Internet Engineering Task Force B. Liu
Internet-Draft S. Jiang
Intended status: Informational Huawei Technologies
Expires: August 9, 2016 February 6, 2016

Considerations For Using Unique Local Addresses


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

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This Internet-Draft will expire on August 9, 2016.

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Table of Contents

1. Introduction

Unique Local Addresses (ULA) is defined in [RFC4193], and it is an alternative to site-local address (deprecated in [RFC3879]). But the two kind of addresses are not the same. 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.

Global scope yet local usage, this special feature has confused network operators more or less. 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 [RFC2119] when they appear in ALL CAPS. When these words are not in ALL CAPS (such as "should" or "Should"), they have their usual English meanings, and are not to be interpreted as [RFC2119] key words.

3. Analysis of ULA Features

3.1. 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).

3.2. Globally Unique

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. Such overlapping was a major drawback to the deployment of private [RFC1918] addresses in IPv4.

Note that, as described in [RFC4864], applications may treat ULAs in practice like global-scope addresses, but address selection algorithms may need to distinguish between ULAs and Global-scope Unicast Addresses (GUAs) to ensure bidirectional communications. As a further note, the default address selection policy table in [RFC6724]) responds to this requirement.

3.3. 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.

3.4. Well Known Prefix

The prefixes of ULAs are well known thus they are easily identified and filtered.

This feature is convenient for management of security policies and troubleshooting. For example, network administrators can segregate packets containing data which must stay in the internal network by assigning ULAs to internal servers. Externally-destined data can be sent to the Internet or telecommunication network by a separate function, through an appropriate gateway/firewall.

3.5. Stable or Temporary Prefix

A ULA prefix can be generated once, at installation time or factory reset, and then possibly never be changed. Alternatively, it can be regenerated regularly, depending on deployment requirements.

4. Analysis and Operational Considerations for Scenarios Using ULAs

4.1. 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 or just temporarily not connected. These networks may include 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.

Serious disadvantages and impact on applications due to the use of ambiguous address space have been well documented in [RFC1918]. However, 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:

Operational considerations:

4.2. Connected Networks

4.2.1. ULA-Only Deployment

In some situations, nodes in one network are assigned ULAs but not Global Unicast Addresses (GUAs), but the nodes also need to communicate with the outside network. There could be two approaches:

Operational considerations:

4.2.2. ULAs along with PA Addresses

Two classes of network might need to use ULA with PA (Provider Aggregated) addresses:

Benefits of Using ULAs in this scenario:


Operational considerations:

4.3. 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 [RFC6555] 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. General Considerations For Using ULAs

5.1. 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 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 4.2.2). 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 4.2.1. 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].

5.2. 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.

6. ULA Usages Considered Helpful

6.1. Used in Isolated Networks

As analyzed in Section 4.1, ULA is very suitable for isolated networks. Especially when there are subnets in the isolated network, ULA is a reasonable choice.

6.2. ULA along with PA

As described in Section 4.2.2, using ULAs along with PA addresses to provide a logically separated local plane can benefit OAM functions and renumbering.

6.3. Some Specific Use Cases

Along with the general scenarios, this section provides some specific use cases that could benefit from using ULA.

6.3.1. 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 described in [I-D.baker-v6ops-b2b-private-routing], 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.

6.3.2. Used as NAT64 Prefix

The NAT64 PREF64 is just a group of local fake addresses for the DNS64 to point traffic to a NAT64. Using a ULA prefix as the PREF64 easily ensures that only local systems can use the translation resources of the NAT64 system since the ULA is not intended to be globally routable. The ULA helps clearly identify traffic that is locally contained and destined to a NAT64. Using ULA for PREF64 is deployed and it is an operational model.

But there is an issue needs to be noted. The NAT64 standard [RFC6146] specifies that the PREF64 should align with [RFC6052], in which the IPv4-Embedded IPv6 Address format was specified. If we pick a /48 for NAT64, it happens to be a standard 48/ part of ULA (7bit ULA well-known prefix+ 1 "L" bit + 40bit Global ID). Then the 40bit of ULA is not violated by being filled with part of the 32bit IPv4 address. This is important, because the 40bit assures the uniqueness of ULA. If the prefix is shorter than /48, the 40bit would be violated, and this could cause conformance issues. But it is considered that the most common use case will be a /96 PREF64, or even /64 will be used. So it seems this issue is not common in current practice.

It is most common that ULA PREF64 will be deployed on a single internal network, where the clients and the NAT64 share a common internal network. ULA will not be effective as PREF64 when the access network must use an Internet transit to receive the translation service of a NAT64 since the ULA will not route across the Internet.

According to the default address selection table specified in [RFC6724], the host would always prefer IPv4 over ULA. This could be a problem in NAT64-CGN scenario as analyzed in Section 8 of [RFC7269]. So administrators need to add additional site-specific address selection rules to the default table to steer traffic flows going through NAT64-CGN. However, updating the default policy tables in all hosts involves significant management cost. This may be possible in an enterprise (using a group policy object, or other configuration mechanisms), but it is not suitable at scale for home networks.

6.3.3. 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:

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.

7. 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 4.2.2, when using NPTv6, the administrators need to know where the firewall is located to set proper filtering rules.

Also as mentioned in Section 4.2.2, 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.

8. IANA Considerations

This memo has no actions for IANA.

9. 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 [RFC2629] (initially prepared using

10. References

10.1. 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.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629, DOI 10.17487/RFC2629, June 1999.
[RFC4193] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast Addresses", RFC 4193, DOI 10.17487/RFC4193, October 2005.

10.2. Informative References

, ", ", ", "
[I-D.baker-v6ops-b2b-private-routing] Baker, F., "Business to Business Private Routing", Internet-Draft draft-baker-v6ops-b2b-private-routing-00, July 2007.
[I-D.ietf-v6ops-dhcpv6-slaac-problem] Liu, B., Jiang, S., Gong, X., Wang, W. and E. Rey, "DHCPv6/SLAAC Interaction Problems on Address and DNS Configuration", Internet-Draft draft-ietf-v6ops-dhcpv6-slaac-problem-06, February 2016.
[I-D.jhlee-mext-mnpp] Tsukada, M., Ernst, T. and J. Lee, Mobile Network Prefix Provisioning", Internet-Draft draft-jhlee-mext-mnpp-00, October 2009.
[I-D.petrescu-autoconf-ra-based-routing] Petrescu, A., Janneteau, C., Demailly, N. and S. Imadali, "Router Advertisements for Routing between Moving Networks", Internet-Draft draft-petrescu-autoconf-ra-based-routing-05, July 2014.
[MIL-STD-1397]Military Standard, Input/Output Interfaces, Standard Digital Data, Navy Systems (MIL-STD-1397B), 3 March 1989"
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G. and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, DOI 10.17487/RFC1918, February 1996.
[RFC2993] Hain, T., "Architectural Implications of NAT", RFC 2993, DOI 10.17487/RFC2993, November 2000.
[RFC3027] Holdrege, M. and P. Srisuresh, "Protocol Complications with the IP Network Address Translator", RFC 3027, DOI 10.17487/RFC3027, January 2001.
[RFC3484] Draves, R., "Default Address Selection for Internet Protocol version 6 (IPv6)", RFC 3484, DOI 10.17487/RFC3484, February 2003.
[RFC3879] Huitema, C. and B. Carpenter, "Deprecating Site Local Addresses", RFC 3879, DOI 10.17487/RFC3879, September 2004.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and More-Specific Routes", RFC 4191, DOI 10.17487/RFC4191, November 2005.
[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.
[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.
[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.
[RFC5902] Thaler, D., Zhang, L. and G. Lebovitz, "IAB Thoughts on IPv6 Network Address Translation", RFC 5902, DOI 10.17487/RFC5902, July 2010.
[RFC6052] Bao, C., Huitema, C., Bagnulo, M., Boucadair, M. and X. Li, "IPv6 Addressing of IPv4/IPv6 Translators", RFC 6052, DOI 10.17487/RFC6052, October 2010.
[RFC6146] Bagnulo, M., Matthews, P. and I. van Beijnum, "Stateful NAT64: Network Address and Protocol Translation from IPv6 Clients to IPv4 Servers", RFC 6146, DOI 10.17487/RFC6146, April 2011.
[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.
[RFC6296] Wasserman, M. and F. Baker, "IPv6-to-IPv6 Network Prefix Translation", RFC 6296, DOI 10.17487/RFC6296, June 2011.
[RFC6555] Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with Dual-Stack Hosts", RFC 6555, DOI 10.17487/RFC6555, April 2012.
[RFC6724] Thaler, D., Draves, R., Matsumoto, A. and T. Chown, "Default Address Selection for Internet Protocol Version 6 (IPv6)", RFC 6724, DOI 10.17487/RFC6724, September 2012.
[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.
[RFC7269] Chen, G., Cao, Z., Xie, C. and D. Binet, "NAT64 Deployment Options and Experience", RFC 7269, DOI 10.17487/RFC7269, June 2014.
[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, """

Authors' Addresses

Bing Liu Huawei Technologies Q14, Huawei Campus, No.156 Beiqing Road Hai-Dian District, Beijing, 100095, P.R. China EMail:
Sheng Jiang Huawei Technologies Q14, Huawei Campus, No.156 Beiqing Road Hai-Dian District, Beijing, 100095, P.R. China EMail: