RE: Gen-ART Last Call review of draft-ietf-isis-mi-bis-02

Orit Levin <oritl@microsoft.com> Thu, 13 April 2017 23:59 UTC

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From: Orit Levin <oritl@microsoft.com>
To: "Les Ginsberg (ginsberg)" <ginsberg@cisco.com>, "gen-art@ietf.org" <gen-art@ietf.org>
CC: "ietf@ietf.org" <ietf@ietf.org>, "draft-ietf-isis-mi-bis.all@tools.ietf.org" <draft-ietf-isis-mi-bis.all@tools.ietf.org>
Subject: RE: Gen-ART Last Call review of draft-ietf-isis-mi-bis-02
Thread-Topic: Gen-ART Last Call review of draft-ietf-isis-mi-bis-02
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Date: Thu, 13 Apr 2017 23:59:00 +0000
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Hi Les,
Sorry for the delay in response.
Your feedback was very helpful. Below is a refresh of my comments. I tried to make them more pointed and some are new.

Summary: This draft is "ready with issues" for publication.

General:
1) For implementers who are familiar with the history and the intent of this extension, the information in the draft is probably sufficient to serve as a check list for implementing a multi-instance IS-IS router. For all other readers, the document doesn't contain an overview of the new mode of operation, i.e. where the instances are not a configuration and an internal implementation choice only, but are exposed through the protocol to achieve the stated objective. Lacking such an overview, the reader needs to reverse-engineer the logic behind the documented guidance.
2) The draft talks about "extensions" in plural. Based on a single extension on the wire and the overall goal of the new mechanism, I would say that it is a single extension only.  How many protocol extensions does this document define? If they can be clearly separated, then it needs to be clarified throughout the document. Otherwise, the language throughout the document needs to be changed from "extensions" to "the extention".
3) Editorial: Please, compare (Diff) the current draft with the published RFC 6822. You will find that various RFC Editor corrections got lost in this bis document. Some repeating examples of the lost corrections are "instance-specific", " topology (or topologies)" and "Type-Length-Value".

My comments below are a result of a reverse-engineering exercise. Please, consider incorporating the suggested clarifications to improve the document readability. I might have misunderstood some of the parts; in such cases, please, provide an alternative text.

Abstract
1) Add clarification: "This document is not backwards compatible with RFC 6822."
2) Par. 2, replace the first two sentences with: "Configuration of multiple protocol instances within a router allow the isolation of resources associated with each instance. This document introduces a new mode of operation where the protocol instances are not a matter of configuration only, but are exposed through the new protocol extension to achieve the objective stated above."
3) Par. 3 uses both present and future time. Does it mean before and after the extension? Please, clarify by spelling out the intent and/or changing the grammar.

1. Introduction
1) Move par. 3 to become the opening paragraph (i.e., par.1 ) in the Introduction to improve its logical flow (also related to the next comment).
2) Par 4, sentence #2 says "This document defines an extension to IS-IS to allow non-zero instances...". My assumption is that the intent of this draft is to define a single extension, which will improve the routing operations in a number of ways. If this is the case, the quote needs to be replaced with "The MI-IS-IS extension, defined in this document, also allows so-called "non-zero instances"..." .
3) Par 4, sentences #4 and #5 are not sufficient to describe the new mode of operation introduced by this extension. Below is my attempt to describe it. Please, correct, if I got it wrong.
"IS-IS router instances that support this extension are preconfigured with unique non-zero Instance Identifiers (IIDs) giving them the name "non-zero instances". In addition, MI RTRs MAY (or SHOULD ?) implement the legacy (or so-called "standard") instance of the IS-IS router for backwards compatibility with legacy IS-IS routers. IID #0 is only used by MI-RTRs to convey information associated with such standard interface if implemented. See section 2.6 Interoperability Considerations for more details."
4) Par 4, replace the two "may" with "can" to clarify the intent.
5) Par 5, change "defined" to "described" since the examples are not normative.
5) Par 5, add references to the (sub)sections containing the description of the two methods.
6) Par 7, move the last paragraph before listing the two examples and adjust the text accordingly, to improve the logical flow.
7) In the end of the Introduction add a reference: "This RFC is not backwards compatible with RFC 6822. Differences between this document and RFC 6822 are described in Annex A." ... so that others don't skip it by mistake as I did earlier...

2. Elements of Procedure
1) It seems that this section (informational) and its subsections (normative) use present and future times and, at times,  the normative language inconsistently both within the sections and among the sub-sections. Please, explain the reasons and improve consistency accordingly. 
2) Par 1, clarify the scope by adding "within a routing domain" at the end of the first sentence.
3) Par 1, after the first sentence add a new sentence alone the following lines "Routers form adjacencies and exchange routing updates only if their IIDs correspond." This explains the basic premise of the whole mechanism.
4) Par 2, change "may" to "can" or "might" for consistency.

Section 2.1
1) Par 2, change "may" to "MAY".
2) Par 3, remove "supported by legacy systems" from the first sentence to avoid confusion. My understanding is that IID #0 is reserved  for use by MI-RTRs that also implement the standard instance and advertise it in IIH using IID-TLV.
3) Par 3, change "except where noted" to "except as noted in section 2.6.2 (?)".  This is an excellent place to explain the logic behind this MUST NOT statement or, at least, state the general circumstances where IID #0 is included in IID-TLV.
4) Par 9 (4th after the picture), change "as described later" to " as described in section 2.6.2 (?)".  
5) Par 13, change "recommended" to "RECOMMENDED".
6) Editorial: Par 13, change "particularf" to "particular".

Section 2.2
1) Add that MI-RTR MAY (or SHOULD) implement the standard instance as well and which packets are used to advertise it.
2) Rephrase "marks ... by including" to "MUST include" to use requirements language.

Section 2.3
Editorial: Replace "normal" with "usual".

Section 2.4.1 
1) Par 1. Replace "IID #0" with "standard instance".
2) Par 1. Replace "instances other than IID #0" with "non-zero instances".
3) Par 2 second sentence. What does it mean "normal expectations"? Is this a network configuration requirement? Please, clarify in the text.

Section 2.4.2 Improve language consistency 
1) Verbs are used inconsistently: some are used in present time, others in future time. 

Section 2.5, replace "exists" with "MUST be performed".
Section 2.5.1, replace "only operates" with "MUST only be performed".
Section 2.5.2, replace "This requires" with "It is REQUIRED".
Section 2.5.2 third sentence, after "inconsistent" insert "due to their configuration". (Please, correct me if I am wrong.)

Section 2.6.1 
1) Editorial: Par 1, first sentence, replace "not to cause" to "to avoid".
2) Par 2, remove "(IID #0)".
3) Par 2, replace "non-zero IID" with "non-zero instance".
4) Par 5 NOTE, replace "IID #0" with "standard instance".

Section 2.6.2 
Replace all four appearances of "IID #0" with "standard instance". 

Section 3.1
Replace the two "MAY" to "can". 

7 Security Considerations
Discuss possible additional security considerations (or the lack of them) related to the exposure of "instances" on the wire.
Reason: Beyond the normal IETF procedure, this is especially important because "multiple instances allow isolation of resources..." Can this isolation, if observed or interfered on the wire, be damaging beyond the previous "standard instance" situation.

Thanks,
Orit.
-----Original Message-----
From: Les Ginsberg (ginsberg) [mailto:ginsberg@cisco.com] 
Sent: Saturday, April 8, 2017 4:23 PM
To: Orit Levin <oritl@microsoft.com>om>; gen-art@ietf.org
Cc: ietf@ietf.org; draft-ietf-isis-mi-bis.all@tools.ietf.org
Subject: RE: Gen-ART Last Call review of draft-ietf-isis-mi-bis-02

Orit -

Thanx for the review.
Responses inline.

> -----Original Message-----
> From: Orit Levin [mailto:oritl@microsoft.com]
> Sent: Thursday, April 06, 2017 8:27 PM
> To: gen-art@ietf.org
> Cc: ietf@ietf.org; draft-ietf-isis-mi-bis.all@tools.ietf.org
> Subject: Gen-ART Last Call review of draft-ietf-isis-mi-bis-02
> 
> I am the assigned Gen-ART reviewer for this draft. The General Area 
> Review Team (Gen-ART) reviews all IETF documents being processed by 
> the IESG for the IETF Chair.  Please treat these comments just like 
> any other last call comments.
> 
> For more information, please see the FAQ at
> 
> <http://wiki.tools.ietf.org/area/gen/trac/wiki/GenArtfaq>.
> 
> Document: draft-ietf-isis-mi-bis-02
> Reviewer: Orit Levin
> Review Date: 2017-04-06
> IETF LC End Date: 2017-04-07
> IESG Telechat date: 2017-04-13
> 
> Summary: This draft is "ready with issues" for publication.
> 
> Major issues: None.
> 
> Minor issues:
> 
> 1. Add text explaining the reason (or reasons) for replacing the 
> original RFC
> 6822 from 2012.
> Reason: It is a "bis" draft and there is no mention about it in the text.

[Les:] Note that the latest revision of the draft correctly identifies the draft as obsoleting RFC 6822. Previous versions had incorrectly identified this as an update to RFC 6822.
This is then the new Standard for the IS-IS MI support.

There are two classes of future readers of this document:

a)Readers who are unfamiliar with RFC 6822. For them what changed between RFC 6822 and this document is irrelevant. 

b)Readers who are familiar with RFC 6822. For them it is useful to know what changed - which is described in Appendix A.

In order not to distract readers of type "a" - as well as to provide an "uninterrupted" description of the normative behavior I believe placement of the change description in an Appendix improves the readability of the document.

Does this make sense to you?

> 2. In Abstract, state clearly that this standard introduces the 
> support for instances vs. other already existing concepts also listed 
> in the Abstract (i.e., circuits, adjacencies,  topologies, etc.).

[Les:] The Abstract currently says:

"This draft describes a mechanism that allows a single router to share
   one or more circuits among multiple Intermediate System To
   Intermediate System (IS-IS) routing protocol instances."

Previous to this extension, a router could have multiple instances of the IS-IS protocol, but multiple instance could not be run over the same interface. 
So we are not introducing "instances", but we are introducing the ability to enable multiple instances on the same interface.

> Reason: The wording is not clear about what is the new feature vs. 
> what are the new benefits vs. what was the original baseline

>3. Throughout the
> document, use "standard instance" instead of "IID = 0" or "IID #0".
> Reason: Expressions "standard instance", "IID = 0" and "IID #0" are 
>used  interchangeably throughout the document. It seems that they all 
>refer to the  same thing - the implementation of the original protocol 
>without the concept  of instances. Please, correct me if I am wrong.

[Les:]  I don't think this is possible without seriously compromising the document. For example:

Section 2.1

" IID #0 is reserved for the standard instance supported by legacy
   systems. "

Changing this to  " Standard instance is reserved for the standard instance ..."

Is clearly nonsensical.

Later in Section 2.1

"When the IID = 0, the list of supported ITIDs MUST NOT be present."

What is being discussed here is what is the correct behavior when an MI-capable router sends a PDU associated IID #0 and includes the new IID TLV. 
Replacing this with "When the standard instance..." loses the important point that the value of the IID in the IID TLV in this case is "0".

Hope this helps clarify things.

> 4. In section 2 par 3, change "support" and "operates" to "MUST 
> support" to use requirements language.

[Les:] I am on the fence as regards this change. Section 2 is an introduction to the following sub-sections - which define the normative behavior. But the introduction itself is not defining normative behavior - it is providing a context in which the protocol extensions defined in the following sub-sections can be understood. 

I am more inclined to change the "MAY" used later in the same paragraph you mention to "may" so it is consistent with the rest of this section.

???


> 5. In section 2 par 2, change "may" to either "can" or "MAY" to 
> clarify the intent.

[Les:] Did you mean Section 2.1 para 2?
If so I agree to the change.

> 6. In section 2.1 par 3, clarify whether IID #0 is ever being used on the wire.

[Les:] There are numerous places in the document where the legal use of IID  #0 is discussed. I do not understand how a reader would conclude that IID #0 is never sent on the wire.

> Explain the concept of the "standard interface" (see previous comment)

[Les:] There is no mention of "standard interface" - did you mean "standard instance"?

If so, Section 1 paragraph 4 states:

"Legacy routers support the standard
   or zero instance of the protocol."

   Les