Re: [Lsr] Proposed Errata for RFCs 8919/8920

["Selderslaghs, Rudy (Nokia - BE/Antwerp)" <[e-mail]>]

Hi Les,

The question remains whether an ISIS Appl-Spec-SRLG TLV with SABML 0 and UDABML 0 means that it is valid for all applications or not.
This is currently not specified in RFC8919.

Secondly the approach to handle ISIS Appl-Spec-SRLG TLVs independently from ASLA sub-TLVs in the context of overruling "All-Appl" attributes, creates confusion in BGP-LS in my opinion.
Take the following example with TLV's belonging to the same link:

IS-Neighbors TLV
         ASLA TLV
                 SABML 0, UDABML 0 (= All Appl)
                 TE-Metric 20
Appl-Spec-SRLG TLV
         SABML 1, UDABML 0, Bitmap SR-Policy
         SRLG 1

Following your answer, SR-Policy has to use TE-Metric 20 and SRLG 1.
Now suppose that the node that receives this ISIS advertisement has to pass this on to a server via BGP-LS, this will be the result:

Link Attributes:
        ASLA TLV
                SABML 0, UDABML 0 (= All Appl)
                TE-Metric 20
        ASLA TLV
                SABML 1, UDABML 0, Bitmap SR-Policy
                SRLG 1

The server that receives this BGP-LS advertisement, has to take into account that it comes from the ISIS protocol in order to treat the ASLA TLV with SRLG info independently (as ISIS is supposed to do). I.e. the ASLA TLV with SRLG info does not overrule the first ASLA TLV and the server has to use TE-Metric 20 and SRLG 1 for SR-Policy.

If the server would receive the same BGP-LS advertisement  for the OSPF protocol, it can only use SRLG 1 for SR-Policy, because the second ASLA TLV overrules the first one in line with the OSPF rules.

I think this is confusing and that it is more consistent to treat an ISIS Appl-Spec-SRLG TLV as an ASLA TLV in the context of overruling of "All-Appl" attributes.
The fact that ISIS transports SRLG info via a separate TLV seems no reason to me to treat it independent of ASLA TLV's.

Thanks,
Rudy

From: Lsr <[e-mail]> On Behalf Of Shraddha Hegde
Sent: Thursday, June 17, 2021 7:14 AM
To: Les Ginsberg (ginsberg) <[e-mail]>; Van De Velde, Gunter (Nokia - BE/Antwerp) <[e-mail]>; [e-mail]
Cc: DECRAENE Bruno IMT/OLN <[e-mail]>
Subject: Re: [Lsr] Proposed Errata for RFCs 8919/8920

Les,

> Whether ASLA sub-TLV is present in IS-Neighbor TLV and whether it has zero-length ABM on non-zero-length ABM is irrelevant to the use of ASLA SRLG TLV - and vice versa.

Can you state this explicitly in the document?

Rgds
Shraddha



Juniper Business Use Only
From: Les Ginsberg (ginsberg) <[e-mail]<mailto:[e-mail]>>
Sent: Wednesday, June 16, 2021 10:31 PM
To: Van De Velde, Gunter (Nokia - BE/Antwerp) <[e-mail]<mailto:[e-mail]>>; Shraddha Hegde <[e-mail]<mailto:[e-mail]>>; [e-mail]<mailto:[e-mail]>
Cc: DECRAENE Bruno IMT/OLN <[e-mail]<mailto:[e-mail]>>
Subject: RE: Proposed Errata for RFCs 8919/8920

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Gunter -

There is no relationship between the ASLA SRLG TLV and IS-Neighbor TLV.
I do not understand why you would think that there is.

Whether ASLA sub-TLV is present in IS-Neighbor TLV and whether it has zero-length ABM on non-zero-length ABM is irrelevant to the use of ASLA SRLG TLV - and vice versa.

   Les


From: Van De Velde, Gunter (Nokia - BE/Antwerp) <[e-mail]<mailto:[e-mail]>>
Sent: Wednesday, June 16, 2021 9:07 AM
To: Shraddha Hegde <[e-mail]<mailto:[e-mail]>>; Les Ginsberg (ginsberg) <[e-mail]<mailto:[e-mail]>>; [e-mail]<mailto:[e-mail]>
Cc: DECRAENE Bruno IMT/OLN <[e-mail]<mailto:[e-mail]>>
Subject: RE: Proposed Errata for RFCs 8919/8920

Another item of ambiguity is whether "wildcarding" applies also to the ISIS TE-Appl-Spec-SRLG TLV.
It seems that the RFC8919 does not specify it.
Note: for OSPF the wildcarding also applies to SRLG info because it is transported via the same container TLV as the other TE attributes.

Example 1
TE-IS-NBRs TLV
     Link x
          ASLA TLV
              SABML 0, UDABML 0 (= All Appl)
              TE-Metric 20
TE-Appl-Spec-SRLG TLV
    Link x
    SABML 1, UDABML 0, Bitmap Flex-Algo
   SRLG 1 2 3

Should TE-Metric 20 be used for Flex-Algo or not ?
In other words, is the wildcard ASLA TLV overruled by the specific TE-Appl-Spec_SRLG TLV or not ?

Example 2
Maybe this is an invalid example if wildcarding does not apply for the TE-Appl-SRLG TLV.
TE-IS-NBRs
     Link x
          ASLA TLV
              SABML 1, UDABML 0, Bitmap Flex-Algo
              TE-Metric 20
TE-Appl-Spec-SRLG
    Link x
    SABML 0, UDABML 0 (= All Appl)
   SRLG 1 2 3

Should SRLG 1 2 3 be used for Flex-Algo or not ?

What is your opinion ?

G/


From: Shraddha Hegde <[e-mail]<mailto:[e-mail]>>
Sent: Wednesday, June 16, 2021 4:46 PM
To: Les Ginsberg (ginsberg) <ginsberg=[e-mail]<mailto:ginsberg=[e-mail]>>; [e-mail]<mailto:[e-mail]>
Cc: DECRAENE Bruno IMT/OLN <[e-mail]<mailto:[e-mail]>>; Van De Velde, Gunter (Nokia - BE/Antwerp) <[e-mail]<mailto:[e-mail]>>
Subject: RE: Proposed Errata for RFCs 8919/8920

Hi,

I think that there may still be some ambiguity arising from the text below due to the fact that
There are attributes such as maximum-link-bandwidth which have special behaviour mentioned in later sections.

"Link attributes MAY be advertised associated with zero-length Application Identifier Bit Masks for both standard applications and user-defined applications.
Such advertisements MUST be used by standard applications and/or user defined applications when no link attribute advertisements with a non-zero-length
Application Identifier Bit Mask and a matching Application Identifier Bit set are present for a given link. Otherwise, such advertisements MUST NOT be used."


For example, If max link bandwidth attribute comes in a
Zero length SABM & UDABM and we have a Flex-algo specific ASLA
that does not have the max-link-bandwidth advertised, can
Flex-algo use max-link-bandwidth attribute?

My interpretation from modified text for ISIS is that,  it cannot use it.
I think there is no harm in re-iterating in order to avoid people reading is differently.

Link attributes MAY be advertised associated with zero-length Application Identifier Bit Masks for both standard applications and user-defined applications.
Such advertisements MUST be used by standard applications and/or user defined applications when no link attribute advertisements with a non-zero-length
Application Identifier Bit Mask and a matching Application Identifier Bit set are present for a given link. Otherwise, such advertisements MUST NOT be used.
In other words,
When an application specific link Attribute sub-TLV is advertised with one or more specific
standard application or user defined application bits set, all link attributes that are allowed in ASLA MUST
be used from the ASLA sub-TLVs having that specific application bit set.
For the purposes of such applications, link attributes MUST NOT be used from
ASLA sub-TLV with zero SABM & UDABM length.


Rgds
Shraddha



Juniper Business Use Only
From: Lsr <[e-mail]<mailto:[e-mail]>> On Behalf Of Les Ginsberg (ginsberg)
Sent: Tuesday, June 15, 2021 8:55 PM
To: [e-mail]<mailto:[e-mail]>
Cc: DECRAENE Bruno IMT/OLN <[e-mail]<mailto:[e-mail]>>; Van De Velde, Gunter (Nokia - BE/Antwerp) <[e-mail]<mailto:[e-mail]>>
Subject: [Lsr] Proposed Errata for RFCs 8919/8920

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Folks -



Recent discussions on the list have highlighted some unintentional ambiguity in how ASLA advertisements are to be used. Please see https://mailarchive.ietf.org/arch/msg/lsr/prSLJDkMUnHm6h7VuCdn_Q7-1vg/<https://urldefense.com/v3/__https:/mailarchive.ietf.org/arch/msg/lsr/prSLJDkMUnHm6h7VuCdn_Q7-1vg/__;!!NEt6yMaO-gk!RK_eZNNu1y0aJvAqIaNwHTIFAjHWFJwW1UqyOO8ACxB0kof3jmD_dRkiPkbVLJyA$>



The following proposed Errata address this ambiguity and aligns language in the two RFCs.



We welcome comments on the proposed Errata before officially filing them.



  Les and Peter


Errata Explanation

Both RFC 8919 and RFC 8920 define advertising link attributes with zero length Standard Application Bit Mask (SABM) and zero length User Defined Application Bit Mask (UDABM)
as a means of advertising link attributes that can be used by any application. However, the text uses the word "permitted", suggesting that the use of such advertisements is "optional".
Such an interpretation could lead to interoperability issues and is not what was intended.

The replacement text below makes explicit the specific conditions when such advertisements MUST be used and the specific conditions under which they MUST NOT be used.

RFC 8919 Section 4.2:

OLD

"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."

NEW

"Link attributes MAY be advertised associated with zero-length Application Identifier Bit Masks for both standard applications and user-defined applications.
Such advertisements MUST be used by standard applications and/or user defined applications when no link attribute advertisements with a non-zero-length
Application Identifier Bit Mask and a matching Application Identifier Bit set are present for a given link. Otherwise, such advertisements MUST NOT be used."

RFC 8919 Section 6.2

OLD

"Link attribute advertisements associated with zero-length Application Identifier Bit Masks for both standard applications and user-defined applications are usable
by any application, subject to the restrictions specified in Section 4.2. 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."


NEW

"Link attribute advertisements associated with zero-length Application Identifier Bit Masks for both standard applications and user-defined applications are usable
by any application, subject to the restrictions specified in Section 4.2. If support for a new application is introduced on any node in a network in the presence of such
advertisements, the new application will use these advertisements, when the aforementioned restrictions are met. 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."



RFC 8920 Section 5

OLD

"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."

NEW

"Link attributes MAY be advertised associated with zero-length Application Identifier Bit Masks for both standard applications and user-defined applications.
Such advertisements MUST be used by standard applications and/or user defined applications when no link attribute advertisements with a non-zero-length
Application Identifier Bit Mask and a matching Application Identifier Bit set are present for a given link. Otherwise, such advertisements MUST NOT be used."



RFC 8920 New Section between 12.1 and 12.2. Current sections following this new section will need to be renumbered.


12.2 Use of Zero-Length Application Identifier Bit Masks

"Link attribute advertisements associated with zero-length Application Identifier Bit Masks for both standard applications and user-defined applications are usable
by any application, subject to the restrictions specified in Section 5. If support for a new application is introduced on any node in a network in the presence of such
advertisements, the new application will use these advertisements, when the aforementioned restrictions are met. 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."