Inter-Domain Routing M. Aelmans
Internet-Draft Juniper Networks
Updates: 4271 (if approved) M. Stucchi
Intended status: Standards Track Independent
Expires: August 22, 2021 J. Snijders
Fastly
February 18, 2021
Revised BGP Maximum Prefix Limits Outbound
draft-sas-idr-maxprefix-outbound-02
Abstract
This document updates RFC4271 by adding a control mechanism which
limits the negative impact of outbound route leaks (RFC7908) in order
to prevent resource exhaustion in Border Gateway Protocol (BGP)
implementations.
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.
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 August 22, 2021.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Changes to RFC4271 Section 6 . . . . . . . . . . . . . . . . 3
3. Changes to RFC4271 Section 8 . . . . . . . . . . . . . . . . 4
4. Changes to RFC4271 Section 9 . . . . . . . . . . . . . . . . 4
5. Use cases . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. Internet use case . . . . . . . . . . . . . . . . . . . . 6
5.2. CE protection . . . . . . . . . . . . . . . . . . . . . . 6
5.3. PE-CE BGP session from operator side . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
9. Implementation status - RFC EDITOR: REMOVE BEFORE PUBLICATION 7
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
10.1. Normative References . . . . . . . . . . . . . . . . . . 9
10.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
This document updates [RFC4271] by adding a control mechanism which
limits the negative impact of outbound route leaks [RFC7908] in order
to prevent resource exhaustion in Border Gateway Protocol (BGP)
implementations. [RFC4271] describes methods to tear down BGP
sessions or discard UPDATES after certain inbound thresholds are
exceeded. In addition to "inbound maximum prefix limits", this
document introduces a specification for "outbound maximum prefix
limits". [I-D.sas-idr-maxprefix-inbound] updates sections in
[RFC4271] to clarify "inbound maximum prefix limits". This documents
updates those sections again to add "outbound maximum prefix limits".
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2. Changes to RFC4271 Section 6
This section updates [RFC4271] to specify what events can result in
AutomaticStop (Event 8) in the BGP FSM.
The following paragraph replaces the second paragraph of Section 6.7
(Cease), which starts with "A BGP speaker MAY support" and ends with
"The speaker MAY also log this locally.":
A BGP speaker MAY support the ability to impose a locally-
configured, upper bound on the number of address prefixes the
speaker is willing to accept from a neighbor (inbound maximum
prefix limit) or send to a neighbor (outbound prefix limit). The
limit on the prefixes accepted from a neighbor can be applied
before policy processing (Pre-Policy) or after policy processing
(Post-Policy). Outbound prefix limits MUST be measured after
policy, since the Policy (even a policy of "send all") is run
before determining what can be sent. When the upper bound is
reached, the speaker, under control of local configuration,
either:
A. Discards new address prefixes being sent to the neighbor while
maintaining the BGP connection with the neighbor. As these
prefixes are discared and their reachability information is
not sent to the neighbor it might lead to inconsistent routing
behaviour;
B. Sent all prefixes exceeding the threshold and generates a log;
C. Terminates the BGP session with the neighbor. This should be
done using a Hard Reset according to [RFC8538].
If the BGP speaker uses option (b), where the limit causes a CEASE
Notification, then the CEASE error codes should use:
+---------+---------------------------------------------------------+
| Subcode | Symbolic Name |
+---------+---------------------------------------------------------+
| 1 | Threshold exceeded: Maximum Number of Prefixes Received |
| TBD | Threshold exceeded: Maximum Number of Prefixes Sent |
+---------+---------------------------------------------------------+
The speaker MAY also log the event locally.
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3. Changes to RFC4271 Section 8
This section updates Section 8 [RFC4271], the paragraph that starts
with "One reason for an AutomaticStop event is" and ends with "The
local system automatically disconnects the peer." is replaced with:
Possible reasons for an AutomaticStop event are: A BGP speaker
receives an UPDATE messages with a number of prefixes for a given
peer such that the total prefixes received exceeds the maximum
number of prefixes configured (either "Pre-Policy" or "Post-
Policy"), or announces more prefixes than through local
configuration allowed to. The local system automatically
disconnects the peer.
4. Changes to RFC4271 Section 9
This section updates [RFC4271] by adding a subsection after
Section 9.4 (Originating BGP routes) to specify various events that
can lead up to an AutomaticStop (Event 8) in the BGP FSM.
9.5 Maximum Prefix Limits
9.5.1 Pre-Policy Inbound Maximum Prefix Limits
The Adj-RIBs-In stores routing information learned from inbound
UPDATE messages that were received from another BGP speaker
Section 3.2 [RFC4271]. The pre-policy limit uses the number of
NLRIs per Address Family Identifier (AFI) per Subsequent
Address Family Identifier (SAFI) as input into its threshold
comparisons. For example, when an operator configures the pre-
policy limit for IPv4 Unicast to be 50 on a given EBGP session,
and the other BGP speaker announces its 51st IPv4 Unicast NLRI,
the session MUST be terminated.
Pre-policy limits are particularly useful to help dampen the
effects of full table route leaks and memory exhaustion when
the implementation stores rejected routes.
9.5.2 Post-Policy Inbound Maximum Prefix Limits
[RFC4271] describes a Policy Information Base (PIB) that
contains local policies that can be applied to the information
in the Routing Information Base (RIB). The post-policy limit
uses the number of NLRIs per Address Family Identifier (AFI)
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per Subsequent Address Family Identifier (SAFI), after
application of the Import Policy as input into its threshold
comparisons. For example, when an operator configures the
post-policy limit for IPv4 Unicast to be 50 on a given EBGP
session, and the other BGP speaker announces a hundred IPv4
Unicast routes of which none are accepted as a result of the
local import policy (and thus not considered for the Loc-RIB by
the local BGP speaker), the session is not terminated.
Post-policy limits are useful to help prevent FIB exhaustion
and prevent accidental BGP session teardown due to prefixes not
accepted by policy anyway.
9.5.3 Outbound Maximum Prefix Limits
An operator MAY configure a BGP speaker to terminate its BGP
session with a neighbor when the number of address prefixes to
be advertised to that neighbor exceeds a locally configured
post-policy upper limit. The BGP speaker then MUST send the
neighbor a NOTIFICATION message with the Error Code "Cease" and
the Error Subcode "Threshold reached: Maximum Number of
Prefixes Sent". Implementations MAY support additional
actions. The Hard Cease action is defined in [RFC8538].
Reporting when thresholds have been exceeded is an
implementation specific consideration, but SHOULD include
methods such as Syslog [RFC5424]. By definition, Outbound
Maximum Prefix Limits are Post-Policy.
The Adj-RIBs-Out stores information selected by the local BGP
speaker for advertisement to its neighbors. The routing
information stored in the Adj-RIBs-Out will be carried in the
local BGP speaker's UPDATE messages and advertised to its
neighbors Section 3.2 [RFC4271]. The Outbound Maximum Prefix
Limit uses the number of NLRIs per Address Family Identifier
(AFI) per Subsequent Address Family Identifier (SAFI), after
application of the Export Policy, as input into its threshold
comparisons. For example, when an operator configures the
Outbound Maximum Prefix Limit for IPv4 Unicast to be 50 on a
given EBGP session, and were about to announce its 51st IPv4
Unicast NLRI to the other BGP speaker as a result of the local
export policy, the session MUST be terminated.
Outbound Maximum Prefix Limits are useful to help dampen the
negative effects of a misconfiguration in local policy. In
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many cases, it would be more desirable to tear down a BGP
session rather than causing or propagating a route leak.
5. Use cases
Egress maximum prefix limits are usefull in a variety of cases. Some
of those are outlined in this section.
5.1. Internet use case
In order to prevent the BGP speaker from leaking a full routing table
to its neighbor operators should implement proper routing policy and
preferably RFC8212. However, even when implementing both
measurements an operator could still (accidentaly) announce more
routes than intended. Setting a maximum prefix outbound value
prevents this.
5.2. CE protection
Residential and many business customers connected to the internet
using a 'simple' CPE and connected to a single Service Provider only
needs to accept a single default route and not the full internet
table. In order to prevent overloading the CPE Control Plane,
maximum outbound limits should be applied on the session on the PE
router.
5.3. PE-CE BGP session from operator side
-- Change this so it explains that it's extra protection towards the
PE so it won't kill the BGP session due to max prefix inbound --
Internet providers PE side gateway PE-CE connections would would
generally set maximum prefix to disconnect if maximum prefix is
reached. This is a secondary protection mechanism as the primary is
prefix length and AS path checks.
6. Security Considerations
Maximum Prefix Limits are an essential tool for routing operations
and SHOULD be used to increase stability. They provide a first-line
mechanism to avoid route leaks and to avoid unintended routing
suggestions to happen between neighbors. Implementing this measures
is only one of the building blocks you need to provide full security,
but it is important to build a modular defense system.
Stability for the routing table is also an important aspect for
implementing the measures included in this draft. Ensuring that
neighbors will not receive an amount of routes that would overload
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their routing platform contributes to the stability of
interconnections and of the Internet as a whole.
7. IANA Considerations
This memo requests that IANA assigns a new subcode named "Threshold
exceeded: Maximum Number of Prefixes Sent" in the "Cease NOTIFICATION
message subcodes" registry under the "Border Gateway Protocol (BGP)
Parameters" group.
8. Acknowledgments
The authors would like to thank Saku Ytti and John Heasley (NTT
Ltd.), Jeff Haas, Colby Barth and John Scudder (Juniper Networks),
Martijn Schmidt (i3D.net), Teun Vink (BIT), Sabri Berisha (eBay),
Martin Pels (Quanza), Steven Bakker (AMS-IX), Aftab Siddiqui (ISOC),
Yu Tianpeng, Ruediger Volk (Deutsche Telekom), Robert Raszuk
(Bloomberg), Jakob Heitz (Cisco), Warren Kumari (Google), Ben
Maddison (Workonline), Randy Bush, Brian Dickson and Gyan Mishra
(Verizon) for their support, insightful reviews, and comments.
9. Implementation status - RFC EDITOR: REMOVE BEFORE PUBLICATION
This section records the status of known implementations of the
protocol defined by this specification at the time of posting of this
Internet-Draft, and is based on a proposal described in RFC7942. The
description of implementations in this section is intended to assist
the IETF in its decision processes in progressing drafts to RFCs.
Please note that the listing of any individual implementation here
does not imply endorsement by the IETF. Furthermore, no effort has
been spent to verify the information presented here that was supplied
by IETF contributors. This is not intended as, and must not be
construed to be, a catalog of available implementations or their
features. Readers are advised to note that other implementations may
exist.
The table below provides an overview (as of the moment of writing) of
which vendors have produced implementations of inbound or outbound
maximum prefix limits. Each table cell shows the applicable
configuration keywords if the vendor implemented the feature.
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+----------+-------------+---------------------+--------------------+
| Vendor | Inbound | Inbound Post-Policy | Outbound |
| | Pre-Policy | | |
+----------+-------------+---------------------+--------------------+
| Cisco | | maximum-prefix | |
| IOS XR | | | |
+----------+-------------+---------------------+--------------------+
| Cisco | | maximum-prefix | |
| IOS XE | | | |
+----------+-------------+---------------------+--------------------+
| Juniper | prefix- | accepted-prefix- | advertise-prefix- |
| Junos OS | limit | limit, or prefix- | limit * |
| | | limit combined with | |
| | | 'keep none' | |
+----------+-------------+---------------------+--------------------+
| Nokia SR | prefix- | | |
| OS | limit | | |
+----------+-------------+---------------------+--------------------+
| NIC.CZ | 'import | 'import limit' or | export limit |
| BIRD | keep | 'receive limit' | |
| | filtered' | | |
| | combined | | |
| | with | | |
| | 'receive | | |
| | limit' | | |
+----------+-------------+---------------------+--------------------+
| OpenBSD | max-prefix | | |
| OpenBGPD | | | |
+----------+-------------+---------------------+--------------------+
| Arista | maximum- | maximum-accepted- | |
| EOS | routes | routes | |
+----------+-------------+---------------------+--------------------+
| Huawei | peer route- | | |
| VRPv5 | limit | | |
+----------+-------------+---------------------+--------------------+
| Huawei | peer route- | peer route-limit | |
| VRPv8 | limit | accept-prefix | |
+----------+-------------+---------------------+--------------------+
First presented by Job Snijders at [RIPE77]
Table 1: Maximum prefix limits capabilities per implementation
*In testing stage
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10. References
10.1. Normative References
[I-D.sas-idr-maxprefix-inbound]
Aelmans, M., stucchi-lists@glevia.com, s., and J.
Snijders, "BGP Maximum Prefix Limits Inbound", draft-sas-
idr-maxprefix-inbound-01 (work in progress), October 2020.
[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>.
[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>.
[RFC8538] Patel, K., Fernando, R., Scudder, J., and J. Haas,
"Notification Message Support for BGP Graceful Restart",
RFC 8538, DOI 10.17487/RFC8538, March 2019,
<https://www.rfc-editor.org/info/rfc8538>.
10.2. Informative References
[I-D.ietf-idr-bgp-model]
Jethanandani, M., Patel, K., Hares, S., and J. Haas, "BGP
YANG Model for Service Provider Networks", draft-ietf-idr-
bgp-model-10 (work in progress), November 2020.
[RFC5424] Gerhards, R., "The Syslog Protocol", RFC 5424,
DOI 10.17487/RFC5424, March 2009,
<https://www.rfc-editor.org/info/rfc5424>.
[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>.
[RIPE77] Snijders, J., "Robust Routing Policy Architecture", May
2018, <https://ripe77.ripe.net/wp-content/uploads/presenta
tions/59-RIPE77_Snijders_Routing_Policy_Architecture.pdf>.
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Authors' Addresses
Melchior Aelmans
Juniper Networks
Boeing Avenue 240
Schiphol-Rijk 1119 PZ
Netherlands
Email: maelmans@juniper.net
Massimiliano Stucchi
Independent
Email: max@stucchi.ch
Job Snijders
Fastly
Amsterdam
Netherlands
Email: job@fastly.com
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