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/rfc9089.

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

  [RFC6790] 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 [RFC8665] and
  OSPFv3 [RFC8666].  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 [RFC8660]), 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
  [RFC8662], 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 [RFC6790] and [RFC8662].

  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.

  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

  [RFC7684] 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) [RFC7684] 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 [RFC7684]
  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

  [RFC5340] 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 [RFC5340] or in the Inter-Area-
  Prefix TLV [RFC8362], 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) [RFC5340], or in the External-Prefix TLV
  [RFC8362], 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
  [RFC8476] using the ERLD-MSD type defined in [RFC9088].

  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 [RFC8476], it MUST be ignored.

  The considerations for advertising the ERLD are specified in
  [RFC8662].

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)
  [RFC7752] using existing BGP-LS TLVs.

  The ELC is advertised using the Prefix Attribute Flags TLV as defined
  in [RFC9085].

  The ERLD-MSD is advertised using the Node MSD TLV as defined in
  [RFC8814].

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 [RFC5340], [RFC7684], [RFC7752],
  [RFC7770], [RFC8476], [RFC8662], [RFC8814], and [RFC9085] 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

  [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>.

  [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
             for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
             <https://www.rfc-editor.org/info/rfc5340>.

  [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and
             L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
             RFC 6790, DOI 10.17487/RFC6790, November 2012,
             <https://www.rfc-editor.org/info/rfc6790>.

  [RFC7684]  Psenak, P., Gredler, H., Shakir, R., Henderickx, W.,
             Tantsura, J., and A. Lindem, "OSPFv2 Prefix/Link Attribute
             Advertisement", RFC 7684, DOI 10.17487/RFC7684, November
             2015, <https://www.rfc-editor.org/info/rfc7684>.

  [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>.

  [RFC7770]  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/RFC7770,
             February 2016, <https://www.rfc-editor.org/info/rfc7770>.

  [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>.

  [RFC8362]  Lindem, A., Roy, A., Goethals, D., Reddy Vallem, V., and
             F. Baker, "OSPFv3 Link State Advertisement (LSA)
             Extensibility", RFC 8362, DOI 10.17487/RFC8362, April
             2018, <https://www.rfc-editor.org/info/rfc8362>.

  [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>.

  [RFC8662]  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/RFC8662, December 2019,
             <https://www.rfc-editor.org/info/rfc8662>.

  [RFC8814]  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>.

  [RFC9085]  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/RFC9085, August 2021,
             <https://www.rfc-editor.org/info/rfc9085>.

  [RFC9088]  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/RFC9088, August 2021,
             <https://www.rfc-editor.org/info/rfc9088>.

8.2.  Informative References

  [RFC8660]  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/RFC8660, December 2019,
             <https://www.rfc-editor.org/info/rfc8660>.

  [RFC8665]  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/RFC8665, December 2019,
             <https://www.rfc-editor.org/info/rfc8665>.

  [RFC8666]  Psenak, P., Ed. and S. Previdi, Ed., "OSPFv3 Extensions
             for Segment Routing", RFC 8666, DOI 10.17487/RFC8666,
             December 2019, <https://www.rfc-editor.org/info/rfc8666>.

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: [email protected]


  Wim Henderickx
  Nokia
  Belgium

  Email: [email protected]


  Keyur Patel
  Arrcus
  United States of America

  Email: [email protected]


Authors' Addresses

  Xiaohu Xu
  Capitalonline

  Email: [email protected]


  Sriganesh Kini

  Email: [email protected]


  Peter Psenak
  Cisco Systems, Inc.
  Eurovea Centre, Central 3
  Pribinova Street 10
  81109 Bratislava
  Slovakia

  Email: [email protected]


  Clarence Filsfils
  Cisco Systems, Inc.
  Brussels
  Belgium

  Email: [email protected]


  Stephane Litkowski
  Cisco Systems, Inc.
  La Rigourdiere
  Cesson Sevigne
  France

  Email: [email protected]


  Matthew Bocci
  Nokia
  740 Waterside Drive
  Aztec West Business Park
  Bristol
  BS32 4UF
  United Kingdom

  Email: [email protected]

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