IDR Working Group J. Tantsura
Internet-Draft Apstra, Inc.
Intended status: Standards Track U. Chunduri
Expires: April 17, 2020 Futurewei Technologies
K. Talaulikar
Cisco Systems
G. Mirsky
ZTE Corp.
N. Triantafillis
Apstra, Inc.
October 15, 2019
Signaling MSD (Maximum SID Depth) using Border Gateway Protocol Link-
State
draft-ietf-idr-bgp-ls-segment-routing-msd-09
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
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 17, 2020.
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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. Manageability Considerations . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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 (described in [RFC8491] ) 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. Manageability Considerations
The new protocol extensions introduced in this document augment the
existing IGP topology information that is distributed via [RFC7752].
Procedures and protocol extensions defined in this document do not
affect the BGP protocol operations and management other than as
discussed in the Manageability Considerations section of [RFC7752].
Specifically, the malformed attribute tests for syntactic checks in
the Fault Management section of [RFC7752] now encompass the new BGP-
LS Attribute TLVs defined in this document. The semantic or content
checking for the TLVs specified in this document and their
association with the BGP-LS NLRI types or their BGP-LS Attribute is
left to the consumer of the BGP-LS information (e.g. an application
or a controller) and not the BGP protocol.
A consumer of the BGP-LS information retrieves this information over
a BGP-LS session (refer Section 1 and 2 of [RFC7752]). The handling
of semantic or content errors by the consumer would be dictated by
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the nature of its application usage and hence is beyond the scope of
this document.
This document only introduces new Attribute TLVs and any syntactic
error in them would result in the BGP-LS Attribute being discarded
with an error log. The MSD information introduced in BGP-LS by this
specification, may be used by BGP-LS consumer applications like a SR
path computation engine (PCE) to learn the SR SID-stack handling
capabilities of the nodes in the topology. This can enable the SR
PCE to perform path computations taking into consideration the size
of SID Stack that the specific headend node may be able to impose.
Errors in the encoding or decoding of the MSD information may result
in the unavailability of such information to the SR PCE or incorrect
information being made available to it. This may result in the
headend node not being able to instantiate the desired SR path in its
forwarding and provide the SR based optimization functionality. The
handling of such errors by applications like SR PCE may be
implementation specific and out of scope of this document.
The extensions specified in this document, do not specify any new
configuration or monitoring aspects in BGP or BGP-LS. The
specification of BGP models BGP and BGP-LS models is an ongoing work
based on the [I-D.ietf-idr-bgp-model]. The management of the MSD
features within an ietf segment-routing stack is also an ongoing work
based on the [I-D.ietf-spring-sr-yang]. Management of the segment
routing in IGPs is ongoing work for ISIS [I-D.ietf-isis-sr-yang] ,
and OSPF [I-D.ietf-ospf-sr-yang].
7. 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.
The document does not introduce additional security issues beyond
discussed in [RFC7752], [RFC8476] and [RFC8491]. However, [RFC7752]
is being revised in [I-D.ietf-idr-rfc7752bis] to provide additional
clarification in several portions of the specification after
receiving feedback from implementers. One of the places that is
being clarified is the error handling and security. It is expected
that after [I-D.ietf-idr-rfc7752bis] is released that implementers
will update all BGP-LS base implementations improving the error
handling for protocol work (including this document) that depend on
this function.
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8. Contributors
Siva Sivabalan
Cisco Systems Inc.
Canada
Email: msiva@cisco.com
9. Acknowledgements
We like to thank Acee Lindem, 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>.
[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>.
10.2. Informative References
[I-D.ietf-idr-bgp-model]
Jethanandani, M., Patel, K., Hares, S., and J. Haas, "BGP
YANG Model for Service Provider Networks", draft-ietf-idr-
bgp-model-07 (work in progress), October 2019.
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[I-D.ietf-idr-rfc7752bis]
Talaulikar, K., "Distribution of Link-State and Traffic
Engineering Information Using BGP", draft-ietf-idr-
rfc7752bis-01 (work in progress), September 2019.
[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-09 (work in progress), October 2019.
[I-D.ietf-isis-sr-yang]
Litkowski, S., Qu, Y., Sarkar, P., Chen, I., and J.
Tantsura, "YANG Data Model for IS-IS Segment Routing",
draft-ietf-isis-sr-yang-06 (work in progress), July 2019.
[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-10 (work in progress), October 2019.
[I-D.ietf-ospf-sr-yang]
Yeung, D., Qu, Y., Zhang, Z., Chen, I., and A. Lindem,
"YANG Data Model for OSPF SR (Segment Routing) Protocol",
draft-ietf-ospf-sr-yang-10 (work in progress), August
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.
[I-D.ietf-spring-sr-yang]
Litkowski, S., Qu, Y., Lindem, A., Sarkar, P., and J.
Tantsura, "YANG Data Model for Segment Routing", draft-
ietf-spring-sr-yang-13 (work in progress), July 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>.
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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
Nikos Triantafillis
Apstra, Inc.
Email: nikos@apstra.com
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