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IETF RFC 9089
Last modified on Saturday, August 14th, 2021
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Internet Engineering Task Force (IETF) X. Xu
Request for Comments: 9089 Capitalonline
Category: Standards Track S. Kini
ISSN: 2070-1721
P. Psenak
C. Filsfils
S. Litkowski
Cisco Systems, Inc.
M. Bocci
Nokia
August 2021
Signaling Entropy Label Capability and Entropy Readable Label Depth
Using OSPF
 Abstract
Multiprotocol Label Switching (MPLS) has defined a mechanism to load-
balance traffic flows using Entropy Labels (EL). An ingress Label
Switching Router (LSR) cannot insert ELs for packets going into a
given Label Switched Path (LSP) unless an egress LSR has indicated
via signaling that it has the capability to process ELs, referred to
as the Entropy Label Capability (ELC), on that LSP. In addition, it
would be useful for ingress LSRs to know each LSR's capability for
reading the maximum label stack depth and performing EL-based load-
balancing, referred to as Entropy Readable Label Depth (ERLD). This
document defines a mechanism to signal these two capabilities using
OSPFv2 and OSPFv3, and Border Gateway Protocol - Link State (BGP-LS).
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/RFC 9089.
Copyright Notice
Copyright (c) 2021 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
2. Terminology
3. Advertising ELC Using OSPF
3.1. Advertising ELC Using OSPFv2
3.2. Advertising ELC Using OSPFv3
4. Advertising ERLD Using OSPF
5. Signaling ELC and ERLD in BGP-LS
6. IANA Considerations
7. Security Considerations
8. References
8.1. Normative References
8.2. Informative References
Acknowledgements
Contributors
Authors' Addresses
1. Introduction
[RFC 6790] describes a method to load-balance Multiprotocol Label
Switching (MPLS) traffic flows using Entropy Labels (EL). It also
introduces the concept of Entropy Label Capability (ELC) and defines
the signaling of this capability via MPLS signaling protocols.
Recently, mechanisms have been defined to signal labels via link-
state Interior Gateway Protocols (IGP) such as OSPFv2 [RFC 8665] and
OSPFv3 [RFC 8666]. This document defines a mechanism to signal the
ELC using OSPFv2 and OSPFv3.
In cases where Segment Routing (SR) is used with the MPLS data plane
(e.g., SR-MPLS [RFC 8660]), it would be useful for ingress LSRs to
know each intermediate LSR's capability of reading the maximum label
stack depth and performing EL-based load-balancing. This capability,
referred to as Entropy Readable Label Depth (ERLD) as defined in
[RFC 8662], may be used by ingress LSRs to determine the position of
the EL label in the stack, and whether it is necessary to insert
multiple ELs at different positions in the label stack. This
document defines a mechanism to signal the ERLD using OSPFv2 and
OSPFv3.
2. Terminology
This memo makes use of the terms defined in [RFC 6790] and [RFC 8662].
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 [RFC 2119] [RFC 8174] when, and only when, they appear in all
capitals, as shown here.
The key word OSPF is used throughout the document to refer to both
OSPFv2 and OSPFv3.
3. Advertising ELC Using OSPF
Even though ELC is a property of the node, in some cases it is
advantageous to associate and advertise the ELC with a prefix. In
multi-area networks, routers may not know the identity of the prefix
originator in a remote area or may not know the capabilities of such
an originator. Similarly, in a multi-domain network, the identity of
the prefix originator and its capabilities may not be known to the
ingress LSR.
If a router has multiple interfaces, the router MUST NOT announce ELC
unless all of its interfaces are capable of processing ELs.
If the router supports ELs on all of its interfaces, it SHOULD
advertise the ELC with every local host prefix it advertises in OSPF.
3.1. Advertising ELC Using OSPFv2
[RFC 7684] defines the OSPFv2 Extended Prefix TLV to advertise
additional attributes associated with a prefix. The OSPFv2 Extended
Prefix TLV includes a one-octet Flags field. A new flag in the Flags
field is used to signal the ELC for the prefix:
0x20 - E-Flag (ELC Flag):
Set by the advertising router to indicate that the prefix
originator is capable of processing ELs.
The ELC signaling MUST be preserved when an OSPF Area Border Router
(ABR) distributes information between areas. To do so, an ABR MUST
originate an OSPFv2 Extended Prefix Opaque Link State Advertisement
(LSA) [RFC 7684] including the received ELC setting.
When an OSPF Autonomous System Border Router (ASBR) redistributes a
prefix from another instance of OSPF or from some other protocol, it
SHOULD preserve the ELC signaling for the prefix if it exists. To do
so, an ASBR SHOULD originate an Extended Prefix Opaque LSA [RFC 7684]
including the ELC setting of the redistributed prefix. The flooding
scope of the Extended Prefix Opaque LSA MUST match the flooding scope
of the LSA that an ASBR originates as a result of the redistribution.
The exact mechanism used to exchange ELC between protocol instances
on an ASBR is outside of the scope of this document.
3.2. Advertising ELC Using OSPFv3
[RFC 5340] defines the OSPFv3 PrefixOptions field to indicate
capabilities associated with a prefix. A new bit in the OSPFv3
PrefixOptions field is used to signal the ELC for the prefix:
0x40 - E-Flag (ELC Flag):
Set by the advertising router to indicate that the prefix
originator is capable of processing ELs.
The ELC signaling MUST be preserved when an OSPFv3 Area Border Router
(ABR) distributes information between areas. The setting of the ELC
Flag in the Inter-Area-Prefix-LSA [RFC 5340] or in the Inter-Area-
Prefix TLV [RFC 8362], generated by an ABR, MUST be the same as the
value the ELC Flag associated with the prefix in the source area.
When an OSPFv3 Autonomous System Border Router (ASBR) redistributes a
prefix from another instance of OSPFv3 or from some other protocol,
it SHOULD preserve the ELC signaling for the prefix if it exists.
The setting of the ELC Flag in the AS-External-LSA, Not-So-Stubby
Area LSA (NSSA-LSA) [RFC 5340], or in the External-Prefix TLV
[RFC 8362], generated by an ASBR, MUST be the same as the value of the
ELC Flag associated with the prefix in the source domain. The exact
mechanism used to exchange ELC between protocol instances on the ASBR
is outside of the scope of this document.
4. Advertising ERLD Using OSPF
The ERLD is advertised in a Node Maximum SID Depth (MSD) TLV
[RFC 8476] using the ERLD-MSD type defined in [RFC 9088].
If a router has multiple interfaces with different capabilities of
reading the maximum label stack depth, the router MUST advertise the
smallest value found across all of its interfaces.
The absence of ERLD-MSD advertisements indicates only that the
advertising node does not support advertisement of this capability.
When the ERLD-MSD type is received in the OSPFv2 or OSPFv3 Link MSD
sub-TLV [RFC 8476], it MUST be ignored.
The considerations for advertising the ERLD are specified in
[RFC 8662].
5. Signaling ELC and ERLD in BGP-LS
The OSPF extensions defined in this document can be advertised via
BGP-LS (distribution of Link-State and TE information using BGP)
[RFC 7752] using existing BGP-LS TLVs.
The ELC is advertised using the Prefix Attribute Flags TLV as defined
in [RFC 9085].
The ERLD-MSD is advertised using the Node MSD TLV as defined in
[RFC 8814].
6. IANA Considerations
IANA has completed the following actions for this document:
* Flag 0x20 in the "OSPFv2 Extended Prefix TLV Flags" registry has
been allocated to the E-Flag (ELC Flag).
* Bit 0x40 in the "OSPFv3 Prefix Options (8 bits)" registry has been
allocated to the E-Flag (ELC Flag).
7. Security Considerations
This document specifies the ability to advertise additional node
capabilities using OSPF and BGP-LS. As such, the security
considerations as described in [RFC 5340], [RFC 7684], [RFC 7752],
[RFC 7770], [RFC 8476], [RFC 8662], [RFC 8814], and [RFC 9085] are
applicable to this document.
Incorrectly setting the E-Flag during origination, propagation, or
redistribution may lead to poor or no load-balancing of the MPLS
traffic or to the MPLS traffic being discarded on the egress node.
Incorrectly setting of the ERLD value may lead to poor or no load-
balancing of the MPLS traffic.
8. References
8.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 5340] Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
for IPv6", RFC 5340, DOI 10.17487/RFC 5340, July 2008,
<https://www.rfc-editor.org/info/RFC 5340>.
[RFC 6790] Kompella, K., Drake, J., Amante, S., Henderickx, W., and
L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
RFC 6790, DOI 10.17487/RFC 6790, November 2012,
<https://www.rfc-editor.org/info/RFC 6790>.
[RFC 7684] Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
Advertisement", RFC 7684, DOI 10.17487/RFC 7684, November
2015, <https://www.rfc-editor.org/info/RFC 7684>.
[RFC 7752] 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/RFC 7752, March 2016,
<https://www.rfc-editor.org/info/RFC 7752>.
[RFC 7770] Lindem, A., Ed., Shen, N., Vasseur, JP., Aggarwal, R., and
S. Shaffer, "Extensions to OSPF for Advertising Optional
Router Capabilities", RFC 7770, DOI 10.17487/RFC 7770,
February 2016, <https://www.rfc-editor.org/info/RFC 7770>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
[RFC 8362] Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
F. Baker, "OSPFv3 Link State Advertisement (LSA)
Extensibility", RFC 8362, DOI 10.17487/RFC 8362, April
2018, <https://www.rfc-editor.org/info/RFC 8362>.
[RFC 8476] Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak,
"Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476,
DOI 10.17487/RFC 8476, December 2018,
<https://www.rfc-editor.org/info/RFC 8476>.
[RFC 8662] Kini, S., Kompella, K., Sivabalan, S., Litkowski, S.,
Shakir, R., and J. Tantsura, "Entropy Label for Source
Packet Routing in Networking (SPRING) Tunnels", RFC 8662,
DOI 10.17487/RFC 8662, December 2019,
<https://www.rfc-editor.org/info/RFC 8662>.
[RFC 8814] 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/RFC 8814, August 2020,
<https://www.rfc-editor.org/info/RFC 8814>.
[RFC 9085] Previdi, S., Talaulikar, K., Ed., Filsfils, C., Gredler,
H., and M. Chen, "Border Gateway Protocol - Link State
(BGP-LS) Extensions for Segment Routing", RFC 9085,
DOI 10.17487/RFC 9085, August 2021,
<https://www.rfc-editor.org/info/RFC 9085>.
[RFC 9088] 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", RFC 9088,
DOI 10.17487/RFC 9088, August 2021,
<https://www.rfc-editor.org/info/RFC 9088>.
8.2. Informative References
[RFC 8660] Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing with the MPLS Data Plane", RFC 8660,
DOI 10.17487/RFC 8660, December 2019,
<https://www.rfc-editor.org/info/RFC 8660>.
[RFC 8665] Psenak, P., Ed., Previdi, S., Ed., Filsfils, C., Gredler,
H., Shakir, R., Henderickx, W., and J. Tantsura, "OSPF
Extensions for Segment Routing", RFC 8665,
DOI 10.17487/RFC 8665, December 2019,
<https://www.rfc-editor.org/info/RFC 8665>.
[RFC 8666] Psenak, P., Ed. and S. Previdi, Ed., "OSPFv3 Extensions
for Segment Routing", RFC 8666, DOI 10.17487/RFC 8666,
December 2019, <https://www.rfc-editor.org/info/RFC 8666>.
Acknowledgements
The authors would like to thank Yimin Shen, George Swallow, Acee
Lindem, Les Ginsberg, Ketan Talaulikar, Jeff Tantsura , Bruno
Decraene, and Carlos Pignataro for their valuable comments.
Contributors
The following people contributed to the content of this document and
should be considered coauthors:
Gunter Van de Velde (editor)
Nokia
Antwerp
Belgium
Email: gunter.van_de_velde@nokia.com
Wim Henderickx
Nokia
Belgium
Email: wim.henderickx@nokia.com
Keyur Patel
Arrcus
United States of America
Email: keyur@arrcus.com
Authors' Addresses
Xiaohu Xu
Capitalonline
Email: xiaohu.xu@capitalonline.net
Sriganesh Kini
Email: sriganeshkini@gmail.com
Peter Psenak
Cisco Systems, Inc.
Eurovea Centre, Central 3
Pribinova Street 10
81109 Bratislava
Slovakia
Email: ppsenak@cisco.com
Clarence Filsfils
Cisco Systems, Inc.
Brussels
Belgium
Email: cfilsfil@cisco.com
Stephane Litkowski
Cisco Systems, Inc.
La Rigourdiere
Cesson Sevigne
France
Email: slitkows@cisco.com
Matthew Bocci
Nokia
740 Waterside Drive
Aztec West Business Park
Bristol
BS32 4UF
United Kingdom
Email: matthew.bocci@nokia.com
RFC TOTAL SIZE: 16303 bytes
PUBLICATION DATE: Saturday, August 14th, 2021
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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