Re: Genart last call review of draft-ietf-isis-reverse-metric-15

["Naiming Shen (naiming)" <naiming@xxxxxxxxx>]

Hi Stewart,

Thanks for detailed review and comments, please see some
replies inline. the modified version diff is attached, please reivew.

thanks.
- Naiming

Title: Diff: draft-ietf-isis-reverse-metric-15.txt - draft-ietf-isis-reverse-metric-15-rev.txt

	draft-ietf-isis-reverse-metric-15.txt		draft-ietf-isis-reverse-metric-15-rev.txt
	Networking Working Group N. Shen		Networking Working Group N. Shen
	Internet-Draft Cisco Systems		Internet-Draft Cisco Systems
	Intended status: Standards Track S. Amante		Intended status: Standards Track S. Amante
	Expires: April 19, 2019 Apple, Inc.		Expires: April 24, 2019 Apple, Inc.
	M. Abrahamsson		M. Abrahamsson
	T-Systems Nordic		T-Systems Nordic
	October 16, 2018		October 21, 2018
	IS-IS Routing with Reverse Metric		IS-IS Routing with Reverse Metric
	draft-ietf-isis-reverse-metric-15		draft-ietf-isis-reverse-metric-15-rev
	Abstract		Abstract
	This document describes a mechanism to allow IS-IS routing to quickly		This document describes a mechanism to allow IS-IS routing to quickly
	and accurately shift traffic away from either a point-to-point or		and accurately shift traffic away from either a point-to-point or
	multi-access LAN interface during network maintenance or other		multi-access LAN interface during network maintenance or other
	operational events. This is accomplished by signaling adjacent IS-IS		operational events. This is accomplished by signaling adjacent IS-IS
	neighbors with a higher reverse metric, i.e., the metric towards the		neighbors with a higher reverse metric, i.e., the metric towards the
	signaling IS-IS router.		signaling IS-IS router.
	skipping to change at page 1, line 38		skipping to change at page 1, line 38
	Internet-Drafts are working documents of the Internet Engineering		Internet-Drafts are working documents of the Internet Engineering
	Task Force (IETF). Note that other groups may also distribute		Task Force (IETF). Note that other groups may also distribute
	working documents as Internet-Drafts. The list of current Internet-		working documents as Internet-Drafts. The list of current Internet-
	Drafts is at http://datatracker.ietf.org/drafts/current/.		Drafts is at http://datatracker.ietf.org/drafts/current/.
	Internet-Drafts are draft documents valid for a maximum of six months		Internet-Drafts are draft documents valid for a maximum of six months
	and may be updated, replaced, or obsoleted by other documents at any		and may be updated, replaced, or obsoleted by other documents at any
	time. It is inappropriate to use Internet-Drafts as reference		time. It is inappropriate to use Internet-Drafts as reference
	material or to cite them other than as "work in progress."		material or to cite them other than as "work in progress."
	This Internet-Draft will expire on April 19, 2019.		This Internet-Draft will expire on April 24, 2019.
	Copyright Notice		Copyright Notice
	Copyright (c) 2018 IETF Trust and the persons identified as the		Copyright (c) 2018 IETF Trust and the persons identified as the
	document authors. All rights reserved.		document authors. All rights reserved.
	This document is subject to BCP 78 and the IETF Trust's Legal		This document is subject to BCP 78 and the IETF Trust's Legal
	Provisions Relating to IETF Documents		Provisions Relating to IETF Documents
	(http://trustee.ietf.org/license-info) in effect on the date of		(http://trustee.ietf.org/license-info) in effect on the date of
	publication of this document. Please review these documents		publication of this document. Please review these documents
	carefully, as they describe your rights and restrictions with respect		carefully, as they describe your rights and restrictions with respect
	to this document. Code Components extracted from this document must		to this document. Code Components extracted from this document must
	include Simplified BSD License text as described in Section 4.e of		include Simplified BSD License text as described in Section 4.e of
	the Trust Legal Provisions and are provided without warranty as		the Trust Legal Provisions and are provided without warranty as
	described in the Simplified BSD License.		described in the Simplified BSD License.
	Table of Contents		Table of Contents
	1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2		1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
	1.1. Node and Link Isolation . . . . . . . . . . . . . . . . . 2		1.1. Node and Link Isolation . . . . . . . . . . . . . . . . . 3
	1.2. Distributed Forwarding Planes . . . . . . . . . . . . . . 3		1.2. Distributed Forwarding Planes . . . . . . . . . . . . . . 3
	1.3. Spine-Leaf Applications . . . . . . . . . . . . . . . . . 3		1.3. Spine-Leaf Applications . . . . . . . . . . . . . . . . . 3
	1.4. LDP IGP Synchronization . . . . . . . . . . . . . . . . . 3		1.4. LDP IGP Synchronization . . . . . . . . . . . . . . . . . 3
	1.5. IS-IS Reverse Metric . . . . . . . . . . . . . . . . . . 3		1.5. IS-IS Reverse Metric . . . . . . . . . . . . . . . . . . 4
	1.6. Specification of Requirements . . . . . . . . . . . . . . 4		1.6. Specification of Requirements . . . . . . . . . . . . . . 4
	2. IS-IS Reverse Metric TLV . . . . . . . . . . . . . . . . . . 4		2. IS-IS Reverse Metric TLV . . . . . . . . . . . . . . . . . . 4
	3. Elements of Procedure . . . . . . . . . . . . . . . . . . . . 6		3. Elements of Procedure . . . . . . . . . . . . . . . . . . . . 6
	3.1. Processing Changes to Default Metric . . . . . . . . . . 6		3.1. Processing Changes to Default Metric . . . . . . . . . . 6
	3.2. Multi-Topology IS-IS Support on Point-to-point links . . 7		3.2. Multi-Topology IS-IS Support on Point-to-point links . . 7
	3.3. Multi-Access LAN Procedures . . . . . . . . . . . . . . . 7		3.3. Multi-Access LAN Procedures . . . . . . . . . . . . . . . 7
	3.4. LDP/IGP Synchronization on LANs . . . . . . . . . . . . . 8		3.4. LDP/IGP Synchronization on LANs . . . . . . . . . . . . . 8
	3.5. Operational Guidelines . . . . . . . . . . . . . . . . . 9		3.5. Operational Guidelines . . . . . . . . . . . . . . . . . 9
	4. Security Considerations . . . . . . . . . . . . . . . . . . . 9		4. Security Considerations . . . . . . . . . . . . . . . . . . . 10
	5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10		5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
	6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10		6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
	7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10		7. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
	7.1. Normative References . . . . . . . . . . . . . . . . . . 10		7.1. Normative References . . . . . . . . . . . . . . . . . . 11
	7.2. Informative References . . . . . . . . . . . . . . . . . 11		7.2. Informative References . . . . . . . . . . . . . . . . . 11
	Appendix A. Node Isolation Challenges . . . . . . . . . . . . . 12		Appendix A. Node Isolation Challenges . . . . . . . . . . . . . 12
	Appendix B. Link Isolation Challenges . . . . . . . . . . . . . 13		Appendix B. Link Isolation Challenges . . . . . . . . . . . . . 13
	Appendix C. Contributors' Addresses . . . . . . . . . . . . . . 14		Appendix C. Contributors' Addresses . . . . . . . . . . . . . . 14
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
	1. Introduction		1. Introduction
	The IS-IS [ISO10589] routing protocol has been widely used in		The IS-IS [ISO10589] routing protocol has been widely used in
	Internet Service Provider IP/MPLS networks. Operational experience		Internet Service Provider IP/MPLS networks. Operational experience
	with the protocol, combined with ever increasing requirements for		with the protocol, combined with ever increasing requirements for
	lossless operations have demonstrated some operational issues. This		lossless operations have demonstrated some operational issues. This
	document describes the issues and a mechanism for mitigating them.		document describes the issues and a mechanism for mitigating them.
			This document defines the IS-IS "Reverse Metric" mechanism that
			allows an IS-IS node to send a "Reverse Metric" TLV through the IS-IS
			IIH PDU to the neighbor or pseudo-node to adjust the routing metric
			on the inbound direction.
	1.1. Node and Link Isolation		1.1. Node and Link Isolation
	IS-IS routing mechanism has the overload-bit, which can be used by		IS-IS routing mechanism has the overload-bit, which can be used by
	operators to perform disruptive maintenance on the router. But in		operators to perform disruptive maintenance on the router. But in
	many operational maintenance cases, it is not necessary to divert all		many operational maintenance cases, it is not necessary to divert all
	the traffic away from this node. It is necessary to avoid only a		the traffic away from this node. It is necessary to avoid only a
	single link during the maintenance. More detailed descriptions of		single link during the maintenance. More detailed descriptions of
	the challenges can be found in Appendix A and Appendix B of this		the challenges can be found in Appendix A and Appendix B of this
	document.		document.
	skipping to change at page 5, line 38		skipping to change at page 5, line 44
	metric offset that a neighbor SHOULD add to the existing, configured		metric offset that a neighbor SHOULD add to the existing, configured
	Default Metric for the IS-IS link [ISO10589]. Refer to "Elements of		Default Metric for the IS-IS link [ISO10589]. Refer to "Elements of
	Procedure", in Section 3 for details on how an IS-IS router should		Procedure", in Section 3 for details on how an IS-IS router should
	process the Metric field in a Reverse Metric TLV.		process the Metric field in a Reverse Metric TLV.
	The Metric field, in the Reverse Metric TLV, is a "reverse offset		The Metric field, in the Reverse Metric TLV, is a "reverse offset
	metric" that will either be in the range of 0 - 63 when a "narrow"		metric" that will either be in the range of 0 - 63 when a "narrow"
	IS-IS metric is used (IS Neighbors TLV, Pseudonode LSP) [RFC1195] or		IS-IS metric is used (IS Neighbors TLV, Pseudonode LSP) [RFC1195] or
	in the range of 0 - (2^24 - 2) when a "wide" Traffic Engineering		in the range of 0 - (2^24 - 2) when a "wide" Traffic Engineering
	metric value is used, (Extended IS Reachability TLV) [RFC5305]		metric value is used, (Extended IS Reachability TLV) [RFC5305]
	[RFC5817].		[RFC5817]. As described below, when the U bit is set, the
			accumulated value of the wide metric is in the range of 0 - (2^24 -
			1), with (2^24 - 1) metric as non-reachable in IS-IS routing. The
			IS-IS metric value of (2^24 - 2) serves as the link of last resort.
	There are currently only two Flag bits defined.		There are currently only two Flag bits defined.
	W bit (0x01): The "Whole LAN" bit is only used in the context of		W bit (0x01): The "Whole LAN" bit is only used in the context of
	multi-access LANs. When a Reverse Metric TLV is transmitted from a		multi-access LANs. When a Reverse Metric TLV is transmitted from a
	node to the Designated Intermediate System (DIS), if the "Whole LAN"		node to the Designated Intermediate System (DIS), if the "Whole LAN"
	bit is set (1), then a DIS SHOULD add the received Metric value in		bit is set (1), then a DIS SHOULD add the received Metric value in
	the Reverse Metric TLV to each node's existing Default Metric in the		the Reverse Metric TLV to each node's existing Default Metric in the
	Pseudonode LSP. If the "Whole LAN" bit is not set (0), then a DIS		Pseudonode LSP. If the "Whole LAN" bit is not set (0), then a DIS
	SHOULD add the received Metric value in the Reverse Metric TLV to the		SHOULD add the received Metric value in the Reverse Metric TLV to the
	skipping to change at page 9, line 34		skipping to change at page 9, line 43
	When the link Traffic Engineering metric is raised to (2^24 - 1)		When the link Traffic Engineering metric is raised to (2^24 - 1)
	[RFC5817], either due to the reverse-metric mechanism or by explicit		[RFC5817], either due to the reverse-metric mechanism or by explicit
	user configuration, this SHOULD immediately trigger the CSPF re-		user configuration, this SHOULD immediately trigger the CSPF re-
	calculation to move the Traffic Engineering traffic away from that		calculation to move the Traffic Engineering traffic away from that
	link. It is RECOMMENDED also that the CSPF does the immediate CSPF		link. It is RECOMMENDED also that the CSPF does the immediate CSPF
	re-calculation when the Traffic Engineering metric is raised to (2^24		re-calculation when the Traffic Engineering metric is raised to (2^24
	- 2) to be the last resort link.		- 2) to be the last resort link.
	It is RECOMMENDED that implementations provide a capability to		It is RECOMMENDED that implementations provide a capability to
	disable any changes by Reverse Metric mechanism through neighbor's		disable any IS-IS metric changes by Reverse Metric mechanism through
	Hello PDUs. It can be to a node's individual interface Default		neighbor's Hello PDUs. It can be to a node's individual interface
	Metric or Traffic Engineering parameters based upon receiving a		Default Metric or Traffic Engineering parameters based upon receiving
	properly formatted Reverse Metric TLVs.		a properly formatted Reverse Metric TLVs.
	If an implementation enables this mechanism by default, it is		If an implementation enables this mechanism by default, it is
	RECOMMENDED that it be disabled by the operators when not explicitly		RECOMMENDED that it be disabled by the operators when not explicitly
	using it.		using it.
	4. Security Considerations		4. Security Considerations
	Security concerns for IS-IS are addressed in [ISO10589], [RFC5304],		Security concerns for IS-IS are addressed in [ISO10589], [RFC5304],
	[RFC5310], and with various deployment and operational security		[RFC5310], and with various deployment and operational security
	considerations in [RFC7645]. The enhancement in this document makes		considerations in [RFC7645]. The enhancement in this document makes
	it possible for one IS-IS router to manipulate the IS-IS Default		it possible for one IS-IS router to manipulate the IS-IS Default
	Metric and, optionally, Traffic Engineering parameters of adjacent		Metric and, optionally, Traffic Engineering parameters of adjacent
	IS-IS neighbors. Although IS-IS routers within a single Autonomous		IS-IS neighbors. Although IS-IS routers within a single Autonomous
	System nearly always are under the control of a single administrative		System nearly always are under the control of a single administrative
	authority, it is highly RECOMMENDED that operators configure		authority, it is highly recommended that operators configure
	authentication of IS-IS PDUs to mitigate use of the Reverse Metric		authentication of IS-IS PDUs to mitigate use of the Reverse Metric
	TLV as a potential attack vector.		TLV as a potential attack vector.
	5. IANA Considerations		5. IANA Considerations
	IANA has allocated IS-IS TLV Codepoints of 16 for the Reverse Metric		IANA has allocated IS-IS TLV Codepoints of 16 for the Reverse Metric
	TLV. This new TLV has the following attributes: IIH = y, LSP = n,		TLV. This new TLV has the following attributes: IIH = y, LSP = n,
	SNP = n, Purge = n.		SNP = n, Purge = n.
	This document also introduces a new registry for sub-TLVs of the		This document also introduces a new registry for sub-TLVs of the
	skipping to change at page 10, line 32		skipping to change at page 10, line 42
	18: Traffic Engineering Metric sub-TLV, as specified in this		18: Traffic Engineering Metric sub-TLV, as specified in this
	document (Section 2)		document (Section 2)
	19-255: Unassigned		19-255: Unassigned
	6. Acknowledgments		6. Acknowledgments
	The authors would like to thank Mike Shand, Dave Katz, Guan Deng,		The authors would like to thank Mike Shand, Dave Katz, Guan Deng,
	Ilya Varlashkin, Jay Chen, Les Ginsberg, Peter Ashwood-Smith, Uma		Ilya Varlashkin, Jay Chen, Les Ginsberg, Peter Ashwood-Smith, Uma
	Chunduri, Alexander Okonnikov, Jonathan Harrison, Dave Ward, Himanshu		Chunduri, Alexander Okonnikov, Jonathan Harrison, Dave Ward, Himanshu
	Shah, Wes George, Danny McPherson, Ed Crabbe, Russ White, Robert		Shah, Wes George, Danny McPherson, Ed Crabbe, Russ White, Robert
	Raszuk, Tom Petch and Acee Lindem for their comments and		Raszuk, Tom Petch, Stewart Bryant and Acee Lindem for their comments
	contributions.		and contributions.
	This document was produced using Marshall Rose's xml2rfc tool.		This document was produced using Marshall Rose's xml2rfc tool.
	7. References		7. References
	7.1. Normative References		7.1. Normative References
	[ISO10589]		[ISO10589]
	ISO, "Intermediate system to Intermediate system routeing		ISO, "Intermediate system to Intermediate system routeing
	information exchange protocol for use in conjunction with		information exchange protocol for use in conjunction with
	skipping to change at page 11, line 38		skipping to change at page 11, line 52
	[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC		[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
	2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,		2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
	May 2017, <https://www.rfc-editor.org/info/rfc8174>.		May 2017, <https://www.rfc-editor.org/info/rfc8174>.
	7.2. Informative References		7.2. Informative References
	[I-D.shen-isis-spine-leaf-ext]		[I-D.shen-isis-spine-leaf-ext]
	Shen, N., Ginsberg, L., and S. Thyamagundalu, "IS-IS		Shen, N., Ginsberg, L., and S. Thyamagundalu, "IS-IS
	Routing for Spine-Leaf Topology", draft-shen-isis-spine-		Routing for Spine-Leaf Topology", draft-shen-isis-spine-
	leaf-ext-03 (work in progress), March 2017.		leaf-ext-07 (work in progress), October 2018.
	[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic		[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
	Authentication", RFC 5304, DOI 10.17487/RFC5304, October		Authentication", RFC 5304, DOI 10.17487/RFC5304, October
	2008, <https://www.rfc-editor.org/info/rfc5304>.		2008, <https://www.rfc-editor.org/info/rfc5304>.
	[RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,		[RFC5310] Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
	and M. Fanto, "IS-IS Generic Cryptographic		and M. Fanto, "IS-IS Generic Cryptographic
	Authentication", RFC 5310, DOI 10.17487/RFC5310, February		Authentication", RFC 5310, DOI 10.17487/RFC5310, February
	2009, <https://www.rfc-editor.org/info/rfc5310>.		2009, <https://www.rfc-editor.org/info/rfc5310>.
End of changes. 14 change blocks.
	18 lines changed or deleted		26 lines changed or added
This html diff was produced by rfcdiff 1.42. The latest version is available from http://tools.ietf.org/tools/rfcdiff/

> On Oct 17, 2018, at 8:59 AM, Stewart Bryant <stewart.bryant@xxxxxxxxx> wrote:
> 
> Reviewer: Stewart Bryant
> Review result: Ready with Issues
> 
> 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
> 
> <https://trac.ietf.org/trac/gen/wiki/GenArtfaq>.
> 
> Document: draft-ietf-isis-reverse-metric-15
> Reviewer: Stewart Bryant
> Review Date: 2018-10-17
> IETF LC End Date: 2018-10-17
> IESG Telechat date: 2018-11-21
> 
> Summary: Generally a well written document, but some earlier text on what a
> reverse metric is and what it does would be very helpful to the reader. Also
> the reader is left with the impression that the use of this gives disruption
> free network changes, and yet it does not discuss microloops.
> 
> Major issues: None
> 
> Minor issues:
> 1.2.  Distributed Forwarding Planes
> 
> <snip>
> Temporarily signaling
>   the 'Reverse Metric' over this link to discourage the traffic via the
> 
> SB> I know it's always chicken and egg, but it would be useful if a
> SB> clearer definition of reverse metric were provided before you
> SB> explained its use.

<NS> I have added a paragraph at the beginning of the Introduction section

> 
>   corresponding line-card will help to reduce the traffic loss in the
>   network.  In the meantime, the remote PE routers will select a
>   different set of PE routers for the BGP best path calculation or use
>   a different link towards the same PE router on which a line-card is
>   resetting.
> 
> SB> Remember that when you change paths you have to deal with
> SB> microloops.

<NS> Well, this â??reverse metricâ?? is just like a normal IS-IS metric change.
<NS> and microloops is implied. This document does not create new
<NS> condition or trying to address the issue. It assumes final convergence
<NS> state will be reached for the mentioned use cases.

> 
> =======
> 
> 1.5.  IS-IS Reverse Metric
> 
>   This document uses the routing protocol itself as the transport
>   mechanism to allow one IS-IS router to advertise a "reverse metric"
>   in an IS-IS Hello (IIH) PDU to an adjacent node on a point-to-point
>   or multi-access LAN link.  This would allow the provisioning to be
>   performed only on a single node, setting a "reverse metric" on a link
>   and have traffic bidirectionally shift away from that link gracefully
>   to alternate, viable paths.
> 
> SB> Again you need to be much clearer what a reverse metric is before
> SB> you cite the use cases and advantages.

<NS> added a new paragraph.

> 
> ===========
> 
> 3.1.  Processing Changes to Default Metric
> 
>   It is important to use the same IS-IS metric type on both ends of the
>   link and in the entire IS-IS area or level.
> 
> SB> Isn't the point about links redundant given that it needs to be the
> SB> same in the area/the level

<NS>  This sentence is talking about it does not deal with the case of
<NS> one side of the link uses IS-IS wide metric, but the other side
<NS> of the link uses a narrow metric. It is saying itâ??s broken and we
<NS> donâ??t support such a mixup, thus â??reverse-metricâ?? also does not handle.

> 
>   On the receiving side of
>   the 'reverse-metric' TLV, the accumulated value of configured metric
>   and the reverse-metric needs to be limited to 63 in "narrow" metric
>   mode and to (2^24 - 2) in "wide" metric mode.
>   This applies to both
>   the Default Metric of Extended IS Reachability TLV and the Traffic
>   Engineering Default Metric sub-TLV in LSP or Pseudonode LSP for the
>   "wide" metric mode case.  If the "U" bit is present in the flags, the
>   accumulated metric value is to be limited to (2^24 - 1) for both the
>   normal link metric and Traffic Engineering metric in IS-IS "wide"
>   metric mode.
> 
> SB> A clarifying note explaining the different usage of 2^24 - 1 and
> SB> 2^24 - 2 would be helpful to the reader.

<NS> Yes, added a sentence in the TLV definition part to describe them
<NS> in some detail.

> 
> =========
> 3.2.  Multi-Topology IS-IS Support on Point-to-point links
> 
>   The Reverse Metric TLV is applicable to Multi-Topology IS-IS (M-ISIS)
>   [RFC5120].  On point-to-point links, if an IS-IS router is configured
>   for M-ISIS, it MUST send only a single Reverse Metric TLV in IIH PDUs
>   toward its neighbor(s) on the designated link.
> 
> SB> Might you not want to use this on a topology by topology basis?
> SB> For example you might want to bring up important typologies first.

<NS> this document does not support that. The main reason is that the
<NS> pnode LSP is shared by all the topologies, and itâ??s not per topology
<NS> pnode definition. For backwards competibility of this draft, it follows
<NS> the same logic. Otherwise we need a flag day in the network to use
<NS> â??reverse-metricâ??.

> 
> =========
> 
>   its
>   inbound metric value to be the maximum and this puts the link of this
>   new node in the last resort position without impacting the other IS-
>   IS nodes on the same LAN.
> 
> SB> It is only down in S3.4 that you provide this simple definition of
> SB> reverse metric as the "inbound metric". It would be helpful to have this
> SB> earlier in the text.

<NS> yes, indeed. a new paragraph is added.

> 
> =========
> 
> It is RECOMMENDED also that the CSPF does the immediate CSPF
>   re-calculation when the Traffic Engineering metric is raised to (2^24
>   - 2) to be the last resort link.
> 
> SB> Again it would help the reader if "link of last resort" was earlier
> SB> in the text,

<NS> â??link of last resortâ?? is only for certain use cases, but not all. added
<NS> also in the TLV definition section.

> =========
>   It is RECOMMENDED that implementations provide a capability to
>   disable any changes by Reverse Metric mechanism through neighbor's
>   Hello PDUs.
> 
> SB> Changes of what? That sentence does not seem to read very well.

<NS> added the â??IS-IS metric changesâ??.

> 
> ==========
> 
>   If an implementation enables this mechanism by default, it is
>   RECOMMENDED that it be disabled by the operators when not explicitly
>   using it.
>  
> SB> Why not RECOMMEND that it be disabled by default, or perhaps
> SB> strengthen that to MUST be disabled by default.

<NS> As also replied also to Alvaroâ??s AD review comments, one of the main
<NS> use case is for operational maintenance window to divert the traffic
<NS> into certain links by using the â??reverse-metricâ?? mechanism. If an operator
<NS> has to track down all the nodes on the LAN side of the links, and to
<NS> manually enable the feature one by one correctly, then it defeats the
<NS> purpose of this to simplify the operational use case here.

> =========
> 
> it is highly RECOMMENDED that operators configure
> authentication of IS-IS PDUs to mitigate use of the Reverse Metric
> TLV as a potential attack vector.
> 
> SB> Not sure that you can qualify RFC2119 RECOMMENDED

<NS> changed to lower case.

> 
> =========
> 
>> From the IANA section
> 
> SB> Why is 18 chosen in an otherwise empty registry?

<NS> Originally, it inherits the same sub-TLV from IS-IS TE sub-TLV RFC
<NS> RFC 5305, and the Traffic Engineering Metric sub-TLV is 18.
<NS> now this â??reverse-metricâ?? TLV has itâ??s own registry, but we keep the
<NS> number the same.

> 
> =========
> Regarding Appendix A and I think Appendix B
> 
> SB> As noted earlier you really need to talk about microloops. There
> SB> is no disruption free lunch available.
> 
> ========
> 
> Nits/editorial comments:
> 
>> From ID-nits
>  ** Downref: Normative reference to an Informational RFC: RFC 5443
>  This is correctly dealt with in the LC
> 
>  == Outdated reference: A later version (-07) exists of
>     draft-shen-isis-spine-leaf-ext-03
> I am sure the RFC Editor will address, but could usefully be fixed in any
> respin.
> 
<NS> Good catch. corrected.