Re: [Lsr] Working Group Adoption of "IGP Unreachable Prefix Announcement" - draft-ppsenak-lsr-igp-unreach-prefix-announce-04

[Huzhibo <[e-mail]>]

From: Les Ginsberg (ginsberg) [mailto:ginsberg=[e-mail]]
Sent: Thursday, August 31, 2023 10:57 AM
To: Huzhibo <[e-mail]>; Peter Psenak (ppsenak) <[e-mail]>; linchangwang <[e-mail]>; Acee Lindem <[e-mail]>; lsr <[e-mail]>
Cc: [e-mail]
Subject: RE: [Lsr] Working Group Adoption of "IGP Unreachable Prefix Announcement" - draft-ppsenak-lsr-igp-unreach-prefix-announce-04


Zhibo -



Please see inline.



> -----Original Message-----

> From: Huzhibo <huzhibo=[e-mail]<mailto:huzhibo=[e-mail]>>

> Sent: Wednesday, August 30, 2023 6:33 PM

> To: Les Ginsberg (ginsberg) <[e-mail]<mailto:[e-mail]>>; Peter Psenak (ppsenak)

> <[e-mail]<mailto:[e-mail]>>; linchangwang <[e-mail]<mailto:[e-mail]>>;

> Acee Lindem <[e-mail]<mailto:[e-mail]>>; lsr <[e-mail]<mailto:[e-mail]>>

> Cc: [e-mail]<mailto:[e-mail]>

> Subject: RE: [Lsr] Working Group Adoption of "IGP Unreachable Prefix

> Announcement" - draft-ppsenak-lsr-igp-unreach-prefix-announce-04

>

> Hi Les:

>

>     I think you may have connected something. Existing routers, on receiving a

> prefix reachability advertisement with a

> U-Flag described in https://datatracker.ietf.org/doc/draft-ppsenak-lsr-igp-<https://datatracker.ietf.org/doc/draft-ppsenak-lsr-igp-ureach-prefix-announce/>

> ureach-prefix-announce/<https://datatracker.ietf.org/doc/draft-ppsenak-lsr-igp-ureach-prefix-announce/> also will interpret that prefix as being reachable.



[LES:] This statement is incorrect.

RFC 5305 states:



<snip>

If a prefix is advertised with a metric larger then MAX_PATH_METRIC

   (0xFE000000, see paragraph 3.0), this prefix MUST NOT be considered

   during the normal SPF computation.  This allows advertisement of a

   prefix for purposes other than building the normal IP routing table.

<end snip>



(Equivalent statement in RFC 5308 for IPv6)



Existing implementations will ignore the advertisement purely on the basis of the metric value - this does not depend upon understanding the U bit.



But existing implementations will NOT ignore a prefix reachability advertisement just because it has a source Router ID set to 0 as draft-wang-lsr-prefix-unreachable-annoucement defines.



It is worth noting that AFTER the publication of draft-ppsenak-lsr-igp-pfx-reach-loss-00 in March 2022 (subsequently renamed as draft-ppsenak-lsr-igp-ureach-prefix-announce), the authors of draft-wang-lsr-prefix-unreachable-annoucement apparently realized they had  an interoperability problem with existing routers (something many of us had been highlighting for years) and in V10 (published in Jul 2022) an option was added to advertise using maximum metric (the solution already proposed by draft-ppsenak). But because the authors apparently didn’t want to abandon the use of "Router ID = 0", the new version of the draft proposed a dependency on how the unreachable prefix should be advertised. If all routers in the network indicated support for the new extension (indicated by yet another protocol extension - a new Router Capability sub-TLV for IS-IS) then the use of Router ID = 0 could be used, but if any router in the network did not advertise the new capability, then the use of max-metric is required. Which means the solution requires routers advertising unreachability to potentially regenerate the advertisement in a different form whenever the state of support by all routers in the network for the extension changes.



[Zhibo:] draft-wang-lsr-prefix-unreachable-annoucement doesn`t use “Router ID = 0” to implement backward compatibility. It only provides two options: capability negotiation and MAX metric. When capability negotiation changes, there is no requirement to update the MAX metric value. It can be retained.



> Both two draft used The 0xFE000000 metric indicates that the prefix is not

> reachable. Doesn't make a difference at this point.



[LES:] The solution defined in draft-ppsenak-lsr-igp-unreach-prefix-announce does not introduce any interoperability issues with existing routers, does not require multiple encoding formats, and does not require a router to regenerate advertisements in a different form based on the state of support by all routers in the network.

I think this makes a big difference. 😊

[Zhibo:] The same applies to draft-wang-lsr-prefix-unreachable-annoucement, which is not the main difference between the two documents.



Thanks

Zhibo



   Les



>

> Thanks

>

> Zhibo Hu

>

> > -----Original Message-----

> > From: Lsr [mailto:[e-mail]] On Behalf Of Les Ginsberg

> > (ginsberg)

> > Sent: Thursday, August 31, 2023 12:31 AM

> > To: Peter Psenak <ppsenak=[e-mail]<mailto:ppsenak=[e-mail]>>; linchangwang

> > <[e-mail]<mailto:[e-mail]>>; Acee Lindem <[e-mail]<mailto:[e-mail]>>;

> > lsr <[e-mail]<mailto:[e-mail]>>

> > Cc: [e-mail]<mailto:[e-mail]>

> > Subject: Re: [Lsr] Working Group Adoption of "IGP Unreachable Prefix

> > Announcement" - draft-ppsenak-lsr-igp-unreach-prefix-announce-04

> >

> > Changwang -

> >

> > It is very important to note ...

> >

> > <snip>

> > > > 2. The Draft #1 utilizes the existing mechanisms [RFC7794] and

> > > > [RFC9084] to

> > > indicate reachability by checking whether the originator information

> > > is

> > > >    NULL.

> > <end snip>

> >

> > This statement is incorrect. There is no existing mechanism defined in the

> > protocol that states that a prefix reachability advertisement sent with a

> > source router ID == 0 implies unreachability.

> > Please see https://www.rfc-editor.org/rfc/rfc7794.html#section-2.2

> >

> > Existing routers, on receiving a prefix reachability advertisement with a

> > Source Router ID == 0 will interpret that prefix as being reachable - which

> > is exactly the opposite of the intent defined in

> > https://www.ietf.org/archive/id/draft-wang-lsr-prefix-unreachable-annouc

> > ement-12.txt

> > This is one of the things which is broken in this draft.

> > This fact has been pointed out to the authors many times over the years -

> > but they have consistently ignored it.

> >

> > On the other hand,

> > https://www.ietf.org/archive/id/draft-ppsenak-lsr-igp-ureach-prefix-annou

> > nce-04.txt uses an existing mechanism defined in RFC 5305 to insure that

> > legacy routers who do not understand the new use case or the new flags

> > will ignore the prefix reachability advertisement. This has been verified by

> > testing against multiple implementations.

> >

> > Please be accurate in the statements that you make.

> >

> >    Les

> >

> >

> > > -----Original Message-----

> > > From: Lsr <[e-mail]<mailto:[e-mail]>> On Behalf Of Peter Psenak

> > > Sent: Wednesday, August 30, 2023 8:43 AM

> > > To: linchangwang <[e-mail]<mailto:[e-mail]>>; Acee Lindem

> > > <[e-mail]<mailto:[e-mail]>>; lsr <[e-mail]<mailto:[e-mail]>>

> > > Cc: [e-mail]<mailto:[e-mail]>

> > > Subject: Re: [Lsr] Working Group Adoption of "IGP Unreachable Prefix

> > > Announcement" - draft-ppsenak-lsr-igp-unreach-prefix-announce-04

> > >

> > > Changwang,

> > >

> > > On 30/08/2023 08:15, linchangwang wrote:

> > > > Hi WG,

> > > >

> > > > When considering adoption, it's important to take into account the

> > > > following

> > > drafts as well.

> > > >

> > > > Draft #1 link：https://www.ietf.org/archive/id/draft-wang-lsr-prefix-

> > > unreachable-annoucement-12.txt

> > > > Draft #2 link：https://www.ietf.org/archive/id/draft-ppsenak-lsr-igp-

> > > ureach-prefix-announce-04.txt

> > > >

> > > > Reasons are as follows:

> > > >

> > > > 1. The two drafts mentioned above are similar in nature.

> > > >    The draft #1 covers more scenarios than the draft #2 as mentioned

> > > > by

> > > Zhibo Hu mail.

> > > >    Therefore, a more in-depth discussion and technical comparison

> > > > should

> > > take place before any adoption decision is made.

> > > >

> > > > 2. The Draft #1 utilizes the existing mechanisms [RFC7794] and

> > > > [RFC9084] to

> > > indicate reachability by checking whether the originator information

> > > is

> > > >    NULL. On the other hand, the draft #2 introduces a new flag to

> > > > indicate

> > > reachability.

> > > >    From an implementation perspective, it would be easier to

> > develop

> > > > using

> > > the existing RFC mechanisms.

> > > >

> > > > 3. The Draft #1 covers more scenarios and can address the

> > > > aggregation issues

> > > of multiple ABRs.

> > > >    However, the Draft #2 explicitly states in Chapter 6 that it does

> > > > not support

> > > this scenario.

> > >

> > > to be more precise, draft #1 talks about more scenarios, it does not

> > > solves any of them, as these scenarios can not be solved by what the

> > > draft #1 introduces.

> > >

> > > draft#2 clearly states the fact that these scenarios are not addressed.

> > >

> > > thanks,

> > > Peter

> > >

> > > >

> > > > 4. If we remove the additional scenarios covered in Draft #1 and

> > > > compare the

> > > two drafts, the only remaining difference is the method of indicating

> > > unreachable prefixes -

> > > >    either through a UPA flag or using the originator TLV.

> > > >

> > > >

> > > > Thanks,

> > > > Changwang

> > > >

> > > >

> > > > -----Original Message-----

> > > > From: Lsr [mailto:[e-mail]] On Behalf Of Acee Lindem

> > > > Sent: Thursday, August 24, 2023 3:58 AM

> > > > To: lsr

> > > > Cc: [e-mail]<mailto:[e-mail]>

> > > > Subject: [Lsr] Working Group Adoption of "IGP Unreachable Prefix

> > > Announcement" - draft-ppsenak-lsr-igp-unreach-prefix-announce-04

> > > >

> > > > LSR Working Group,

> > > >

> > > > This begins the working group adoption call for “IGP Unreachable

> > > > Prefix

> > > Announcement” - draft-ppsenak-lsr-igp-unreach-prefix-announce-04.

> > > > Please indicate your support or objection on this list prior to

> > > > September 7th,

> > > 2023.

> > > >

> > > > Thanks,

> > > > Acee

> > > > _______________________________________________

> > > > Lsr mailing list

> > > > [e-mail]<mailto:[e-mail]>

> > > > https://www.ietf.org/mailman/listinfo/lsr

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