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Versions: 00 01 02 03 04 05 draft-ietf-v6ops-ula-usage-recommendations

Network Working Group                                           B. Liu
Internet Draft                                                S. Jiang
Intended status: Best Current Practice             Huawei Technologies
Expires: August 28, 2013                                      C. Byrne
                                                          T-Mobile USA
                                                     February 25, 2013

                 Guidance of Using Unique Local Addresses
                 draft-liu-v6ops-ula-usage-analysis-05.txt


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 August 28, 2013.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors. All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   (http://trustee.ietf.org/license-info) in effect on the date of
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   document.

Abstract

   This document provides guidance of how to use ULA. It analyzes ULA
   usage scenarios and recommends use cases where ULA address may be
   beneficially used.




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Table of Contents


   1. Introduction ................................................. 2
   2. ULA usage analysis ........................................... 3
      2.1. The features of ULA ..................................... 3
         2.1.1. Self-assigned ...................................... 3
         2.1.2. Globally unique .................................... 3
         2.1.3. Independent address space .......................... 3
         2.1.4. Well known prefix .................................. 4
         2.1.5. Stable or Temporary Prefix ......................... 4
      2.2. Enumeration of ULA use scenarios ........................ 4
         2.2.1. Isolated network ................................... 4
         2.2.2. Connected network .................................. 5
            2.2.2.1. ULA-only Deployment ........................... 5
            2.2.2.2. ULA along with GUA ............................ 6
   3. Recommended ULA Use Cases .................................... 6
      3.1. Used in Isolated Networks ............................... 6
      3.2. ULA along with GUA ...................................... 6
      3.3. Special Use Cases ....................................... 7
         3.3.1. Special routing .................................... 7
         3.3.2. Used as NAT64 prefix ............................... 7
         3.3.3. Used as identifier ................................. 8
   4. Security Considerations ...................................... 8
   5. IANA Considerations .......................................... 8
   6. Conclusions .................................................. 9
   7. References ................................................... 9
      7.1. Normative References .................................... 9
      7.2. Informative References .................................. 9
   8. Acknowledgments ............................................. 10

1. Introduction

   Unique Local Addresses (ULAs) are defined in [RFC4193] as provider-
   independent prefixes that can be used on isolated networks, internal
   networks, and VPNs. Although ULAs may be treated like global scope by
   applications, normally they are not used on the publicly routable
   internet.

   However, the ULAs haven't been widely used since IPv6 hasn't been
   widely deployed yet.

   The use of ULA addresses in various types of networks has been confused
   for network operators. Some network operators believe ULAs are not
   useful at all while other network operators run their entire networks on
   ULA address space. This document attempts to clarify the advantages and
   disadvantages of ULAs and how they can be most appropriately used.


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2. ULA usage analysis

2.1. The features of ULA

2.1.1. Self-assigned

   ULA is self-assigned, this feature allows automatic address
   allocation, which is beneficial for some lightweight systems and can
   leverage minimal human management.

2.1.2. Globally unique

   ULA is intended to be globally unique to avoid collision. Since the
   hosts assigned with ULA may occasionally be merged into one network,
   this uniqueness is necessary. The prefix uniqueness is based on
   randomization of 40 bits and is considered random enough to ensure a
   high degree of uniqueness (refer to [RFC4193] section 3.2.3 for
   details)and make merging of networks simple and without the need to
   renumbering overlapping IP address space. Overlapping is cited as a
   deficiency with how [RFC1918] addresses were deployed, and ULA was
   designed to overcome this deficiency.

   Notice that, as described in [RFC4864], in practice, applications may
   treat ULAs like global-scope addresses, but address selection
   algorithms may need to distinguish between ULAs and ordinary GUA
   (Global-scope Unicast Address) to ensure bidirectional communications.

2.1.3. Independent address space

   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. But the ability to merge two ULA
   networks without renumbering (because of the uniqueness) is a big
   advantage over [RFC1918].

   On the other hand, many organizations have security policies and
   architectures based around the local-only routing of [RFC1918] addresses
   and those policies may directly map to ULA. ULA can be used for
   internal communications without having any permanent or only
   intermittent Internet connectivity. And it needs no registration so
   that it can support on-demand usage and does not carry any RIR
   documentation burden or disclosures.






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2.1.4. Well known prefix

   The prefixes of ULAs are well known and they are easy to be
   identified and easy to be filtered.

   This feature may be convenient to management of security policies and
   troubleshooting. For example, the administrators can decide what
   parameters have to be assembled or transmitted globally, by a
   separate function, through an appropriate gateway/firewall, to the
   Internet or to the telecom network.

2.1.5. Stable or Temporary Prefix

   A ULA prefix can be generated once, at installation time or "factory
   reset", and then never change unless the network manager wants to
   change. Alternatively, it could be regenerated regularly, if desired
   for some reason.

2.2. Enumeration of ULA use scenarios

   In this section, we try to cover plausible possible ULA use case.
   Some of them might have been discussed in other documents and are
   briefly reviewed in this document as well as other potential valid
   usage is discussed.

2.2.1. Isolated network

   IP is used ubiquitously. Some networks like RS-485, or other type of
   industrial control bus, or even non-networked digital interface like
   MIL-STD-1397 began to use IP protocol. In such kind of networks, the
   system might lack the ability/requirement of connecting to the
   Internet.

   Besides, some networks are explicitly designed to not connect to the
   internet. These networks may include machine-to-machine, sensor networks,
   or other types of SCADA networks which may include very large numbers of
   addresses and explicitly prohibited from connect to the global internet
   (electricity meters...).

   ULA is a straightforward way to assign the IP addresses in these
   kinds of networks with minimal administrative cost or burden.








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2.2.2. Connected network

2.2.2.1. ULA-only Deployment

   In some situations, hosts/interfaces are assigned with ULA-only, but
   the networks need to communicate with the outside. For example, just
   like many implementations of private IPv4 address space [RFC1918]. One
   important reason of using private address space is the lack of IPv4
   addresses, but this it is not an issue any more in IPv6. Another reason
   is regarding with security, private address space is designed by some
   administrators as one layer of a multilayer security. Such design is
   also applicable in IPv6 with using ULAs.

   But we should eliminate the misunderstanding that ULA is designed to
   be the IPv6 version of [RFC1918] deployment model. If you chose non-
   globally routable address space for some reasons, ULA is a nature
   selection, but we need to know ULA itself is not designed for this
   intention.

   ULA-only in connected network may include the following two models.

   o  Using Network Prefix Translation

   Network Prefix Translation (NPTv6) [RFC6296] is an experimental
   specification that provides a stateless one to one mapping between
   internal addresses and external addresses.

   In some very constrained situations(for example, in the sensors), the
   network needs ULA as the on-demand and stable addressing which
   doesn't need much code to support address assignment mechanisms like
   DHCP or ND. And the network also needs to connect to the outside,
   then there can be a gateway to be the NAT which may not be so
   sensitive to the constrained resource. This behavior could refer
   NPTv6 [RFC6296].

   o  Using application-layer proxies

   The proxies terminate the network-layer connectivity of the hosts and
   delegate the outgoing/incoming connections.

   There may be some scenarios that need this kind of deployment for
   some special purpose (strict application access control, content
   monitoring, e.g.).






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2.2.2.2. ULA along with GUA

   There are two classes of network probably to use ULA with GUA
   addresses:

   o  Home network. Home networks are normally assigned with one or more
      globally routed PA prefixes to connect to the uplink of some an
      ISP. And besides, they may need internal routed networking even
      when the ISP link is down. Then ULA is a proper tool to fit the
      requirement. And in [RFC6204], it requires the CPE to support ULA.

   o  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 could be used for internal
      connectivity redundancy and better internal connectivity or
      isolation of certain functions like OAM of servers.

3. Recommended ULA Use Cases

3.1. Used in Isolated Networks

   As analyzed in section 2.2.1, ULA is very suitable for isolated
   networks. Especially when you have subnets in the isolated networks,
   ULA is almost the only choice.

3.2. ULA along with GUA

   For either home networks or enterprise networks, the main purpose of
   using ULA along with GUA is to provide a logically local routing
   plane separated from the globally 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 private OAM function in an enterprise.

   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 whole
   network because of changing ISPs, ISPs restructure their address
   allocations, or whatever reasons. In these situations, ULA is an
   effective tool for the internal-only nodes.

   Besides the internal-only nodes, the public nodes can also benefit
   from ULA for renumbering. When renumbering, as RFC4192 suggested, it
   has a period to keep using the old prefix(es) before 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 the local
   communication immune of global routing plane change. If we use ULA


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   for the local communication, the separated local routing plane can
   isolate the affecting by global routing change.

   But for the separated local routing plane, there always be some
   argument that in practice the ULA+PA makes terrible operational
   complexity. But it is not a ULA-specific problem; the multiple-
   addresses-per-interface is an important feature of IPv6 protocol.
   Running multiple prefixes in IPv6 might be very common, and we need
   to adapt this new operational model than that in IPv4.

   Another issue is 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. In [RFC6724] , it claimed that a
   site-specific policy entry can be used to cause ULAs within a site to
   be preferred over global addresses.

3.3. Special Use Cases

3.3.1. Special routing

   If you have a special routing scenario, of which
   [draft-baker-v6ops-b2b-private-routing] is an example, for various
   reasons you might want to have routing that you control and is
   separate from other routing. In the b2b case, even though two
   companies each have at least one ISP, they might choose to also 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 obtain such a prefix that would be used in accordance
   with an agreement between the parties.

3.3.2. Used as NAT64 prefix

   Since the NAT64 pref64 is just a group of local fake addresses for
   the DNS64 to point traffic to a NAT64, the pref64 is a very good use
   of ULA. It ensures that only local systems can use the translation
   resources of the NAT64 system since the ULA is not globally routable
   and helps clearly identify traffic that is locally contained and
   destine to a NAT64. Using ULA for Pref64 is deployed and it is an
   operational model.

   But there's an issue should be noticed. The NAT64 standard [RFC6146)
   mentioned the pref64 should align with [RFC6052], in which the IPv4-
   Embedded IPv6 Address format was specified. If we pick a /48 for
   NAT64, it happened to be a standard 48/ part of ULA (7bit ULA famous
   prefix+ 1 "L" bit + 40bit Global ID). Then the 40bit of ULA is not
   violated to be filled with part of the 32bit IPv4 address. This is


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   important, because the 40bit assures the uniqueness of ULA, if the
   prefix is shorter than /48, the 40bit would be violated, and this may
   cause conformance issue. 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.

3.3.3. Used as identifier

   Since ULA could be self-generated and easily grabbed from the
   standard IPv6 stack, it is 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:

   o  TCP connections between two end hosts wish to survive in network
      changes.

   o  Sometimes one needs a constant identifier to be associated with a
      key so that the Security Association can survive the location
      changes.

   It should be noticed again that in theory ULA has the possibility of
   collision. However, the probability is desirable small enough and
   could be ignored by most of the cases when used as identifiers.

4. Security Considerations

   Security considerations regarding ULAs, in general, please refer to
   the ULA specification [RFC4193].

5. IANA Considerations

   None.






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6. Conclusions

   ULA is a useful tool, it have been successfully deployed in a diverse
   set of circumstances including large private machine-to-machine type
   networks, enterprise networks with private systems, and within
   service providers to limit Internet communication with non-public
   services such as caching DNS servers and NAT64 translation resources.

   We should eliminate the misunderstanding that ULA is just an IPv6
   version of [RFC1918]. The features of ULA could be beneficial for
   various use cases.

7. References

7.1. Normative References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
             Requirement Levels", RFC 2119, BCP14, March 1997.

   [RFC4193] Hinden, R., B. Haberman, "Unique Local IPv6 Unicast
             Addresses", RFC 4193, October 2005.

7.2. Informative References

   [RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
             E. Lear, "Address Allocation for Private Internets",BCP 5,
             RFC 1918, February 1996.

   [RFC4864] Van de Velde, G., Hain, T., Droms, R., Carpenter, B., and
             E. Klein, "Local Network Protection for IPv6", RFC 4864,
             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, July 2008.

   [RFC6281] Cheshire, S., Zhu, Z., Wakikawa, R., and L. Zhang,
             "Understanding Apple's Back to My Mac (BTMM) Service", RFC
             6281, June 2011.

   [RFC6296] Wasserman, M., and F. Baker, "IPv6-to-IPv6 Network Prefix
             Translation", RFC 6296, June 2011.

   [RFC6724] Thaler, D., Draves, R., Matsumoto, A., and Tim Chown,
             "Default Address Selection for Internet Protocol version 6
             (IPv6)", RFC 6724, September, 2012.


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   [draft-baker-v6ops-b2b-private-routing]
             F. Baker, "Business to Business Private Routing", Expired


8. Acknowledgments

   Many valuable comments were received in the mail list, especially
   from Fred Baker, Brian Carpenter, Anders Brandt and Wesley George.

   This document was prepared using 2-Word-v2.0.template.dot.






































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   Authors' Addresses

   Bing Liu
   Huawei Technologies Co., Ltd
   Huawei Q14 Building, No.156 Beiqing Rd.,
   Zhong-Guan-Cun Environmental Protection Park, Beijing
   P.R. China

   EMail: leo.liubing@huawei.com


   Sheng Jiang
   Huawei Technologies Co., Ltd
   Huawei Q14 Building, No.156 Beiqing Rd.,
   Zhong-Guan-Cun Environmental Protection Park, Beijing
   P.R. China

   EMail: jiangsheng@huawei.com


   Cameron Byrne
   T-Mobile USA
   Bellevue, Washington  98006
   USA

   Email: cameron.byrne@t-mobile.com


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