Re: [Lsr] New Version Notification for draft-ietf-lsr-isis-area-proxy-02.txt

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

From: Lsr <[e-mail]> on behalf of "Les Ginsberg (ginsberg)" <ginsberg=[e-mail]>
Date: Monday, August 3, 2020 at 12:32 PM
To: Bruno Decraene <[e-mail]>, Tony Li <[e-mail]>
Cc: "[e-mail]" <[e-mail]>
Subject: Re: [Lsr] New Version Notification for draft-ietf-lsr-isis-area-proxy-02.txt

Bruno –

Inline.

From: [e-mail] <[e-mail]>
Sent: Monday, August 03, 2020 8:52 AM
To: Les Ginsberg (ginsberg) <[e-mail]>; [e-mail]
Cc: [e-mail]
Subject: RE: [Lsr] New Version Notification for draft-ietf-lsr-isis-area-proxy-02.txt

Les,


From: Les Ginsberg (ginsberg) [mailto:[e-mail]]
Sent: Monday, August 3, 2020 5:22 PM
To: DECRAENE Bruno TGI/OLN <[e-mail]<mailto:[e-mail]>>; [e-mail]<mailto:[e-mail]>
Cc: [e-mail]<mailto:[e-mail]>
Subject: RE: [Lsr] New Version Notification for draft-ietf-lsr-isis-area-proxy-02.txt

Bruno –

The concept of “Area SID” – at least to me – is “please forward to any node in the Area advertising the Area SID”.
[Bruno] How is that different from “anycast”? i.e. your option b?

[Les:] It would differ because it would not be associated with a prefix but with an area. Any node in the area could make use of the SID.

You, however, seem to be asking for either:

a)The node originating the Proxy LSP to advertise a Node SID for a loopback address on that node

OR

b)The node originating the Proxy LSP to advertise an anycast SID which is shared by the IERs in the Proxy Area

To do so, it seems to me we need not invent a new type of SID, we can simply advertise using the existing Prefix Reachability/Prefix SID sub-TLVs.

So it seems you are not interested in an “Area SID”.

Can you please clarify?

[Bruno] I’m interested in the forwarding behavior defined in the draft: “all of the Inside Edge Nodes […] should consume this SID at forwarding time.” How we call this, I don’t really care. Could be area SID, or proxy SID or anycast SID… but for the external L2 nodes, there is no anycast behavior: just one single node/LSP/SID, so calling it anycast could be strange.

[Les:] Agreed – which is why a new type of SID was chosen.

[Acee] I know you guys have all thought about this a lot more than myself. However, I’d envisioned this new Area SID as taking one to the closest entry to the abstracted area. The next SID in the stack would either take you to a destination inside the area or would use the abstracted area as transit to another SID.

Thanks,
Acee


For me, the reason to invent a new type of SID and new forms of SID advertisements is because we were defining a SID with new functionality i.e., send this to another area – which entry point into the area is used should not matter.
[Bruno] From the external L2 nodes, the “area” is seen as a single node. “which entry point into the area is used ” equals “which interface of the (proxy) node is used. I’m not sure that we need a new concept.
From within area internal nodes, we do see the detail of the topology and need to advertise & agree on the area SID.
[Les:] You are correct in terms of usage within Area Proxy.
But I was allowing that other use cases for Area SID might be defined in the future and the abstraction that is present in Area Proxy would not be relevant.
But if the proposal is to use a SID associated with a prefix then I see no need to invent a new SID advertisement.

This seems like a property which might be useful – and might be useful outside of Area Proxy use cases as well. If however, we (the WG) see no need for this type of SID then we should remove these definitions and simply use the existing Prefix-SID advertisements.
[Bruno] The property is useful. I’m fine with the name & encoding in current and past draft.
I’m simply raising the point that this new advertisement will not be understood by vanilla external L2 nodes. Hence unless you upgrade all those external nodes, you have regression in the network, at least in the following use case: replacing a big chassis with a set of smaller nodes grouped in one area.

[Les:] A fair point. But currently the draft is defining a SID which is NOT associated with a prefix. And you are correctly pointing out that this has an impact on both the IER nodes (which MUST support the Area Proxy extensions) and the external L2 nodes – which theoretically need not be upgraded at all.
If the impact on external nodes is unacceptable, then a prefix-SID advertisement MUST be used. Any new advertisement – whether associated with a prefix or not – would raise the same interoperability issue you mention.


One other comment regarding your reference to https://tools.ietf.org/html/draft-ietf-spring-mpls-anycast-segments-03 .

https://tools.ietf.org/html/draft-ietf-lsr-isis-area-proxy-02#section-2.1 already specifies that all inside nodes MUST have the “same SRGB”. So I do not see that your reference is relevant – unless you are proposing to change that.
[Bruno] You can safely forget about this reference.
I was trying to say that anycast SID is not new. And no, I’m not proposing to change that all node must use the same SRGB. (that been said, sometimes you need to use the implementation that are on the market. The concept of index + SRGB has only been designed because some nodes could not support a common SRGB.)
[Les:] Understood. But there is a reason why the anycast draft has not been progressing. 😊

  Les

--Bruno

   Les

From: Lsr <[e-mail]<mailto:[e-mail]>> On Behalf Of [e-mail]<mailto:[e-mail]>
Sent: Monday, August 03, 2020 1:10 AM
To: [e-mail]<mailto:[e-mail]>
Cc: [e-mail]<mailto:[e-mail]>
Subject: Re: [Lsr] New Version Notification for draft-ietf-lsr-isis-area-proxy-02.txt

Hi Tony,

From: Tony Li [mailto:[e-mail]] On Behalf Of [e-mail]<mailto:[e-mail]>
Subject: Re: [Lsr] New Version Notification for draft-ietf-lsr-isis-area-proxy-02.txt


Hi Bruno,

Thank you for the clarification.
[Bruno] You are very welcome. Thank you for listening.

 I understand completely what you’re trying to do and I agree that it’s valuable.

The downside of your approach is that the Area Leader will now need configuration of a new prefix to advertise as the Node SID.
[Bruno] Agreed.
I believe that the Area SID equally needs to be configured, so configuration is required in all cases. Given this, the extra effort seems marginal to me.
Not unthinkable.

What do the Inside Nodes do with this prefix, if anything?
[Bruno] good question. 2 options:
-          Drop traffic to control plane (i.e. IP is not supposed to be used)
-          Nothing really new: it’s an anycast IP/SID. There is even a draft for the more complex case where the SRGB is different on the anycast nodes . https://tools.ietf.org/html/draft-ietf-spring-mpls-anycast-segments-03

Regards,
--Bruno

I am open to this approach, either in addition to, or instead of the approach currently in the draft.  I await feedback from the WG.

Regards,
Tony

<IS-IS bigotry>

p.s. The fact that the node SID requires a prefix is just a side effect of the IP address space excluding hosts from addressing. The one, true
way within IS-IS is the system ID, a separate, independent namespace for nodes that simply avoids ALL of these problems.  If RFC 8667
encoded node SIDs as their own TLV without the unnecessary prefix that OSPF’s style mandates, this would be trivial.

</IS-IS bigotry>

On Jul 31, 2020, at 2:24 AM, [e-mail]<mailto:[e-mail]> wrote:

Hi Tony,

Thank you for your reply.
Top posting the description of the use case that I have in mind.

>  First off, the Area SID is 100% optional. If you choose not to use it, then the Proxy LSP should be 100% compatible with a standard L2 node.
Good. But I think that the idea of the Area SID is a good one, and I choose to use it. Then I’d like to get it for free ;-)


Please fine below a network topology:
<image003.jpg>


My understanding is that the L2 topology seen by node S is the following
<image004.jpg>

P been the Proxy LSP.

S wants to protect from the failure of link S-C by using TI-LFA. For the destination C, it would push 2 (*) node SIDs: P, C
So S needs a Node SID for P:
a)      If P does not redistribute the Node SIDs from its area members, P does not seem to advertise any node SID currently. There is a chance that C’s TI-LFA implementation would not like it and hence would not install protection for this (link, destination)
b)      If P redistributes the Node SIDs from its area members, P advertises 3 node SIDs (1,2, 3). S could pick any one at random. If it picks 3, the forwarding path would be S, A,B, 1, 2, 3, 2 , 1, C,  which is suboptimal.

Two solutions I could think of:
- when redistributing the node SID, P changes the SIDs values and replace them with the value of the Area SID. This works for this use case, but it is borderline. (e.g. if some a L2 node learn both the original and ‘NATed’ SID, we have some SID conflict. Let’s try to avoid this subject).
- P could advertise its own Node-SID with the Area SID value. This is what I’m proposing. Both the IP loopback and the Area SID of this Node SID  are likely configured by the network operator so this does not seem like a significant effort from the implementation.

As you say, this does not involve any protocol extension. But the goal is to improve interop with existing/legacy L2 nodes so this may be valuable in the draft. This point could be pushed to a deployment consideration section if you don’t want any impact on the protocol extension.

Thanks,
--Bruno

(*) Assuming the right metrics on the links. This is not the subject of this thread.


From: Tony Li [mailto:[e-mail]] On Behalf Of [e-mail]<mailto:[e-mail]>
Sent: Thursday, July 30, 2020 7:39 PM
To: DECRAENE Bruno TGI/OLN <[e-mail]<mailto:[e-mail]>>
Cc: [e-mail]<mailto:[e-mail]>
Subject: Re: [Lsr] New Version Notification for draft-ietf-lsr-isis-area-proxy-02.txt


Hi Bruno,

Thank you for your comments.




On Jul 30, 2020, at 9:22 AM, [e-mail]<mailto:[e-mail]> wrote:

Hi Tony,

Thanks for the updated draft.

“ The Area SID Sub-TLV allows the Area Leader to advertise a SID that
   represents the entirety of the Inside Area to the Outside Area.  This
   sub-TLV is learned by all of the Inside Edge Nodes who should consume
   this SID at forwarding time.”

Excellent, from my perspective.

>  - The Area Segment SID TLV has been replaced by extending the Binding SID TLV.


“When SR is enabled, it may be useful to advertise an Area SID which

   will direct traffic to any of the Inside Edge Routers.  The Binding/

   MT Binding TLVs described in RFC 8667 Section 2.4<https://tools.ietf.org/html/rfc8667#section-2.4> are used to

   advertise such a SID.



   The following extensions to the Binding TLV are defined in order to

   support Area SID:



      A new flag is defined:



         T-flag: The SID directs traffic to an area.  (Bit 5) »




This works.


Excellent.




However I may have a different deployment environment than the one you have in mind. Even if those issues may be mine, allow me to share them with you.
In many WAN networks than I’m used to, there are routers from different vendors, platforms, software, generations. Requiring all those routers to support the new Binding SID TLV T-Flag will take time. Some platform may even be end of engineering (evolutions) so would never support such new features.
In my environment, ideally, I would prefer a solution which do not require any new feature on external L2 nodes, while all existing L2 features keep working, in particular SR, SR-TE, TI-LFA, SR uloop avoidance… This would require the Proxy LSP to be not (significantly) different than the LSP of a vanilla L2 node.


First off, the Area SID is 100% optional. If you choose not to use it, then the Proxy LSP should be 100% compatible with a standard L2 node.



I cannot claim that we’ve exhaustively tested our implementation against all of the features that you cite, so there may still be corner cases, but our intent is to make that doable.  For exaple, the Proxy LSP can still contain a node SID, adjacency SID, and prefix SID as before. There’s no change there.




For SR, I think that this would require this Proxy LSP to advertise a Prefix/Node SID with the Area SID attached. One drawback is that a Node-SID is advertised with an IP address that would need to be provisioned.


That’s certainly doable and requires no new protocol machinery. If the WG prefers this mode of operation, I’m not opposed.




Both approaches are not mutually exclusives. I’d be happy enough with an option for the Proxy LSP to advertise an Area Node SID with the Area SID attached.

Finally, there is no requirement to make me happy ;-) . The above could also be a local implementation knob not mentioned in the draft.


Our goal is to make as many customers as happy as possible.  ;-)

Tony


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