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INTERNET-DRAFT                                        Y. Rekhter, Cisco
February 22, 1996                                 P. Lothberg, STUPI.AB
                                            R. Hinden, Ipsilon Networks
                                                 S. Deering, Xerox PARC
                                                         J. Postel, ISI
                                                                Editors




             An IPv6 Provider-Based Unicast Address Format

              <draft-ietf-ipngwg-unicast-addr-fmt-03.txt>



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.

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Please check the 1id-abstracts.txt listing contained in the internet-
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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.






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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.

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.




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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.


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.



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

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



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     provider will request a Provider ID assignment.

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


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 providers 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 recommended that subscribers use IPv6 auto-configuration
capabilities [AUTO] to generate addresses using 48 bit IEEE-802 MAC
addresses as Interface IDs.  In this case 16 bits is left for the Subnet



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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   |
   +--------------------------------+-----------+------------------+

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 a follows:



        | 3 |  5 bits  |       n bits        |    m bits   |
        +---+----------+---------------------+-------------+
        |010|RegistryID|National-Registry ID | Provider 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 providers
expected to be in each country.












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6.0 Acknowledgments

The editors would like to express our thanks to Jim Bound (DEC), 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",
          Internet Draft.


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:  yakov@cisco.com

     Peter Lothberg
     STUPI.AB
     Box 9129
     S-102 72 Stockholm
     Sweden
     Phone:+46 8 6699720
     email: roll@Stupi.SE







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     Robert M. Hinden
     Ipsilon Networks, Inc.
     2191 E. Bayshore Road
     Palo Alto, CA 94303
     USA
     phone: +1 415 846 4604
     email: hinden@ipsilon.com

     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: deering@parc.xerox.com

     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: postel@isi.edu


























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