Re: [Idr] WG adoption call - draft-kaliraj-idr-multinexthop-attribute-10 - (11/10/2023 to 11/24/2023)

[Igor Malyushkin <[e-mail]>]

Hi Kaliraj,

My responses are below ([IM2]).


пт, 17 нояб. 2023 г. в 13:32, Kaliraj Vairavakkalai <[e-mail]>:

> Hi Igor,
>
>
>
> Please find some responses inline. KV>
>
>
>
> Thanks
>
> Kaliraj
>
> Juniper Business Use Only
>
> *From: *Igor Malyushkin <[e-mail]>
> *Date: *Wednesday, November 15, 2023 at 11:45 PM
> *To: *Kaliraj Vairavakkalai <[e-mail]>
> *Cc: *Susan Hares <[e-mail]>, [e-mail] <[e-mail]>
> *Subject: *Re: [Idr] WG adoption call -
> draft-kaliraj-idr-multinexthop-attribute-10 - (11/10/2023 to 11/24/2023)
>
> *[External Email. Be cautious of content]*
>
>
>
> Hi Kaliraj,
>
> Thanks for your response! In general, I agree with the list above and
> personally find your work promising. I just want to clarify several things.
> Please, see my inline.
>
>
>
> чт, 16 нояб. 2023 г. в 05:14, Kaliraj Vairavakkalai <[e-mail]>:
>
> Hi Igor, thanks for your comments.
>
>
>
> This MNH draft applies to single path carrying an ordered set of one or
> more nexthops.
>
> [IM] That's about outgoing messages I believe. I asked about the sources
> of it. Let's say, we have two paths for the same destination: x.x.x.x/y
> {1;2;3} and x.x.x.x/y {5} from different peers (and with the different
> next-hops as well). A node selects the latter due to its shorter AS_PATH,
> would it be possible to propagate the route further with the MNH attribute
> and both next-hops?
>
>     KV> Good question. The node may readvertise the best path with a MNH
> containing the nexthop (EP1) of best path as active leg, and nexthop (EP2)
> of inactive path as backup leg. E.g.,
>
>         MNH-X:
>
>           +NFI-X <Primary, Num-Nexthops=2>
>
>                 +FI-X <Action=Push, RelativePref=1>
>
>                      -FA-U <Type=1, EP=EP1>
>
>                 +FI-Y <Action=Push, RelativePref=2>
>
>                      -FA-X <Type=1, EP=EP2>
>

 [IM2] Ok, thank you, now I understand it better.


> Yes Addpath advertises multiple paths along with their attributes, but
> only if they have unique nexthops. So as I see it, the real use of Addpath
> is also to advertise multiple nexthops. But Addpath does not specify any
> relationship/order in the nexthops sent, and uses high RIB-out/in scale,
> because of using all attributes.
>
>
>
> Conceptually, if the sending BGP speaker is able to come up with “what
> kind of multiple-nexthop forwarding” is needed at the receiving nodes, then
> that can be conveyed in a single route update in a more expressive manner,
> instead of carrying all paths to receiver and further consuming CPU in
> multipath computation at all receiving-nodes. MNH just provides that
> expressiveness on the wire. How the sending speaker arrives at the
> forwarding-info to be sent in MNH (ECMP, Ordered fallback, WECMP, etc) -
> there can be more than one ways to figure that out, based on different
> usecases - as we are discovering. This just opens up a BGP based standard
> API to the box’s BGP RIB.
>
> [IM] Thanks for the additional clarification.
>
>
>
> And about the point on carrying labels in unlabled families: nodes not
> supporting MNH will just ignore it. Only those that have configured an AF
> to use MNH will use it. So I think there is no surprise element to any old
> BGP speakers receiving the label/MNH. If a receiving node that understands
> MNH does not understand enough of the contents to safely use it, they don’t
> use it (‘Attribute Discard’ approach). But I agree, the error handling will
> evolve as the draft matures.
>
> [IM] I'm worried about the approach when we define some new attributes and
> significantly change the very nature of an address family. Where does this
> road lead us? I consider an address family not only as an encoding
> container but also as some semantics behind it, and it looks like
> additional attributes can alter it in any possible way.
>
>    KV> I get your concern. I consider an address family as having a core
> business logic. And these encoding enhancements should not alter that
> business logic. They should just help in achieving that business logic
> better.
>
>    KV> IOW, encoding deficiencies should not come in the way of achieving
> the business logic better. I agree with you that the core business logic of
> an AF should not be diluted. But I also feel giving any AF a little more
> expressability
>
>    KV> should not hurt its business logic.
>
[IM2] Well, we can already send prefixes for one AFI with next-hops from
another. A more detailed text about the path resolution of unlabeled
families with labeled next-hops would be helpful. Additionally, I'm curious
about the process of withdrawing a label when a connected with it next-hop
is alive it shouldn't be withdrawn.
>
> I feel the flexibility (carrying labels in unlabled families) itself is
> beneficial. Even today we have usecases (LU-EPE, 6PE, 4PE) where we want to
> impose labels on unlabeled service routes by resolving over labeled
> families. And some of these usecases (6PE, 4PE) require redistribution
> between Internet-families and Labeled-AFs which is risky. Using MNH allows
> safe walled gardens with consistent service, transport AF-layers, avoiding
> such redistribtions. And, I agree that interaction/precedence of the label
> carried in MNH with the label carried in other places on the route need to
> be specified.
>
> [IM] At this very moment, I can't see where this flexibility is justified.
> It more looks like the approach to get rid of the LU.
>
> KV> I feel having transport layer families (like LU) and resolving service
> families over them has the advantage of indirection and BGP PIC.
>
> KV> So I think we cannot get rid of LU as a transport-family, unless the
> indirection is provided by some other means.
>
>      KV> Its core business logic is to carry transport end-points.
>
>      KV> Having flexibility to carry label on service-routes does not mean
> we need to eliminate a transport layer family. it just allows additional
> expressability for AFs at each layer.
>
> [IM2] I agree with everything about the LU and for exactly this reason I
want to better understand when the LU doesn't suit and we should allocate
extra labels for service routes.

> I also don't think that 4PE/6PE require any redistribution between AFIs,
> no one requires us to disseminate service routes as labeled ones, it is
> enough only for their next-hops. In other words, all the described cases
> are already solved by the LU.
>
> KV> I suppose you are mentioning the LU-EPE (
> https://datatracker.ietf.org/doc/html/draft-gredler-idr-bgplu-epe-15) way
> (nexthop-unchanged on SAFI 1 routes) of doing 6PE? I agree in that way,
> redistribution between AFs is not needed.
>
> KV> If the same approach needs to be used with nexthop-self on SAFI 1
> routes, it may need multiple loopbacks, one to advertise explicit-null, and
> another to advertise implicit-null
>
>  [IM2] Yes, the idea behind my previous comment can be expressed in this
draft (although, I don't like the way it's written). Yes, to preserve a NH
in this case we may need an additional loopback. I'm not sure that to
overcome this "issue" we need to invent something really new. Maybe there
should be more convincing use cases. :)

> KV> But in mechanisms described in rfc4798 or in
> https://www.ietf.org/archive/id/draft-mishra-idr-v4-islands-v6-core-4pe-06.html#section-7.4,
> AFI redistribution is needed. SAFI 1  routes are redistributed into SAFI 4.
>
> [IM2] RFC4798 requires us to send service routes via SAFI4, yes. But as we
agreed above, it is not necessary, there are options. Moreover, vendors
already give us all we need (Juniper is also).
Speaking about the latter document, Section 7 explicitly states that we
actually can send service routes without any labels attached to them. I've
spent much time discussing it with the authors because the original
versions required the sending of service routes as labeled ones.

> KV> This dilutes the business logic of these AFs. Ability to carrying
> explicit-null in SAFI 1 using MNH helps with these scenarios, by not
> diluting SAFI 4 business logic to carry service routes. Thanks.
>
> [IM2] So, having the alternative (the LU), I cannot agree with you on this
part in the way, that we have to create something new to overcome the
described problem.

>
>
> Thanks
>
> Kaliraj
>
>
>
>
>
>
>
>
>
>
>
>
>
> Juniper Business Use Only
>
> *From: *Idr <[e-mail]> on behalf of Igor Malyushkin <
> [e-mail]>
> *Date: *Wednesday, November 15, 2023 at 11:45 AM
> *To: *Susan Hares <[e-mail]>
> *Cc: *[e-mail] <[e-mail]>
> *Subject: *Re: [Idr] WG adoption call -
> draft-kaliraj-idr-multinexthop-attribute-10 - (11/10/2023 to 11/24/2023)
>
> *[External Email. Be cautious of content]*
>
>
>
> Hello folks,
>
> I have some questions. First, it is unclear whether this draft applies to
> several paths and their next-hop addresses or a single path and its
> potential next-hop addresses. The text, especially in Section 3, refers to
> the Add-Paths mechanism as today's alternative, but Add-Paths allows us to
> propagate several paths without losing any attributes.
> Second, the draft specifies that for unlabeled families there can be a
> labeled next-hop attached via the new attribute. I think that moment should
> be carefully considered by the WG. From my side, it is strange to advertise
> any labels to a receiver that does not expect that via families that are
> not about that.
>
> Thank you!
>
>
>
> пт, 10 нояб. 2023 г. в 13:19, Susan Hares <[e-mail]>:
>
> This begins a WG adoption call for
> draft-kaliraj-idr-multinexthop-attribute-10.txt.
>
>
>
> Each author should reply to this message with a message
>
> that indicates whether you know of any IPR on this topic.
>
>
>
> During your consideration,  please consider:
>
>
>
> a.       Are there any errors or problems with this specification?
>
> b.      Will this specification aid operational networks?
>
>
>
> Cheerily, Sue Hares
>
> _______________________________________________
> Idr mailing list
> [e-mail]
> https://www.ietf.org/mailman/listinfo/idr
> <https://urldefense.com/v3/__https:/www.ietf.org/mailman/listinfo/idr__;!!NEt6yMaO-gk!EqDZ5Xgy6P1r1DqauJ3-G_fdixf-M42OMPDJ41fgj9c3Cgs3EuxCPdUgGTckEw2m2hSbX7P6jwCMDhXb1FPb$>
>
>