BGP-LS MSD: Signaling Maximum SID Depth (MSD) Using the Border Gateway Protocol - Link State
Two new BGP-LS TLVs let routers tell a central controller how many SIDs they can stack onto a packet.
Tantsura, J., Chunduri, U., Talaulikar, K., Mirsky, G., and N. Triantafillis, "Signaling Maximum SID Depth (MSD) Using the Border Gateway Protocol - Link State", RFC 8814, DOI 10.17487/RFC8814, August 2020, <https://www.rfc-editor.org/info/rfc8814>.
@misc{rfc8814,
series = {Request for Comments},
number = 8814,
howpublished = {RFC 8814},
publisher = {RFC Editor},
doi = {10.17487/RFC8814},
url = {https://www.rfc-editor.org/info/rfc8814},
author = {J. Tantsura and U. Chunduri and K. Talaulikar and G. Mirsky and N. Triantafillis},
title = {{Signaling Maximum SID Depth (MSD) Using the Border Gateway Protocol - Link State}},
pagetotal = 9,
year = 2020,
month = aug,
} RFC text
Contents
Internet Engineering Task Force (IETF) J. Tantsura
Request for Comments: 8814 Apstra, Inc.
Category: Standards Track U. Chunduri
ISSN: 2070-1721 Futurewei Technologies
K. Talaulikar
Cisco Systems
G. Mirsky
ZTE Corp.
N. Triantafillis
Amazon Web Services
August 2020
Signaling Maximum SID Depth (MSD) Using the Border Gateway Protocol -
Link State
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 is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8814.
Copyright Notice
Copyright (c) 2020 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
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
1.1. Conventions Used in This Document
1.1.1. Terminology
1.1.2. Requirements Language
2. Advertisement of MSD via BGP-LS
3. Node MSD TLV
4. Link MSD TLV
5. IANA Considerations
6. Manageability Considerations
7. Security Considerations
8. References
8.1. Normative References
8.2. Informative References
Acknowledgements
Contributors
Authors' Addresses
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.
[RFC8664] defines how to signal MSD in the Path Computation Element
Protocol (PCEP). The OSPF and IS-IS extensions for the signaling of
MSD are defined in [RFC8476] and [RFC8491], respectively.
However, if PCEP is not supported/configured on the head-end of an SR
tunnel or a Binding-SID anchor node, and the 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 expose 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 MSDs
are known to be useful. For example, [OSPF-ELC] and [ISIS-ELC]
define Entropy Readable Label Depth (ERLD), which 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
MSD: Maximum SID Depth - the number of SIDs supported by a node or a
link on a node
PCE: Path Computation Element
PCEP: Path Computation Element Protocol
SID: Segment Identifier as defined in [RFC8402]
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:
* 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 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 [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 based on their MSD
capabilities. When a BGP-LS speaker is originating the topology
learnt via link-state routing protocols such as 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 extensions introduced in this document allow for advertisement of
different MSD-Types, which are defined elsewhere and were introduced
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
The Node MSD ([RFC8476] [RFC8491]) is encoded in a new Node Attribute
TLV [RFC7752] to carry the provisioned SID depth of the router
identified by the corresponding Router-ID. Node MSD is the smallest
MSD supported by the node on the set of interfaces configured for
use. MSD values may be learned via a hardware API or may be
provisioned. The following format is used:
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:
Type: 266
Length: variable (multiple of 2); represents the total length of
the value field in octets.
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 "IGP MSD-Types"
registry defined in [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
The Link MSD ([RFC8476] [RFC8491]) is defined to carry the MSD of the
interface associated with the link. It 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:
Type: 267
Length: variable (multiple of 2); represents the total length of
the value field in octets.
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 "IGP MSD-Types"
registry defined in [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
IANA has assigned code points from the registry "BGP-LS Node
Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs"
based on the table below.
+==========+=============+===========================+===========+
| TLV Code | Description | IS-IS TLV/Sub-TLV | Reference |
| Point | | | |
+==========+=============+===========================+===========+
| 266 | Node MSD | 242/23 | This |
| | | | document |
+----------+-------------+---------------------------+-----------+
| 267 | Link MSD | (22,23,25,141,222,223)/15 | This |
| | | | document |
+----------+-------------+---------------------------+-----------+
Table 1: BGP-LS MSD TLV Code Points
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 Section 6 (Manageability Considerations) of [RFC7752].
Specifically, the malformed attribute tests for syntactic checks in
Section 6.2.2 (Fault Management) 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 Network Layer Reachability Information
(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 to Sections 1 and 2 of [RFC7752]).
This document only introduces new Attribute TLVs, and any syntactic
error in them would result in the BGP-LS Attribute being discarded
[RFC7752]. The MSD information introduced in BGP-LS by this
specification, may be used by BGP-LS consumer applications like an SR
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 head-end 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 head-end 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 is an ongoing work based on the
[BGP-MODEL].
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 procedures and protocol extensions defined in this document do
not affect the BGP security model. See the "Security Considerations"
Section of [RFC4271] for a discussion of BGP security. Also, refer
to [RFC4272] and [RFC6952] for analyses of security issues for BGP.
Security considerations for acquiring and distributing BGP-LS
information are discussed in [RFC7752]. The TLVs introduced in this
document are used to propagate the MSD IGP extensions defined in
[RFC8476] and [RFC8491]. It is assumed that the IGP instances
originating these TLVs will support all the required security (as
described in [RFC8476] and [RFC8491]) in order to prevent any
security issues when propagating the TLVs into BGP-LS. The
advertisement of the node and link attribute information defined in
this document presents no significant additional risk beyond that
associated with the existing node and link attribute information
already supported in [RFC7752].
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>.
[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>.
8.2. Informative References
[BGP-MODEL]
Jethanandani, M., Patel, K., Hares, S., and J. Haas, "BGP
YANG Model for Service Provider Networks", Work in
Progress, Internet-Draft, draft-ietf-idr-bgp-model-09, 28
June 2020,
<https://tools.ietf.org/html/draft-ietf-idr-bgp-model-09>.
[ISIS-ELC] Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S.,
and M. Bocci, "Signaling Entropy Label Capability and
Entropy Readable Label Depth Using IS-IS", Work in
Progress, Internet-Draft, draft-ietf-isis-mpls-elc-13, 28
May 2020,
<https://tools.ietf.org/html/draft-ietf-isis-mpls-elc-13>.
[OSPF-ELC] Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S.,
and M. Bocci, "Signaling Entropy Label Capability and
Entropy Readable Label Depth Using OSPF", Work in
Progress, Internet-Draft, draft-ietf-ospf-mpls-elc-15, 1
June 2020,
<https://tools.ietf.org/html/draft-ietf-ospf-mpls-elc-15>.
[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>.
[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC4272] Murphy, S., "BGP Security Vulnerabilities Analysis",
RFC 4272, DOI 10.17487/RFC4272, January 2006,
<https://www.rfc-editor.org/info/rfc4272>.
[RFC6952] Jethanandani, M., Patel, K., and L. Zheng, "Analysis of
BGP, LDP, PCEP, and MSDP Issues According to the Keying
and Authentication for Routing Protocols (KARP) Design
Guide", RFC 6952, DOI 10.17487/RFC6952, May 2013,
<https://www.rfc-editor.org/info/rfc6952>.
[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>.
[RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
and J. Hardwick, "Path Computation Element Communication
Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
DOI 10.17487/RFC8664, December 2019,
<https://www.rfc-editor.org/info/rfc8664>.
Acknowledgements
We would like to thank Acee Lindem, Stephane Litkowski, Bruno
Decraene, and Alvaro Retana for their reviews and valuable comments.
Contributors
Siva Sivabalan
Cisco Systems Inc.
Canada
Email: msiva@cisco.com
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
Amazon Web Services
Email: nikost@amazon.com
AI ELI5
On this page
§1The Problem: Knowing How Deep a SID Stack Can Go
When a centralized controller computes Segment Routing (SR) paths, it must know the Maximum SID Depth (MSD) that each node or link can impose. Otherwise the stack of Segment Identifiers (SIDs) it builds could be longer than the node can handle.
MSD signaling already exists for PCEP (RFC 8664), OSPF (RFC 8476) and IS-IS (RFC 8491). But if PCEP is not supported or configured on the head-end of an SR tunnel, and the controller does not take part in IGP routing, the controller has no way to learn MSD. BGP-LS (RFC 7752) already exposes topology information to controllers, so this RFC extends it to carry MSD.
The RFC expects further MSD-Types in future, for example for entropy labels, and notes that in a non-SR MPLS network, MSD defines the maximum label depth.
A planner writes a list of steps for a messenger. Each messenger can remember only so many steps, so the planner needs to know that number first.
§1.1Key Terms
MSD is the number of SIDs supported by a node or by a link on a node. A SID is a Segment Identifier as defined in RFC 8402.
Label imposition is the act of modifying and/or adding labels to the outgoing label stack of a packet. It includes replacing the label at the top of the stack and pushing new labels. The number of labels imposed is the sum of those replaced and those pushed.
MSD is how many instruction cards a router can stick on a packet at once.
§2Advertising MSD via BGP-LS
The extensions let BGP-LS speakers tell a consumer of topology data, such as a centralized controller, about the MSD capabilities of nodes and their links. The controller can use this when computing SR paths.
When a speaker originates topology learned through OSPF or IS-IS, the MSD information comes from the underlying IGP extensions in RFC 8476 and RFC 8491. The TLVs can carry any MSD-Type from the "IGP MSD-Types" registry introduced in RFC 8491, including types defined later by the IGPs.
Routers already tell each other their limits. This RFC lets them pass those limits on to the planner too.
§3The Node MSD TLV
The Node MSD TLV is a new Node Attribute TLV with Type 266. It carries the provisioned SID depth of the router identified by the Router-ID. Node MSD is the smallest MSD supported by the node on the set of interfaces configured for use. Values may be learned through a hardware API or provisioned.
The Length field is variable, a multiple of 2, and is the total length of the value in octets. The value is one or more pairs of a 1-octet MSD-Type and a 1-octet MSD-Value.
MSD-Value is a number from 0 to 255. A value of 0 means the node cannot impose an MSD stack of any depth. Any other value MUST be the lowest value supported by any link configured for use by the advertising protocol instance.
The node says one number for the whole router: the smallest number any of its doors can handle.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
TYPE | Type | 2 bytes | 266 | 266 for the Node MSD TLV. |
LENGTH | Length | 2 bytes | Total length of the value field in octets; a multiple of 2. | |
MSD-TYPE | MSD-Type | 1 byte | A value from the IGP MSD-Types registry. | |
MSD-VALUE | MSD-Value | 1 byte | 0 to 255; 0 means no ability to impose any depth. | |
MORE | More pairs | 2 bytes | Further MSD-Type and MSD-Value pairs may follow. |
§4The Link MSD TLV
The Link MSD TLV is a new Link Attribute TLV with Type 267. It carries the MSD of the interface associated with the link. Its layout is the same as the Node MSD TLV: a variable Length that is a multiple of 2, followed by pairs of a 1-octet MSD-Type and a 1-octet MSD-Value.
The MSD-Value range is again 0 to 255, with 0 meaning no ability to impose an MSD stack of any depth. Any other value is the MSD of the link when used as an outgoing interface.
A link gets its own number too, because one door may handle fewer instructions than the router as a whole.
§5IANA Code Points
IANA assigned the two TLV code points in the registry "BGP-LS Node Descriptor, Link Descriptor, Prefix Descriptor, and Attribute TLVs". The table also records the matching IS-IS TLV or sub-TLV: Node MSD maps to 242/23, and Link MSD maps to (22,23,25,141,222,223)/15.
The two new labels were written into the official list, each with its own number.
| Description | TLV code point | IS-IS TLV/Sub-TLV |
|---|---|---|
| Node MSD | 266 | 242/23 |
| Link MSD | 267 | (22,23,25,141,222,223)/15 |
§6Manageability
The extensions only add new Attribute TLVs and do not affect BGP operation beyond what RFC 7752 already covers. The malformed-attribute checks of RFC 7752 now include these TLVs, and a syntax error in them causes the BGP-LS Attribute to be discarded. Checking the meaning of the TLVs is left to the consumer, such as a controller, and not the BGP protocol.
Encoding or decoding errors may leave a PCE without the information, or with incorrect information, so a head-end may be unable to instantiate the desired SR path. How applications handle this is implementation specific and out of scope. The RFC adds no new BGP or BGP-LS configuration or monitoring.
If a number gets scrambled, the planner might hand a router a list that is too long. How the planner copes is up to the planner.
§7Security Considerations
An incorrect MSD value can do harm. If it is smaller than what is supported, path computation may fail to find a viable path. If it is larger, a path may be attempted that the head-end cannot support. The presence of this information may also show an attacker how to induce either condition.
The extensions do not change the BGP security model. The TLVs propagate the IGP MSD extensions, and it is assumed the IGP instances that originate them provide the required security. The RFC concludes that the new information adds no significant risk beyond the node and link attribute information already supported in RFC 7752.
If someone lies about the number, the planner makes bad plans. So the number needs to come from a source you trust.