RE: Genart last call review of draft-ietf-rtgwg-backoff-algo-07

[<[e-mail]>]

Hi Elwyn, Acee,

Thanks for your review and comments.
Please see inline [Bruno]

 > -----Original Message-----
 > From: Acee Lindem (acee) [mailto:[e-mail]]
 > Sent: Friday, February 16, 2018 1:31 AM
 > To: Elwyn Davies; [e-mail]
 > Cc: [e-mail]; [e-mail]; [e-mail]
 > Subject: Re: Genart last call review of draft-ietf-rtgwg-backoff-algo-07
 > 
 > Hi Elwyn,
 > 
 > On 2/15/18, 2:12 PM, "Elwyn Davies" <[e-mail]> 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 &lt;= SHORT_SPF_DELAY and SHORT_SPF_DELAY &lt;= 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 &lt;= SHORT_SPF_DELAY, SHORT_SPF_DELAY &lt;= 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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