INTERNET-DRAFT Thomas Narten
IBM
<draft-narten-ipv6-3177bis-48boundary-02.txt> Geoff Huston
APNIC
Lea Roberts
Stanford University
June 26, 2006
IPv6 Address Allocation to End Sites
<draft-narten-ipv6-3177bis-48boundary-02.txt>
Status of this Memo
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Abstract
RFC 3177 argued that in IPv6, end sites should be assigned /48 blocks
in most cases. The Regional Internet Registries (RIRs) adopted those
recommendations in 2002 and they have been in effect ever since.
This document revisits and updates the IAB/IESG recommendations on
the assignment of IPv6 address space to end sites. The exact choice
of how much address space to assign end sites is a policy issue under
the purview of the RIRs, subject to IPv6 architectural and
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operational considerations. This document reviews the architectural
considerations and some of the operational issues and reiterates that
changing the /48 recommendation is one of policy, and has minimal
impact on the IPv6 architecture and on IETF Standards.
This document obsoletes RFC 3177 and reclassifies it as historic.
Contents
Status of this Memo.......................................... 1
1. Introduction............................................. 2
2. On /48 Assignments to End Sites.......................... 3
3. Other RFC 3177 considerations............................ 4
4. Impact on IPv6 Standards................................. 5
4.1. RFC3056: Connection of IPv6 Domains via IPv4 Clouds. 5
4.2. IPv6 Multicast Addressing........................... 5
5. Summary.................................................. 5
6. Security Considerations.................................. 6
7. IANA Considerations...................................... 6
8. Acknowledgments.......................................... 6
9. Normative References..................................... 6
10. Informative References.................................. 6
11. Author's Address........................................ 7
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. Likewise, giving out an excessive
amount of address space could result in premature depletion. 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.
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RFC 3177 [RFC3177] called for a default end site IPv6 assignment size
of /48. Subsequently, the RIRs developed and adopted IPv6 address
assignment and allocation policies consistent with the RFC 3177
recommendations [RIR-IPV6]. Additional history and discussion of IPv6
address policy and its long-term implications can be found in
[IPV6-HISTORY].
This document performs two functions:
1) It revisits the RFC 3177 recommendations and concludes that the
default IPv6 assignment size could be changed from /48 to some
other value (e.g., /56) with essentially no impact on existing
IPv6 standards and no impact to (standards-compliant)
implementations.
2) It obsoletes RFC 3177 and reclassifies it historic.
2. On /48 Assignments to End Sites
This document does not make a specific recommendation on what the
assignment size should be. The exact choice of how much address
space to assign end sites is a policy issue under the purview of the
RIRs, subject to IPv6 architectural and opertational considerations.
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.
Looking back at some of the original motivations behind the /48
recommendation [RFC 3177], there were two 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 typically significantly more
expensive either in terms of the justification effort needed to
obtain additional addresses, or in the actual monthly service cost.
(It should be noted that an end-user additional "cost" for obtaining
more than one address is difficult to justify by the actual effective
per-address cost charged by the RIRs, but providing additional
addresses is frequently only available as part of a different type or
"higher grade" of service, for which an additional charge is levied.)
Thus, an important goal in IPv6 is to significantly change the
default and minimal end site assignment, from "a single address" to
"multiple networks".
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A second motivation behind the original /48 recommendation was to
simplify the management of an end site's numbering plan in the
presence of renumbering (e.g., when switching ISPs) and multiple
prefixes. 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 presences of multiple
prefixes, it is signficantly less complex to manage a numbering plans
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 addition, renumbering from a larger set of
"subnet bits" into a smaller set is relatively painful, as it it can
require making changes to the network itself (e.g., collapsing
subnets). In contrast, renumbering a site into a prefix that has the
same number (or more) of subnet bits is more straightforward, because
only the top-level bits of the address need to change. Thus, another
goal of the RFC 3177 recommendation 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.
The above concerns were met by the original /48 recommendation, but
could also be realized through a more conservative approach. A key
goal, however, is to avoid the need for a site to renumber into a
smaller number of subnet bits when adding a new prefix. This could be
achieved, for instance, by having it be easy for an end site to
obtain an address block of the same size (or larger) as any existing
assignments it already has.
3. Other RFC 3177 considerations
RFC3177 suggested that some multihoming approaches (e.g., GSE) might
benefit from having a fixed /48 boundary. This no longer appears to
be a consideration. There is no such requirement coming out of the
IETF multi6 or shim6 efforts.
RFC3177 argued that having a "one size fits all" default assignment
size reduced the need for customers to continually or repeatedly
justify 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 consistent with
reasonable growth projections over multi-year time frames. Thus, it
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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. RFC3056: Connection of IPv6 Domains via IPv4 Clouds
RFC3056 [RFC 3056] 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 an RFC3056 address numbering scheme 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 left
most bits only if the size of the site made that necessary.
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 a
policy issue under the purview of the RIRs, subject to IPv6
architectural and operational considerations. However, the RFC 3177
recommendation 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. The IETF recommends that any policy on IPv6
address assignment policy to end sites take into consideration:
- 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)
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- the default assignment size should take into consideration the
likelihood that an end site will have need to have multiple
subnets in the future and avoid the IPv4 practice of having
frequent and continual justification for obtaining small amounts
of additional space
- it is generally undesirable to have one ISP assign a longer
prefix to an end site when compared with the existing prefixes
the end site already has assigned to it
- the operational considerations of managing and delegating the
reverse DNS tree under ip6.arpa on nibble vs. non-nibble
boundaries should be given adequate consideration
6. Security Considerations
This document has no known security implications.
7. IANA Considerations
This document makes no requests to IANA.
8. Acknowledgments
This document was motivated by and benefited from numerous
conversations held during the ARIN XV and RIPE 50 meetings in April-
May, 2005.
9. Normative References
10. Informative References
[HUSTON-RIPE] "Report from the ARIN XV IPv6 Roundtable"
http://www.ripe.net/ripe/meetings/ripe-50/presentations/ripe50-plenary-
wed-ipv6-roundtable-report.pdf
[IPV6-HISTORY] Issues Related to the Management of IPv6 Address
Space, draft-narten-iana-rir-
ipv6-considerations-00.txt
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[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
[RFC 3056] "Connection of IPv6 Domains via IPv4 Clouds," B.
Carpenter, K. Moore, RFC 3056, February 2001.
[RFC 3306] "Unicast-Prefix-based IPv6 Multicast Addresses," B.
Haberman, D. Thaler, RFC 3306, August 2002.
[RFC 3177] IAB/IESG Recommendations on IPv6 Address Allocations to
Sites. IAB, IESG. September 2001.
[ULA-ADDRESSES] RFC 4193 "Unique Local IPv6 Unicast Addresses," R.
Hinden, B. Haberman, RFC 4193, October 2005.
11. Author's Address
Thomas Narten
IBM Corporation
3039 Cornwallis Ave.
PO Box 12195 - BRQA/502
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
241 Panama Street
Pine Hall, room 175B
Stanford, CA 94305-4102
Email: lea.roberts@stanford.edu
Phone: +1-650-723-3352
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