Internet DRAFT - draft-ietf-isis-segment-routing-msd
draft-ietf-isis-segment-routing-msd
IS-IS Working Group J. Tantsura
Internet-Draft Apstra, Inc.
Intended status: Standards Track U. Chunduri
Expires: April 12, 2019 Huawei Technologies
S. Aldrin
Google, Inc
L. Ginsberg
Cisco Systems
October 9, 2018
Signaling MSD (Maximum SID Depth) using IS-IS
draft-ietf-isis-segment-routing-msd-19
Abstract
This document defines a way for an Intermediate System to
Intermediate System (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 (Segment ID) stack can be
supported in a given network. This document only defines one type of
MSD (Base MPLS Imposition), but defines an encoding that can support
other MSD types. This document focuses on MSD use in a Segment
Routing enabled network, but MSD may also be useful when Segment
Routing is not enabled.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 12, 2019.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Node MSD Advertisement . . . . . . . . . . . . . . . . . . . 4
3. Link MSD Advertisement . . . . . . . . . . . . . . . . . . . 5
4. Procedures for Defining and Using Node and Link MSD
Advertisements . . . . . . . . . . . . . . . . . . . . . . . 6
5. Base MPLS Imposition MSD . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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 of a given SR path
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
imposing.
[I-D.ietf-pce-segment-routing] defines how to signal MSD in the Path
Computation Element communication Protocol (PCEP). 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
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(Distribution of Link-State and TE Information using Border Gateway
Protocol) [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 capabilities SHOULD be
advertised by every Intermediate System to Intermediate System(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 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.
MSD advertisements MAY be useful even if Segment Routing itself is
not enabled. For example, in a non-SR MPLS network, MSD defines the
maximum label depth.
1.1. Terminology
BMI: Base MPLS Imposition is the number of MPLS labels which can be
imposed inclusive of all service/transport/special labels
MSD: Maximum SID Depth - the number of SIDs supported by a node or a
link on a node
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
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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
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 .
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 (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 and
1 octet MSD-Value.
MSD-Type is a value defined in the IGP MSD-Types registry created by
the IANA Section of this document.
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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
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.
If there exist multiple Node MSD advertisements for the same MSD-Type
originated by the same router, the procedures defined in [RFC7981]
apply. These procedures may result in different MSD values being
used by (for example) different controllers - but this does not
create any interoperability issue.
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 signaled by the forwarding plane 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 2: Link MSD Sub-TLV
Type: 15 (allocated by IANA via the early assignment process)
Length: variable (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 and 1
octet MSD-Value.
MSD-Type is a value defined in the MSD-Types registry created by the
IANA Section of this document.
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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
other value represents that of the particular link when used as an
outgoing interface.
This sub-TLV is optional.
If multiple Link MSD advertisements for the same MSD-Type and the
same link are received, the procedure used to select which copy is
used is undefined.
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
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
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
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 which can be imposed, including all service/transport/special
labels.
Absence of BMI-MSD advertisements indicates solely that the
advertising node does not support advertisement of this capability.
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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:
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 the
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 "IGP MSD-Types". Types are
an unsigned 8 bit number. The following values are defined by this
document:
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Value Name Reference
----- --------------------- -------------
0 Reserved This document
1 Base MPLS Imposition MSD This document
2-250 Unassigned This document
251-254 Experimental Use This document
255 Reserved This document
Figure 6: MSD-Types Codepoints Registry
General guidance for the Designated Experts is as defined in
[RFC7370]
7. Security Considerations
Security considerations as specified by [RFC7981] are applicable to
this document.
Advertisement of an incorrect MSD value may have negative
consequences. If the value is smaller than supported, path
computation may fail to compute a viable path. If the value is
larger than supported, an attempt to instantiate a path that can't be
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
to induce any of the aforementioned conditions.
8. Contributors
The following people contributed to this document:
Peter Psenak
Email: ppsenak@cisco.com
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
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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>.
[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
Registry", RFC 7370, DOI 10.17487/RFC7370, September 2014,
<https://www.rfc-editor.org/info/rfc7370>.
[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>.
[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>.
[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>.
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
10.2. Informative References
[I-D.ietf-idr-bgp-ls-segment-routing-msd]
Tantsura, J., Chunduri, U., Mirsky, G., and S. Sivabalan,
"Signaling MSD (Maximum SID Depth) using Border Gateway
Protocol Link-State", draft-ietf-idr-bgp-ls-segment-
routing-msd-02 (work in progress), August 2018.
[I-D.ietf-isis-mpls-elc]
Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
Litkowski, "Signaling Entropy Label Capability and Entropy
Readable Label Depth Using IS-IS", draft-ietf-isis-mpls-
elc-06 (work in progress), September 2018.
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[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-12 (work in progress), June
2018.
[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>.
Authors' Addresses
Jeff Tantsura
Apstra, Inc.
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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