Re: [Lsr] Working Group Last Call for draft-ietf-lsr-ip-flexalgo-04 - "IGP Flexible Algorithms (Flex-Algorithm) In IP Networks"

[Ketan Talaulikar <[e-mail]>]

Hi Acee,

Regarding the draft-ietf-lsr-flex-algo, there are two aspects that I had
brought up as part of the discussion on the IP FlexAlgo draft:

1) A purely editorial thing: the suggestion to use a term other than
"application" for the data plane or forwarding mechanism related to
FlexAlgo. Peter has suggested "data plane" and I am ok with that or a
similar term (something other than "application" so it doesn't get mixed up
with "application" as in ASLA).

2) The clarification on a per algo per "forwarding plane" computation
requirement. This is not something specified clearly in
draft-ietf-lsr-flex-algo and it is important that we do so using normative
language since that document forms the flex algo base.

Apart from the above, I do not have any other concerns or comments on that
draft.

Thanks,
Ketan


On Sun, Apr 17, 2022 at 3:15 AM Acee Lindem (acee) <[e-mail]> wrote:

> Speaking as WG member and document shepherd:
>
>
>
> Hi Gyan,
>
>
>
> Thanks for the explanation. My position is that the initial draft,
> draft-ietf-lsr-flex-algo,  and the encodings with their attendant
> constraints on those encodings were targeted towards MPLS and SRv6. Given
> that the draft has been in progress for a couple years and that there are
> implementations, we don’t really gain anything by generalizing this draft.
> This can come with draft-ietf-lsr-ip-flexalgo. Am I missing anything here?
> This question is for Ketan as well…
>
> Thanks,
>
> Acee
>
>
>
>
>
> *From: *Lsr <[e-mail]> on behalf of Gyan Mishra <
> [e-mail]>
> *Date: *Friday, April 15, 2022 at 10:38 PM
> *To: *Acee Lindem <[e-mail]>
> *Cc: *"[e-mail]" <[e-mail]>, Ketan Talaulikar <
> [e-mail]>, "[e-mail]" <
> [e-mail]>, "Peter Psenak (ppsenak)" <
> [e-mail]>
> *Subject: *Re: [Lsr] Working Group Last Call for
> draft-ietf-lsr-ip-flexalgo-04 - "IGP Flexible Algorithms (Flex-Algorithm)
> In IP Networks"
>
>
>
>
>
> Hi Acee
>
>
>
> Fixing a typo
>
>
>
> On Fri, Apr 15, 2022 at 10:34 PM Gyan Mishra <[e-mail]>
> wrote:
>
>
>
> Hi Acee
>
>
>
> My question cane up from the list of questions posed by Ketan and Peter’s
> response to question #3.
>
>
>
> See excerpt below.
>
>
>
> I am confused by what Ketan stated in his question below and also Peter’s
> response which is why I am asking the question again.
>
>
>
> I believe the goal of the draft is for Flex Ago to be deployed
> independently of SR filling the gap of IGP Flex Algo providing that
> solution.  So based on what Ketan stated in his question that IGP Flex Algo
> is data plane agnostic and can be used with IP data plane then there is no
> gap to be filled by this draft.
>
>
>
> Maybe I am misreading Ketan’s question.
>
>
>
> So this is a very important point made by Ketan that if IGP Flex Algo is
> open to usage “outside of SR”,  then it is very important to restate the
> goal of this draft, removing assertions in the draft that this draft is for
> non SR IP data planes, as that can be accomplished today by IGP Flex Algo,
> and the gap or new solution being filled by this draft is for IP prefix
> based Flex Algo with Native IP Forwarding.
>
>
>
> This as well is quite confusing to me as if IGP Flex Algo can be used
> outside of SR then can do everything that this draft is supposed to
> accomplish.
>
>
>
> So what then is the purpose of this draft?
>
>
>
> In Peter’s response is stated that each Flex Algo data plane / app can be
> deployed independently meaning this draft and IGP flex Algo can act as 2
> ships in the night.  Also confusing.
>
>
>
> 3) This draft makes assertions that IGP FlexAlgo cannot be deployed
> > without SR. This is not true since the base IGP FlexAlgo spec explicitly
> > opens it up for usage outside of the SR forwarding plane. We already
> > have BIER and MLDP forwarding planes as users of the IGP FlexAlgo. My
> > suggestion is to remove such assertions from the document. It is
> > sufficient to just say that the document enables the use of IGP FlexAlgo
> > for IP prefixes with native IP forwarding.
>
> ##PP
> where do you see such assertion? Each flex-algo data-plane/app can be
> deployed independently.
>
>
>
>
>
>
>
> On Fri, Apr 15, 2022 at 7:51 PM Acee Lindem (acee) <[e-mail]> wrote:
>
> Gyan,
>
>
>
> What is your point here? Is this a trick question?
>
>
>
> Thanks,
>
> Acee
>
>
>
> *From: *Gyan Mishra <[e-mail]>
> *Date: *Friday, April 15, 2022 at 5:31 PM
> *To: *"Peter Psenak (ppsenak)" <[e-mail]>
> *Cc: *Acee Lindem <[e-mail]>, Ketan Talaulikar <
> [e-mail]>, "[e-mail]" <
> [e-mail]>, "[e-mail]" <[e-mail]>
> *Subject: *Re: [Lsr] Working Group Last Call for
> draft-ietf-lsr-ip-flexalgo-04 - "IGP Flexible Algorithms (Flex-Algorithm)
> In IP Networks"
>
>
>
>
>
> Hi Peter
>
>
>
> My understanding is that the main goal of this draft is to be able to use
> flex algo over IPv4 or IPv6 data plane as that is not possible with
> existing Flex Algo which can only be used on SR data plane.
>
>
>
> Is that correct or am I missing something?
>
>
>
> https://datatracker.ietf.org/doc/html/draft-ietf-lsr-ip-flexalgo
>
>
>
>
>
> Abstract
>
>
>
>    An IGP Flexible Algorithm (Flex-Algorithm) allows IGP to compute
>
>    constraint-based paths.  As currently defined, IGP Flex-Algorithm is
>
>    used with Segment Routing (SR) data planes - SR MPLS and SRv6.
>
>    Therefore, Flex-Algorithm cannot be deployed in the absence of SR.
>
>
>
>    This document extends IGP Flex-Algorithm, so that it can be used for
>
>    regular IPv4 and IPv6 prefixes.  This allows Flex-Algorithm to be
>
>    deployed in any IP network, even in the absence of SR.
>
>
>
> https://datatracker.ietf.org/doc/html/draft-ietf-lsr-flex-algo-19
>
>
>
> Abstract
>
>
>
>    IGP protocols traditionally compute best paths over the network based
>
>    on the IGP metric assigned to the links.  Many network deployments
>
>    use RSVP-TE based or Segment Routing based Traffic Engineering to
>
>    steer traffic over a path that is computed using different metrics or
>
>    constraints than the shortest IGP path.  This document proposes a
>
>    solution that allows IGPs themselves to compute constraint-based
>
>    paths over the network.  This document also specifies a way of using
>
>    Segment Routing (SR) Prefix-SIDs and SRv6 locators to steer packets
>
>    along the constraint-based paths.
>
>
>
> Kind Regards
>
>
>
> Gyan
>
>
>
>
>
> On Mon, Apr 11, 2022 at 3:37 AM Peter Psenak <ppsenak=
> [e-mail]> wrote:
>
> Hi Ketan,
>
> please responses to some of your comments inline (##PP):
>
> On 11/04/2022 08:25, Ketan Talaulikar wrote:
> > Hello All,
> >
> > Following are some comments on this draft:
> >
> > 1) Is this draft about opening the use of all IGP Algorithms for IP
> > (Algo) Routing or intended to be specific to Flexible Algorithms (i.e.
> > algo 128-255) alone. I think it is important to specify the scope
> > unambiguously. Perhaps it makes sense to restrict the usage in this
> > particular document to FlexAlgorithms alone. If not, the draft probably
> > needs an update and we need to also cover algo 1 (Strict SPF)
> > applicability and update the text to refer more generically to
> > algo-specific IP routing.
>
> ##PP
> the intent is to use FlexAlgorithms  only.
>
> >
> > 2) The relationship between the algo usage for IP FlexAlgo and other
> > data planes (e.g. FlexAlgo with SR) is not very clear. There arise
> > complications when the algo usage for IP FlexAlgo overlap with other
> > (say SR) data planes since the FAD is shared but the node participation
> > is not shared. While Sec 9 suggests that we can work through these
> > complications, I question the need for such complexity. The FlexAlgo
> > space is large enough to allow it to be shared between various data
> > planes without overlap. My suggestion would be to neither carve out
> > parallel algo spaces within IGPs for various types of FlexAlgo data
> > planes nor allow the same algo to be used by both IP and SR data planes.
> > So that we have a single topology computation in the IGP for a given
> > algo based on its FAD and data plane participation and then when it
> > comes to prefix calculation, the results could involve programming of
> > entries in respective forwarding planes based on the signaling of the
> > respective prefix reachabilities. The coverage of these aspects in a
> > dedicated section upfront will help.
>
> ##PP
> I strongly disagree.
>
> FAD is data-pane/app independent. Participation is data-plane/app
> dependent. Base flex-algo specification is very clear about that. That
> has advantages and we do not want to modify that part.
>
> Topology calculation for algo/data-plane needs to take both FAD and
> participation into account. You need independent calculation for each
> data-plane/app in the same algo.
>
> The fact that the same FAD is shareable between all apps has it
> advantages and use cases - e.g. if the participation for algo X is the
> same in SR and IP data-planes, one can use SR to protect IP in that algo.
>
>
> >
> > 3) This draft makes assertions that IGP FlexAlgo cannot be deployed
> > without SR. This is not true since the base IGP FlexAlgo spec explicitly
> > opens it up for usage outside of the SR forwarding plane. We already
> > have BIER and MLDP forwarding planes as users of the IGP FlexAlgo. My
> > suggestion is to remove such assertions from the document. It is
> > sufficient to just say that the document enables the use of IGP FlexAlgo
> > for IP prefixes with native IP forwarding.
>
> ##PP
> where do you see such assertion? Each flex-algo data-plane/app can be
> deployed independently.
>
> >
> > 4) The draft is mixing up "address" and "prefix" in some places. IGP
> > path computation is for prefixes and not addresses. There are a few
> > instances where "address" should be replaced by "prefix". References to
> > RFC791 and RFC8200 seem unnecessary.
> >
> > 5) The draft does not cover the actual deployment use-case or
> > applicability of IP FlexAlgo. The text in Sec 3 is not clear and
> > insufficient. What is the point/use of sending traffic to a loopback of
> > the egress router? Perhaps it makes sense in a deployment where IP-in-IP
> > encapsulation is used for delivering an overlay service? If so, would be
> > better to clarify this. The other deployment scenario is where
> > "external" or "host/leaf prefixes" are associated with a FlexAlgo to
> > provide them a different/appropriate routing path through the network.
> > Yet another is the use of IP FlexAlgo along with LDP. Sec 9 does not
> > address the topic well enough. I would suggest expanding and clarifying
> > this and perhaps other such deployment use cases (or applicability) in
> > the document in one of the earlier sections to provide a better context
> > to the reader.
> >
> > 6) It would be better to move the common (i.e. not protocol specific)
> > text from 5.1 and 5.2 under 5. This might also apply to some extent to
> > the contents of sec 6.
> >
> > 7) Most of the text with MUSTs in sec 5 doesn't really make sense in
> > repeating - this is covered in the base FlexAlgo spec already. The only
> > key/important MUST is the one related to using separate algo for IP
> > FlexAlgo over SR data planes. See my previous comment (2) above.
> >
> > 8) Sec 5.1, the SHOULD needs to be MUST in the text below.
> >
> >     A router receiving multiple IP Algorithm
> >     sub-TLVs from the same originator SHOULD select the first
> >     advertisement in the lowest-numbered LSP and subsequent instances of
> >     the IP Algorithm Sub-TLV MUST be ignored.
> >
> >
> > 9) Sec 5.1, I would suggest changing the following text as indicated.
> > Also, perhaps add that the algo 0 MUST NOT be advertised and a receiver
> > MUST ignore if it receives algo 0.
> > OLD
> >
> >     The IP Algorithm Sub-TLV could be used to advertise
> >     support for non-zero standard algorithms, but that is outside the
> >     scope of this document.
> >
> > NEW
> >
> >     The use of IP Algorithm Sub-TLV to advertise support for algorithms
> >
> >     outside the flex-algorithm range is outside the
> >     scope of this document.
> >
> >
> > 10) Sec 5.1, the SHOULD needs to be MUST in the text below
> >
> >     The IP Algorithm TLV is optional.  It SHOULD only be advertised once
> >     in the Router Information Opaque LSA.
> >
> >
> > 11) Sec 6. The following text is better moved into the respective
> > protocol sub-sections. OSPFv3 is not covered anyway by it.
> >
> >     Two new top-level TLVs are defined in ISIS [ISO10589  <
> https://datatracker.ietf.org/doc/html/draft-ietf-lsr-ip-flexalgo#ref-ISO10589>]
> to advertise
> >     prefix reachability associated with a Flex-Algorithm.
> >
> >     *  The IPv4 Algorithm Prefix Reachability TLV
> >
> >     *  The IPv6 Algorithm Prefix Reachability TLV
> >
> >     New top-level TLV of OSPFv2 Extended Prefix Opaque LSA [RFC7684  <
> https://datatracker.ietf.org/doc/html/rfc7684>] is
> >     defined to advertise prefix reachability associated with a Flex-
> >     Algorithm in OSPFv2.
> >
> > 12) Sec 6.1 & 6.2. There is no discussion regd the use of the Prefix
> > Attribute Flags sub-TLV with the new top-level TLVs.
> >
> > I think their usage MUST (or at least SHOULD) be included as it helps
> > determine the route type and prefix attributes that
> >
> > have proven to be quite useful for various use cases and deployments.
> >
> >
> > 13) Sec 6.2 what happens when the same prefix is advertised as SRv6
> > Locator as well as IPv6 Algo Prefix (same or conflicting algos). Perhaps
> > both must be ignored?
> >
> > The same applies for OSPFv3 as well.
> >
> >
> > 14) Sec 6.3, OSPFv2 MT-ID reference should be RFC4915. Perhaps the range
> > of MT should be mentioned since it is a 8 bit field here.
> >
> >
> > 15) Sec 6.4, the metric field in the sub-TLV has to be 32-bit. While
> > 24-bit is ok when the FAD uses IGP metric, it will not suffice for other
> > IGP metric types.
> >
> >
> > 16) Sec 6.3 & 6.4, the conflict checking with base algo 0 prefix
> > reachability cannot be limited only to the OSPFv2/3 Extended LSAs but
> > should also cover the base fixed form
> >
> > OSPFv2/v3 LSAs. We could use a more generic term like "normal prefix
> > reachability" advertisements perhaps to cover the different LSAs?
> >
> >
> > 17) Sec 7 and 8, suggest to not use the term "application" to avoid
> > confusion with ASLA. My understanding is that there is a single FlexAlgo
> > application when it comes to ASLA.
> >
> > Perhaps the intention here is "data plane" ?
> >
> >
> > 18) The relationship between the BIER IPA and this draft needs some
> > clarifications - should the BIER WG be notified if they want to update
> > draft-ietf-bier-bar-ipa?
>
> ##PP
> what is the relationship? I see none.
>
>
> >
> > This (in some way) goes back to my comment (2) above.
> >
> >
> > 19) Sec 8, what prevents the use of IP FlexAlgo paths programmed by LDP
> > as well. Or if the intention is to use them strictly for IP forwarding
> only
> >
> > then this needs to be clarified.
> >
> >
> > 20) The following text in Sec 9 is about using the same FlexAlgo (with a
> > common definition) for multiple data-planes at the same time. The key is
> > that we only are able to use
> >
> > prefix in one algo/data-plane? I am wondering if we can improve this
> > text to bring this out in a better way. Or altogether remove this if we
> > agree to not allow sharing of algo
> >
> > between different data planes to keep things simple.
> >
> >     Multiple application can use the same Flex-Algorithm value at the
> >
> >     same time and and as such share the FAD for it.  For example SR-MPLS
> >     and IP can both use such common Flex-Algorithm.  Traffic for SR-MPLS
> >     will be forwarded based on Flex-algorithm specific SR SIDs.  Traffic
> >     for IP Flex-Algorithm will be forwarded based on Flex-Algorithm
> >     specific prefix reachability announcements.
>
>
> ##PP
> above text does not talk about the same prefix. It talks in general how
> forwarding works in presence of multiple data-planes/apps using the same
> algo.
>
> thanks,
> Peter
>
> >
> >
> > Thanks,
> >
> > Ketan
> >
> >
> >
> > On Fri, Apr 8, 2022 at 12:38 AM Acee Lindem (acee)
> > <acee=[e-mail] <mailto:[e-mail]>>
> wrote:
> >
> >     This begins a WG last call for draft-ietf-lsr-ip-flexalgo-04.  The
> >     draft had a lot of support and discussion initially and has been
> >     stable for some time. Please review and send your comments, support,
> >     or objection to this list before 12 AM UTC on April 22^nd , 2022.____
> >
> >     __ __
> >
> >     Thanks,
> >     Acee____
> >
> >     _______________________________________________
> >     Lsr mailing list
> >     [e-mail] <mailto:[e-mail]>
> >     https://www.ietf.org/mailman/listinfo/lsr
> >     <https://www.ietf.org/mailman/listinfo/lsr>
> >
>
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>
> --
>
> *Error! Filename not specified.* <http://www.verizon.com/>
>
> *Gyan Mishra*
>
> *Network Solutions Architect *
>
> *Email [e-mail] <[e-mail]>*
>
> *M 301 502-1347*
>
>
>
> --
>
> [image: Image removed by sender.] <http://www.verizon.com/>
>
> *Gyan Mishra*
>
> *Network Solutions Architect *
>
> *Email [e-mail] <[e-mail]>*
>
> *M 301 502-1347*
>
>
>
> --
>
> [image: Image removed by sender.] <http://www.verizon.com/>
>
> *Gyan Mishra*
>
> *Network Solutions Architect *
>
> *Email [e-mail] <[e-mail]>*
>
> *M 301 502-1347*
>
>
>