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BGP Router Capabilities Attribute
draft-ietf-idr-entropy-label-05

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Authors Bruno Decraene , John Scudder , Wim Henderickx , Kireeti Kompella , SATYA R MOHANTY , Jim Uttaro , Bin Wen
Last updated 2023-07-10 (Latest revision 2023-07-07)
Replaces draft-scudder-idr-entropy-label, draft-ietf-idr-next-hop-capability
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draft-ietf-idr-entropy-label-05
Internet Engineering Task Force                         B. Decraene, Ed.
Internet-Draft                                                    Orange
Updates: 6790, 7447 (if approved)                     J. G. Scudder, Ed.
Intended status: Standards Track                        Juniper Networks
Expires: 11 January 2024                                   W. Henderickx
                                                                   Nokia
                                                             K. Kompella
                                                        Juniper Networks
                                                              S. Mohanty
                                                           Cisco Systems
                                                               J. Uttaro
                                                 Independent Contributor
                                                                  B. Wen
                                                                 Comcast
                                                            10 July 2023

                   BGP Router Capabilities Attribute
                    draft-ietf-idr-entropy-label-05

Abstract

   RFC 5492 allows a BGP speaker to advertise its capabilities to its
   peer.  When a route is propagated beyond the immediate peer, it is
   useful to allow certain capabilities to be conveyed further.  In
   particular, it is useful to advertise forwarding plane features.

   This specification defines a new BGP transitive attribute to carry
   such capability information, the "Router Capabilities Attribute," or
   RCA.  Unlike the capabilities defined by RFC 5492, those conveyed in
   the RCA apply solely to the routes advertised by the BGP UPDATE that
   contains the particular RCA.

   This specification also defines an RCA capability 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/.

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   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 11 January 2024.

Copyright Notice

   Copyright (c) 2023 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.  BGP Router Capabilities Attribute . . . . . . . . . . . . . .   4
     2.1.  Encoding  . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Sending the RCA . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Receiving the RCA . . . . . . . . . . . . . . . . . . . .   6
     2.4.  Attribute Error Handling  . . . . . . . . . . . . . . . .   7
     2.5.  Network Operation Considerations  . . . . . . . . . . . .   7
   3.  Entropy Label Capability (ELCv3)  . . . . . . . . . . . . . .   8
     3.1.  Encoding  . . . . . . . . . . . . . . . . . . . . . . . .   8
     3.2.  Sending the ELCv3 . . . . . . . . . . . . . . . . . . . .   8
     3.3.  Receiving the ELCv3 . . . . . . . . . . . . . . . . . . .   9
     3.4.  ELCv3 Error Handling  . . . . . . . . . . . . . . . . . .   9
   4.  Legacy ELC  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
     6.1.  Considerations for the RCA  . . . . . . . . . . . . . . .  10
     6.2.  Considerations for the ELCv3 Capability . . . . . . . . .  11
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Other Means of Signaling EL Capability . . . . . . .  13
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  13
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

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1.  Introduction

   [RFC5492] allows a BGP speaker to advertise its capabilities to its
   peer.  When a route is propagated beyond the immediate peer, it is
   useful to allow certain capabilities to be conveyed further.  In
   particular, it may be useful to advertise forwarding plane features.

   This specification defines a new BGP optional transitive attribute to
   carry such capability information, the "Router Capabilities
   Attribute", or RCA.  (This somewhat ponderous name is regrettable but
   is needed in order to be descriptive while still distinguishing it
   from RFC 5492 BGP Capabilities.)

   Since the RCA is intended chiefly for conveying information about
   forwarding plane features, it needs to be regenerated whenever the
   BGP route's next hop is changed.  Since owing to the properties of
   BGP transitive attributes this can't be guaranteed (an intermediate
   router that doesn't implement this specification would be expected to
   propagate the RCA as opaque data), the RCA identifies itself with the
   next hop of its originator.  If the RCA passes through a router that
   changes the next hop without regenerating the RCA, they will fail to
   match when later examined, and the recipient can act accordingly.
   This scheme allows RCA support to be introduced into a network
   incrementally.  Complete details are provided in Section 2.

   An RCA carried in a given BGP UPDATE message conveys information that
   relates to all NLRI advertised in that particular UPDATE, and only to
   those NLRI.  A different UPDATE message originated by the same source
   might not include an RCA, and if so, NLRI carried in that UPDATE
   would not be affected by the RCA.  By implication, if a router wishes
   to use RCA to describe all NLRI it originates, it needs to include an
   RCA with each UPDATE it sends.  In this respect, despite its similar
   naming, the RCA is unlike RFC 5492 BGP Capabilities.

   This specification also defines an RCA to advertise the ability to
   process the MPLS Entropy Label as an egress 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 3.

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.

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2.  BGP Router Capabilities Attribute

2.1.  Encoding

   The BGP Router Capabilities attribute (RCA attribute, or just RCA) is
   an optional, transitive BGP path attribute with type code 39.  The
   RCA has as its data a network layer address, representing the next
   hop of the route the RCA accompanies.  The RCA signals potentially
   useful optimizations, so it is desirable to make it transitive; the
   next hop data is to ensure correctness if it traverses BGP speakers
   that do not understand the RCA.

   The Attribute Data field of the RCA attribute is encoded as a header
   portion that identifies the originator of the attribute, followed by
   one or more capability TLVs.

        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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Address Family Identifier   |     SAFI      | Next Hop Len  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~             Network Address of Next Hop (variable)            ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                   Capability TLVs (variable)                  ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                            Figure 1: RCA Format

   The meanings of the header fields (Address Family Identifier, SAFI or
   Subsequent Address Family Identifier, Length of Next Hop, and Network
   Address of Next Hop) are as given in Section 3 of [RFC4760].

   In turn, each Capability is a triple (Capability Code, Capability
   Length, Capability 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |        Capability Code        |        Capability Length      |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ~                  Capability Value (variable)                  ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 2: Capability TLV Format

   Capability Code: a two-octet unsigned binary integer that indicates
   the type of Capability advertised and unambiguously identifies an
   individual capability.

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   Capability Length: a two-octet unsigned binary integer that indicates
   the length, in octets, of the Capability Value field.  A length of 0
   indicates that no Capability Value field is present.

   Capability Value: a variable-length field.  It is interpreted
   according to the value of the Capability Code.

   A BGP speaker MUST NOT include more than one instance of a capability
   with the same Capability Code, Capability Length, and Capability
   Value.  Note, however, that processing multiple instances of such a
   capability does not require special handling, as additional instances
   do not change the meaning of the announced capability; thus, a BGP
   speaker MUST be prepared to accept such multiple instances.

   BGP speakers MAY include more than one instance of a capability (as
   identified by the Capability Code) with different Capability Value
   and either the same or different Capability Length.  Processing of
   these capability instances is specific to the Capability Code and
   MUST be described in the document introducing the new capability.

   Capability TLVs MUST be placed in the RCA in increasing order of
   Capability Code.  (In the event of multiple instances of a capability
   with the same Capability Code as discussed above, no further sorting
   order is defined here.)  Although the major sorting order is
   mandated, an implementation MAY elect to be prepared to consume
   capabilities in any order, for robustness reasons.

2.2.  Sending the RCA

   Suppose a BGP speaker S has a route R it wishes to advertise with
   next hop N to its peer.

   If S is originating R into BGP, it MAY include an RCA attribute with
   it, that carries capability TLVs that describe aspects of R.  S MUST
   set the header portion of the RCA to be equal to N, using the
   encoding given above.

   If S has received R from some other BGP speaker, two possibilities
   exist.  First, S could be propagating R without changing N.  In that
   case, S need take no special action, it SHOULD simply propagate the
   RCA unchanged unless specifically configured otherwise.  Indeed, we
   observe that this is no different from the default action a BGP
   speaker takes with an unrecognized optional transitive attribute --
   it is treated as opaque data and propagated.

   Second, S could be changing R in some way, and in particular, it
   could be changing N.  If S has changed N it MUST NOT propagate the
   RCA unchanged.  It MAY include a newly-constructed RCA attribute with

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   R, constructed as described above in the "originating R into BGP"
   case.  Any given capability TLV carried by the newly-constructed RCA
   attribute might use information from the received RCA attribute as
   input to its construction; the details of this are specific to the
   definition of each capability.  In effect, this means that a given
   capability TLV will only be propagated along a path where it is
   supported by every BGP next-hop along the path.

   The RCA MAY be sent by default to IBGP peers.  It MUST NOT be sent by
   default to peers not under the administrative control of the local
   network administrator (so, generally, to EBGP peers).

   We note that due to the nature of BGP optional transitive path
   attributes, any BGP speaker that does not implement this
   specification will propagate the RCA, the requirements of this
   section notwithstanding.  Such a speaker will not update the RCA,
   however.

2.3.  Receiving the RCA

   By default, the RCA MUST NOT be accepted from peers not under the
   administrative control of the local network administrator (so,
   generally, from EBGP peers); if received it MUST be discarded without
   further processing, except that the contents MAY be logged.  An
   implementation MAY enable RCA processing by default from peers under
   the administrative control of the local network administrator (so,
   generally, from IBGP peers).  An implementation SHOULD provide the
   ability to modify these default settings by configuration.

   When a BGP speaker receives a BGP route that includes the RCA, it
   MUST compare the address given in the header portion of the RCA to
   the next hop of the BGP route.  If the two match, the RCA may be
   further processed.  If the two do not match, it means some
   intermediate BGP speaker that handled the route in transit both does
   not support RCA, and changed the next hop of the route.  In this
   case, the contents of the RCA cannot be used, and the RCA MUST be
   discarded without further processing, except that the contents MAY be
   logged.

   In considering whether the next hop "matches", a semantic match is
   sought.  While bit-for-bit equality is a trivial test of matching,
   there may be certain cases where the two are not bit-for-bit equal,
   but still "match".  An example is when a MP_REACH Next Hop encodes
   both a global and a link-local IPv6 address.  In that case the link-
   local address might be removed during IBGP propagation, the two would
   still be considered to match if they were equal on the global part.
   See Section 3 of [RFC2545].

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   A BGP speaker receiving a Capability Code that it supports behaves as
   defined in the document defining the Capability Code.  A BGP speaker
   receiving a Capability Code that it does not support MUST ignore that
   Capability Code.  In particular, it MUST NOT be handled as an error.

   The presence of a Capability SHOULD NOT influence route selection or
   route preference, unless tunneling is used to reach the BGP next hop
   or the selected route has been learned from External BGP (that is,
   the next hop is in a different Autonomous System).  Indeed, it is in
   general impossible for a node to know that all BGP routers of the
   Autonomous System (AS) will understand a given capability, and if
   different routers within an AS were to use a different preference for
   a route, forwarding loops could result unless tunneling is used to
   reach the BGP next hop.

2.4.  Attribute Error Handling

   An RCA is considered malformed if the length of the attribute is
   inconsistent with the lengths of the contained capability TLVs.

   A BGP UPDATE message with a malformed RCA SHALL be handled using the
   approach of "attribute discard" defined in [RFC7606].

   Unknown Capability Codes MUST NOT be considered to be an error.

   An RCA that contains no capability TLVs MAY be considered malformed,
   although it is observed that the prescribed behavior of "attribute
   discard" is semantically no different from that of having no TLVs to
   process.

   A document that specifies a new RCA Capability should provide
   specifics regarding what constitutes an error for that RCA
   Capability.

   If a capability TLV is malformed, that capability TLV MUST be ignored
   and removed.  Other capability TLVs MUST be processed as usual.

2.5.  Network Operation Considerations

   In the corner case where multiple nodes use the same IP address as
   their BGP next hop, such as with anycast nodes as described in
   [RFC4786], a BGP speaker MUST NOT advertise a given capability unless
   all nodes sharing this same IP address support this capability.  The
   network operator operating those anycast nodes is responsible for
   ensuring that an anycast node does not advertise a capability not
   supported by all nodes sharing this anycast address.  The means for
   accomplishing this are beyond the scope of this document.

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3.  Entropy Label Capability (ELCv3)

   When BGP [RFC4271] is used for distributing labeled Network Layer
   Reachability Information (NLRI) as described in, for example,
   [RFC8277], the route may include the ELCv3 as part of the RCA.  The
   inclusion of this capability with a route indicates that the egress
   of the associated Label Switched Path (LSP) can process entropy
   labels as an egress Label Switched Router (LSR) for that route -- see
   Section 4.2 of [RFC6790].  Below, we refer to this for brevity as
   being "EL-capable."

   For historical reasons, this capability 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].

3.1.  Encoding

   The ELCv3 has capability code 1, capability 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Capability Code = 1      |       Capability Length = 0   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 3: ELCv3 TLV Format

3.2.  Sending the ELCv3

   When a BGP speaker S has a route R it wishes to advertise with next
   hop N to its peer, it MUST NOT include the ELCv3 capability except if
   it knows that the egress of the associated LSP L is EL-capable.
   Specifically, this will be true 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
      capability, and is not changing the value of N, or

   *  Is re-advertising a BGP route it received with a valid ELCv3
      capability, and is changing the value of N, and knows (for
      example, through configuration) that the router represented by N
      is either the LSP egress and is EL-capable, or that it will simply
      swap labels without popping the BGP-advertised label stack and
      processing the label below, as with a transit LSR, or

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   *  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 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.

3.3.  Receiving the ELCv3

   (Below, we assume that "includes the ELCv3" implies that the
   containing RCA has passed the checks specified in Section 2.3.  If it
   had not passed, then the RCA 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 that includes the ELCv3,
   that indicates the LSP supports entropy labels, which implies that
   the receiving BGP speaker, if acting as ingress, MAY insert an
   entropy label as per Section 4.2 of [RFC6790].

3.4.  ELCv3 Error Handling

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

4.  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
   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.

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5.  IANA Considerations

   IANA has made a temporary allocation in the BGP Path Attributes
   registry of the Border Gateway Protocol (BGP) Parameters group.  IANA
   is requested to make this allocation permanent.

          +=======+===============================+============+
          | Value | Code                          | Reference  |
          +=======+===============================+============+
          | 39    | BGP Router Capabilities (RCA) | (this doc) |
          +-------+-------------------------------+------------+

                                 Table 1

   IANA is requested to create a new registry called "BGP Router
   Capability Codes" within the Border Gateway Protocol (BGP) Parameters
   group.  The registry's allocation policy is First Come, First Served.
   It is seeded with the following values:

   +===============+==================+============+===================+
   | Value         | Description      | Reference  | Change Controller |
   +===============+==================+============+===================+
   | 0             | reserved         | (this doc) | IETF              |
   +---------------+------------------+------------+-------------------+
   | 1             | ELCv3            | (this doc) | IETF              |
   +---------------+------------------+------------+-------------------+
   | 65500 -       | reserved for     | (this doc) | IETF              |
   | 65534         | experimental use |            |                   |
   +---------------+------------------+------------+-------------------+
   | 65535         | reserved         | (this doc) | IETF              |
   +---------------+------------------+------------+-------------------+

                                  Table 2

6.  Security Considerations

6.1.  Considerations for the RCA

   The header portion of the RCA contains the next hop the attribute's
   originator included when sending it.  This will typically be an IP
   address of the router in question.  This may be an infrastructure
   address the network operator does not intend to announce beyond the
   border of its Autonomous System, and it may even be considered in
   some weak sense, confidential information.  Although the desired
   operation of the protocol is for the attribute's propagation scope to
   be limited to the network operator's own Autonomous System, this
   can't be guaranteed in all cases -- if a border router doesn't
   implement this specification, the attribute, like all BGP optional

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   transitive attributes, will propagate to neighboring Autonomous
   Systems.  Similarly, if a border router receiving the attribute from
   an external Autonomous System doesn't implement this specification,
   it will store and propagate the attribute, the requirements of
   Section 2.3 notwithstanding.  So, sometimes this information could
   leak beyond its intended scope.  (Note that it will only propagate as
   far as the first router that does support this specification, at
   which point it will be discarded per Section 2.3.)

   If the attribute leaks beyond its intended scope, capabilities within
   it would potentially be exposed.  Specifications for individual
   capabilities should consider the consequences of such unintended
   exposure.

6.2.  Considerations for the ELCv3 Capability

   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>.

   [RFC2545]  Marques, P. and F. Dupont, "Use of BGP-4 Multiprotocol
              Extensions for IPv6 Inter-Domain Routing", RFC 2545,
              DOI 10.17487/RFC2545, March 1999,
              <https://www.rfc-editor.org/rfc/rfc2545>.

   [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>.

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   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760,
              DOI 10.17487/RFC4760, January 2007,
              <https://www.rfc-editor.org/rfc/rfc4760>.

   [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>.

   [RFC7606]  Chen, E., Ed., Scudder, J., Ed., Mohapatra, P., and K.
              Patel, "Revised Error Handling for BGP UPDATE Messages",
              RFC 7606, DOI 10.17487/RFC7606, August 2015,
              <https://www.rfc-editor.org/rfc/rfc7606>.

   [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>.

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>.

   [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast
              Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
              December 2006, <https://www.rfc-editor.org/rfc/rfc4786>.

   [RFC5492]  Scudder, J. and R. Chandra, "Capabilities Advertisement
              with BGP-4", RFC 5492, DOI 10.17487/RFC5492, February
              2009, <https://www.rfc-editor.org/rfc/rfc5492>.

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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>.

Appendix A.  Other Means of Signaling EL Capability

   A router that supports this specification could also have other means
   to know that an egress is EL-capable, for example, it could support
   ELCv2 [I-D.scudder-bgp-entropy-label], or it could know through
   configuration.  If a router learns through any means that an egress
   is EL-capable, it MAY treat the egress as EL-capable.  For example,
   reception of a valid ELCv2 would be sufficient (even if a valid ELCv3
   is not received), and similarly, reception of a valid ELCv3 would be
   sufficient (even if a valid ELCv2 is not received).  The details of
   which methods are accepted for signaling EL capability are beyond the
   scope of this specification but SHOULD be configurable by the user.

Acknowledgements

   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

   Serge Krier
   Cisco Systems
   Email: sekrier@cisco.com

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   Kevin Wang
   Juniper Networks
   Email: kfwang@juniper.net

Authors' Addresses

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

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

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

   Kireeti Kompella
   Juniper Networks
   Email: kireeti@juniper.net

   Satya Mohanty
   Cisco Systems
   Email: satyamoh@cisco.com

   James Uttaro
   Independent Contributor
   Email: juttaro@ieee.org

   Bin Wen
   Comcast
   Email: Bin_Wen@comcast.com

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