INTERNET-DRAFT R. Hinden, Nokia
January 20, 2004 Brian Haberman, Caspian
Unique Local IPv6 Unicast Addresses
<draft-ietf-ipv6-unique-local-addr-02.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. 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 months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
To view the list Internet-Draft Shadow Directories, see
http://www.ietf.org/shadow.html.
This internet draft expires on July 25, 2004.
Abstract
This document defines an unicast address format that is globally
unique and is intended for local communications, usually inside of a
site. They are not expected to be routable on the global Internet
given current routing technology.
Table of Contents
1.0 Introduction....................................................2
2.0 Acknowledgments.................................................3
3.0 Local IPv6 Unicast Addresses....................................3
3.1 Format..........................................................3
3.1.1 Background....................................................4
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3.2 Global ID.......................................................4
3.2.1 Centrally Assigned Global IDs.................................5
3.2.2 Locally Assigned Global IDs...................................6
3.2.3 Sample Code for Pseudo-Random Global ID Algorithm.............7
3.2.4 Analysis of the Uniqueness of Global IDs......................7
3.3 Scope Definition................................................8
4.0 Routing.........................................................8
5.0 Renumbering and Site Merging....................................9
6.0 Site Border Router and Firewall Packet Filtering................9
7.0 DNS Issues......................................................9
8.0 Application and Higher Level Protocol Issues...................10
9.0 Use of Local IPv6 Addresses for Local Communications...........10
10.0 Use of Local IPv6 Addresses with VPNs.........................11
11.0 Advantages and Disadvantages..................................12
12.0 Security Considerations.......................................12
13.0 IANA Considerations...........................................13
14.0 References....................................................13
14.1 Normative References..........................................13
14.2 Informative References........................................14
15.0 Authors' Addresses............................................14
16.0 Change Log....................................................15
1.0 Introduction
This document defines an IPv6 unicast address format that is globally
unique and is intended for local communications [IPV6]. These
addresses are called Unique Local IPv6 Unicast Addresses and are
abbreviated in this document as Local IPv6 addresses. They are not
expected to be routable on the global Internet given current routing
technology. They are routable inside of a more limited area such as
a site. They may also be routed between a limited set of sites.
Local IPv6 unicast addresses have the following characteristics:
- Globally unique prefix.
- Well known prefix to allow for easy filtering at site
boundaries.
- Allows sites to be combined or privately interconnected without
creating any address conflicts or require renumbering of
interfaces using these prefixes.
- Internet Service Provider independent and can be used for
communications inside of a site without having any permanent or
intermittent Internet connectivity.
- If accidentally leaked outside of a site via routing or DNS,
there is no conflict with any other addresses.
- In practice, applications may treat these addresses like global
scoped addresses.
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This document defines the format of Local IPv6 addresses, how to
allocate them, and usage considerations including routing, site
border routers, DNS, application support, VPN usage, and guidelines
for how to use for local communication inside a site.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC 2119].
2.0 Acknowledgments
The underlying idea of creating Local IPv6 addresses described in
this document been proposed a number of times by a variety of people.
The authors of this draft do not claim exclusive credit. Credit goes
to Brian Carpenter, Christian Huitema, Aidan Williams, Andrew White,
Charlie Perkins, and many others. The authors would also like to
thank Brian Carpenter, Charlie Perkins, Harald Alvestrand, Keith
Moore, Margaret Wasserman, Shannon Behrens, Alan Beard, Hans Kruse,
and Geoff Huston for their comments and suggestions on this document.
3.0 Local IPv6 Unicast Addresses
3.1 Format
The Local IPv6 addresses are created using a centrally allocated
global ID. They have the following format:
| 7 bits | 41 bits | 16 bits | 64 bits |
+--------+------------+-----------+-----------------------------+
| prefix | global ID | subnet ID | interface ID |
+--------+------------+-----------+-----------------------------+
Where:
prefix FC00::/7 prefix to identify Local IPv6 unicast
addresses.
global ID 41-bit global identifier used to create a
globally unique prefix. See section 3.2 for
additional information.
subnet ID 16-bit subnet ID is an identifier of a subnet
within the site.
interface ID 64-bit IID as defined in [ADDARCH].
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3.1.1 Background
There were a range of choices available when choosing the size of the
prefix and Global ID field length. There is a direct tradeoff
between having a Global ID field large enough to support foreseeable
future growth and not using too much of the IPv6 address space
needlessly. A reasonable way of evaluating a specific field length
is to compare it to a projected 2050 world population of 9.3 billion
[POPUL] to compare the number of resulting /48 prefixes per person.
A range of prefix choices is shown in the following table:
Prefix Global ID Number /48 Prefixes % of IPv6
Length Prefixes per Person Address Space
/11 37 137,438,953,472 15 0.049%
/10 38 274,877,906,944 30 0.098%
/9 39 549,755,813,888 59 0.195%
/8 40 1,099,511,627,776 118 0.391%
/7 41 2,199,023,255,552 236 0.781%
/6 42 4,398,046,511,104 473 1.563%
A very high utilization ratio of these allocations can be assumed
because no internal structure is required in the field nor is there
any reason to be able to aggregate the prefixes.
The authors believes that a /7 prefix resulting in a 41 bit Global ID
is a good choice. It provides for a large number of assignments
(i.e., 2.2 trillion) and at the same time uses less than .8% of the
total IPv6 address space. It is unlikely that this space will be
exhausted. If more than this was needed, then additional IPv6
address space could be allocated for this purpose.
3.2 Global ID
The allocation of global IDs should be pseudo-random [RANDOM]. They
should not be assigned sequentially or with well known numbers. This
to ensure that there is not any relationship between allocations and
to help clarify that these prefixes are not intended to be routed
globally. Specifically, these prefixes are designed to not
aggregate.
There are two ways to allocate Global IDs. These are centrally by a
allocation authority and locally by the site. The Global ID is split
into two parts for each type of allocation. The prefixes for each
type are:
FC00::/8 Centrally assigned
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FD00::/8 Locally assigned
Each results in a 40-bit space to allocate.
Two assignment methods are included because they have different
properties. The centrally assigned global IDs are uniquely assigned
and the assignments can be escrowed to resolve any disputes regarding
duplicate assignments. The local assignments are free and do not
need any central coordination or assignment, but have a lower (but
still adequate) probability of being unique. It is expected that
large managed sites will prefer central assignments and small or
disconnected sites will prefer local assignments. It is recommended
that sites planning to use Local IPv6 addresses for extensive inter-
site communication, initially or as a future possibility, use a
centrally assigned prefix as there is no possibility of assignment
conflicts. Sites are free to choose either approach.
3.2.1 Centrally Assigned Global IDs
Centrally assigned global IDs MUST be generated with a pseudo-random
algorithm consistent with [RANDOM]. They should not be assigned
sequentially or by locality. This is to ensure that there is no
relationship between allocations and to help clarify that these
prefixes are not intended to be routed globally by eliminating the
possibility of aggregation. Specifically, these prefixes are
designed to not aggregate.
Global IDs should be assigned centrally by a single allocation
authority because they are pseudo-random and without any structure.
This is easiest to accomplish if there is a single source of the
assignments.
The requirements for centrally assigned global ID allocations are:
- Available to anyone in an unbiased manner.
- Permanent with no periodic fees.
- One time non-refundable allocation fee per allocation that is
affordable by a broad spectrum of end users when considered
globally.
- Provide mechanisms that prevent hoarding of these allocations.
- The ownership of each individual allocation should be private,
but should be escrowed.
The allocation authority should permit allocations to be obtained
without having any sort of Internet connectivity. For example in
addition to web based registration they should support some methods
like telephone, postal mail, fax, telex, etc. They should also
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accept a variety of payment methods and currencies.
The reason for the use of an allocation fee for each prefix is to
allow this service function to operate on a cost recovery basis. An
acknowledged side-effect of such a mechanism is that it provides some
impediment to any hoarding of the these allocations. The
consideration of affordability is intended to keep the cost low
enough so as not create a barrier to anyone needing one. The charge
is one time to eliminate the need for an ongoing relationship with
the allocation authority, while acknowledging the service costs for
both the registration function and the enduring escrow service. The
charge is non-refundable in order to keep overhead low.
The ownership of the allocations is not needed to be public since the
resulting addresses are intended to be used for local communication.
It is escrowed to insure there are no duplicate allocations and in
case it is needed in the future (e.g., to resolve duplicate
allocation disputes, or to support a change of the central allocation
authority).
Note, there are many possible ways of of creating an allocation
authority. It is important to keep in mind when reviewing
alternatives that the goal is to pick one that can do the job. It
doesn't have to be perfect, only good enough to do the job at hand.
This document directs the IANA, in section 13.0, to delegate the
FC00::/8 prefix to an allocation authority to allocate centrally
assigned /48 prefixes consistent with the requirements defined in
this section.
3.2.2 Locally Assigned Global IDs
Global IDs can also be generated locally by an individual site. This
makes it easy to get a prefix with out the need to contact an
assignment authority or internet service provider. There is not as
high a degree of assurance that the prefix will not conflict with
another locally generated prefix, but the likelihood of conflict is
small. Sites that are not comfortable with this degree of
uncertainty should use a centrally assigned global ID.
Locally assigned global IDs MUST be generated with a pseudo-random
algorithm consistent with [RANDOM]. Section 3.2.3 describes a
suggested algorithm. It is important to insure a reasonable
likelihood uniqueness that all sites generating a Global IDs use a
functionally similar algorithm.
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3.2.3 Sample Code for Pseudo-Random Global ID Algorithm
The algorithm described below is intended to be used for centrally
and locally assigned Global IDs. In each case the resulting global
ID will be used in the appropriate prefix as defined in section 3.2.
1) Obtain the current time of day in 64-bit NTP format [NTP].
2) Obtain an EUI-64 identifier from the system running this
algorithm. If an EUI-64 does not exist, one can be created from
a 48-bit MAC address as specified in [ADDARCH]. If an EUI-64
cannot be obtained or created, a suitably unique identifier,
local to the node, should be used (e.g. system serial number).
3) Concatenate the time of day with the system-specific identifier
creating a key.
4) Compute an MD5 digest on the key as specified in [MD5DIG].
5) Use the least significant 40 bits as the Global ID.
This algorithm will result in a global ID that is reasonably unique
and can be used as a Global ID.
3.2.4 Analysis of the Uniqueness of Global IDs
The selection of a pseudo random global ID is similar to the
selection of an SSRC identifier in RTP/RTCP defined in section 8.1 of
[RTP]. This analysis is adapted from that document.
Since the global ID is chosen randomly, it is possible that two or
more networks that have an inter-network connection using globally-
unique local addresses will chose the same global ID. The
probability of collision can be approximated based on the number of
inter-connections using globally-unique local addresses and the
length of the ID (40 bits). The formula
P = 1 - exp(-N**2 / 2**(L+1))
approximates the probability of collision (where N is the number
inter-connections and L is the length of the global ID).
The following table shows the probability of a collision for a range
of inter-connections using a 40 bit global ID field.
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Inter-connections Probability of Collision
2 1.81*10^-12
10 4.54*10^-11
100 4.54*10^-09
1000 4.54*10^-07
10000 4.54*10^-05
Based on this analysis the uniqueness of locally generated global IDs
is adequate for sites planning a small to moderate amount inter-site
communication using locally generated global IDs. Sites planning
more extensive inter-site communication should consider using the
centrally assigned global ID.
3.3 Scope Definition
By default, the scope of these addresses is global. That is, they
are not limited by ambiguity like the site-local addresses defined in
[ADDARCH]. Rather, these prefixes are globally unique, and as such,
their applicability is greater than site-local addresses. Their
limitation is in the routability of the prefixes, which is limited to
a site and any explicit routing agreements with other sites to
propagate them. Also, unlike site-locals, a site may have more than
one of these prefixes and use them at the same time.
4.0 Routing
Local IPv6 addresses are designed to be routed inside of a site in
the same manner as other types of unicast addresses. They can be
carried in any IPv6 routing protocol without any change.
It is expected that they would share the same subnet IDs with
provider based global unicast addresses if they were being used
concurrently [GLOBAL].
Any routing protocol that is used between sites MUST filter out any
incoming or outgoing Local IPv6 unicast routes. The exception to
this is if specific /48 IPv6 local unicast routes have been
configured to allow for inter-site communication.
If BGP is being used at the site border with an ISP, filters MUST be
installed by default in the BGP configuration to keep any Local IPv6
address prefixes from being advertised outside of the site or for
these prefixes to be learned from another site. The exception to
this is if there are specific /48 routes created for one or more
Local IPv6 prefixes.
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5.0 Renumbering and Site Merging
The use of Local IPv6 addresses in a site results in making
communication using these addresses independent of renumbering a
site's provider based global addresses.
When merging multiple sites none of the addresses created with these
prefixes need to be renumbered because all of the addresses are
unique. Routes for each specific prefix would have to be configured
to allow routing to work correctly between the formerly separate
sites.
6.0 Site Border Router and Firewall Packet Filtering
While no serious harm will be done if packets with these addresses
are sent outside of a site via a default route, it is recommended
that they be filtered to keep any packets with Local IPv6 destination
addresses from leaking outside of the site and to keep any site
prefixes from being advertised outside of their site.
Site border routers SHOULD install a black hole route for the Local
IPv6 prefix FC00::/7. This will insure that packets with Local IPv6
destination addresses will not be forwarded outside of the site via a
default route. Site border routers SHOULD respond with the
appropriate ICMPv6 Destination Unreachable message to inform the
source that the packet was not forwarded [ICMPV6]. This feedback is
important to avoid transport protocol timeouts.
Site border routers and firewalls SHOULD NOT forward any packets with
Local IPv6 source or destination addresses outside of the site unless
they have been explicitly configured with routing information about
other Local IPv6 prefixes. The default behavior of these devices
SHOULD be to filter them. Site border routers SHOULD respond with
the appropriate ICMPv6 Destination Unreachable message to inform the
source that the packet was not forwarded.
Additionally, domain border routers connecting autonomous systems
throughout the Internet SHOULD obey these recommendations for site
border routers.
7.0 DNS Issues
AAAA records for Local IPv6 addresses SHOULD NOT be installed in the
global DNS. They may be installed in a naming system local to the
site or kept separate from the global DNS using techniques such as
"two-faced" DNS.
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If Local IPv6 address are configured in the global DNS, no harm is
done because they are unique and will not create any confusion. They
may not be reachable, but this is a property that is common to all
types of global IPv6 unicast addresses.
For future study names with Local IPv6 addresses may be resolved
inside of the site using dynamic naming systems such as Multicast
DNS.
8.0 Application and Higher Level Protocol Issues
Application and other higher level protocols can treat Local IPv6
addresses in the same manner as other types of global unicast
addresses. No special handling is required. This type of addresses
may not be reachable, but that is no different from other types of
IPv6 global unicast addresses. Applications need to be able to
handle multiple addresses that may or may not be reachable any point
in time. In most cases this complexity should be hidden in APIs.
From a host's perspective this difference shows up as different
reachability than global unicast and could be handled by default that
way. In some cases it is better for nodes and applications to treat
them differently from global unicast addresses. A starting point
might be to give them preference over global unicast, but fall back
to global unicast if a particular destination is found to be
unreachable. Much of this behavior can be controlled by how they are
allocated to nodes and put into the DNS. However it is useful if a
host can have both types of addresses and use them appropriately.
Note that the address selection mechanisms of [ADDSEL], and in
particular the policy override mechanism replacing default address
selection, are expected to be used on a site where Local IPv6
addresses are configured.
9.0 Use of Local IPv6 Addresses for Local Communications
Local IPv6 addresses, like global scope unicast addresses, are only
assigned to nodes if their use has been enabled (via IPv6 address
autoconfiguration [ADDAUTO], DHCPv6 [DHCP6], or manually) and
configured in the DNS. They are not created automatically the way
that IPv6 link-local addresses are and will not appear or be used
unless they are purposely configured.
In order for hosts to autoconfigure Local IPv6 addresses, routers
have to be configured to advertise Local IPv6 /64 prefixes in router
advertisements, or a DHCPv6 server must have been configured to
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assign them. In order for a node to learn the Local IPv6 address of
another node, the Local IPv6 address must have been installed in the
DNS. For these reasons, it is straight forward to control their
usage in a site.
To limit the use of Local IPv6 addresses the following guidelines
apply:
- Nodes that are to only be reachable inside of a site: The local
DNS should be configured to only include the Local IPv6
addresses of these nodes. Nodes with only Local IPv6 addresses
must not be installed in the global DNS.
- Nodes that are to be limited to only communicate with other
nodes in the site: These nodes should be set to only
autoconfigure Local IPv6 addresses via [ADDAUTO] or to only
receive Local IPv6 addresses via [DHCP6]. Note: For the case
where both global and Local IPv6 prefixes are being advertised
on a subnet, this will require a switch in the devices to only
autoconfigure Local IPv6 addresses.
- Nodes that are to be reachable from inside of the site and from
outside of the site: The DNS should be configured to include
the global addresses of these nodes. The local DNS may be
configured to also include the Local IPv6 addresses of these
nodes.
- Nodes that can communicate with other nodes inside of the site
and outside of the site: These nodes should autoconfigure global
addresses via [ADDAUTO] or receive global address via [DHCP6].
They may also obtain Local IPv6 addresses via the same
mechanisms.
10.0 Use of Local IPv6 Addresses with VPNs
Local IPv6 addresses can be used for inter-site Virtual Private
Networks (VPN) if appropriate routes are set up. Because the
addresses are unique these VPNs will work reliably and without the
need for translation. They have the additional property that they
will continue to work if the individual sites are renumbered or
merged.
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11.0 Advantages and Disadvantages
11.1 Advantages
This approach has the following advantages:
- Provides Local IPv6 prefixes that can be used independently of
any provider based IPv6 unicast address allocations. This is
useful for sites not always connected to the Internet or sites
that wish to have a distinct prefix that can be used to localize
traffic inside of the site.
- Applications can treat these addresses in an identical manner as
any other type of global IPv6 unicast addresses.
- Sites can be merged without any renumbering of the Local IPv6
addresses.
- Sites can change their provider based IPv6 unicast address
without disrupting any communication using Local IPv6 addresses.
- Well known prefix that allows for easy filtering at site
boundary.
- Can be used for inter-site VPNs.
- If accidently leaked outside of a site via routing or DNS, there
is no conflict with any other addresses.
11.2 Disadvantages
This approach has the following disadvantages:
- Not possible to route Local IPv6 prefixes on the global Internet
with current routing technology. Consequentially, it is
necessary to have the default behavior of site border routers to
filter these addresses.
- There is a very low probability of non-unique locally assigned
global IDs being generated by the algorithm in section 3.2.3.
This risk can be ignored for all practical purposes, but it
leads to a theoretical risk of clashing address prefixes.
12.0 Security Considerations
Local IPv6 addresses do not provide any inherent security to the
nodes that use them. They may be used with filters at site
boundaries to keep Local IPv6 traffic inside of the site, but this is
no more or less secure than filtering any other type of global IPv6
unicast addresses.
Local IPv6 addresses do allow for address-based security mechanisms,
including IPSEC, across end to end VPN connections.
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13.0 IANA Considerations
The IANA is instructed to allocate the FC00::/7 prefix for Unique
Local IPv6 unicast addresses.
The IANA is instructed to delegate, within a reasonable time, the
prefix FC00::/8 to an allocation authority for Unique Local IPv6
Unicast prefixes of length /48. This allocation authority shall
comply with the requirements described in section 3.2 of this
document, including in particular the charging of a modest one-time
fee that is to be aligned with service costs associated with the
allocation function and enduring escrow of the allocation data.
14.0 References
14.1 Normative References
[ADDARCH] Hinden, R., S. Deering, S., "IP Version 6 Addressing
Architecture", RFC 3515, April 2003.
[GLOBAL] Hinden, R., S. Deering, E. Nordmark, "IPv6 Global Unicast
Address Format", RFC 3587, August 2003.
[ICMPV6] Conta, A., S. Deering, "Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6)
Specification", RFC1885, December 1998.
[IPV6] Deering, S., R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[MD5DIG] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
April 1992.
[NTP] Mills, David L., "Network Time Protocol (Version 3)
Specification, Implementation and Analysis", RFC 1305,
March 1992.
[POPUL] Population Reference Bureau, "World Population Data Sheet
of the Population Reference Bureau 2002", August 2002.
[RANDOM] Eastlake, D. 3rd, S. Crocker, J. Schiller, "Randomness
Recommendations for Security", RFC 1750, December 1994.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", RFC 2026, BCP00009, October 1996.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, BCP14, March 1997.
14.2 Informative References
[ADDAUTO] Thomson, S., T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[ADDSEL] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[DHCP6] Droms, R., et. al., "Dynamic Host Configuration Protocol
for IPv6 (DHCPv6)", RFC3315, July 2003.
[RTP] Schulzrinne, H., S. Casner, R. Frederick, V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications"
RFC1889, January 1996.
15.0 Authors' Addresses
Robert M. Hinden
Nokia
313 Fairchild Drive
Mountain View, CA 94043
USA
phone: +1 650 625-2004
email: bob.hinden@nokia.com
Brian Haberman
Caspian Networks
1 Park Drive, Suite 300
Research Triangle Park, NC 27709
USA
phone: +1-929-949-4828
email: brian@innovationslab.net
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16.0 Change Log
Draft <draft-ietf-ipv6-unique-local-addr-02.txt>
o Removed mention of 10 euro charge and changed text in section
3.2.1 and IANA considerations to restate the requirement for low
cost allocations and added specific requirement to prevent
hording.
o Added need to send ICMPv6 destination unreachable messages if
packets are filtered or dropped at site boundaries.
o Changed format section to list prefix sizes and definition, and
changed discussion of prefix sizes to new background section.
o Various editorial changes.
Draft <draft-ietf-ipv6-unique-local-addr-01.txt>
o Removed example of PIR as an example of an allocation authority
and clarified the text that the IANA should delegate the
centrally assigned prefix to an allocation authority.
o Changed sample code for generating pseudo random Global IDs to
not require any human input. Changes from using birthday to
unique token (e.g., EUI-64, serial number, etc.) available on
machine running the algorithm.
o Added a new section analyzing the uniqueness properties of the
pseudo random number algorithm.
o Added text to recommend that centrally assigned local addresses
be used for site planning extensive inter-site communication.
o Clarified that domain border routers should follow site border
router recommendations.
o Clarified that AAAA DNS records should not be installed in the
global DNS.
o Several editorial changes.
Draft <draft-ietf-ipv6-unique-local-addr-00.txt>
o Changed file name to become an IPv6 w.g. group document.
o Clarified language in Routing and Firewall sections.
o Several editorial changes.
Draft <draft-hinden-ipv6-global-local-addr-02.txt>
o Changed title and name of addresses defined in this document to
"Unique Local IPv6 Unicast Addresses" with abbreviation of
"Local IPv6 addresses".
o Several editorial changes.
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Draft <draft-hinden-ipv6-global-local-addr-01.txt>
o Added section on scope definition and updated application
requirement section.
o Clarified that, by default, the scope of these addresses is
global.
o Renumbered sections and general text improvements
o Removed reserved global ID values
o Added pseudo code for local allocation submitted by Brian
Haberman and added him as an author.
o Split Global ID values into centrally assigned and local
assignments and added text to describe local assignments
Draft <draft-hinden-ipv6-global-local-addr-00.txt>
o Initial Draft
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