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RFC 2374 - An IPv6 Aggregatable Global Unicast Address Format

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Network Working Group                                        R. Hinden
Request for Comments: 2374                                       Nokia
Obsoletes: 2073                                              M. O'Dell
Category: Standards Track                                        UUNET
                                                            S. Deering
                                                             July 1998

           An IPv6 Aggregatable Global Unicast Address Format

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

1.0 Introduction

   This document defines an IPv6 aggregatable global unicast address
   format for use in the Internet.  The address format defined in this
   document is consistent with the IPv6 Protocol [IPV6] and the "IPv6
   Addressing Architecture" [ARCH].  It is designed to facilitate
   scalable Internet routing.

   This documented replaces RFC 2073, "An IPv6 Provider-Based Unicast
   Address Format".  RFC 2073 will become historic.  The Aggregatable
   Global Unicast Address Format is an improvement over RFC 2073 in a
   number of areas.  The major changes include removal of the registry
   bits because they are not needed for route aggregation, support of
   EUI-64 based interface identifiers, support of provider and exchange
   based aggregation, separation of public and site topology, and new
   aggregation based terminology.

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

2.0 Overview of the IPv6 Address

   IPv6 addresses are 128-bit identifiers for interfaces and sets of
   interfaces.  There are three types of addresses: Unicast, Anycast,
   and Multicast.  This document defines a specific type of Unicast

   In this document, fields in addresses are given specific names, for
   example "subnet".  When this name is used with the term "ID" (for
   "identifier") after the name (e.g., "subnet ID"), it refers to the
   contents of the named field.  When it is used with the term "prefix"
   (e.g.  "subnet prefix") it refers to all of the addressing bits to
   the left of and including this field.

   IPv6 unicast addresses are designed assuming that the Internet
   routing system makes forwarding decisions based on a "longest prefix
   match" algorithm on arbitrary bit boundaries and does not have any
   knowledge of the internal structure of IPv6 addresses.  The structure
   in IPv6 addresses is for assignment and allocation.  The only
   exception to this is the distinction made between unicast and
   multicast addresses.

   The specific type of an IPv6 address is indicated by the leading bits
   in the address.  The variable-length field comprising these leading
   bits is called the Format Prefix (FP).

   This document defines an address format for the 001 (binary) Format
   Prefix for Aggregatable Global Unicast addresses. The same address
   format could be used for other Format Prefixes, as long as these
   Format Prefixes also identify IPv6 unicast addresses.  Only the "001"
   Format Prefix is defined here.

3.0 IPv6 Aggregatable Global Unicast Address Format

   This document defines an address format for the IPv6 aggregatable
   global unicast address assignment.  The authors believe that this
   address format will be widely used for IPv6 nodes connected to the
   Internet.  This address format is designed to support both the
   current provider-based aggregation and a new type of exchange-based
   aggregation.  The combination will allow efficient routing
   aggregation for sites that connect directly to providers and for
   sites that connect to exchanges.  Sites will have the choice to
   connect to either type of aggregation entity.

   While this address format is designed to support exchange-based
   aggregation (in addition to current provider-based aggregation) it is
   not dependent on exchanges for it's overall route aggregation
   properties.  It will provide efficient route aggregation with only
   provider-based aggregation.

   Aggregatable addresses are organized into a three level hierarchy:

      - Public Topology
      - Site Topology
      - Interface Identifier

   Public topology is the collection of providers and exchanges who
   provide public Internet transit services.  Site topology is local to
   a specific site or organization which does not provide public transit
   service to nodes outside of the site.  Interface identifiers identify
   interfaces on links.

        ______________                  ______________
    --+/              \+--------------+/              \+----------
      (       P1       )    +----+    (       P3       )  +----+
      +\______________/     |    |----+\______________/+--|    |--
      |                  +--| X1 |                       +| X2 |
      | ______________  /   |    |-+    ______________  / |    |--
      +/              \+    +-+--+  \  /              \+  +----+
      (       P2       )     / \     +(      P4        )
    --+\______________/     /   \      \______________/
           |               /     \           |      |
           |              /       |          |      |
           |             /        |          |      |
          _|_          _/_       _|_        _|_    _|_
         /   \        /   \     /   \      /   \  /   \
        ( S.A )      ( S.B )   ( P5  )    ( P6  )( S.C )
         \___/        \___/     \___/      \___/  \___/
                                  |          / \
                                 _|_       _/_  \   ___
                                /   \     /   \  +-/   \
                               ( S.D )   ( S.E )  ( S.F )
                                \___/     \___/    \___/

   As shown in the figure above, the aggregatable address format is
   designed to support long-haul providers (shown as P1, P2, P3, and
   P4), exchanges (shown as X1 and X2), multiple levels of providers
   (shown at P5 and P6), and subscribers (shown as S.x) Exchanges
   (unlike current NAPs, FIXes, etc.) will allocate IPv6 addresses.
   Organizations who connect to these exchanges will also subscribe
   (directly, indirectly via the exchange, etc.) for long-haul service
   from one or more long-haul providers.  Doing so, they will achieve

   addressing independence from long-haul transit providers.  They will
   be able to change long-haul providers without having to renumber
   their organization.  They can also be multihomed via the exchange to
   more than one long-haul provider without having to have address
   prefixes from each long-haul provider.  Note that the mechanisms used
   for this type of provider selection and portability are not discussed
   in the document.

3.1 Aggregatable Global Unicast Address Structure

   The aggregatable global unicast address format is as follows:

     | 3|  13 | 8 |   24   |   16   |          64 bits               |
     |FP| TLA |RES|  NLA   |  SLA   |         Interface ID           |
     |  | ID  |   |  ID    |  ID    |                                |

     <--Public Topology--->   Site
                                     <------Interface Identifier----->


      FP           Format Prefix (001)
      TLA ID       Top-Level Aggregation Identifier
      RES          Reserved for future use
      NLA ID       Next-Level Aggregation Identifier
      SLA ID       Site-Level Aggregation Identifier
      INTERFACE ID Interface Identifier

   The following sections specify each part of the IPv6 Aggregatable
   Global Unicast address format.

3.2 Top-Level Aggregation ID

   Top-Level Aggregation Identifiers (TLA ID) are the top level in the
   routing hierarchy.  Default-free routers must have a routing table
   entry for every active TLA ID and will probably have additional
   entries providing routing information for the TLA ID in which they
   are located.  They may have additional entries in order to optimize
   routing for their specific topology, but the routing topology at all
   levels must be designed to minimize the number of additional entries
   fed into the default free routing tables.

   This addressing format supports 8,192 (2^13) TLA ID's.  Additional
   TLA ID's may be added by either growing the TLA field to the right
   into the reserved field or by using this format for additional format

   The issues relating to TLA ID assignment are beyond the scope of this
   document.  They will be described in a document under preparation.

3.3 Reserved

   The Reserved field is reserved for future use and must be set to

   The Reserved field allows for future growth of the TLA and NLA fields
   as appropriate.  See section 4.0 for a discussion.

3.4 Next-Level Aggregation Identifier

   Next-Level Aggregation Identifier's are used by organizations
   assigned a TLA ID to create an addressing hierarchy and to identify
   sites.  The organization can assign the top part of the NLA ID in a
   manner to create an addressing hierarchy appropriate to its network.
   It can use the remainder of the bits in the field to identify sites
   it wishes to serve.  This is shown as follows:

      |  n  |      24-n bits     |   16   |    64 bits      |
      |NLA1 |      Site ID       | SLA ID | Interface ID    |

   Each organization assigned a TLA ID receives 24 bits of NLA ID space.
   This NLA ID space allows each organization to provide service to
   approximately as many organizations as the current IPv4 Internet can
   support total networks.

   Organizations assigned TLA ID's may also support NLA ID's in their
   own Site ID space.  This allows the organization assigned a TLA ID to
   provide service to organizations providing public transit service and
   to organizations who do not provide public transit service.  These
   organizations receiving an NLA ID may also choose to use their Site
   ID space to support other NLA ID's.  This is shown as follows:

   |  n  |      24-n bits     |   16   |    64 bits      |
   |NLA1 |      Site ID       | SLA ID | Interface ID    |

         |  m  |    24-n-m    |   16   |    64 bits      |
         |NLA2 |   Site ID    | SLA ID | Interface ID    |

               |  o  |24-n-m-o|   16   |    64 bits      |
               |NLA3 | Site ID| SLA ID | Interface ID    |

   The design of the bit layout of the NLA ID space for a specific TLA
   ID is left to the organization responsible for that TLA ID.  Likewise
   the design of the bit layout of the next level NLA ID is the
   responsibility of the previous level NLA ID.  It is recommended that
   organizations assigning NLA address space use "slow start" allocation
   procedures similar to [RFC2050].

   The design of an NLA ID allocation plan is a tradeoff between routing
   aggregation efficiency and flexibility.  Creating hierarchies allows
   for greater amount of aggregation and results in smaller routing
   tables.  Flat NLA ID assignment provides for easier allocation and
   attachment flexibility, but results in larger routing tables.

3.5 Site-Level Aggregation Identifier

   The SLA ID field is used by an individual organization to create its
   own local addressing hierarchy and to identify subnets.  This is
   analogous to subnets in IPv4 except that each organization has a much
   greater number of subnets.  The 16 bit SLA ID field support 65,535
   individual subnets.

   Organizations may choose to either route their SLA ID "flat" (e.g.,
   not create any logical relationship between the SLA identifiers that
   results in larger routing tables), or to create a two or more level
   hierarchy (that results in smaller routing tables) in the SLA ID
   field.  The latter is shown as follows:

   |  n  |   16-n     |              64 bits                |
   |SLA1 |   Subnet   |            Interface ID             |

         | m  |16-n-m |              64 bits                |
         |SLA2|Subnet |            Interface ID             |

   The approach chosen for structuring an SLA ID field is the
   responsibility of the individual organization.

   The number of subnets supported in this address format should be
   sufficient for all but the largest of organizations.  Organizations
   which need additional subnets can arrange with the organization they
   are obtaining Internet service from to obtain additional site
   identifiers and use this to create additional subnets.

3.6 Interface ID

   Interface identifiers are used to identify interfaces on a link.
   They are required to be unique on that link.  They may also be unique
   over a broader scope.  In many cases an interfaces identifier will be
   the same or be based on the interface's link-layer address.
   Interface IDs used in the aggregatable global unicast address format
   are required to be 64 bits long and to be constructed in IEEE EUI-64
   format [EUI-64].  These identifiers may have global scope when a
   global token (e.g., IEEE 48bit MAC) is available or may have local
   scope where a global token is not available (e.g., serial links,
   tunnel end-points, etc.).  The "u" bit (universal/local bit in IEEE
   EUI-64 terminology) in the EUI-64 identifier must be set correctly,
   as defined in [ARCH], to indicate global or local scope.

   The procedures for creating EUI-64 based Interface Identifiers is
   defined in [ARCH].  The details on forming interface identifiers is
   defined in the appropriate "IPv6 over <link>" specification such as
   "IPv6 over Ethernet" [ETHER], "IPv6 over FDDI" [FDDI], etc.

4.0 Technical Motivation

   The design choices for the size of the fields in the aggregatable
   address format were based on the need to meet a number of technical
   requirements.  These are described in the following paragraphs.

   The size of the Top-Level Aggregation Identifier is 13 bits.  This
   allows for 8,192 TLA ID's.  This size was chosen to insure that the
   default-free routing table in top level routers in the Internet is

   kept within the limits, with a reasonable margin, of the current
   routing technology.  The margin is important because default-free
   routers will also carry a significant number of longer (i.e., more-
   specific) prefixes for optimizing paths internal to a TLA and between

   The important issue is not only the size of the default-free routing
   table, but the complexity of the topology that determines the number
   of copies of the default-free routes that a router must examine while
   computing a forwarding table.  Current practice with IPv4 it is
   common to see a prefix announced fifteen times via different paths.

   The complexity of Internet topology is very likely to increase in the
   future.  It is important that IPv6 default-free routing support
   additional complexity as well as a considerably larger internet.

   It should be noted for comparison that at the time of this writing
   (spring, 1998) the IPv4 default-free routing table contains
   approximately 50,000 prefixes.  While this shows that it is possible
   to support more routes than 8,192 it is matter of debate if the
   number of prefixes supported today in IPv4 is already too high for
   current routing technology.  There are serious issues of route
   stability as well as cases of providers not supporting all top level
   prefixes.  The technical requirement was to pick a TLA ID size that
   was below, with a reasonable margin, what was being done with IPv4.

   The choice of 13 bits for the TLA field was an engineering
   compromise.  Fewer bits would have been too small by not supporting
   enough top level organizations.  More bits would have exceeded what
   can be reasonably accommodated, with a reasonable margin, with
   current routing technology in order to deal with the issues described
   in the previous paragraphs.

   If in the future, routing technology improves to support a larger
   number of top level routes in the default-free routing tables there
   are two choices on how to increase the number TLA identifiers.  The
   first is to expand the TLA ID field into the reserved field.  This
   would increase the number of TLA ID's to approximately 2 million.
   The second approach is to allocate another format prefix (FP) for use
   with this address format.  Either or a combination of these
   approaches allows the number of TLA ID's to increase significantly.

   The size of the Reserved field is 8 bits.  This size was chosen to
   allow significant growth of either the TLA ID and/or the NLA ID

   The size of the Next-Level Aggregation Identifier field is 24 bits.

   This allows for approximately sixteen million NLA ID's if used in a
   flat manner.  Used hierarchically it allows for a complexity roughly
   equivalent to the IPv4 address space (assuming an average network
   size of 254 interfaces).  If in the future additional room for
   complexity is needed in the NLA ID, this may be accommodated by
   extending the NLA ID into the Reserved field.

   The size of the Site-Level Aggregation Identifier field is 16 bits.
   This supports 65,535 individual subnets per site.  The design goal
   for the size of this field was to be sufficient for all but the
   largest of organizations.  Organizations which need additional
   subnets can arrange with the organization they are obtaining Internet
   service from to obtain additional site identifiers and use this to
   create additional subnets.

   The Site-Level Aggregation Identifier field was given a fixed size in
   order to force the length of all prefixes identifying a particular
   site to be the same length (i.e., 48 bits).  This facilitates
   movement of sites in the topology (e.g., changing service providers
   and multi-homing to multiple service providers).

   The Interface ID Interface Identifier field is 64 bits.  This size
   was chosen to meet the requirement specified in [ARCH] to support
   EUI-64 based Interface Identifiers.

5.0 Acknowledgments

   The authors would like to express our thanks to Thomas Narten, Bob
   Fink, Matt Crawford, Allison Mankin, Jim Bound, Christian Huitema,
   Scott Bradner, Brian Carpenter, John Stewart, and Daniel Karrenberg
   for their review and constructive comments.

6.0 References

   [ALLOC]   IAB and IESG, "IPv6 Address Allocation Management",
             RFC 1881, December 1995.

   [ARCH]    Hinden, R., "IP Version 6 Addressing Architecture",
             RFC 2373, July 1998.

   [AUTH]    Atkinson, R., "IP Authentication Header", RFC 1826, August

   [AUTO]    Thompson, S., and T. Narten., "IPv6 Stateless Address
             Autoconfiguration", RFC 1971, August 1996.

   [ETHER]   Crawford, M., "Transmission of IPv6 Packets over Ethernet
             Networks", Work in Progress.

   [EUI64]   IEEE, "Guidelines for 64-bit Global Identifier (EUI-64)
             Registration Authority",
             March 1997.

   [FDDI]    Crawford, M., "Transmission of IPv6 Packets over FDDI
             Networks", Work in Progress.

   [IPV6]    Deering, S., and R. Hinden, "Internet Protocol, Version 6
             (IPv6) Specification", RFC 1883, December 1995.

   [RFC2050] Hubbard, K., Kosters, M., Conrad, D., Karrenberg, D.,
             and J. Postel, "Internet Registry IP Allocation
             Guidelines", BCP 12, RFC 1466, November 1996.

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

7.0 Security Considerations

   IPv6 addressing documents do not have any direct impact on Internet
   infrastructure security.  Authentication of IPv6 packets is defined
   in [AUTH].

8.0 Authors' Addresses

   Robert M. Hinden
   232 Java Drive
   Sunnyvale, CA 94089

   Phone: 1 408 990-2004
   EMail: hinden@iprg.nokia.com

   Mike O'Dell
   UUNET Technologies, Inc.
   3060 Williams Drive
   Fairfax, VA 22030

   Phone: 1 703 206-5890
   EMail: mo@uunet.uu.net

   Stephen E. Deering
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134-1706

   Phone: 1 408 527-8213
   EMail: deering@cisco.com

9.0  Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an


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