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

[Peter Psenak <[e-mail]>]

Hi Ketan,


On 13/05/2022 15:32, Ketan Talaulikar wrote:
> Hi Peter,
> 
> Thanks for your updates to the draft and your responses below.
> 
> I would like to point out a few remaining points to be fixed/addressed.
> 
> a) There is a discrepancy regarding the size of the Metric field for the 
> OSPFv2 IP Algo Reachability sub-TLV between the figure and the text 
> description. The text needs to be fixed to reflect 4 octets size.

fixed

> 
> b) For the OSPFv3 IP Algo Prefix Reachability sub-TLV the Type should be 
> 2 octets and the discrepancy in the sub-TLV name in the Figure needs to 
> be corrected. Length should now become 8.

fixed.

> 
> c) The references to the sections of draft-lsr-flex-algo in this 
> document need corrections in Sec 7 ? In general, I think the references 
> to the base draft sections 11, 12, and 13 (except that M-flag is always 
> used) would be helpful.

I removed the references to any particular sections and kept only the 
references to the draft itself. This is better as the sections tends to 
change and the result is that the references to sections are incorrect.

thanks,
Peter


> 
> Thanks,
> Ketan
> 
> On Wed, May 11, 2022 at 3:20 PM Peter Psenak <[e-mail] 
> <mailto:[e-mail]>> wrote:
> 
>     Hi Ketan,
> 
> 
>     please see 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
>     Done
> 
>      >
>      > 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
>     this has been discussed previously in this thread.
> 
> 
>      >
>      > 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
>     Done
> 
>      >
>      > 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.
> 
> 
>     ##PP
>     Done
> 
>      >
>      > 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.
> 
> 
>     ##PP
>     Done
> 
> 
>      >
>      > 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.
> 
> 
>     ##PP
>     Done. For section 6, I would prefer to keep it in the protocol specific
>     sections.
> 
>      >
>      > 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.
> 
>     ##PP
>     I prefer to keep the MUSTs there
> 
>      >
>      > 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.
> 
>     ##PP
>     Done.
> 
>      >
>      >
>      > 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.
> 
>     ##PP
>     Done
> 
>      >
>      >
>      > 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.
> 
>     ##PP
>     Done
> 
>      >
>      >
>      > 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
>     <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
>     <https://datatracker.ietf.org/doc/html/rfc7684>>] is
>      >     defined to advertise prefix reachability associated with a Flex-
>      >     Algorithm in OSPFv2.
> 
>     ##PP
>     Done
> 
>      >
>      > 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.
> 
>     ##PP
> 
>     Why? We have a text that says:
> 
>     "This new TLV shares the sub-TLV space defined for TLVs 135, 235, 236
>     and 237."
> 
>     Why do we need to describe the usage of the specific sub-TLV?
> 
>      >
>      >
>      > 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.
> 
>     ##PP
>     Done
> 
>      >
>      >
>      > 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.
> 
>     ##PP
>     Done
> 
>      >
>      >
>      > 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.
> 
>     ##PP
>     Done
> 
>      >
>      >
>      > 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?
> 
>     ##PP
>     Done
> 
> 
>      >
>      >
>      > 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" ?
> 
>     ##PP
>     Done
> 
>      >
>      >
>      > 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?
>      >
>      > This (in some way) goes back to my comment (2) above.
> 
>     ##PP
>     I don't see the relationship to BIER IPA here.
> 
>      >
>      >
>      > 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.
> 
>     ##PP
>     nothing prevents someone to advertise LDP label for the IP algo-prefix
>     and use it with the labeled forwarding. I don't see a problem. But this
>     specification does not specify any of it.
> 
>      >
>      >
>      > 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
>     Done.
> 
>     thanks,
>     Peter
>      >
>      >
>      > Thanks,
>      >
>      > Ketan
>      >
>      >
>      >
>      > On Fri, Apr 8, 2022 at 12:38 AM Acee Lindem (acee)
>      > <acee=[e-mail]
>     <mailto:[e-mail]>
>     <mailto:[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____
>      >
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