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Keywords: internet protocol, local communication
Network Working Group R. Hinden
Request for Comments: 4193 Nokia
Category: Standards Track B. Haberman
JHU-APL
October 2005
Unique Local IPv6 Unicast Addresses
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.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document defines an IPv6 unicast address format that is globally
unique and is intended for local communications, usually inside of a
site. These addresses are not expected to be routable on the global
Internet.
Table of Contents
1. Introduction ....................................................2
2. Acknowledgements ................................................3
3. Local IPv6 Unicast Addresses ....................................3
3.1. Format .....................................................3
3.1.1. Background ..........................................4
3.2. Global ID ..................................................4
3.2.1. Locally Assigned Global IDs .........................5
3.2.2. Sample Code for Pseudo-Random Global ID Algorithm ...5
3.2.3. Analysis of the Uniqueness of Global IDs ............6
3.3. Scope Definition ...........................................6
4. Operational Guidelines ..........................................7
4.1. Routing ....................................................7
4.2. Renumbering and Site Merging ...............................7
4.3. Site Border Router and Firewall Packet Filtering ...........8
4.4. DNS Issues .................................................8
4.5. Application and Higher Level Protocol Issues ...............9
4.6. Use of Local IPv6 Addresses for Local Communication ........9
4.7. Use of Local IPv6 Addresses with VPNs .....................10
Hinden & Haberman Standards Track [Page 1]
RFC 4193 Unique Local IPv6 Unicast Addresses October 2005
5. Global Routing Considerations ..................................11
5.1. From the Standpoint of the Internet .......................11
5.2. From the Standpoint of a Site .............................11
6. Advantages and Disadvantages ...................................12
6.1. Advantages ................................................12
6.2. Disadvantages .............................................13
7. Security Considerations ........................................13
8. IANA Considerations ............................................13
9. References .....................................................13
9.1. Normative References ......................................13
9.2. Informative References ....................................14
1. 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. 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 (with high probability of uniqueness).
- Well-known prefix to allow for easy filtering at site
boundaries.
- Allow sites to be combined or privately interconnected without
creating any address conflicts or requiring renumbering of
interfaces that use 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.
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.
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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 [RFC2119].
2. Acknowledgements
The underlying idea of creating Local IPv6 addresses described in
this document has been proposed a number of times by a variety of
people. The authors of this document 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, Geoff Huston, Pekka Savola, Christian Huitema, Tim
Chown, Steve Bellovin, Alex Zinin, Tony Hain, Bill Fenner, Sam
Hartman, and Elwyn Davies for their comments and suggestions on this
document.
3. Local IPv6 Unicast Addresses
3.1. Format
The Local IPv6 addresses are created using a pseudo-randomly
allocated global ID. They have the following format:
| 7 bits |1| 40 bits | 16 bits | 64 bits |
+--------+-+------------+-----------+----------------------------+
| Prefix |L| Global ID | Subnet ID | Interface ID |
+--------+-+------------+-----------+----------------------------+
Where:
Prefix FC00::/7 prefix to identify Local IPv6 unicast
addresses.
L Set to 1 if the prefix is locally assigned.
Set to 0 may be defined in the future. See
Section 3.2 for additional information.
Global ID 40-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 Interface ID 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] and the number of resulting /48 prefixes per person. A range
of prefix choices is shown in the following table:
Prefix Global ID Number of Prefixes % of IPv6
Length /48 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 the Global ID field does not require internal structure, and
there is no reason to be able to aggregate the prefixes.
The authors believe that a /7 prefix resulting in a 41-bit Global ID
space (including the L bit) 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 were to be
needed, then additional IPv6 address space could be allocated for
this purpose.
3.2. Global ID
The allocation of Global IDs is pseudo-random [RANDOM]. They MUST
NOT be assigned sequentially or with well-known numbers. This is 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 not designed to
aggregate.
This document defines a specific local method to allocate Global IDs,
indicated by setting the L bit to 1. Another method, indicated by
clearing the L bit, may be defined later. Apart from the allocation
method, all Local IPv6 addresses behave and are treated identically.
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The local assignments are self-generated and do not need any central
coordination or assignment, but have an extremely high probability of
being unique.
3.2.1. Locally Assigned Global IDs
Locally assigned Global IDs MUST be generated with a pseudo-random
algorithm consistent with [RANDOM]. Section 3.2.2 describes a
suggested algorithm. It is important that all sites generating
Global IDs use a functionally similar algorithm to ensure there is a
high probability of uniqueness.
The use of a pseudo-random algorithm to generate Global IDs in the
locally assigned prefix gives an assurance that any network numbered
using such a prefix is highly unlikely to have that address space
clash with any other network that has another locally assigned prefix
allocated to it. This is a particularly useful property when
considering a number of scenarios including networks that merge,
overlapping VPN address space, or hosts mobile between such networks.
3.2.2. Sample Code for Pseudo-Random Global ID Algorithm
The algorithm described below is intended to be used for 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
in order to create a key.
4) Compute an SHA-1 digest on the key as specified in [FIPS, SHA1];
the resulting value is 160 bits.
5) Use the least significant 40 bits as the Global ID.
6) Concatenate FC00::/7, the L bit set to 1, and the 40-bit Global
ID to create a Local IPv6 address prefix.
This algorithm will result in a Global ID that is reasonably unique
and can be used to create a locally assigned Local IPv6 address
prefix.
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3.2.3. 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 Global IDs are chosen randomly (and independently), it is
possible that separate networks have chosen the same Global ID. For
any given network, with one or more random Global IDs, that has
inter-connections to other such networks, having a total of N such
IDs, the probability that two or more of these IDs will collide can
be approximated using the formula:
P = 1 - exp(-N**2 / 2**(L+1))
where P is the probability of collision, N is the number of
interconnected Global IDs, and L is the length of the Global ID.
The following table shows the probability of a collision for a range
of connections using a 40-bit Global ID field.
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 of
inter-site communication using locally generated Global IDs.
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 see Section 4.1). Also, unlike site-locals, a
site may have more than one of these prefixes and use them at the
same time.
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4. Operational Guidelines
The guidelines in this section do not require any change to the
normal routing and forwarding functionality in an IPv6 host or
router. These are configuration and operational usage guidelines.
4.1. 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].
The default behavior of exterior routing protocol sessions between
administrative routing regions must be to ignore receipt of and not
advertise prefixes in the FC00::/7 block. A network operator may
specifically configure prefixes longer than FC00::/7 for inter-site
communication.
If BGP is being used at the site border with an ISP, the default BGP
configuration must filter out any Local IPv6 address prefixes, both
incoming and outgoing. It must be set both to keep any Local IPv6
address prefixes from being advertised outside of the site as well as
to keep these prefixes from being learned from another site. The
exception to this is if there are specific /48 or longer routes
created for one or more Local IPv6 prefixes.
For link-state IGPs, it is suggested that a site utilizing IPv6 local
address prefixes be contained within one IGP domain or area. By
containing an IPv6 local address prefix to a single link-state area
or domain, the distribution of prefixes can be controlled.
4.2. Renumbering and Site Merging
The use of Local IPv6 addresses in a site results in making
communication that uses these addresses independent of renumbering a
site's provider-based global addresses.
When merging multiple sites, the addresses created with these
prefixes are unlikely to need to be renumbered because all of the
addresses have a high probability of being unique. Routes for each
specific prefix would have to be configured to allow routing to work
correctly between the formerly separate sites.
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4.3. 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 routers be configured by default to keep any packets with Local
IPv6 addresses from leaking outside of the site and to keep any site
prefixes from being advertised outside of their site.
Site border routers and firewalls should be configured to 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 specific /48 or longer Local IPv6 prefixes. This
will ensure that packets with Local IPv6 destination addresses will
not be forwarded outside of the site via a default route. The
default behavior of these devices should be to install a "reject"
route for these prefixes. 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.
Routers that maintain peering arrangements between Autonomous Systems
throughout the Internet should obey the recommendations for site
border routers, unless configured otherwise.
4.4. DNS Issues
At the present time, AAAA and PTR records for locally assigned local
IPv6 addresses are not recommended to be installed in the global DNS.
For background on this recommendation, one of the concerns about
adding AAAA and PTR records to the global DNS for locally assigned
Local IPv6 addresses stems from the lack of complete assurance that
the prefixes are unique. There is a small possibility that the same
locally assigned IPv6 Local addresses will be used by two different
organizations both claiming to be authoritative with different
contents. In this scenario, it is likely there will be a connection
attempt to the closest host with the corresponding locally assigned
IPv6 Local address. This may result in connection timeouts,
connection failures indicated by ICMP Destination Unreachable
messages, or successful connections to the wrong host. Due to this
concern, adding AAAA records for these addresses to the global DNS is
thought to be unwise.
Reverse (address-to-name) queries for locally assigned IPv6 Local
addresses MUST NOT be sent to name servers for the global DNS, due to
the load that such queries would create for the authoritative name
servers for the ip6.arpa zone. This form of query load is not
specific to locally assigned Local IPv6 addresses; any current form
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of local addressing creates additional load of this kind, due to
reverse queries leaking out of the site. However, since allowing
such queries to escape from the site serves no useful purpose, there
is no good reason to make the existing load problems worse.
The recommended way to avoid sending such queries to nameservers for
the global DNS is for recursive name server implementations to act as
if they were authoritative for an empty d.f.ip6.arpa zone and return
RCODE 3 for any such query. Implementations that choose this
strategy should allow it to be overridden, but returning an RCODE 3
response for such queries should be the default, both because this
will reduce the query load problem and also because, if the site
administrator has not set up the reverse tree corresponding to the
locally assigned IPv6 Local addresses in use, returning RCODE 3 is in
fact the correct answer.
4.5. 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 address
may not be reachable, but that is no different from other types of
IPv6 global unicast address. Applications need to be able to handle
multiple addresses that may or may not be reachable at any point in
time. In most cases, this complexity should be hidden in APIs.
From a host's perspective, the difference between Local IPv6 and
other types of global unicast addresses shows up as different
reachability and could be handled by default in 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.
4.6. Use of Local IPv6 Addresses for Local Communication
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). They are
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RFC 4193 Unique Local IPv6 Unicast Addresses October 2005
not created automatically in 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
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 a
naming system (e.g., DNS, proprietary naming system, etc.) For these
reasons, controlling their usage in a site is straightforward.
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.
4.7. 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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5. Global Routing Considerations
Section 4.1 provides operational guidelines that forbid default
routing of local addresses between sites. Concerns were raised to
the IPv6 working group and to the IETF as a whole that sites may
attempt to use local addresses as globally routed provider-
independent addresses. This section describes why using local
addresses as globally-routed provider-independent addresses is
unadvisable.
5.1. From the Standpoint of the Internet
There is a mismatch between the structure of IPv6 local addresses and
the normal IPv6 wide area routing model. The /48 prefix of an IPv6
local addresses fits nowhere in the normal hierarchy of IPv6 unicast
addresses. Normal IPv6 unicast addresses can be routed
hierarchically down to physical subnet (link) level and only have to
be flat-routed on the physical subnet. IPv6 local addresses would
have to be flat-routed even over the wide area Internet.
Thus, packets whose destination address is an IPv6 local address
could be routed over the wide area only if the corresponding /48
prefix were carried by the wide area routing protocol in use, such as
BGP. This contravenes the operational assumption that long prefixes
will be aggregated into many fewer short prefixes, to limit the table
size and convergence time of the routing protocol. If a network uses
both normal IPv6 addresses [ADDARCH] and IPv6 local addresses, these
types of addresses will certainly not aggregate with each other,
since they differ from the most significant bit onwards. Neither
will IPv6 local addresses aggregate with each other, due to their
random bit patterns. This means that there would be a very
significant operational penalty for attempting to use IPv6 local
address prefixes generically with currently known wide area routing
technology.
5.2. From the Standpoint of a Site
There are a number of design factors in IPv6 local addresses that
reduce the likelihood that IPv6 local addresses will be used as
arbitrary global unicast addresses. These include:
- The default rules to filter packets and routes make it very
difficult to use IPv6 local addresses for arbitrary use across
the Internet. For a site to use them as general purpose unicast
addresses, it would have to make sure that the default rules
were not being used by all other sites and intermediate ISPs
used for their current and future communication.
Hinden & Haberman Standards Track [Page 11]
RFC 4193 Unique Local IPv6 Unicast Addresses October 2005
- They are not mathematically guaranteed to be unique and are not
registered in public databases. Collisions, while highly
unlikely, are possible and a collision can compromise the
integrity of the communications. The lack of public
registration creates operational problems.
- The addresses are allocated randomly. If a site had multiple
prefixes that it wanted to be used globally, the cost of
advertising them would be very high because they could not be
aggregated.
- They have a long prefix (i.e., /48) so a single local address
prefix doesn't provide enough address space to be used
exclusively by the largest organizations.
6. Advantages and Disadvantages
6.1. 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 that uses 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.
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6.2. 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.
7. 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.
8. IANA Considerations
The IANA has assigned the FC00::/7 prefix to "Unique Local Unicast".
9. References
9.1. Normative References
[ADDARCH] Hinden, R. and S. Deering, "Internet Protocol Version 6
(IPv6) Addressing Architecture", RFC 3513, April 2003.
[FIPS] "Federal Information Processing Standards Publication",
(FIPS PUB) 180-1, Secure Hash Standard, 17 April 1995.
[GLOBAL] Hinden, R., Deering, S., and E. Nordmark, "IPv6 Global
Unicast Address Format", RFC 3587, August 2003.
[ICMPV6] Conta, A. and S. Deering, "Internet Control Message
Protocol (ICMPv6) for the Internet Protocol Version 6
(IPv6) Specification", RFC 2463, December 1998.
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[IPV6] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[NTP] Mills, D., "Network Time Protocol (Version 3)
Specification, Implementation and Analysis", RFC 1305,
March 1992.
[RANDOM] Eastlake, D., 3rd, Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
June 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[SHA1] Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1
(SHA1)", RFC 3174, September 2001.
9.2. Informative References
[ADDAUTO] Thomson, S. and 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., Bound, J., Volz, B., Lemon, T., Perkins, C., and
M. Carney, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
[POPUL] Population Reference Bureau, "World Population Data Sheet
of the Population Reference Bureau 2002", August 2002.
[RTP] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
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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
Johns Hopkins University
Applied Physics Lab
11100 Johns Hopkins Road
Laurel, MD 20723
USA
Phone: +1 443 778 1319
EMail: brian@innovationslab.net
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Full Copyright Statement
Copyright (C) The Internet Society (2005).
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specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Hinden & Haberman Standards Track [Page 16]