“If link attributes are advertised with zero-length Application
Identifier Bit Masks for both standard applications and user-defined
applications, then any standard application and/or any user-defined
application is permitted to use that set of link attributes. If support
for a new application is introduced on any node in a network in the
presence of such advertisements, these advertisements are permitted to
be used by the new application. If this is not what is intended, then
existing advertisements MUST be readvertised with an explicit set of
applications specified before a new application is introduced.An
application-specific advertisement (Application Identifier Bit Mask with
a matching Application Identifier Bit set) for an attribute MUST always
be preferred over the advertisement of the same attribute with the
zero-length Application Identifier Bit Masks for both standard
applications and user-defined applications on the same link.”
Not sure whether there was a technical reason for this difference, but
it didn’t help be clarify by myself the RFC 8919 formulation.
Thank you,
--Bruno
-----Original Message-----
From: Peter Psenak [mailto:[email protected]]
Sent: Thursday, June 3, 2021 3:28 PM
To: DECRAENE Bruno INNOV/NET <[email protected]>; [email protected]
Subject: Re: [Lsr] RFC 8919 clarification
Hi Bruno,
On 03/06/2021 14:55, [email protected] wrote:
> Hi all,
>
> In order to (try to) avoid interop issues, I have a clarification
> question on RFC 8919.
>
> “If link attributes are advertised associated with zero-length
> Application Identifier Bit Masks for both standard applications and
> user-defined applications, then any standard application and/or any
> user-defined application is permitted to use that set of link attributes
> so long as there is not another set of attributes advertised on that
> same link that is associated with a non-zero-length Application
> Identifier Bit Mask with a matching Application Identifier Bit set.”
>
> https://www.rfc-editor.org/rfc/rfc8919.html#name-application-specific-link-a
>
> My reading is that if one ALSA “S1” with a specific application bit set
> (e.g. X-Flag/FlexAlgo) advertises a set of attributes (A1, A2), then
> this ALSA (or the set of all ASLA occurrence with the X-Flag set) needs
> to advertise _/all/_ the attributes used by this application.
right.
>
> So:
>
> - if we have another ALSA “S2” advertising zero-length Application
> Identifier Bit Masks for both standard applications and user-defined
> applications, that application (FlexAlgo) is not permitted to fall back
> to “S2” in order to learn a another attribute (A3). Regardless of
> whether S2 advertises the L-flag (legacy) or its own Link Attribute
> sub-sub-TLVs.
yes, that is correct.
>
> - if we don’t have such S2 ASLA, that application (FlexAlgo) is not
> permitted to fall back to legacy attributes.
well, if you have S1 as you mentioned above, S2 becomes irrelevant from
the flex-algo perspective. So you are right, flex-algo is not allowed to
use legacy advertisement in this case.
>
> IOW, an ALSA for application X, does not advertise additional/more
> specific attributes for application X, but _/all/_ the attributes that
> application X is allowed to read.
correct.
> As a consequence, if an attribute is
> common to N set of applications, it needs to be advertised N times in N
> ASLA.
depends. If the set of attributes that are common across N applications
represent the full set of attributes for all these N applications, you
simply advertise them once with SAMB/UDAMB including the bits for all N
applications. If the set of attributes for each application is
different, but there is one attribute that is common, you need to
include that attribute in ASLA advertisement for every app.
thanks,
Peter
>
> Thanks for correcting/confirming.
> >
> > Thanks,
> >
> > Regards,
> >
> > Bruno
> >
> >
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