Internet Engineering Task Force S. Jiang
Internet-Draft BUPT
Intended status: Informational B. Liu
Expires: 28 December 2023 Huawei Technologies
N. Buraglio
ForwardingPlane, LLC
26 June 2023
Considerations For Using Unique Local Addresses
draft-ietf-v6ops-ula-usage-considerations-03
Abstract
This document provides considerations for using IPv6 Unique Local
Addresses (ULAs). Based on an analysis of different ULA usage
scenarios, this document identifies use cases where ULA addresses are
helpful as well as potential problems caused by using them.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 28 December 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 3
3. General Considerations For Using ULAs . . . . . . . . . . . . 3
3.1. Do Not Treat ULA Equal to RFC1918 . . . . . . . . . . . . 3
3.2. Using ULAs in a Limited Scope . . . . . . . . . . . . . . 4
4. Analysis and Operational Considerations for Scenarios Using
ULAs . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. ULA-only in Isolated Networks . . . . . . . . . . . . . . 4
4.2. ULA+PA in Connected Networks . . . . . . . . . . . . . . 5
4.3. ULA-Only in Connected Networks . . . . . . . . . . . . . 7
4.4. Some Specific Use Cases . . . . . . . . . . . . . . . . . 8
4.4.1. Special Routing . . . . . . . . . . . . . . . . . . . 8
4.4.2. Used as Identifier . . . . . . . . . . . . . . . . . 8
4.5. IPv4 Co-existence Considerations . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
Unique Local Addresses (ULA) is defined in [RFC4193], and it is an
alternative to site-local address (deprecated in [RFC3879]). ULAs
have the following features:
- Automatically Generated
ULA prefixes can be automatically generated using the algorithms
described in [RFC4193]. This feature allows automatic prefix
allocation. Thus one can get a network working immediately
without applying for prefix(es) from an RIR/LIR (Regional Internet
Registry/Local Internet Registry).
- Globally Unique
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ULAs are defined as a global scope address space. However, they
are not intended to be used globally on the public Internet; in
contrast, they are mostly used locally, for example, in isolated
networks, internal networks, or VPNs.
ULAs are intended to have an extremely low probability of
collision. The randomization of 40 bits in a ULA prefix is
considered sufficient enough to ensure a high degree of uniqueness
(refer to [RFC4193] Section 3.2.3 for details) and simplifies
merging of networks by avoiding the need to renumber overlapping
IP address space.
- Provider Independent Address Space
ULAs can be used for internal communications even without Internet
connectivity. They need no registration, so they can support on-
demand usage and do not carry any RIR/LIR burden of documentation
or fees.
- Well Known Prefix
The prefixes of ULAs are well known thus they are easily
identified and filtered.
This document aims to introduce the usage of ULAs in various
scenarios, provide some operational considerations, and clarify the
advantages and disadvantages of the usage in each scenario. Thus,
the administrators could choose to use ULAs in a certain way that
considered benificial for them.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. General Considerations For Using ULAs
3.1. Do Not Treat ULA Equal to RFC1918
ULA and [RFC1918] are similar in some aspects. The most obvious one
is as described in Section 3.1.3 that ULA provides an internal
address independence capability in IPv6 that is similar to how
[RFC1918] is commonly used. ULA allows administrators to configure
the internal network of each platform the same way it is configured
in IPv4. Many organizations have security policies and architectures
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based around the local-only routing of [RFC1918] addresses and those
policies may directly map to ULA [RFC4864].
But this does not mean that ULA is equal to an IPv6 version of
[RFC1918] deployment. [RFC1918] usually combines with NAT/NAPT for
global connectivity. But it is not necessary to combine ULAs with
any kind of NAT. Operators can use ULA for local communications
along with global addresses for global communications (see
Section 4.2). This is a big advantage brought by default support of
multiple-addresses-per-interface feature in IPv6. (People may still
have a requirement for NAT with ULA, this is discussed in
Section 4.3. But people also need to keep in mind that ULA is not
intentionally designed for this kind of use case.)
Another important difference is the ability to merge two ULA networks
without renumbering (because of the uniqueness), which is a big
advantage over [RFC1918].
3.2. Using ULAs in a Limited Scope
A ULA is by definition a prefix that is never advertised outside a
given domain, and is used within that domain by agreement of those
networked by the domain.
So when using ULAs in a network, the administrators need to clearly
set the scope of the ULAs and configure ACLs on relevant border
routers to block them out of the scope. And if internal DNS is
enabled, the administrators might also need to use internal-only DNS
names for ULAs and might need to split the DNS so that the internal
DNS server includes records that are not presented in the external
DNS server.
4. Analysis and Operational Considerations for Scenarios Using ULAs
4.1. ULA-only in Isolated Networks
IP is used ubiquitously. Some networks like industrial control bus
(e.g. [RS-485], [SCADA], or even non-networked digital interfaces
like [MIL-STD-1397] have begun to use IP. In these kinds of
networks, the system may lack the ability to communicate with the
public networks.
As another example, there may be some networks in which the equipment
has the technical capability to connect to the Internet, but is
prohibited by administration. These networks may include data center
networks, separate financial networks, lab networks. machine-to-
machine (e.g. vehicle networks), sensor networks, or even normal
LANs, and can include very large numbers of addresses.
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ULA is a straightforward way to assign the IP addresses in the kinds
of networks just described, with minimal administrative cost or
burden. Also, ULAs fit in multiple subnet scenarios, in which each
subnet has its own ULA prefix. For example, when assigning vehicles
with ULAs, it is then possible to separate in-vehicle embedded
networks into different subnets depending on real-time situation.
However, each isolated network has the possibility to be connected in
the future. Administrators need to consider the following before
deciding whether to use ULAs:
* If the network eventually connects to another isolated or private
network, the potential for address collision arises. However, if
the ULAs were generated in the standard way, this will not be a
big problem.
* If the network eventually connects to the global Internet, then
the operator will need to add a new global prefix and ensure that
the address selection policy is properly set up on all interfaces.
Operational considerations:
* Prefix generation: randomly generated according to the algorithms
defined in [RFC4193] or manually assigned. Normally, automatic
generation of the prefixes is recommended, following [RFC4193].
If there are some specific reasons that call for manual
assignment, administrators have to plan the prefixes carefully to
avoid collision.
* Prefix announcement: in some cases, networks might need to
announce prefixes to each other. For example, in vehicle networks
with infrastructure-less settings such as Vehicle-to-Vehicle (V2V)
communication, prior knowledge of the respective prefixes is
unlikely. Hence, a prefix announcement mechanism is needed to
enable inter-vehicle communications based on IP. As one
possibility, such announcements could rely on extensions to the
Router Advertisement message of the Neighbor Discovery Protocol
[RFC4861].
4.2. ULA+PA in Connected Networks
Two classes of network might need to use ULA with PA (Provider
Aggregated) addresses:
* Home network. Home networks are normally assigned with one or
more globally routed PA prefixes to connect to the uplink of an
ISP. In addition, they may need internal routed networking even
when the ISP link is down. Then ULA is a proper tool to fit the
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requirement. [RFC7084] requires the CPE to support ULA. Note:
ULAs provide more benefit for multiple-segment home networks; for
home networks containing only one segment, link-local addresses
are better alternatives.
* Enterprise network. An enterprise network is usually a managed
network with one or more PA prefixes or with a PI prefix, all of
which are globally routed. The ULA can be used to improve
internal connectivity and make it more resilient, or to isolate
certain functions like OAM for servers.
Benefits of Using ULAs in this scenario:
* Separated local communication plane: for either home networks or
enterprise networks, the main purpose of using ULAs along with PA
addresses is to provide a logically local routing plane separated
from the global routing plane. The benefit is to ensure stable
and specific local communication regardless of the ISP uplink
failure. This benefit is especially meaningful for the home
network or for private OAM function in an enterprise.
* Renumbering: in some special cases such as renumbering, enterprise
administrators may want to avoid the need to renumber their
internal-only, private nodes when they have to renumber the PA
addresses of the rest of the network because they are changing
ISPs, because the ISP has restructured its address allocations, or
for some other reason. In these situations, ULA is an effective
tool for addressing internal-only nodes. Even public nodes can
benefit from ULA for renumbering, on their internal interfaces.
When renumbering, as [RFC4192] suggests, old prefixes continue to
be valid until the new prefix(es) is(are) stable. In the process
of adding new prefix(es) and deprecating old prefix(es), it is not
easy to keep local communication disentangled from global routing
plane change. If we use ULAs for local communication, the
separated local routing plane can isolate the effects of global
routing change.
Drawbacks:
* Operational Complexity: there are some arguments that in practice
the use of ULA+PA creates additional operational complexity. This
is not a ULA-specific problem; the multiple-addresses-per-
interface is an important feature of IPv6 protocol. Nevertheless,
running multiple prefixes needs more operational consideration
than running a single one.
Operational considerations:
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* Default Routing: connectivity may be broken if ULAs are used as
default route. When using RIO (Route Information Option) in
[RFC4191], specific routes can be added without a default route,
thus avoiding bad user experience due to timeouts on ICMPv6
redirects. This behavior was well documented in [RFC7084] as rule
ULA-5 "An IPv6 CE router MUST NOT advertise itself as a default
router with a Router Lifetime greater than zero whenever all of
its configured and delegated prefixes are ULA prefixes." and along
with rule L-3 "An IPv6 CE router MUST advertise itself as a router
for the delegated prefix(es) (and ULA prefix if configured to
provide ULA addressing) using the "Route Information Option"
specified in Section 2.3 of [RFC4191]. This advertisement is
independent of having or not having IPv6 connectivity on the WAN
interface.". However, it needs to be noticed that current OSes
don't all support [RFC4191].
* SLAAC/DHCPv6 co-existing: Since SLAAC and DHCPv6 might be enabled
in one network simultaneously; the administrators need to
carefully plan how to assign ULA and PA prefixes in accordance
with the two mechanisms. The administrators need to know the
current issue of the SLAAC/DHCPv6 interaction.
* Address selection: As mentioned in [RFC5220], there is a
possibility that the longest matching rule will not be able to
choose the correct address between ULAs and global unicast
addresses for correct intra-site and extra-site communication.
[RFC6724] claims that a site-specific policy entry can be used to
cause ULAs within a site to be preferred over global addresses.
* DNS relevant: if administrators choose not to do reverse DNS
delegation inside of their local control of ULA prefixes, a
significant amount of information about the ULA population may
leak to the outside world. Because reverse queries will be made
and naturally routed to the global reverse tree, so external
parties will be exposed to the existence of a population of ULA
addresses. [ULA-IN-WILD] provides more detailed situations on
this issue. Administrators may need a split DNS to separate the
queries from internal and external for ULA entries and GUA
entries.
4.3. ULA-Only in Connected Networks
In theory, a site numbered with ULAs only can get connected via a
NPTv6[RFC6296] (which is an experimental specification that provides
a stateless one-to-one mapping between internal addresses and
external addresses) or application-layer proxy. This approach could
get provider independent addresses or get connected from the isolated
stage without applying to any RIRs/LIRs. This might make small
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organizations saving time and address fee.
However, this approach breaks the end-to-end transparence. People
have suffered from the NAT/Proxy middle boxes so much in the IPv4
ear, there is no reason to continue the suffering when IPv6 is
available. This document does not consider ULA+NPTv6/Proxy as a good
choice for normal cases. Rather, this document considers ULA+PA
(Provider Aggregated) as a better approach to connect to the global
network when ULAs are expected to be retained.
4.4. Some Specific Use Cases
Along with the general scenarios, this section provides some specific
use cases that could benefit from using ULA.
4.4.1. Special Routing
For various reasons the administrators may want to have private
routing be controlled and separated from other routing. For example,
in the business-to-business case, two companies might want to use
direct connectivity that only connects stated machines, such as a
silicon foundry with client engineers that use it. A ULA provides a
simple way to assign prefixes that would be used in accordance with
an agreement between the parties.
4.4.2. Used as Identifier
ULAs could be self-generated and easily grabbed from the standard
IPv6 stack. And ULAs don't need to be changed as the GUA prefixes
do. So they are very suitable to be used as identifiers by the up
layer applications. And since ULA is not intended to be globally
routed, it is not harmful to the routing system.
Such kind of benefit has been utilized in real implementations. For
example, in [RFC6281], the protocol BTMM (Back To My Mac) needs to
assign a topology-independent identifier to each client host
according to the following considerations:
* TCP connections between two end hosts wish to survive in network
changes.
* Sometimes one needs a constant identifier to be associated with a
key so that the Security Association can survive the location
changes.
It needs to be noticed again that in theory ULA has the possibility
of collision. However, the probability is desirably small enough and
can be ignored in most cases when ULAs are used as identifiers.
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4.5. IPv4 Co-existence Considerations
Generally, this document does not consider IPv4 to be in scope. But
regarding ULA, there is a special case needs to be recognized, which
is described in Section 3.2.2 of [RFC5220]. When an enterprise has
IPv4 Internet connectivity but does not yet have IPv6 Internet
connectivity, and the enterprise wants to provide site-local IPv6
connectivity, a ULA is the best choice for site-local IPv6
connectivity. Each employee host will have both an IPv4 global or
private address and a ULA. Here, when this host tries to connect to
an outside node that has registered both A and AAAA records in the
DNS, the host will choose AAAA as the destination address and the ULA
for the source address according to the IPv6 preference of the
default policy table defined in the old address selection standard
[RFC3484]. This will clearly result in a connection failure. The
new address selection standard [RFC6724] has corrected this behavior
by preferring IPv4 than ULAs in the default policy table. However,
there are still lots of hosts using the old standard [RFC3484], thus
this could be an issue in real networks.
Happy Eyeballs [RFC8305] solves this connection failure problem, but
unwanted timeouts will obviously lower the user experience. One
possible approach to eliminating the timeouts is to deprecate the
IPv6 default route and simply configure a scoped route on hosts (in
the context of this document, only configure the ULA prefix routes).
Another alternative is to configure IPv4 preference on the hosts, and
not include DNS A records but only AAAA records for the internal
nodes in the internal DNS server. Then outside nodes have both A and
AAAA records and can be connected through IPv4 as default and
internal nodes can always connect through IPv6. But since IPv6
preference is default, changing the default in all nodes is not
suitable at scale.
5. Security Considerations
Security considerations regarding ULAs, in general, please refer to
the ULA specification [RFC4193]. Also refer to [RFC4864], which
shows how ULAs help with local network protection.
As mentioned in Section 4.2, when using NPTv6, the administrators
need to know where the firewall is located to set proper filtering
rules.
Also as mentioned in Section 4.2, if administrators choose not to do
reverse DNS delegation inside their local control of ULA prefixes, a
significant amount of information about the ULA population may leak
to the outside world.
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6. IANA Considerations
This memo has no actions for IANA.
7. Acknowledgements
Many valuable comments were received in the IETF v6ops WG mail list,
especially from Cameron Byrne, Fred Baker, Brian Carpenter, Lee
Howard, Victor Kuarsingh, Alexandru Petrescu, Mikael Abrahamsson, Tim
Chown, Jen Linkova, Christopher Palmer Jong-Hyouk Lee, Mark Andrews,
Lorenzo Colitti, Ted Lemon, Joel Jaeggli, David Farmer, Doug Barton,
Owen Delong, Gert Doering, Bill Jouris, Bill Cerveny, Dave Thaler,
Nick Hilliard, Jan Zorz, Randy Bush, Anders Brandt, , Sofiane Imadali
and Wesley George.
Some test of using ULA in the lab was done by our research partner
BNRC-BUPT (Broad Network Research Centre in Beijing University of
Posts and Telecommunications). Thanks for the work of Prof.
Xiangyang Gong and student Dengjia Xu.
Tom Taylor did a language review and revision throught the whole
document. The authors appreciate a lot for his help.
This document was produced using the xml2rfc tool [RFC7991]
(initially prepared using 2-Word-v2.0.template.dot.).
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005,
<https://www.rfc-editor.org/info/rfc4193>.
[RFC7991] Hoffman, P., "The "xml2rfc" Version 3 Vocabulary",
RFC 7991, DOI 10.17487/RFC7991, December 2016,
<https://www.rfc-editor.org/info/rfc7991>.
8.2. Informative References
[MIL-STD-1397]
"Military Standard, Input/Output Interfaces, Standard
Digital Data, Navy Systems (MIL-STD-1397B), 3 March 1989".
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[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.
J., and E. Lear, "Address Allocation for Private
Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918,
February 1996, <https://www.rfc-editor.org/info/rfc1918>.
[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484,
DOI 10.17487/RFC3484, February 2003,
<https://www.rfc-editor.org/info/rfc3484>.
[RFC3879] Huitema, C. and B. Carpenter, "Deprecating Site Local
Addresses", RFC 3879, DOI 10.17487/RFC3879, September
2004, <https://www.rfc-editor.org/info/rfc3879>.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191,
November 2005, <https://www.rfc-editor.org/info/rfc4191>.
[RFC4192] Baker, F., Lear, E., and R. Droms, "Procedures for
Renumbering an IPv6 Network without a Flag Day", RFC 4192,
DOI 10.17487/RFC4192, September 2005,
<https://www.rfc-editor.org/info/rfc4192>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.
[RFC4864] Van de Velde, G., Hain, T., Droms, R., Carpenter, B., and
E. Klein, "Local Network Protection for IPv6", RFC 4864,
DOI 10.17487/RFC4864, May 2007,
<https://www.rfc-editor.org/info/rfc4864>.
[RFC5220] Matsumoto, A., Fujisaki, T., Hiromi, R., and K. Kanayama,
"Problem Statement for Default Address Selection in Multi-
Prefix Environments: Operational Issues of RFC 3484
Default Rules", RFC 5220, DOI 10.17487/RFC5220, July 2008,
<https://www.rfc-editor.org/info/rfc5220>.
[RFC6281] Cheshire, S., Zhu, Z., Wakikawa, R., and L. Zhang,
"Understanding Apple's Back to My Mac (BTMM) Service",
RFC 6281, DOI 10.17487/RFC6281, June 2011,
<https://www.rfc-editor.org/info/rfc6281>.
[RFC6296] Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix
Translation", RFC 6296, DOI 10.17487/RFC6296, June 2011,
<https://www.rfc-editor.org/info/rfc6296>.
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[RFC6724] Thaler, D., Ed., Draves, R., Matsumoto, A., and T. Chown,
"Default Address Selection for Internet Protocol Version 6
(IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012,
<https://www.rfc-editor.org/info/rfc6724>.
[RFC7084] Singh, H., Beebee, W., Donley, C., and B. Stark, "Basic
Requirements for IPv6 Customer Edge Routers", RFC 7084,
DOI 10.17487/RFC7084, November 2013,
<https://www.rfc-editor.org/info/rfc7084>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
Better Connectivity Using Concurrency", RFC 8305,
DOI 10.17487/RFC8305, December 2017,
<https://www.rfc-editor.org/info/rfc8305>.
[RS-485] "Electronic Industries Association (1983). Electrical
Characteristics of Generators and Receivers for Use in
Balanced Multipoint Systems. EIA Standard RS-485.".
[SCADA] "Boyer, Stuart A. (2010). SCADA Supervisory Control and
Data Acquisition. USA: ISA - International Society of
Automation.".
[ULA-IN-WILD]
"G. Michaelson, "conference.apnic.net/data/36/apnic-
36-ula_1377495768.pdf"".
Authors' Addresses
Sheng Jiang
Beijing University of Posts and Telecommunications
No. 10 Xitucheng Road
Haidian District
Beijing
100083
China
Email: shengjiang@bupt.edu.cn
Bing Liu
Huawei Technologies
Q14, Huawei Campus, No.156 Beiqing Road
Hai-Dian District, Beijing, 100095
P.R. China
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Email: leo.liubing@huawei.com
Nick Buraglio
ForwardingPlane, LLC
Email: buraglio@forwardingplane.net
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