IDR and SIDR K. Sriram, Ed.
Internet-Draft USA NIST
Intended status: Standards Track A. Azimov, Ed.
Expires: January 26, 2020 Yandex
July 25, 2019
Methods for Detection and Mitigation of BGP Route Leaks
draft-ietf-grow-route-leak-detection-mitigation-01
Abstract
Problem definition for route leaks and enumeration of types of route
leaks are provided in [RFC7908]. This document describes a new well-
known Large Community that provides a way for route leak prevention,
detection, and mitigation. The configuration process for this
Community can be automated with the methodology for setting BGP roles
that is described in ietf-idr-bgp-open-policy draft.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Peering Relationships . . . . . . . . . . . . . . . . . . . . 2
3. Community vs Attribute . . . . . . . . . . . . . . . . . . . 3
4. Down Only Community . . . . . . . . . . . . . . . . . . . . . 4
4.1. Route Leak Mitigation . . . . . . . . . . . . . . . . . . 5
4.2. Only Marking . . . . . . . . . . . . . . . . . . . . . . 6
5. Implementation Considerations . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Informative References . . . . . . . . . . . . . . . . . . . 7
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 8
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
[RFC7908] provides a definition of the route leak problem and
enumerates several types of route leaks. For this document, the
definition that is applied is that a route leak occurs when a route
received from a transit provider or a lateral peer is forwarded
(against commonly used policy) to another transit provider or a
lateral peer. The commonly used policy is that a route received from
a transit provider or a lateral peer MAY be forwarded only to
customers.
This document describes a solution for prevention, detection and
mitigation route leaks which is based on conveying route-leak
detection information in a well-known BGP Large Community. The
configuration process for the Large Community MUST be defined
according to peering relations between ISPs. This process can be
automated with the methodology for setting BGP roles that is
described in [I-D.ietf-idr-bgp-open-policy].
The techniques described in this document can be incrementally
deployed. If a group of big ISPs and/or Internet Exchanges (IXes)
deploy the proposed techniques, then they would be helping each other
by blocking route leaks that can happen between them.
2. Peering Relationships
As described in [I-D.ietf-idr-bgp-open-policy] there are several
common peering relations between eBGP neighbors:
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o Provider - sender is a transit provider of the neighbor;
o Customer - sender is a customer of the neighbor;
o Route Server (RS) - sender is route server at an internet exchange
(IX)
o RS-client - sender is client of an RS at an IX
o Peer - sender and neighbor are lateral (non-transit) peers;
If a route is received from a provider, peer or RS-client, it MUST
follow the 'down only' rule, i.e., it MAY be advertised only to
customers. If a route is sent to a customer, peer or RS-client, it
also MUST follow the 'down only' rule at each subsequent AS in the AS
path.
A standardized transitive route-leak detection signal is needed that
will prevent Autonomous Systems (ASes) from leaking and also inform a
remote ISP (or AS) in the AS path that a received route violates
'down only' policy. This signal would facilitate a way to stop the
propagation of leaked prefixes.
To improve reliability and cover for non-participating preceding
neighbor, the signal should be set on both receiver and sender sides.
3. Community vs Attribute
This section presents a brief discussion of the advantages and
disadvantages of communities and BGP path attributes for the purpose
of route leak detection.
A transitive path attribute is a native way to implement the route-
leak detection signal. Based on the way BGP protocol works, the use
of a transitive attribute makes it more certain that the route-leak
detection signal would pass unaltered through non-participating
(i.e., not updated) BGP routers. The main disadvantage of this
approach is that the deployment of a new BGP attribute requires a
software update in router OS which may delay wide adoption for years.
On the other hand, BGP communities do not require a router OS update.
The potential disadvantage of using a Community for the route-leak
detection signal is that it is more likely to be dropped somewhere
along the way in the AS path. Currently, the use of BGP Communities
is somewhat overloaded. BGP Communities are already used for
numerous applications: different types of route marking, route policy
control, black-holing, etc. It is observed that some ASes seem to
purposefully or accidentally remove transitive communities on
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receipt, sometimes well-known ones. Perhaps this issue may be
mitigated with strong policy guidance related to the handling of
Communities.
Due to frequently occurring regional and global disruptions in
Internet connectivity, it is critical to move forward with a solution
that is viable in the near term. That solution would be route leak
detection using BGP Community.
Large Communities have much higher capacity, and therefore they are
likely to be less overloaded. Hence, Large Community is proposed to
be used for route-leak detection. This document suggests reserving
<TBD1> class for the purpose of transitive well-known Large
Communities that MUST not be stripped on ingress or egress.
While it is not part of this document, the route-leak detection
signal described here can also be carried in a BGP path attribute,
and the same prevention and mitigation techniques as described here
would apply. The authors are pursuing a separate internet draft in
the IDR WG on that approach.
4. Down Only Community
This section specifies the semantics of route-leak-detection
Community and its usage. This Community is given the specific name
Down Only (DO) Community. The DO Community is carried in a BGP Large
Community with a format as shown in Figure 1.
0 1 2 3
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 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBD1 (class for well-known transit communities) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TBD2 (subclass for DO) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ASN |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: Format of the DO Community using a Large Community
[RFC8092].
The authors studied different options for route leak mitigation. The
main options considered are (1) drop detected route leaks and (2)
deprioritize detected route leaks. It can be demonstrated that the
loose mode that uses deprioritization is not safe. Traffic
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Engineering (TE) technique which limit prefix visibility are quite
common. It may happen that a more specific TE prefix is sent only to
downstream ASes or to IX(es)/selected peers, and a control Community
is used to restrict its propagation. If such a more specific prefix
is leaked, deprioritization will not stop such a route leak from
propagating. In addition, propagation of leaked prefixes based on
deprioritization may result in priority loops leading to BGP wedgies
[RFC4264] or even persistent route oscillations.
So, the only truly safe way to implement route leak mitigation is to
drop detected route leaks. The ingress and egress policies
corresponding to 'drop detected route leaks' is described in
Section 4.1. This policy SHOULD be used as a default behavior.
Nevertheless, early adopters might want to deploy only the signaling
and perhaps use it only for diagnostics before applying any route
leak mitigation policy. They are also encouraged to use slightly
limited marking, which is described in Section 4.2.
4.1. Route Leak Mitigation
This section describes the eBGP ingress and egress policies that MUST
be used to perform route leak prevention, detection and mitigation
using the DO Community.
The ingress policy MUST use the following procedure:
1. If a route with DO Community set (i.e., DO is attached) is
received from a Customer or RS-client, then it is a route leak
and MUST be rejected. The procedure halts.
2. If a route with DO Community set is received from Peer (non-
transit) and DO value is not equal to the sending neighbor's ASN,
then it is a route leak and MUST be rejected. The procedure
halts.
3. If a route is received from a Provider, Peer or RS, then the DO
Community MUST be added with a value equal to the sending
neighbor's ASN.
The egress policy MUST use the following procedure:
1. A route with DO Community set MUST not be sent to Providers,
Peers, and RS.
2. If a route is sent to a Customer or Peer, then the DO Community
MUST be added with a value equal to the ASN of the sender.
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The above procedures comprehensively provide route-leak prevention,
detection and mitigation. Policy consisting of these procedures
SHOULD be used as a default behavior.
4.2. Only Marking
This section describes eBGP ingress and egress marking policies that
MUST be used if an AS is not performing route-leak mitigation (i.e.,
dropping detected route leaks) as described in Section 4.1, but wants
to use only marking with DO Community. The slightly limited DO
marking (compared to that in Section 4.1) described below guarantees
that this DO marking will not limit the leak detection opportunities
for subsequent ASes in the AS path.
The ingress policy MUST use the following procedure:
1. If a route with DO Community set is received from a Customer or
RS-client, then it is a route leak. The procedure halts.
2. If a route with DO Community set is received from a Peer and DO
value is not equal to the sending neighbor's ASN, then it is a
route leak. The procedure halts.
3. If a route is received from a Provider, Peer or RS, then the DO
Community MUST be added with value equal to the sending
neighbor's ASN.
The egress policy MUST use the following procedure:
1. If a route is sent to a Customer or RS-client, then the DO
Community MUST be added with value equal to the ASN of the
sender.
2. If DO Community is not set and the route is sent to a Peer, then
the DO Community MUST be added with value equal to the ASN of the
sender.
These above procedures specify setting DO signal in a way that can be
used to evaluate the potential impact of route leak mitigation policy
before deploying strict dropping of detected route leaks.
5. Implementation Considerations
It was observed that the majority of BGP implementations does not
support negative match for communities like a:b:!c. Considering that
it is suggested to replace the second rule from ingress policy with
the following:
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If a route with DO Community set is received from a Peer and DO value
is equal to the sending neighbor's ASN, then it is a valid route,
otherwise it is a route leak. The procedure halts.
This rule is based on a weaker assumption that a peer that is doing
marking is also doing filtering (dropping detected leaks). That is
why networks that do not follow the route leak mitigation policy in
Section 4.1 MUST carefully follow marking rules described in
Section 4.2.
6. Security Considerations
In specific circumstances in a state of partial adoption, route leak
mitigation mechanism can result in Denial of Service (DoS) for the
victim prefix. Such a scenario may happen only for a prefix that has
a single path from the originator to a Tier-1 ISP and only when the
prefix is not covered with a less specific prefix with multiple paths
to the Tier-1 ISP. If, in such unreliable topology, route leak is
injected somewhere inside this single path, then it may be rejected
by upper layer providers in the path, thus limiting prefix
visibility. While such anomaly is unlikely to happen, such an issue
should be easy to debug, since it directly affects the sequence of
originator's providers.
With the use of BGP Community, there is often a concern that the
Community propagates beyond its intended perimeter and causes harm
[streibelt]. However, that concern does not apply to the DO
Community because it is a transitive Community that must propagate as
far as the update goes.
7. IANA Considerations
The draft suggests to reserve a Global Administrator ID <TBD1> for
transitive well-known Large Community registry. IANA is requested to
register a subclass <TBD2> for DO Community in this registry.
8. Informative References
[I-D.ietf-idr-bgp-open-policy]
Azimov, A., Bogomazov, E., Bush, R., Patel, K., and K.
Sriram, "Route Leak Prevention using Roles in Update and
Open messages", draft-ietf-idr-bgp-open-policy-06 (work in
progress), July 2019.
[RFC4264] Griffin, T. and G. Huston, "BGP Wedgies", RFC 4264,
DOI 10.17487/RFC4264, November 2005,
<https://www.rfc-editor.org/info/rfc4264>.
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[RFC7908] Sriram, K., Montgomery, D., McPherson, D., Osterweil, E.,
and B. Dickson, "Problem Definition and Classification of
BGP Route Leaks", RFC 7908, DOI 10.17487/RFC7908, June
2016, <https://www.rfc-editor.org/info/rfc7908>.
[RFC8092] Heitz, J., Ed., Snijders, J., Ed., Patel, K., Bagdonas,
I., and N. Hilliard, "BGP Large Communities Attribute",
RFC 8092, DOI 10.17487/RFC8092, February 2017,
<https://www.rfc-editor.org/info/rfc8092>.
[streibelt]
Streibelt et al., F., "BGP Communities: Even more Worms in
the Routing Can", ACM IMC, October 2018,
<https://archive.psg.com//181101.imc-communities.pdf>.
Acknowledgements
The authors wish to thank John Scudder, Susan Hares, Ruediger Volk,
Mat Ford, Greg Skinner for their review and comments.
Contributors
The following people made significant contributions to this document
and should be considered co-authors:
Brian Dickson
Independent
Email: brian.peter.dickson@gmail.com
Doug Montgomery
USA National Institute of Standards and Technology
Email: dougm@nist.gov
Keyur Patel
Arrcus
Email: keyur@arrcus.com
Andrei Robachevsky
Internet Society
Email: robachevsky@isoc.org
Eugene Bogomazov
Qrator Labs
Email: eb@qrator.net
Randy Bush
Internet Initiative Japan
Email: randy@psg.com
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Authors' Addresses
Kotikalapudi Sriram (editor)
USA National Institute of Standards and Technology
100 Bureau Drive
Gaithersburg, MD 20899
United States of America
Email: ksriram@nist.gov
Alexander Azimov (editor)
Yandex
Ulitsa Lva Tolstogo 16
Moscow 119021
Russia
Email: a.e.azimov@gmail.com
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