Re: [Lsr] WG Last Call fo "IS-IS Flood Reflection" -draft-ietf-lsr-isis-flood-reflection-05

[Aijun Wang <[e-mail]>]

Hi, Tony:

The advantage of IGP is that it can cure itself when there is any topology change, no additional  operator intervention involved.
But from your explanation below, there are situations that the draft doesn’t covered for the possible loop scenarios. Additional route compare rules or configuration requirements needed to be added when such thing happens. Then can we call the current solution is not final?
And, if we lack one trust theory to avoid the loop, we will be stuck in the emerged loop situations.

Some additional replies inline.

Aijun Wang
China Telecom

> On Dec 11, 2021, at 20:08, Tony Przygienda <[e-mail]> wrote:
> 
> 
> let's be more precise beyond sensationalist headlines ... Though it's nice you found our documentation and based on that make first technical argument ;-) 
> 
> a) transitory loops during convergence/tunnel setup may exist, those also exist during normal ISIS convergence. Simple implementation techniques used in other contexts since many years exist like e.g. holding the router in overload until converged. That can be also easily implemented if e.g. a FR client has no FR tunnel established. A draft/spec is not an implementation description however so techniques deployed can be very different and starting to put that in will lead to overspecification. 

[WAJ] From the example that illustrated in the mentioned example, it seems the loop is not transient. It seems when R1 is not act as FR client, the loop will occur then?

> b) let's split the rest assuming full convergence scenario into tunnel; and no L1 tunnels case 
> ib.) L1 tunnel case when all tunnels in place/converged is simple, it's identical really to any shortcut forwarding where the next hop is a tunnel, the packet just shows up magically @ the egress. this is AFAIS simpler option to understand and deploy correctly. If you harbor fears and doubts about the whole thing or looping potential, just use this. 

[WAJ] Then the merit that you compared with TTZ will go away?

> b.ii) no tunnel case is more delicate (but hotly desired by many [arguably sophisticated] customers hence the work). If the SPF is implemented correctly no routing loops exist when converged.
> This preconditions  also correct leaking of L1 into L2 (hence the "all egress MUST be clients", it could work with a mix but the draft gets unnecessarily complex) and section 5.2 of the draft provides the necessary implementation on SPF to prevent loops in every case.
[WAJ] As Acee pointe out also, this section is harder to understand and not in clear logic. I doubt and am unconvinced that it can cover every case.

> However, as the documentation you found says it is easier to ensure that L1 metrics when deployed are shorter than L2 metrics so the L1->L2 route always takes preference towards the egress and not rely on the correct SPF preferece outside metric comparison.

[WAJ] To achieve this, the design should be verified by manual carefully?

> As footnote: redistributing prefixes in ISIS is always delicate, nothing new here, L1->L2->L1 was originally forbidden and only works with the RFC and lots of tightly kept pref rules. With enough inventive re-distribution on a system around the RFC and fiddiling via policy with redistributed route properties you can loop up normal ISIS as well BTW. 

[WAJ] Can I say that your solution is based on the original forbidden design? To explore the forbidden, you introduce the additional SPF rules. When the topology or interconnection scenario become complex, will the SPF rules complex also?

> c) ECMP, yes, as our doc says it's not looping but it can be changed when using b.ii) unless whole network is forklifted to flood reflection and e.g. omits ECMP computed over FR adjacencies which again, is an implementation technique that is optional complexity since FR adjacencies are not forwarding (one could make them taht and stuff would still work but then we would have all kind of restrictions on tunnels like e.g. XoUDP to ECMP inside L1 and ultimately shove e'thing though FRs, most customers think that doing L2 hot potato forwarding out the area is actually far preferred IME).
> 
> And no, you will never achieve the same "globally optimal" routing through the whole topology in L2 when abstracting some L1 things except when you advertise a full mesh of links reflecting all ingress-egress metrics in L2 being basically L1 shortest path (or assume that all ingress/egress connections have infinite/same metric, it's called a chassis or more precisely a switching fabric then ;-). Beside scaling very poorly as the draft explains this can also cause a single link failing in L1 to cause a havoc of advertisements in L2 and if you are experienced in operational matters this is the last things you desire. In more abstract control theory terms you are building with such a solution a negatively stable system of control loops or in simpler terms an invisible big blast radius. Good for fighter planes, very bad for networking if you experienced those effects. 
> 
> hope that's enough details, none of this is BTW pure speculation but output of implemenation and extensive testing. 
> 
> --- tony 
> 
>> On Sat, Dec 11, 2021 at 8:48 AM Aijun Wang <[e-mail]> wrote:
>> Please refer to “Routing Loops” section that described in your company’s documents:
>>   https://www.juniper.net/documentation/en_US/junos/topics/concept/understanding-isis-flood-reflectors.html#jd0e162
>> 
>> This is just the simplest transit scenario. I am worrying for its further applications. 
>> 
>> The overall routing path for L2 topologies that connected by several L1 is not determined by one L2 SPF algorithm, how can you assure no loop occur?
>> 
>> Is it nightmare or mess for the operator to run its network in such challenges situations?
>> 
>> Aijun Wang
>> China Telecom
>> 
>>>> On Dec 11, 2021, at 02:02, Tony Przygienda <[e-mail]> wrote:
>>>> 
>>> 
>>> What you describe is not technical reality when you're dealing with SPF in L2 ... You seem to be deeply confused (but honestly, I have a hard time to even figure out what kind of assumptions you're making about all kind of things in all those threads about IGPs these days and on what but personal preferences all those claims of "better" things are based) by the simple fact that there is only a single connected L2 area ever in ISIS, if you partition L2 you do not run ISIS within its viable envelope anymore, with or without extensions. And L2 SPF will never loop and flood reflector draft does not change the path taken in L2 so by first order logic you won't loop. That is BTW one of the reasons why you will want to deploy multiple flood reflectors per L1 (to not partition L2). 
>>> 
>>> As Acee pointed out, unlike OSPF (discounting alternate ABR behavior) L2 could in ISIS from day one use L1 to "traverse across" (if you squint a bit ;-)  in a sense and those drafts just improve the behavior so we are not even architecturally changing the topological properties of the protocol (whatever that means without delving deeply into graph theory definitions ;-) 
>>> 
>>> -- tony 
>>> 
>>>> On Fri, Dec 10, 2021 at 1:40 AM Aijun Wang <[e-mail]> wrote:
>>>> Hi, Acee:
>>>> 
>>>> No, I think the WG participations would like to enjoy the interested and correct direction topics. 
>>>> One technical considerations is the following: if IGP evolved into this direction, then the following connections loop diagram is also possible:
>>>> L2-L1-L2-L1-L2-L1-…..-L2(back to the origin)
>>>> Then, will we introduce the “AS number” to avoid loop, and various “Path Attributes” to influence the path selection within the IGP?
>>>> 
>>>> There is a rush to adopt this draft, we should not rush again to accomplish its WG last call.
>>>> 
>>>> 
>>>> Aijun Wang
>>>> China Telecom
>>>> 
>>>>>> On Dec 9, 2021, at 10:07, Acee Lindem (acee) <acee=[e-mail]> wrote:
>>>>>> 
>>>>> 
>>>>> Hi Aijun,
>>>>> 
>>>>>  
>>>>> 
>>>>> At least I’ve had several discussions with authors on the draft and you’ll note my comments on the list. The lack of further discussion on the list is a general problem in LSR. It seems many WG participants only want to discuss the draft that they have authored and it is hard to get comment without forcing the issue without an adoption or last call. I’m expecting at least one more review on the draft. Technical comments on the draft would be appreciated.
>>>>> 
>>>>>  
>>>>> 
>>>>> Thanks,
>>>>> Acee
>>>>> 
>>>>>  
>>>>> 
>>>>> From: Aijun Wang <[e-mail]>
>>>>> Date: Wednesday, December 8, 2021 at 11:15 AM
>>>>> To: Acee Lindem <[e-mail]>
>>>>> Cc: "Les Ginsberg (ginsberg)" <[e-mail]>, "[e-mail]" <[e-mail]>
>>>>> Subject: Re: [Lsr] WG Last Call fo "IS-IS Flood Reflection" -draft-ietf-lsr-isis-flood-reflection-05
>>>>> 
>>>>>  
>>>>> 
>>>>> Hi, Acee:
>>>>> 
>>>>>  
>>>>> 
>>>>> I found there is no any technical discussion on the list for the mentioned draft after its adoption(from 2020-7-6), even before its adoption.
>>>>> 
>>>>> There is only one presentation for its update at the IETF 111 meeting(1 pages, 5 minutes) and after it’s presentation at IETF 112, the WG Last Call is issued.
>>>>> 
>>>>>  
>>>>> 
>>>>> From the authors’ statement, there is only the author’s affiliation implemented it, no interoperability problems can be found.
>>>>> 
>>>>>  
>>>>> 
>>>>> From the discussion in these days, it is doubtful for IGP to evolve into this direction as behaved by BGP.
>>>>> 
>>>>>  
>>>>> 
>>>>> From the POV of the operator, we will not consider such strange design to scale the IS-IS.
>>>>> 
>>>>>  
>>>>> 
>>>>> From the documents itself, there are unsolved problems (for example in section 7) which can only be solved by “sending alarm and declare misconfiguration).
>>>>> 
>>>>>  
>>>>> 
>>>>> Then, why it is ready to WG Last Call? 
>>>>> 
>>>>>  
>>>>> 
>>>>>  
>>>>> 
>>>>> Aijun Wang
>>>>> 
>>>>> China Telecom
>>>>> 
>>>>> 
>>>>> 
>>>>> 
>>>>> On Dec 8, 2021, at 06:28, Acee Lindem (acee) <acee=[e-mail]> wrote:
>>>>> 
>>>>> Hi Les,
>>>>> 
>>>>>  
>>>>> 
>>>>> From: "Les Ginsberg (ginsberg)" <[e-mail]>
>>>>> Date: Tuesday, December 7, 2021 at 5:10 PM
>>>>> To: "Acee Lindem (acee)" <acee=[e-mail]>, Acee Lindem <[e-mail]>, "[e-mail]" <[e-mail]>
>>>>> Subject: RE: [Lsr] WG Last Call fo "IS-IS Flood Reflection" -draft-ietf-lsr-isis-flood-reflection-05
>>>>> 
>>>>>  
>>>>> 
>>>>> Let me try to respond to Acee/Tony/Tony in one email.
>>>>> 
>>>>>  
>>>>> 
>>>>> Acee – I don’t like any of the proposals. I believe they are all abusing the protocols to some degree.
>>>>> 
>>>>>  
>>>>> 
>>>>> That’s fine if you have technical concerns with the individual drafts. The problem space discussion is a moot point since the WG has already adopted these documents.
>>>>> 
>>>>>  
>>>>> 
>>>>> Thanks,
>>>>> 
>>>>> Acee
>>>>> 
>>>>>  
>>>>> 
>>>>> I fully believe that the authors are clever enough to figure out how to make this work – but that doesn’t mean any of them are desirable.
>>>>> 
>>>>> So if I have to “vote” – I vote “no”.
>>>>> 
>>>>>  
>>>>> 
>>>>> Tony Li –
>>>>> 
>>>>>  
>>>>> 
>>>>> Area Proxy Introduction says in part:
>>>>> 
>>>>>  
>>>>> 
>>>>> “Following the current rules of IS-IS, all spine routers would
>>>>> 
>>>>>    necessarily be part of the Level 2 topology, plus all links between a
>>>>> 
>>>>>    Level 2 leaf and the spines.  In the limit, where all leaves need to
>>>>> 
>>>>>    support Level 2, it implies that the entire L3LS topology becomes
>>>>> 
>>>>>    part of Level 2.  This is seriously problematic as it more than
>>>>> 
>>>>>    doubles the LSDB held in the L3LS topology and eliminates any
>>>>> 
>>>>>    benefits of the hierarchy.”
>>>>> 
>>>>>  
>>>>> 
>>>>> Flood Reflection says:
>>>>> 
>>>>>  
>>>>> 
>>>>> “In such scenarios, an
>>>>> 
>>>>>    alternative approach is to consider multiple L2 flooding domains
>>>>> 
>>>>>    connected together via L1 flooding domains.  In other words, L2
>>>>> 
>>>>>    flooding domains are connected by "L1/L2 lanes" through the L1 areas
>>>>> 
>>>>>    to form a single L2 backbone again.  Unfortunately, in its simplest
>>>>> 
>>>>>    implementation, this requires the inclusion of most, or all, of the
>>>>> 
>>>>>    transit L1 routers as L1/L2 to allow traffic to flow along optimal
>>>>> 
>>>>>    paths through such transit areas.  Consequently, this approach fails
>>>>> 
>>>>>    to reduce the number of L2 routers involved, so it fails to increase
>>>>> 
>>>>>    the scalability of the L2 backbone.”
>>>>> 
>>>>>  
>>>>> 
>>>>>  
>>>>> 
>>>>> These problem statements seem fundamentally similar to me.
>>>>> 
>>>>>  
>>>>> 
>>>>> No question the two drafts define very different solutions – but the problem statements seem quite similar to me.
>>>>> 
>>>>>  
>>>>> 
>>>>> Tony P – I would argue that a “20K perspective” is useful when deciding whether the overall approach is a good one.
>>>>> 
>>>>>  
>>>>> 
>>>>> And in response to Tony Li’s statement:  “…the IETF is at its best when it is documenting de facto standards”
>>>>> 
>>>>>  
>>>>> 
>>>>> 1) I fully believe that our customers understand their requirements(sic) better than we (vendors) do. But that does not mean that they understand what is the best solution better than we do.
>>>>> 
>>>>> When a customer comes to me and says “Can you do this?” my first response is usually “Please fully describe your requirements independent of the solution.”
>>>>> 
>>>>>  
>>>>> 
>>>>> 2)Not clear to me that there is an existing “de facto standard” here – which is reinforced by the fact that we have so many different solutions proposed.
>>>>> 
>>>>>  
>>>>> 
>>>>>    Les
>>>>> 
>>>>>  
>>>>> 
>>>>>  
>>>>> 
>>>>>  
>>>>> 
>>>>> From: Acee Lindem (acee) <acee=[e-mail]> 
>>>>> Sent: Tuesday, December 7, 2021 10:31 AM
>>>>> To: Les Ginsberg (ginsberg) <[e-mail]>; Acee Lindem (acee) <[e-mail]>; [e-mail]
>>>>> Subject: Re: [Lsr] WG Last Call fo "IS-IS Flood Reflection" -draft-ietf-lsr-isis-flood-reflection-05
>>>>> 
>>>>>  
>>>>> 
>>>>> Hi Les,
>>>>> 
>>>>>  
>>>>> 
>>>>> From: Lsr <[e-mail]> on behalf of "Les Ginsberg (ginsberg)" <ginsberg=[e-mail]>
>>>>> Date: Tuesday, December 7, 2021 at 1:17 PM
>>>>> To: "Acee Lindem (acee)" <acee=[e-mail]>, "[e-mail]" <[e-mail]>
>>>>> Subject: Re: [Lsr] WG Last Call fo "IS-IS Flood Reflection" -draft-ietf-lsr-isis-flood-reflection-05
>>>>> 
>>>>>  
>>>>> 
>>>>> When I look at this request, I see it in a larger context.
>>>>> 
>>>>>  
>>>>> 
>>>>> There are two drafts which attempt to address the same problem in very different ways:
>>>>> 
>>>>>  
>>>>> 
>>>>> https://datatracker.ietf.org/doc/draft-ietf-lsr-isis-flood-reflection/
>>>>> 
>>>>>  
>>>>> 
>>>>> and
>>>>> 
>>>>>  
>>>>> 
>>>>> https://datatracker.ietf.org/doc/draft-ietf-lsr-isis-area-proxy/
>>>>> 
>>>>>  
>>>>> 
>>>>> Both of them discuss in their respective introductions the motivation – which is to address scaling issues in deployment scenarios where the existing IS-IS hierarchy is being asked to “stand on its head” i.e., interconnection between different L1 areas is not to be achieved by utilizing an L2 backbone – rather it is the L1 areas themselves which are required to be used for interconnection of sites (e.g., two datacenters) and the scaling properties of the existing protocol hierarchy when used in this way are not attractive.
>>>>> 
>>>>>  
>>>>> 
>>>>> I find no technical basis on which to choose between the two proposed solutions – so in my mind a last call for “Flood-Reflection” presupposes a last call for “Area Proxy” – and therein lies my angst.
>>>>> 
>>>>> The end result will be that multiple incompatible solutions to the same problem will be defined. It will then be left to customers to try to determine which of the solutions seems best to them – which in turn will put the onus on vendors to support both solutions (depending on the set of customers each vendor supports).
>>>>> 
>>>>> This – to me – represents an utter failure of the standards process. We are reduced to a set of constituencies which never find common ground – the end result being sub-optimal for the industry as a whole.
>>>>> 
>>>>>  
>>>>> 
>>>>> It seems to me that the proper role of the WG is to address the big questions first:
>>>>> 
>>>>>  
>>>>> 
>>>>> 1)Is this a problem which needs to be solved by link-state protocols?
>>>>> 
>>>>> We certainly have folks who are clever enough to define solutions – the two drafts are a proof of that.
>>>>> 
>>>>> But whether this is a wise use of the IGPs I think has never been fully discussed/answered.
>>>>> 
>>>>> Relevant to this point is past experience with virtual links in OSPF – use of which was problematic and which has largely fallen out of use.
>>>>> 
>>>>> Also, many datacenters use BGP (w or w/o IGP) and therefore have other ways to address such issues.
>>>>> 
>>>>> Although I am familiar with the “one protocol is simpler” argument, whether that justifies altering the IGPs in any of the proposed ways is still an important question to discuss.
>>>>> 
>>>>>  
>>>>> 
>>>>> Given the discussions of these solutions over the last two years in LSR, I don’t think we need to rehash this – especially on the experimental track.  
>>>>> 
>>>>>  
>>>>> 
>>>>> 2)If link state protocols do need to solve this problem, what is the preferred way to do that?
>>>>> 
>>>>> This requires meaningful dialogue and a willingness to engage on complex technical issues.
>>>>> 
>>>>>  
>>>>> 
>>>>> The alternative is to do what we seem to be doing – allowing multiple solutions to move forward largely without comment. In which case I see no basis on which to object – anyone who can demonstrate a deployment case should then be allowed to move a draft forward – and there are then no standardized solutions.
>>>>> 
>>>>> (The Experimental Track status for these drafts reflects that reality.)
>>>>> 
>>>>>  
>>>>> 
>>>>> Are you saying you don’t want any experimental solutions unless we have one standardized solution that everybody agrees on? Please review this one as part of WG last call.
>>>>> 
>>>>>  
>>>>> 
>>>>> Thanks,
>>>>> 
>>>>> Acee
>>>>> 
>>>>>  
>>>>> 
>>>>>    Les
>>>>> 
>>>>>  
>>>>> 
>>>>> P.S.  (Aside: There is a third draft offering a solution in this space https://datatracker.ietf.org/doc/draft-ietf-lsr-isis-ttz/  - but as that draft continues to promote its primary usage as a means of more easily changing area boundaries (merging/splitting) I have not discussed it here. However, if the authors of that draft claim it as a solution to the same problem space claimed by Area Proxy/Flood Reflection then the WG would have no basis but to also progress it – which would result in three solutions being advanced.)
>>>>> 
>>>>>  
>>>>> 
>>>>>  
>>>>> 
>>>>>  
>>>>> 
>>>>> From: Lsr <[e-mail]> On Behalf Of Acee Lindem (acee)
>>>>> Sent: Monday, November 22, 2021 11:47 AM
>>>>> To: [e-mail]
>>>>> Subject: [Lsr] WG Last Call fo "IS-IS Flood Reflection" -draft-ietf-lsr-isis-flood-reflection-05
>>>>> 
>>>>>  
>>>>> 
>>>>> This begins the WG Last for draft-ietf-lsr-isis-flood-reflection-05. Please post your support or objection to this list by 12:00 AM UTC on Dec 14th , 2021. Also please post your comments on the draft. I’m allowing as extra week as I like to get some additional reviews – although my comments have been addressed. 
>>>>> 
>>>>>  
>>>>> 
>>>>> Thanks,
>>>>> Acee
>>>>> 
>>>>>  
>>>>> 
>>>>> _______________________________________________
>>>>> Lsr mailing list
>>>>> [e-mail]
>>>>> https://www.ietf.org/mailman/listinfo/lsr
>>>>> 
>>>> _______________________________________________
>>>> Lsr mailing list
>>>> [e-mail]
>>>> https://www.ietf.org/mailman/listinfo/lsr
>>> _______________________________________________
>>> Lsr mailing list
>>> [e-mail]
>>> https://www.ietf.org/mailman/listinfo/lsr
> _______________________________________________
> Lsr mailing list
> [e-mail]
> https://www.ietf.org/mailman/listinfo/lsr