BGP Entropy Label Characteristic
draft-ietf-idr-elc-01
| Document | Type | Active Internet-Draft (idr WG) | |
|---|---|---|---|
| Authors | Bin Wen , Kevin Wang , John Scudder , SATYA R MOHANTY , Serge Krier , Kireeti Kompella , Bruno Decraene | ||
| Last updated | 2026-06-22 | ||
| Replaces | draft-scudder-idr-elc, draft-ietf-idr-entropy-label | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
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| Additional resources | Mailing list discussion | ||
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draft-ietf-idr-elc-01
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
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Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Copyright (c) 2026 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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
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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].
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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].)
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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
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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".
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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,
<https://www.rfc-editor.org/rfc/rfc2119>.
[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>.
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[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.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,
<https://datatracker.ietf.org/doc/html/draft-ietf-idr-nhc-
07>.
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,
<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>.
[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,
<https://www.rfc-editor.org/rfc/rfc7942>.
[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
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.
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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
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