Private Autonomous System (AS) Removal Requirements
draft-mitchell-grow-remove-private-as-00

Abstract

This document specifies operator's requirements for implementations that remove Private Use Autonomous System (AS) numbers from the AS path of routes sent to external Border Gateway Protocol (BGP) peers.

Status of This Memo

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This Internet-Draft will expire on January 16, 2014.

Copyright Notice

Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved.

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1. Introduction

After the original IANA reservation of Autonomous System Numbers (ASNs) for Private Use was allocated via [RFC1930] implementation specific features were released that removed Autonomous System Numbers (ASNs) from the Border Gateway Protocol AS_PATH attribute. The details of such implementations were driven by multiple operators use cases and varied accordingly. At times, implementation differences and undocumented behaviors have led to operators leaking Private Use ASNs to the Internet. Now that a new range of Private Use ASNs has become available via [I-D.ietf-idr-as-private-reservation] implementations will likely require update and even more variation is possible.

This document captures operator's requirements across various use cases, being cognizant of the operations of current implementations that remove Private Use ASNs, and provides a set of requirements for Private Use ASN removal implementations in the hopes of reducing inconsistencies and variations between implementations.

2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

3. Basic Requirements

An implementation that removes Private Use ASNs MUST provide a configuration option to remove them from both the AS_PATH attribute of [RFC4271] and if Four-Octet AS Support [RFC6793] is present, the AS4_PATH attribute of the route. This configuration option MUST be configurable at the External Border Gateway Protocol (EBGP) peering session level, i.e. per neighbor, and will impact the as path attributes associated with any NLRI sent to the router to which is configured. The implementation MUST remove all Private Use ASNs from the as path attributes up to the first non-Private Use AS in the as path, except as dictated by Section 4. An implementation MAY remove Private Use ASNs from the entire as path (past the first ASN in the as path attributes), however if it does so, it SHOULD provide an operator configurable option to disable this behavior if desired. The reason for this behavior is that operators would prefer visibility to which network is leaking Private Use ASNs to the global Internet (or any other network) so the behavior can be corrected directly by the upstream network providing connectivity to the Private Use ASN rather than masking the issue, which may not fully correct the problem if the upstream network has multiple providers.

4. Loop Prevention when using Private Use ASN Removal

Implementations of the Private Use ASN removal feature SHOULD provide basic loop prevention to prevent a multi-homed network with a Private Use ASN from accepting a prefix that was originated by its other location when the route is passed back to it from an adjacent ASN who may have the "Remove Private AS" functionality enabled, and has removed the network's Private AS from the path. Due to the standard BGP path selection process described in Section 9.1.2.2 of [RFC4271] EBGP routes will be preferred over IBGP routes which may have been from IBGP neighbors within the AS, so without further attribute manipulation, this can pose a risk of a routing information loop to some networks. Therefore a router SHOULD NOT remove Private Use ASN's from an AS_PATH or AS4_PATH attribute if it encounters the EBGP AS of the neighbor on which it is configured in the AS_PATH or AS4_PATH that would be removed.

5. Unnecessary Restrictions on Local or Peer AS

Implementations of this feature SHOULD NOT have any unnecessary restrictions on Private Use ASN use on either the local ASN of the router that is configuring the feature or the peer ASN that will be receiving the routes. Both use cases are prevalent in some networks as Private Use ASN removal features have sometimes been used in network mergers or other situations where masking the Private Use ASN's behind a particular AS is necessary to avoid conflict with Private Use ASN's behind the upstream network. In these cases, as long as both the router configuring the feature and the peer have a unique Private ASN from each other, all routes originated from behind their networks containing Private ASN's can be masked to be their ASN. In the case where the AS where the feature is configured is a Private Use ASN and the router also has policy configured to prepend the local AS to the as path, an implementation SHOULD NOT remove the ASN's that have been locally prepended as per policy configuration.

6. Behavior Towards other Special-Use ASNs

Implementations of this feature SHOULD NOT remove Documentation ASNs [RFC5398] as this may encourage their use by operators. These ASNs are not reserved for Private Use and use of them is likely the result of a misconfiguration. Due to historical reasons and lack of operator guidance on Last ASNs prior to [I-D.jhjm-idr-last-as-reservations] implementations MAY remove Last ASNs, which are deployed in some networks as if they are Private Use ASNs, even though this is not recommended to operators for the reasons specified in that document. If implementations choose to do this, the behavior towards Last ASNs SHOULD be consistent with the behavior of the implementation towards Private Use ASNs as specified in this document.

7. Operational Considerations

It should be noted that removing items from the AS_PATH or AS4_PATH poses some risk and could introduce the chance of a routing loop. Further operational considerations for the use of Private Use ASNs are documented in [I-D.ietf-idr-as-private-reservation].

8. IANA Considerations

There are no IANA actions required by this document.

9. Security Considerations

There are no new security concerns in relation to the feature described in this document. General BGP security considerations are discussed in [RFC4271] and [RFC4272]. Identification of the originator of a route with a Private Use ASN in the AS path would have to be done by tracking the route back to the neighboring globally unique AS in the path or by inspecting other attributes.

10. References

10.1. Normative References

10.2. Informative References

Appendix A. Acknowledgements

JM - Placeholder.

Author's Address

Table of Contents

• 1. Introduction
• 2. Requirements Language
• 3. Basic Requirements
• 4. Loop Prevention when using Private Use ASN Removal
• 5. Unnecessary Restrictions on Local or Peer AS
• 6. Behavior Towards other Special-Use ASNs
• 7. Operational Considerations
• 8. IANA Considerations
• 9. Security Considerations
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Appendix A. Acknowledgements
• Author's Address