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LinuxDig.com Request For Comments

RFC Number : 2073

Title : An IPv6 Provider-Based Unicast Address Format.






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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RFC 2073 IPv6 Provider-Based Unicast Address Format January 1997


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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RFC 2073 IPv6 Provider-Based Unicast Address Format January 1997


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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RFC 2073 IPv6 Provider-Based Unicast Address Format January 1997


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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RFC 2073 IPv6 Provider-Based Unicast Address Format January 1997


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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RFC 2073 IPv6 Provider-Based Unicast Address Format January 1997


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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RFC 2073 IPv6 Provider-Based Unicast Address Format January 1997


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

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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