IDR Working Group J. Tantsura
Internet-Draft Apstra, Inc.
Intended status: Standards Track U. Chunduri
Expires: December 3, 2019 Futurewei Technologies
K. Talaulikar
Cisco Systems
G. Mirsky
ZTE Corp.
N. Triantafillis
Apstra, Inc.
June 1, 2019
Signaling MSD (Maximum SID Depth) using Border Gateway Protocol Link-
State
draft-ietf-idr-bgp-ls-segment-routing-msd-05
Abstract
This document defines a way for a Border Gateway Protocol Link-State
(BGP-LS) speaker 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 Segment Identifier (SID) stack can be
supported in a given network.
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 3, 2019.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions used in this document . . . . . . . . . . . . 3
1.1.1. Terminology . . . . . . . . . . . . . . . . . . . . . 3
1.1.2. Requirements Language . . . . . . . . . . . . . . . . 4
2. Advertisement of MSD via BGP-LS . . . . . . . . . . . . . . . 4
3. Node MSD TLV . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Link MSD TLV . . . . . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction
When Segment Routing (SR) [RFC8402] 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 on a
given SR path. This ensures that the Segment Identifier (SID) stack
depth of a computed path doesn't 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 Protocol (PCEP). The OSPF and IS-IS extensions
for signaling of MSD are defined in [RFC8476] and [RFC8491]
respectively.
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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 of learning the MSD of
nodes and links. BGP-LS [RFC7752] defines a way to advertise
topology and associated attributes and capabilities of the nodes in
that topology to a centralized controller. This document defines
extensions to BGP-LS to advertise one or more types of MSDs at node
and/or link granularity.
Other types of MSD are known to be useful. For example,
[I-D.ietf-ospf-mpls-elc] and [I-D.ietf-isis-mpls-elc] define Readable
Label Depth Capability (RLDC) that is used by a head-end to insert an
Entropy Label (EL) at a depth that can be read by transit nodes.
In the future, it is expected that new MSD-Types will be defined to
signal additional capabilities, e.g., ELs, SIDs that can be imposed
through recirculation, or SIDs associated with another data plane
such as IPv6. MSD advertisements MAY be useful even if SR itself is
not enabled. For example, in a non-SR MPLS network, MSD defines the
maximum label depth.
1.1. Conventions used in this document
1.1.1. Terminology
BGP-LS: Distribution of Link-State and TE Information using Border
Gateway Protocol
MSD: Maximum SID Depth
PCC: Path Computation Client
PCE: Path Computation Element
PCEP: Path Computation Element Protocol
SID: Segment Identifier
SR: Segment routing
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.
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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 that are
replaced and the number of labels that are pushed. See [RFC3031]
for further details.
1.1.2. 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. Advertisement of MSD via BGP-LS
This document describes extensions that enable BGP-LS speakers to
signal the MSD capabilities of nodes and their links in a network to
a BGP-LS consumer of network topology such as a centralized
controller. The centralized controller can leverage this information
in computation of SR paths and their instantiation on network nodes
based on their MSD capabilities. When a BGP-LS speaker is
originating the topology learnt via link-state routing protocols like
OSPF or IS-IS, the MSD information for the nodes and their links is
sourced from the underlying extensions as defined in [RFC8476] and
[RFC8491] respectively. The BGP-LS speaker may also advertise the
MSD information for the local node and its links when not running any
link-state IGP protocol e.g. when running BGP as the only routing
protocol.
The extensions introduced in this document allow for advertisement of
different MSD-Types. This document does not define these MSD-Types
but leverages the definition, guidelines and the code-point registry
specified in [RFC8491]. This enables sharing of MSD-Types that may
be defined in the future by the IGPs in BGP-LS.
3. Node MSD TLV
Node MSD is encoded in a new Node Attribute TLV [RFC7752] using the
following format:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSD-Type | MSD-Value | MSD-Type... | MSD-Value... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Node MSD TLV Format
Where:
o Type: 266
o Length: variable (multiple of 2); represents the total length of
the value field in octets.
o Value : consists of one or more pairs of a 1-octet MSD-Type and
1-octet MSD-Value.
* MSD-Type : one of the values defined in the IANA registry
titled "IGP MSD-Types" under the "Interior Gateway Protocol
(IGP) Parameters" registry created by [RFC8491].
* MSD-Value : a number in the range of 0-255. For all MSD-Types,
0 represents the lack of ability to impose an MSD 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 protocol instance.
4. Link MSD TLV
Link MSD is encoded in a new Link Attribute TLV [RFC7752] using the
following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSD-Type | MSD-Value | MSD-Type... | MSD-Value... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: Link MSD TLV Format
Where:
o Type: 267
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o Length: variable (multiple of 2); represents the total length of
the value field in octets.
o Value : consists of one or more pairs of a 1-octet MSD-Type and
1-octet MSD-Value.
* MSD-Type : one of the values defined in the IANA registry
titled "IGP MSD-Types" under the "Interior Gateway Protocol
(IGP) Parameters" registry created by [RFC8491].
* MSD-Value : a number in the range of 0-255. For all MSD-Types,
0 represents the lack of ability to impose an MSD stack of any
depth; any other value represents that of the link when used as
an outgoing interface.
5. IANA Considerations
This document requests assigning code-points from the registry "BGP-
LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute
TLVs" based on table below. Early allocation for these code-points
have been done by IANA.
+------------+-----------------+---------------------------+
| Code Point | Description | IS-IS TLV/Sub-TLV |
+------------+-----------------+---------------------------+
| 266 | Node MSD | 242/23 |
| 267 | Link MSD | (22,23,25,141,222,223)/15 |
+------------+-----------------+---------------------------+
6. Security Considerations
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 may also inform an
attacker of how to induce any of the aforementioned conditions.
This document does not introduce security issues beyond those
discussed in [RFC7752], [RFC8476] and [RFC8491]
7. Contributors
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Siva Sivabalan
Cisco Systems Inc.
Canada
Email: msiva@cisco.com
8. Acknowledgements
We like to thank Acee Lindem, Stephane Litkowski and Bruno Decraene
for their reviews and valuable comments.
9. References
9.1. 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>.
[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>.
[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>.
[RFC8476] Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak,
"Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476,
DOI 10.17487/RFC8476, December 2018,
<https://www.rfc-editor.org/info/rfc8476>.
[RFC8491] Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
"Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491,
DOI 10.17487/RFC8491, November 2018,
<https://www.rfc-editor.org/info/rfc8491>.
9.2. Informative References
[I-D.ietf-isis-mpls-elc]
Xu, X., Kini, S., Psenak, P., Filsfils, C., and S.
Litkowski, "Signaling Entropy Label Capability and Entropy
Readable Label Depth Using IS-IS", draft-ietf-isis-mpls-
elc-07 (work in progress), May 2019.
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[I-D.ietf-ospf-mpls-elc]
Xu, X., Kini, S., Psenak, P., Filsfils, C., and S.
Litkowski, "Signaling Entropy Label Capability and Entropy
Readable Label-stack Depth Using OSPF", draft-ietf-ospf-
mpls-elc-08 (work in progress), May 2019.
[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-16 (work in progress),
March 2019.
[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>.
[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>.
Authors' Addresses
Jeff Tantsura
Apstra, Inc.
Email: jefftant.ietf@gmail.com
Uma Chunduri
Futurewei Technologies
Email: umac.ietf@gmail.com
Ketan Talaulikar
Cisco Systems
Email: ketant@cisco.com
Greg Mirsky
ZTE Corp.
Email: gregimirsky@gmail.com
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Nikos Triantafillis
Apstra, Inc.
Email: nikos@apstra.com
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