IS-IS Working Group J. Tantsura
Internet-Draft Individual
Intended status: Standards Track U. Chunduri
Expires: July 14, 2018 Huawei Technologies
S. Aldrin
Google, Inc
L. Ginsberg
Cisco Systems
January 10, 2018
Signaling MSD (Maximum SID Depth) using IS-IS
draft-ietf-isis-segment-routing-msd-09
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 is
supportable in a given network. This document only defines one type
of MSD (maximum label imposition) - but defines an encoding which can
support other MSD types.
Status of This Memo
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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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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 . . . . . . . . . . . . . . . . . . . . . . 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
10.1. Normative References . . . . . . . . . . . . . . . . . . 7
10.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
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) which can be imposed at the node/link a given SR path is
applied so as 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 a 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 which has been
configured. 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
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capabilites should be advertised to every IS-IS router in the
network.
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 Entropy Label (EL) at
appropriate depth, so it 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
one MSD type called Base MPLS Imposition MSD. 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.
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 any service/transport 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", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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2. Node MSD Advertisement
A new sub-TLV "Node MSD sub-TLV" is defined within the body of the
IS-IS Router Capability TLV [RFC7981], to carry the provisioned
MSD(s) of the router originating the Router Capability TLV. Node MSD
is the lowest MSD supported by the node on any interface. 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
The Type (1 byte) of this sub-TLV has value of 23.
Length is 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 associated with the node.
This sub-TLV is optional. The scope of the advertisement is specific
to the deployment.
3. Link MSD Advertisement
A new sub-TLV - Link MSD sub-TLV is defined for TLVs 22, 23, 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
The Type (1 byte) of this sub-TLV has value of 15.
Length is 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.
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 be used in preference to the Node MSD.
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
MSD type is not supported. The correct interpretation MUST be
specified when an MSD type is defined.
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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 any service/transport
labels.
Absence of BMI-MSD advertisements indicates only that the advertising
node does not support advertisement of this capability.
6. IANA Considerations
This document requests IANA to allocate a sub-TLV type code for the
new sub TLV proposed in Section 2 of this document from IS-IS Router
Capability TLV Registry as defined by [RFC7981].
The following value has been allocated by IANA:
Value Description Reference
----- --------------- -------------
23 Node MSD This document
Figure 3: Node MSD
This document requests IANA to allocate a sub-TLV type code as
defined in Section 3 from Sub-TLVs for TLVs 22, 23, 141, 222 and 223
registry.
The following value has been allocated by IANA:
Value Description Reference
----- --------------- -------------
15 Link MSD This document
Figure 4: Link MSD
Per TLV information where Link MSD sub-TLV can be part of:
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 a new IANA managed registry under
a new category of "Interior Gateway Protocol (IGP) Parameters" IANA
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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
8. Contributors
The following people contributed to this document:
Peter Psenak
Email: ppsenak@cisco.com
9. Acknowledgements
The authors would like to thank 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>.
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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.
[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
Individual
Email: jefftant.ietf@gmail.com
Uma Chunduri
Huawei Technologies
Email: uma.chunduri@huawei.com
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Sam Aldrin
Google, Inc
Email: aldrin.ietf@gmail.com
Les Ginsberg
Cisco Systems
Email: ginsberg@cisco.com
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