Selection of Loop-Free Alternates for Multi-Homed Prefixes
RFC 8518

Received changes through RFC Editor sync (created alias RFC 8518, changed title to 'Selection of Loop-Free Alternates for Multi-Homed Prefixes', changed abstract to 'Deployment experience gained from implementing algorithms to determine Loop-Free Alternates (LFAs) for multi-homed prefixes (MHPs) has revealed some avenues for potential improvement.  This document provides explicit inequalities that can be used to evaluate neighbors as potential alternates for MHPs.  It also provides detailed criteria for evaluating potential alternates for external prefixes advertised by OSPF ASBRs.  This document updates Section 6 of RFC 5286 by expanding some of the routing aspects.', changed pages to 20, changed standardization level to Proposed Standard, changed state to RFC, added RFC published event at 2019-03-11, changed IESG state to RFC Published, created updates relation between draft-ietf-rtgwg-multihomed-prefix-lfa and RFC 5286)

[Ballot comment]
Thanks for the work on this. I have a couple of minor comments:

- There is an IPR disclosure with possible royalties. The shepherd report says there were no comments in the WG. Was the WG reminded of it?

§9: "Existing OSPF security considerations and stronger authentication and
manual key management mechanisms are specified in [RFC7474] SHOULD be
considered for OSPF deployments."

Could this say something stronger than "SHOULD be considered"? I'm fine with the SHOULD part, but simply considering it may not be that helpful.

[Ballot comment]

Thanks for the work everyone did on this document.

---------------------------------------------------------------------------

Please expand the following acronyms upon first use and in the title;
see https://www.rfc-editor.org/materials/abbrev.expansion.txt for guidance.

- LFA - Loop-Free Alternate
- FRR - Fast Reroute
- AS - Autonomous System
- LSA - Link State Advertisement

[Ballot comment]
Thanks for doing this work!

(1) I've read rfc5286, but I still had to go back to it to refresh my memory of the Inequalities...even to make sure that I was understanding/remembering the inequalities in §2 correctly.  I think it would be good to (1) include some text to go over (in plain English) what the inequalities are saying, and, (2) "show your work" (at least reference where the corresponding Inequalities came from in rfc5286).  IOW, add text to describe the inequalities in Figures 1, 6 and 7.

(2) §3 says both that "a computing router S MUST follow one of the appropriate procedures below, for each alternate neighbor N", and "the computing router SHOULD evaluate one of the corresponding LFA inequalities, as mentioned in Figure 1, once for each remote node that originated the prefix".  First off, MUST != SHOULD.  Also, it seems to me that "each alternate neighbor" is not the same as "each...node that originated the prefix".  However §3.1 points out that they are the same (or, I guess, at least equivalent): "[RFC5286] Section 6.1 recommends that a router computes the alternate next-hop for an IGP multi-homed prefix by considering alternate paths via all routers that have announced that prefix and the same has been elaborated with appropriate inequalities in the above section".  Please clarify what is meant with the different terminology, or (maybe better) use the same words.

(3) The document doesn't explain what a multi-homed prefix is.  There are some hints throughout the text, but I couldn't find a clear definition.  rfc5286 has a description in §6.1, but about "multi-homed routes".  For clarity, the document would benefit from a couple of sentences in the Introduction...

(4) This document uses MAX_METRIC to describe constants in IS-IS and OSPF: "0xffffff /2^24 - 1 for IS-IS and 0xffff for OSPF" (§5.1).  However neither protocol use MAX_METRIC as the name for those values: OSPF uses MaxLinkMetric (rfc6987) and IS-IS simply "maximum link metric" (rfc5305).  I think that it is ok to use the term MAX_METRIC in this document, but please be clear on what it is from the start (in the Introduction).

BTW, Figure 8 uses MAX_MET (not MAX_METRIC)

(5) §3.1: "The approach specified here MAY also be applicable for handling default routes as explained in Section 3.2." I think that MAY is out of place because it is pointing out a fact, and there doesn't seem to be a normative behavior attached. s/MAY/may

(6) §3.2: "...when multiple ECMP L1/L2 routers are reachable in an L1 area corresponding best LFAs SHOULD be given for each primary next-hop associated with default route".  I don't know what this sentence says.  Please clarify.  Specifically, what does "SHOULD be given" mean?

(7) §4.1: "LFA evaluation for multi-homed external prefixes in IS-IS is similar to the multi-homed internal prefixes."  The text says "similar", but no differences are mentioned...is it similar, or the same?

(8) Do we really still have to worry about RFC1583Compatibility?  Just wondering...

(9) The document could benefit from a grammar-focused review as many articles (the, an, a) are missing.

(10) "proceed to step 4c" in Figure 5 seems to not be needed.

[Ballot comment]
I was forced to make lots of inferences while reading this document, so
it's pretty likely that some of my comments will not make sense because I
misunderstood the technology.  My apologies in advance, and please help me
to become un-confused.  I think part of this is because the document relies
very heavily on prior knowledge, and could be avoided by sprinkling a few
more words here and there, such as "downstream-paths-only (for micro-loop
avoidance)" or "alternate neighbor N of S".  I've tried to note some more
cases in the section-by-section comments.


The Abstract and Introduction sound like they should be attached to an
Informational document; why is this going for PS?

Section 1.1

  SPF    -  Shortest Path First PDU

Is the "PDU" really entirely silent in the acronym?

Section 2

It might be nice to super-briefly reframe the situation, maybe:

The scenario for LFA protection for MHPs involves protecting a node N in
the path from source S to MHP prefix P, providing an alternate path subject
to constraints on the distance metric, measured as the sum of the cost of
the links traversed by the path.

Perhaps also expand the inequality headers slightly, as "Link-Protection
for PO_i to P is possible via N if:"

      Cost(X,P)    - Cost of reaching the prefix P from prefix
                    originating node X.

Isn't it important to note that the Cost differs from a D_opt in that the
Cost is only defined for what we essentially model here as a link and not a
multi-link path?

Section 3

Are these procedures supposed to terminate when the first LFA is found, or
produce multiple LFAs as output?  (Perhaps this is just an editorial nit,
but "for each alternate neighbor N" and "a valid LFA" would then be a
singular/plural mismatch", and perhaps the "select N as" steps in the
various procedures as well.

The phrase "If alternate neighbor N is also prefix-originator of P"
confused me a fair amount -- the "also" makes me think that what N is a
neighbor of should be the "primary" prefix-originator of P, i.e., PO_best,
but text elsewhere in the document implies that N is a neighbor of *S*, to
be used as the next-hop for forwarding to P.  I guess the intended sense of
the "also" could be more like "if, in addition to being a neighbor, N is
also a prefix-originator of P", but as written it seems ambiguous to me.

  However, if N is not a prefix-originator of P, the computing router
  SHOULD evaluate one of the corresponding LFA inequalities, as

Why is this a SHOULD?

Section 3.1

      In this scenario, E and F both are pre-failure optimal points of
  attachment and share the same primary next-hop.  Hence, an
  implementation MAY compare the kind of protection A provides to F
  (link-and-node protection) with the kind of protection C provides to
  E (link protection) and inherit the better alternative to prefix P
  and here it is A.

I find that this chunk makes more sense if I read it as "kind of protection
A provides to E" (not F).  Am I just confused?

                                                                  In
  this case, prefix P MUST inherit corresponding LFAs of each primary
  next-hop calculated for the router advertising the same respectively.

Is this MUST a new requirement imposed by this specification or a
restatement of something from (e.g.) RFC 5286?
 
  In summary, if there are multiple pre-failure points of attachment
  for a MHP and primary next-hop of a MHP is same as that of the
  primary next-hop of the router that was pre-failure optimal point of
  attachment, an implementation MAY provide a better protection to MHP
  without incurring any additional computation cost.

I had a hard time understanding why this is the case, most likely because I
don't have a clear picture about what the reference point is for
computational cost (that this is not "additional" to).

Section 4.2.1

This procedure is not super-well-defined if a cost type other than 1 or 2
is encountered.
 
  5. If route type (type 5/type 7)
          5a. If route type is same, check route p-bit,
                  forwarding address field for routes from both
                  ASBRs match. [...]

nit: "check if the route p-bit and the forwarding address field for"

Section 4.2.1.2

  If there are multiple ASBRs not pruned via rules defined in
  Section 4.2.1.1, [...]

nit: I wouldn't exactly say that the rules are *defined* in 4.2.1.1, rather
that they are "described in" or "referred to by" it.

Section 4.2.2.1

Similarly to a previous comment, if we started out with "A neighbor N of S
is a valid LFA to P under the named conditions when the corresponding
inequality holds:", that would seem to help readability a lot.

Section 5.2

  In Multi Topology (MT) IGP deployments, for each MT ID, a separate
  shortest path tree (SPT) is built with topology specific adjacencies,
  the LFA principles laid out in [RFC5286] are actually applicable for
  MT IS-IS [RFC5120] LFA SPF. [...]

nit: I think this is formally a comma splice as written, and would be
better by adding "so" after the last comma, and maybe expanding to "so the
LFA principles laid out in [RFC5286] are actually applicable on a per-MT-ID
basis for MT IS-IS [RFC5120] LFA SPF."

Section 9

I don't see any new security considerations to mention here, but would
suggest a rewording of the current text for readability:
 
  The existing OSPF security considerations continue to apply, as do the
  recommended manual key management mechanisms specified in [RFC7474].  The
  existing security considerations for IS-IS also continue to apply, as
  specified in [RFC5304] and [RFC5310] and extended by [RFC7645] for KARP.
  This document does not change any of the discussed protocol specifications
  [RFC1195] [RFC5120] [RFC2328] [RFC5838], and the security considerations of
  the LFA base specification [RFC5286] therefore continue to apply.

(Via drafts-lastcall@iana.org): IESG/Authors/WG Chairs:

The IANA Functions Operator has reviewed draft-ietf-rtgwg-multihomed-prefix-lfa-07, which is currently in Last Call, and has the following comments:

We understand that this document doesn't require any registry actions.

While it's often helpful for a document's IANA Considerations section to remain in place upon publication even if there are no actions, if the authors strongly prefer to remove it, we do not object.

If this assessment is not accurate, please respond as soon as possible.

Thank you,

Sabrina Tanamal
Senior IANA Services Specialist

The following Last Call announcement was sent out (ends 2018-10-09):

From: The IESG
To: IETF-Announce
CC: Stewart Bryant , rtgwg-chairs@ietf.org, stewart.bryant@gmail.com, draft-ietf-rtgwg-multihomed-prefix-lfa@ietf.org, martin.vigoureux@nokia.com, rtgwg@ietf.org
Reply-To: ietf@ietf.org
Sender:
Subject: Last Call:  (LFA selection for Multi-Homed Prefixes) to Proposed Standard


The IESG has received a request from the Routing Area Working Group WG
(rtgwg) to consider the following document: - 'LFA selection for Multi-Homed
Prefixes'
  as Proposed Standard

The IESG plans to make a decision in the next few weeks, and solicits final
comments on this action. Please send substantive comments to the
ietf@ietf.org mailing lists by 2018-10-09. Exceptionally, comments may be
sent to iesg@ietf.org instead. In either case, please retain the beginning of
the Subject line to allow automated sorting.

Abstract


  This document shares experience gained from implementing algorithms
  to determine Loop-Free Alternates for multi-homed prefixes.  In
  particular, this document provides explicit inequalities that can be
  used to evaluate neighbors as a potential alternates for multi-homed
  prefixes.  It also provides detailed criteria for evaluating
  potential alternates for external prefixes advertised by OSPF ASBRs.
  This documents updates and expands some of the "Routing Aspects" as
  specified in Section 6 of RFC 5286.





The file can be obtained via
https://datatracker.ietf.org/doc/draft-ietf-rtgwg-multihomed-prefix-lfa/

IESG discussion can be tracked via
https://datatracker.ietf.org/doc/draft-ietf-rtgwg-multihomed-prefix-lfa/ballot/

The following IPR Declarations may be related to this I-D:

  https://datatracker.ietf.org/ipr/2747/
  https://datatracker.ietf.org/ipr/2477/



The document contains these normative downward references.
See RFC 3967 for additional information:
    rfc5714: IP Fast Reroute Framework (Informational - IETF stream)

As required by RFC 4858, this is the current template for the Document
Shepherd Write-Up.

Changes are expected over time. This version is dated 24 February 2012.

(1) What type of RFC is being requested (BCP, Proposed Standard,
Internet Standard, Informational, Experimental, or Historic)?  Why
is this the proper type of RFC?  Is this type of RFC indicated in the
title page header?

It is a standards draft, which is appropriate for its contents and
this track is needed to update RFC5286 which is also standards
track.

This is correctly indicated on the front page.

(2) The IESG approval announcement includes a Document Announcement
Write-Up. Please provide such a Document Announcement Write-Up. Recent
examples can be found in the "Action" announcements for approved
documents. The approval announcement contains the following sections:

Technical Summary

This document shares experience gained from implementing algorithms
to determine Loop-Free Alternates for multi-homed prefixes.  In
particular, this document provides explicit inequalities that can be
used to evaluate neighbors as a potential alternates for multi-homed
prefixes.  It also provides detailed criteria for evaluating
potential alternates for external prefixes advertised by OSPF ASBRs.
This documents updates and expands some of the "Routing Aspects" as
specified in Section 6 of RFC5286.

Working Group Summary

This passed through the WG smoothly. Between the author team and
the reviewers many of the subject matter experts have read the text
and thus I have confidence that it is technically correct and a
useful extension to the work published in RFC5286

Document Quality

This is a clear and well written document. That enhances a widely
implemented IP fast-reroute protocol.

Personnel

Stewart Bryant is the Document Shepherd.

Alia Atlas is the responsible AD at the time of publication request.

(3) Briefly describe the review of this document that was performed by
the Document Shepherd.  If this version of the document is not ready
for publication, please explain why the document is being forwarded to
the IESG.

I read it through line by line and fed back a small number of detailed comments to the
authors who addressed these.

The text is now ready for publication.

(4) Does the document Shepherd have any concerns about the depth or
breadth of the reviews that have been performed?

No, this document has been adequately reviewed.

(5) Do portions of the document need review from a particular or from
broader perspective, e.g., security, operational complexity, AAA, DNS,
DHCP, XML, or internationalization? If so, describe the review that
took place.

The document does not need a specialist review beyond the set of reviews it will
ordinarily get at the next stage of the publication process.

(6) Describe any specific concerns or issues that the Document Shepherd
has with this document that the Responsible Area Director and/or the
IESG should be aware of? For example, perhaps he or she is uncomfortable
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is a need for it. In any event, if the WG has discussed those issues and
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concerns here.

I have no concerns regarding this document.

(7) Has each author confirmed that any and all appropriate IPR
disclosures required for full conformance with the provisions of BCP 78
and BCP 79 have already been filed. If not, explain why.

Yes they have.

(8) Has an IPR disclosure been filed that references this document?
If so, summarize any WG discussion and conclusion regarding the IPR
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There have been two IPR disclosures. The Working Group has made no
comment on these.

(9) How solid is the WG consensus behind this document? Does it
represent the strong concurrence of a few individuals, with others
being silent, or does the WG as a whole understand and agree with it? 

There is consensus amongst the subject matter experts that this
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if they had concerns.

(10) Has anyone threatened an appeal or otherwise indicated extreme
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No has threatened an appeal, and no one has expressed
discontent.

(11) Identify any ID nits the Document Shepherd has found in this
document. (See https://www.ietf.org/tools/idnits/ and the Internet-Drafts
Checklist). Boilerplate checks are not enough; this check needs to be
thorough.

There is one minor stylistic warning.

(12) Describe how the document meets any required formal review
criteria, such as the MIB Doctor, media type, and URI type reviews.

These are not needed for this document.

(13) Have all references within this document been identified as
either normative or informative?

Yes they have.

(14) Are there normative references to documents that are not ready for
advancement or are otherwise in an unclear state? If such normative
references exist, what is the plan for their completion?

Both normative references are RFCs.

(15) Are there downward normative references references (see RFC 3967)?
If so, list these downward references to support the Area Director in
the Last Call procedure.

There are no downrefs.

(16) Will publication of this document change the status of any
existing RFCs? Are those RFCs listed on the title page header, listed
in the abstract, and discussed in the introduction? If the RFCs are not
listed in the Abstract and Introduction, explain why, and point to the
part of the document where the relationship of this document to the
other RFCs is discussed. If this information is not in the document,
explain why the WG considers it unnecessary.

It will update RFC5286 and this is noted in the top left corner and in
the Abstract.

(17) Describe the Document Shepherd's review of the IANA considerations
section, especially with regard to its consistency with the body of the
document. Confirm that all protocol extensions that the document makes
are associated with the appropriate reservations in IANA registries.
Confirm that any referenced IANA registries have been clearly
identified. Confirm that newly created IANA registries include a
detailed specification of the initial contents for the registry, that
allocations procedures for future registrations are defined, and a
reasonable name for the new registry has been suggested (see RFC 5226).

There are no IANA requests.

(18) List any new IANA registries that require Expert Review for future
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There are no IANA requests.

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This is not applicable.

Request for posting confirmation emailed to previous authors: Shraddha Hegde , Bruno Decraene , Pushpasis Sarkar , rtgwg-chairs@ietf.org, Uma Chunduri , Hannes Gredler , Jeff Tantsura , Chris Bowers

Request for posting confirmation emailed to previous authors: Shraddha Hegde , Pushpasis Sarkar , Bruno Decraene , Uma Chunduri , Hannes Gredler , Jeff Tantsura , Chris Bowers

Request for posting confirmation emailed to previous authors: Shraddha Hegde , Pushpasis Sarkar , Bruno Decraene , Uma Chunduri , Hannes Gredler , Jeff Tantsura , Chris Bowers

Request for posting confirmation emailed to previous authors: "Shraddha Hegde" , "Jeff Tantsura" , "Pushpasis Sarkar" , rtgwg-chairs@ietf.org, "Hannes Gredler" , "Chris Bowers" , "Bruno Decraene" , "Uma Chunduri"