[Docs] [txt|pdf]



[Note that this file is a concatenation of more than one RFC.]




Network Working Group                                       J. Hawkinson
Request for Comments: 1930                                    BBN Planet
BCP: 6                                                          T. Bates
Category: Best Current Practice                                      MCI
                                                              March 1996


          Guidelines for creation, selection, and registration
                      of an Autonomous System (AS)

Status of this Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Abstract

   This memo discusses when it is appropriate to register and utilize an
   Autonomous System (AS), and lists criteria for such.  ASes are the
   unit of routing policy in the modern world of exterior routing, and
   are specifically applicable to protocols like EGP (Exterior Gateway
   Protocol, now at historical status; see [EGP]), BGP (Border Gateway
   Protocol, the current de facto standard for inter-AS routing; see
   [BGP-4]), and IDRP (The OSI Inter-Domain Routing Protocol, which the
   Internet is expected to adopt when BGP becomes obsolete; see [IDRP]).
   It should be noted that the IDRP equivalent of an AS is the RDI, or
   Routing Domain Identifier.

Table of Contents

   1. Introduction ............................................    2
   2. Motivation ..............................................    2
   3. Definitions .............................................    2
   4. Common errors in allocating ASes ........................    5
   5. Criteria for the decision -- do I need an AS?  ..........    5
   5.1 Sample Cases ...........................................    6
   5.2 Other Factors ..........................................    7
   6. Speculation .............................................    7
   7. One prefix, one origin AS ...............................    8
   8. IGP issues ..............................................    8
   9. AS Space exhaustion .....................................    8
   10. Reserved AS Numbers ....................................    9
   11. Security Considerations ................................    9
   12. Acknowledgments ........................................    9
   13. References .............................................    9
   14. Authors' Addresses .....................................   10




Hawkinson & Bates        Best Current Practice                  [Page 1]

RFC 1930            Guidelines for creation of an AS          March 1996


1. Introduction

   This memo discusses when it is appropriate to register and utilize an
   Autonomous System (AS), and lists criteria for such.  ASes are the
   unit of routing policy in the modern world of exterior routing, and
   are specifically applicable to protocols like EGP (Exterior Gateway
   Protocol, now at historical status; see [EGP]), BGP (Border Gateway
   Protocol, the current de facto standard for inter-AS routing; see
   [BGP-4]), and IDRP (The OSI Inter-Domain Routing Protocol, which the
   Internet is expected to adopt when BGP becomes obsolete; see [IDRP]).
   It should be noted that the IDRP equivalent of an AS is the RDI, or
   Routing Domain Identifier.

2. Motivation

   This memo is aimed at network operators and service providers who
   need to understand under what circumstances they should make use of
   an AS.  It is expected that the reader is familiar with routing
   protocols and will be someone who configures and operates Internet
   networks.  Unfortunately, there is a great deal of confusion in how
   ASes should be used today; this memo attempts to clear up some of
   this confusion, as well as acting as a simple guide to today's
   exterior routing.

3. Definitions

   This document refers to the term "prefix" throughout. In the current
   classless Internet (see [CIDR]), a block of class A, B, or C networks
   may be referred to by merely a prefix and a mask, so long as such a
   block of networks begins and ends on a power-of-two boundary. For
   example, the networks:

        192.168.0.0/24
        192.168.1.0/24
        192.168.2.0/24
        192.168.3.0/24

   can be simply referred to as:

        192.168.0.0/22

   The term "prefix" as it is used here is equivalent to "CIDR block",
   and in simple terms may be thought of as a group of one or more
   networks. We use the term "network" to mean classful network, or "A,
   B, C network".

   The definition of AS has been unclear and ambiguous for some time.
   [BGP-4] states:



Hawkinson & Bates        Best Current Practice                  [Page 2]

RFC 1930            Guidelines for creation of an AS          March 1996


      The classic definition of an Autonomous System is a set of routers
      under a single technical administration, using an interior gateway
      protocol and common metrics to route packets within the AS, and
      using an exterior gateway protocol to route packets to other ASes.
      Since this classic definition was developed, it has become common
      for a single AS to use several interior gateway protocols and
      sometimes several sets of metrics within an AS.  The use of the
      term Autonomous System here stresses the fact that, even when
      multiple IGPs and metrics are used, the administration of an AS
      appears to other ASes to have a single coherent interior routing
      plan and presents a consistent picture of what networks are
      reachable through it.

   To rephrase succinctly:

      An AS is a connected group of one or more IP prefixes run by one
      or more network operators which has a SINGLE and CLEARLY DEFINED
      routing policy.

   Routing policy here is defined as how routing decisions are made in
   the Internet today.  It is the exchange of routing information
   between ASes that is subject to routing policies. Consider the case
   of two ASes, X and Y exchanging routing information:

                NET1 ......  ASX  <--->  ASY  ....... NET2

   ASX knows how to reach a prefix called NET1.  It does not matter
   whether NET1 belongs to ASX or to some other AS which exchanges
   routing information with ASX, either directly or indirectly; we just
   assume that ASX knows how to direct packets towards NET1.  Likewise
   ASY knows how to reach NET2.

   In order for traffic from NET2 to NET1 to flow between ASX and ASY,
   ASX has to announce NET1 to ASY using an exterior routing protocol;
   this means that ASX is willing to accept traffic directed to NET1
   from ASY. Policy comes into play when ASX decides to announce NET1 to
   ASY.

   For traffic to flow, ASY has to accept this routing information and
   use it.  It is ASY's privilege to either use or disregard the
   information that it receives from ASX about NET1's reachability. ASY
   might decide not to use this information if it does not want to send
   traffic to NET1 at all or if it considers another route more
   appropriate to reach NET1.

   In order for traffic in the direction of NET1 to flow between ASX and
   ASY, ASX must announce that route to ASY and ASY must accept it from
   ASX:



Hawkinson & Bates        Best Current Practice                  [Page 3]

RFC 1930            Guidelines for creation of an AS          March 1996


                    resulting packet flow towards NET1
                  <<===================================
                                    |
                                    |
                     announce NET1  |  accept NET1
                    --------------> + ------------->
                                    |
                        AS X        |    AS Y
                                    |
                     <------------- + <--------------
                       accept NET2  |  announce NET2
                                    |
                                    |
                   resulting packet flow towards NET2
                   ===================================>>

   Ideally, though seldom practically, the announcement and acceptance
   policies of ASX and ASY are symmetrical.

   In order for traffic towards NET2 to flow, announcement and
   acceptance of NET2 must be in place (mirror image of NET1). For
   almost all applications connectivity in just one direction is not
   useful at all.

   It should be noted that, in more complex topologies than this
   example, traffic from NET1 to NET2 may not necessarily take the same
   path as traffic from NET2 to NET1; this is called asymmetrical
   routing.  Asymmetrical routing is not inherently bad, but can often
   cause performance problems for higher level protocols, such as TCP,
   and should be used with caution and only when necessary. However,
   assymetric routing may be a requirement for mobile hosts and
   inherently asymmetric siutation, such a satelite download and a modem
   upload connection.

   Policies are not configured for each prefix separately but for groups
   of prefixes.  These groups of prefixes are ASes.

   An AS has a globally unique number (sometimes referred to as an ASN,
   or Autonomous System Number) associated with it; this number is used
   in both the exchange of exterior routing information (between
   neighboring ASes), and as an identifier of the AS itself.

   In routing terms, an AS will normally use one or more interior
   gateway protocols (IGPs) when exchanging reachability information
   within its own AS. See "IGP Issues".






Hawkinson & Bates        Best Current Practice                  [Page 4]

RFC 1930            Guidelines for creation of an AS          March 1996


4. Common errors in allocating ASes

   The term AS is often confused or even misused as a convenient way of
   grouping together a set of prefixes which belong under the same
   administrative umbrella, even if within that group of prefixes there
   are various different routing policies. Without exception, an AS must
   have only one routing policy.

   It is essential that careful consideration and coordination be
   applied during the creation of an AS. Using an AS merely for the sake
   of having an AS is to be avoided, as is the worst-case scenario of
   one AS per classful network (the IDEAL situation is to have one
   prefix, containing many longer prefixes, per AS). This may mean that
   some re-engineering may be required in order to apply the criteria
   and guidelines for creation and allocation of an AS that we list
   below; nevertheless, doing so is probably the only way to implement
   the desired routing policy.

   If you are currently engineering an AS, careful thought should be
   taken to register appropriately sized CIDR blocks with your
   registration authority in order to minimize the number of advertised
   prefixes from your AS.  In the perfect world that number can, and
   should, be as low as one.

   Some router implementations use an AS number as a form of tagging to
   identify interior as well as exterior routing processes.  This tag
   does not need to be unique unless routing information is indeed
   exchanged with other ASes. See "IGP Issues".

5. Criteria for the decision -- do I need an AS?

   *    Exchange of external routing information

        An AS must be used for exchanging external routing information
        with other ASes through an exterior routing protocol. The cur-
        rent recommended exterior routing protocol is BGP, the Border
        Gateway Protocol. However, the exchange of external routing
        information alone does not constitute the need for an AS. See
        "Sample Cases" below.

   *    Many prefixes, one AS

        As a general rule, one should try to place as many prefixes as
        possible within a given AS, provided all of them conform to the
        same routing policy.






Hawkinson & Bates        Best Current Practice                  [Page 5]

RFC 1930            Guidelines for creation of an AS          March 1996


   *    Unique routing policy

        An AS is only needed when you have a routing policy which is
        different from that of your border gateway peers. Here routing
        policy refers to how the rest of the Internet makes routing
        decisions based on information from your AS. See "Sample
        Cases" below to see exactly when this criteria will apply.

5.1 Sample Cases

   *    Single-homed site, single prefix

        A separate AS is not needed; the prefix should be placed in an
        AS of the provider. The site's prefix has exactly the same rout-
        ing policy as the other customers of the site's service
        provider, and there is no need to make any distinction in rout-
        ing information.

        This idea may at first seem slightly alien to some, but it high-
        lights the clear distinction in the use of the AS number as a
        representation of routing policy as opposed to some form of
        administrative use.

        In some situations, a single site, or piece of a site, may find
        it necessary to have a policy different from that of its
        provider, or the rest of the site. In such an instance, a sepa-
        rate AS must be created for the affected prefixes. This situa-
        tion is rare and should almost never happen. Very few stub sites
        require different routing policies than their parents. Because
        the AS is the unit of policy, however, this sometimes occurs.

   *    Single-homed site, multiple prefixes

        Again, a separate AS is not needed; the prefixes should be
        placed in an AS of the site's provider.

   *    Multi-homed site

        Here multi-homed is taken to mean a prefix or group of prefixes
        which connects to more than one service provider (i.e. more than
        one AS with its own routing policy). It does not mean a network
        multi-homed running an IGP for the purposes of resilience.

        An AS is required; the site's prefixes should be part of a
        single AS, distinct from the ASes of its service providers.
        This allows the customer the ability to have a different repre-
        sentation of policy and preference among the different service
        providers.



Hawkinson & Bates        Best Current Practice                  [Page 6]

RFC 1930            Guidelines for creation of an AS          March 1996


        This is ALMOST THE ONLY case where a network operator should
        create its own AS number. In this case, the site should ensure
        that it has the necessary facilities to run appropriate routing
        protocols, such as BGP4.

5.2 Other factors


   *    Topology

        Routing policy decisions such as geography, AUP (Acceptable Use
        Policy) compliance and network topology can influence decisions
        of AS creation. However, all too often these are done without
        consideration of whether or not an AS is needed in terms of
        adding additional information for routing policy decisions by
        the rest of the Internet. Careful consideration should be taken
        when basing AS creation on these type of criteria.

   *    Transition / "future-proofing"

        Often a site will be connected to a single service provider but
        has plans to connect to another at some point in the future.
        This is not enough of a reason to create an AS before you really
        need it.  The AS number space is finite and the limited amount
        of re-engineering needed when you connect to another service
        provider should be considered as a natural step in transition.

   *    History

        AS number application forms have historically made no reference
        to routing policy. All too often ASes have been created purely
        because it was seen as "part of the process" of connecting to
        the Internet. The document should be used as a reference from
        future application forms to show clearly when an AS is needed.

6. Speculation

   1) If provider A and provider B have a large presence in a
   geographical area (or other routing domain), and many customers are
   multi-homed between them, it makes sense for all of those customers
   to be placed within the same AS. However, it is noted that case
   should only be looked at if practical to do so and fully coordinated
   between customers and service providers involved.

   2) Sites should not be forced to place themselves in a separate AS
   just so that someone else (externally) can make AS-based policy
   decisions. Nevertheless, it may occasionally be necessary to split
   up an AS or a prefix into two ASes for policy reasons. Those making



Hawkinson & Bates        Best Current Practice                  [Page 7]

RFC 1930            Guidelines for creation of an AS          March 1996


   external policy may request the network operators make such AS
   changes, but the final decision is up to those network operators
   who manage the prefixes in question, as well as the ASes containing
   them. This is, of course, a trade off -- it will not always be
   possible to implement all desired routing policies.

7. One prefix, one origin AS

   Generally, a prefix can should belong to only one AS. This is a
   direct consequence of the fact that at each point in the Internet
   there can be exactly one routing policy for traffic destined to each
   prefix. In the case of an prefix which is used in neighbor peering
   between two ASes, a conscious decision should be made as to which AS
   this prefix actually resides in.

   With the introduction of aggregation it should be noted that a prefix
   may be represented as residing in more than one AS, however, this is
   very much the exception rather than the rule. This happens when
   aggregating using the AS_SET attribute in BGP, wherein the concept of
   origin is lost. In some cases the origin AS is lost altogether if
   there is a less specific aggregate announcement setting the
   ATOMIC_AGGREGATE attribute.

8. IGP Issues

   As stated above, many router vendors require an identifier for
   tagging their IGP processes. However, this tag does not need to be
   globally unique. In practice this information is never seen by
   exterior routing protocols. If already running an exterior routing
   protocol, it is perfectly reasonable to use your AS number as an IGP
   tag; if you do not, choosing from the private use range is also
   acceptable (see "Reserved AS Numbers"). Merely running an IGP is not
   grounds for registration of an AS number.

   With the advent of BGP4 it becomes necessary to use an IGP that can
   carry classless routes. Examples include OSPF [OSPF] and ISIS [ISIS].

9. AS Space exhaustion

   The AS number space is a finite amount of address space. It is
   currently defined as a 16 bit integer and hence limited to 65535
   unique AS numbers. At the time of writing some 5,100 ASes have been
   allocated and a little under 600 ASes are actively routed in the
   global Internet. It is clear that this growth needs to be continually
   monitored. However, if the criteria applied above are adhered to,
   then there is no immediate danger of AS space exhaustion. It is
   expected that IDRP will be deployed before this becomes an issue.
   IDRP does not have a fixed limit on the size of an RDI.



Hawkinson & Bates        Best Current Practice                  [Page 8]

RFC 1930            Guidelines for creation of an AS          March 1996


10. Reserved AS Numbers

   The Internet Assigned Numbers Authority (IANA) has reserved the
   following block of AS numbers for private use (not to be advertised
   on the global Internet):

                           64512 through 65535

11. Security Considerations

   There are few security concerns regarding the selection of ASes.

   AS number to owner mappings are public knowledge (in WHOIS), and
   attempting to change that would serve only to confuse those people
   attempting to route IP traffic on the Internet.

12. Acknowledgments

   This document is largely based on [RIPE-109], and is intended to have
   a wider scope than purely the RIPE community; this document would not
   exist without [RIPE-109].

13. References

   [RIPE-109]
        Bates, T., Lord, A., "Autonomous System Number Application
        Form & Supporting Notes", RIPE 109, RIPE NCC, 1 March 1994.
        URL: ftp://ftp.ripe.net/ripe/docs/ripe-109.txt.

   [BGP-4]
        Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
        RFC 1654, T.J. Watson Research Center, cisco Systems, July 1994.

   [EGP]
        Mills, D., "Exterior Gateway Protocol Formal Specifications",
        STD 18, RFC 904, International Telegraph and Telephone Co.,
        April 1984.

   [IDRP]
        Kunzinger, C., Editor, "OSI Inter-Domain Routing Protocol
        (IDRP)", ISO/IEC 10747, Work In Progress, October 1993.

   [CIDR]
        Fuller, V., T. Li, J. Yu, and K. Varadhan, "Classless
        Inter-Domain Routing (CIDR): an Address Assignment and
        Aggregation Strategy", RFC 1519, BARRnet, cisco, MERIT, OARnet,
        September 1993.




Hawkinson & Bates        Best Current Practice                  [Page 9]

RFC 1930            Guidelines for creation of an AS          March 1996


   [OSPF]
        Moy, J., "OSPF Version 2", RFC 1583, March 1994.

   [ISIS]
        Callon, R., "Use of OSI IS-IS for Routing in TCP/IP and Multi-
        Protocol Environments", RFC 1195, Digital Equipment
        Corporation, December 1990.

14. Authors' Addresses

   John Hawkinson
   BBN Planet Corporation
   150 CambridgePark Drive
   Cambridge, MA 02139

   Phone:  +1 617 873 3180
   EMail: jhawk@bbnplanet.com


   Tony Bates
   MCI
   2100 Reston Parkway
   Reston, VA 22094

   Phone: +1 703 715 7521
   EMail: Tony.Bates@mci.net

























Hawkinson & Bates        Best Current Practice                 [Page 10]
=========================================================================





Internet Engineering Task Force (IETF)                       J. Mitchell
Request for Comments: 6996                         Microsoft Corporation
BCP: 6                                                         July 2013
Updates: 1930
Category: Best Current Practice
ISSN: 2070-1721


           Autonomous System (AS) Reservation for Private Use

Abstract

   This document describes the reservation of Autonomous System Numbers
   (ASNs) that are for Private Use only, known as Private Use ASNs, and
   provides operational guidance on their use.  This document enlarges
   the total space available for Private Use ASNs by documenting the
   reservation of a second, larger range and updates RFC 1930 by
   replacing Section 10 of that document.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   BCPs is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6996.

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
   (http://trustee.ietf.org/license-info) 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.




Mitchell                  Best Current Practice                 [Page 1]

RFC 6996               Private Use AS Reservation              July 2013


1.  Introduction

   The original IANA reservation of Autonomous System Numbers (ASNs) for
   Private Use was a block of 1023 ASNs.  This was also documented by
   the IETF in Section 10 of [RFC1930].  Since the time that the range
   was reserved, the Border Gateway Protocol (BGP) [RFC4271] has seen
   deployment in new application domains, such as data center networks,
   which require a larger Private Use AS space.

   Since the introduction of "BGP Support for Four-Octet Autonomous
   System (AS) Number Space" [RFC6793], the total size of ASN space has
   increased dramatically.  A larger subset of the space is available to
   network operators to deploy in these Private Use cases.  The existing
   range of Private Use ASNs is widely deployed, and the ability to
   renumber this resource in existing networks cannot be coordinated
   given that these ASNs, by definition, are not registered.  Therefore,
   this RFC documents the existing Private Use ASN reservation while
   also introducing a second, larger range that can also be utilized.

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 RFC 2119 [RFC2119].

3.  Private Use ASNs

   To allow the continued growth of BGP protocol usage in new network
   applications that utilize Private Use ASNs, two ranges of ASNs are
   reserved by Section 5 of this document.  The first is part of the
   original 16-bit Autonomous System range previously defined in
   [RFC1930], and the second is a larger range out of the Four-Octet AS
   Number Space [RFC6793].

4.  Operational Considerations

   If Private Use ASNs are used and prefixes originate from these ASNs,
   Private Use ASNs MUST be removed from AS path attributes (including
   AS4_PATH if utilizing a four-octet AS number space) before being
   advertised to the global Internet.  Operators SHOULD ensure that all
   External Border Gateway Protocol (EBGP) speakers support the
   extensions described in [RFC6793] and that implementation-specific
   features that recognize Private Use ASNs have been updated to
   recognize both ranges prior to making use of the newer, numerically
   higher range of Private Use ASNs in the four-octet AS number space.
   Some existing implementations that remove Private Use ASNs from the
   AS_PATH are known to not remove Private Use ASNs if the AS_PATH
   contains a mixture of Private Use and Non-Private Use ASNs.  If such



Mitchell                  Best Current Practice                 [Page 2]

RFC 6996               Private Use AS Reservation              July 2013


   implementations have not been updated to recognize the new range of
   ASNs in this document and a mix of old and new range Private Use ASNs
   exist in the AS4_PATH, these implementations will likely cease to
   remove any Private Use ASNs from either of the AS path attributes.
   Normal AS path filtering MAY also be used to prevent prefixes
   originating from Private Use ASNs from being advertised to the global
   Internet.

5.  IANA Considerations

   IANA has reserved, for Private Use, a contiguous block of 1023
   Autonomous System numbers from the "16-bit Autonomous System Numbers"
   registry, namely 64512 - 65534 inclusive.

   IANA has also reserved, for Private Use, a contiguous block of
   94,967,295 Autonomous System numbers from the "32-bit Autonomous
   System Numbers" registry, namely 4200000000 - 4294967294 inclusive.

   These reservations have been documented in the IANA "Autonomous
   System (AS) Numbers" registry [IANA.AS].

6.  Security Considerations

   Private Use ASNs do not raise any unique security concerns.  Loss of
   connectivity might result from their inappropriate use, specifically
   outside of a single organization, since they are not globally unique.
   This loss of connectivity is limited to the organization using
   Private Use ASNs inappropriately or without reference to Section 4.
   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.

7.  References

7.1.  Normative References

   [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 2012.




Mitchell                  Best Current Practice                 [Page 3]

RFC 6996               Private Use AS Reservation              July 2013


7.2.  Informative References

   [IANA.AS]  IANA, "Autonomous System (AS) Numbers",
              <http://www.iana.org/assignments/as-numbers/>.

   [RFC1930]  Hawkinson, J. and T. Bates, "Guidelines for creation,
              selection, and registration of an Autonomous System (AS)",
              BCP 6, RFC 1930, March 1996.

   [RFC4272]  Murphy, S., "BGP Security Vulnerabilities Analysis",
              RFC 4272, January 2006.

8.  Acknowledgements

   The author would like to acknowledge Christopher Morrow, Jason
   Schiller, and John Scudder for their advice on how to pursue this
   change.  The author would also like to thank Brian Dickson, David
   Farmer, Jeffrey Haas, Nick Hilliard, Joel Jaeggli, Warren Kumari, and
   Jeff Wheeler for their comments and suggestions.

Author's Address

   Jon Mitchell
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   USA

   EMail: Jon.Mitchell@microsoft.com






















Mitchell                  Best Current Practice                 [Page 4]
=========================================================================





Internet Engineering Task Force (IETF)                           J. Haas
Request for Comments: 7300                              Juniper Networks
BCP: 6                                                       J. Mitchell
Updates: 1930                                      Microsoft Corporation
Category: Best Current Practice                                July 2014
ISSN: 2070-1721


           Reservation of Last Autonomous System (AS) Numbers

Abstract

   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.  This document updates Section 10 of RFC 1930.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It has been approved for publication by the Internet
   Engineering Steering Group (IESG).  Further information on BCPs is
   available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7300.

Copyright Notice

   Copyright (c) 2014 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
   (http://trustee.ietf.org/license-info) 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.







Haas & Mitchell           Best Current Practice                 [Page 1]

RFC 7300                   Last AS Reservation                 July 2014


1.  Introduction

   Over a decade ago, IANA reserved the last Autonomous System Number
   (ASN) of the 16-bit ASN range, 65535, with the intention that it not
   be used by network operators running BGP [RFC4271].  Since the
   introduction of "BGP Support for Four-Octet Autonomous System (AS)
   Number Space" [RFC6793], IANA has also reserved the last ASN of the
   32-bit autonomous system number range, 4294967295.  This reservation
   has been documented in the IANA "Autonomous System (AS) 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 [IANA.AS].  This document describes the reasoning for
   reserving 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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Reasons for Reservation of the Last ASNs

   A subset of the BGP communities of ASN 65535, the last ASN of the
   16-bit range, 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 another Special
   Use ASN that is not designed to be globally routable, or for the
   associated BGP communities of such an ASN, ASN 4294967295 could be a
   valid candidate for such purpose.  This document does not prescribe
   any such Special Use to this ASN at the time of publication.

4.  Operational Considerations

   Operators SHOULD NOT use these Last ASNs for any other purpose or as
   Private Use ASNs.  Operational use of these Last ASNs could have
   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 values [IANA.WK], causing undesirable routing behavior.

   Last ASNs MUST NOT be advertised to the global Internet within
   AS_PATH or AS4_PATH attributes.  Operators SHOULD filter Last ASNs
   within the AS_PATH and AS4_PATH attributes.








Haas & Mitchell           Best Current Practice                 [Page 2]

RFC 7300                   Last AS Reservation                 July 2014


5.  Implementation Considerations

   While Last ASNs are reserved, they remain valid ASNs from a BGP
   perspective.  Therefore, implementations of BGP [RFC4271] SHOULD NOT
   treat the use of Last ASNs as any type of protocol error.  However,
   if a Last ASN is configured as the local AS, implementations MAY
   generate a warning message indicating 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 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.

   These reservations have been documented in the IANA "Autonomous
   System (AS) Numbers" registry [IANA.AS] and the IANA "Special-Purpose
   Autonomous System (AS) Numbers" registry [IANA.SpecialAS].

7.  Security Considerations

   This document does not introduce any additional security concerns in
   regards to usage of Last ASNs.  Although the BGP is designed to allow
   usage of 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",
              <http://www.iana.org/assignments/as-numbers/>.

   [IANA.SpecialAS]
              IANA, "Special-Purpose Autonomous System (AS) Numbers",
              <http://www.iana.org/assignments/
              iana-as-numbers-special-registry/>.

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




Haas & Mitchell           Best Current Practice                 [Page 3]

RFC 7300                   Last AS Reservation                 July 2014


   [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
              2012.

8.2.  Informative References

   [IANA.WK]  IANA, "Border Gateway Protocol (BGP) Well-known
              Communities", <http://www.iana.org/assignments/
              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.

































Haas & Mitchell           Best Current Practice                 [Page 4]

RFC 7300                   Last AS Reservation                 July 2014


Appendix A.  Acknowledgments

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

Authors' Addresses

   Jeffrey Haas
   Juniper Networks

   EMail: jhaas@juniper.net


   Jon Mitchell
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   USA

   EMail: Jon.Mitchell@microsoft.com






























Haas & Mitchell           Best Current Practice                 [Page 5]


Html markup produced by rfcmarkup 1.109, available from https://tools.ietf.org/tools/rfcmarkup/