Network Working Group G. Van de Velde
Internet-Draft C. Popoviciu
Expires: December 3, 2006 Cisco Systems
T. Chown
University of Southampton
O. Bonness
C. Hahn
T-Systems Enterprise Services GmbH
June 2006
IPv6 Unicast Address Assignment Considerations
<draft-ietf-v6ops-addcon-01.txt>
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Copyright (C) The Internet Society (2006).
Abstract
One fundamental aspect of any IP communications infrastructure is its
addressing plan. With its new address architecture and allocation
policies, the introduction of IPv6 into a network means that network
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designers and operators need to reconsider their existing approaches
to network addressing. Lack of guidelines on handling this aspect of
network design could slow down the integration of IPv6. This draft
aims to provide the information and recommendations relevant to
planning the addressing aspects of IPv6 deployments. The draft also
provides IPv6 addressing case studies for both an enterprise and an
ISP network. In this first version of the draft we aim to provoke
discussion on this important topic; more detailed case study texts
will follow.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Network Level Addressing Design Considerations . . . . . . . . 5
2.1. Global Unique Addresses . . . . . . . . . . . . . . . . . 5
2.2. Unique Local IPv6 Addresses . . . . . . . . . . . . . . . 5
2.3. 6Bone Address Space . . . . . . . . . . . . . . . . . . . 6
2.4. Network Level Design Considerations . . . . . . . . . . . 7
2.4.1. Sizing the Network Allocation . . . . . . . . . . . . 8
2.4.2. Address Space Conservation . . . . . . . . . . . . . . 8
3. Subnet Prefix Considerations . . . . . . . . . . . . . . . . . 8
3.1. Considerations for subnet prefixes shorter then /64 . . . 8
3.2. Considerations for /64 prefixes . . . . . . . . . . . . . 9
3.3. Considerations for subnet prefixes longer then /64 . . . . 9
3.3.1. Anycast addresses . . . . . . . . . . . . . . . . . . 9
3.3.2. Addresses used by Embedded-RP (RFC3956) . . . . . . . 11
3.3.3. ISATAP addresses . . . . . . . . . . . . . . . . . . . 11
3.3.4. /126 addresses . . . . . . . . . . . . . . . . . . . . 12
3.3.5. /127 addresses . . . . . . . . . . . . . . . . . . . . 12
3.3.6. /128 addresses . . . . . . . . . . . . . . . . . . . . 12
4. Allocation of the IID of an IPv6 Address . . . . . . . . . . . 12
4.1. Automatic EUI-64 Format Option . . . . . . . . . . . . . . 13
4.2. Using Privacy Extensions . . . . . . . . . . . . . . . . . 13
4.3. Cryptographically Generated IPv6 Addresses . . . . . . . . 13
4.4. Manual/Dynamic Assignment Option . . . . . . . . . . . . . 14
5. Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1. Enterprise Considerations . . . . . . . . . . . . . . . . 14
5.1.1. Obtaining general IPv6 network prefixes . . . . . . . 14
5.1.2. Forming an address (subnet) allocation plan . . . . . 15
5.1.3. Other considerations . . . . . . . . . . . . . . . . . 16
5.1.4. Node configuration considerations . . . . . . . . . . 16
5.1.5. Observations . . . . . . . . . . . . . . . . . . . . . 17
5.2. Service Provider Considerations . . . . . . . . . . . . . 17
5.2.1. Investigation of objective Requirements for an
IPv6 addressing schema of a Service Provider . . . . . 17
5.2.2. IPv6 address allocation plan for a Service Provider . 19
5.2.3. Additional Remarks . . . . . . . . . . . . . . . . . . 23
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
7. Security Considerations . . . . . . . . . . . . . . . . . . . 24
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
9.1. Normative References . . . . . . . . . . . . . . . . . . . 25
9.2. Informative References . . . . . . . . . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28
Intellectual Property and Copyright Statements . . . . . . . . . . 30
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1. Introduction
The Internet Protocol Version 6 (IPv6) Addressing Architecture [23]
defines three main types of addresses: unicast, anycast and
multicast. This document focuses on unicast addresses, for which
there are currently three principal allocated types: Global Unique
Addresses [12] ('globals'), Unique Local IPv6 Addresses [22] (ULAs)
and 6bone address space [3].
The document covers aspects that should be considered during IPv6
deployment for the design and planning of an addressing scheme for an
IPv6 network. The network's IPv6 addressing plan may be for an IPv6-
only network, or for a dual-stack infrastructure where some or all
devices have addresses in both protocols. These considerations will
help an IPv6 network designer to efficiently and prudently assign the
IPv6 address space that has been allocated to its organization.
The address assignment considerations are analyzed separately for the
two major components of the IPv6 unicast addresses, namely 'Network
Level Addressing' (the allocation of subnets) and the 'Subnet Prefix'
(address usage within a subnet). Thus the document includes a
discussion of aspects of address assignment to nodes and interfaces
in an IPv6 network. Finally the document will provide two examples
of successfully deployed address plans in a service provider (ISP)
and an enterprise network.
Parts of this document highlight the differences that an experienced
IPv4 network designer should consider when planning an IPv6
deployment, for example:
o IPv6 devices will more likely be multi-addressed in comparison
with their IPv4 counterparts.
o The practically unlimited size of an IPv6 subnet (2^64 bits)
reduces the requirement to size subnets to device counts for the
purposes of (IPv4) address conservation.
o Even though there is no broadcast for the IPv6 protocol, there is
still need to consider the number of devices in a given subnet due
to traffic storm and level of traffic generated by hosts.
o The implications of the reduced threat of address-based host
scanning, as discussed in [26].
We do not discuss here how a site or ISP should proceed with
acquiring its globally routable IPv6 address prefix. However, one
should note that IPv6 networks receive their global unicast address
allocation from their 'upstream' provider, which may be another ISP,
a Local Internet Registry (LIR) or a Regional Internet Registry
(RIR). In each case the prefix received is provider assigned (PA);
there is currently no provider independent (PI) address space for
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IPv6. Thus an IPv6 network which changes provider will need to
undergo a renumbering process, as described in [21]. A separate
document [28] makes recommendations to ease the IPv6 renumbering
process.
This document neither discusses implementation aspects between ULA
addresses and Site-local addresses. Most implementations know about
Site-local addresses even though they are deprecated, and do not know
about ULAs - even though they are according current specification.
As result transitioning between these types of addresses may cause
difficulties.
2. Network Level Addressing Design Considerations
This section discusses the kind of IPv6 addresses used at the network
level for the IPv6 infrastructure. The kind of addresses that can be
considered are Global Unique Addresses, ULAs and 6bone address space.
2.1. Global Unique Addresses
The most commonly used unicast addresses will be Global Unique
Addresses ('globals'). No significant considerations are necessary
if the organization has an address space assignment and a single
prefix is deployed through a single upstream provider.
However, a multihomed site may deploy addresses from two or more
Service Provider assigned IPv6 address ranges. Here, the network
Administrator must have awareness on where and how these ranges are
used on the multihomed infrastructure environment. The nature of the
usage of multiple prefixes may depend on the reason for multihoming
(e.g. resilience failover, load balancing, policy-based routing, or
multihoming during an IPv6 renumbering event). IPv6 introduces
improved support for multi-addressed hosts through the IPv6 default
address selection methods described in RFC3484 [10]. A multihomed
host may thus have two addresses, one per prefix (provider), and
select source and destination addresses to use as described in that
RFC.
2.2. Unique Local IPv6 Addresses
ULAs have replaced the originally conceived Site Local addresses in
the IPv6 addressing architecture, for reasons described in [17].
ULAs improve on site locals by offering a high probability of the
global uniqueness of the prefix used, which can be beneficial in the
case of (deliberate or accidental) leakage, or where networks are
merged. ULAs are akin to the private address space [1] assigned for
IPv4 networks.
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The ULA address range allows a network administrator to deploy IPv6
addresses on their network without asking for a globally unique
registered IPv6 address range. A ULA prefix is 48 bits, i.e. a /48,
the same as the currently recommended allocation for a site from the
globally routable IPv6 address space [7].
ULAs provide the means to deploy a fixed addressing scheme that is
not affected by a change in service provider and the corresponding PA
global addresses. Internal operation of the network is thus
unaffected during renumbering events. Nevertheless, this type of
address must be used with caution.
A site using ULAs may or may not also deploy globals. In an isolated
network ULAs may be deployed on their own. In a connected network,
that also deploys global addresses, both may be deployed, such that
hosts become multiaddressed (one global and one ULA address) and the
IPv6 default address selection algorithm will pick the appropriate
source and destination addresses to use, e.g. ULAs will be selected
where both the source and destination hosts have ULA addresses.
Because a ULA and a global site prefix are both /48 length, an
administrator can choose to use the same subnetting (and host
addressing) plan for both prefixes.
As an example of the problems ULAs may cause, when using IPv6
multicast within the network, the IPv6 default address selection
algorithm prefers the ULA address as the source address for the IPv6
multicast streams. This is NOT a valid option when sending an IPv6
multicast stream to the IPv6 Internet for two reasons. For one,
these addresses are not globally routable so RPF checks for such
traffic will fail outside the internal network. The other reason is
that the traffic will likely not cross the network boundary due to
multicast domain control and perimeter security policies.
In principal ULAs allow easier network mergers than RFC1918 addresses
do for IPv4 because ULA prefixes have a high probability of
uniqueness, if the prefix is chosen as described in the RFC.
The usage of ULAs should be carefully considered even when not
attached to the IPv6 Internet due to the potential for added
complexity when connecting to the Internet at some point in the
future.
2.3. 6Bone Address Space
The 6Bone address space was used before the RIRs started to
distribute 'production' IPv6 prefixes. The 6Bone prefixes have a
common first 16 bits in the IPv6 Prefix of 3FFE::/16. This address
range is deprecated as of 6th June 2006 [15] and should be avoided on
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any new IPv6 network deployments. Sites using 6bone address space
should renumber to production address space using procedures as
defined in [21].
2.4. Network Level Design Considerations
IPv6 provides network administrators with a significantly larger
address space, enabling them to be very creative in how they can
define logical and practical address plans. The subnetting of
assigned prefixes can be done based on various logical schemes that
involve factors such as:
o Geographical Boundaries - by assigning a common prefix to all
subnets within a geographical area.
o Organizational Boundaries - by assigning a common prefix to an
entire organization or group within a corporate infrastructure.
o Service Type - by reserving certain prefixes for predefined
services such as: VoIP, Content Distribution, wireless services,
Internet Access, etc.
Such logical addressing plans have the potential to simplify network
operations and service offerings, and to simplify network management
and troubleshooting. A very large network would also have no need to
consider using private address space for its infrastructure devices,
simplifying network management.
The network designer must however keep in mind several factors when
developing these new addressing schemes:
o Prefix Aggregation - The larger IPv6 addresses can lead to larger
routing tables unless network designers are actively pursuing
aggregation. While prefix aggregation will be enforced by the
service provider, it is beneficial for the individual
organizations to observe the same principles in their network
design process.
o Network growth - The allocation mechanism for flexible growth of a
network prefix, documented in RFC3531 [11] can be used to allow
the network infrastructure to grow and be numbered in a way that
is likely to preserve aggregation (the plan leaves 'holes' for
growth).
o ULA usage in large networks - Networks which have a large number
of 'sites' that each deploy a ULA prefix which will by default be
a 'random' /48 under fc00::/7 will have no aggregation of those
prefixes. Thus the end result may be cumbersome because the
network will have large amounts of non-aggregated ULA prefixes.
However, there is no rule to disallow large networks to use a
single ULA for all 'sites', as a ULA still provides 16 bits for
subnetting to be used internally.
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2.4.1. Sizing the Network Allocation
We do not discuss here how a network designer sizes their application
for address space. By default a site will receive a /48 prefix [7].
The default provider allocation via the RIRs is currently a /32 [27].
These allocations are indicators for a first allocation for a
network. Different sizes may be obtained based on the anticipated
address usage [27]. There are examples of allocations as large as
/19 having been made from RIRs to providers at the time of writing.
2.4.2. Address Space Conservation
Despite the large IPv6 address space which enables easier subnetting,
it still is important to ensure an efficient use of this resource.
Some addressing schemes, while facilitating aggregation and
management, could lead to significant numbers of addresses being
unused. Address conservation requirements are less stringent in IPv6
but they should still be observed.
The proposed HD [8] value for IPv6 is 0.94 compared to the current
value of 0.96 for IPv4. Note that for IPv6 HD is calculated for
sites (i.e. on a basis of /48), instead of based on addresses like
with IPv4.
3. Subnet Prefix Considerations
This section analyzes the considerations applied to define the subnet
prefix of the IPv6 addresses. The boundaries of the subnet prefix
allocation are specified in RFC4291 [23]. In this document we
analyze their practical implications. Based on RFC4291 [23] it is
legal for any IPv6 unicast address starting with binary address '000'
to have a subnet prefix larger than, smaller than or of equal to 64
bits. Each of these three options is discussed in this document.
3.1. Considerations for subnet prefixes shorter then /64
An allocation of a prefix shorter then 64 bits to a node or interface
is bad practice. The shortest subnet prefix that could theoretically
be assigned to an interface or node is limited by the size of the
network prefix allocated to the organization.
A possible reason for choosing the subnet prefix for an interface
shorter then /64 is that it would allow more nodes to be attached to
that interface compared to a prescribed length of 64 bits. This
however is unnecessary considering that 2^64 provides plenty of node
addresses for a well designed IPv6 network. Layer two technologies
are unlikely to support such large numbers of nodes within a single
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link (e.g. Ethernet limited to 48-bits of hosts)
The subnet prefix assignments can be made either by manual
configuration, by a stateful Host Configuration Protocol [9] or by a
stateful prefix delegation mechanism [14].
3.2. Considerations for /64 prefixes
Based on RFC3177 [7], 64 bits is the prescribed subnet prefix length
to allocate to interfaces and nodes.
When using a /64 subnet length, the address assignment for these
addresses can be made either by manual configuration, by a stateful
Host Configuration Protocol [9] [16] or by stateless
autoconfiguration [2].
Note that RFC3177 strongly prescribes 64 bit subnets for general
usage, and that stateless autoconfiguration option is only defined
for 64 bit subnets.
3.3. Considerations for subnet prefixes longer then /64
Address space conservation is the main motivation for using a subnet
prefix length longer than 64 bits.
The address assignment can be made either by manual configuration or
by a stateful Host Configuration Protocol [9].
When assigning a subnet prefix of more then 80 bits, according to
RFC4291 [23] "u" and "g" bits (respectively the 81st and 82nd bit)
need to be taken into consideration and should be set correctly. In
currently implemented IPv6 protocol stacks, the relevance of the "u"
(universal/local) bit and "g" (the individual/group) bit are marginal
and typically will not show an issue when configured wrongly, however
future implementations may turn out differently.
When using subnet lengths longer then 64 bits, it is important to
avoid selecting addresses that may have a predefined use and could
confuse IPv6 protocol stacks. The alternate usage may not be a
simple unicast address in all cases. The following points should be
considered when selecting a subnet length longer then 64 bits.
3.3.1. Anycast addresses
3.3.1.1. Subnet Router Anycast Address
RFC4291 [23] provides a definition for the required Subnet Router
Anycast Address as follows:
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| n bits | 128-n bits |
+--------------------------------------------+----------------+
| subnet prefix | 00000000000000 |
+--------------------------------------------+----------------+
It is recommended to avoid allocating this IPv6 address to a device
which is not a router. No additional dependencies for the subnet
prefix while the EUI-64 and an IID dependencies will be discussed
later in this document.
3.3.1.2. Reserved IPv6 Subnet Anycast Addresses
RFC2526 [4] stated that within each subnet, the highest 128 interface
identifier values are reserved for assignment as subnet anycast
addresses.
The construction of a reserved subnet anycast address depends on the
type of IPv6 addresses used within the subnet, as indicated by the
format prefix in the addresses.
The first type of Subnet Router Anycast addresses have been defined
as follows for EUI-64 format:
| 64 bits | 57 bits | 7 bits |
+------------------------------+------------------+------------+
| subnet prefix | 1111110111...111 | anycast ID |
+------------------------------+------------------+------------+
The anycast address structure implies that it is important to avoid
creating a subnet prefix where the bits 65 to 121 are defined as
"1111110111...111" (57 bits in total) so that confusion can be
avoided.
For other IPv6 address types (that is, with format prefixes other
than those listed above), the interface identifier is not in EUI-64
format and may be other than 64 bits in length; these reserved subnet
anycast addresses for such address types are constructed as follows:
| n bits | 121-n bits | 7 bits |
+------------------------------+------------------+------------+
| subnet prefix | 1111111...111111 | anycast ID |
+------------------------------+------------------+------------+
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| interface identifier field |
In the case discussed above there is no additional dependency for the
subnet prefix with the exception of the EUI-64 and an IID dependency.
These will be discussed later in this document.
3.3.2. Addresses used by Embedded-RP (RFC3956)
Embedded-RP [18] reflects the concept of integrating the Rendezvous
Point (RP) IPv6 address into the IPv6 multicast group address. Due
to this embedding and the fact that the length of the IPv6 address
AND the IPv6 multicast address are 128 bits, it is not possible to
have the complete IPv6 address of the multicast RP embedded as such.
This resulted in a restriction of 15 possible RP-addresses per prefix
that can be used with embedded-RP. The space assigned for the
embedded-RP is based on the 4 low order bits, while the remainder of
the Interface ID is set to all '0'.
[IPv6-prefix (64 bits)][60 bits all '0'][RIID]
Where: [RIID] = 4 bit.
This format implies that when selecting subnet prefixes longer then
64, and the bits beyond the 64th one are none-zero, the subnet can
not use embedded-RP.
In addition it is discouraged to assign a matching embedded-RP IPv6
address to a device that is not a real Multicast Rendezvous Point.
3.3.3. ISATAP addresses
ISATAP [25] is an automatic tunneling protocol used to provide IPv6
connectivity over an IPv4 campus or enterprise environment. In order
to leverage the underlying IPv4 infrastructure, the IPv6 addresses
are constructed in a special format.
An IPv6 ISATAP [25] address has the IPv4 address embedded, based on a
predefined structure policy that identifies them as an ISATAP [25]
address.
[IPv6 Prefix (64 bits)][0000:5EFE][IPv4 address]
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When using subnet prefix length longer then 64 bits it is recommended
that that the portion of the IPv6 prefix from bit 65 to the end of
the subnet prefix does not match with the well-known ISATAP [0000:
5EFE] address portion.
In its actual definition there is no multicast support on ISATAP
3.3.4. /126 addresses
The 126 bit subnet prefixes are typically used for point-to-point
links similar to the RFC3021 [5] recommendations for IPv4. The usage
of this subnet address length does not lead to any additional
considerations other than the ones discussed earlier in this section,
particularly those related to the "u" and "g" bits.
3.3.5. /127 addresses
The usage of the /127 addresses is not valid and should be strongly
discouraged as documented in RFC3627 [13].
3.3.6. /128 addresses
The 128 bit address prefix may be used in those situations where we
know that one, and only one address is sufficient. Example usage
would be the off-link loopback address of a network device.
When choosing a 128 bit prefix, it is recommended to take the "u" and
"g" bits into consideration and to make sure that there is no overlap
with either the following well-known addresses:
o Subnet Router Anycast Address
o Reserved Subnet Anycast Address
o Addresses used by Embedded-RP
o ISATAP Addresses
4. Allocation of the IID of an IPv6 Address
In order to have a complete IPv6 address, an interface must be
associated a prefix and an Interface Identifier (IID). Section 3 of
this document analyzed the prefix selection considerations. This
section discusses the elements that should be considered when
assigning the IID portion of the IPv6 address.
There are various ways to allocate an IPv6 address to a device or
interface. The option with the least amount of caveats for the
network administrator is that of EUI-64 [2] based addresses. For the
manual or dynamic options, the overlap with well known IPv6 addresses
should be avoided.
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4.1. Automatic EUI-64 Format Option
When using this method the network administrator has to allocate a
valid 64 bit subnet prefix. The EUI-64 [2] allocation procedure can
from that moment onwards assign the remaining 64 IID bits in a
stateless manner. All the considerations for selecting a valid IID
have been incorporated in the EUI-64 methodology.
4.2. Using Privacy Extensions
The main purpose of IIDs generated based on RFC3041 [6] is to provide
privacy to the entity using this address. While there are no
particular constraints in the usage of these addresses as defined in
[6] there are some implications to be aware of when using privacy
addresses as documented in section 4 of RFC3041 [6]:
o The privacy extension algorithm may complicate flexibility in
future transport protocols
o These addresses may add complexity to the operational management
and troubleshooting of the infrastructure (i.e. which address
belongs to which real host)
o A reverse DNS lookup check may be broken when using privacy
extensions
4.3. Cryptographically Generated IPv6 Addresses
Cryptographically Generated Addresses (CGAs) are based upon RFC3972
[20] and provide a method for binding a public signature key to an
IPv6 address in the Secure Neighbor Discovery (SEND) protocol [19].
The basic idea is to generate the interface identifier (i.e. the
rightmost 64 bits) of the IPv6 address by computing a cryptographic
hash of the public key. The resulting IPv6 address is called a
cryptographically generated address (CGA). The corresponding private
key can then be used to sign messages sent from that address.
Implications to be aware of when using CGA addresses are found in
section 7 of RFC3972 [20]:
o When using CGA addresses the values of the "u" and "g" bits are
ignored however it does not add any security or implementation
implications
o There is no mechanism for proving that an address is not a CGA
o When it is discovered that a node has been compromised, a new
signature key and a new CGA should be generated
Due to the fact that CGA generated addresses are almost
indistinguishable from a privacy address and has similar properties
for many purposes, the same considerations as with privacy addresses
are also valid for CGA generated addresses.
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4.4. Manual/Dynamic Assignment Option
This section discusses those IID allocations that are not implemented
through stateless address configuration (Section 4.1). They are
applicable regardless of the prefix length used on the link. It is
out of scope for this section to discuss the various assignment
methods (e.g. manual configuration, DHCPv6, etc).
In this situation the actual allocation is done by human intervention
and consideration needs to be given to the complete IPv6 address so
that it does not result in overlaps with any of the well known IPv6
addresses:
o Subnet Router Anycast Address
o Reserved Subnet Anycast Address
o Addresses used by Embedded-RP
o ISATAP Addresses
When using an address assigned by human intervention it is
recommended to choose IPv6 addresses which are not obvious to guess
and/or avoid any IPv6 addresses that embed IPv4 addresses used in the
current infrastructure. Following these two recommendations will
make it more difficult for malicious third parties to guess targets
for attack, and thus reduce security threats to a certain extent.
5. Case Studies
tbc.
5.1. Enterprise Considerations
In this section we consider a case study of a campus network that is
deploying IPv6 in parallel with existing IPv4 protocols in a dual-
stack environment. The specific example is the University of
Southampton (UK). The case study is a 'work in progress' as the
deployment is an evolving one, currently covering around 1,500 hosts.
5.1.1. Obtaining general IPv6 network prefixes
In the case of a campus network, the site will typically take its
connectivity from its National Research and Education Network (NREN).
Southampton connects to JANET, the UK academic network. JANET
currently has a /32 allocation from RIPE of 2001:630::/32. The
current recommended practice is for sites to receive a /48
allocation, and on this basis Southampton has received such a prefix
for its own use, specifically 2001:630:d0::/48.
No ULA addressing is used on site. The campus does not expect to
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change service provider, and thus does not plan to use ULAs for the
(perceived) benefit of easing network renumbering. Indeed, the
campus has renumbered following the aforementioned renumbering
procedure [21] on two occasions, and this has proven adequate (with
provisos documented in [28]. We also do not see any need to deploy
ULAs for in or out of band network management; there are enough IPv6
prefixes available in the site allocation for the infrastructure.
No 6bone addressing is used on site. This was phased out some time
ago. We note that as of 6th June 2006 transit ISPs will likely
filter any attempted use of such prefixes.
Southampton does participate in global and organization scope IPv6
multicast networks. Multicast address allocations are not discussed
here as they are not in scope for the document. Embedded RP is in
use, and has been tested successfully across providers between sites.
5.1.2. Forming an address (subnet) allocation plan
The campus has a /16 prefix for IPv4 use; in principle 256 subnets of
256 addresses. In reality the subnetting is muddier, because of
concerns of IPv4 address conservation; subnets are sized to the hosts
within them, e.g. a /26 IPv4 prefix is used if a subnet has 35 hosts
in it. While this is efficient, it increases management burden when
physical deployments change, and IPv4 subnets require resizing (up or
down), even with DHCP in use.
The /48 IPv6 prefix is considerably larger than the IPv4 allocation
already in place at the site. It is loosely equivalent to a 'Class
A' IPv4 prefix in that it has 2^16 (over 65,000) subnets, but has an
effectively unlimited subnet address size (2^64) compared to 256 in
the IPv4 equivalent. The increased subnet size means that /64 IPv6
prefixes can be used on all subnets, without any requirement to
resize them at a later date. The increased subnet volume allows
subnets to be allocated more generously to schools and departments in
the campus. While address conservation is still important, it is no
longer an impediment on network management. Rather, address (subnet)
allocation is more about planning for future expansion.
In a dual-stack network, we chose to deploy our IP subnets
congruently for IPv4 and IPv6. This is because the systems are still
in the same administrative domains and the same geography. We do not
expect to have IPv6-only subnets in production use for a while yet,
outside test beds and our early Mobile IPv6 trials. The firewall
would ideally be a single dual-stack device with consistent policies
(by host rather than IP version), however this is currently
implemented as a firewall per IP protocol due to vendor limitations
(Nokia/Checkpoint for IPv4, BSD pf tool for IPv6).
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The subnet allocation plan required a division of the address space
per school or department. Here a /56 was allocated to the school
level of the university; there are around 30 schools currently.
Further allocations were made for central IT infrastructure, for the
network infrastructure and the server side systems.
5.1.3. Other considerations
The network uses a Demilitarized Zone (DMZ) topology for some level
of protection of 'public' systems. Again, this topology is congruent
with the IPv4 network.
There are no specific transition methods deployed internally to the
campus; everything is using the conventional dual-stack approach.
There is no use of tools such as ISATAP for example.
For the Mobile IPv6 early trails, we have allocated one prefix for
Home Agent (HA) use. We have not yet considered how Mobile IPv6
usage may grow, and whether more or even every subnet will require HA
support.
The university operates a tunnel broker service on behalf of UKERNA.
This uses separate address space from JANET, not the main university
allocation.
5.1.4. Node configuration considerations
We currently use stateless autoconfiguration on most subnets for IPv6
hosts. There is no DHCPv6 service deployed yet, beyond tests of
early code releases. We do seek a common integrated DHCP/DNS
management platform, even if the servers themselves are not co-
located. Currently we add client statelessly autoconfigured
addresses to the DNS manually. Our administrators would prefer the
use of DHCP because they believe it gives them some management
control.
Regarding the [26] implications, we note that all our hosts are dual-
stack, and thus are potentially exposed over both protocols anyway.
We publish all addresses in DNS, and do not operate a two faced DNS.
We have internal usage of RFC3041 privacy addresses currently, but
may wish to administratively disable this (perhaps via DHCP), but we
need to determine the feasibility of this on all systems, e.g. for
WLAN guests or other user-maintained systems. Network management
should be simpler without RFC3041 in operation. Note RFC3041 is only
an issue for outbound connections.
We manually configure server addresses to avoid address changes on a
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change of network adaptor. With IPv6 you can choose to pick ::53 for
a DNS server, or can pick 'random' addresses for obfuscation, though
that's not an issue for publicly advertised addresses (dns, mx, web,
etc).
5.1.5. Observations
The site is not (yet) using prefix delegation tools for IPv6.
5.2. Service Provider Considerations
In this section an IPv6 addressing schema is sketched that could
serve as an example by a Service Provider offering Internet Services
as well as Network Access services to millions of customers. In this
example, the Service Provider is assumed to operate an MPLS based
backbone and implements 6PE to provide IPv6 backbone transport
between the different locations (POPs) of a fully dual-stacked
network access and aggregation area.
Besides that it is assumed that the Service Provider
o has received a /20 from its RIR
o operates its own LIR
o has to address its own IPv6 infrastructure
o delegates prefixes from this aggregate to its customers.
Hence this addressing schema covers the numbering of the Service
Provider IPv6 network devices as well the customer aggregates chosen
out of the /20 prefix of the SP.
5.2.1. Investigation of objective Requirements for an IPv6 addressing
schema of a Service Provider
The first step of the IPv6 addressing plan design for a Service
provider should be the identification of all technical, operational,
political and business requirements that have to be satisfied by the
services supported by this addressing schema.
According to the different technical constraints and business models
as well as the different weights of these requirements (from the
point of view of the corresponding Service Provider) it is very
likely that different addressing schemas will be developed and
deployed by different ISPs. Nevertheless the addressing schema of
this section is one possible example.
5.2.1.1. Requirements for an IPv6 addressing schema from the LIR
perspective of the Service Provider
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In their role as LIR the Service Providers have to care about the
policy constraints of the RIRs and the standards of the IETF
regarding IPv6 addressing. In this context, the following basic
requirements and recommendations have to be taken into account and
should be satisfied by the IPv6 address allocation plan of a Service
Provider:
o As recommended in RFC 3177 [7] and in several RIR policies
"Common" customers sites (normally private customers) should
receive a /48 prefix from the aggregate of the Service Provider.
(Note: The addressing plan must be flexible enough and take into
account the possible change of the minimum allocation size for end
users currently under definition by the RIRs.)
o "Big customers" (like big enterprises, governmental agencies etc.)
may receive shorter prefixes according to their needs when this
need could be documented and justified to the RIR.
o The IPv6 address allocation schema has to be able to meet the HD-
ratio of 0.94 as it is defined for IPv6. This requirement
corresponds to the demand for an efficient usage of the IPv6
address aggregate by the Service Provider. (Note: A HD-ratio of
0.94 means an effective usage of about 31% of the /20 of the
Service Provider on the basis of /48 assignments.)
o All assignments to customers have to be documented and stored into
a database that can also be queried by the RIR.
o The LIR has to make available means for supporting the reverse DNS
mapping of the customer prefixes.
5.2.1.2. IPv6 addressing schema requirements from the ISP perspective
of the Service Provider
From ISP perspective the following basic requirements could be
identified:
o The IPv6 address allocation schema must be able to realize a
maximal aggregation of all IPv6 address delegations to customers
into the /20 of the Service Provider. Only this /20 will be
routed and injected from the Service Provider into the global
routing table (DFZ). This strong aggregation keeps the routing
tables of the DFZ small and eases filtering and access control
very much. (Note: A strong aggregation e.g. on POP or LER basis
limits as well the numbers of customer routes that are visible
within the ISP network.)
o The IPv6 addressing schema of the SP should contain maximal
flexibility since the infrastructure of the SP will change over
the time with new customers, transport technologies and business
cases. The requirement of maximal flexibility is contrary to the
requirements of strong IPv6 address aggregation and efficient
address usage, but at this point each SP has to decide which of
these requirements to prioritize.
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o Keeping the multilevel network hierarchy of an ISP in mind, due to
addressing efficiency reasons not all hierarchy levels can and
should be mapped into the IPv6 addressing schema of an ISP.
Sometimes it is much better to implement "flat" addressing for the
ISP network than to loose big chunks of the IPv6 address aggregate
in addressing each level of network hierarchy. Besides that a
decoupling of provider network addressing and customer addressing
is recommended.
5.2.1.3. IPv6 addressing schema requirements from the Network Access
provider perspective of the Service Provider
As already done for the LIR and the ISP roles of the SP it is also
necessary to identify requirements that come from its Network Access
Provider role. Some of the basic requirements are:
o The IPv6 addressing schema of the SP must be flexible enough to
adapt changes that are injected from the customer side. This
covers changes that are based on the raising IPv6 address needs of
the customer as well as changes that come from topological
modifications (e.g. when the customer moves from one point of
network attachment (POP) to another).
o For each IPv6 address assignment to customers a "buffer zone" must
be reserved that allows the customer to grow in its addressing
range without renumbering or assignment of additional prefixes.
o The IPv6 addressing schema of the SP must deal with multiple-
attachments of a single customer to the SP network infrastructure
(i.e. multi-homed network access with the same SP).
These few requirements are only part of all the requirements a
Service Provider has to investigate and keep in mind during the
definition phase of its addressing architecture. Each SP will most
likely add more constraints to this list.
5.2.2. IPv6 address allocation plan for a Service Provider
This section illustrates how the /20 IPv6 prefix of the SP can be
used to address the SP-own infrastructure and to delegate IPv6
prefixes to its customers following the above mentioned requirements
as far as possible.
The below figure summarizes the device types in an SP network and the
typical network design. The network hierarchy of the SP has to be
taken into account for the design of an IPv6 addressing schema and
defines its basic shape.
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+------------------------------------------------------------------+
| LSRs of the MPLS Backbone of the SP |
+------------------------------------------------------------------+
| | | | |
| | | | |
+-----+ +-----+ +--------+ +--------+ +--------+
| LER | | LER | | LER-BB | | LER-BB | | LER-BB |
+-----+ +-----+ +--------+ +--------+ +--------+
| | | | | | / | | |
| | | | | | / | | |
| | | | +------+ +------+ +------+ | |
| | | | |BB-RAR| |BB-RAR| | AG | | |
| | | | +------+ +------+ +------+ | |
| | | | | | | | | | | |
| | | | | | | | | | | |
| | | | | | | | +-----+ +-----+ +-----+ +-----+
| | | | | | | | | RAR | | RAR | | RAR | | RAR |
| | | | | | | | +-----+ +-----+ +-----+ +-----+
| | | | | | | | | | | | | | | |
| | | | | | | | | | | | | | | |
+-------------------------------------------------------------------+
| Customer networks |
+-------------------------------------------------------------------+
Figure: Exemplary Service Provider Network
LSR ... Label Switch Router
LER ... Label Edge Router
LER-BB ... Broadband Label Edge Router
RAR ... Remote Access Router
BB-RAR ... Broadband Remote Access Router
AG ... Aggregation Router
Basic design decisions for the SP IPv6 address plan regarding
customer prefixes take into consideration:
o The prefixes assigned to all customers behind the same LER (e.g.
LER or LER-BB) are aggregated under one prefix. This ensures that
the number of labels that have to be used for 6PE is limited and
hence provides a strong MPLS label conservation.
o The /20 prefix of the SP is separated into 3 different pools that
are used to allocate IPv6 prefixes to the customers of the SP:
* A pool (e.g. /24) for satisfying the addressing needs of real
"big" customers (as defined in 5.2.2.1 sub-section A.) that
need IPv6 prefixes larger than /48 (e.g. /32). These customers
are assumed to be connected to several POPs of the access
network, so that this customer prefix will be visible in each
of these POPs.
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* A pool (e.g. /24) for the LERs with direct customer connections
(e.g. dedicated line access) and without an additional
aggregation area between the customer and the LER. (These LERs
are mostly connected to a limited number of customers because
of the limited number of interfaces/ports.)
* A larger pool (e.g. 14*/24) for LERs (e.g. LER-BB) that serve
a high number of customers that are normally connected via some
kind of aggregation network (e.g. DSL customers behind a BB-
RAR or Dial-In customers behind a RAR).
* The IPv6 address delegation within each Pool (end customer
delegation or also the aggregates that are dedicated to the
LERs itself) should be chosen with an additional buffer zone of
300% for future growth.
5.2.2.1. Defining an IPv6 address allocation plan for customers of the
Service Provider
5.2.2.1.1. 'Big' customers
SP's "big" customers receive their prefix from the /24 IPv6 address
aggregate that has been reserved for their "big" customers. A
customer is considered as "big" customer if it has a very complex
network infrastructure and/or huge IPv6 address needs (e.g. because
of very large customer numbers) and/or several uplinks to different
POPs of the SP network.
The assigned IPv6 address prefixes can have a prefix length in the
range 32-48 and for each assignment a 300% future growing zone is
marked as "reserved" for this customer. This means that for instance
with a delegation of a /34 to a customer the /32 that contains this
/34 is reserved for the customer for future usage.
The prefixes for the "big" customers can be chosen from the
corresponding LER customer pool by either using an equidistant
algorithm or using mechanisms simililar to the Sparse Alocation
Algorithm (SAA) [27].
5.2.2.1.2. 'Common' customers
All customers that are not "big" customers are considered as "common"
customers. They represent the majority of customers hence they
receive a /48 out of the IPv6 customer address pool of the LER where
they are directly connected or aggregated.
Again a 300% future growing IPv6 address range is reserved for each
customer, so that a "common" customer receives a /48 allocation but
has a /46 reserved.
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In the network access scenarios where the customer is directly
connected to the LER the customer prefix is directly taken out of the
customer IPv6 address aggregate (e.g. /38) of the corresponding LER.
In all other cases (e.g. the customer is attached to a RAR that is
themselves aggregated to an AG or to a LER) at least 2 different
approaches are possible.
1.) Mapping of Aggregation Network Hierarchy into Customer IPv6
Addressing Schema. The aggregation network hierarchy could be mapped
into the design of the customer prefix pools of each network level in
order to achieve a maximal aggregation at the LER level as well as at
the intermediate levels. (Example: Customer - /48, RAR - /38, AG -
/32, LER-BB - /30). At each network level an adequate growing zone
should be reserved. (Note: This approach requires of course some
"fine tuning" of the addressing schema based on a very good knowledge
of the Service Provider network topology including actual growing
ranges and rates.)
When the IPv6 customer address pool of a LER (or another device of
the aggregation network - AG or RAR) is exhausted, the related LER
(or AG or RAR) prefix is shortened by 1 or 2 bits (e.g. from /38 to
/37 or /36) so that the originally reserved growing zone can be used
for further IPv6 address allocations to customers. In the case where
the growing zone is exhausted as well a new prefix range from the
corresponding pool of the next higher hierarchy level can be
requested.
2.) "Flat" Customer IPv6 Addressing Schema. The other option is to
allocate all the customer prefixes directly out of the customer IPv6
address pool of the LER where the customers are attached and
aggregated and ignore the intermediate aggregation network
infrastructure. This approach leads of course to a higher amount of
customer routes at LER and aggregation network level but takes a
great amount of complexity out of the addressing schema.
Nevertheless the aggregation of the customer prefixes to one prefix
at LER level is realized as required above.
If the actual observed growing rates show that the reserved growing
zones are not needed than these growing areas can be freed and used
for assignments for prefix pools to other devices at the same level
of the network hierarchy.
5.2.2.2. Defining an IPv6 address allocation plan for the Service
Provider Network Infrastructure
For the IPv6 addressing of SPs own network infrastructure a /32 (or
/40) from the "big" customers address pool can be chosen.
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This SP infrastructure prefix is used to code the network
infrastructure of the SP by assigning a /48 to every POP/location and
using for instance a /56 for coding the corresponding router within
this POP. Each SP internal link behind a router interface could be
coded using a /64 prefix. (Note: While it is suggested to chose a
/48 for addressing the POP/location of the SP network it is left to
each SP to decide what prefix length to assign to the routers and
links within this POP.)
The IIDs of the router interfaces may be generated by using EUI-64 or
through plain manual configuration e.g. for coding additional network
or operational information into the IID.
It is assumed that a 300% growing zones for each level of network
hierarchy and additional prefixes may be assigned to POPs and/or
routers if needed.
Loopback interfaces of routers may be chosen from the first /64 of
the /56 router prefix (in the example above).
(Note: The /32 prefix that has been chosen for addressing SPs own
IPv6 network infrastructure gives enough place to code additional
functionalities like security levels or private and test
infrastructure although such approaches haven't been considered in
more detail for the above described SP until now.)
Point-to-point links to customers (e.g. PPP links, dedicated line
etc.) may be addressed using /126 prefixes out of the first /64 of
the access routers that could be reserved for this reason.
5.2.3. Additional Remarks
5.2.3.1. ULA
From the actual view point of SP there is no compelling reason why
ULAs should be used from a SP. Look at section 2.2.
ULAs could be used inside the SP network in order to have an
additional "site-local scoped" IPv6 address for SPs own
infrastructure for instance for network management reasons and maybe
also in order to have an addressing schema that couldn't be reached
from outside the SP network.
In the case when ULAs are used it is possible to map the proposed
internal IPv6 addressing of SPs own network infrastructure as
described in 5.2.2.2 above directly to the ULA addressing schema by
substituting the /48 POP prefix with a /48 ULA site prefix.
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5.2.3.2. Multicast
IPv6 Multicast-related addressing issues are out of the scope of this
document.
5.2.3.3. POP Multi-homing or Change of POP
POP (or better LER) Multi-homing of customers with the same SP can be
realized within the proposed IPv6 addressing schema of the SP by
assigning multiple LER-dependent prefixes to this customer (i.e.
considering each customer location as a single-standing customer) or
by choosing a customer prefix out of the pool of "big" customers.
The second solution has the disadvantage that in every LER where the
customer is attached this prefix will appear inside the IGP routing
table requiring an explicit MPLS label.
An equal effect happens when a customer changes its point of
attachment to another POP/LER since in this case the customer prefix
could not be aggregated into the LER prefix and needs to be
advertised more specific in the IGP.
(Note: The described negative POP/LER Multi-homing effects to the
addressing architecture in the SP access network are not tackled by
implementing the Shim6 Site Multi-homing approach since this approach
targets only on a mechanism for dealing with multiple prefixes in end
systems -- the SP will nevertheless have unaggregated customer
prefixes in its internal routing tables.)
5.2.3.4. Extensions needed for the later IPv6 migration phases
The proposed IPv6 addressing schema for a SP needs some slight
enhancements / modifications for the later phases of IPv6
integration, for instance in the case when the whole MPLS backbone
infrastructure (LDP, IGP etc.) is realized over IPv6 transport an
addressing of the LSRs is needed. Other changes may be necessary as
well but should not be explained at this point.
6. IANA Considerations
There are no extra IANA consideration for this document.
7. Security Considerations
This IPv6 addressing documents does not have any direct impact on
Internet infrastructure security.
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8. Acknowledgements
Constructive feedback and contributions have been received from Stig
Venaas, Pekka Savola, John Spencer, Patrick Grossetete and Carlos
Garcia Braschi.
9. References
9.1. Normative References
9.2. Informative References
[1] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and E.
Lear, "Address Allocation for Private Internets", BCP 5,
RFC 1918, February 1996.
[2] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[3] Hinden, R., Fink, R., and J. Postel, "IPv6 Testing Address
Allocation", RFC 2471, December 1998.
[4] Johnson, D. and S. Deering, "Reserved IPv6 Subnet Anycast
Addresses", RFC 2526, March 1999.
[5] Retana, A., White, R., Fuller, V., and D. McPherson, "Using 31-
Bit Prefixes on IPv4 Point-to-Point Links", RFC 3021,
December 2000.
[6] Narten, T. and R. Draves, "Privacy Extensions for Stateless
Address Autoconfiguration in IPv6", RFC 3041, January 2001.
[7] IAB and IESG, "IAB/IESG Recommendations on IPv6 Address
Allocations to Sites", RFC 3177, September 2001.
[8] Durand, A. and C. Huitema, "The H-Density Ratio for Address
Assignment Efficiency An Update on the H ratio", RFC 3194,
November 2001.
[9] Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C., and M.
Carney, "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
[10] Draves, R., "Default Address Selection for Internet Protocol
version 6 (IPv6)", RFC 3484, February 2003.
[11] Blanchet, M., "A Flexible Method for Managing the Assignment of
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Bits of an IPv6 Address Block", RFC 3531, April 2003.
[12] Hinden, R., Deering, S., and E. Nordmark, "IPv6 Global Unicast
Address Format", RFC 3587, August 2003.
[13] Savola, P., "Use of /127 Prefix Length Between Routers
Considered Harmful", RFC 3627, September 2003.
[14] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic Host
Configuration Protocol (DHCP) version 6", RFC 3633,
December 2003.
[15] Fink, R. and R. Hinden, "6bone (IPv6 Testing Address
Allocation) Phaseout", RFC 3701, March 2004.
[16] Droms, R., "Stateless Dynamic Host Configuration Protocol
(DHCP) Service for IPv6", RFC 3736, April 2004.
[17] Huitema, C. and B. Carpenter, "Deprecating Site Local
Addresses", RFC 3879, September 2004.
[18] Savola, P. and B. Haberman, "Embedding the Rendezvous Point
(RP) Address in an IPv6 Multicast Address", RFC 3956,
November 2004.
[19] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[20] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
[21] Baker, F., Lear, E., and R. Droms, "Procedures for Renumbering
an IPv6 Network without a Flag Day", RFC 4192, September 2005.
[22] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
[23] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, February 2006.
[24] Templin, F., Gleeson, T., Talwar, M., and D. Thaler, "Intra-
Site Automatic Tunnel Addressing Protocol (ISATAP)",
draft-ietf-ngtrans-isatap-24 (work in progress), January 2005.
[25] Templin, F., Gleeson, T., Talwar, M., and D. Thaler, "Intra-
Site Automatic Tunnel Addressing Protocol
(draft-ietf-ngtrans-isatap-24.txt)", July 2005.
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[26] Chown, T., "IPv6 Implications for TCP/UDP Port Scanning (chown-
v6ops- port-scanning-implications-02.txt)", October 2005.
[27] APNIC, ARIN, RIPE NCC, "IPv6 Address Allocation and Assignment
Policy (www.ripe.net/ripe/docs/ipv6policy.html)", January 2003.
[28] Chown, T., Thompson, M., Ford, A., and S. Venaas, "Things to
think about when Renumbering an IPv6 network
(draft-chown-v6ops-renumber-thinkabout-03.txt)", July 2005.
[29] Paul Wilson, Raymond Plzak, Axel Pawlik, "IPv6 Address Space
Management (www.ripe.net/ripe/docs/ipv6-sparse.html)",
February 2005.
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Authors' Addresses
Gunter Van de Velde
Cisco Systems
De Kleetlaan 6a
Diegem 1831
Belgium
Phone: +32 2704 5473
Email: gunter@cisco.com
Ciprian Popoviciu
Cisco Systems
7025-6 Kit Creek Road
Research Triangle Park, North Carolina PO Box 14987
USA
Phone: +1 919 392-3723
Email: cpopovic@cisco.com
Tim Chown
University of Southampton
Highfield
Southampton, SO17 1BJ
United Kingdom
Phone: +44 23 8059 3257
Email: tjc@ecs.soton.ac.uk
Olaf Bonness
T-Systems Enterprise Services GmbH
Goslarer Ufer 35
Berlin, 10589
Germany
Phone: +49 30 3497 3124
Email: Olaf.Bonness@t-systems.com
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Christian Hahn
T-Systems Enterprise Services GmbH
Goslarer Ufer 35
Berlin, 10589
Germany
Phone: +49 30 3497 3164
Email: HahnC@t-systems.com
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Van de Velde, et al. Expires December 3, 2006 [Page 30]
