BGP-4 Path Attribute Filtering Capability
draft-haas-idr-path-attribute-filtering-01

idr                                                              J. Haas
Internet-Draft                                                J. Scudder
Intended status: Standards Track                                     HPE
Expires: 22 January 2026                                    21 July 2025

               BGP-4 Path Attribute Filtering Capability
               draft-haas-idr-path-attribute-filtering-01

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

   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
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   This Internet-Draft will expire on 22 January 2026.

Copyright Notice

   Copyright (c) 2025 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
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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
   extracted from this document must include Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Specification of Requirements . . . . . . . . . . . . . .   3
   2.  BGP Path Attribute Filtering Capability . . . . . . . . . . .   3
   3.  Operation . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Selecting supported Path Attributes . . . . . . . . . . . . .   5
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   8.  Recommendations for Filtering Path Attributes . . . . . . . .   6
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   9
   References  . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     Normative References  . . . . . . . . . . . . . . . . . . . . .   9
     Informative References  . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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.  Specification of Requirements

   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 0..32 octets.

   *  Capability Value contains a bit-string padded to an octet boundary
      where a bit is set if this BGP speaker considers the corresponding
      BGP Path Attribute from the remote BGP speaker to be unwanted.

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

       In this example, bits are clear 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).

       Other path attributes through attribute 23 are unwanted.
       Path attributes 24 and beyond are accepted.

              Figure 1: Example encoding for Capability Value

   Bits 1, 2, 3, 6, 7, 14, 15, 17, 18 MUST be clear (value 0), because
   support for [RFC4271], [RFC4760], and [RFC6793] procedures are
   required when this specification is in use.

   Any bit not explicitly represented (e.g., bits 24 and beyond in the
   above example) is deemed to be clear (value 0).  That is, the default
   is to accept any path attribute not explicitly unwanted.

3.  Operation

   A clear (value 0) bit in the Path Attribute Filtering capability
   indicates that the BGP speaker advertising it is willing to accept
   the corresponding Path Attribute and will process it according to the
   normal rules of the BGP protocol and the attribute in question.

   A set (value 1) bit in the Path Attribute Filtering capability
   indicates that the BGP speaker advertising it is not willing to
   accept the corresponding Path Attribute.  We refer to such Path
   Attributes as "unwanted Path Attributes".

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   A BGP speaker MUST NOT send an unwanted Path Attribute to its peer.
   (Of course, this expectation will be met only by BGP speakers that
   support this specification; therefore a BGP speaker that implements
   this specification SHOULD be prepared for the possibility it will
   receive unwanted Path Attributes; this is discussed below.)

   One strategy to accomplish the above requirement is for the BGP
   speaker to not advertise BGP routes containing the unwanted 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 unwanted 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 an unwanted Path
   Attribute SHOULD use treat-as-withdraw procedures.  Receiving BGP
   speakers MAY accept the route and discard the unwanted Path Attribute
   if permitted to by local configuration.

4.  Selecting supported Path Attributes

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

   Common BGP features that are defined for Internet use SHOULD be clear
   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 clear
   when that address family is configured.  An example of this is the
   BGP-LS attribute (29) when the BGP-LS feature is in use.

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5.  Operational Considerations

   The feature described in this document does not change the semantics
   of when a Path Attribute is intended to be transitive per RFC-4271
   definition.  However, it does act as a policy to limit the
   distribution of routes containing a transitive Path Attribute, or may
   cause that attribute to be filtered.

   eBGP speakers using this features must be cognizant of the impact
   their filtering policies will have on the incremental deployment of
   new BGP features.

6.  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.

7.  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.

   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.

8.  Recommendations for Filtering Path Attributes

   BGP Path Attributes that are in the table below have a suggested
   default filtering behavior.  It is recommended that they should not
   be filtered by default.

   Is is not recommended that BGP Path Attributes that are deprecated
   should be filtered by default.  This permits their long term
   reassignment and re-use.  Operators with a strong filtering policy
   may consider filtering these to block routes from older
   implementations of these features that may still be deployed.

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   +=======+===============================================+===========+
   | Code  | Name                                          | Should    |
   | Point |                                               | Filter    |
   |       |                                               | By        |
   |       |                                               | Default   |
   +=======+===============================================+===========+
   | 0     | Reserved                                      | Yes       |
   +-------+-----------------------------------------------+-----------+
   | 1     | ORIGIN [RFC4271]                              | Never     |
   +-------+-----------------------------------------------+-----------+
   | 2     | AS_PATH [RFC4271]                             | Never     |
   +-------+-----------------------------------------------+-----------+
   | 3     | NEXT_HOP [RFC4271]                            | Never     |
   +-------+-----------------------------------------------+-----------+
   | 4     | MULTI_EXIT_DISC [RFC4271]                     | No        |
   +-------+-----------------------------------------------+-----------+
   | 5     | LOCAL_PREF [RFC4271]                          | Yes       |
   +-------+-----------------------------------------------+-----------+
   | 6     | ATOMIC_AGGREGATE [RFC4271]                    | No        |
   +-------+-----------------------------------------------+-----------+
   | 7     | AGGREGATOR [RFC4271]                          | No        |
   +-------+-----------------------------------------------+-----------+
   | 8     | COMMUNITIES [RFC1997]                         | No        |
   +-------+-----------------------------------------------+-----------+
   | 9     | ORIGINATOR_ID [RFC4456]                       | Yes       |
   +-------+-----------------------------------------------+-----------+
   | 10    | CLUSTER_LIST [RFC4456]                        | Yes       |
   +-------+-----------------------------------------------+-----------+
   | 11    | DPA (deprecated) [RFC6938]                    | -         |
   +-------+-----------------------------------------------+-----------+
   | 12    | ADVERTISER (historic) (deprecated)            | -         |
   |       | [RFC1863][RFC4223][RFC6938]                   |           |
   +-------+-----------------------------------------------+-----------+
   | 13    | RCID_PATH / CLUSTER_ID (Historic)             | -         |
   |       | (deprecated) [RFC1863][RFC4223][RFC6938]      |           |
   +-------+-----------------------------------------------+-----------+
   | 14    | MP_REACH_NLRI [RFC4760]                       | Never     |
   +-------+-----------------------------------------------+-----------+
   | 15    | MP_UNREACH_NLRI [RFC4760]                     | Never     |
   +-------+-----------------------------------------------+-----------+
   | 16    | EXTENDED COMMUNITIES [Eric_Rosen][draft-      | No        |
   |       | ramachandra-bgp-ext-communities-00][RFC4360]  |           |
   +-------+-----------------------------------------------+-----------+
   | 17    | AS4_PATH [RFC6793]                            | Never     |
   +-------+-----------------------------------------------+-----------+
   | 18    | AS4_AGGREGATOR [RFC6793]                      | Never     |
   +-------+-----------------------------------------------+-----------+
   | 19    | SAFI Specific Attribute (SSA) (deprecated)    | -         |

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   |       | [Gargi_Nalawade][draft-kapoor-nalawade-idr-   |           |
   |       | bgp-ssa-00][draft-nalawade-idr-mdt-safi-      |           |
   |       | 00][draft-wijnands-mt-discovery-00]           |           |
   +-------+-----------------------------------------------+-----------+
   | 20    | Connector Attribute (deprecated) [RFC6037]    | -         |
   +-------+-----------------------------------------------+-----------+
   | 21    | AS_PATHLIMIT (deprecated) [draft-ietf-idr-as- | -         |
   |       | pathlimit-02]                                 |           |
   +-------+-----------------------------------------------+-----------+
   | 22    | PMSI_TUNNEL [RFC6514]                         | Yes       |
   +-------+-----------------------------------------------+-----------+
   | 23    | Tunnel Encapsulation [RFC9012]                | Yes       |
   +-------+-----------------------------------------------+-----------+
   | 24    | Traffic Engineering [RFC5543]                 | Yes       |
   +-------+-----------------------------------------------+-----------+
   | 25    | IPv6 Address Specific Extended Community      | No        |
   |       | [RFC5701]                                     |           |
   +-------+-----------------------------------------------+-----------+
   | 26    | AIGP [RFC7311]                                | Yes       |
   +-------+-----------------------------------------------+-----------+
   | 27    | PE Distinguisher Labels [RFC6514]             | Yes       |
   +-------+-----------------------------------------------+-----------+
   | 28    | BGP Entropy Label Capability Attribute        | -         |
   |       | (deprecated) [RFC6790][RFC7447]               |           |
   +-------+-----------------------------------------------+-----------+
   | 29    | BGP-LS Attribute [RFC9552]                    | Yes       |
   +-------+-----------------------------------------------+-----------+
   | 30    | Deprecated [RFC8093]                          | -         |
   +-------+-----------------------------------------------+-----------+
   | 31    | Deprecated [RFC8093]                          | -         |
   +-------+-----------------------------------------------+-----------+
   | 32    | LARGE_COMMUNITY [RFC8092]                     | No        |
   +-------+-----------------------------------------------+-----------+
   | 33    | BGPsec_Path [RFC8205]                         | Never     |
   +-------+-----------------------------------------------+-----------+
   | 34    | BGP Community Container Attribute (TEMPORARY  | No        |
   |       | - registered 2017-07-28, extension registered |           |
   |       | 2024-08-22, expires 2025-07-28) [draft-ietf-  |           |
   |       | idr-wide-bgp-communities-11]                  |           |
   +-------+-----------------------------------------------+-----------+
   | 35    | Only to Customer (OTC) [RFC9234]              | Never     |
   +-------+-----------------------------------------------+-----------+
   | 36    | BGP Domain Path (D-PATH) (TEMPORARY -         | Yes       |
   |       | registered 2019-07-08, extension registered   |           |
   |       | 2025-06-06, expires 2026-07-08) [draft-ietf-  |           |
   |       | bess-evpn-ipvpn-interworking-10]              |           |
   +-------+-----------------------------------------------+-----------+
   | 37    | SFP attribute [RFC9015]                       | Yes       |

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   +-------+-----------------------------------------------+-----------+
   | 38    | BFD Discriminator [RFC9026]                   | Yes       |
   +-------+-----------------------------------------------+-----------+
   | 39    | BGP Next Hop Dependent Capabilities (NHC)     | Yes       |
   |       | (TEMPORARY - registered 2022-12-20, extension |           |
   |       | registered 2024-12-10, expires 2025-12-20)    |           |
   |       | [draft-ietf-idr-entropy-label-13]             |           |
   +-------+-----------------------------------------------+-----------+
   | 40    | BGP Prefix-SID [RFC8669]                      | Yes       |
   +-------+-----------------------------------------------+-----------+
   | 41    | BIER [RFC9793]                                | Yes       |
   +-------+-----------------------------------------------+-----------+
   | 42    | Edge Metadata Path Attribute (TEMPORARY -     | Yes       |
   |       | registered 2025-04-23, expires 2026-04-23)    |           |
   |       | [draft-ietf-idr-5g-edge-service-metadata-27]  |           |
   +-------+-----------------------------------------------+-----------+
   | 128   | ATTR_SET [RFC6368]                            | Yes       |
   +-------+-----------------------------------------------+-----------+
   | 129   | Deprecated [RFC8093]                          | -         |
   +-------+-----------------------------------------------+-----------+
   | 241   | Deprecated [RFC8093]                          | -         |
   +-------+-----------------------------------------------+-----------+
   | 242   | Deprecated [RFC8093]                          | -         |
   +-------+-----------------------------------------------+-----------+
   | 243   | Deprecated [RFC8093]                          | -         |
   +-------+-----------------------------------------------+-----------+
   | 255   | Reserved for development [RFC2042]            | Yes       |
   +-------+-----------------------------------------------+-----------+

                                  Table 1

Acknowledgements

   TBD.

References

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>.

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   [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>.

   [RFC6793]  Vohra, Q. and E. Chen, "BGP Support for Four-Octet
              Autonomous System (AS) Number Space", RFC 6793,
              DOI 10.17487/RFC6793, December 2012,
              <https://www.rfc-editor.org/rfc/rfc6793>.

   [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>.

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>.

   [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>.

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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