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Versions: (draft-tantsura-isis-segment-routing-msd)
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12 13 14 15 16 17 18 19 RFC 8491
IS-IS Working Group J. Tantsura
Internet-Draft Nuage Networks
Intended status: Standards Track U. Chunduri
Expires: November 11, 2018 Huawei Technologies
S. Aldrin
Google, Inc
L. Ginsberg
Cisco Systems
May 10, 2018
Signaling MSD (Maximum SID Depth) using IS-IS
draft-ietf-isis-segment-routing-msd-11
Abstract
This document defines a way for an IS-IS Router to advertise multiple
types of supported Maximum SID Depths (MSDs) at node and/or link
granularity. Such advertisements allow entities (e.g., centralized
controllers) to determine whether a particular SID stack can be
supported in a given network. This document only defines one type of
MSD maximum label imposition, but defines an encoding that can
support other MSD types.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 11, 2018.
Copyright Notice
Copyright (c) 2018 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions used in this document . . . . . . . . . . . . 3
1.1.1. Terminology . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Node MSD Advertisement . . . . . . . . . . . . . . . . . . . 4
3. Link MSD Advertisement . . . . . . . . . . . . . . . . . . . 4
4. Using Node and Link MSD Advertisements . . . . . . . . . . . 5
5. Base MPLS Imposition MSD . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 7
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
When Segment Routing(SR) paths are computed by a centralized
controller, it is critical that the controller learns the Maximum SID
Depth(MSD) that can be imposed at each node/link a given SR path to
insure that the SID stack depth of a computed path doesn't exceed the
number of SIDs the node is capable of imposing.
PCEP SR extensions draft [I-D.ietf-pce-segment-routing] signals MSD
in SR PCE Capability TLV and METRIC Object. However, if PCEP is not
supported/configured on the head-end of an SR tunnel or a Binding-SID
anchor node and controller does not participate in IGP routing, it
has no way to learn the MSD of nodes and links. BGP-LS [RFC7752]
defines a way to expose topology and associated attributes and
capabilities of the nodes in that topology to a centralized
controller. MSD signaling by BGP-LS has been defined in
[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
head-ends. In order for BGP-LS to signal MSD for all the nodes and
links in the network MSD is relevant, MSD capabilites should be
advertised by every IS-IS router in the network.
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Other types of MSD are known to be useful. For example,
[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
a depth, that could be read by transit nodes.
This document defines an extension to IS-IS used to advertise one or
more types of MSD at node and/or link granularity. It also creates
an IANA registry for assigning MSD type identifiers. It also defines
the Base MPLS Imposition MSD type. In the future it is expected,
that new MSD types will be defined to signal additional capabilities
e.g., entropy labels, SIDs that can be imposed through recirculation,
or SIDs associated with another dataplane e.g., IPv6. Although MSD
advertisements are associated with Segment Routing, the
advertisements MAY be present even if Segment Routing itself is not
enabled.
1.1. Conventions used in this document
1.1.1. Terminology
BGP-LS: Distribution of Link-State and TE Information using Border
Gateway Protocol
BMI: Base MPLS Imposition is the number of MPLS labels which can be
imposed inclusive of all service/transport/special labels
IS-IS: Intermediate System to Intermediate System
MSD: Maximum SID Depth - the number of SIDs a node or a link on a
node can support
PCC: Path Computation Client
PCE: Path Computation Element
PCEP: Path Computation Element Protocol
SID: Segment Identifier
SR: Segment Routing
1.2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here .
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2. Node MSD Advertisement
The node MSD sub-TLV is defined within the body of the IS-IS Router
Capability TLV [RFC7981], to carry the provisioned SID depth of the
router originating the Router Capability TLV. Node MSD is the
smallest MSD supported by the node on the set of interfaces
configured for use by the advertising IGP instance. MSD values may
be learned via a hardware API or may be provisioned.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSD-Type | MSD Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// ................... //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSD-Type | MSD Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Node MSD Sub-TLV
Type: 23 (allocated by IANA via the early assignment process)
Length: variable (minimum of 2, multiple of 2 octets) and represents
the total length of value field.
Value: field consists of one or more pairs of a 1 octet MSD-Type
(IANA Registry) and 1 octet Value.
Node MSD value is a number in the range of 0-255. 0 represents lack
of the ability to support SID stack of any depth; any other value
represents that of the node. This value MUST represent the lowest
value supported by any link configured for use by the advertising IS-
IS instance.
This sub-TLV is optional. The scope of the advertisement is specific
to the deployment.
3. Link MSD Advertisement
The link MSD sub-TLV is defined for TLVs 22, 23, 25, 141, 222, and
223 to carry the MSD of the interface associated with the link. MSD
values may be learned via a hardware API or may be provisioned.
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0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSD-Type | MSD Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
// ................... //
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSD-Type | MSD Value |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Link MSD Sub-TLV
Type: 15 (allocated by IANA via the early assignment process)
Length: variable (minimum of 2, multiple of 2 octets) and represents
the total length of value field.
Value: consists of one or more pairs of a 1 octet MSD-Type (IANA
Registry) and 1 octet Value.
Link MSD value is a number in the range of 0-255. 0 represents lack
of the ability to support SID stack of any depth; any other value
represents that of the link when used as an outgoing link.
This sub-TLV is optional. The scope of the advertisement is specific
to the deployment.
4. Using Node and Link MSD Advertisements
When Link MSD is present for a given MSD type, the value of the Link
MSD MUST take preference over the Node MSD. When a Link MSD type is
not signalled but the Node MSD type is, then the Node MSD type value
MUST be considered as the MSD value for that link.
In order to increase flooding efficiency, it is RECOMMENDED that
routers with homogenous link MSD values advertise just the Node MSD
value.
The meaning of the absence of both Node and Link MSD advertisements
for a given MSD type is specific to the MSD type. Generally it can
only be inferred that the advertising node does not support
advertisement of that MSD type. However, in some cases the lack of
advertisement might imply that the functionality associated with the
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MSD type is not supported. The correct interpretation MUST be
specified when an MSD type is defined.
5. Base MPLS Imposition MSD
Base MPLS Imposition MSD (BMI-MSD) signals the total number of MPLS
labels a node is capable of imposing, including all
service/transport/special labels.
Absence of BMI-MSD advertisements indicates solely that the
advertising node does not support advertisement of this capability.
6. IANA Considerations
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
Capability TLV Registry as defined by [RFC7981].
IANA has allocated the following value through the early assignment
process:
Value Description Reference
----- --------------- -------------
23 Node MSD This document
Figure 3: Node MSD
This document requests IANA to allocate a sub-TLV type as defined in
Section 3 from Sub-TLVs for TLVs 22, 23, 25, 141, 222 and 223
registry.
IANA has allocated the following value through the early assignment
process:
Value Description Reference
----- --------------- -------------
15 Link MSD This document
Figure 4: Link MSD
Per TLV information where Link MSD sub-TLV can be part of:
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TLV 22 23 25 141 222 223
--- --------------------
y y y y y y
Figure 5: TLVs where LINK MSD Sub-TLV can be present
This document requests creation of an IANA managed registry under a
new category of "Interior Gateway Protocol (IGP) Parameters" IANA
registries to identify MSD types as proposed in Section 2 and
Section 3. The registration procedure is "Expert Review" as defined
in [RFC8126]. Suggested registry name is "MSD types". Types are an
unsigned 8 bit number. The following values are defined by this
document
Value Name Reference
----- --------------------- -------------
0 Reserved This document
1 Base MPLS Imposition MSD This document
2-250 Unassigned This document
251-254 Experimental This document
255 Reserved This document
Figure 6: MSD Types Codepoints Registry
7. Security Considerations
Security considerations as specified by [RFC7981] are applicable to
this document.
Advertisement of the additional information defined in this document
that is false, e.g., an MSD that is incorrect, may result in a path
computation failing, having a service unavailable, or instantiation
of a path that can't be supported by the head-end (the node
performing the imposition).
8. Contributors
The following people contributed to this document:
Peter Psenak
Email: ppsenak@cisco.com
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9. Acknowledgements
The authors would like to thank Acee Lindem, Ketan Talaulikar,
Stephane Litkowski and Bruno Decraene for their reviews and valuable
comments.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7981] Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions
for Advertising Router Information", RFC 7981,
DOI 10.17487/RFC7981, October 2016,
<https://www.rfc-editor.org/info/rfc7981>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
10.2. Informative References
[I-D.ietf-idr-bgp-ls-segment-routing-msd]
Tantsura, J., Chunduri, U., Mirsky, G., and S. Sivabalan,
"Signaling Maximum SID Depth using Border Gateway Protocol
Link-State", draft-ietf-idr-bgp-ls-segment-routing-msd-01
(work in progress), October 2017.
[I-D.ietf-isis-mpls-elc]
Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
Litkowski, "Signaling Entropy Label Capability and
Readable Label-stack Depth Using IS-IS", draft-ietf-isis-
mpls-elc-03 (work in progress), January 2018.
[I-D.ietf-pce-segment-routing]
Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
and J. Hardwick, "PCEP Extensions for Segment Routing",
draft-ietf-pce-segment-routing-11 (work in progress),
November 2017.
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[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016,
<https://www.rfc-editor.org/info/rfc7752>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
Authors' Addresses
Jeff Tantsura
Nuage Networks
Email: jefftant.ietf@gmail.com
Uma Chunduri
Huawei Technologies
Email: uma.chunduri@huawei.com
Sam Aldrin
Google, Inc
Email: aldrin.ietf@gmail.com
Les Ginsberg
Cisco Systems
Email: ginsberg@cisco.com
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