Re: [Lsr] "Prefix Unreachable Announcement" and "IS-IS and OSPF Extension for Event Notification"

["Acee Lindem (acee)" <[e-mail]>]

Speaking as a mere WG member...

Assuming that there are enough remote PEs that need this unreachability information and we want to use the IGPs for this, here is what I don't like about PUA (Les has taken away much of my thunder __): 

    1. Usage of the prefix-originator for unreachability notification requires that every router in the domain support the extension before it can be used. If a router don't support it and ignores the prefix-originator sub-TLV, it will actually prefer the advertising ABR (due to LPM) and blackhole the data. 
    2. The non-deterministic nature of the notification. Unreachability is advertised for any route that is subsumed by a range and become unreachable. Is this advertised forever if the route in question is taken out of service? What about a route that is mistakenly put into service - will we advertise unreachability forever? What if the PE is already unreachable when the ABR comes up - no reachability information will be advertised. 
    3. Like the event notification draft, the unreachability notification will trigger BGP reconvergence. Additionally, an ABR that has the route is supposed to advertise a more specific route. However, by the time this happens, BGP reconvergence should have already taken place. 
    4. The interaction of MAA and reachable prefixes could cause quite a bit of churn when there are oscillations. However, given 1-3, I don't think we'll have to worry about this. 

Thanks,
Acee

On 10/13/21, 2:44 PM, "Les Ginsberg (ginsberg)" <ginsberg=[e-mail]> wrote:

    This thread is becoming "diverse".
    We are trying to talk about many different solutions (IGP, BGP, BFD) - all of which may be useful and certainly are not mutually exclusive.

    If we can agree that an IGP solution is useful, then we can use this thread to set a direction for the IGP solution - which seems to me to be something we should do independent of whether the other solutions are also pursued.

    With that in mind,  here is my input on the IGP solutions:

    PUA
    -------

    For me, the solution has two major drawbacks:

    1)It tries to repurpose an existing (and fundamental) Reachability Advertisement into an UnReachability advertisement under certain conditions

    The interoperability risks associated with this make me very reluctant to go down this path.
    And the use of the same advertisement to indicate Reachability and Unreachability is IMO highly undesirable.

    2)The withdrawal of the "Unreachability Advertisement" after some delay (which is necessary)  remains problematic despite the authors attempts to address thus

    Event Notification
    ------------------------

    This avoids the drawbacks of PUA and is flexible enough to handle future and unforeseen types of notifications. 

    I would like to extend the offer already made by Peter to the authors of PUA to join us and work on the Event Notification draft.
    The authors of PUA certainly deserve credit for raising awareness of the problem space and it would be good to have them working with us on a single IGP solution.

    But PUA is not an alternative that I can support.

        Les

    > -----Original Message-----
    > From: Lsr <[e-mail]> On Behalf Of Acee Lindem (acee)
    > Sent: Wednesday, October 13, 2021 9:49 AM
    > To: Peter Psenak <ppsenak=[e-mail]>; [e-mail]
    > Subject: Re: [Lsr] "Prefix Unreachable Announcement" and "IS-IS and OSPF
    > Extension for Event Notification"
    > 
    > Hi Peter,
    > 
    > See inline.
    > 
    > On 10/13/21, 4:42 AM, "Peter Psenak"
    > <ppsenak=[e-mail]> wrote:
    > 
    >     Hi Acee,
    > 
    >     On 12/10/2021 21:05, Acee Lindem (acee) wrote:
    >     > Speaking as WG Chairs:
    >     >
    >     > The authors of “Prefix Unreachable Announcement” have requested an
    >     > adoption. The crux of the draft is to signal unreachability of a prefix
    >     > across OSPF or IS-IS areas when area summarization is employed and
    >     > prefix is summarised. We also have “IS-IS and OSPF Extension for Event
    >     > Notification” which can be used to address the same use case. The drafts
    >     > take radically different approaches to the problem and the authors of
    >     > both drafts do not wish to converge on the other draft’s method so it is
    >     > understandable that merging the drafts really isn’t an option.
    > 
    >     just for the record, I offered authors of "Prefix Unreachable
    >     Announcement" co-authorship on "Event notification" draft, arguing the
    >     the event base solution addresses their use case in a more elegant and
    >     scalable way. They decided to push their idea regardless.
    > 
    > One solution to this problem would have definitely been better.
    > 
    >     > Before an adoption call for either draft, I’d like to ask the WG:
    >     >
    >     >  1. Is this a problem that needs to be solved in the IGPs? The use case
    >     >     offered in both drafts is signaling unreachability of a BGP peer.
    >     >     Could this better solved with a different mechanism  (e.g., BFD)
    >     >     rather than flooding this negative reachability information across
    >     >     the entire IGP domain?
    > 
    >     we have looked at the various options. None of the existing ones would
    >     fit the large scale deployment with summarization in place. Using BFD
    >     end to end to track reachability between PEs simply does not scale.
    > 
    > It seems to me that scaling of BFD should be "roughly" proportional to BGP
    > session scaling but I seem to be in the minority. My opinion is based on
    > SDWAN tunnel scaling, where BFD is used implicitly in our solution. How
    > many other PEs does a BGP edge PE maximally peer with?
    > Thanks,
    > Acee
    > 
    > 
    >     Some people believe this should be solved by BGP, but it is important to
    >     realize that while the problem statement at the moment is primarily
    >     targeted for egress PE reachability loss detection for BGP, the
    >     mechanism proposed is generic enough and can be used to track the peer
    >     reachablity loss for other cases (e.g GRE endpoint, etc) that do not
    >     involve BGP.
    > 
    >     We went even further and explored the option to use completely out of
    >     band mechanism that do not involve any existing protocols.
    > 
    >     Simply, the advantage of using IGP is that it follows the existing MPLS
    >     model, where the endpoint reachability is provided by IGPs. Operators
    >     are familiar with IGPs and know how to operate them.
    > 
    >     On top of the above, IGP event notification can find other use cases in
    >     the future, the mechanism defined in draft is generic enough.
    > 
    > 
    >     >  2. Assuming we do want to take on negative advertisement in the IGP,
    >     >     what are the technical merits and/or detriments of the two
    > approaches?
    > 
    >     we have listed some requirements at:
    > 
    >     https://datatracker.ietf.org/doc/html/draft-ppsenak-lsr-igp-event-
    > notification-00#section-3
    > 
    >      From my perspective the solution should be optimal in terms of amount
    >     of data and state that needs to be maintained, ideally separated from
    >     the traditional LS data. I also believe that having a generic mechanism
    >     to distribute events has it own merits.
    > 
    >     thanks,
    >     Peter
    > 
    >     >
    >     > We’ll reserve any further discussion to “WG member” comments on the
    > two
    >     > approaches.
    >     >
    >     > Thanks,
    >     > Acee and Chris
    >     >
    > 
    > 
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