Network Working Group                                         Y. Rekhter
Request for Comments: 2073                                         cisco
Category: Standards Track                                    P. Lothberg
                                                               STUPI.AB
                                                              R. Hinden
                                                       Ipsilon Networks
                                                             S. Deering
                                                             Xerox PARC
                                                              J. Postel
                                                                    ISI
                                                                Editors
                                                           January 1997


            An IPv6 Provider-Based 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.

1.0 Introduction

  This document defines an IPv6 provider-based unicast address format
  for use in the Internet.  The address format defined in this document
  is consistent with the "IPv6 Addressing Architecture" [ARCH] and the
  "An Architecture for IPv6 Unicast Address Allocation" [ALLOC], and is
  intended to facilitate scalable Internet routing.

  The unicast address format defined in this document doesn't preclude
  the use of other unicast address formats.

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
  address.

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



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  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 010 (binary) Format
  Prefix for Provider-Based 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 "010" Format
  Prefix is defined here.

3.0 IPv6 Provider-Based Unicast Address Format

  This document defines an address format for the IPv6 provider-based
  unicast address assignment.  It is expected that this address format
  will be widely used for IPv6 nodes connected to the Internet.

  The address format defined in this document conforms to the
  "Architecture for IPv6 Unicast Address Allocation" [ALLOC].
  Specifically, the format is designed to support aggregation of
  network layer reachability information at multiple levels of routing
  hierarchy.

  For addresses of the format described in this document the address
  administration is organized into a three level hierarchy -- registry,
  provider, and subscriber.  The address format defined here allows
  flexible address allocation at each level of the address
  administration hierarchy in such a way as to support a wide spectrum
  of demands for address allocation.

  This document assumes that the Internet routing system doesn't make
  any assumptions about the specific structure and semantics of an IPv6
  address, except for the structure and semantics of the Format Prefix
  part of the address, and the use of the "longest prefix match"
  algorithm (on arbitrary bit boundaries) for making a forwarding
  decision.

  The address format defined in this document is intended to facilitate
  scalable Internet-wide routing that does not impose any constraints
  on connectivity among the providers, as well as among the providers
  and subscribers.











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3.1 Provider-Based Unicast Address Structure

  For the purpose of address allocation, the address format defined in
  this document consists of the following parts:  Format Prefix,
  Registry ID, Provider ID, Subscriber ID, and an Intra-Subscriber
  part.  The Intra-Subscriber part definition is the responsibility of
  the subscriber and is not defined in this document.  The provider-
  based unicast address format is as follows:

     | 3 |  5 bits  |   n bits   |   56-n bits  |        64 bits     |
     +---+----------+------------+--------------+--------------------+
     |010|RegistryID| ProviderID | SubscriberID |  Intra-Subscriber  |
     +---+----------+------------+--------------+--------------------+

  The following sections specify each part of the IPv6 Provider-Based
  Unicast address format.  In general other allocation strategies are
  possible within this framework, but the ones described in this
  document will be used to assign IPv6 provider-based addresses.

  3.2 Registry ID

  With the growth of the Internet and its increasing globalization,
  much thought has been given to the evolution of the Network Layer
  address space allocation and assignment process.  RFC 1466,
  "Guidelines for Management of IP Address Space", proposes a plan that
  defines distributed allocation and assignment of the IPv4 address
  space.

  As the Internet transitions to IPv6, the plan for distributed
  allocation and assignment of the IPv4 address space established in
  RFC1466 forms a base for the distributed allocation and assignment of
  the IPv6 address space.

  The Internet Assigned Number Authority (IANA) is the principal
  registry for the IPv6 address space.  The IANA may allocate blocks of
  IPv6 addresses and delegate the assignment of those blocks to
  qualified Regional Registries.  The IANA will serve as the default
  registry in cases where no delegated registration authority has been
  identified.

  The Registry ID of the IPv6 provider-based unicast address format is
  intended to facilitate a broad geographic address allocation and
  facilitate the operations of the distributed Regional Registries.

  The Registry ID immediately follows the format prefix part of an IPv6
  address.





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  At present there are three Regional Registries: INTERNIC, RIPE NCC,
  and APNIC.  In addition, address allocation could be done directly by
  the IANA.  Corresponding to this division of address allocation, this
  document defines the following Registry IDs:

       Regional Registry                     Value (binary)
       --------------------                  --------------

       Multi-Regional (IANA)                 10000
       RIPE NCC                              01000
       INTERNIC                              11000
       APNIC                                 00100

  All other values of the Registry ID are reserved by the IANA.

  Use of the Multi-regional Registry ID permits flexibility in address
  assignments which are outside of the geographical regions already
  allocated.  The IANA will be responsible for managing address space
  registration under the Multi-Regional Registry ID.

  It is expected that the IANA, and any designated Regional Registries,
  allocate addresses in conformance with this overall scheme.  Where
  there are qualifying Regional Registries established, primary
  responsibility for allocation from within that block will be
  delegated to that registry.

  A Regional Registry may have more than one block of addresses
  allocated to it (as a result the Registry would have multiple
  Registry IDs associated with it).

3.3 Provider ID and Subscriber ID

  This document leaves the organization of the Provider ID and
  Subscriber ID portions of address up to individual registries.
  Particularly the registry needs to define how much address space is
  given to providers and their subscribers.  There are several issues
  which must be addressed when doing this.  These include:

     o There will likely be a mixture of providers of different sizes.
     o Small providers will grow to become large providers.
     o Large providers will lose customers and become small providers.
       In extreme cases, the registry will require them to return some
       of their address space to the registry.
     o Organizations which need to be multi-homed to more than one
       provider will request a Provider ID assignment.

  It is important that a registry design its Provider ID space to allow
  flexibility and at the same time use the address space efficiently.



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3.3.1 Provider ID

  The value of the Provider ID associated with an address block a
  registry allocates to a particular provider uniquely identifies this
  provider within the registry.

  This document assumes that some subscribers may decide to acquire
  their address space directly from a registry, thus making their
  addresses independent of the provider(s) they are directly attached.

3.3.2 Subscriber ID

  The structure and assignment strategy of Subscriber ID's is specified
  by each provider.

  A (direct) provider may decide to group its subscribers into regions.
  This grouping may be useful when the (direct) provider is attached to
  another (indirect) provider at multiple points, as it allows the
  direct provider to exert a certain degree of control over the
  coupling between the attachment points and flow of the traffic
  destined to a particular subscriber (see Section 5.3.1 of [ALLOC]).

  To accommodate such a grouping the (direct) provider may allocate
  some small number of high-order bits of the Subscriber ID as a
  Subscriber-Region ID.  The purpose of a Subscriber-Region ID is to
  identify a group of subscribers that are within a close topological
  proximity to each other (from the provider's point of view), and thus
  could be reachable through a particular attachment point between the
  (direct) provider and other (indirect) provider(s).

3.4 Intra-Subscriber Part

  This document leaves the organization of Intra-Subscriber portion of
  the address up to individual subscribers.

  The provider-based unicast address format described in this document
  leaves 64 bits for the local portion of the address.  The editors of
  this document recommend that subscribers use IPv6 auto-configuration
  capabilities [AUTO] to generate addresses using link-specific
  addresses as Interface ID such as 48 bit IEEE-802 MAC addresses.  In
  this case 16 bits are left for the Subnet ID.  This should sufficient
  (e.g., 65,535 subnets) for all but the largest of subscribers.  This
  is shown as follows:

     |            64 bits             |  16 bits  |     48 bits      |
     +--------------------------------+-----------+------------------+
     |       Subscriber Prefix        | Subnet ID |   Interface ID   |
     +--------------------------------+-----------+------------------+



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  Subscribers who need additional subnets (and who desire to continue
  to use 48 bit IEEE-802 MAC addresses for Interface ID's) can be
  accommodated by having the provider assign them a block of subscriber
  prefixes.  Alternatively, an extremely large subscriber could be
  assigned its own Provider ID which would give it additional bits of
  address space to create its own local address hierarchy.

4.0 National Registries

  A Regional Registry may allocate blocks of address space to several
  National Registries.  The National Registry then becomes the entity
  that allocates address space to individual providers within the
  country served by the National Registry.

  To create National Registries the Regional Registry may add a layer
  of hierarchy in the Provider ID field to create National Registries.
  The resulting Provider Prefix is as follows:

  | 3 |  5 bits  |  n bits  |  m bits  |   56-n-m   |    64 bits     |
  +---+----------+----------+----------+------------+----------------+
  |010|RegistryID| National | Provider | Subscriber |Intra-Subscriber|
  |   |          |RegistryID|   ID     |     ID     |                |
  +---+----------+----------+----------+------------+----------------+

  This document assumes that within each regional registry there will
  be a relatively small number of national registries.  The size of the
  National-Registry ID should be related to the number of countries in
  the region administrated by the regional registry and the number of
  providers expected to be in each country.

5.0 Acknowledgments

  The editors would like to express our thanks to Jim Bound (Digital),
  Scott Bradner (Harvard), Brian Carpenter (CERN), Geoff Huston
  (AANET), and Tony Li (cisco) for their review and constructive
  comments.

6.0 References

  [ALLOC] Rekhter, Y., Li, T., "An Architecture for IPv6 Unicast
          Address Allocation", RFC 1887, December 1995.

  [ARCH]  Hinden, R., "IP Version 6 Addressing Architecture",
          RFC 1884, December 1995.

  [AUTO]  Thompson, S., "IPv6 Stateless Address Autoconfiguration",
          RFC 1972, August 1996.




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7.0 Security Considerations

  Security issues are not discussed in this memo.

8.0 Editors' Addresses

  Yakov Rekhter
  Cisco Systems, Inc.
  170 West Tasman Drive
  San Jose, CA 95134-1706
  USA
  Phone:  +1 914 528-0090
  EMail:  [email protected]

  Peter Lothberg
  STUPI.AB
  Box 9129
  S-102 72 Stockholm
  Sweden
  Phone:+46 8 6699720
  EMail: [email protected]

  Robert M. Hinden
  Ipsilon Networks, Inc.
  2191 E. Bayshore Road
  Palo Alto, CA 94303
  USA
  Phone: +1 415 846 4604
  EMail: [email protected]

  Stephen E. Deering
  Xerox Palo Alto Research Center
  3333 Coyote Hill Road
  Palo Alto, CA 94304
  USA
  Phone: +1 415 812 4839
  Fax:   +1 415 812 4471
  EMail: [email protected]

  Jon Postel
  Information Sciences Institute
  4676 Admiralty Way
  Marina del Rey, CA 90292-6695
  USA
  Phone: +1 310 822 1511
  Fax:   +1 310 823 6714
  EMail: [email protected]




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