BGP Maximum Prefix Limits Inbound
draft-sas-idr-maxprefix-inbound-00

Abstract

This document describes mechanisms to limit the negative impact of route leaks and/or resource exhaustion in BGP [RFC4271] 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.

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This Internet-Draft will expire on October 17, 2020.

Copyright Notice

Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

• 1. Introduction
• 2. Changes to RFC4271 Section 6
• 3. Changes to RFC4271 Section 8
• 4. BGP Yang Model Considerations - PERHAPS REMOVE BEFORE PUBLICATION
• 5. Changes to RFC4271 Section 9
• 6. Security Considerations
• 7. IANA Considerations
• 8. Acknowledgments
• 9. Implementation status - RFC EDITOR: REMOVE BEFORE PUBLICATION
• 10. Appendix: Implementation Guidance
• 11. References
• 11.1. Normative References
• 11.2. Informative References
• Authors' Addresses

1. Introduction

This document updates [RFC4271] by revising control mechanism which limit the negative impact of route leaks and/or resource exhaustion in Border Gateway Protocol (BGP) implementations. While [RFC4271] described methods to tear down BGP sessions or discard UPDATES after certain thresholds are exceeded, some nuances in this specification were missing resulting in inconsistencies between BGP implementations.

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). The limit on the prefixes accepted from a neighbor can be applied before policy processing (Pre-Policy) or after policy processing (Post-Policy). When the upper bound is reached, the speaker, under control of local configuration, either:
  1. Discards new address prefixes from the neighbor (while maintaining the BGP connection with the neighbor).
  2. Terminates the BGP connection with the neighbor.

• If the BGP speaker decides to terminate its BGP connection with a neighbor because the number of address prefixes received from the neighbor exceeds the locally-configured, upper bound, then the speaker MUST send the neighbor a NOTIFICATION message with the Error Code Cease.

• The speaker MAY also log this locally.

3. Changes to RFC4271 Section 8

This section updates Section 8, 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"). The local system automatically disconnects the peer.

4. BGP Yang Model Considerations - PERHAPS REMOVE BEFORE PUBLICATION

According to [I-D.ietf-idr-bgp-model], in the container 'prefix-limit', a leaf named "max-prefixes" exists. The authors recommend the BGP Yang Model to be revised to contain the following leaves:

max-prefixes-inbound-pre-policy

max-prefixes-inbound-post-policy

In addition to the above, the authors suggest that the BGP Yang Model is extended in such a way that for each peer, for each AFI/SAFI pair, an operator can specify whether to tear down the session or discard received updates.

5. 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 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. 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) 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.

6. Security Considerations

Maximum Prefix Limits are an essential tool for routing operations and SHOULD be used to increase stability for the global routing ecosystem.

7. IANA Considerations

This memo requests that IANA updates the name of subcode "Maximum Number of Prefixes Reached" to "Threshold exceeded: Maximum Number of Prefixes Received" 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) and Jakob Heitz (Cisco) 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 below table provides an overview (as of the moment of writing) of which vendors have produced implementation of inbound prefix limits. Each table cell shows the applicable configuration keywords if the vendor implemented the feature.

First presented by Snijders at [RIPE77]

10. Appendix: Implementation Guidance

TBD

11. References

11.1. Normative References

11.2. Informative References

Authors' Addresses