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

[Gyan Mishra <[e-mail]>]

Hi Aijun

Section 6 describes why as Les has mentioned that you need a new top level
TLV for OSPF and ISIS for IP Flex Algo Reachability advertisement to
disambiguate the data plane IP from existing SR-MPLS and SRV6 data planes.

The problem is that IP Flex Algo cannot use the existing prefix
reachability advertisement because IGP Flex Algo advertise the prefix
reachability in default Algo 0.  Thus the existing top level TLVs cannot be
used  and new Top Level TLVs are being  allocated with the IP Flex Algo
draft,
.

https://datatracker.ietf.org/doc/html/draft-ietf-lsr-ip-flexalgo#section-6

6 <https://datatracker.ietf.org/doc/html/draft-ietf-lsr-ip-flexalgo#section-6>.
Advertising IP Flex-Algorthm Reachability

   To be able to associate the prefix with the Flex-Algorithm, the
   existing prefix reachability advertisements can not be used, because
   they advertise the prefix reachability in default algorithm 0.
   Instead, a new IP Flex-Algorithm reachability advertisements are
   defined in ISIS and OSPF.

   The M-flag in FAD is not applicable to IP Algorithm Prefixes.  Any IP
   Algorithm Prefix advertisement includes the Algorithm and Metric
   fields.  When IP Algorithm Prefix is advertised between areas or
   domains, the metric field in the IP Algorithm Prefix advertisement
   MUST be used irrespective of the M-flag in the FAD advertisement.

   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.



Section 14 LSR Flex Algo helps describe the association of Flex Algo to

SR-MPLS prefix SID and SRv6 Locator.


This is most pertinent as LSR Flex Algo base draft is applicable to

SR data plane but as this is a base draft it is extensible to future

innovations such as IP Flex Algo and other future data planes.


https://datatracker.ietf.org/doc/html/draft-ietf-lsr-flex-algo-19#section-14



Thanks


Gyan


On Wed, May 4, 2022 at 7:07 AM Aijun Wang <[e-mail]> wrote:

> Hi, Les:
>
> Aijun Wang
> China Telecom
>
> On May 4, 2022, at 07:12, Les Ginsberg (ginsberg) <[e-mail]>
> wrote:
>
> 
>
> Aijun –
>
>
>
> I am not an author of the draft – and so cannot speak on behalf of the
> draft authors.
>
> But here is my response as WG member.
>
>
>
> You need to focus on the dataplane.
>
>
>
> Suppose a node advertises 1.1.1.1/32 in (IS-IS) TLV 135.
>
> If a packet addressed to 1.1.1.1 arrives unlabeled, it can be forwarded
> using the Algo 0 path(s) installed in forwarding.
>
> If a packet addressed to 1.1.1.1 arrives with MPLS label encap, you can
> use the algorithm specific SID to do a lookup and forward based on the
> flex-algo specific paths.
>
>
>
> But if you do not have an SR encap, what about the packet will tell you to
> forward via (for example) Flex-algo 130 paths rather than Algo 0 paths? And
> how would you install such paths in the dataplane along with Algo 0 paths?
>
> [WAJ] For IP-Flexalgo, I think the only indicator for different flex-algo
> is the destination itself. That is to say, if you associated the prefixes
> with one flex-algo, then the forwarding path/nexthop is determined via this
> flex-algo.
> That is to say, one prefix must be only associated with one flex-algo.
>
>
>
> IP Flex-Algo addresses those issues by assigning a given address to
> one-and-only-one algo. This is why new Reachability advertisements are
> required.
>
> [WAJ] We can achieve the same effect via associated the FAPM within the
> existing TLVs. I haven’t seen the necessary to define the new top TLV.
>
> So, for example, the same node could advertise 1.1.1.2/32 in the new IPv4
> Algorithm Prefix Reachability TLV, associate it with algorithm 130, and a
> forwarding entry can be installed for that prefix that follows Algo 130
> paths.
>
> Since this address is not used by any other algorithm (and especially not
> by Algo 0), there is no ambiguity in the dataplane.
>
>
> [WAJ] Can get the same effect via the FAPM within the existing TLVs
>
>
>
> This is also why the statement you quote below regarding the same prefix
> advertised in both IPv4 Reachability and IPv4 Algorithm Prefix Reachability
> is necessary. The dataplane cannot support both for the same prefix.
>
>
> [WAJ] I think the above considerations is related to the flex-algo
> priority, that is, algo 0 has the higher priority. Such considerations can
> also be achieved via the FAPM. It can’t convince the persons that you must
> define one new top TLV.
>
>
>
> HTH
>
>
>
>     Les
>
>
>
>
>
> *From:* Aijun Wang <[e-mail]>
> *Sent:* Tuesday, May 3, 2022 3:10 PM
> *To:* Les Ginsberg (ginsberg) <[e-mail]>
> *Cc:* Peter Psenak (ppsenak) <[e-mail]>; Acee Lindem (acee) <
> [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 and Les:
>
> Prefix Segment Identifier sub-TLV and FAPM sub-TLV are two independent
> sub-TLVs for TLV135, 235,236 and 237.
>
> They are not required to be exist at the same time.
>
> FAPM just describes the metrics that associated with different Flex-Algo.
> Isn’t it more straightforward to associate it with the intended prefixes to
> achieve the same results as the newly defined top-TLV?
>
> I want to know the technical reason why we can’t follow this direction?
>
> Anyway, the router can use such information to calculate the different
> forwarding path based on the algorithm and metric.
>
> If there is any obstacles to achieve this, I think we should update the
> https://datatracker.ietf.org/doc/html/draft-ietf-lsr-flex-algo-19 to
> cover it.
>
>
>
> And, in the WGLC document, there is the following description:
>
>
>
>   In cases where a prefix advertisement is received in both a IPv4
>
>    Prefix Reachability TLV and an IPv4 Algorithm Prefix Reachability
>
>    TLV, the IPv4 Prefix Reachability advertisement MUST be preferred
>
>    when installing entries in the forwarding plane.
>
>
> It is obvious that any prefixes can be advertised in either TLVs, what’s
> the necessary to define the new one?
>
>
>
> Aijun Wang
>
> China Telecom
>
>
>
> On May 3, 2022, at 22:48, Les Ginsberg (ginsberg) <
> ginsberg=[e-mail]> wrote:
>
> 
>
> Peter -
>
>
>
> I am in agreement.
>
>
>
> However, the IANA section of the draft is missing some necessary
> information.
>
> The new top level TLVs in IS-IS - I am assuming you want these to share
> the sub-TLV space defined in
> https://www.iana.org/assignments/isis-tlv-codepoints/isis-tlv-codepoints.xhtml#isis-tlv-codepoints-advertising-prefix-reachability
>
> In which case you need to provide a list of the existing sub-TLVs and an
> indication (Y/N) as to whether they are allowed in the new TLVs.
>
>
>
> Here is my initial take:
>
>
>
> 1    32-bit Administrative Tag Sub-TLV    Y
>
> 2    64-bit Administrative Tag Sub-TLV    y
>
> 3    Prefix Segment Identifier            n
>
> 4    Prefix Attribute Flags              y
>
> 5    SRv6 End SID                        n
>
> 6    Flex-Algorithm Prefix Metric        n
>
> 11   IPv4 Source Router ID               y
>
> 12   IPv6 Source Router ID               y
>
> 32   BIER Info                          n(??)
>
>
>
>     Les
>
>
>
> > -----Original Message-----
>
> > From: Lsr <[e-mail]> On Behalf Of Peter Psenak
>
> > Sent: Tuesday, May 3, 2022 7:14 AM
>
> > To: Aijun Wang <[e-mail]>; Peter Psenak
>
> > <ppsenak=[e-mail]>
>
> > Cc: Acee Lindem (acee) <acee=[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"
>
> >
>
> > Aijun,
>
> >
>
> > On 03/05/2022 15:52, Aijun Wang wrote:
>
> > > Hi, Peter:
>
> > > I think the logic is the following:
>
> > > FAPM is the sub-TLV of TLV 135,235,236 and 237, then it extends these
> TLVs
>
> > for advertising prefixes in algorithm 0 to other Flexible Algorithm.
>
> > > Then I see no reason to define the new top-TLV to encoding the similar
>
> > information.
>
> >
>
> > FAPM is used in SR-MPLS case where algo 0 prefix has multiple flex-algo
>
> > SIDs. So the algo 0 reachability is always advertised in legacy TLV and
>
> > FAPM is used to advertise additional flex-algo metric for inter-area or
>
> > external prefixes.
>
> >
>
> > We can not use it for IP flex-algo.
>
> >
>
> > thanks,
>
> > Peter
>
> >
>
> > >
>
> > > Aijun Wang
>
> > > China Telecom
>
> > >
>
> > >> On May 3, 2022, at 19:16, Peter Psenak
>
> > <ppsenak=[e-mail]> wrote:
>
> > >>
>
> > >> Hi Aijun,
>
> > >>
>
> > >>> On 03/05/2022 11:57, Aijun Wang wrote:
>
> > >>> Hi, Peter：
>
> > >>> Different data planes use different Flex-Algorithm and associated
>
> > metric, they can’t be mixed.
>
> > >>> Or, would you like to point out why the following scenarios can’t be
>
> > achieved via the FAPM?
>
> > >>> 1) The PE router has three loopback addresses(Lo1-Lo3), each
> associated
>
> > with different Flex-ALgorithhms, and also different metrics. They are
>
> > advertised via the FAPM, no MPLS SIDs are associated with these loopack
>
> > prefixes advertisements.
>
> > >>> 2) The PE router has also another inter-area/inter-domain
>
> > prefixes(IPextra), with the FAPM and MPLS SID advertised via the prefixes
>
> > advertisements.
>
> > >>> When the PE in other ends want to send the traffic to theses
> addresses:
>
> > >>> 1)  To the formers three destinations(Lo1-Lo3), the FIB that are
> formed
>
> > by the associated FAPM will be used, that is, the IP-based forwarding
> will be
>
> > selected.
>
> > >>> 2) To the Inter-area/inter-domain prefixes the FIB that are formed
> via
>
> > the FAPM and the associated SID, the MPLS-based forwarding will be
>
> > selected.
>
> > >>> Why can’t they coexist?
>
> > >>
>
> > >> FAPM Sub-TLV is a sub-TLV of TLVs 135, 235, 236, and 237. These TLVs
>
> > advertise the reachability of the prefix in algorithm 0.
>
> > >>
>
> > >> For an IP algo prefix, which is associated with the flex-algorithm,
> the
>
> > reachability in algorithm 0 must not be advertised. So we have to use a
>
> > different top level TLV.
>
> > >>
>
> > >>
>
> > >> thanks,
>
> > >> Peter
>
> > >>
>
> > >>
>
> > >>
>
> > >>> Aijun Wang
>
> > >>> China Telecom
>
> > >>>>> On May 3, 2022, at 16:05, Peter Psenak
>
> > <ppsenak=[e-mail]> wrote:
>
> > >>>>
>
> > >>>> Aijun,
>
> > >>>>
>
> > >>>>> On 03/05/2022 09:59, Aijun Wang wrote:
>
> > >>>>> Hi, Peter:
>
> > >>>>> The definition of FAPM for IS-IS and OSPF doesn’t prevent from it
> is
>
> > used for the intra-area prefixes.
>
> > >>>>> If we advertise the different loopback addresses via the FAPM,
>
> > associate them to different Flex-Algo and related metrics, and does not
>
> > allocate the MPLS SID, we can achieve the IP-Flex effect then.
>
> > >>>>
>
> > >>>> as I said, we can not mix metrics for different data-planes.
>
> > >>>>
>
> > >>>>> So, what’s the additional value of the IP-Flexalgo draft then?
>
> > >>>>
>
> > >>>> please read the draft. It defines the flex-algo for IP data plane.
>
> > >>>>
>
> > >>>> thanks,
>
> > >>>> Peter
>
> > >>>>
>
> > >>>>
>
> > >>>>
>
> > >>>>> Aijun Wang
>
> > >>>>> China Telecom
>
> > >>>>>>> On May 3, 2022, at 14:46, Peter Psenak
>
> > <ppsenak=[e-mail]> wrote:
>
> > >>>>>>
>
> > >>>>>> Aijun,
>
> > >>>>>>
>
> > >>>>>>> On 03/05/2022 00:47, Aijun Wang wrote:
>
> > >>>>>>> Hi, Acee:
>
> > >>>>>>> The questions raised at
>
> > https://mailarchive.ietf.org/arch/msg/lsr/RlHphXCwxMbgGvcBV_m24xiDzS0
>
> > /
> <https://mailarchive.ietf.org/arch/msg/lsr/RlHphXCwxMbgGvcBV_m24xiDzS0/>
>
> > <https://mailarchive.ietf.org/arch/msg/lsr/RlHphXCwxMbgGvcBV_m24xiDzS
> <https://mailarchive.ietf.org/arch/msg/lsr/RlHphXCwxMbgGvcBV_m24xiDzS0/>
>
> > 0/
> <https://mailarchive.ietf.org/arch/msg/lsr/RlHphXCwxMbgGvcBV_m24xiDzS0/>>
> has not been answered.
>
> > >>>>>>
>
> > >>>>>> IS-IS Flexible Algorithm Prefix Metric Sub-TLV” and “OSPF Flexible
>
> > Algorithm Prefix Metric Sub-TLV” are defined for advertisement of
> algorithm
>
> > specific metric for inter-area inter-AS prefixes for SR-MPLS data-plane.
>
> > >>>>>>
>
> > >>>>>> SR MPLS and IP are independent data-planes used for flex-algo. We
>
> > can not mix their metric.
>
> > >>>>>>
>
> > >>>>>> thanks,
>
> > >>>>>> Peter
>
> > >>>>>>
>
> > >>>>>>> Aijun Wang
>
> > >>>>>>> China Telecom
>
> > >>>>>>>>> On May 2, 2022, at 23:00, Acee Lindem (acee)
>
> > <acee=[e-mail]> wrote:
>
> > >>>>>>>>
>
> > >>>>>>>> The WG last call has completed. We will submit an updated
>
> > version of the document for publication with the terminology changes
> based
>
> > on the discussion amongst the authors, Ketan, Robert, Gyan, and others.
>
> > >>>>>>>>
>
> > >>>>>>>> Thanks,
>
> > >>>>>>>> Acee
>
> > >>>>>>>>
>
> > >>>>>>>> *From: *Lsr <[e-mail]> on behalf of "Acee Lindem
>
> > (acee)" <acee=[e-mail]>
>
> > >>>>>>>> *Date: *Thursday, April 7, 2022 at 3:07 PM
>
> > >>>>>>>> *To: *"[e-mail]" <[e-mail]>
>
> > >>>>>>>> *Cc: *"[e-mail]" <draft-ietf-lsr-ip-
> <[e-mail]>
>
> > [e-mail] <[e-mail]>>
>
> > >>>>>>>> *Subject: *[Lsr] Working Group Last Call for draft-ietf-lsr-ip-
>
> > flexalgo-04 - "IGP Flexible Algorithms (Flex-Algorithm) In IP Networks"
>
> > >>>>>>>>
>
> > >>>>>>>> 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]
>
> > >>>>>>>> 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
>
> _______________________________________________
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-- 

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*Gyan Mishra*

*Network Solutions A**rchitect *

*Email [e-mail] <[e-mail]>*



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