Re: draft-ietf-rtgwg-segment-routing-ti-lfa : A simple pathological network fragment

[Ahmed Bashandy <[e-mail]>]

Stewart,

Bruno and Sasha have already responded to your original email.

I have other responses to this email which, at best, shows a severe lack 
of understanding of the draft. See inline starting with #Ahmed


Ahmed


On 11/2/23 12:29 AM, Stewart Bryant wrote:
> Let me ask a fundamental question.
>
> The whole point of Ti-LFA as sold to the community was that repairing 
> along the post convergence path as opposed to repairing along a 
> convenient temporary path avoided micro loops.

#Ahmed: Incorrect statement (as well as lack of understanding of the 
draft). A repair path along the post convergence path is one of the 
important advantages but definitely NOT "The whole point" of Ti-LFA.

> The repair path constraint and subsequent segment optimisations add 
> complexity to the calculation of the path.

#Ahmed: Incorrect statement again (likewise shows lack of understanding 
of the draft). I was not able to see any wording in the draft that 
suggests repairing along the post-convergence is a **constraint**. Using 
the post-convergence path is a highly desired optimization, NOT a 
constraint. If there is wording in the draft that indicates that a 
Ti-LFA repair path MUST or SHOULD be along the post convergence path, I 
would be very happy to look at it.

>
> Are we are now saying that micro loops can form elsewhere and as a 
> consequence we need a micro loop avoidance strategy?

#Ahmed: Incorrect phrase: "as a consequence". Ti-LFA is NOT the source 
and has nothing to do with uloops forming "elsewhere" and hence using or 
not using uloop avoidance mechanism(s) is independent of ti-lfa. Again, 
your own RFCs indicate that uloop is a different problem and is not be 
addressed within the scope of IP FRR

Also the entire sentence or question is a clear off-topic digression. 
The objective of all IP FRR proposals as it is clearly mentioned in the 
ti-lfa draft and the RFCs that you co-authored is to protect traffic 
**passing through the PLR** (that implements IP FRR) against some or all 
failures **local** to that PLR. I am not really sure why are you 
referring to other places in the network in the context of local 
failures. Besides, if you think that a routing correction or failure 
mitigation/avoidance mechanism must or should also include a uloop 
avoidance mechanism, we might as well throw away all routing protocols 
that have been running for past 25-30 years together with any 
optimizations that were developed after (many of which you have 
proposed/co-authored).

>
> If so the fundamental premise behind TiLFA is broken and the repair 
> can simply become: use SR to expeditiously route the packets into Q 
> space and run a micro loop avoidance strategy.

#Ahmed: A sentence with multiple mistakes (one more obvious indication 
of lack of understanding).

1. The PQ algorithm is **one** of the algorithms to calculate one or 
more repair paths. The draft does NOT say you must or should use the PQ 
algorithm. Just like other claims, if there is wording that indicates 
that, I would like to see it

2. "Premise behind Ti-LFA is broken": Why do you think that there is 
something called a "premise" for ti-lfa? And what is that "premise"?

3. Using "and" to combine Ti-LFA and uloop avoidance: The draft does NOT 
say that you must or should use both at the same time. In fact the last 
sentence in the first paragraph in the Introduction says that that uloop 
is addressed in a separate document. Your own RFCs indicate the same. 
Again if there is wording that indicates a MUST or SHOULD use uloop 
avoidance in conjunction with ti-lfa, I will be happy to look at it

> This approach removes the complexity of constraining the repair to the 
> post convergence path.

#Ahmed: Again the **incorrect** claim that post convergence is a constraint

>
> Of course an implementor could cheat and just used the simplified 
> strategy I describe above and almost certainly very few operators 
> would notice because:

#Ahmed: There are more than one ti-lfa implementation deployed. Can you 
point to one of these implementors that have "cheated" and any 
indication of such "cheat" (at least according to your definition of 
"cheat")?

>
> 1) In many cases the two paths would be congruent
>
> 2) The transient is short and quite difficult to instrument
>
> 3) Unless there was some security reason or traffic management reason 
> for the path constraint few would care.
>
> I will look at the proposed differences later, but this sounds like it 
> should be a topic for discussion in RTGWG before the text is finalised 
> and sent the RFC editor.
#Ahmed: Pushing a clear off-topic discussion down the throat of the 
ti-lfa draft is not OK. Feel free to start any discussion away from 
ti-lfa, unless it is within the scope of ti-lfa
>
> - Stewart
>
>> On 2 Nov 2023, at 05:09, Gyan Mishra <[e-mail]> wrote:
>>
>>
>> Hi Sasha, Bruno & Stewart
>>
>> Thank you for going over my OPSDIR review in detail.
>>
>> I am good with the latest updated verbiage that Bruno had given.
>>
>> Comments in-line
>>
>> On Mon, Oct 23, 2023 at 8:41 AM Alexander Vainshtein 
>> <[e-mail]> wrote:
>>
>>     Bruno,
>>
>>     Lots of thanks for a prompt and very encouraging response!
>>
>>     Your version of the text is definitely better than mine, I am all
>>     for using it.
>>
>>     As for where the clarifying text could be inserted, I see two
>>     options:
>>
>>       * A common “Applicability Statement” section (there is no such
>>         section in the draft)
>>
>>      *
>>
>>
>>       * A dedicated section on relationship between TI-LFA and
>>         micro-loops.
>>
>>         Gyan> I think this option would  be best.  This would fix the
>>     existing gap on uLoop.  I did mention but not sure if possible-
>>     as TI-LFA and uLoop are tightly coupled as a overall post
>>     convergence solution is it possible to combine the drafts and
>>     issue another WGLC.  (Question for authors)
>>
>>     In any case, I defer to you and the rest of the authors to decide
>>     what, if anything should be done for clarifying the relationship
>>     between TI-LFA and micro-loops.
>>
>>     Regards,
>>
>>     Sasha
>>
>>     *From:* [e-mail] <[e-mail]>
>>     *Sent:* Monday, October 23, 2023 3:27 PM
>>     *To:* Alexander Vainshtein <[e-mail]>
>>     *Cc:* [e-mail]; rtgwg-chairs <[e-mail]>;
>>     [e-mail]; Stewart Bryant
>>     <[e-mail]>
>>     *Subject:* [EXTERNAL] RE: draft-ietf-rtgwg-segment-routing-ti-lfa
>>     : A simple pathological network fragment
>>
>>     Sasha,
>>
>>     Thanks for the summary and the constructive proposal.
>>
>>     Speaking for myself, this makes sense and I agree.
>>
>>     ØTI-LFA is a local operation applied by the PLR when it detects
>>     failure of one of its local links. As such,  it does not affect:
>>
>>     oMicro-loops that appear – or do not appear –on the paths to the
>>     destination that do not pass thru TI-LFA paths
>>
>>     As an editorial comment, depending on where such text would be
>>     inserted, I would propose the following change:
>>
>>     OLD: Micro-loops that appear – or do not appear –
>>
>>     NEW: Micro-loops that appear – or do not appear – as part of the
>>     distributed IGP convergence [RFC5715]
>>
>>     Motivation: some reader could wrongly understand that such
>>     micro-loops are caused by TI-LFA
>>
>>     Thanks,
>>
>>     Regards,
>>
>>     --Bruno
>>
>>     Orange Restricted
>>
>>     *From:*Alexander Vainshtein <[e-mail]>
>>     *Sent:* Sunday, October 22, 2023 4:21 PM
>>     *To:* DECRAENE Bruno INNOV/NET <[e-mail]>;
>>     Stewart Bryant <[e-mail]>
>>     *Cc:* [e-mail]; rtgwg-chairs <[e-mail]>;
>>     [e-mail]
>>     *Subject:* RE: draft-ietf-rtgwg-segment-routing-ti-lfa : A simple
>>     pathological network fragment
>>     *Importance:* High
>>
>>     Bruno, Stewart and all,
>>
>>     I think that most of the things about TI-LFA and micro-loops have
>>     been said already (if in a slightly different context)  and are
>>     mainly self-evident.
>>
>>     However, I share the feeling that somehow the relationship
>>     between TI-LFA and micro-loop avoidance has become somewhat muddled.
>>
>>     Therefore, I would like to suggest adding some text to the TI-LFA
>>     draft that clarifies this relationship, e.g., along the following
>>     lines:
>>
>>     1.TI-LFA is a local operation applied by the PLR when it detects
>>     failure of one of its local links. As such,  it does not affect:
>>
>>     a.Micro-loops that appear – or do not appear –on the paths to the
>>     destination that do not pass thru TI-LFA paths
>>
>>     i.As explained in RFC 5714, such micro-loops may result in the
>>     traffic not reaching the PLR and therefore not following TI-LFA paths
>>
>>     ii.Segment Routing may be used for prevention of such micro-loops
>>     as described in the micro-loop avoidance draft
>>
>>     b.Micro-loops that appear – or do not appear - when the failed
>>     link is repaired (/aside: the need for this line is based on
>>     personal experience//☹/)
>>
>>     2.TI-LFA paths are loop-free. What’s more, they follow the
>>     post-convergence paths, and, therefore, not subject to
>>     micro-loops due to difference in the IGP convergence times of the
>>     nodes thru which they pass
>>
>>     3.TI-LFA paths are applied from the moment the PLR detects
>>     failure of a local link and until IGP convergence at the PLR is
>>     completed. Therefore, early (relative to the other nodes) IGP
>>     convergence at the PLR and the consecutive ”early” release of
>>     TI-LFA paths may cause micro-loops, especially if these paths
>>     have been computed using the methods described in Section 6.2,
>>     6.3 or 6.4 of the draft. One of the possible ways to prevent such
>>     micro-loops is local convergence delay (RFC 8333).
>>
>>     4.TI-LFA procedures are complementary to application of any
>>     micro-loop avoidance procedures in the case of link or node failure:
>>
>>     a.Link or node failure requires some urgent action to restore the
>>     traffic that passed thru the failed resource. TI-LFA paths are
>>     pre-computed and pre-installed and therefore suitable for urgent
>>     recovery
>>
>>     b.The paths used in the micro-loop avoidance procedures typically
>>     cannot be pre-computed.
>>
>>     Hopefully these notes would be useful.
>>
>>     Regards,
>>
>>     Sasha
>>
>>     *From:* rtgwg <[e-mail]> *On Behalf Of
>>     *[e-mail]
>>     *Sent:* Thursday, October 19, 2023 7:34 PM
>>     *To:* Stewart Bryant <[e-mail]>
>>     *Cc:* [e-mail]; rtgwg-chairs <[e-mail]>;
>>     [e-mail]
>>     *Subject:* [EXTERNAL] RE: draft-ietf-rtgwg-segment-routing-ti-lfa
>>     : A simple pathological network fragment
>>
>>     Hi Stewart,
>>
>>     I agree with you on the technical points, so the first part of
>>     your email up to “So I think”.
>>
>>     But I don’t quite follow why you want to mix IGP Convergence
>>     issues with this Fast ReRoute Solution.
>>
>>     To quote RFC 5714 « IP Fast Reroute Framework”
>>
>>     In order to reduce packet disruption times to a duration commensurate
>>
>>        with the failure detection times, two mechanisms may be required:
>>
>>        a.  A mechanism for the router(s) adjacent to the failure to
>>     rapidly
>>
>>     invoke a repair path, which is unaffected by any subsequent re-
>>
>>     convergence.
>>
>>        b.  In topologies that are susceptible to micro-loops, a
>>     micro-loop
>>
>>     control mechanism may be required [RFC5715
>>     <https://datatracker.ietf.org/doc/html/rfc5715>].
>>
>>     Performing the first task without the second may result in the repair
>>
>>        path being starved of traffic and hence being redundant.
>>
>>     https://datatracker.ietf.org/doc/html/rfc5714#section-4
>>
>>       
>>
>>     I would assume that you agree with the above (as you are an
>>     author of this RFC and my guess would be that you wrote that text)
>>
>>       
>>
>>     My point is that there are two different mechanisms involved, in
>>     two different time periods:
>>
>>     -Fast ReRoute (“a”): this is the scope of
>>     draft-ietf-rtgwg-segment-routing-ti-lfa
>>
>>     oTiming: from detection time , to start of the IGP convergence
>>
>>     -IGP Micro-loop avoidance (“b”)
>>
>>     oTiming: from start of IGP convergence to end of IGP convergence
>>
>>     The scope of draft-ietf-rtgwg-segment-routing-ti-lfa is FRR /
>>     “a”. IGP micro-loop is out of scope. Other documents are
>>     proposing solutions for this. (and for those Micro-loop
>>     documents, FRR is similarly out of scope).
>>
>>     Personally I agree with you that both mechanisms are needed. But
>>     I think that this is already highlighted in RFC 5714, and that
>>     this is no different than RFC 7490 (RLFA). Therefore, I don’t see
>>     why the outcome/text should be different. Hence my proposition to
>>     reuse the text from RFC 7490 (RLFA). I find it adequate. You
>>     wrote it so probably find it adequate.
>>
>>     On a side note, RFC5715, that you also wrote, seems to already
>>     cover what you are asking for. Quoting the abstract, it
>>
>>     provides a summary of the causes and consequences of
>>
>>     micro-loops and enables the reader to form a judgement on whether
>>
>>     micro-looping is an issue that needs to be addressed in specific
>>
>>     networks.
>>
>>     Note that this RFC5715 is already cited in the proposed text.
>>
>>     PS: If you were ready to wrote a 5715bis, I would support this.
>>
>>     Best regards,
>>
>>     --Bruno
>>
>>     Orange Restricted
>>
>>     *From:* Stewart Bryant <[e-mail]>
>>     *Sent:* Tuesday, October 17, 2023 1:48 PM
>>     *To:* DECRAENE Bruno INNOV/NET <[e-mail]>
>>     *Cc:* Stewart Bryant <[e-mail]>; [e-mail];
>>     rtgwg-chairs <[e-mail]>;
>>     [e-mail]
>>     *Subject:* Re: draft-ietf-rtgwg-segment-routing-ti-lfa : A simple
>>     pathological network fragment
>>
>>     Hi Bruno
>>
>>     I was thinking about this some more. It is something that was
>>     recognised in the early days, but somewhat swept aside.
>>
>>     The case that Gyan bought up was an ECMP case, but I fear that
>>     the case is more common and I think we should characterise it as
>>     part of the text rather that giving the impression it is unusual.
>>
>>     I think the problem occurs whenever there are two or more nodes
>>     between the point of packet entry and the failure.
>>
>>     CE1 - R1 - R2 - R3 - R4 -/- R5 - CE2
>>
>>       |                     |
>>
>>       R6 - R7 - R8 - R9 — R10
>>
>>     The normal path CE1-CE2 is via R2
>>
>>     When R4-R5 fails it is trivial to see how the repair works with
>>     R7 as the entry into Q space.
>>
>>     However unless R1, R2,  R3 converge in that order there will be
>>     microloops for traffic entering via any of those three nodes.
>>
>>     So I think we can say that unless the PLR is only receiving
>>     traffic to be protected directly or from its immediate neighbour
>>     it is not guaranteed that there  will not be micro loops that are
>>     not addressable by the propose strategy of aligning the repair
>>     path with the post convergence path.
>>
>>     Now thinking about the text you have below, I think we need to
>>     write in in terms of - Unless the operator is certain that no
>>     micro loops will form over any path the protected traffic will
>>     traverse between entry to the network and arrival at the PLR a
>>     micro loop avoidance method MUST be deployed. Of course I think
>>     that it would be helpful to the operator community for the text
>>     to provide some guidance on how to ascertain whether there is a
>>     danger of the formation of micro loops.
>>
>>     I would note that the long chains of nodes show in the example
>>     above were probably not present in the test topologies which as I
>>     remember were all national scale provider networks, but unless we
>>     provide guidance otherwise Ti-LFA could reasonably be deployed in
>>     edge networks and in the case of cell systems these are often
>>     ring topologies.
>>
>>     So I think we need to agree (as a WG) on the constrains that we
>>     are prepared to specify in the text and the degree of warning we
>>     need to provide to the operator community and then we can polish
>>     the text below.
>>
>>     Best regards
>>
>>     Stewart
>>
>>         On 16 Oct 2023, at 17:25, [e-mail] wrote:
>>
>>         Hi Stewart,
>>
>>         Please see inline
>>
>>         Orange Restricted
>>
>>         *From:*Stewart Bryant <[e-mail]
>>         <mailto:[e-mail]>>
>>         *Sent:*Monday, October 16, 2023 2:08 PM
>>         *To:*[e-mail] <mailto:[e-mail]>; rtgwg-chairs
>>         <[e-mail]
>>         <mailto:[e-mail]>>;[e-mail]
>>         <mailto:[e-mail]>
>>         *Cc:*Stewart Bryant <[e-mail]
>>         <mailto:[e-mail]>>
>>         *Subject:*draft-ietf-rtgwg-segment-routing-ti-lfa : A simple
>>         pathological network fragment
>>
>>         During the operations directorate early review
>>         of draft-ietf-rtgwg-segment-routing-ti-lfa
>>         Gyan Mishra points to a simple pathological network fragment
>>         that I think deserves wider discussion.
>>
>>         https://datatracker.ietf.org/doc/review-ietf-rtgwg-segment-routing-ti-lfa-11-opsdir-early-mishra-2023-08-25/
>>         <https://datatracker.ietf.org/doc/review-ietf-rtgwg-segment-routing-ti-lfa-11-opsdir-early-mishra-2023-08-25>
>>
>>         I am not aware of any response to the RTGWG by the draft
>>         authors concerning the review comment and I cannot see
>>         obvious new text addressing this concern.
>>
>>
>>         The fragment is as follows
>>
>>
>>         CE1 –R1- R2-/-R3-CE2
>>              |         |
>>              R4 – R5 -R6
>>
>>         In the pre converged network R4 is ECMP CE2 via R5 (cost 4)
>>         and via R1 (cost also 4).
>>
>>         We can easily build a TI-LFA repair path from R2 under link
>>         failure to CE2 (so long as we remember that R4 is an ECMP
>>         path to CE2), but the problem occurs during convergence. If
>>         R1 converges before R4, R4 may ECMP packets addressed to CE2
>>         back to R1 in a micro loop. Meanwhile since no packets for R3
>>         are reaching R2 the Ti-LFA repair is not doing anything useful.
>>
>>         The Ti-LFA text leads the reader to conclude that it is a
>>         loop-free solution, but gives no guidance on how to determine
>>         when this assumption breaks down. There is an informational
>>         reference to
>>         draft-bashandy-rtgwg-segment-routing-uloop, but this short
>>         individual draft does little in the way of helping the reader
>>         determine when  loop avoidance strategy needs to be deployed
>>         and the loop-free approach it describes does not seem to be
>>         fully developed.
>>
>>         I am worried that proceeding with the Ti-LFA draft without
>>         noting that there is a real risk that simple network
>>         fragments can micoloop, and providing a fully formed
>>         mitigation strategy is a disservice to the operator community
>>         given the industry interest in Ti-LDA and the insidious
>>         nature of unexpected micro loop network transients, I am
>>         wondering what the view of the working group is on how to
>>         proceed.
>>
>>         One approach would be for the Ti-LFA draft to incorporate
>>         detailed guidance on how to determine the risk of a micro
>>         loop in a specific operator network, and to provide specific
>>         mitigation advice. Another approach would be to  reference a
>>         developed loop avoidance strategy and recommending its
>>         preemptive deployment. Another approach would be to make
>>         draft-bashandy-rtgwg-segment-routing-uloop a normative
>>         reference and tie the fate of the two drafts. Another
>>         approach would be to elaborate on the risks and their
>>         manifestations but declare it a currently unsolved problem. I
>>         am sure there are other options that the WG may formulate.
>>
>>         What is the opinion of the working group on how we should
>>         proceed with draft-ietf-rtgwg-segment-routing-ti-lfa when
>>         considering the possible formation of micro loops?
>>
>>         FRR takes place between the failure (detection) and the IGP
>>         reconvergence. Those are two consecutive steps that the WG
>>         has so far addressed with different solutions and documents.
>>
>>         That’s not new and that’s not specific to TI-LFA. E.g.,
>>         that’s applicable to RLFA.
>>
>>         Would the below text, taken verbatim from RFC 7490 (RLFA),
>>         work for you? Or would you say that the text is not good enough?
>>
>>         “When the network reconverges, micro-loops [RFC5715 <https://datatracker.ietf.org/doc/html/rfc5715>] can form due to
>>
>>             transient inconsistencies in the forwarding tables of different
>>
>>             routers.  If it is determined that micro-loops are a significant
>>
>>             issue in the deployment, then a suitable loop-free convergence
>>
>>             method, such as one of those described in [RFC5715 <https://datatracker.ietf.org/doc/html/rfc5715>], [RFC6976 <https://datatracker.ietf.org/doc/html/rfc6976>], or
>>
>>             [ULOOP-DELAY
>>         <https://datatracker.ietf.org/doc/html/rfc7490#ref-ULOOP-DELAY>], should be implemented.”
>>
>>         https://datatracker.ietf.org/doc/html/rfc7490#section-10
>>         <https://datatracker.ietf.org/doc/html/rfc7490#section-10>
>>
>>         Of course, we could update the list of informative references.
>>
>>         E.g., by adding another informative reference to
>>         draft-bashandy-rtgwg-segment-routing-uloop and by removing
>>         informative references to [RFC6976] and [ULOOP-DELAY] which
>>         are probably outdated.
>>
>>         --Bruno
>>
>>         - Stewart
>>
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