Re: [Lsr] Opsdir last call review of draft-ietf-ospf-te-link-attr-reuse-12

Peter Psenak <ppsenak@cisco.com> Thu, 28 May 2020 14:09 UTC

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To: Scott Bradner <sob@sobco.com>, ops-dir@ietf.org
Cc: draft-ietf-ospf-te-link-attr-reuse.all@ietf.org, lsr@ietf.org, last-call@ietf.org
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From: Peter Psenak <ppsenak@cisco.com>
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Date: Thu, 28 May 2020 16:09:13 +0200
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Subject: Re: [Lsr] Opsdir last call review of draft-ietf-ospf-te-link-attr-reuse-12
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Hi Scott,

please see inline (##PP):

On 27/05/2020 17:17, Scott Bradner via Datatracker wrote:
> Reviewer: Scott Bradner
> Review result: Not Ready
> 
> This is an OPS-DIR review of OSPF Link Traffic Engineering Attribute Reuse
> (draft-ietf-ospf-te-link-attr-reuse)
> 
> This ID describes application-specific attribute advertisements for use in OSPF.
> 
> I found this ID hard to read and recommend that it be reviewed for readability.
> 
> I have a basic question about this proposal – the ID describes specific
> advertisements to be used when particular applications want to make use of
> specific link attributes and says that other applications should not make of
> the information in the advertisement without saying why such use would be a
> problem.  I can imagine some reasons but it seems to me that it would be good
> if this document just explained the problem it is trying to solve.

##PP

There are two problems mentioned in the Introduction section:

1) Ambiguity in terms of which of the advertisements are to be used by 
RSVP-TE and which ones should not be. When RSVP was the only application 
using these attributes, advertisement of these attributes meant that 
RSVP was enabled on the link. Such link was considered as a part of the 
RSVP-TE topology. With other applications using and advertising these 
link attributes, this logic can not be used anymore, which created the 
mentioned ambiguity.

2) Lack of support for application specific values for the link attribute.

I thought this was clear, but if it is not, please suggest what else 
needs to be said.

> 
> Some specific issues in the document
> 
> Page 6 – the text says “Standard Application Identifier Bit Mask: Optional set
> of bits, where each bit represents a single standard application.  Bits are
> defined in [I-D.ietf-isis-te-app].”  - it seems to me that this should be in an
> IANA registry for extensibility but it does not seem to be in the referenced ID
> but I could not actually tell

##PP
that's a fair point. I have changed to:

"Bits are defined in the Link Attribute Application Identifier Registry, 
which has been defined in [I-D.ietf-isis-te-app]."


> 
> Page 6 – text says “The bits are repeated here for informational purpose”
> maybe point to a IANA registry or say “current assignments”

##PP
sure, changed to "current assignments”

> 
> Page 6 – text says “If the link attribute advertisement is limited to be used
> by a specific set of applications”  - maybe say “intended” rather than
> “limited” since I do not see a way to actually limit a future application from
> eavesdropping on the advertisement

##PP
changed to "intended to be only used by"

> 
> Page 7 – the text says “If the SABM or UDABM length is other than 0, 4, or 8,
> the ASLA sub-TLV MUST be ignored by the receiver.”  - it would seem to be
> useful operations-wise to say that an indication of an error should be recorded
> somewhere

##PP
fair enough, changed to:

"MUST be ignored by the receiver and an error SHOULD be logged (subject 
to rate limiting)."

> 
> Page 7 – a “User Defined Application Identifier” is introduced but never
> described – what uses it and what is it used for
##PP
we provide a way for the user to advertise link attributes for the 
purpose of something that is not defined as standardized application. 
What such application might be is not subject to the specification.

> 
> Section 11 – I found this discussion of the relationship between the existence
> of a particular advertisement and the possible existence of an application to
> use that advertisement to be quite confusing – if the existence of a particular
> advertisement does not indicate that any application is listening why not just
> say that?

##PP
there are applications where the application enablement on the link is 
relevant - e.g. RSVP-TE - one need to make sure that RSVP is enabled on 
the link before sending a RSVP signaling message over it.

There are applications, where the enablement of the application on the 
link is irrelevant and has nothing to do with the fact that some link 
attributes are advertised for the purpose of such application - e.g. LFA.

We have provided full flexibility to support both.


> 
> Section 12.1 – it would help to say what problem is trying to be solved – why
> is the use of RSVP-TE LSA advertisements a problem?

##PP
please see my response above in the context of the Introduction section.

> 
> Section 12.3.3 – I could not tell if this section is saying that the
> application specific attribute advertisements could not be used if there is
> even a single legacy router present of if the presence of such a router means
> that the application specific attribute advertisements can be used but the old
> advertisements must also be used 

##PP
a) as long as there is a single legacy router present, all routers MUST 
continue to advertise link attributes using legacy advertisements to 
allow the legacy router to function properly.

b) new advertisements can be used in parallel and they can be used by 
the routers that do understand them.

The text in 12.3.3 says:

    "Send application specific advertisements while continuing to
     advertise using legacy (all advertisements are then duplicated)."


> Section 14 – it might help to say how new Sub-TLV types can be added to the registry

##PP
we are not defining a nynew registry, we only use existing ones. These 
registries have their own registration procedures.

thanks,
Peter





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