rfc9917v1.txt		rfc9917.txt
	skipping to change at line 21 ¶		skipping to change at line 21 ¶
	Abstract		Abstract
	An IGP Flexible Algorithm (Flex-Algorithm) enables the computation of		An IGP Flexible Algorithm (Flex-Algorithm) enables the computation of
	constraint-based paths within an IGP domain, allowing operators to		constraint-based paths within an IGP domain, allowing operators to
	influence path selection according to administrative policies. This		influence path selection according to administrative policies. This
	document defines an extension to Flex-Algorithm that allows the		document defines an extension to Flex-Algorithm that allows the
	inclusion or exclusion of links from path computation based on		inclusion or exclusion of links from path computation based on
	Administrative Groups (also known as link affinities) associated with		Administrative Groups (also known as link affinities) associated with
	the reverse direction of the path under computation.		the reverse direction of the path under computation.
	This extension enhances the path selection capabilities of Flex-
	Algorithm by enabling reverse-affinity-based constraints, which are
	particularly useful for scenarios where path symmetry or directional
	link attributes are operationally significant.
	This document updates RFCs 9350 and 9843 by introducing the new IANA		This document updates RFCs 9350 and 9843 by introducing the new IANA
	registry that specifies the ordered set of rules that are used to		registry that specifies the ordered set of rules that are used to
	prune links from the topology during the Flex-Algorithm path		prune links from the topology during the Flex-Algorithm path
	computation.		computation.
	Status of This Memo		Status of This Memo
	This is an Internet Standards Track document.		This is an Internet Standards Track document.
	This document is a product of the Internet Engineering Task Force		This document is a product of the Internet Engineering Task Force
	skipping to change at line 75 ¶		skipping to change at line 70 ¶
	4. Deployment Consideration		4. Deployment Consideration
	5. IS-IS Flexible Algorithm Exclude Reverse Admin Group Sub-TLV		5. IS-IS Flexible Algorithm Exclude Reverse Admin Group Sub-TLV
	6. IS-IS Flexible Algorithm Include-Any Reverse Admin Group		6. IS-IS Flexible Algorithm Include-Any Reverse Admin Group
	Sub-TLV		Sub-TLV
	7. IS-IS Flexible Algorithm Include-All Reverse Admin Group		7. IS-IS Flexible Algorithm Include-All Reverse Admin Group
	Sub-TLV		Sub-TLV
	8. OSPF Flexible Algorithm Exclude Reverse Admin Group Sub-TLV		8. OSPF Flexible Algorithm Exclude Reverse Admin Group Sub-TLV
	9. OSPF Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV		9. OSPF Flexible Algorithm Include-Any Reverse Admin Group Sub-TLV
	10. OSPF Flexible Algorithm Include-All Reverse Admin Group Sub-TLV		10. OSPF Flexible Algorithm Include-All Reverse Admin Group Sub-TLV
	11. Calculation of Flexible Algorithm Paths		11. Calculation of Flexible Algorithm Paths
	12. IGP Flex-Algorithm Path Computation Rules Registry		12. IANA Considerations
	13. IANA Considerations		12.1. Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV
	13.1. Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV		12.2. OSPF Flexible Algorithm Definition TLV Sub-TLV Registry
	13.2. OSPF Flexible Algorithm Definition TLV Sub-TLV Registry		12.3. IGP Flex-Algorithm Path Computation Rules Registry
	13.3. IGP Flex-Algorithm Path Computation Rules Registry		12.3.1. Guidance for Designated Experts
	14. Security Considerations		13. Security Considerations
	15. Normative References		14. References
	16. Informative References		14.1. Normative References
	Acknowledgments		14.2. Informative References
	Authors' Addresses		Authors' Addresses
	1. Introduction		1. Introduction
	IGP protocols historically computed the best paths over the network		IGP protocols historically computed the best paths over the network
	solely based on the IGP metric assigned to the links. An IGP Flex-		solely based on the IGP metric assigned to the links. An IGP Flex-
	Algorithm as specified in [RFC9350] allows IGPs to compute		Algorithm as specified in [RFC9350] allows IGPs to compute
	constraint-based paths. Several mechanisms to include or exclude the		constraint-based paths. Several mechanisms to include or exclude the
	link during the Flex-Algorithm path calculation have been defined		link during the Flex-Algorithm path calculation have been defined
	already:		already:
	skipping to change at line 113 ¶		skipping to change at line 108 ¶
	* link exclusion based on maximum delay [RFC9843]		* link exclusion based on maximum delay [RFC9843]
	This document extends IGP Flex-Algorithm with additional constraints		This document extends IGP Flex-Algorithm with additional constraints
	for inclusion or exclusion of links in the path based on Admin Groups		for inclusion or exclusion of links in the path based on Admin Groups
	associated with the reverse direction of the Shortest Path First		associated with the reverse direction of the Shortest Path First
	(SPF) computation.		(SPF) computation.
	This document updates [RFC9350] and [RFC9843] by creating the new		This document updates [RFC9350] and [RFC9843] by creating the new
	IANA registry that specifies the ordered set of rules that are used		IANA registry that specifies the ordered set of rules that are used
	to prune links from the topology during the Flex-Algorithm path		to prune links from the topology during the Flex-Algorithm path
	computation (refer to Section 12).		computation (refer to Section 12.3.1).
	The term OSPF in this document is used to cover both OSPFv2 and		The term OSPF in this document is used to cover both OSPFv2 and
	OSPFv3 protocols.		OSPFv3 protocols.
	2. Requirements Language		2. Requirements Language
	The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",		The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
	"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and		"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
	"OPTIONAL" in this document are to be interpreted as described in		"OPTIONAL" in this document are to be interpreted as described in
	BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all		BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
	skipping to change at line 170 ¶		skipping to change at line 165 ¶
	Algorithm path recalculation.		Algorithm path recalculation.
	One possible approach is to use two different thresholds when setting		One possible approach is to use two different thresholds when setting
	and unsetting the Extended Administrative Group on the link. For		and unsetting the Extended Administrative Group on the link. For
	example, when setting the Extended Administrative Group on the link		example, when setting the Extended Administrative Group on the link
	based on the percentage of certain incoming errors, higher threshold		based on the percentage of certain incoming errors, higher threshold
	is used for setting and lower threshold is used for unsetting the		is used for setting and lower threshold is used for unsetting the
	Extended Administrative Group on the link.		Extended Administrative Group on the link.
	Many implementations provide a throttling mechanism that limits the		Many implementations provide a throttling mechanism that limits the
	rate at which the Label Switched Path (LSP) in case of IS-IS, or Link		rate at which the Link State PDU (LSP) in case of IS-IS, or Link
	State Advertisement (LSA) in case of OSPFv2 and OSPFv3, is updated at		State Advertisement (LSA) in case of OSPFv2 and OSPFv3, is updated at
	the originator. Such mechanism is typically not specific to any		the originator. Such mechanism is typically not specific to any
	particular link attribute, but rather considers any change in the LSP		particular link attribute, but rather considers any change in the LSP
	or LSA. Usage of such throttling mechanism can also be used to avoid		or LSA. Application of such throttling mechanism can also be used to
	frequent changes in the setting of the Extended Administrative Group		avoid frequent changes in the setting of the Extended Administrative
	on a link to affect the stability of the receivers.		Group on a link to affect the stability of the receivers.
	5. IS-IS Flexible Algorithm Exclude Reverse Admin Group Sub-TLV		5. IS-IS Flexible Algorithm Exclude Reverse Admin Group Sub-TLV
	The IS-IS Flexible Algorithm Exclude Reverse Admin Group (FAERAG)		The IS-IS Flexible Algorithm Exclude Reverse Admin Group (FAERAG)
	Sub-TLV is used to advertise the exclude rule that is used during the		Sub-TLV is used to advertise the exclude rule that is used during the
	Flex-Algorithm path calculation as specified in Section 11.		Flex-Algorithm path calculation as specified in Section 11.
	The IS-IS FAERAG Sub-TLV is a sub-TLV of the IS-IS FAD Sub-TLV. It		The IS-IS FAERAG Sub-TLV is a sub-TLV of the IS-IS FAD Sub-TLV. It
	has the following format:		has the following format:
	skipping to change at line 368 ¶		skipping to change at line 363 ¶
	is set on the link in the reverse direction, the link MUST be		is set on the link in the reverse direction, the link MUST be
	pruned from the computation.		pruned from the computation.
	* Check if any include-all reverse Admin Group rule is part of the		* Check if any include-all reverse Admin Group rule is part of the
	Flex-Algorithm definition. If such include-all rule exists, check		Flex-Algorithm definition. If such include-all rule exists, check
	if all Admin Groups that are part of the include-all rule are also		if all Admin Groups that are part of the include-all rule are also
	set on the link in the reverse direction. If all such Admin		set on the link in the reverse direction. If all such Admin
	Groups are not set on the link in the reverse direction, the link		Groups are not set on the link in the reverse direction, the link
	MUST be pruned from the computation.		MUST be pruned from the computation.
	Please see rules 8, 9, and 10 in Section 13.3 for these added rules.		Please see rules 8, 9, and 10 in Section 12.3 for these added rules.
	12. IGP Flex-Algorithm Path Computation Rules Registry
	Since the original Flex-Algorithm specification [RFC9350], many Flex-
	Algorithm extensions have been proposed and standardized. Many of
	them added additional rules to the Flex-Algorithm path computation.
	Maintaining an IANA registry for these rules allows specification
	across multiple documents progressing independently. The new "IGP
	Flex-Algorithm Path Computation Rules" registry has been created and
	is specified in Section 13.3.
	This section provides the guidance for designated experts on
	evaluating new registrations in the "IGP Flex-Algorithm Path
	Computation Rules" registry:
	1. When a new constraint is defined, the rule associated with that
	constraint MAY be inserted at any position. Backwards
	compatibility is guaranteed because nodes that don't support the
	new constraint will not participate in an algorithm where the FAD
	specifies a constraint they don't support.
	2. The relative ordering of existing rules MUST NOT be altered.
	Doing so has the potential to create backwards compatibility
	issues.
	3. Deletion of the rules MUST NOT be done. Given that the rules are
	only used conditionally based on the information carried in the
	winning FAD, deletion of the rule is not necessary.
	4. Merging or repeating of the rules MUST NOT be done.
	13. IANA Considerations		12. IANA Considerations
	13.1. Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV		12.1. Sub-Sub-TLVs for Flexible Algorithm Definition Sub-TLV
	IANA has registered the following in the "IS-IS Sub-Sub-TLVs for		IANA has registered the following in the "IS-IS Sub-Sub-TLVs for
	Flexible Algorithm Definition Sub-TLV" registry under the "IS-IS TLV		Flexible Algorithm Definition Sub-TLV" registry under the "IS-IS TLV
	Codepoints" registry group:		Codepoints" registry group:
	Type: 10		Type: 10
	Description: Flexible Algorithm Exclude Reverse Admin Group		Description: Flexible Algorithm Exclude Reverse Admin Group
	MP: n		MP: n
	Reference: RFC 9917, Section 5		Reference: RFC 9917, Section 5
	Type: 11		Type: 11
	Description: Flexible Algorithm Include-Any Reverse Admin Group		Description: Flexible Algorithm Include-Any Reverse Admin Group
	MP: n		MP: n
	Reference: RFC 9917, Section 6		Reference: RFC 9917, Section 6
	Type: 12		Type: 12
	Description: Flexible Algorithm Include-All Reverse Admin Group		Description: Flexible Algorithm Include-All Reverse Admin Group
	MP: n		MP: n
	Reference: RFC 9917, Section 7		Reference: RFC 9917, Section 7
	13.2. OSPF Flexible Algorithm Definition TLV Sub-TLV Registry		12.2. OSPF Flexible Algorithm Definition TLV Sub-TLV Registry
	This document makes the following registration in the "OSPF Flexible		This document makes the following registration in the "OSPF Flexible
	Algorithm Definition TLV Sub-TLVs" registry under the "Open Shortest		Algorithm Definition TLV Sub-TLVs" registry under the "Open Shortest
	Path First (OSPF) Parameters" registry group:		Path First (OSPF) Parameters" registry group:
	Type: 10		Type: 10
	Description: Flexible Algorithm Exclude Reverse Admin Group		Description: Flexible Algorithm Exclude Reverse Admin Group
	Reference: RFC 9917, Section 8		Reference: RFC 9917, Section 8
	Type: 11		Type: 11
	Description: Flexible Algorithm Include-Any Reverse Admin Group		Description: Flexible Algorithm Include-Any Reverse Admin Group
	Reference: RFC 9917, Section 9		Reference: RFC 9917, Section 9
	Type: 12		Type: 12
	Description: Flexible Algorithm Include-All Reverse Admin Group		Description: Flexible Algorithm Include-All Reverse Admin Group
	Reference: RFC 9917, Section 10		Reference: RFC 9917, Section 10
	13.3. IGP Flex-Algorithm Path Computation Rules Registry		12.3. IGP Flex-Algorithm Path Computation Rules Registry
	IANA has created a new registry called "IGP Flex-Algorithm Path		IANA has created a new registry called "IGP Flex-Algorithm Path
	Computation Rules" within the "Interior Gateway Protocol (IGP)		Computation Rules" within the "Interior Gateway Protocol (IGP)
	Parameters" registry group. The registration procedure for the new		Parameters" registry group. The registration procedure for the new
	registry is Expert Review [RFC8126]. Section 12 provides guidance		registry is Expert Review [RFC8126]. Section 12.3.1 provides
	for designated experts.		guidance for designated experts.
	The "IGP Flex-Algorithm Path Computation Rules" registry specifies		The "IGP Flex-Algorithm Path Computation Rules" registry specifies
	the ordered set of rules that are used to prune links from the		the ordered set of rules that are used to prune links from the
	topology during the Flex-Algorithm path computation.		topology during the Flex-Algorithm path computation.
	There is no upper bound on the number of rules that the registry		There is no upper bound on the number of rules that the registry
	supports.		supports.
			In Table 1, "FAEMB" means "Flex-Algorithm Exclude Minimum Bandwidth",
			and "FAEMD" means "Flex-Algorithm Exclude Maximum Delay".
	+==========+===========================================+===========+		+==========+===========================================+===========+
	| Sequence | Description | Reference |		| Sequence | Description | Reference |
	| Number | | |		| Number | | |
	+==========+===========================================+===========+		+==========+===========================================+===========+
	| 1 | Check if any exclude Administrative Group | Section |		| 1 | Check if any exclude Administrative Group | Section |
	| | rule is part of the Flex-Algorithm | 13 of |		| | rule is part of the Flex-Algorithm | 13 of |
	| | Definition. If such exclude rule exists, | [RFC9350] |		| | Definition. If such exclude rule exists, | [RFC9350] |
	| | check if any color that is part of the | |		| | check if any color that is part of the | |
	| | exclude rule is also set on the link. If | |		| | exclude rule is also set on the link. If | |
	| | such a color is set, the link MUST be | |		| | such a color is set, the link MUST be | |
	skipping to change at line 553 ¶		skipping to change at line 521 ¶
	| | exists, check if all Admin Groups that | |		| | exists, check if all Admin Groups that | |
	| | are part of the include-all rule are also | |		| | are part of the include-all rule are also | |
	| | set on the link in the reverse direction. | |		| | set on the link in the reverse direction. | |
	| | If all such Admin Groups are not set on | |		| | If all such Admin Groups are not set on | |
	| | the link in the reverse direction, the | |		| | the link in the reverse direction, the | |
	| | link MUST be pruned from the computation. | |		| | link MUST be pruned from the computation. | |
	+----------+-------------------------------------------+-----------+		+----------+-------------------------------------------+-----------+
	Table 1: IGP Flex-Algorithm Path Calculation Rules		Table 1: IGP Flex-Algorithm Path Calculation Rules
	14. Security Considerations		12.3.1. Guidance for Designated Experts
			Since the original Flex-Algorithm specification [RFC9350], many Flex-
			Algorithm extensions have been proposed and standardized. Many of
			them added additional rules to the Flex-Algorithm path computation.
			Maintaining an IANA registry for these rules allows specification
			across multiple documents progressing independently. The new "IGP
			Flex-Algorithm Path Computation Rules" registry has been created and
			is specified in Section 12.3.
			This section provides the guidance for designated experts on
			evaluating new registrations in the "IGP Flex-Algorithm Path
			Computation Rules" registry:
			1. When a new constraint is defined, the rule associated with that
			constraint MAY be inserted at any position. Backwards
			compatibility is guaranteed because nodes that don't support the
			new constraint will not participate in an algorithm where the FAD
			specifies a constraint they don't support.
			2. The relative ordering of existing rules MUST NOT be altered.
			Doing so has the potential to create backwards compatibility
			issues.
			3. Deletion of the rules MUST NOT be done. Given that the rules are
			only used conditionally based on the information carried in the
			winning FAD, deletion of the rule is not necessary.
			4. Merging or repeating of the rules MUST NOT be done.
			13. Security Considerations
	This document inherits security considerations from [RFC9350].		This document inherits security considerations from [RFC9350].
	15. Normative References		14. References
			14.1. Normative References
	[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate		[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
	Requirement Levels", BCP 14, RFC 2119,		Requirement Levels", BCP 14, RFC 2119,
	DOI 10.17487/RFC2119, March 1997,		DOI 10.17487/RFC2119, March 1997,
	<https://www.rfc-editor.org/info/rfc2119>.		<https://www.rfc-editor.org/info/rfc2119>.
	[RFC7308] Osborne, E., "Extended Administrative Groups in MPLS		[RFC7308] Osborne, E., "Extended Administrative Groups in MPLS
	Traffic Engineering (MPLS-TE)", RFC 7308,		Traffic Engineering (MPLS-TE)", RFC 7308,
	DOI 10.17487/RFC7308, July 2014,		DOI 10.17487/RFC7308, July 2014,
	<https://www.rfc-editor.org/info/rfc7308>.		<https://www.rfc-editor.org/info/rfc7308>.
	skipping to change at line 583 ¶		skipping to change at line 583 ¶
	[RFC9350] Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K.,		[RFC9350] Psenak, P., Ed., Hegde, S., Filsfils, C., Talaulikar, K.,
	and A. Gulko, "IGP Flexible Algorithm", RFC 9350,		and A. Gulko, "IGP Flexible Algorithm", RFC 9350,
	DOI 10.17487/RFC9350, February 2023,		DOI 10.17487/RFC9350, February 2023,
	<https://www.rfc-editor.org/info/rfc9350>.		<https://www.rfc-editor.org/info/rfc9350>.
	[RFC9843] Hegde, S., Britto, W., Shetty, R., Decraene, B., Psenak,		[RFC9843] Hegde, S., Britto, W., Shetty, R., Decraene, B., Psenak,
	P., and T. Li, "IGP Flexible Algorithms: Bandwidth, Delay,		P., and T. Li, "IGP Flexible Algorithms: Bandwidth, Delay,
	Metrics, and Constraints", RFC 9843, DOI 10.17487/RFC9843,		Metrics, and Constraints", RFC 9843, DOI 10.17487/RFC9843,
	September 2025, <https://www.rfc-editor.org/info/rfc9843>.		September 2025, <https://www.rfc-editor.org/info/rfc9843>.
	16. Informative References		14.2. Informative References
	[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for		[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
	Writing an IANA Considerations Section in RFCs", BCP 26,		Writing an IANA Considerations Section in RFCs", BCP 26,
	RFC 8126, DOI 10.17487/RFC8126, June 2017,		RFC 8126, DOI 10.17487/RFC8126, June 2017,
	<https://www.rfc-editor.org/info/rfc8126>.		<https://www.rfc-editor.org/info/rfc8126>.
	Acknowledgments
	TBD
	Authors' Addresses		Authors' Addresses
	Peter Psenak		Peter Psenak
	Cisco Systems		Cisco Systems
	Apollo Business Center		Apollo Business Center
	Mlynske nivy 43		Mlynske nivy 43
	82109 Bratislava		82109 Bratislava
	Slovakia		Slovakia
	Email: ppsenak@cisco.com		Email: ppsenak@cisco.com
End of changes. 16 change blocks.
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