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rfc:rfc6177

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.

Narten, et al. Best Current Practice [Page 1] RFC 6177 IPv6 Address Assignment to End Sites March 2011

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
 Provisions Relating to IETF Documents
 (http://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 and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 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

Narten, et al. Best Current Practice [Page 2] RFC 6177 IPv6 Address Assignment to End Sites March 2011

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.

Narten, et al. Best Current Practice [Page 3] RFC 6177 IPv6 Address Assignment to End Sites March 2011

 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.

Narten, et al. Best Current Practice [Page 4] RFC 6177 IPv6 Address Assignment to End Sites March 2011

 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

Narten, et al. Best Current Practice [Page 5] RFC 6177 IPv6 Address Assignment to End Sites March 2011

 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.

Narten, et al. Best Current Practice [Page 6] RFC 6177 IPv6 Address Assignment to End Sites March 2011

 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:
  1. 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).
  1. 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.
  1. 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.
  1. 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.

Narten, et al. Best Current Practice [Page 7] RFC 6177 IPv6 Address Assignment to End Sites March 2011

  1. 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.

Narten, et al. Best Current Practice [Page 8] RFC 6177 IPv6 Address Assignment to End Sites March 2011

 [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

Narten, et al. Best Current Practice [Page 9]

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