Internet Engineering Task Force (IETF)                             X. Xu
Request for Comments: 9088                                 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 IS-IS

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
  IS-IS 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/rfc9088.

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 IS-IS
  4.  Advertising ERLD Using IS-IS
  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 IS-IS [RFC8667].  This
  document defines a mechanism to signal the ELC using IS-IS.

  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's necessary to insert
  multiple ELs at different positions in the label stack.  This
  document defines a mechanism to signal the ERLD using IS-IS.

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.

3.  Advertising ELC Using IS-IS

  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 a
  multi-area network, routers may not know the identity of the prefix
  originator in a remote area or may not know the capabilities of such
  originator.  Similarly, in a multi-domain network, the identity of
  the prefix originator and its capabilities may not be known to the
  ingress LSR.

  Bit 3 in the Prefix Attribute Flags [RFC7794] is used as the ELC Flag
  (E-Flag), as shown in Figure 1.  If a router has multiple interfaces,
  the router MUST NOT announce the ELC for any local host prefixes
  unless all of its interfaces are capable of processing ELs.  If a
  router supports ELs on all of its interfaces, it SHOULD set the ELC
  for every local host prefix it advertises in IS-IS.

         0 1 2 3 4 5 6 7...
        +-+-+-+-+-+-+-+-+...
        |X|R|N|E|        ...
        +-+-+-+-+-+-+-+-+...

                     Figure 1: Prefix Attribute Flags

  E-Flag:
     ELC Flag (Bit 3) - Set for local host prefix of the originating
     node if it supports ELC on all interfaces.

  The ELC signaling MUST be preserved when a router propagates a prefix
  between IS-IS levels [RFC5302].

  When redistributing a prefix between two IS-IS protocol instances or
  redistributing from another protocol to an IS-IS protocol instance, a
  router SHOULD preserve the ELC signaling for that prefix if it
  exists.  The exact mechanism used to exchange ELC between protocol
  instances running on an Autonomous System Border Router is outside of
  the scope of this document.

4.  Advertising ERLD Using IS-IS

  A new MSD-Type [RFC8491], called ERLD-MSD, is defined to advertise
  the ERLD [RFC8662] of a given router.  An MSD-Type code 2 has been
  assigned by IANA for ERLD-MSD.  The MSD-Value field is set to the
  ERLD in the range between 0 to 255.  The scope of the advertisement
  depends on the application.  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 its
  interfaces.

  The absence of ERLD-MSD advertisements indicates only that the
  advertising node does not support advertisement of this capability.

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

  If the ERLD-MSD type is received in the Link MSD sub-TLV, it MUST be
  ignored.

5.  Signaling ELC and ERLD in BGP-LS

  The IS-IS 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:

  *  Bit 3 in the "Bit Values for Prefix Attribute Flags Sub-TLV"
     registry has been assigned to the ELC Flag.  IANA has updated the
     registry to reflect the name used in this document: ELC Flag
     (E-Flag).

  *  Type 2 in the "IGP MSD-Types" registry has been assigned for the
     ERLD-MSD.  IANA has updated the registry to reflect the name used
     in this document: ERLD-MSD.

7.  Security Considerations

  This document specifies the ability to advertise additional node
  capabilities using IS-IS and BGP-LS.  As such, the security
  considerations as described in [RFC7752], [RFC7794], [RFC7981],
  [RFC8491], [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 MPLS traffic being discarded on the egress node.

  Incorrectly setting 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>.

  [RFC5302]  Li, T., Smit, H., and T. Przygienda, "Domain-Wide Prefix
             Distribution with Two-Level IS-IS", RFC 5302,
             DOI 10.17487/RFC5302, October 2008,
             <https://www.rfc-editor.org/info/rfc5302>.

  [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>.

  [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>.

  [RFC7794]  Ginsberg, L., Ed., Decraene, B., Previdi, S., Xu, X., and
             U. Chunduri, "IS-IS Prefix Attributes for Extended IPv4
             and IPv6 Reachability", RFC 7794, DOI 10.17487/RFC7794,
             March 2016, <https://www.rfc-editor.org/info/rfc7794>.

  [RFC7981]  Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions
             for Advertising Router Information", RFC 7981,
             DOI 10.17487/RFC7981, October 2016,
             <https://www.rfc-editor.org/info/rfc7981>.

  [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>.

  [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>.

  [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>.

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>.

  [RFC8667]  Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
             Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
             Extensions for Segment Routing", RFC 8667,
             DOI 10.17487/RFC8667, December 2019,
             <https://www.rfc-editor.org/info/rfc8667>.

Acknowledgements

  The authors would like to thank Yimin Shen, George Swallow, Acee
  Lindem, Les Ginsberg, Ketan Talaulikar, Jeff Tantsura, Bruno
  Decraene, Carlos Pignataro, Wim Hendrickx, and Gunter Van de Velde
  for their valuable comments.

Contributors

  The following people contributed to the content of this document and
  should be considered as 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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