wdiff rfc8491.alt-original rfc8491.txt

IS-IS Working Group
Internet Engineering Task Force (IETF) J. Tantsura
Internet-Draft
Request for Comments: 8491 Apstra, Inc.
Intended status:
Category: Standards Track U. Chunduri
Expires: April 12, 2019
ISSN: 2070-1721 Huawei Technologies
S. Aldrin
Google, Inc Inc.
L. Ginsberg
Cisco Systems
October 9,
November 2018

Signaling MSD (Maximum Maximum SID Depth) using Depth (MSD) 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) Segment ID (SID) stack can be
supported in a given network. This document only defines one type of
MSD (Base
MSD: Base MPLS Imposition), but Imposition. However, it defines an encoding that can
support other MSD types. This document focuses on MSD use in a
network that is Segment Routing enabled network, (SR) enabled, but MSD may also be
useful when Segment
Routing SR is not enabled.

Status of This Memo

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This Internet-Draft will expire on April 12, 2019.
https://www.rfc-editor.org/info/rfc8491.

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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. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 9 8
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Contributors
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 10
Contributors . . 9
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10

1. Introduction

When Segment Routing (SR) paths are computed by a centralized
controller, it is critical that the controller learns learn the Maximum SID
Depth (MSD) that can be imposed at each node/link of a given SR path
to ensure path.
This ensures 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]

[PCEP-EXT] defines how to signal MSD in the Path Computation Element communication
Communication Protocol (PCEP). However, if PCEP is not supported/configured supported/
configured on the head-end of an SR tunnel or a Binding-SID anchor node
node, and the controller does not participate in IGP routing, it has
no way to learn of learning the MSD of nodes and links. BGP-LS (Distribution
of Link-State and TE Information using 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]. [MSD-BGP].
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 for which MSD is relevant,
MSD capabilities SHOULD be advertised by every Intermediate System to
Intermediate System(IS-IS) System (IS-IS) router in the network.

Other types of MSD MSDs are known to be useful. For example,
[I-D.ietf-isis-mpls-elc] [ELC-ISIS]
defines Entropy Readable Label Depth Capability
(RLDC) that (ERLD), which is used by a head-end head-
end to insert an Entropy Label (EL) at a depth, that could depth where it can be read
by transit nodes.

This document defines an extension to IS-IS used to advertise one or
more types of MSD MSDs at node and/or link granularity. It also creates
an IANA registry for assigning MSD-type identifiers. It also MSD-Type identifiers and defines the
Base MPLS Imposition MSD-type. MSD-Type. In the future future, it is expected that
new MSD-types MSD-Types will be defined to signal additional capabilities capabilities,
e.g., entropy labels, SIDs that can be imposed through recirculation,
or SIDs associated with another dataplane e.g., data plane such as 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 that can be
imposed inclusive of all service/transport/special labels labels.

MSD: Maximum SID Depth - is the number of SIDs supported by a node or
a link on a node node.

SID: Segment Identifier as is defined in [RFC8402] [RFC8402].

Label Imposition: Imposition is the act of modifying and/or adding
labels to the outgoing label stack associated with a packet.
This includes:

* replacing the label at the top of the label stack with a new
label

* 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
that are replaced and the number of labels which that 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 . here.

2. Node MSD Advertisement

The node Node MSD sub-TLV is defined within the body of the IS-IS Router
Capability
CAPABILITY TLV [RFC7981], [RFC7981] to carry the provisioned SID depth of the
router originating the IS-IS Router Capability 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

Figure 1: Node MSD Sub-TLV

Type: 23 (allocated by IANA via the early assignment process)

Length: variable (multiple of 2 octets) and octets); represents the total length
of value field. the Value field

Value: field consists of one or more pairs of a 1 octet 1-octet MSD-Type and
1 octet MSD-Value.

MSD-Type is a
1-octet MSD-Value

MSD-Type: value defined in the IGP MSD-Types "IGP MSD-Types" registry created by
the IANA Section Considerations section of this document.

MSD-Value is a document Section 6
MSD-Value: number in the range of 0-255. For 0-255 (for all MSD-Types, 0
represents the lack of the ability to support a 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. 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
used, for example, by (for example) different controllers - but this controllers. This does not not, however,
create any interoperability issue.

3. Link MSD Advertisement

The link 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

Figure 2: Link MSD Sub-TLV

Type: 15 (allocated by IANA via the early assignment process)

Length: variable (multiple of 2 octets) and octets); represents the total length
of value field. the Value field

Value: field consists of one or more pairs of a 1 octet 1-octet MSD-Type and 1
octet MSD-Value.

MSD-Type is a
1-octet MSD-Value

MSD-Type: value defined in the MSD-Types "IGP MSD-Types" registry created by
the IANA Section Considerations section of this document.

MSD-Value is a document Section 6
MSD-Value: number in the range of 0-255. For 0-255 (for all MSD-Types, 0
represents the lack of the ability to support a SID stack of any depth;
any other value represents that of the particular link when used as
an outgoing interface. 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 to use 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
per-link values. In such a case 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, MSD-Type, the value of the Link
MSD MUST take precedence over the Node MSD. When If a Link MSD-type MSD-Type is
not signaled signaled, but the Node MSD-type MSD-Type is, then the Node MSD-type MSD-Type value
MUST be considered as to be the MSD value for that link.

In order to increase flooding efficiency, it is RECOMMENDED that
routers with homogenous link 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 MSD-Type is specific to the MSD-type. Generally MSD-Type. Generally, it can
only be inferred that the advertising node does not support
advertisement of that MSD-type. However, in MSD-Type. In some cases cases, however, the lack of
advertisement might imply that the functionality associated with the
MSD-type
MSD-Type is not supported. The correct interpretation MUST be
specified when an MSD-type MSD-Type is defined.

5. Base MPLS Imposition MSD

Base MPLS Imposition MSD (BMI-MSD) signals the total number of MPLS
labels which that can be imposed, including all service/transport/special
labels.

Absence

The absence of BMI-MSD advertisements indicates solely only that the
advertising node does not support advertisement of this capability.

6. IANA Considerations

This document requests

IANA to allocate has allocated a sub-TLV type for the new
sub TLV sub-TLV proposed in
Section 2 of this document from IS-IS Router
Capability the "Sub-TLVs for TLV Registry 242 (IS-IS
Router CAPABILITY TLV)" registry as defined by [RFC7981].

IANA has allocated the following value through the early assignment
process: value:

Value Description Reference
----- --------------- -------------
23 Node MSD This document

Figure 3: Node MSD

This document requests

IANA to allocate has allocated a sub-TLV type as defined in Section 3 from Sub-TLVs the
"Sub-TLVs for TLVs 22, 23, 25, 141, 222, and 223 (Extended IS
reachability, IS Neighbor Attribute, L2 Bundle Member Attributes,
inter-AS reachability information, MT-ISN, and MT IS Neighbor
Attribute TLVs)" registry.

IANA has allocated the following value through the early assignment
process: value:

Value Description Reference
----- --------------- -------------
15 Link MSD This document

Figure 4: Link MSD

Per TLV

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 Where LINK MSD Sub-TLV can be present

This document requests creation of an Can Be Present
IANA managed has created an IANA-managed registry titled "IGP MSD-Types"
under the
category of "Interior Gateway Protocol (IGP) Parameters" IANA
registries registry to
identify MSD-types MSD-Types as proposed in Section Sections 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 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 Use This document
255 Reserved This document

Figure 6: MSD-Types Codepoints Registry

General guidance for the Designated Experts designated experts is as defined in
[RFC7370] [RFC7370].

7. Security Considerations

Security considerations as specified by [RFC7981] are applicable to
this document.

The 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 also inform an attacker of how
to induce any of the aforementioned conditions.

8. References

8.1. 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>.

8.2. Informative References

[IS-IS]

[ELC-ISIS]
Xu, X., Kini, S., Sivabalan, S., Filsfils, C., and S.
Litkowski, "Signaling Entropy Label Capability and Entropy
Readable Label Depth Using IS-IS", Work in Progress,
draft-ietf-isis-mpls-elc-06, September 2018.

[MSD]

[MSD-BGP] Tantsura, J., Chunduri, U., Mirsky, G., and S. Sivabalan,
"Signaling MSD (Maximum SID Depth) using Border Gateway
Protocol Link-State", Work in Progress, draft-ietf-idr-
bgp-ls-segment-routing-msd-02, August 2018.

[PCEP-EXTENSIONS]

[PCEP-EXT]
Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
and J. Hardwick, "PCEP Extensions for Segment Routing",
Work in Progress, draft-ietf-pce-segment-routing-13,
October 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>.

Contributors

The following people contributed to this document:

Peter Psenak

Email: ppsenak@cisco.com

Acknowledgements

The authors would like to thank Acee Lindem, Ketan Talaulikar,
Stephane Litkowski Litkowski, and Bruno Decraene for their reviews and valuable
comments.

Contributors

The following people contributed to this document:

Peter Psenak

Email: ppsenak@cisco.com

Authors' Addresses

Jeff Tantsura
Apstra, Inc.

Email: jefftant.ietf@gmail.com

Uma Chunduri
Huawei Technologies

Email: uma.chunduri@huawei.com

Sam Aldrin
Google, Inc Inc.

Email: aldrin.ietf@gmail.com

Les Ginsberg
Cisco Systems

Email: ginsberg@cisco.com