< draft-ietf-isis-segment-routing-msd-17.txt		draft-ietf-isis-segment-routing-msd-18.txt >
	IS-IS Working Group J. Tantsura		IS-IS Working Group J. Tantsura
	Internet-Draft Nuage Networks		Internet-Draft Nuage Networks
	Intended status: Standards Track U. Chunduri		Intended status: Standards Track U. Chunduri
	Expires: March 30, 2019 Huawei Technologies		Expires: April 7, 2019 Huawei Technologies
	S. Aldrin		S. Aldrin
	Google, Inc		Google, Inc
	L. Ginsberg		L. Ginsberg
	Cisco Systems		Cisco Systems
	September 26, 2018		October 4, 2018
	Signaling MSD (Maximum SID Depth) using IS-IS		Signaling MSD (Maximum SID Depth) using IS-IS
	draft-ietf-isis-segment-routing-msd-17		draft-ietf-isis-segment-routing-msd-18
	Abstract		Abstract
	This document defines a way for an Intermediate System to		This document defines a way for an Intermediate System to
	Intermediate System (IS-IS) router to advertise multiple types of		Intermediate System (IS-IS) router to advertise multiple types of
	supported Maximum SID Depths (MSDs) at node and/or link granularity.		supported Maximum SID Depths (MSDs) at node and/or link granularity.
	Such advertisements allow entities (e.g., centralized controllers) to		Such advertisements allow entities (e.g., centralized controllers) to
	determine whether a particular SID stack can be supported in a given		determine whether a particular SID (Segment ID) stack can be
	network. This document only defines one type of MSD (Base MPLS		supported in a given network. This document only defines one type of
	Imposition), but defines an encoding that can support other MSD		MSD (Base MPLS Imposition), but defines an encoding that can support
	types. This document focuses on MSD use in a Segment Routing enabled		other MSD types. This document focuses on MSD use in a Segment
	network, but MSD may also be useful when Segment Routing is not		Routing enabled network, but MSD may also be useful when Segment
	enabled.		Routing is not enabled.
	Status of This Memo		Status of This Memo
	This Internet-Draft is submitted in full conformance with the		This Internet-Draft is submitted in full conformance with the
	provisions of BCP 78 and BCP 79.		provisions of BCP 78 and BCP 79.
	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 https://datatracker.ietf.org/drafts/current/.		Drafts is at https://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 March 30, 2019.		This Internet-Draft will expire on April 7, 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
	(https://trustee.ietf.org/license-info) in effect on the date of		(https://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. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3		1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
	1.2. Requirements Language . . . . . . . . . . . . . . . . . . 3		1.2. Requirements Language . . . . . . . . . . . . . . . . . . 4
	2. Node MSD Advertisement . . . . . . . . . . . . . . . . . . . 4		2. Node MSD Advertisement . . . . . . . . . . . . . . . . . . . 4
	3. Link MSD Advertisement . . . . . . . . . . . . . . . . . . . 5		3. Link MSD Advertisement . . . . . . . . . . . . . . . . . . . 5
	4. Procedures for using Node and Link MSD Advertisements . . . . 6		4. Procedures for Defining and Using Node and Link MSD
			Advertisements . . . . . . . . . . . . . . . . . . . . . . . 6
	5. Base MPLS Imposition MSD . . . . . . . . . . . . . . . . . . 6		5. Base MPLS Imposition MSD . . . . . . . . . . . . . . . . . . 6
	6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6		6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
	7. Security Considerations . . . . . . . . . . . . . . . . . . . 8		7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
	8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 8		8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 8
	9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8		9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
	10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8		10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
	10.1. Normative References . . . . . . . . . . . . . . . . . . 8		10.1. Normative References . . . . . . . . . . . . . . . . . . 9
	10.2. Informative References . . . . . . . . . . . . . . . . . 9		10.2. Informative References . . . . . . . . . . . . . . . . . 9
	Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9		Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
	1. Introduction		1. Introduction
	When Segment Routing (SR) paths are computed by a centralized		When Segment Routing (SR) paths are computed by a centralized
	controller, it is critical that the controller learns the Maximum SID		controller, it is critical that the controller learns the Maximum SID
	Depth (MSD) that can be imposed at each node/link of a given SR path		Depth (MSD) that can be imposed at each node/link of a given SR path
	to ensure that the Segment Identifier (SID) stack depth of a computed		to ensure that the Segment Identifier (SID) stack depth of a computed
	path does not exceed the number of SIDs the node is capable of		path does not exceed the number of SIDs the node is capable of
	imposing.		imposing.
	[I-D.ietf-pce-segment-routing] defines how to signal MSD in the Path		[I-D.ietf-pce-segment-routing] defines how to signal MSD in the Path
	Computation Element Protocol (PCEP). However, if PCEP is not		Computation Element communication Protocol (PCEP). However, if PCEP
	supported/configured on the head-end of an SR tunnel or a Binding-SID		is not supported/configured on the head-end of an SR tunnel or a
	anchor node and controller does not participate in IGP routing, it		Binding-SID anchor node and controller does not participate in IGP
	has no way to learn the MSD of nodes and links. BGP-LS (Distribution		routing, it has no way to learn the MSD of nodes and links. BGP-LS
	of Link-State and TE Information using Border Gateway Protocol)		(Distribution of Link-State and TE Information using Border Gateway
			Protocol) [RFC7752] defines a way to expose topology and associated
	[RFC7752] defines a way to expose topology and associated attributes		attributes and capabilities of the nodes in that topology to a
	and capabilities of the nodes in that topology to a centralized		centralized controller. MSD signaling by BGP-LS has been defined in
	controller. MSD signaling by BGP-LS has been defined in
	[I-D.ietf-idr-bgp-ls-segment-routing-msd]. Typically, BGP-LS is		[I-D.ietf-idr-bgp-ls-segment-routing-msd]. Typically, BGP-LS is
	configured on a small number of nodes that do not necessarily act as		configured on a small number of nodes that do not necessarily act as
	head-ends. In order for BGP-LS to signal MSD for all the nodes and		head-ends. In order for BGP-LS to signal MSD for all the nodes and
	links in the network MSD is relevant, MSD capabilities SHOULD be		links in the network MSD is relevant, MSD capabilities SHOULD be
	advertised by every Intermediate System to Intermediate System(IS-IS)		advertised by every Intermediate System to Intermediate System(IS-IS)
	router in the network.		router in the network.
	Other types of MSD are known to be useful. For example,		Other types of MSD are known to be useful. For example,
	[I-D.ietf-isis-mpls-elc] defines Readable Label Depth Capability		[I-D.ietf-isis-mpls-elc] defines Readable Label Depth Capability
	(RLDC) that is used by a head-end to insert an Entropy Label (EL) at		(RLDC) that is used by a head-end to insert an Entropy Label (EL) at
	skipping to change at page 3, line 42 ¶		skipping to change at page 3, line 42 ¶
	1.1. Terminology		1.1. Terminology
	BMI: Base MPLS Imposition is the number of MPLS labels which can be		BMI: Base MPLS Imposition is the number of MPLS labels which can be
	imposed inclusive of all service/transport/special labels		imposed inclusive of all service/transport/special labels
	MSD: Maximum SID Depth - the number of SIDs supported by a node or a		MSD: Maximum SID Depth - the number of SIDs supported by a node or a
	link on a node		link on a node
	SID: Segment Identifier as defined in [RFC8402]		SID: Segment Identifier as defined in [RFC8402]
			Label Imposition: Imposition is the act of modifying and/or adding
			labels to the outgoing label stack associated with a packet. This
			includes:
			o replacing the label at the top of the label stack with a new label
			o pushing one or more new labels onto the label stack
			The number of labels imposed is then the sum of the number of labels
			which are replaced and the number of labels which are pushed. See
			[RFC3031] for further details.
	1.2. Requirements Language		1.2. 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 BCP		"OPTIONAL" in this document are to be interpreted as described in BCP
	14 [RFC2119] [RFC8174] when, and only when, they appear in all		14 [RFC2119] [RFC8174] when, and only when, they appear in all
	capitals, as shown here .		capitals, as shown here .
	2. Node MSD Advertisement		2. Node MSD Advertisement
	skipping to change at page 5, line 41 ¶		skipping to change at page 6, line 7 ¶
	length of value field.		length of value field.
	Value: consists of one or more pairs of a 1 octet MSD-Type and 1		Value: consists of one or more pairs of a 1 octet MSD-Type and 1
	octet MSD-Value.		octet MSD-Value.
	MSD-Type is a value defined in the MSD-Types registry created by the		MSD-Type is a value defined in the MSD-Types registry created by the
	IANA Section of this document.		IANA Section of this document.
	MSD-Value is a number in the range of 0-255. For all MSD-Types, 0		MSD-Value is a number in the range of 0-255. For all MSD-Types, 0
	represents lack of the ability to support SID stack of any depth; any		represents lack of the ability to support SID stack of any depth; any
	other value represents that of the link.		other value represents that of the particular link when used as an
			outgoing interface.
	This sub-TLV is optional.		This sub-TLV is optional.
	If multiple Link MSD advertisements for the same MSD-Type and the		If multiple Link MSD advertisements for the same MSD-Type and the
	same link are received, the procedure used to select which copy is		same link are received, the procedure used to select which copy is
	used is undefined.		used is undefined.
	4. Procedures for using Node and Link MSD Advertisements		If the advertising router performs label imposition in the context of
			the ingress interface, it is not possible to meaningfully advertise
			per link values. In such a case only the Node MSD SHOULD be
			advertised.
			4. Procedures for Defining and Using Node and Link MSD Advertisements
	When Link MSD is present for a given MSD-type, the value of the Link		When Link MSD is present for a given MSD-type, the value of the Link
	MSD MUST take precedence over the Node MSD. When a Link MSD-type is		MSD MUST take precedence over the Node MSD. When a Link MSD-type is
	not signaled but the Node MSD-type is, then the Node MSD-type value		not signaled but the Node MSD-type is, then the Node MSD-type value
	MUST be considered as the MSD value for that link.		MUST be considered as the MSD value for that link.
	In order to increase flooding efficiency, it is RECOMMENDED that		In order to increase flooding efficiency, it is RECOMMENDED that
	routers with homogenous link MSD values advertise just the Node MSD		routers with homogenous link MSD values advertise just the Node MSD
	value.		value.
	skipping to change at page 6, line 28 ¶		skipping to change at page 6, line 44 ¶
	only be inferred that the advertising node does not support		only be inferred that the advertising node does not support
	advertisement of that MSD-type. However, in some cases the lack of		advertisement of that MSD-type. However, in some cases the lack of
	advertisement might imply that the functionality associated with the		advertisement might imply that the functionality associated with the
	MSD-type is not supported. The correct interpretation MUST be		MSD-type is not supported. The correct interpretation MUST be
	specified when an MSD-type is defined.		specified when an MSD-type is defined.
	5. Base MPLS Imposition MSD		5. Base MPLS Imposition MSD
	Base MPLS Imposition MSD (BMI-MSD) signals the total number of MPLS		Base MPLS Imposition MSD (BMI-MSD) signals the total number of MPLS
	labels which can be imposed, including all service/transport/special		labels which can be imposed, including all service/transport/special
	labels. The value advertised MUST indicate what can be imposed under		labels.
	all conditions e.g., if label popping/swapping affects the number of
	labels which can be imposed this MUST be accounted for in the value
	which is advertised.
	If the advertising router performs label imposition in the context of
	the ingress interface, it is not possible to meaningfully advertise
	per link values. In such a case only the Node MSD SHOULD be
	advertised.
	Absence of BMI-MSD advertisements indicates solely that the		Absence of BMI-MSD advertisements indicates solely that the
	advertising node does not support advertisement of this capability.		advertising node does not support advertisement of this capability.
	6. IANA Considerations		6. IANA Considerations
	This document requests IANA to allocate a sub-TLV type for the new		This document requests IANA to allocate a sub-TLV type for the new
	sub TLV proposed in Section 2 of this document from IS-IS Router		sub TLV proposed in Section 2 of this document from IS-IS Router
	Capability TLV Registry as defined by [RFC7981].		Capability TLV Registry as defined by [RFC7981].
	skipping to change at page 8, line 13 ¶		skipping to change at page 8, line 23 ¶
	Figure 6: MSD-Types Codepoints Registry		Figure 6: MSD-Types Codepoints Registry
	General guidance for the Designated Experts is as defined in		General guidance for the Designated Experts is as defined in
	[RFC7370]		[RFC7370]
	7. Security Considerations		7. Security Considerations
	Security considerations as specified by [RFC7981] are applicable to		Security considerations as specified by [RFC7981] are applicable to
	this document.		this document.
	Advertisement of the additional information defined in this document		Advertisement of an incorrect MSD value may have negative
	that is false, e.g., an MSD that is incorrect, may result in a path		consequences. If the value is smaller than supported, path
	computation failing, having a service unavailable, or calculation of		computation may fail to compute a viable path. If the value is
	a path that cannot be supported by the head-end (the node performing		larger than supported, an attempt to instantiate a path that can't be
	the imposition).		supported by the head-end (the node performing the SID imposition)
			may occur.
	The presence of this information also may inform an attacker of how		The presence of this information also may inform an attacker of how
	to induce any of the aforementioned conditions.		to induce any of the aforementioned conditions.
	8. Contributors		8. Contributors
	The following people contributed to this document:		The following people contributed to this document:
	Peter Psenak		Peter Psenak
	skipping to change at page 8, line 37 ¶		skipping to change at page 9, line 4 ¶
	Email: ppsenak@cisco.com		Email: ppsenak@cisco.com
	9. Acknowledgements		9. Acknowledgements
	The authors would like to thank Acee Lindem, Ketan Talaulikar,		The authors would like to thank Acee Lindem, Ketan Talaulikar,
	Stephane Litkowski and Bruno Decraene for their reviews and valuable		Stephane Litkowski and Bruno Decraene for their reviews and valuable
	comments.		comments.
	10. References		10. References
	10.1. Normative References		10.1. 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>.
			[RFC3031] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
			Label Switching Architecture", RFC 3031,
			DOI 10.17487/RFC3031, January 2001,
			<https://www.rfc-editor.org/info/rfc3031>.
	[RFC7370] Ginsberg, L., "Updates to the IS-IS TLV Codepoints		[RFC7370] Ginsberg, L., "Updates to the IS-IS TLV Codepoints
	Registry", RFC 7370, DOI 10.17487/RFC7370, September 2014,		Registry", RFC 7370, DOI 10.17487/RFC7370, September 2014,
	<https://www.rfc-editor.org/info/rfc7370>.		<https://www.rfc-editor.org/info/rfc7370>.
	[RFC7981] Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions		[RFC7981] Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions
	for Advertising Router Information", RFC 7981,		for Advertising Router Information", RFC 7981,
	DOI 10.17487/RFC7981, October 2016,		DOI 10.17487/RFC7981, October 2016,
	<https://www.rfc-editor.org/info/rfc7981>.		<https://www.rfc-editor.org/info/rfc7981>.
	[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for		[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
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