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: 24 December 2026 HPE S. Mohanty Zscaler S. Krier Cisco Systems K. Kompella HPE B. Decraene, Ed. Orange 22 June 2026 BGP Entropy Label Characteristic draft-ietf-idr-elc-01 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. About This Document This note is to be removed before publishing as an RFC. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-ietf-idr-elc/. Discussion of this document takes place on the IDR Working Group mailing list (mailto:idr@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/idr/. Subscribe at https://www.ietf.org/mailman/listinfo/idr/. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Wen, et al. Expires 24 December 2026 [Page 1] Internet-Draft ELC June 2026 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 24 December 2026. Copyright Notice Copyright (c) 2026 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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 2. Entropy Label Characteristic (ELCv3) . . . . . . . . . . . . 3 2.1. Encoding . . . . . . . . . . . . . . . . . . . . . . . . 4 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. Implementation Status . . . . . . . . . . . . . . . . . . . . 6 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 7.1. Normative References . . . . . . . . . . . . . . . . . . 7 7.2. Informative References . . . . . . . . . . . . . . . . . 8 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 8 Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9 Wen, et al. Expires 24 December 2026 [Page 2] Internet-Draft ELC June 2026 1. Introduction [I-D.ietf-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. 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.ietf-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]. Wen, et al. Expires 24 December 2026 [Page 3] Internet-Draft ELC June 2026 2.1. Encoding The ELCv3 has characteristic code 1, characteristic length 0, and carries no value: 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].) Wen, et al. Expires 24 December 2026 [Page 4] Internet-Draft ELC June 2026 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. 2.2.1. Aggregation When forming an aggregate (see Section 2.2.2 of [I-D.ietf-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.ietf-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. 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 Wen, et al. Expires 24 December 2026 [Page 5] Internet-Draft ELC June 2026 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.ietf-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. Implementation Status | RFC Editor: Please remove this entire section before | publication, as well as the reference to RFC 7942. This section refers the reader to the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in [RFC7942]. The description of implementations referenced by this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist. According to [RFC7942], "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit". Wen, et al. Expires 24 December 2026 [Page 6] Internet-Draft ELC June 2026 Implementations are reported at the IDR implementation status Wiki (https://wiki.ietf.org/group/idr/implementations/draft-ietf-idr- entropy-label). 6. 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. 7. References 7.1. 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, . [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, . [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, . [RFC7447] Scudder, J. and K. Kompella, "Deprecation of BGP Entropy Label Capability Attribute", RFC 7447, DOI 10.17487/RFC7447, February 2015, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . Wen, et al. Expires 24 December 2026 [Page 7] Internet-Draft ELC June 2026 [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, . [I-D.ietf-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-ietf-idr-nhc-07, 4 June 2026, . 7.2. 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, . [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, . [RFC7942] Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", BCP 205, RFC 7942, DOI 10.17487/RFC7942, July 2016, . [RFC8277] Rosen, E., "Using BGP to Bind MPLS Labels to Address Prefixes", RFC 8277, DOI 10.17487/RFC8277, October 2017, . Acknowledgements This specification derives from two earlier documents, [I-D.ietf-idr-next-hop-capability] and [I-D.scudder-bgp-entropy-label]. The authors of The authors of the present specification thank Randy Bush, Mach Chen, Wes Hardaker, Jeff Haas, Susan Hares, Ketan Talaulikar, and Gyan Mishra for their review and comments. Wen, et al. Expires 24 December 2026 [Page 8] Internet-Draft ELC June 2026 Contributors James Uttaro Independent Contributor Email: juttaro@ieee.org Wim Henderickx Nokia Email: wim.henderickx@nokia.com Authors' Addresses Bin Wen Comcast Email: Bin_Wen@comcast.com Kevin Wang HPE Email: kevin.wang@hpe.com John G. Scudder (editor) HPE Email: john.scudder@hpe.com Satya Mohanty Zscaler Email: smohanty@zscaler.com Serge Krier Cisco Systems Email: sekrier@cisco.com Kireeti Kompella HPE Email: kireeti.kompella@hpe.com Bruno Decraene (editor) Orange Email: bruno.decraene@orange.com Wen, et al. Expires 24 December 2026 [Page 9]