IPv6 Address Assignment to End Sites
RFC 6177

Internet Engineering Task Force (IETF)                         T. Narten
Request for Comments: 6177                                           IBM
BCP: 157                                                       G. Huston
Obsoletes: 3177                                                    APNIC
Category: Best Current Practice                               L. Roberts
ISSN: 2070-1721                                      Stanford University
                                                              March 2011

                  IPv6 Address Assignment to End Sites

Abstract

   RFC 3177 argued that in IPv6, end sites should be assigned /48 blocks
   in most cases.  The Regional Internet Registries (RIRs) adopted that
   recommendation in 2002, but began reconsidering the policy in 2005.
   This document obsoletes the RFC 3177 recommendations on the
   assignment of IPv6 address space to end sites.  The exact choice of
   how much address space to assign end sites is an issue for the
   operational community.  The IETF's role in this case is limited to
   providing guidance on IPv6 architectural and operational
   considerations.  This document reviews the architectural and
   operational considerations of end site assignments as well as the
   motivations behind the original recommendations in RFC 3177.
   Moreover, this document clarifies that a one-size-fits-all
   recommendation of /48 is not nuanced enough for the broad range of
   end sites and is no longer recommended as a single default.

   This document obsoletes RFC 3177.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   BCPs is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6177.

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Copyright Notice

   Copyright (c) 2011 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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   Without obtaining an adequate license from the person(s) controlling
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   than English.

Table of Contents

   1. Introduction ....................................................3
   2. On /48 Assignments to End Sites .................................4
   3. Other RFC 3177 Considerations ...................................6
   4. Impact on IPv6 Standards ........................................6
      4.1. RFC 3056: Connection of IPv6 Domains via IPv4 Clouds .......6
      4.2. IPv6 Multicast Addressing ..................................7
   5. Summary .........................................................7
   6. Security Considerations .........................................8
   7. Acknowledgments .................................................8
   8. Informative References ..........................................8

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1.  Introduction

   There are a number of considerations that factor into address
   assignment policies.  For example, to provide for the long-term
   health and scalability of the public routing infrastructure, it is
   important that addresses aggregate well [ROUTE-SCALING].  Likewise,
   giving out an excessive amount of address space could result in
   premature depletion of the address space.  This document focuses on
   the (more narrow) question of what is an appropriate IPv6 address
   assignment size for end sites.  That is, when end sites request IPv6
   address space from ISPs, what is an appropriate assignment size.

   RFC 3177 [RFC3177] called for a default end site IPv6 assignment size
   of /48.  Subsequently, the Regional Internet Registries (RIRs)
   developed and adopted IPv6 address assignment and allocation policies
   consistent with the recommendations of RFC 3177 [RIR-IPV6].  In 2005,
   the RIRs began discussing IPv6 address assignment policy again.
   Since then, APNIC [APNIC-ENDSITE], ARIN [ARIN-ENDSITE], and RIPE
   [RIPE-ENDSITE] have revised the end site assignment policy to
   encourage the assignment of smaller (i.e., /56) blocks to end sites.

   This document obsoletes RFC 3177, updating its recommendations in the
   following ways:

      1) It is no longer recommended that /128s be given out.  While
         there may be some cases where assigning only a single address
         may be justified, a site, by definition, implies multiple
         subnets and multiple devices.

      2) RFC 3177 specifically recommended using prefix lengths of /48,
         /64, and /128.  Specifying a small number of fixed boundaries
         has raised concerns that implementations and operational
         practices might become "hard-coded" to recognize only those
         fixed boundaries (i.e., a return to "classful addressing").
         The actual intention has always been that there be no hard-
         coded boundaries within addresses, and that Classless Inter-
         Domain Routing (CIDR) continues to apply to all bits of the
         routing prefixes.

      3) This document moves away from the previous recommendation that
         a single default assignment size (e.g., a /48) makes sense for
         all end sites in the general case.  End sites come in different
         shapes and sizes, and a one-size-fits-all approach is not
         necessary or appropriate.

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   This document does, however, reaffirm an important assumption behind
   RFC 3177:

      A key principle for address management is that end sites always be
      able to obtain a reasonable amount of address space for their
      actual and planned usage, and over time ranges specified in years
      rather than just months.  In practice, that means at least one
      /64, and in most cases significantly more.  One particular
      situation that must be avoided is having an end site feel
      compelled to use IPv6-to-IPv6 Network Address Translation or other
      burdensome address conservation techniques because it could not
      get sufficient address space.

   This document does not make a formal recommendation on what the exact
   assignment size should be.  The exact choice of how much address
   space to assign end sites is an issue for the operational community.
   The IETF's role in this case is limited to providing guidance on IPv6
   architectural and operational considerations.  This document provides
   input into those discussions.  The focus of this document is to
   examine the architectural issues and some of the operational
   considerations relating to the size of the end site assignment.

2.  On /48 Assignments to End Sites

   Looking back at some of the original motivations behind the /48
   recommendation [RFC3177], there were three main concerns.  The first
   motivation was to ensure that end sites could easily obtain
   sufficient address space without having to "jump through hoops" to do
   so.  For example, if someone felt they needed more space, just the
   act of asking would at some level be sufficient justification.  As a
   comparison point, in IPv4, typical home users are given a single
   public IP address (though even this is not always assured), but
   getting any more than one address is often difficult or even
   impossible -- unless one is willing to pay a (significantly)
   increased fee for what is often considered to be a "higher grade" of
   service.  (It should be noted that increased ISP charges to obtain a
   small number of additional addresses cannot usually be justified by
   the real per-address cost levied by RIRs, but additional addresses
   are frequently only available to end users as part of a different
   type or "higher grade" of service, for which an additional charge is
   levied.  The point here is that the additional cost is not due to the
   RIR fee structures, but to business choices ISPs make.) An important
   goal in IPv6 is to significantly change the default and minimal end
   site assignment, from "a single address" to "multiple networks" and
   to ensure that end sites can easily obtain address space.

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   A second motivation behind the original /48 recommendation was to
   simplify the management of an end site's addressing plan in the
   presence of renumbering (e.g., when switching ISPs).  In IPv6, a site
   may simultaneously use multiple prefixes, including one or more
   public prefixes from ISPs as well as Unique Local Addresses
   [ULA-ADDRESSES].  In the presence of multiple prefixes, it is
   significantly less complex to manage a numbering plan if the same
   subnet numbering plan can be used for all prefixes.  That is, for a
   link that has (say) three different prefixes assigned to it, the
   subnet portion of those prefixes would be identical for all assigned
   addresses.  In contrast, renumbering from a larger set of "subnet
   bits" into a smaller set is often painful, as it can require making
   changes to the network itself (e.g., collapsing subnets).  Hence,
   renumbering a site into a prefix that has (at least) the same number
   of subnet bits is more straightforward, because only the top-level
   bits of the address need to change.  A key goal of the
   recommendations in RFC 3177 is to ensure that upon renumbering, one
   does not have to deal with renumbering into a smaller subnet size.

   It should be noted that similar arguments apply to the management of
   zone files in the DNS.  In particular, managing the reverse
   (ip6.arpa) tree is simplified when all links are numbered using the
   same subnet ids.

   A third motivation behind the /48 recommendation was to better
   support network growth common at many sites.  In IPv4, it is usually
   difficult (or impossible) to obtain public address space for more
   than a few months worth of projected growth.  Thus, even slow growth
   over several years can lead to the need to renumber into a larger
   address block.  With IPv6's vast address space, end sites can easily
   be given more address space (compared with IPv4) to support expected
   growth over multi-year time periods.

   While the /48 recommendation does simplify address space management
   for end sites, it has also been widely criticized as being wasteful.
   For example, a large business (which may have thousands of employees)
   would, by default, receive the same amount of address space as a home
   user, who today typically has a single (or small number of) LAN and a
   small number of devices (dozens or less).  While it seems likely that
   the size of a typical home network will grow over the next few
   decades, it is hard to argue that home sites will make use of 65K
   subnets within the foreseeable future.  At the same time, it might be
   tempting to give home sites a single /64, since that is already
   significantly more address space compared with today's IPv4 practice.
   However, this precludes the expectation that even home sites will
   grow to support multiple subnets going forward.  Hence, it is
   strongly intended that even home sites be given multiple subnets

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   worth of space, by default.  Hence, this document still recommends
   giving home sites significantly more than a single /64, but does not
   recommend that every home site be given a /48 either.

   A change in policy (such as above) would have a significant impact on
   address consumption projections and the expected longevity for IPv6.
   For example, changing the default assignment from a /48 to /56 (for
   the vast majority of end sites, e.g., home sites) would result in a
   savings of up to 8 bits, reducing the "total projected address
   consumption" by (up to) 8 bits or two orders of magnitude.  (The
   exact amount of savings depends on the relative number of home users
   compared with the number of larger sites.)

   The above-mentioned goals of RFC 3177 can easily be met by giving
   home users a default assignment of less than /48, such as a /56.

3.  Other RFC 3177 Considerations

   RFC 3177 suggested that some multihoming approaches (e.g.,
   Generalized Structure Element (GSE)) might benefit from having a
   fixed /48 boundary.  This no longer appears to be a consideration.

   RFC 3177 argued that having a "one-size-fits-all" default assignment
   size reduced the need for customers to continually or repeatedly
   justify the usage of existing address space in order to get "a little
   more".  Likewise, it also reduces the need for ISPs to evaluate such
   requests.  Given the large amount of address space in IPv6, there is
   plenty of space to grant end sites enough space to be consistent with
   reasonable growth projections over multi-year time frames.  Thus, it
   remains highly desirable to provide end sites with enough space (on
   both initial and subsequent assignments) to last several years.
   Fortunately, this goal can be achieved in a number of ways and does
   not require that all end sites receive the same default size
   assignment.

4.  Impact on IPv6 Standards

4.1.  RFC 3056: Connection of IPv6 Domains via IPv4 Clouds

   RFC 3056 [RFC3056] describes a way of generating IPv6 addresses from
   an existing public IPv4 address.  That document describes an address
   format in which the first 48 bits concatenate a well-known prefix
   with a globally unique public IPv4 address.  The "SLA ID" field is
   assumed to be 16 bits, consistent with a 16-bit "subnet id" field.
   To facilitate transitioning from the address numbering scheme in RFC
   3056 to one based on a prefix obtained from an ISP, an end site would
   be advised to number out of the right most bits first, using the
   leftmost bits only if the size of the site made that necessary.

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   Similar considerations apply to other documents that allow for a
   subnet id of 16 bits, including [ULA-ADDRESSES].

4.2.  IPv6 Multicast Addressing

   Some IPv6 multicast address assignment schemes embed a unicast IPv6
   prefix into the multicast address itself [RFC3306].  Such documents
   do not assume a particular size for the subnet id, per se, but do
   assume that the IPv6 prefix is a /64.  Thus, the relative size of the
   subnet id has no direct impact on multicast address schemes.

5.  Summary

   The exact choice of how much address space to assign end sites is an
   issue for the operational community.  The recommendation in RFC 3177
   [RFC3177] to assign /48s as a default is not a requirement of the
   IPv6 architecture; anything of length /64 or shorter works from a
   standards perspective.  However, there are important operational
   considerations as well, some of which are important if users are to
   share in the key benefit of IPv6: expanding the usable address space
   of the Internet.  The IETF recommends that any policy on IPv6 address
   assignment policy to end sites take into consideration the following:

      - it should be easy for an end site to obtain address space to
        number multiple subnets (i.e., a block larger than a single /64)
        and to support reasonable growth projections over long time
        periods (e.g., a decade or more).

      - the default assignment size should take into consideration the
        likelihood that an end site will have need for multiple subnets
        in the future and avoid the IPv4 practice of having frequent and
        continual justification for obtaining small amounts of
        additional space.

      - Although a /64 can (in theory) address an almost unlimited
        number of devices, sites should be given sufficient address
        space to be able to lay out subnets as appropriate, and not be
        forced to use address conservation techniques such as using
        bridging.  Whether or not bridging is an appropriate choice is
        an end site matter.

      - assigning a longer prefix to an end site, compared with the
        existing prefixes the end site already has assigned to it, is
        likely to increase operational costs and complexity for the end
        site, with insufficient benefit to anyone.

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      - the operational considerations of managing and delegating the
        reverse DNS tree under ip6.arpa on nibble versus non-nibble
        boundaries should be given adequate consideration.

6.  Security Considerations

   This document has no known security implications.

7.  Acknowledgments

   This document was motivated by and benefited from numerous
   conversations held during the ARIN XV and RIPE 50 meetings in April-
   May, 2005.

8.  Informative References

   [APNIC-ENDSITE] "prop-031: Proposal to amend APNIC IPv6 assignment
                   and utilisation requirement policy,"
                   http://www.apnic.net/policy/proposals/prop-031

   [ARIN-ENDSITE]  "2005-8: Proposal to amend ARIN IPv6 assignment and
                   utilisation requirement",
                   http://www.arin.net/policy/proposals/2005_8.html

   [RIR-IPV6]      ARIN: http://www.arin.net/policy/nrpm.html#ipv6; RIPE
                   Document ID: ripe-267, Date: 22 January 2003
                   http://www.ripe.net/ripe/docs/ipv6policy.html; APNIC:
                   http://www.apnic.net/docs/policy/ipv6-address-
                   policy.html

   [RFC3056]       Carpenter, B. and K. Moore, "Connection of IPv6
                   Domains via IPv4 Clouds", RFC 3056, February 2001.

   [RFC3306]       Haberman, B. and D. Thaler, "Unicast-Prefix-based
                   IPv6 Multicast Addresses", RFC 3306, August 2002.

   [RFC3177]       IAB and IESG, "IAB/IESG Recommendations on IPv6
                   Address Allocations to Sites", RFC 3177, September
                   2001.

   [RIPE-ENDSITE]  "Proposal to Amend the IPv6 Assignment and
                   Utilisation Requirement Policy", 2005-8,
                   http://www.ripe.net/ripe/policies/proposals/2005-08.

   [ROUTE-SCALING] "Routing and Addressing Problem Statement", Work in
                   Progress, February 2010.

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   [ULA-ADDRESSES] Hinden, R. and B. Haberman, "Unique Local IPv6
                   Unicast Addresses", RFC 4193, October 2005.

Authors' Addresses

   Thomas Narten
   IBM Corporation
   3039 Cornwallis Ave.
   PO Box 12195
   Research Triangle Park, NC 27709-2195

   Phone: 919-254-7798
   EMail: narten@us.ibm.com

   Geoff Huston
   APNIC

   EMail: gih@apnic.net

   Rosalea G Roberts
   Stanford University, Networking Systems
   P.O. Box 19131
   Stanford, CA  94309-9131

   EMail: lea.roberts@stanford.edu
   Phone: +1-650-723-3352

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