BGP-4 Path Attribute Filtering Capability
draft-haas-idr-path-attribute-filtering-00

Internet Engineering Task Force                                  J. Haas
Internet-Draft                                                J. Scudder
Intended status: Standards Track                                     HPE
Expires: 7 January 2026                                      6 July 2025

               BGP-4 Path Attribute Filtering Capability
               draft-haas-idr-path-attribute-filtering-00

Abstract

   Path Attributes in the BGP-4 protocol [RFC 4271] carry data
   associated with BGP routes.  Many of the Path Attributes carried in
   BGP are intended for limited scope deployment.  However, the
   extension mechanism defined by BGP that carries these attributes
   often carries them farther than necessary, sometimes with unfortunate
   results.

   This document defines a mechanism using BGP Capabilities [RFC 5492]
   that permits eBGP speakers to determine what Path Attributes should
   be permitted to cross external BGP routing boundaries.

Status of This Memo

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   This Internet-Draft will expire on 7 January 2026.

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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.  BGP Path Attribute Filtering Capability . . . . . . . . . . .   3
   3.  Operation . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  If both eBGP speakers exchange the Path Attribute Filtering
           capability  . . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  If only one peer supports the Path Attribute Filtering
           capability  . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Selecting supported Path Attributes . . . . . . . . . . . . .   5
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     7.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   7
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   A BGP Route (Section 1.1 of [RFC4271]) is a tuple consisting of a set
   of Path Attributes (Section 5 of [RFC4271]) and sets of network
   reachability carried as Network Layer Reachability Information
   (NLRI).  Some of these Path Attributes are defined as part of the
   core BGP-4 protocol.  Path Attributes are the main extension
   mechanism defined by BGP, and may be scoped as "transitive" or "non-
   transitive."

   Non-Transitive Path Attributes require the BGP speaker to understand
   the attribute in order to determine if it will be locally used and
   perhaps later propagated to additional BGP speakers.  Unrecognized
   non-transitive Path Attributes are discarded by the receiving BGP
   speaker.

   Transitive Path Attributes, when not understood by the receiving BGP
   speaker, are required to be propagated to other BGP speakers.

   Some Path Attributes defined by BGP extensions are intended to be
   used in limited scopes, such as a single BGP Autonomous System (AS).
   When such attributes are distributed beyond the expected scope, this
   is called an "Attribute Escape" [I-D.haas-idr-bgp-attribute-escape].

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   Such attribute escapes may lead to improper BGP protocol behavior
   when received outside of their expected scope, and may lead to
   incorrect forwarding, or be a serious security consideration.

   This document defines a mechanism exchanged through BGP Capabilities
   [RFC5492] where BGP speakers can more appropriately scope both Path
   Attributes to prevent escape, and to limit the distribution of routes
   that carry escaped Path Attributes.

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.  BGP Path Attribute Filtering Capability

   The BGP Path Attribute Filtering Capability is encoded as follows:

   *  Capability Code of (TBD).
   *  Capability Length of 1..32.
   *  Capability Value contains a bit-string where a bit is set if the
      underlying BGP Path Attribute is desired to be advertised by this
      BGP speaker to the remote BGP speaker.

             Example encoding for Capability Value:

    0                   1                   2
    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|1|1|1|1|0|1|1|1|0|0|0|0|0|1|1|0|1|1|0|0|0|0|0|0|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Bits are set for Path Attributes:

      Origin (1),
      AS_PATH (2),
      NEXT_HOP (3),
      MULTI_EXIT_DISCR (4),
      ATOMIC_AGGREGATE (6),
      AGGREGATOR (7),
      COMMUNITIES (8),
      MP_REACH_NLRI (14),
      MP_UNREACH_NLRI (15),
      AS4_PATH (17),
      AS4_AGGREGATOR (18).

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                                  Figure 1

   Bits 1, 2, 3, 6, 7, 14, 15, 17, 18 MUST be set, because support for
   [RFC4271], [RFC4760], and [RFC6783] procedures are required when this
   specification is in use.

3.  Operation

3.1.  If both eBGP speakers exchange the Path Attribute Filtering
      capability

   The intersection of the sent and the received Path Attribute
   Filtering capabilies's bits represents the set of Path Attributes
   that the BGP speakers are willing to exchange with each other.  When
   both bits for a given Path Attribute Type Code are 1, the Path
   Attribute is negotiated; otherwise it is not negotiated.

   Routes with Path Attributes that have been negotiated MAY be
   exchanged between both BGP speakers, depending on the procedures
   associated with those Path Attributes.

   When a Path Attribute has not been negotiated, a BGP speaker MUST NOT
   send a BGP route that contains a Path Attribute for that Path
   Attribute Type Code.

   One strategy to accomplish the above requirement is for the BGP
   speaker to not advertise BGP routes containing that Path Attribute in
   question.  This might require a withdraw to be sent instead.  This is
   similar to treat-as-withdraw as defined in [RFC7606].

   Another strategy that could be used, when appropriate for the
   procedure covering a given BGP Path Attribute, is for the BGP speaker
   to remove the not negotiated Path Attributes when it distributes the
   route to the remote BGP speaker.  This is similar to the Attribute
   Discard behavior defined in [RFC7606].

   Receiving BGP speakers SHOULD filter routes or discard unwanted Path
   Attributes if they are incorrectly sent by the remote BGP speaker.
   Minimally, a receiving BGP speaker receiving a Path Attribute that
   was not negotiated SHOULD use treat-as-withdraw procedures.
   Receiving BGP speakers MAY accept the route and discard the not
   negotiated Path Attribute if permitted to by local configuration.

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3.2.  If only one peer supports the Path Attribute Filtering capability

   When only one BGP speaker supports or sends the Path Attribute
   Filtering capability that BGP implementation is demonstrating a
   preference to not receive Path Attributes where the bit representing
   the Path Attribute Type Code is zero.  It is, in fact, representing a
   filtering policy for routes containing those Path Attributes.

   BGP speakers sending a Path Attribute Filtering capability but not
   receiving one from the remote BGP speaker SHOULD filter the routes
   according to their own locally set and supported bits for the
   capability, as described in the final paragraph of the previous
   subsection.

4.  Selecting supported Path Attributes

   Implementations MUST, as described in Figure 1, exchange the required
   bits covering required core eBGP Path Attributes.

   Common BGP features that are defined for Internet use SHOULD be
   included by default between two BGP speakers.  These include:

   *  Communities (8)
   *  Extended Communities (16)
   *  Large BGP Communities (32)
   *  BGPsec_Path (33)
   *  Only To Customer (OTC) (35)

   BGP features required to support a given AFI/SAFI MUST also be
   included when that address family is configured.  An example of this
   is the BGP-LS attribute (29) when the BGP-LS feature is in use.

   Other BGP features desiring eBGP distribution MAY include their bits
   in the capability when their procedures require it.  It is
   RECOMMENDED that this require explicit configuration to prevent
   attribute escape.

5.  IANA Considerations

   This document requests a new BGP Capability Code to be allocated from
   the First Come First Served range of the Capability Codes registry.

6.  Security Considerations

   The motivation for this feature is to attempt to address the numerous
   BGP security implications where BGP Path Attributes propagate beyond
   their intended scope.

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   The definition of a feature that limits the distribution of BGP Path
   Attributes unfortunately moves BGP's default behavior away from
   "distribute unknown things easily" and thus hampers incremental
   deployment of new features.  However, operators have already begun
   indiscriminate filtering of Path Attributes they do not themselves
   require.  This feature attempts to provide a more flexible negotiated
   mode to permit such filtering while at the same time not completely
   precluding incremental deployment of new features.

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/info/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/info/rfc4271>.

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

   [RFC6783]  Levine, J. and R. Gellens, "Mailing Lists and Non-ASCII
              Addresses", RFC 6783, DOI 10.17487/RFC6783, November 2012,
              <https://www.rfc-editor.org/info/rfc6783>.

   [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/info/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/info/rfc8174>.

7.2.  Informative References

   [I-D.haas-idr-bgp-attribute-escape]
              Haas, J., "BGP Attribute Escape", Work in Progress,
              Internet-Draft, draft-haas-idr-bgp-attribute-escape-03, 9
              April 2025, <https://datatracker.ietf.org/doc/html/draft-
              haas-idr-bgp-attribute-escape-03>.

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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/info/rfc4760>.

Acknowledgements

   TBD.

Contributors

   TBD.

Authors' Addresses

   Jeffrey Haas
   HPE
   1133 Innovation Way
   Sunnyvale, CA 94089
   United States of America
   Email: jhaas@juniper.net

   John Scudder
   HPE
   1133 Innovation Way
   Sunnyvale, CA 94089
   United States of America
   Email: jgs@juniper.net

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