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BGP Entropy Label Capability, Version 3

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This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors John Scudder , Kireeti Kompella , SATYA R MOHANTY , Jim Uttaro , Bin Wen
Last updated 2022-09-29
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Internet Engineering Task Force                       J. G. Scudder, Ed.
Internet-Draft                                               K. Kompella
Intended status: Standards Track                        Juniper Networks
Expires: 2 April 2023                                         S. Mohanty
                                                           Cisco Systems
                                                               J. Uttaro
                                                                  B. Wen
                                                       29 September 2022

                BGP Entropy Label Capability, Version 3


   This specification defines the Entropy Label Capability Attribute
   version 3 (ELCv3), a BGP attribute that can be used to inform an LSP
   ingress router about an LSP egress router's ability to process
   entropy labels.  This version of the attribute corrects a
   specification error in the first version, and an improper code point
   reuse in the second.

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
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   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 2 April 2023.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Entropy Label Capability Path Attribute, Version 3  . . . . .   3
     2.1.  Sending the ELCv3 . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Receiving the ELCv3 . . . . . . . . . . . . . . . . . . .   5
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  Other Means of Signaling EL Capability . . . . . . .   7
     A.1.  Backward Compatibility with ELCv2 . . . . . . . . . . . .   8
   Appendix B.  Contributors . . . . . . . . . . . . . . . . . . . .   8
   Appendix C.  Acknowledgements . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   [RFC6790] defines the Entropy Label Capability attribute (ELC), an
   optional, transitive BGP path attribute.  For correct operation, it
   is necessary that any intermediate node modifying the next hop of a
   route must remove the ELC unless the node so doing is able to process
   entropy labels.  Sadly, these requirements cannot be fulfilled with
   the ELC as specified, because it is an optional, transitive
   attribute: by definition, a node that does not support the ELC will
   propagate the attribute.  But such a node might be exactly the one
   that we desire to remove it.  For this reason, [RFC7447] deprecated
   the attribute.

   Roughly concurrently with the development and advancement of RFC
   7447, Juniper Networks began shipping routing code that implements
   what is documented in [I-D.scudder-bgp-entropy-label] and dubbed
   Entropy Label Capability version 2 (ELCv2).  That implementation uses
   the code point that was assigned by RFC 6790 and deprecated by RFC
   7447.  At time of writing, the functionality is in use in operational

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   The present specification is based on ELCv2 but moves to a new,
   previously unallocated, code point.

   A related solution to the problem of signaling entropy label
   capability is [I-D.ietf-idr-next-hop-capability].  That specification
   is based on an optional, non-transitive path attribute.  In contrast,
   ELCv3 (and ELCv2) is based on an optional, transitive path attribute.
   This expands the deployment options available -- in many cases (for
   example, route reflectors) it's fine that an intermediate node does
   propagate an ELCv3 even if it doesn't itself have the ability to
   process entropy labels.

   In order to prevent use of the signaled information beyond the
   intended perimeter (the problem that led to the deprecation of ELC,
   and which is inherently solved by
   [I-D.ietf-idr-next-hop-capability]'s use of a non-transitive
   attribute), in this specification we take the approach of carrying a
   copy of the next hop information in the ELCv3.  This allows the node
   processing it to know if it can rely on the information carried
   therein, while still allowing it to be propagated by all intermediate

1.1.  Requirements Language

   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.

2.  Entropy Label Capability Path Attribute, Version 3

   The Entropy Label Capability Path Attribute, Version 3 (ELCv3) is an
   optional, transitive BGP attribute with type code TBD1.  The ELCv3
   has as its data a network layer address, representing the next hop of
   the route the ELCv3 accompanies.  The ELCv3 signals a useful
   optimization, 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 ELCv3.

   The Attribute Data field of the ELCv3 path attribute is encoded as
   shown below:

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      | Address Family Identifier (2 octets)                    |
      | Subsequent Address Family Identifier (1 octet)          |
      | Length of Next Hop Network Address (1 octet)            |
      | Network Address of Next Hop (variable)                  |

   The meanings of the fields are as given in Section 3 of [RFC4760].

   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 Path
   Attributes.  The inclusion of this attribute 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."

2.1.  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 attribute 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
      attribute, and is not changing the value of N, or

   *  Is re-advertising a BGP route it received with a valid ELCv3
      attribute, 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 process the
      outer label(s) without processing the entropy label below, as with
      a transit LSR, 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 LDP ELC TLV,
      Section 5.1 of [RFC6790]).

   In any event, when sending an ELCv3, S MUST set the data portion of
   the ELCv3 to be equal to N, using the encoding given in Section 2.

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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.

   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 ELCv3, the requirements of this
   section notwithstanding.  However, such a speaker will not update the
   data part of the ELCv3.

2.2.  Receiving the ELCv3

   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,
   it MUST compare the address given in the ELCv3's data portion to the
   next hop of the route.  If the two are equal, the egress of the LSP
   supports entropy labels, which implies that the receiving BGP
   speaker, if acting as ingress, MAY insert an entropy label below the
   advertised label, as per Section 4.2 of [RFC6790].  If the two are
   not equal, either some intermediate router that does not implement
   this specification modified the next hop, or some router on the path
   had an incorrect implementation.  In either case, the action taken is
   the same: the ELCv3 MUST be discarded.  The Partial bit MAY be
   inspected -- if it is equal to zero, then the mismatch must have been
   caused by an incorrect implementation, and the error MAY be logged.

   When a BGP speaker receives a route that includes an ELCv3 whose
   Attribute Length is less than 4, whose Attribute Length is not
   greater than or equal to 4 plus the value encoded in the Length of
   Next Hop Network Address carried in the Attribute Data, or whose
   Attribute Data is otherwise inconsistent with the encoding specified
   in Section 2, it MUST discard the ELCv3.

   If an ELCv3 includes data beyond the Network Address of Next Hop
   field, such data MUST be disregarded.  If the ELCv3 is propagated,
   the unknown data MUST be included with it.

3.  IANA Considerations

   IANA is requested to make a new allocation in the BGP Path Attributes

   *  Value = TBD1

   *  Code = ELCv3

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   *  Reference = (this document)

4.  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.

   The Attribute Data portion of the ELCv3 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, it will not always be so -- indeed, that is the
   reason this specification had to be written.  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 2.2.)

5.  References

5.1.  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,

   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760,
              DOI 10.17487/RFC4760, January 2007,

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   [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,

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

5.2.  Informative References

              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,

              Scudder, J. G. and K. Kompella, "BGP Entropy Label
              Capability, Version 2", Work in Progress, Internet-Draft,
              draft-scudder-bgp-entropy-label-00, 28 April 2022,

   [RFC7447]  Scudder, J. and K. Kompella, "Deprecation of BGP Entropy
              Label Capability Attribute", RFC 7447,
              DOI 10.17487/RFC7447, February 2015,

   [RFC8277]  Rosen, E., "Using BGP to Bind MPLS Labels to Address
              Prefixes", RFC 8277, DOI 10.17487/RFC8277, October 2017,

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.

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A.1.  Backward Compatibility with ELCv2

   As was noted in Section 1, there are networks in which ELCv2
   (documented in [I-D.scudder-bgp-entropy-label]) is already in use.

   Any node that sends the ELCv2 format may also include an ELCv3 per
   Section 2.1, so that both formats are sent.  The exact set of formats
   to send SHOULD be user-configurable.

   As discussed above, a route received with either a valid ELCv2 or
   ELCv3 may be considered EL-capable.

Appendix B.  Contributors

   Serge Krier
   Cisco Systems

   Kevin Wang
   Juniper Networks

Appendix C.  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.

Authors' Addresses

   John G. Scudder (editor)
   Juniper Networks

   Kireeti Kompella
   Juniper Networks

   Satya Mohanty
   Cisco Systems

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   James Uttaro

   Bin Wen

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