Hi Loa, The -02 version of draft-ietf-trill-ecn-support, just posted, included the improvements we have discussed.
Thanks, Donald =============================== Donald E. Eastlake 3rd +1-508-333-2270 (cell) 155 Beaver Street, Milford, MA 01757 USA d3e...@gmail.com On Sat, Jan 28, 2017 at 3:44 AM, Loa Andersson <l...@pi.nu> wrote: > Donald, > > I'm pretty happy with your response on my comments. I think that the > agreed updates will go a long way to improve readability of the > document. > > /Loa > > > On 2017-01-27 06:09, Donald Eastlake wrote: > >> Hi Loa, >> >> Thanks for your comments. See below >> >> On Sat, Jan 21, 2017 at 12:02 AM, Loa Andersson <l...@pi.nu> 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 :). >>> >> >> OK :-) >> >> Other than the Comment/Minor Issues I find the document pretty solid, >>> >> >> Thanks. >> >> 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? >>> >> >> I think the point of that paragraph is to overcome the presumption >> many readers might have that ECN marking would be useless if the >> Destination does not understand ECN marking. >> >> 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]. >>> >> >> OK. How about this: >> >> 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). >> >> 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. >>> >> >> OK. >> >> 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 >>> >> >> I'm not sure how much explanation/definition needs to appear in this >> document for bits that are irrelevant to the document. All of your "no >> expansion found in document" comments are true and probably a bit more >> should be said but the draft does, in connection with bits other than >> those it specifies, say >> >> See [RFC7780] and [RFC7179] for the meaning of the other >> bits. >> >> The format seems moderately clear and is the same as in Section 10.2 >> of RFC 7780. Adding a mention that the TRILL Header Extension Flags >> Word is 32-bits and the bit numbers are across the top might >> help. Also, for those wishing to see a more tabular presentation, a >> pointer to the IANA Registry >> http://www.iana.org/assignments/trill-parameters/trill- >> parameters.xhtml#extended-header-flags >> might help. >> >> 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. >>> >> >> At the beginning of Section 2, it says "a two-bit TRILL-ECN >> field". Since the name of the TRILL Header "Extension Flags Word" is >> now embedded in multiple RFCs, I don't think that should be changed >> but I wouldn't have any problem reviewing the draft to see that >> multi-bit regions of that 32-bit word are consistently referred to as >> fields. >> >> 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. >>> >> >> That distinction between ECT(0) and ECT(1) only applies if the PCN >> variant of ECN is in use which is why the text you quote is in a >> subsection of Section 4 (TRILL Support for ECN Variants). There is a >> reference to RFC 6660 which talks about this. Perhaps we can make that >> clearer. >> >> 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. >>> >> >> This distincition is in RFC 6660 on PCN (Pre-Congestion Notification). >> >> 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)..." >>> >> >> OK. >> >> 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? >>> >> >> Because the meaning of the other three code points is the same in >> TRILL and IP. >> >> ECN SUpport >>> ----------- >>> >>> The first paragraph has "logically" at three places, what would be the >>> difference if these "logically" are dropped? >>> >> >> Probably not. I think that word was included to be careful not to >> overly constrain the order of processing within a TRILL switch. >> >> 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:" >>> >> >> OK. >> >> 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". >>> >> >> I think "set to zero" is clearer. >> >> First line section 3,2 >>> ---------------------- >>> s/ahow/shown >>> Caveat I normally don't English grammar reviews, but sometimes I can't >>> stop >>> myself :) >>> >> >> OK. >> >> Second line first main bullet in section 3.2 >>> -------------------------------------------- >>> >>> I prefer the format "guidelines in RFC 7567 [RFC7567]" >>> >> >> I really don't see any benefit to that and I think "RFC xxxx >> [RFCxxxx]" is not used in TRILL RFCs. >> >> 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); >>> >> >> I agree with your earlier comment that "field" is generally better. >> >> 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." >>> >> >> Yes, although it might be even clearer in the first line to say >> "... ECN, that RBridge will ..." >> >> TRILL Support for ECN Variants >>> ------------------------------ >>> The second 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]. >>> >> >> Interestingly, although RFC Editor meta-data consistencly shows RFC >> 4301 as updating RFC 3168, this is not indicated on the first page of >> RFC 4301. While ECN is mentioned a number of times in RFC 4301 and >> that RFC could be viewed as extending ECN, I'm not quite sure what, if >> any, change RFC 4301 makes to the behavior standardized RFC in 3168. >> >> I'll look at this more closely but I think the text currently in the >> draft is OK and need not refer to either RFC 4301 or RFC 6040. >> >> Nits: >>> >>> Codepoints >>> ---------- >>> at several places "codepoint(s)" I think the words IANA and the >>> RFC Editor use is "code point(s)" >>> >> >> While I didn't to an exhaustive search, everywhere I looked in the ECN >> and PCN RFCs, "codepoint" is used so I would prefer to leave this >> unchanged for now. >> >> Thanks, >> Donald >> =============================== >> Donald E. Eastlake 3rd +1-508-333-2270 (cell) >> 155 Beaver Street, Milford, MA 01757 USA >> d3e...@gmail.com >> >> /Loa >>> -- >>> >>> Loa Andersson email: l...@mail01.huawei.com >>> Senior MPLS Expert l...@pi.nu >>> Huawei Technologies (consultant) phone: +46 739 81 21 64 >>> >> > -- > > > Loa Andersson email: l...@mail01.huawei.com > Senior MPLS Expert l...@pi.nu > Huawei Technologies (consultant) phone: +46 739 81 21 64 >
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