Deprecation of AS_SET and AS_CONFED_SET in BGP
draft-ietf-idr-deprecate-as-set-confed-set-16

Network Working Group                                          W. Kumari
Internet-Draft                                              Google, Inc.
Obsoletes: 6472 (if approved)                                  K. Sriram
Updates: 4271 5065 (if approved)                             L. Hannachi
Intended status: Standards Track                                USA NIST
Expires: 28 February 2025                                        J. Haas
                                                  Juniper Networks, Inc.
                                                          27 August 2024

             Deprecation of AS_SET and AS_CONFED_SET in BGP
             draft-ietf-idr-deprecate-as-set-confed-set-16

Abstract

   BCP 172 (i.e., RFC 6472) recommends not using AS_SET and
   AS_CONFED_SET AS_PATH segment types in the Border Gateway Protocol
   (BGP).  This document advances that recommendation to a standards
   requirement in BGP; it prohibits the use of the AS_SET and
   AS_CONFED_SET path segment types in the AS_PATH.  This is done to
   simplify the design and implementation of BGP and to make the
   semantics of the originator of a BGP route clearer.  This will also
   simplify the design, implementation, and deployment of various BGP
   security mechanisms.  This document updates RFC 4271 by deprecating
   the origination of BGP routes with AS_SET (Type 1 AS_PATH segment)
   and updates RFC 5065 by deprecating the origination of BGP routes
   with AS_CONFED_SET (Type 4 AS_PATH segment).  Finally, it obsoletes
   RFC 6472.

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/.

   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 28 February 2025.

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Copyright Notice

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Recommendations . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Treatment of Routes with AS_SET in RPKI-based BGP Security  .   4
   5.  BGP AS_PATH "Brief" Aggregation . . . . . . . . . . . . . . .   4
     5.1.  Issues with "Brief" AS_PATH Aggregation and RPKI-ROV  . .   5
     5.2.  Recommendations to Mitigate Unpredictable AS_PATH Origins
           for RPKI-ROV Purposes . . . . . . . . . . . . . . . . . .   6
   6.  Operational Considerations  . . . . . . . . . . . . . . . . .   6
     6.1.  Implementing Consistent Brief Aggregation . . . . . . . .   6
     6.2.  Not Advertising Aggregate Routes to Contributing ASes . .   6
     6.3.  Mitigating Forwarding Loops . . . . . . . . . . . . . . .   6
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   7
     10.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Appendix A.  Example of Route Filtering for Aggregate Routes and
           its Contributors  . . . . . . . . . . . . . . . . . . . .   9
   Appendix B.  Examples of Consistent and Inconsistent BGP Origin-AS
           Generated by Traditional Brief Aggregation  . . . . . . .  10
     B.1.  Scenario 1: First one route, then another, each with a
           fully disjoint AS_PATH  . . . . . . . . . . . . . . . . .  11
     B.2.  Scenario 2: First one route, then another, the AS_PATHs
           overlap at the origin AS. . . . . . . . . . . . . . . . .  12
     B.3.  Scenario 3: First one route, then another, the AS_PATHs
           overlap at the neighbor AS  . . . . . . . . . . . . . . .  12
     B.4.  Achieving Consistent Origin-AS During Aggregation . . . .  12
   Appendix C.  Discussion on Forwarding Loops and AS_SETs . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

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1.  Introduction

   BCP 172 [RFC6472] makes a recommendation for not using AS_SET (see
   [RFC4271]) and AS_CONFED_SET (see [RFC5065]) AS_PATH path segment
   types in the Border Gateway Protocol (BGP).  This document advances
   the BCP recommendation to a standards requirement in BGP; it
   proscribes the use of the AS_SET and AS_CONFED_SET types of path
   segments in the AS_PATH.  The purpose is to simplify the design and
   implementation of BGP and to make the semantics of the originator of
   a BGP route clearer.  This will also simplify the design,
   implementation, and deployment of various BGP security mechanisms.
   In particular, the prohibition of AS_SETs and AS_CONFED_SETs removes
   the possibility of ambiguity about origin AS in RPKI-based route
   origin validation (RPKI-ROV) [RFC6811] [RFC6907] [RFC9319].

   The AS_SET path segment in the AS_PATH attribute (Sections 4.3 and
   5.1.2 of [RFC4271]) is created by a router that is performing route
   aggregation and contains an unordered set of Autonomous Systems
   (ASes) that contributing prefixes in the aggregate have traversed.

   The AS_CONFED_SET path segment (see [RFC5065]) in the AS_PATH
   attribute is created by a router that is performing route aggregation
   and contains an unordered set of Member AS Numbers in the local
   confederation that contributing prefixes in the aggregate have
   traversed.  It is very similar to an AS_SET but is used within a
   confederation.

   By performing aggregation, a router is combining multiple BGP routes
   for more specific destinations into a new route for a less specific
   destination ([RFC4271], Section 9.1.2.2.).  Aggregation may blur the
   semantics of the origin AS for the prefix being announced by
   producing an AS_SET or AS_CONFED_SET.  Such sets can cause
   operational issues, such as not being able to authenticate a route
   origin for the aggregate prefix in new BGP security technologies such
   as those that take advantage of X.509 extensions for IP addresses and
   AS identifiers ([RFC3779], [RFC6480], [RFC6811], [RFC6907],
   [RFC8205], [RFC9319]).  This could result in reachability problems
   for the destinations covered by the aggregated prefix.

   From analysis of historical Internet routing data, it is apparent
   that aggregation that involves AS_SETs is very seldom used in
   practice on the public Internet (see [Analysis]).  When it is used,
   it is often used incorrectly; only a single AS in the AS_SET is the
   most common case [Analysis].  Also, very often the same AS appears in
   the AS_SEQUENCE and the AS_SET in the BGP update.  The occurrence of
   reserved AS numbers ([IANA-SP-ASN]) is also somewhat frequent.

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2.  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.

3.  Recommendations

   Unless explicitly configured by a network operator to do otherwise
   (e.g., during a transition phase), BGP speakers:

   *  MUST NOT advertise BGP UPDATE messages containing AS_SETs or
      AS_CONFED_SETs, and

   *  Upon reception of BGP UPDATE messages containing AS_SETs or
      AS_CONFED_SETs in the AS_PATH or AS4_PATH [RFC6793], MUST use the
      "treat-as-withdraw" error handling behavior as per [RFC7606].

   Per above specifications, this document updates RFC 4271 and RFC 5065
   by deprecating AS_SET (see [RFC4271], Section 4.3) and AS_CONFED_SET
   (see [RFC5065], Section 3), respectively.

4.  Treatment of Routes with AS_SET in RPKI-based BGP Security

   Resource Public Key Infrastructure (RPKI) [RFC6480] uses X.509
   extensions for IP addresses and AS identifiers [RFC3779].  RPKI-based
   Route Origin Validation (ROV) [RFC6811] [RFC6907] is a BGP security
   technology that never allows a route with AS_SET to be considered
   Valid.  BGPsec [RFC8205] and Autonomous System Provider Authorization
   (ASPA) [I-D.ietf-sidrops-aspa-verification] are also BGP security
   technologies based on RPKI.  BGPsec does not support AS_SETs.  In
   ASPA-based AS_PATH verification, a route with AS_SET is always
   considered Invalid and hence ineligible for route selection.

5.  BGP AS_PATH "Brief" Aggregation

   Sections 9.1.4 and 9.2.2.2 of [RFC4271] describe BGP aggregation
   procedures.  Appendix F.6 in [RFC4271] describes a generally less
   utilized "Complex AS_PATH Aggregation" procedure.

   [RFC4271], Section 5.1.6, describing the ATOMIC_AGGREGATE Path
   Attribute, notes that:

   |  When a BGP speaker aggregates several routes for the purpose of
   |  advertisement to a particular peer, the AS_PATH of the aggregated
   |  route normally includes an AS_SET formed from the set of ASes from

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   |  which the aggregate was formed.  In many cases, the network
   |  administrator can determine if the aggregate can safely be
   |  advertised without the AS_SET, and without forming route loops.
   |  
   |  If an aggregate excludes at least some of the AS numbers present
   |  in the AS_PATH of the routes that are aggregated as a result of
   |  dropping the AS_SET, the aggregated route, when advertised to the
   |  peer, SHOULD include the ATOMIC_AGGREGATE attribute.

   When BGP AS_PATH aggregation is done according to the [RFC4271],
   Section 9.2.2.2 procedures, and any resulting AS_SETs are discarded,
   this is typically referred to as "brief" aggregation in
   implementations.  Brief aggregation results in an AS_PATH that has
   the property (from [RFC4271], Section 9.2.2.2):

   |  determine the longest leading sequence of tuples (as defined
   |  above) common to all the AS_PATH attributes of the routes to be
   |  aggregated.  Make this sequence the leading sequence of the
   |  aggregated AS_PATH attribute.

   The ATOMIC_AGGREGATE Path Attribute is subsequently attached to the
   BGP route, if AS_SETs are dropped.

5.1.  Issues with "Brief" AS_PATH Aggregation and RPKI-ROV

   While brief AS_PATH aggregation has the desirable property of not
   containing AS_SETs, the resulting aggregated AS_PATH may contain an
   unpredictable origin AS.  This is because the aggregating AS may be
   different from the purported origin AS (for the aggregate), which may
   vary as explained below.  Such an unpredictable origin ASes may
   result in RPKI-ROV validation issues:

   *  Depending on the contributing routes to the aggregate route, the
      resulting origin AS may vary.

   *  The presence of expected contributing routes may be unpredictable
      due to route availability from BGP neighbors.

   *  In the presence of such varying origin ASes, it would be necessary
      for the resource holder to register Route Origin Authorizations
      (ROAs) [RFC9582] for each potential origin AS that may result from
      the expected aggregated AS_PATHs.

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5.2.  Recommendations to Mitigate Unpredictable AS_PATH Origins for
      RPKI-ROV Purposes

   To ensure a consistent BGP origin AS is announced for aggregate BGP
   routes for implementations of "brief" BGP aggregation, the
   implementation MUST be configured to truncate the AS_PATH after the
   right-most instance of the desired origin AS for the aggregate.  The
   desired origin AS could be the aggregating AS itself.  A ROA would be
   necessary for the aggregate prefix with the desired origin AS.

   This form of brief aggregation is referred to as "consistent brief"
   BGP aggregation.

   If the resulting AS_PATH would be truncated from the otherwise
   expected result of BGP AS_PATH aggregation (an AS_SET would not be
   generated and possibly some ASes are removed from the "longest
   leading sequence" of ASes), the ATOMIC_AGGREGATE Path Attribute
   SHOULD be attached.  This is consistent with the intent of [RFC4271],
   Section 5.1.6.

6.  Operational Considerations

6.1.  Implementing Consistent Brief Aggregation

   When aggregating prefixes, network operators MUST use consistent
   brief aggregation as described in Section 5.2.  In consistent brief
   aggregation, the AGGREGATOR and ATOMIC_AGGREGATE Path Attributes are
   included, but the AS_PATH does not have AS_SET or AS_CONFED_SET path
   segment types.  See Appendix B for examples of brief aggregation
   while keeping the origin AS unambiguous and generating appropriate
   ROAs.

6.2.  Not Advertising Aggregate Routes to Contributing ASes

   An aggregate prefix MUST NOT be announced to the contributing ASes.
   Instead, more specific prefixes (from the aggregate) MUST be
   announced to each contributing AS, excluding any that were learned
   from the contributing AS in consideration.  See Appendix A for an
   example of this filtering policy.

6.3.  Mitigating Forwarding Loops

   As discussed in Section 5.1 of [RFC4632], the presence of both less
   specific and more specific destinations has the possibility to create
   forwarding loops between networks.

   BGP implementations performing aggregation MUST follow Rule #2 of
   Section 5.1 of [RFC4632].

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   Further discussion of forwarding loops and their relationship to
   AS_SETs can be found in Appendix C.

7.  Security Considerations

   This document deprecates the use of aggregation techniques that
   create AS_SETs or AS_CONFED_SETs.  Obsoleting these path segment
   types from BGP and removal of the related code from implementations
   would potentially decrease the attack surface for BGP.  Deployments
   of new BGP security technologies (e.g., [RFC6480], [RFC6811],
   [RFC8205]) benefit greatly if AS_SETs and AS_CONFED_SETs are not used
   in BGP.

8.  IANA Considerations

   This document requires no IANA actions.

9.  Acknowledgements

   The authors would like to thank Alvaro Retana, Ketan Talaulikar,
   Keyur Patel, Susan Hares, Claudio Jeker, Nick Hilliard, Robert
   Raszuk, John Heasley, Job Snijders, Jared Mauch, Jakob Heitz, Douglas
   Montgomery, Randy Bush, John Scudder, Curtis Villamizar, Danny
   McPherson, Chris Morrow, Tom Petch, Ilya Varlashkin, Enke Chen, Tony
   Li, Florian Weimer, John Leslie, Paul Jakma, Rob Austein, Russ
   Housley, Sandra Murphy, Steve Bellovin, Steve Kent, Steve Padgett,
   and Alfred Hoenes for comments and suggestions.

10.  References

10.1.  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>.

   [RFC4632]  Fuller, V. and T. Li, "Classless Inter-domain Routing
              (CIDR): The Internet Address Assignment and Aggregation
              Plan", BCP 122, RFC 4632, DOI 10.17487/RFC4632, August
              2006, <https://www.rfc-editor.org/info/rfc4632>.

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   [RFC5065]  Traina, P., McPherson, D., and J. Scudder, "Autonomous
              System Confederations for BGP", RFC 5065,
              DOI 10.17487/RFC5065, August 2007,
              <https://www.rfc-editor.org/info/rfc5065>.

   [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/info/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/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>.

10.2.  Informative References

   [Analysis] Hannachi, L. and K. Sriram, "Detailed analysis of AS_SETs
              in BGP updates", NIST Robust Inter-domain Routing Project
              Website , October 2019,
              <https://github.com/ksriram25/IETF/blob/main/Detailed-
              AS_SET-analysis.txt>.

   [I-D.ietf-sidrops-aspa-verification]
              Azimov, A., Bogomazov, E., Bush, R., Patel, K., Snijders,
              J., and K. Sriram, "BGP AS_PATH Verification Based on
              Autonomous System Provider Authorization (ASPA) Objects",
              Work in Progress, Internet-Draft, draft-ietf-sidrops-aspa-
              verification-18, 8 July 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-sidrops-
              aspa-verification-18>.

   [IANA-SP-ASN]
              "Special-Purpose Autonomous System (AS) Numbers",
              <https://www.iana.org/assignments/iana-as-numbers-special-
              registry/iana-as-numbers-special-registry.xhtml>.

   [RFC3779]  Lynn, C., Kent, S., and K. Seo, "X.509 Extensions for IP
              Addresses and AS Identifiers", RFC 3779,
              DOI 10.17487/RFC3779, June 2004,
              <https://www.rfc-editor.org/info/rfc3779>.

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   [RFC6472]  Kumari, W. and K. Sriram, "Recommendation for Not Using
              AS_SET and AS_CONFED_SET in BGP", BCP 172, RFC 6472,
              DOI 10.17487/RFC6472, December 2011,
              <https://www.rfc-editor.org/info/rfc6472>.

   [RFC6480]  Lepinski, M. and S. Kent, "An Infrastructure to Support
              Secure Internet Routing", RFC 6480, DOI 10.17487/RFC6480,
              February 2012, <https://www.rfc-editor.org/info/rfc6480>.

   [RFC6811]  Mohapatra, P., Scudder, J., Ward, D., Bush, R., and R.
              Austein, "BGP Prefix Origin Validation", RFC 6811,
              DOI 10.17487/RFC6811, January 2013,
              <https://www.rfc-editor.org/info/rfc6811>.

   [RFC6907]  Manderson, T., Sriram, K., and R. White, "Use Cases and
              Interpretations of Resource Public Key Infrastructure
              (RPKI) Objects for Issuers and Relying Parties", RFC 6907,
              DOI 10.17487/RFC6907, March 2013,
              <https://www.rfc-editor.org/info/rfc6907>.

   [RFC8205]  Lepinski, M., Ed. and K. Sriram, Ed., "BGPsec Protocol
              Specification", RFC 8205, DOI 10.17487/RFC8205, September
              2017, <https://www.rfc-editor.org/info/rfc8205>.

   [RFC9319]  Gilad, Y., Goldberg, S., Sriram, K., Snijders, J., and B.
              Maddison, "The Use of maxLength in the Resource Public Key
              Infrastructure (RPKI)", BCP 185, RFC 9319,
              DOI 10.17487/RFC9319, October 2022,
              <https://www.rfc-editor.org/info/rfc9319>.

   [RFC9582]  Snijders, J., Maddison, B., Lepinski, M., Kong, D., and S.
              Kent, "A Profile for Route Origin Authorizations (ROAs)",
              RFC 9582, DOI 10.17487/RFC9582, May 2024,
              <https://www.rfc-editor.org/info/rfc9582>.

Appendix A.  Example of Route Filtering for Aggregate Routes and its
             Contributors

   Presented here is an illustration of how an AS_SET is not used when
   aggregating and still data-plane route loops are avoided.  Consider
   that p1/24 (from AS 64501), p2/24 (from AS 64502), p3/24 (from AS
   64503), and p4/24 (from AS 64504) are aggregated by AS 64505 to p/22.
   AS_SET is not used with the aggregate p/22 but AGGREGATOR and ATOMIC
   AGGREGATE are used.  Data-plane route loops are avoided by not
   announcing the aggregate p/22 to the contributing ASes, i.e., AS
   64501, AS 64502, AS 64503, and AS 64504.  Instead, as further
   illustration, p1/24, p2/24, and p4/24 are announced to AS 64503.  The
   routing tables (post aggregation) of each of the ASes are depicted in

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   the diagram below.

         (       )     (       )           (       )     (       )
        ( AS64501 )   ( AS64502 )         ( AS64503 )   ( AS64504 )
         (       )     (       )           (       )     (       )
           p1/24         p2/24               p3/24         p4/24
             |             |                   |             |
             |             +-->  (       )  <--+             |
             |                  ( AS64505 )                  |
             +---------------->  (       )  <----------------+
                                    p/22
                                     |
                                     V

        AS 64501                      AS 64502
        ==========================    ==========================
        p1/24 AS_PATH ""              p1/24 AS_PATH "64505 64501"
        p2/24 AS_PATH "64505 64502"   p2/24 AS_PATH ""
        p3/24 AS_PATH "64505 64503"   p3/24 AS_PATH "64505 64503"
        p4/24 AS_PATH "64505 64504"   p4/24 AS_PATH "64505 64504"

        AS 64503                      AS 64504
        ==========================    ==========================
        p1/24 AS_PATH "64505 64501"   p1/24 AS_PATH "64505 64501"
        p2/24 AS_PATH "64505 64502"   p2/24 AS_PATH "64505 64502"
        p3/24 AS_PATH ""              p3/24 AS_PATH "64505 64503"
        p4/24 AS_PATH "64505 64504"   p4/24 AS_PATH ""

        AS 64505
        ==========================
        p/22  AS_PATH "" AGGREGATOR 64505 ATOMIC_AGGREGATE
        p1/24 AS_PATH "64501"
        p2/24 AS_PATH "64502"
        p3/24 AS_PATH "64503"
        p4/24 AS_PATH "64504"

Appendix B.  Examples of Consistent and Inconsistent BGP Origin-AS
             Generated by Traditional Brief Aggregation

   In the examples below, it is illustrated how traditional brief
   aggregation may result in an inconsistent origin AS.

   AS 64500 aggregates more specific routes into 192.0.2.0/24.

   Consider the following scenarios where brief aggregation is done by
   AS 64500 and what the resultant origin ASes would be.

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   Routes:
   R1 - 192.0.2.0/26   AS_PATH "64501"
   R2 - 192.0.2.64/26  AS_PATH "64502"
   R3 - 192.0.2.128/26 AS_PATH "64504 64502"
   R4 - 192.0.2.192/26 AS_PATH "64504 64501"

              (       )                        (       )
             ( AS64501 )                      ( AS64502 )
              (       )                        (       )
   192.0.2.0/26    192.0.2.192/26    192.0.2.128/26  192.0.2.64/26
        |                      |      |                 |
        |                      |      |                 |
        |                      \/    \/                 |
        |                     (        )                |
        |                    (  AS64504 )               |
        |                     (        )                |
        |                      |      |                 |
        |                   R4 |      | R3              |
        |                      |      |                 |
        |                      \/    \/                 |
        |             R1      (         )      R2       |
        +------------------->(  AS64500  )<-------------+
                              (         )
                                   |
                                   | (announcing
                                   |  aggregate 192.0.2.0/24)
                                  \/

B.1.  Scenario 1: First one route, then another, each with a fully
      disjoint AS_PATH

   Receive R1.  Aggregate 192.0.2.0/24 AS_PATH "64501"

   Alternate "bug?": Aggregate 192.0.2.0/24 AS_PATH "[ 64501 ]"

   (Note: AS numbers within square brackets represent an AS_SET.)

   Receive R2.  Aggregate 192.0.2.0/24 AS_PATH "[ 64501 64502 ]"

   If brief aggregation is in use, the AS_PATH would be truncated to the
   empty AS_PATH, "".

   The resulting AS_PATH is thus not stable and depends on the presence
   of specific routes.

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B.2.  Scenario 2: First one route, then another, the AS_PATHs overlap at
      the origin AS.

   Receive R1.  Aggregate 192.0.2.0/24 AS_PATH "64501"

   Receive R4.  Aggregate 192.0.2.0/24 AS_PATH "[ 64504 64501 ]"

   If brief aggregation is in use, the AS_PATH is truncated to "".

   The resulting AS_PATH is thus not stable and depends on the presence
   of specific routes.

B.3.  Scenario 3: First one route, then another, the AS_PATHs overlap at
      the neighbor AS

   Receive R3.  Aggregate 192.0.2.0/24 AS_PATH "64504 64501".

   Receive R4.  Aggregate 192.0.2.0/24 AS_PATH "64504 [ 64501 64502 ]"

   If brief aggregation is in use, the AS_PATH is truncated to "64504".

   The resulting AS_PATH is thus not stable and depends on the presence
   of specific routes.

B.4.  Achieving Consistent Origin-AS During Aggregation

   In the three scenarios above, the aggregating AS 64500 is using
   traditional brief aggregation.  This results in inconsistent origin
   ASes as the contributing routes are learned.  This motivates the
   "consistent brief" BGP aggregation mentioned in Section 5.2 and
   discussed further with examples below.

   The trivial solution to addressing the issue is to simply discard all
   of the ASes for the contributing routes.  In simple BGP aggregation
   topologies, this is likely the correct thing to do.  The AS
   originating the aggregate, 192.0.2.0/24 in this example, is likely
   the resource holder for the route in question.  In such a case,
   simply originating the route to its BGP upstream neighbors in the
   Internet with its own AS, 64500, means that a consistent Route Origin
   Authorization (ROA) could be registered in the RPKI for this prefix.
   This satisfies the need for a consistent (unambiguous) origin AS.

   If the contributing ASes are themselves multihomed to the Internet
   outside of their connections to AS 64500, then additional ROAs would
   need to be created for each of the more specific prefixes.

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   In more complex proxy aggregation scenarios, there may be a desire to
   permit some stable (i.e., common) portion of the contributing
   AS_PATHs to be kept in the aggregate route.  Consider the case for
   Scenario 3, where the neighbor AS is the same for both R3 and R4 - AS
   64504.  In such a case, an implementation may permit the aggregate's
   brief AS_PATH to be "64504", and a ROA would be created for the
   aggregate prefix with 64504 as the origin AS.

Appendix C.  Discussion on Forwarding Loops and AS_SETs

   Although BGP-4 was designed to carry CIDR routes, [RFC4271] does not
   discuss the installation of "discard" or "null" routes when
   implementing its aggregation procedures.  Implementations could
   originate an aggregate prefix without a covering route for a more-
   specific prefix (subsumed by the aggregate prefix) present in the
   local routing table.

   When aggregating more specific routes according to [RFC4271]
   aggregation procedures, the aggregating BGP speaker will place
   contributing routes into the generated AS_PATH, perhaps using
   AS_SETs.  As a result, a contributing AS will not install the
   aggregated route into its RIB since the route is an AS_PATH loop.
   This provides a form of protection against forwarding loops created
   by BGP aggregation.

   When brief aggregation methods are used, a BGP speaker may receive a
   route containing such less specific destination covering a local more
   specific destination and install it in its routing table since it is
   not prevented from doing so by BGP AS_PATH loop detection.  This
   gives rise to the possibility of forwarding loops.  To help prevent
   forwarding loops, it is critical to adhere to the following:

   1.  Rule #2 of Section 5.1 of [RFC4632]: A router that generates an
       aggregate route for multiple, more-specific routes must discard
       packets that match the aggregate route, but not any of the more-
       specific routes.  In other words, the "next hop" for the
       aggregate route should be the null destination.

   2.  Not advertising aggregate routes to contributing ASes as
       specified in Section 6.2 of this document (also see Appendix A).

Authors' Addresses

   Warren Kumari
   Google, Inc.
   1600 Amphitheatre Parkway
   Mountain View, CA 94043
   United States of America

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   Phone: +1 571 748 4373
   Email: warren@kumari.net

   Kotikalapudi Sriram
   USA NIST
   100 Bureau Drive
   Gaithersburg, MD 20899
   United States of America
   Phone: +1 301 975 3973
   Email: ksriram@nist.gov

   Lilia Hannachi
   USA NIST
   100 Bureau Drive
   Gaithersburg, MD 20899
   United States of America
   Phone: +1 301 975 3259
   Email: lilia.hannachi@nist.gov

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

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