IDR                                                              J. Haas
Internet-Draft                                          Juniper Networks
Intended status: Informational                               J. Mitchell
Expires: January 16, April 18, 2014                            Microsoft Corporation
                                                        October 15, 2013

           Reservation of Last Autonomous System (AS) Numbers


   This document reserves two Autonomous System numbers (ASNs) at the
   end of the 16 bit and 32 bit ranges, described in this document as
   "Last ASNs" and provides guidance to implementers and operators on
   their use.

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

   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 January 16, April 18, 2014.

Copyright Notice

   Copyright (c) 2013 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
   ( 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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Introduction

   IANA has reserved the last Autonomous System Number (ASN), 65535, of
   the 16 bit autonomous system number range for over a decade with the
   intention that it not be used by network operators running BGP
   [RFC4271].  Since the introduction of BGP Support for Four-Octet AS
   Number Space [RFC6793], IANA has also reserved the ASN of the 32 bit
   autonomous system number range, 4294967295.  This reservation has
   been documented in the IANA Autonomous System Numbers Registry
   [IANA.AS].  Although these "Last ASNs" border on Private Use ASN
   [RFC6996] ranges, they are not defined or reserved as Private Use
   ASNs by
   [I-D.ietf-idr-as-private-reservation]. [IANA.AS].  This document describes the reasoning for
   reserving these Last ASNs and provides guidance both to operators and
   to implementers on their use.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Reasons for Last ASNs Reservation

   The primary reason for reserving the Last ASN of both the 16 bit and
   32 bit ASN ranges is that these numbers are also at the end of
   typical computational data structures holding the underlying number.
   Programmatic errors are more common when handling of end of range
   values, and sometimes last values (binary all ones) have been used as
   "magic numbers", to represent a different number or behavior.

   Secondly, a

   A subset of the standard BGP communities of ASN 65535, the last ASN of the 16
   bit range, 65535, are reserved for use by Well-known communities as
   described in [RFC1997] and [IANA.WK].  Although this is not currently
   true of ASN 4294967295, if there is a future need for a another Special
   Use ASN that is not designed to be globally routable, or the
   associated BGP attributes (such as communities) communities of such an ASN, this ASN 4294967295 could be a
   valid candidate for such purpose.  This document does not prescribe
   any such purpose Special Use to this ASN. ASN at the time of publication.

4.  Operational Considerations

   Operators MUST SHOULD NOT use these Last ASNs as if they are Private Use
   ASNs, and SHOULD NOT use these Last
   ASNs or for any other purpose,
   except a Special Uses defined elsewhere.  Any other operational purpose.  Operational use of these Last ASNs
   could have unpredictable or undesirable results.  For example; use of AS 65535 as if
   it were a Private Use ASN, may result in inadvertent use of BGP Well-known community Well-
   known Community values [IANA.WK], causing undesired undesireable routing

   These last ASNs MUST NOT be advertised to the global Internet within
   AS_PATH or AS4_PATH attributes.  Operators that choose to filter do inbound
   filtering of Private Use ASNs within the AS_PATH and AS4_PATH
   attributes SHOULD also filter these Last ASNs.  These
   last ASNs MUST NOT be advertised to the global Internet within
   AS_PATH or AS4_PATH attributes.

5.  Implementation Considerations
   While these Last ASNs are reserved, they remain valid ASNs from a BGP
   protocol perspective.  Therefore, implementations of BGP [RFC4271]
   SHOULD NOT treat the use of these Last ASNs as any type of protocol
   error.  However, implementations MAY generate a local warning message
   indicating probable improper use of a reserved ASN.

   Implementations that provide tools that filter Private Use ASNs
   within the AS_PATH and AS4_PATH attributes MAY also include these
   Last ASNs.

6.  IANA Considerations

   IANA has reserved last Autonomous System number 65535 from the
   "16-bit Autonomous System Numbers" registry for the reasons described
   in this document.

   IANA has also reserved last Autonomous System number 4294967295 from
   the "32-bit Autonomous System Numbers" registry for the reasons
   described in this document.

   This reservation has been documented in the IANA Autonomous System
   Numbers Registry [IANA.AS].

7.  Security Considerations

   This document does not introduce any additional security concerns in
   regards to usage of Last ASNs.  Although the BGP protocol is designed
   to allow usage of these Last ASNs, security issues related to BGP
   implementation errors could be triggered by Last ASN usage.

8.  References

8.1.  Normative References

   [IANA.AS]  IANA, ., "Autonomous System (AS) Numbers", July October 2013,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC6793]  Vohra, Q. and E. Chen, "BGP Support for Four-Octet
              Autonomous System (AS) Number Space", RFC 6793, December

8.2.  Informative References

              Mitchell, J., "Autonomous System (AS) Reservation for
              Private Use", draft-ietf-idr-as-private-reservation-05
              (work in progress), May 2013.

   [IANA.WK]  IANA, ., "Border Gateway Protocol (BGP) Well-known
              Communities", July October 2013, <
              bgp-well-known-communities/>. <

   [RFC1997]  Chandrasekeran, R., Traina, P., and T. Li, "BGP
              Communities Attribute", RFC 1997, August 1996.

   [RFC6996]  Mitchell, J., "Autonomous System (AS) Reservation for
              Private Use", BCP 6, RFC 6996, July 2013.

Appendix A.  Acknowledgements

   The authors would like to thank Michelle Cotton and Elwyn Davis for
   encouraging the proper documentation of the reservation of these
   ASNs.  The authors would also like to thank ASNs
   and David Farmer for his contributions to the document.

Authors' Addresses

   Jeffrey Haas
   Juniper Networks


   Jon Mitchell
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052