Internet Engineering Task Force                            J. G. Scudder
Internet-Draft                                               K. Kompella
Intended status: Informational                          Juniper Networks
Expires: 30 October 2022                                   28 April 2022

                BGP Entropy Label Capability, Version 2


   RFC 6790 defined the Entropy Label Capability Attribute (ELC); RFC
   7447 deprecated that attribute.  This specification, dubbed "Entropy
   Label Capability Attribute version 2" (ELCv2), was intended to be
   offered for standardization, to replace the ELC as a way to signal
   that a BGP protocol speaker is capable of processing entropy labels.

   Although ultimately a different specification was chosen for that
   purpose, at least one implementation of ELCv2 was shipped by Juniper
   Networks and is currently in use in service provider networks.  This
   document is published in order to document what was implemented.

Status of This Memo

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   This Internet-Draft will expire on 30 October 2022.

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   Please review these documents carefully, as they describe your rights

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   and restrictions with respect to this document.  Code Components
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Entropy Label Capability Path Attribute, Version 2  . . . . .   3
     2.1.  Sending the ELCv2 . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Receiving the ELCv2 . . . . . . . . . . . . . . . . . . .   4
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

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.

   Ultimately the IDR working group adopted
   [I-D.ietf-idr-next-hop-capability] as a proposed solution for this
   and similar problems.  However, prior to that, at least one
   implementation of this specification was shipped, by Juniper
   Networks.  The shipping implementation uses the code point that was
   assigned by RFC 6790, and deprecated by RFC 7447.  This document
   explains what was implemented and deployed, dubbed "Entropy Label
   Capability Attribute version 2" (ELCv2).

   Although [I-D.ietf-idr-next-hop-capability] uses an optional, non-
   transitive path attribute, at the time ELCv2 was developed it was
   decided that an optional, non-transitive solution would over-
   constrain the deployment options available -- in many cases (for
   example, route reflectors) it's fine that an intermediate node does
   propagate an ELC even if it doesn't itself have the ability to
   process entropy labels.

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   Instead, in this specification, we take the approach of carrying a
   copy of the next hop information in the ELCv2.  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 2

   The Entropy Label Capability Path Attribute, Version 2 (ELCv2) is an
   optional, transitive BGP attribute (for the attribute type code, see
   Section 3).  The ELCv2 has as its data a network layer address,
   representing the next hop of the route the ELCv2 accompanies.  The
   ELCv2 signals a useful optimization, so it is desirable to make it
   transitive; the next hop data is to ensure correctness across BGP
   speakers that do not understand the ELCv2.

   The Attribute Data field of the ELCv2 path attribute is encoded as
   shown below:

      | 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 ELCv2 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."

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2.1.  Sending the ELCv2

   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 ELCv2 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 ELCv2
      attribute, and is not changing the value of N, or

   *  Is re-advertising a BGP route it received with a valid ELCv2
      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 ELCv2, S MUST set the data portion of
   the ELCv2 to be equal to N, using the encoding given in Section 2.

   The ELCv2 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 ELCv2, the requirements of this
   section notwithstanding.  However, such a speaker will not update the
   data part of the ELCv2.

2.2.  Receiving the ELCv2

   When a BGP speaker receives an unlabeled route that includes the
   ELCv2, it MUST discard the ELCv2.

   When a BGP speaker receives a labeled route that includes the ELCv2,
   it MUST compare the ELCv2'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

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   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
   ELCv2 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 ELCv2 whose
   Attribute Length is less than 4, whose Attribute Length is not 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 ELCv2.

3.  IANA Considerations

   As per [RFC7447], IANA has deprecated BGP attribute 28.  That
   deprecated type code is used by implementations of this
   specification.  IANA is requested to update the references for
   attribute 28 to include this specification.

4.  Security Considerations

   Insertion of an ELCv2 by an attacker could cause forwarding to fail.
   Deletion of an ELCv2 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 ELCv2.  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.  In sum, the ELCv2 attribute creates no
   significant issues beyond those analyzed in [RFC4272].

5.  Acknowledgements

   Thanks to Alia Atlas, Bruno Decraene, Martin Djernaes, John Drake,
   Adrian Farrell, Keyur Patel, Ravi Singh, and Jim Uttaro for their
   discussion of this issue.  Particular thanks to Kevin Wang for his
   many valuable contributions.

6.  References

6.1.  Normative References

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   [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,

   [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, <>.

6.2.  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-07, 8 December
              2021, <

   [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis",
              RFC 4272, DOI 10.17487/RFC4272, January 2006,

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

Authors' Addresses

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   John G. Scudder
   Juniper Networks

   Kireeti Kompella
   Juniper Networks

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