Re: [Lsr] New Version Notification for draft-hegde-lsr-asla-any-app-00.txt

[Tony Przygienda <[e-mail]>]

My quick take:

1. yes, A bit will necessitate being either deployed in a well understood
part of the network (because as Les says old nodes will basically see _no_
application [which seems desirable, they basically take themselves out]) or
forklifting. Not that different from flex-algo being same kind of forklift
AFAIS.
2. any application introduced after that will precondition implementation
of A bit if we don't want to get into the rathole of "do not encode using
A, encode using repetition per application if you have old routers".

I see a value in the "A" bit from multiple angles (which I do not read as
"all applications" but "any application". The distinction is subtle but
important)

a) it fits what flex-algo needs in ASLA architecture. E'one wins AFAIS.
b) if we want to replace A with X|Y|Z we need to know on a router about
_all_ applications on all routers and that may be non-trivial and on every
change may force re-adverts (unless we set all bits 1 on a 8 bytes ASLA
mask [as in _all_ applications]. That does not seem like a good idea given
the encoding sizes). A as "any" basically means "any application on this
router uses this metric" and avoids both problems. Significantly simpler
AFAIS.

A very, very subtle point (I didn't read the 'a' draft yet so it may be
taken care of) is whether SABM length is 1 with all 0s or length is 0 on A
bit presence and if 0 will the current implementations hold up to that ;-)

Les, correct me if I'm off somewhere, I was watching lots of that just from
the corner of my eye ;-)

-- tony



On Sat, Aug 21, 2021 at 2:06 AM Les Ginsberg (ginsberg) <ginsberg=
[e-mail]> wrote:

> Robert -
>
>
>
> *From:* Robert Raszuk <[e-mail]>
> *Sent:* Friday, August 20, 2021 5:01 PM
> *To:* Les Ginsberg (ginsberg) <[e-mail]>
> *Cc:* Ron Bonica <[e-mail]>; [e-mail]
> *Subject:* Re: [Lsr] New Version Notification for
> draft-hegde-lsr-asla-any-app-00.txt
>
>
>
> Hi Les,
>
>
>
> *The point being is that “A-bit” is no different than introducing any
> other new application bit. Until all routers in the network understand it
> you cannot safely use it.*
>
>
>
> That is true.
>
>
>
> But the entire point of A-bit is that you are doing this exercise to make
> sure your routers understand A-bit only one time.
>
> *[LES:] This does not mean that you can introduce support for a new
> application (call it “bit N”) w/o upgrading your routers simply because you
> already have A-bit support. I hope that is obvious. **😊*
>
>
>
> *My original point was simply  that the statement about “backwards
> compatibility” regarding A-bit isn’t accurate. Good that we now agree on
> that.*
>
>
>
> *   Les*
>
>
>
> Otherwise you need to do it each time you invent a new bit.
>
>
>
> Thx,
>
> R.
>
>
>
>
>
>
>
>
>
>
>
>
>
> On Sat, Aug 21, 2021 at 1:34 AM Les Ginsberg (ginsberg) <
> [e-mail]> wrote:
>
> Robert –
>
>
>
> Inline.
>
>
>
> *From:* Robert Raszuk <[e-mail]>
> *Sent:* Friday, August 20, 2021 1:29 PM
> *To:* Les Ginsberg (ginsberg) <[e-mail]>
> *Cc:* Ron Bonica <[e-mail]>; [e-mail]
> *Subject:* Re: [Lsr] New Version Notification for
> draft-hegde-lsr-asla-any-app-00.txt
>
>
>
> Hi Les,
>
>
>
> Please see below.
>
>
>
> It is not just that a new application wants to use the same link attribute
> value that allows you to use the "all applications" encoding. It is also
> necessary for the set of links used by the new application to be identical
> to the set of links used by the existing applications.
>
>
>
> Not really. You can use subset of links when you apply affinity bits to
> it.
>
> *[LES:] This isn’t relevant.*
>
> *Let me try explaining this a different way.*
>
>
>
> *Suppose I have 1000 links in my network. *
>
> *On 500 of those links I have Attribute #1 advertised using “all
> applications”. (For the purposes of this discussion it does not matter
> whether I use the existing 0 length ABM format or the proposed new “A-bit”
> format)*
>
> *There are currently two applications, X and Y, deployed in the network
> and they are both using the same value of attribute #1 on the same set of
> 500 links.*
>
> *All is well.*
>
> *Now, I want to enable application Z. If I do so and make no changes to
> the existing link attribute advertisements, application Z will think it can
> use Attribute #1 on all 500 of the links on which the “all” form of the
> ASLA sub-TLV is being advertised.*
>
> *If application Z is intended to use all of those 500 links all is well.
> But if application Z is NOT meant to use one or more of the links on which
> the ALL ASLA sub-TLVs are being advertised then I have to make changes to
> at least some of the existing advertisements.*
>
>
>
> *This is why, in RFC 8919/8920, we advise caution in using the “all” form
> – and why we do not allow both the “all” form and the “app-specific” form
> to be used by a given application. It is too easy for mistakes to occur,
> especially when enabling a new application.*
>
>
>
> *Implementations that I am aware of do not send the “ALL” form for this
> reason i.e., it introduces dependencies between applications which are hard
> to validate.*
>
>
>
> Likewise as Peter confirmed you also need to use affinities to select
> subset of links carrying given flex-algo metric to be used only by some
> selective flex-algo topologies.
>
>
>
>
>
> " The solution described in this document is backward compatible with
>    [RFC8919] and [RFC8920]."
>
> This is FALSE.
>
>
>
> Well I am not sure what Shraddha wanted to express by this sentence or
> what "backwards" means here. But if you delete "backwards" the rest of the
> sentence seems just fine.
>
>
>
> Let's observe that even if you define a new application and define new bit
> participating nodes need to support it. That means that you must keep
> upgrading your OS on all participating nodes each time new new bit is
> invented.
>
>
>
> *[LES:] Again, a simple example should suffice.*
>
> *All routers in my network support application X and application Y.*
>
> *Some of the routers support the proposed A-bit, some do not.*
>
> *For the set of links on which applications X and Y are using the same
> attribute we will then have some links using A-bit ASLA, some not using
> A-bit ASLA.*
>
> *For those routers which support the A-bit, they will see links with both
> styles of ASLA advertisements as usable by applications X and Y.*
>
> *For those routers which do NOT support A-bit, they will see only the
> links w/0 A-bit ASLA as usable by applications X and Y.*
>
>
>
> *The point being is that “A-bit” is no different than introducing any
> other new application bit. Until all routers in the network understand it
> you cannot safely use it.*
>
>
>
> *   Les*
>
>
>
>
>
> Don't you think this is pretty bad ?
>
>
>
> How often do you think operators upgrade their core routers ?
>
>
>
> With A-bit and affinities at least your OS is ready to support any
> application based on already defined metrics without keep inventing new
> bits.
>
>
>
> Of course if we assume velocity of inventing new applications is near zero
> then this is not a problem. But then the usefulness of ASLA also can be
> challenged.
>
>
>
> Thx,
> R.
>
>
>
> _______________________________________________
> Lsr mailing list
> [e-mail]
> https://www.ietf.org/mailman/listinfo/lsr
>