INTERNET-DRAFT                              R. Hinden, Ipsilon Networks
June 12, 1997                                          M. O'Dell, UUNET
                                                      S. Deering, Cisco

           An IPv6 Aggregatable Global Unicast Address Format


Status of this Memo

   This document is an Internet Draft.  Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its Areas,
   and its Working Groups.  Note that other groups may also distribute
   working documents as Internet Drafts.

   Internet Drafts are draft documents valid for a maximum of six
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   Please check the 1id-abstracts.txt listing contained in the internet-
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   current status of any Internet Draft.

   This internet draft expires on December 13, 1997.

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 key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

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   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 both sites that connect directly to providers and
   sites that connect to exchanges.  Sites will have the choice to
   connect to either type of aggregation entity.

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   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.D )
         \___/        \___/     \___/      \___/  \___/
                                  |          / \
                                 _|_       _/_  \   ___
                                /   \     /   \  +-/   \
                               ( S.E )   ( S.F )  ( S.G )
                                \___/     \___/    \___/

   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 [EXCH] (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

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   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 |    32     |   16   |          64 bits               |
      |FP | TLA |   NLA*    |  SLA*  |         Interface ID           |

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


      FP           Format Prefix (001)
      TLA          Top-Level Aggregator
      NLA*         Next-Level Aggregator(s)
      SLA*         Site-Level Aggregator(s)
      INTERFACE ID Interface Identifier

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

3.2 Top-Level Aggregator

   Top-Level Aggregators (TLA) are the top level in the routing
   hierarchy.  Default-free routers must have a routing table entry for
   every active TLA.  They may have additional entries, 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's.  Additional TLA
   may be added by using this format for additional format prefixes.
   The addition of another FP will add another 8,192 TLA's.

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3.2.1 Assignment of TLAs

   TLAs are assigned to organizations providing public transit topology.
   They are specifically not assigned to organizations only providing
   leaf or private transit topology.  TLA assignment does not imply
   ownership.  It does imply stewardship over valuable Internet

   The IAB and IESG have authorized the Internet Assigned Numbers
   Authority (IANA) as the appropriate entity to have the responsibility
   for the management of the IPv6 address space as defined in [ALLOC].

   The IANA will assign small blocks of TLAs to IPv6 registries.  The
   registries will assign the TLAs to organizations meeting the
   requirements for TLAs.  When the registries have assigned all of
   their TLAs they can request that the IANA give them another block.
   The blocks do not have to be contiguous.  The IANA may also assign
   TLAs to organizations directly.

   Organizations assigned TLAs are required to meet the following

    - Must have a plan to offer public native IPv6 service within 6
      months from assignment.  Plan must include plan for NLA

    - Plan or track record providing public internet transit service on
      fair, reasonable, and non-discriminatory terms, to other
      providers.  TLAs must not be assigned to organizations that are
      only providing leaf service even if multihomed.

    - Must provide registry services on fair, reasonable, and non-
      discriminatory terms, for the NLA address space it is responsible
      for under its TLA.  This must include both sites and next level

    - Must provide transit routing and forwarding to all assigned TLAs
      on fair, reasonable, and non-discriminatory terms.  Organizations
      are not allowed to filter out any specific TLA's (except
      temporarily for diagnostic purposes or emergency repair purposed).

    - Periodically (interval set by registry) provide to registry
      utilization statistics of the TLA it has custody of.  The
      organization must also show evidence of carrying TLA routing and
      transit traffic.  This can be in the form of traffic statistics,
      traceroutes, routing table dumps, or similar means.

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   Organizations which are given custody of a TLA and fail to continue
   to meet these may have the TLA custody revoked.

3.3 Next-Level Aggregator(s)

   Next-Level Aggregator(s) are used by TLA's to create an addressing
   hierarchy and to identify sites.  The TLA can assign the top part of
   the NLA 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  |      32-n bits     |   16   |    64 bits      |
         |NLA1 |       Site         |  SLA*  | Interface ID    |

   Each TLA receives 32 bits of NLA* space.  This NLA* space allows each
   TLA to provide service to about as many organizations as the current
   IPv4 internet can support total nodes.

   The TLAs may also support NLAs in their own Site ID space.  This
   allows the TLAs to provide service to organizations providing public
   transit service and organizations who do not.  The organizations
   providing public transit service become NLA's themselves.  These NLAs
   may also choose to use their Site ID space to support other NLAs.
   This is shown as follows:

         |  n  |      32-n bits     |   16   |    64 bits      |
         |NLA1 |       Site         |  SLA*  | Interface ID    |

               |  m  |    32-n-m    |   16   |    64 bits      |
               |NLA2 |    Site      |  SLA*  | Interface ID    |

                     |  o  |32-n-m-o|   16   |    64 bits      |
                     |NLA3 |  Site  |  SLA*  | Interface ID    |

   The NLA delegation works in the same manner as CIDR delegation in
   IPv4 [CIDR].  TLAs are required to assume registry duties for the
   NLAs.  Each level of NLA is required to assume registry duties for
   the next level NLA.

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   The design of the bit layout of the NLA space for a specific TLA is
   left to the organization responsible for that TLA.  Likewise the
   design of the bit layout of the next level NLA is the responsibility
   of the previous level NLA.  It is recommended that organizations
   assigning NLA address space use "slow start" allocation procedures as
   is currently done with IPV4 CIDR blocks.

   The design of an NLA 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 assignment provides for easier allocation and
   attachment flexibility but results in larger routing tables.

3.4 Site-Level Aggregator(s)

   The SLA* 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* field support 65,535
   individual subnets.

   Organizations may choose to either route their SLA* "flat" (e.g., not
   create any logical relationship between the SLA identifiers which
   results in larger routing tables), or to create a two or more level
   hierarchy (which results in smaller routing tables) in the SLA*
   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 how to the structure of an SLA* field is the
   responsibility of the individual organization.

   The number of subnets supported 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.

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3.5 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 interface's identifier will
   be the same as that 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.

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4.0 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, and John Stewart.  for their review
   and constructive comments.

5.0 References

     [ALLOC]   IAB and IESG, "IPv6 Address Allocation Management",
              RFC1881, December 1995.

     [ARCH]    Hinden, R., "IP Version 6 Addressing Architecture",
              Internet Draft, <draft-ietf-ipngwg-addr-arch-00.txt>, May

     [AUTO]    Thompson, S., Narten T., "IPv6 Stateless Address
              Autoconfiguration", RFC1971, August 1996.

     [CIDR]    Fuller, V., T. Li, K. Varadhan, J. Yu, "Supernetting: an
              Address Assignment and Aggregation Strategy", RFC1338.

     [ETHER]   Crawford, M., "Transmission of IPv6 Packets over Ethernet
              Networks", Internet Draft, <draft-ietf-ipngwg-trans-
              ethernet-00.txt>, March 1997.

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

     [EXCH]    Hinden, R., Huitema, C. "Internet Exchanges", document
              under preparation.

     [FDDI]    Crawford, M., "Transmission of IPv6 Packets over FDDI
              Networks", Internet Draft, <draft-ietf-ipngwg-trans-fddi-
              net-00.txt>, March 1997.

     [IPV6]    Deering, S., Hinden, R., Editors, "Internet Protocol,
              Version 6 (IPv6) Specification", RFC1883, December 1995.

     [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", RFC2119, BCP14, March 1997.

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6.0 Security Considerations

   Documents of this type do not directly impact the security of the
   Internet infrastructure or its applications.

7.0 Authors' Addresses

   Robert M. Hinden                     phone: 1 408 990-2004
   Ipsilon Networks, Inc.               email:
   232 Java Drive
   Sunnyvale, CA 94089

   Mike O'Dell                          phone: 1 703 206-5890
   UUNET Technologies, Inc.             email:
   3060 Williams Drive
   Fairfax, VA 22030

   Stephen E. Deering                   phone: 1 408 527-8213
   Cisco Systems, Inc.                  email:
   170 West Tasman Drive
   San Jose, CA 95134-1706

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