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.
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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.
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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
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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
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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.
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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
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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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