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Signaling Entropy Label Capability and Entropy Readable Label Depth Using IS-IS
RFC 9088

Document Type RFC - Proposed Standard (August 2021)
Authors Xiaohu Xu , Sriganesh Kini , Peter Psenak , Clarence Filsfils , Stephane Litkowski , Matthew Bocci
Last updated 2021-08-14
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
IESG Responsible AD Alvaro Retana
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RFC 9088

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
                                                             August 2021

  Signaling Entropy Label Capability and Entropy Readable Label Depth
                              Using IS-IS


   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

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
   ( 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
   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",
   "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

      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

   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

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

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

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

   *  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

   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,

   [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,

   [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,

   [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,

   [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, <>.

   [RFC7981]  Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions
              for Advertising Router Information", RFC 7981,
              DOI 10.17487/RFC7981, October 2016,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [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,

   [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,

   [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,

   [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,

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,

   [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,


   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.


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

   Gunter Van de Velde (editor)


   Wim Henderickx


   Keyur Patel
   United States of America


Authors' Addresses

   Xiaohu Xu


   Sriganesh Kini


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


   Clarence Filsfils
   Cisco Systems, Inc.


   Stephane Litkowski
   Cisco Systems, Inc.
   La Rigourdiere
   Cesson Sevigne


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