



Internet Engineering Task Force                                   B. Wen
Internet-Draft                                                   Comcast
Updates: 6790, 7447 (if approved)                                K. Wang
Intended status: Standards Track                      J. G. Scudder, Ed.
Expires: 17 April 2026                                               HPE
                                                              S. Mohanty
                                                                 Zscaler
                                                                S. Krier
                                                           Cisco Systems
                                                             K. Kompella
                                                                     HPE
                                                        B. Decraene, Ed.
                                                                  Orange
                                                         14 October 2025


                    BGP Entropy Label Characteristic
                        draft-scudder-idr-elc-00

Abstract

   The BGP Next Hop Dependent Characteristics Attribute (NHC) provides a
   way for a BGP speaker to advertise certain characteristics of routes.
   In particular, it is useful to advertise forwarding plane features.

   This specification defines an NHC characteristic that can be used to
   advertise the ability to process the MPLS Entropy Label as an egress
   LSR for all NLRI advertised in the BGP UPDATE.  It updates RFC 6790
   and RFC 7447 concerning this BGP signaling.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 17 April 2026.





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Copyright Notice

   Copyright (c) 2025 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
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Entropy Label Characteristic (ELCv3)  . . . . . . . . . . . .   3
     2.1.  Encoding  . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Sending the ELCv3 . . . . . . . . . . . . . . . . . . . .   4
       2.2.1.  Aggregation . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Receiving the ELCv3 . . . . . . . . . . . . . . . . . . .   5
     2.4.  ELCv3 Error Handling  . . . . . . . . . . . . . . . . . .   5
   3.  Legacy ELC  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   8
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   [I-D.scudder-idr-nhc] provides a way for a BGP speaker to advertise
   certain characteristics of routes.  In particular, it is useful to
   advertise forwarding plane features.












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   This specification defines an NHC characteristic, called "ELCv3", to
   advertise the ability to process the Multiprotocol Label Switching
   (MPLS) Entropy Label as an egress Label Switching Router (LSR) for
   all NLRI advertised in the BGP UPDATE.  It updates [RFC6790] and
   [RFC7447] with regard to this BGP signaling; this is further
   discussed in Section 2.  ELCv3 is only relevant to NLRI of labeled
   address families.  (The term "labeled address family" is defined in
   the first paragraph of Section 3.5 of [RFC9012].  In this document,
   we use the term "labeled NLRI" as a short form of "NLRI of a labeled
   address family.")

1.1.  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.  Entropy Label Characteristic (ELCv3)

   [I-D.scudder-idr-nhc] defines the NHC as a container for
   characteristic TLVs.  The Entropy Label Characteristic is one such
   characteristic.

   When BGP [RFC4271] is used for distributing labeled NLRI as described
   in, for example, [RFC8277], the route may include the ELCv3 as part
   of the NHC.  The inclusion of this characteristic with a route
   indicates that the egress of the associated Label Switched Path (LSP)
   can process entropy labels as an egress LSR for that route -- see
   Section 4.1 of [RFC6790].  Below, we refer to this for brevity as
   being "EL-capable."

   For historical reasons, this characteristic is referred to as
   "ELCv3", to distinguish it from the prior Entropy Label Capability
   (ELC) defined in [RFC6790] and deprecated in [RFC7447], and the ELCv2
   described in [I-D.scudder-bgp-entropy-label].

   This section (and its subsections) replaces Section 5.2 of [RFC6790],
   which was previously deprecated by [RFC7447].

2.1.  Encoding

   The ELCv3 has characteristic code 1, characteristic length 0, and
   carries no value:






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       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Characteristic Code = 1    |   Characteristic Length = 0   |
      +-------------------------------+-------------------------------+

                         Figure 1: ELCv3 TLV Format

2.2.  Sending the ELCv3

   When a BGP speaker S has a route R it wishes to advertise with next
   hop N to its peer, it MAY include the ELCv3 characteristic if it
   knows that the egress of the associated LSP L is EL-capable;
   otherwise, it MUST NOT include the ELCv3 characteristic.  Specific
   conditions where S would know that the egress is EL-capable are if S:

   *  Is itself the egress, and knows itself to be EL-capable, or

   *  Is re-advertising a BGP route it received with a valid ELCv3
      characteristic, and is preserving the value of N as received, or

   *  Is re-advertising a BGP route it received with a valid ELCv3
      characteristic, and is changing the next hop that it has received
      to N, and knows that this new next hop (normally itself) is EL-
      capable, or

   *  Is re-advertising a BGP route it received with a valid ELCv3
      characteristic, and is changing the next hop that it has received
      to N, and knows (for example, through configuration) that the new
      next hop (normally itself) even if not EL-capable will simply swap
      labels without popping the BGP-advertised label stack and
      processing the label below, as with a transit LSR, or

   *  Knows by implementation-specific means that the egress is EL-
      capable, or

   *  Is redistributing a route learned from another protocol, and that
      other protocol conveyed the knowledge that the egress of L was EL-
      capable.  (For example, this might be known through the Label
      Distribution Protocol (LDP) ELC TLV, Section 5.1 of [RFC6790].)

   The ELCv3 MAY be advertised with routes that are labeled, such as
   those using SAFI 4 [RFC8277].  It MUST NOT be advertised with
   unlabeled routes.







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2.2.1.  Aggregation

   When forming an aggregate (see Section 2.2.2 of
   [I-D.scudder-idr-nhc]), the aggregate route thus formed MUST NOT
   include the ELCv3 unless each constituent route would be eligible to
   include the ELCv3 according to the criteria given above.

2.3.  Receiving the ELCv3

   (Below, we assume that "includes the ELCv3" implies that the
   containing NHC has passed the checks specified in Section 2.3 of
   [I-D.scudder-idr-nhc].  If it had not passed, then the NHC would have
   been discarded and the ELCv3 would be deemed not to have been
   included.)

   When a BGP speaker receives an unlabeled route that includes the
   ELCv3, it MUST discard the ELCv3.

   When a BGP speaker receives a labeled route, if it includes the
   ELCv3, that indicates it can safely insert an entropy label into the
   label stack of the associated LSP.  This implies that the receiving
   BGP speaker, if acting as ingress, MAY insert an entropy label as per
   Section 4.2 of [RFC6790].

2.4.  ELCv3 Error Handling

   The ELCv3 is considered malformed and must be disregarded if its
   length is other than zero.

   If more than one instance of the ELCv3 is included in an NHC,
   instances beyond the first MUST be disregarded.

3.  Legacy ELC

   The ELCv3 functionality introduced in this document replaces the "BGP
   Entropy Label Capability Attribute" (ELC attribute) that was
   introduced by [RFC6790], and deprecated by [RFC7447].  The latter RFC
   specifies that the ELC attribute, BGP path attribute 28, "MUST be
   treated as any other unrecognized optional, transitive attribute".
   This specification revises that requirement.











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   As the current specification was developed, it became clear that due
   to incompatibilities between how the ELC attribute is processed by
   different fielded implementations, the most prudent handling of
   attribute 28 is not to propagate it as an unrecognized optional,
   transitive attribute, but to discard it.  Therefore, this
   specification updates [RFC7447] by instead requiring that an
   implementation that receives the ELC attribute MUST discard any
   received ELC attribute.

4.  IANA Considerations

   [I-D.scudder-idr-nhc] created a new registry called "BGP Next Hop
   Dependent Characteristic Codes" within the Border Gateway Protocol
   (BGP) Parameters group and seeded it with values including:

         +=======+=============+============+===================+
         | Value | Description | Reference  | Change Controller |
         +=======+=============+============+===================+
         | 1     | ELCv3       | (this doc) | IETF              |
         +-------+-------------+------------+-------------------+

                                 Table 1

   IANA is requested to update the reference to this document.

5.  Security Considerations

   Insertion of an ELCv3 by an attacker could cause forwarding to fail.
   Deletion of an ELCv3 by an attacker could cause one path in the
   network to be overutilized and another to be underutilized.  However,
   we note that an attacker able to accomplish either of these (below,
   an "on-path attacker") could equally insert or remove any other BGP
   path attribute or message.  The former attack described above denies
   service for a given route, which can be accomplished by an on-path
   attacker in any number of ways, even absent ELCv3.  The latter attack
   defeats an optimization but nothing more; it seems dubious that an
   attacker would go to the trouble of doing so rather than launching
   some more damaging attack.

6.  References

6.1.  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/rfc/rfc2119>.




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   [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/rfc/rfc4271>.

   [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/rfc/rfc6790>.

   [RFC7447]  Scudder, J. and K. Kompella, "Deprecation of BGP Entropy
              Label Capability Attribute", RFC 7447,
              DOI 10.17487/RFC7447, February 2015,
              <https://www.rfc-editor.org/rfc/rfc7447>.

   [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/rfc/rfc8174>.

   [RFC9012]  Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
              "The BGP Tunnel Encapsulation Attribute", RFC 9012,
              DOI 10.17487/RFC9012, April 2021,
              <https://www.rfc-editor.org/rfc/rfc9012>.

   [I-D.scudder-idr-nhc]
              Decraene, B., Kompella, K., Krier, S., Satya, M. R.,
              Scudder, J., Wang, K., and B. Wen, "BGP Next Hop Dependent
              Characteristics Attribute", Work in Progress, Internet-
              Draft, draft-scudder-idr-nhc-00, 14 October 2025,
              <https://datatracker.ietf.org/doc/html/draft-scudder-idr-
              nhc-00>.

6.2.  Informative References

   [I-D.ietf-idr-next-hop-capability]
              Decraene, B., Kompella, K., and W. Henderickx, "BGP Next-
              Hop dependent capabilities", Work in Progress, Internet-
              Draft, draft-ietf-idr-next-hop-capability-08, 8 June 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-idr-
              next-hop-capability-08>.

   [I-D.scudder-bgp-entropy-label]
              Scudder, J. and K. Kompella, "BGP Entropy Label
              Capability, Version 2", Work in Progress, Internet-Draft,
              draft-scudder-bgp-entropy-label-00, 28 April 2022,
              <https://datatracker.ietf.org/doc/html/draft-scudder-bgp-
              entropy-label-00>.




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   [RFC8277]  Rosen, E., "Using BGP to Bind MPLS Labels to Address
              Prefixes", RFC 8277, DOI 10.17487/RFC8277, October 2017,
              <https://www.rfc-editor.org/rfc/rfc8277>.

Acknowledgements

   The authors of this specification thank Randy Bush, Mach Chen, Wes
   Hardaker, Jeff Haas, Susan Hares, Ketan Talaulikar, and Gyan Mishra
   for their review and comments.

   This specification derives from two earlier documents,
   [I-D.ietf-idr-next-hop-capability] and
   [I-D.scudder-bgp-entropy-label].

   [I-D.ietf-idr-next-hop-capability] included the following
   acknowledgements:

     The Entropy Label Next-Hop Capability defined in this document is
     based on the ELC BGP attribute defined in section 5.2 of [RFC6790].

     The authors wish to thank John Scudder for the discussions on this
     topic and Eric Rosen for his in-depth review of this document.

     The authors wish to thank Jie Dong and Robert Raszuk for their
     review and comments.

   [I-D.scudder-bgp-entropy-label] included the following
   acknowledgements:

       Thanks to Swadesh Agrawal, Alia Atlas, Bruno Decraene, Martin
       Djernaes, John Drake, Adrian Farrell, Keyur Patel, Toby Rees, and
       Ravi Singh, for their discussion of this issue.

Contributors

   James Uttaro
   Independent Contributor
   Email: juttaro@ieee.org


   Wim Henderickx
   Nokia
   Email: wim.henderickx@nokia.com


Authors' Addresses





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   Bin Wen
   Comcast
   Email: Bin_Wen@comcast.com


   Kevin Wang
   HPE
   Email: kfwang@juniper.net


   John G. Scudder (editor)
   HPE
   Email: jgs@juniper.net


   Satya Mohanty
   Zscaler
   Email: smohanty@zscaler.com


   Serge Krier
   Cisco Systems
   Email: sekrier@cisco.com


   Kireeti Kompella
   HPE
   Email: kireeti@juniper.net


   Bruno Decraene (editor)
   Orange
   Email: bruno.decraene@orange.com


















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