Hi.
Thanks for the quick response.
I think the message I just sent to Acee covers most of this. I'll await the final text on HOLDDOWN_INTERVAL etc.
I would just say that making it even clearer that the example values for timer intervals are not defaults.. implementers may understand the language but they can be lazy.
Cheers,
Elwyn
Sent from Samsung tablet.
-------- Original message --------
From: bruno.decraene@xxxxxxxxxx
Date: 16/02/2018 14:00 (GMT+00:00)
To: "Acee Lindem (acee)" <acee@xxxxxxxxx>, Elwyn Davies <elwynd@xxxxxxxxxxxxxx>
Cc: draft-ietf-rtgwg-backoff-algo.all@xxxxxxxx, ietf@xxxxxxxx, rtgwg@xxxxxxxx, gen-art@xxxxxxxx
Subject: RE: Genart last call review of draft-ietf-rtgwg-backoff-algo-07
Thanks for your review and comments.
Please see inline [Bruno]
> -----Original Message-----
> From: Acee Lindem (acee) [mailto:acee@xxxxxxxxx]
> Sent: Friday, February 16, 2018 1:31 AM
> To: Elwyn Davies; gen-art@xxxxxxxx
> Cc: draft-ietf-rtgwg-backoff-algo.all@xxxxxxxx; ietf@xxxxxxxx; rtgwg@xxxxxxxx
> Subject: Re: Genart last call review of draft-ietf-rtgwg-backoff-algo-07
>
> Hi Elwyn,
>
> On 2/15/18, 2:12 PM, "Elwyn Davies" <elwynd@xxxxxxxxxxxxxx> wrote:
>
> Reviewer: Elwyn Davies
> Review result: Ready with Nits
>
> I am the assigned Gen-ART reviewer for this draft. The General Area
> Review Team (Gen-ART) reviews all IETF documents being processed
> by the IESG for the IETF Chair. Please treat these comments just
> like any other last call comments.
>
> For more information, please see the FAQ at
>
> <https://trac.ietf.org/trac/gen/wiki/GenArtfaq>.
>
> Document: draft-ietf-rtgwg-backoff-algo-07.txt
> Reviewer: Elwyn Davies
> Review Date: 2018/02/15
> IETF LC End Date: 2018/02/14
> IESG Telechat date: 2016/02/22
>
> Summary: Ready with nits. The draft does not refer to OSPFv3 - i am not sure if
> this is an oversight or because ODSPFv3 already has this mechanism - either way
> it should be mentioned.
[Bruno] This is an oversight and has been added.
> One question that occurred to me is whether the draft
> could be considered as updating the OSPFv2/v3 and ISIS standards (not that IETF
> has any control over ISIS).
[Bruno] I personally don't think so. I'd leave this to OSPF/IS-IS/LSR chairs and routing AD.
> Major issues:
> None
>
> Minor issues:
> (Non-)Relation between HOLDDOWN_INTERVAL and *_SPF_DELAY values: I notced that
> Benjamin Kaduk's SECDIR review of this document
> (https://datatracker.ietf.org/doc/review-ietf-rtgwg-backoff-algo-07-secdir-lc-kaduk-2018-02-14/)
> was concerned that certain state transitions would never occur. I loooked at
> this and realized that his assumption that LONG_SPF_DELAY < HOLDDOWN_INTERVAL
> is not required by the document and s6 explicitly resiles from offering
> suggested default values. Without this assumption, the state machine appears
> to be correct. Not being familiar with the consequences of setting the
> HOLDDOWN_INTERVAL relative to the *_SPF_DELAY, I am not sure if anything could
> be said about such consequences,
[Bruno] There is no "significant" bad consequence.
Thinking about this, I could see 2 consequences:
- There could be a SPF computation scheduled after the HOLDDOWN_INTERVAL. I don't see this as an issue as, as stated below by Acee, the HOLDDOWN_INTERVAL is defined as related to the period with no IGP events. It's not defined as the period with no SPF computations (not to mention further computations e.g., from BGP).
- Perhaps more importantly, if an IGP event occurs at t1 in the interval [LONG_SPF_DELAY;HOLDDOWN_INTERVAL], then the SPF computation would be triggered HOLDDOWN_INTERVAL-t1 after the IGP event. While the intuition is that it should be triggered after INITIAL_SPF_DELAY.
> but I think it would avoid other people making
> the same assumption as the SECDIR reviewer if it was explicitly stated that
> HOLDDOWN_INTERVAL is not necessarily bigger than any of the *_SPF_DELAY values
> and adding any advice from experience about how to choose appropriate values.
> This might also avoid naive implementers shortcutting the state machine
> implementation if they made the same assumption.
[Bruno] ok, I would propose the following change:
OLD: In order to satisfy the goals stated in Section 2, operators are RECOMMENDED to configure delay intervals such that INITIAL_SPF_DELAY <= SHORT_SPF_DELAY and SHORT_SPF_DELAY <= LONG_SPF_DELAY.
NEW: In order to satisfy the goals stated in Section 2, operators are RECOMMENDED to configure delay intervals such that INITIAL_SPF_DELAY <= SHORT_SPF_DELAY, SHORT_SPF_DELAY <= LONG_SPF_DELAY and HOLDDOWN_TIMER greater than INITIAL_SPF_DELAY, SHORT_SPF_DELAY, LONG_SPF_DELAY.
> The definition of HOLDDOWN_INTERVAL explicitly states:
>
> HOLDDOWN_INTERVAL: The time required with no received IGP events
> before considering the IGP to be stable again and allowing the
> SPF_DELAY to be restored to INITIAL_SPF_DELAY. e.g., 3 seconds. The
> HOLDDOWN_INTERVAL MUST be defaulted or configured to be longer than
> the TIME_TO_LEARN_INTERVAL.
>
> Perhaps, it should be restated in the third paragraph of section 6.
[Bruno] ok, added.
>
> Requirements Language: Suggest s/RFC2119/RFC8174/ as there are uses of lower
> case versions of the reserved words.
>
> I believe this was brought up before and we will make this change. If not, we'll change to the RFC
> 8174 language.
[Bruno] ok, changed.
> Default values for parameters: There is a possible conflict between s3, where
> example values for the various interval parameters are given and s6 which
> states that no default values are specified in the document. The difference in
> termnology maybe too subtle for some implementers.
>
> I would expect those implementing IGPs to know the different between an "example" and a
> "default".
[Bruno] I agree with Acee.
In s3, I think the examples are useful for the understanding.
In s6, I think the text is clear enough:
" This document does not propose default values for the parameters
because these values are expected to be context dependent.
Implementations are free to propose their own default values."
> Aborting or otherwise of SPF calculation if an IGP event occurs while an SPF
> calculation is in progress. A note about whether this should happen (if it is
> possible) would be desirable.
>
> This is certainly out scope and I'm not sure why anyone would deduce from this draft that an
> implementation should or shouldn't do this.
[Bruno] I agree with Acee. I think adding some text on this may bring confusion.
>
> OSPFv3: Does this (not) equally apply to OSPF v3 for IPv6? If so it should be
> mentioned and RFC 5340 included in the references.
>
> Yes. This will be added as reference.
[Bruno] done.
> s12: I suspect (although it could be arguable) that the ISIS definition, RFC
> 2328 (OSPFv2) and (if added) RFC5340 are normative as you need to understand
> how they work. This work could even be considered to update these documents.
>
> While implemented from the beginning, SPF Backoff is not specified by the IGP protocol
> specifications.
[Bruno] Personally, I'd argue that one does not really need to understand any specific link state protocol in order to add a delay before its SPF computation. In all cases, I'd leave this to the IESG.
> Bruno and I will look at the editorial comments and let you know which ones we do and don't
> incorporate.
Under way....
Thanks,
--Bruno
> Thanks,
> Acee
>
> Nits/editorial comments:
> General: The term 'back-off' may not be familiar to non-Emglish mother tongue
> speakers and on first occurrence needs a little explanation for naive readers
> to indicate what it means and to what the back-off is being applied. I have
> suggested some additional text to this end for the abstract and s1.
>
> Abstract:
> OLD:
> This document defines a standard algorithm to back-off link-state IGP
> Shortest Path First (SPF) computations.
> NEW:
> This document defines a standard algorithm to temporararily postpone or
> 'back-off' link-state IGP Shortest Path First (SPF) computations to reduce
> the computational load on IGP nodes if network events occurring at closely
> spaced times would otherwise lead to multiple, essentially redundant
> recalculations of the routing tables.
> ENDS
>
> s1, para 1: s/at the same time/essentially at the same time/
>
> s1, para 2: s/new Shortest Path First (SPF)/new Shortest Path First (SPF)
> routing table/
>
> s1, para 2:
> OLD:
> experiencing multiple temporally close failures over a short
> period of time
> NEW:
> experiencing multiple temporally close failures (that is, eventuating over a
> short period of time)
> ENDS
>
> s1, para 2: There is a right bracket missing in the following and starting a
> clause with 'such as' and ending it with an ellipsis ('...') is redundant. >
> such as LDP [RFC5036], RSVP-TE [RFC3209], > BGP [RFC4271], Fast ReRoute
> computations (e.g. Loop Free Alternates > (LFA) [RFC5286], FIB updates...
> It is unclear to me where the bracket should go: maybe after [RFC5286] or at
> the end. Please clarify.
>
> s1, para 2: the phrase
> > This also reduces the churn on
> > routers and in the network and.
> is useless, vague jargon. The previous sentence expresses what I suspect is
> meant by 'churn'. so this is redundant and can be omitted.
>
> s1, para 3:
> OLD:
> To allow for this, IGPs implement an SPF back-off algorithm.
> NEW:
> To allow for this, IGPs usually implement an SPF back-off algorithm that
> postpones or backs-off the running of the SPF calculation when the algorithm
> predicts that a run would be essentially redundant or even counter-productive
> because it appears that multiple closely timed routing-affecting events can be
> expected. ENDS
>
> s1, para 3: s/choosen/chosen/
>
> s2, last bullet: SPF_DELAY is not defined at this point:
> s/SPF_DELAY timers values/values for any timers used to back-off SPF
> calculations/
>
> s2, last bullet: s/Even though/This is important even though/
>
> s3, para 1: Undesirable ellipsis:
> s/a metric change on a link or prefix.../and a metric change on a link or
> prefix./
>
> s3:Need to expand SRLG on first use - it isn't deemed to be well-known.
>
> s3, INITIAL_SPF_DELAY bullet: s/A very small delay to quickly handle link
> failure/A very small delay to quickly handle a single isolated link failure/
>
> s3, SHORT_SPF_DELAY bullet:
> OLD:
> SHORT_SPF_DELAY: A small delay to have a fast convergence in case of
> a single failure (node, SRLG..), e.g., 50-100 milliseconds.
> NEW:
> SHORT_SPF_DELAY: A small delay to provide fast convergence in the case of
> a single component failure (node, SRLG..) that leads to multiple IGP events,
> e.g., 50-100 milliseconds.
> ENDS
>
> s5/s5.1: There is currently no text in s5: this is generally considered
> inappropriate. Suggest removing the first sentence in s5.1 ("This section
> describes the state machine.") and adding to s5: NEW: This section describes
> the abstract finite state machine (FSM) intended to control the timing of the
> running of SPF calculations in response to IGP events.
>
> s5.1, QUIET bullet: s/occured/occurred/
>
> s5.2: There is no need for 3 expansions of FSM - the expansion can be moved to
> s5 as suggested above.
>
> s5.3 title: s/States/State/
>
> s6, next to last para: s/it's RECOMMENDED to play it safe/it is recommended
> that timer intervals should be chosen conservatively/ (this is an operational
> recommendation).
>
> s6, last para: s/RECOMMENDED/recommended/ (ditto).
>
> s7, para 1: s/is based on/is dependent on/, s/RECOMMENDED/recommended/
> (operational again)
>
> s8: Other documents (e.g., from vendors) have used the terms SPF wait time and
> SPF hold time. It might be useful to mention that this document essentially
> provides ways to implement these settings.
>
>
>
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