Re: [RTG-DIR] Rtgdir early review of draft-ietf-idr-segment-routing-te-policy-18

[[e-mail]]

Hi Ketan,

Thanks for the follow-up.

Will monitor when the new version is available and react if I have any further comments.

Cheers,
Med

De : rtg-dir <[e-mail]> De la part de Ketan Talaulikar
Envoyé : mardi 19 juillet 2022 19:55
À : BOUCADAIR Mohamed INNOV/NET <[e-mail]>
Cc : [e-mail]; [e-mail]; idr@ietf. org <[e-mail]>
Objet : Re: [RTG-DIR] Rtgdir early review of draft-ietf-idr-segment-routing-te-policy-18

Hi Mohamed,

Thanks for your very detailed review and helpful suggestions. Please check inline below for responses.

We will post the update once the submission tool reopens.


On Fri, Jul 8, 2022 at 6:35 PM Mohamed Boucadair via Datatracker <[e-mail]<mailto:[e-mail]>> wrote:
Reviewer: Mohamed Boucadair
Review result: Has Issues

Document: draft-ietf-idr-segment-routing-te-policy-18
Reviewer: Mohamed Boucadair
Review Date: 08/07/2022
IETF LC End Date: N/A
Intended Status: Standards Track

I appreciate the effort that was spent to progress this draft since more than 6
years!

Before reviewing the document, I started first by re-reading RFC8024/RFC9012
and reading draft-ietf-spring-segment-routing-policy for establishing the
context. Overall, the approach documented in
draft-ietf-idr-segment-routing-te-policy is sound and straightforward.

I didn’t find major concerns from a routing standpoint other than the need to
motivate some few claims (see the detailed review file about RRs, for example)
and the lack of considerations related to the handling of the various sub-TLVs
by intermediate routers (if any).

However, there are a number of generic issues that I would recommend to
consider (see the detailed review for the full list). All these are easy-to-fix
issues.

# General Comments (in no specific order)

## Consistency

### Single or multiple paths

There is an apparent inconsistency in the document about the handling of
multiple paths. For example, Section 1 says :"Selection of the best candidate
path for an SR Policy" while the same section says also “this will result in
one or more candidate paths being installed into ..”.

KT> The first is about the selection of the best candidate path for an SR Policy by the SRPM - this is what gets installed in the forwarding. The second is about the installation of the received candidate paths into the BGP table. There is no inconsistency.


If multipath is supported, then please add an explicit statement and make sure
the overall text is consistent.

KT> Only a single CP is selected for a given SR Policy. This is per the draft-ietf-spring-segment-routing-policy and this document does not change that.


### Value 0 is marked as reserved for some registries, while that value is
associated with a meaning for other registries.

Is there any reason why a consistent approach isn’t followed here? what is the
issues if value 0 is open for assignment?

KT> It is normal routing protocol practice to not assign the TLV 0 values. Can you indicate where the TLV code point 0 is being assigned?


## Modifications to the format of the Color Extended Community

The text says that you are modifying the format the Color Extended Community,
while this is not true. What this draft does is just associating a meaning with
some bits. I would update the text accordingly.

KT> We are changing the format of only the Flags field and not of the entire EC. Flags are normally independent bits and here we are combining two bits to convey 4 values. Clarified this in the Introduction section.


## Normative language

The use of the normative language should be double-checked. The most apparent
concern is related the statement related to the handling of the reserved bits
(SHOULD) while this RFC9012 uses MUST (which is correct, IMO).

KT> Ack. I will fix it and change it to MUST.


I tagged many others in the full review, fwiw.

## Lack of description

Many fields are provided without acceptable description (e.g., “Local IPv4
Address: a 4-octet IPv4 address.” or “Preference: a 4-octet value” !!).

KT> These fields are in the context of a sub-TLV. There is text in the description of that sub-TLV that provides a reference (e.g., to the draft-ietf-spring-segment-routing-policy section or a segment type, etc.) There is no need to repeat a detailed description for each field IMO.


Also, some fields are provided with a structure but the text says also that
these are reserved (e.g., 2.4.2 says “TC, S and TTL (Total of 12 bits) are
RESERVED”).

KT> This is the MPLS label field. I am not sure that I follow your concern here.


I wonder whether you can add a statement to say that multiple flags can be set
simultaneously unless this is precluded by future flag assignments.

KT> Not sure that is necessary. In most cases, the bits/flags are independent. Where they are not, there is generally text explaining their relationship or dependency.


Last, the document does not include the expected behavior of intermediate
routers (e.g., whether it is allowed or not to alter some fields). I guess,
they must not touch the content of the attributes but it is better if this is
explicitly mentioned in the text.

KT> Yes, the contents must not be altered. Will clarify in sec 4.2.4.


## Reserved vs. Unassigned

Almost all the “reserved” bits in the spec can be assigned in the future. I
would use “Unassigned” as per RFC8126.

KT> Ack. Will change in a few places where this has been missed.


FWIW, 8126 says the following:

      Unassigned:  Not currently assigned, and available for assignment
            via documented procedures.

      Reserved:  Not assigned and not available for assignment.
            Reserved values are held for special uses, such as to extend
            the namespace when it becomes exhausted.

## Deprecated values

The document includes notes about some “deprecated” codepoints. I’m not sure
there is a value in having such notes in the final RFC.

KT> Yes, there is a need. One is to avoid them being used for any other sub-TLV in the future. Two is that there are early implementations out there that have some degree of support - even if they are just doing some parsing/showing.


## IANA considerations

### The document uses a mix of TBD statements (e.g., Section 2.4.3) and
hard-coded values (early assignments). Not sure what’s was the rationale
especially that code 20 was assigned but not listed as such.

KT> Fixed.


### The IANA actions should be more explicit and ask IANA to update existing
entries. For example, the current registry for code 73 points to
[draft-previdi-idr-segment-routing-te-policy]. Need to update that entry and
similar ones.

KT> Have fixed the text. IANA will update "This document" to the RFC number before publication. There is no need to keep changing the draft name through its lifecycle.


### The document lists (under IANA section) some values that are deprecated.
The document should be clear whether these codes are available for future
assignment or not.

KT> Deprecated means they are not available for assignment by IANA unless the IETF changes that via an RFC.


### Many sub-TLVs have flag bits but not all of them have a registry to track
future flag bit assignments.

KT> The registries would be added by future documents that start using those flags.


## Manageability considerations

No such considerations are included in the document.

KT> Will add.


# Detailed review

FWIW, you can find my full review at:

* pdf:
https://github.com/boucadair/IETF-Drafts-Reviews/raw/master/draft-ietf-idr-segment-routing-te-policy-18-rev%20Med.pdf
* doc:
https://github.com/boucadair/IETF-Drafts-Reviews/raw/master/draft-ietf-idr-segment-routing-te-policy-18-rev%20Med.doc


KT> This was helpful and have incorporated most of those suggestions.

Thanks,
Ketan


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