Deprecation of AS_SET and AS_CONFED_SET in BGP
draft-ietf-idr-deprecate-as-set-confed-set-14
| Document | Type | Active Internet-Draft (idr WG) | |
|---|---|---|---|
| Authors | Warren "Ace" Kumari , Kotikalapudi Sriram , Lilia Hannachi , Jeffrey Haas | ||
| Last updated | 2024-07-12 (Latest revision 2024-05-05) | ||
| Replaces | draft-kumari-deprecate-as-set-confed-set | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Additional resources |
GitHub Repository
Mailing list discussion |
||
| Stream | WG state | In WG Last Call | |
| Document shepherd | Keyur Patel | ||
| Shepherd write-up | Show Last changed 2024-07-12 | ||
| IESG | IESG state | I-D Exists | |
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | keyur@arrcus.com |
draft-ietf-idr-deprecate-as-set-confed-set-14
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: 6 November 2024 J. Haas
Juniper Networks, Inc.
5 May 2024
Deprecation of AS_SET and AS_CONFED_SET in BGP
draft-ietf-idr-deprecate-as-set-confed-set-14
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 proscribes 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) and also updates
RFC 5065 by deprecating the origination of BGP routes with
AS_CONFED_SET (type 4) AS_PATH segments. 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 6 November 2024.
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Copyright Notice
Copyright (c) 2024 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
Provisions Relating to IETF Documents (https://trustee.ietf.org/
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 . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 4
3. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 4
4. Updates to Existing RFCs . . . . . . . . . . . . . . . . . . 4
4.1. BGP AS_PATH "Brief" Aggregation . . . . . . . . . . . . . 5
4.2. Issues with "Brief" AS_PATH Aggregation and RPKI-ROV . . 5
4.3. Recommendations to Mitigate Unpredictable AS_PATH Origins
for RPKI-ROV Purposes . . . . . . . . . . . . . . . . . . 6
4.4. Interactions with Four-octet AS Numbers . . . . . . . . . 6
5. Operational Considerations . . . . . . . . . . . . . . . . . 7
5.1. Implementing Consistent Brief Aggregation . . . . . . . . 7
5.2. Not Advertising Aggregate Routes to Contributing ASes . . 7
5.3. Mitigating Forwarding Loops . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 8
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
9.1. Normative References . . . . . . . . . . . . . . . . . . 8
9.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. Example of Route Filtering for Aggregate Routes and
its Contributors . . . . . . . . . . . . . . . . . . . . 10
Appendix B. Examples of Consistent and Inconsistent BGP Origin-AS
Generated by Traditional Brief Aggregation . . . . . . . 11
B.1. Scenario 1: First one route, then another, each with a
fully disjoint AS_PATH . . . . . . . . . . . . . . . . . 12
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 . . . . 13
Appendix C. Discussion on Forwarding Loops and AS_SETs . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
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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 proscription 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
BGP speakers conforming to this document (i.e., conformant BGP
speakers) SHOULD NOT locally generate BGP UPDATE messages containing
AS_SETs or AS_CONFED_SETs. Conformant BGP speakers SHOULD NOT send
BGP UPDATE messages containing AS_SETs or AS_CONFED_SETs. Upon
receipt of such messages, conformant BGP speakers SHOULD use the
"treat-as-withdraw" error handling behavior as per [RFC7606].
The document uses normative language such as "SHOULD NOT send" rather
than "MUST NOT send" with the intention of allowing some transition
time for existing implementations and avoiding abrupt disruptions for
the operators currently using AS_SETs or AS_CONFED_SETs. However, it
is strongly urged that operators stop sending UPDATEs with AS_SETs or
AS_CONFED_SETs as quickly as possible to avoid having UPDATEs dropped
by BGP security mechanisms such as RPKI-ROV and BGPsec.
If a network operator wishes to consider BGP UPDATE messages with
AS_SETs or AS_CONFED_SETs received from an external BGP peers, they
MAY have a feature (knob) in their implementation to do so on a per-
peer basis. The operator should understand the full implications of
choosing this option.
Network operators SHOULD NOT locally generate any new announcements
containing AS_SETs or AS_CONFED_SETs.
BGP security technologies (such as those that take advantage of X.509
extensions for IP addresses and AS identifiers ([RFC3779], [RFC6480],
[RFC6811], [RFC8205]) might not support routes with AS_SETs or
AS_CONFED_SETs in them. Routes with AS_SETs have no possibility of
ever being considered RPKI-ROV valid [RFC6811] [RFC6907].
4. Updates to Existing RFCs
This document deprecates the origination of BGP routes with AS_SET
(type 1) (see [RFC4271], Section 4.3).
This document also deprecates the origination of BGP routes with
AS_CONFED_SET (type 4) AS_PATH segments (see [RFC5065], Section 3).
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BGP speakers conforming to this document — i.e., conformant BGP
speakers — SHOULD NOT originate BGP UPDATE messages containing
AS_SETs or AS_CONFED_SETs. Upon receipt of BGP routes containing
AS_SETs, conformant BGP speakers SHOULD use the "treat-as-withdraw"
error handling behavior as per [RFC7606].
4.1. 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
unimplemented "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
| 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.
4.2. 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. Such an unpredictable origin ASes may
result in RPKI-ROV validation issues:
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* 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) [RFC6482] for each potential origin AS that may result from
the expected aggregated AS_PATHs.
4.3. Recommendations to Mitigate Unpredictable AS_PATH Origins for
RPKI-ROV Purposes
In order to ensure a consistent BGP origin AS is announced for
aggregate BGP routes for implementations of "brief" BGP aggregation,
the implementation should 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.
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 ASes are removed from the "longest leading sequence" of
ASes), the ATOMIC_AGGREGATE Path Attribute SHALL be attached. This
is consistent with the intent of Section 5.1.6 of [RFC4271].
4.4. Interactions with Four-octet AS Numbers
[RFC4893] created support for four-octet AS numbers in BGP. A BGP
speaker not supporting four-octet AS numbers, termed an "OLD speaker"
in that document, might have routes that carry the AS4_PATH Path
Attribute. This attribute is used to carry four-octet AS paths
across OLD speakers and may contain AS_PATH segments of type AS_SET.
BGP speakers conforming to this specification MUST NOT use the
information contained in the AS4_PATH for treat-as-withdraw purposes.
Instead, only the AS_PATH should be used.
Aggregation of routes containing the AS4_PATH by OLD speakers can
result in inconsistencies between the AS_PATH and the AS4_PATH which
cause problems for reconstruction of the AS_PATH by a NEW speaker
that has received an OLD speaker's AS_PATH and AS4_PATH. See
Section 4.2.3 of [RFC4893] for details.
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5. Operational Considerations
5.1. Implementing Consistent Brief Aggregation
When aggregating prefixes, network operators MUST use consistent
brief aggregation as described in Section 4.3. 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.
5.2. 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.
5.3. 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].
Further discussion of forwarding loops and their relationship to
AS_SETs can be found in Appendix C.
6. 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.
7. IANA Considerations
This document requires no IANA actions.
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8. Acknowledgements
The authors would like to thank John Heasley, Job Snijders, Jared
Mauch, Jakob Heitz, Keyur Patel, Douglas Montgomery, Randy Bush,
Susan Hares, 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, Alfred Hoenes, and
Alvaro Retana for comments and suggestions.
9. References
9.1. 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>.
[RFC4893] Vohra, Q. and E. Chen, "BGP Support for Four-octet AS
Number Space", RFC 4893, DOI 10.17487/RFC4893, May 2007,
<https://www.rfc-editor.org/info/rfc4893>.
[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>.
9.2. 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>.
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[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>.
[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>.
[RFC6482] Lepinski, M., Kent, S., and D. Kong, "A Profile for Route
Origin Authorizations (ROAs)", RFC 6482,
DOI 10.17487/RFC6482, February 2012,
<https://www.rfc-editor.org/info/rfc6482>.
[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>.
[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>.
[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>.
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[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>.
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
the diagram below.
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( ) ( ) ( ) ( )
( 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"
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 ]"
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.
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.
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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 4.3 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.
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 a destination without a covering route 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.
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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 5.2 of this document.
Authors' Addresses
Warren Kumari
Google, Inc.
1600 Amphitheatre Parkway
Mountain View, CA 94043
United States of America
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
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Jeffrey Haas
Juniper Networks, Inc.
1133 Innovation Way
Sunnyvale, CA 94089
United States of America
Email: jhaas@juniper.net
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