I think the document is on the right track, for a reader not an expert
>> in the area there are a lot of abbreviations that are not properly
>> expanded. I also think that it would be a good idea to more clearly
>> make the the case why the document is needed (abstract and/or
>> introduction).
>>
>> After a while I stop trying to distinguish between "Minor issues" and
>> "Nits" and placed everything as Minor Issues. I guess I could have
>> done everything as Nits :).
>>
>> Other than the Comment/Minor Issues I find the document pretty solid,
>> though I sometimes found it hard to parse sentences.
>> If you want I can take a look at that aspect once what is in this
>> review has been addressed.
>>
>>
>> Comments:
>>
>> Last paragraph of the Introduction
>> ----------------------------------
>>
>>    Whichever RBridges do not support ECN, this
>>    specification ensures congestion notification will propagate safely
>>    to Destination because the packet will be dropped if explicit
>>    congestion notification cannot be propagated and drop is itself an
>>    implicit form of congestion notification.
>>
>> Is this logic really watertight? What if the packet is dropped because
>> of a checksum error?
>>
>>
>>
>> Major Issues:
>>
>>
>> Minor Issues:
>>
>> Abstract
>> --------
>> I find the Abstract a bit meager, a little more context would be good.
>>
>>    Maybe lead with some short words about what ECN is good for.
>>
>> And maybe use this from the Introduction:
>>
>>    This specification provides for any ECN marking in the traffic at the
>>    ingress to be copied into the TRILL Extension Header Flags Word. It
>>    also enables congestion marking by a congested RBridge such as RBn or
>>    RB1 above in the TRILL Header Extension Flags Word [RFC7179].
>>
>> ECNencapGuide
>> -------------
>>
>> This reference has expired, probably not a problem since Bob is a
>> co-author of both documents.
>>
>> TRILL Header
>> ------------
>>
>> Referred to in section in the Introduction, I think a reference would be
>> good.
>>
>> The ECN Specific Extended Header Flags
>> --------------------------------------
>>
>> The pictures is less than intuitive, it took me quite some time de-code
>> it.
>> I did the following:
>> Critical (?) flags
>>  0 - CRHbH (no expansion found in document)
>>  1 - CRItE (no expansion found in document)
>>  2 - CRRsv (no expansion found in document)
>>
>> CHbH flags (Critical Hop by Hop flags - no expansion found in document)
>>  3 - unassigned
>>  4 - unassigned
>>  5 - unassigned
>>  6 - unassigned
>>  7 - CRCAF (no expansion found in document)
>>
>> NCHbH flags = Non-Critical Hop-by-Hop flags
>>  8 - NCCAF (no expansion found in document)
>>  9 - unassigned
>> 10 - unassigned
>> 11 - unassigned
>> -------------------------------------------
>> 12 - ECN = Explicit Congestion Notification
>> 13   (two bit flags)
>> -------------------------------------------
>>
>> CRSV flags (no expansion found in document)
>> -------------------------------------------
>> 14 - Ext Hop Cnt (no expansion found in document)
>> 15   three bit field
>> 16
>> ------------------------------------------
>>
>> NCRSV flags (no expansion found in document)
>> 17 - unassigned
>> 18 - unassigned
>> 19 - unassigned
>> 20 - unassigned
>> ------------------------------------------
>>
>> CItE flags = Critical Ingress-to-Egress
>> ------------------------------------------
>> 21 - unassigned
>> 22 - unassigned
>> 23 - unassigned
>> 24 - unassigned
>> 25 - unassigned
>> 26 - CCE = Critical Congestion Experienced
>> ------------------------------------------
>>
>> NCItE flags = Non Critical Ingress-to-Egress
>> --------------------------------------------
>> 27 - Ext Clr (no expansion found in document)
>> 28   two bit field
>> --------------------------------------------
>> 29 - unassigned
>> 30 - unassigned
>> 31 - unassigned
>>
>> Multi-bit flags
>> ---------------
>>
>> In the context I've been active "flags" are one bit, if there are
>> multiple bits they are called fields. I understand that in the context
>> this is written there are multiple bit flags.
>>
>> Bit 11 and 12
>> -------------
>>
>> Bit 11 and 12 has the following code points:
>>
>>           Binary  Name     Meaning
>>           ------  -------  -----------------------------------
>>             00     Not-ECT Not ECN-Capable Transport
>>             01     ECT(1)  ECN-Capable Transport (1)
>>             10     ECT(0)  ECN-Capable Transport (0)
>>             11     NCCE    Non-Critical Congestion Experienced
>>
>>                     Table 1. TRILL-ECN Field Codepoints
>>
>> There is no good explanation what ECT(0) and ECT(1) means, even though
>> it is (page 9) said that "ECT(1) as a lesser severity level, termed the
>> Threshold-Marked (ThM) codepoint". It could be inferred that ECT(0) is
>> a higher severity level, but this should be clearly spelled out.
>>
>> RFC 3168 does not make the distinction between ECT(0) and ECT(1), but
>> says that it will be done in future RFCs; since this is about 3000 RFCs
>> ago it might have happened but I couldn't find it. If this has been done
>> I think a reference would be good.
>>
>> Code Point 0b11
>> ---------------
>> The text above Table 1 says:
>> OLD
>> "However codepoint 11 is called Non-Critical Congestion Experienced
>> (NCCE)..."
>> I think this should be:
>> However code point 0b11 is called Non-Critical Congestion Experienced
>> (NCCE)..."
>>
>> The text further says that the code point is call NCCE to distinguish
>> it from Congestion Experienced in IP. The question I have is why it is
>> necessary to distinguish code point 0b11, but not 0b00, 0b01 and 0b10?
>>
>> ECN SUpport
>> -----------
>>
>> The first paragraph has "logically" at three places, what would be the
>> difference if these "logically" are dropped?
>>
>>
>> First sentence in sectuion 3.1
>> ------------------------------
>>
>> The sentence says:
>> "The ingress behavior is as follows:"
>>
>> I would say
>> "The behavior of an ingress RBridge is as follows:"
>> or even
>> "The behavior of an ingress RBridge MUST be as follows:"
>>
>> cleared vs set to zero
>> ----------------------
>> The last sub-bullet in the first main bullet of section 3.1 says:
>> "ensure the CCE flag is cleared to zero (Flags Word bit 26)." I would
>> have used "cleared" or "swt to zero".
>>
>> First line section 3,2
>> ----------------------
>> s/ahow/shown
>> Caveat I normally don't English grammar reviews, but sometimes I can't
>> stop myself :)
>>
>> Second line first main bullet in section 3.2
>> --------------------------------------------
>>
>> I prefer the format "guidelines in RFC 7567 [RFC7567]"
>>
>> Third sub-bullet in the third main bullet of section 3.2
>> ---------------------------------------------------------
>>
>> It says:
>> "+  set the TRILL-ECN field to Not-ECT (00);"
>>
>> Here you use "field" instead of "flag", which is fine, but the document
>> should be consistent. Either:
>> +  set the TRILL-ECN field to Not-ECT (0b00);
>> or
>> +  set the TRILL-ECN flag to Not-ECT (0b00);
>>
>> Egress ECN Support
>> ------------------
>> First sentence:
>>   "If the egress RBridge does not support ECN, it will ignore bits 12
>>    and 13 of any Flags Word that is present, because it does not contain
>>    any special ECN logic."
>>
>> in "it will ignore" what does "it" refer to?
>>
>> SHould it be:
>>
>>   "If the egress RBridge does not support ECN, the RBridge will ignore
>>    the TRILL-ECN field (bits 12 and 13) if a Flags Word that is
>>    present, because it has no ECN logic."
>>
>> TRILL Support for ECN Variants
>> ------------------------------
>> The sedond sentence of section four says:
>>
>>    Section 3 specifies interworking between TRILL and the original
>>    standardized form of ECN in IP [RFC3168].
>>
>> RFC 3168 is updated by RFC 4301, RFC 6040, does section 3 relate to
>> RFC 3168 or does the updates come into plan. IF the updates are in
>> scope I think the sentence should say:
>>
>>    Section 3 specifies interworking between TRILL and the original
>>    standardized form of ECN in IP RFC 3168 [RFC3168], and the updates
>>    in RFC4310 [RFC4301] and RFC 6040 [6040].
>>
>>
>>
>>
>>
>> Nits:
>>
>> Codepoints
>> ----------
>> at several places "codepoint(s)" I think the words IANA and the
>> RFC Editor use is "code point(s)"
>>
>>
>>
>> /Loa
>>
>
> --
>
>
> Loa Andersson                        email: [e-mail]
> Senior MPLS Expert                          [e-mail]
> Huawei Technologies (consultant)     phone: [phone]
>