Internet DRAFT - draft-white-auto-subnet
draft-white-auto-subnet
ipv6 A. White
Internet-Draft A. Williams
Expires: May 2, 2003 Motorola
November 2002
Automatic Globally Unique Site Local Subnet Allocation
draft-white-auto-subnet-00
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This memo specifies an automatically generated globally unique site
local address format based on IEEE EUI-48 identifiers.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Address Format . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1 Existing Site Local Space . . . . . . . . . . . . . . . . . . 4
2.2 New Allocation . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1 Multihomed links . . . . . . . . . . . . . . . . . . . . . . . 5
3.2 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 8
A. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
Full Copyright Statement . . . . . . . . . . . . . . . . . . . 10
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1. Introduction
Site local addresses are specified by the IPv6 addressing
architecture [1]. As specified site local addresses are not globally
unique and valid only within a "site".
If sites freely allocate from the fec0::/48 space, problems can occur
when addresses leak outside a sit or when sites merge.
Leakage: Even with strong filtering, it is possible for site local
addresses to leak outside the site. Because site local addresses
are ambiguous (non-unique), multiple devices may share the same
address. When a device outside the site attempts to use such an
address, it is desirable that communication reliably fails instead
of attempting communciation with an incorrect device.
Merging: Since site locals are only required to be unique within a
site, problems can occur when merging sites. If the sets of site
local addresses used between the two sites are not disjoint,
renumbering may be required to merge the sites.
Manual allocation of site local prefixes by system administrators
results in clustering. Many administrators will allocate subnets
fec0:0:0:1::/64, fec0:0:0:2::/64, etc, increasing the likelihood
of collision during a merge.
This memo specifies a mechanism for generating stable globally unique
site local subnet addresses (64 bits) without requiring any
registration mechanism, based on router interface MAC address. These
addresses are not aggregable, but this is not expected to be a
problem within a site.
1.1 Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
2. Address Format
A router generates a unique 64 bit subnet prefix based on a 48 bit
device identifier, such as an IEEE EUI-48 identifier (e.g. ethernet
MAC address). For each such identifier the router may allocate a
number of subnets.
Routers with multiple identifiers (e.g. multiple ethernet
interfaces) may choose to use a single identifier to address all
attached links (and subnets) or to use each interface's identifier to
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address that link, or a combination thereof. Routers using per-
interface allocation may set the subnet bits 0 or to an arbitrary
value.
Two models are available, depending on the prefix used.
Inside existing site local space.
New allocation
2.1 Existing Site Local Space
The first option is to reserve some of the existing fec0::/10 site
local address space for globally unique addresses. The proposed
prefix is fef0:/12. The 64 bit subnet prefix consists of the 12 bit
prefix "1111 1110 1111" (fef0::/12), the 48 bit device identifier and
4 bits of subnet identifier.
---------------------------------------------------------------------
| 12 bits | 48 bits | 4 bits | 64 bits |
+---------+------------------+----------+----------------------+
| fef | router device ID | sub ID | machine interface ID |
+---------+------------------+----------+----------------------+
| <- subnet prefix -> | |
Figure 1: Address Format: fef0::/12
---------------------------------------------------------------------
2.2 New Allocation
The second option is to allocate a new non-aggregable non-globally-
routable address space. The proposed space is fe00::/10. The 64 bit
subnet prefix consists of the 10 bit prefix "1111 1110 00" (fe00::/
10), the 48 bit device identifier and 6 bits of subnet identifier.
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---------------------------------------------------------------------
| 10 bits | 48 bits | 6 bits | 64 bits |
+---------+------------------+----------+----------------------+
| fe0 | router device ID | sub ID | machine interface ID |
+---------+------------------+----------+----------------------+
| <- subnet prefix -> | |
Figure 2: Address Format: fe00::/10
---------------------------------------------------------------------
3. Discussion
Subnet prefixes allocated by this mechanism are globally unique to
the extent that IEEE EUI-48 device identifiers are globally unique.
They persist on the link as long as the allocating router is not
changed. They can be configured by the router upon startup without
any external communication.
Subnet prefixes allocated by this mechanism are generally non-
aggregable. This is not expected to be a problem for addressing
realms small enough to be considered a 'site'.
This proposal provides for globally unique site local addresses
without requiring any additional registration. Because addresses are
globally unique and generated in a predictable manner, merging does
not require renumbering. Out of site hosts attempting communication
are more likely to fail, since the addresses are unambiguous and will
thus be filtered rather than finding an incorrect device.
This mechanism also specifies subnet based allocation, as opposed to
site based allocation for global or existing site local address.
This allows subnet allocation to be zero-configuration, as the subnet
address is allocated by the router rather than additional
administrative policy.
3.1 Multihomed links
In the case where a link contains two (or more) routers, two options
are available. Either the link may be multi-homed, with each router
providing a generated site local prefix, or one router may be
instructed not to use its automatic prefix and be configured with the
prefix from the master router on the link. Multihoming is
advantageous in a zero-configuration environment, while the
administrator may prefer configuration.
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3.2 Limitations
Tying the subnet identifier router EUIs has two potential
limitations. Routes are unlikely to be aggregable, and changing
routers may cause renumbering.
It is envisaged that site scoped networks will be small enough
(orders of magnitude less than the global internet) that aggregation
is unnecessary for effective routing table operation. Site scoped
networks with thousands of links / subnets may cause problems.
Router change may be dealt with in several ways. Fully zero-
configuration systems should be able to reconfigure themselves to
compensate for readdressing. In non-zero-configuration systems, the
new router could be configured with the old routers addresses. This
assumes that the old router is not being re-used (and thus the EUI-48
remains unique). Alternatively, manually configured subnet prefixes
could be allocated to subnets attached to critical servers.
4. Example
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---------------------------------------------------------------------
+----------+
| Router |
| A |
+--+----+--+
00:80:ab:cd:ef:02| |00:80:ab:cd:ef:01
| |
fef0:080a:bcde:f020::/64| |fef0:080a:bcde:f010::/64
+---------------------------+ +-----+
|fef0:0505:566a:abb0::/64 |
| |
| | fef0:0409:8675
|00:50:55:66:aa:bb +-----+----+ :4231::/64
+-----+----+ | Router +---------------
| Router | 00:40:98:67:54:23| C |
| B | +--+----+--+ fef0:0409:8675
+-----+----+ | | :4233::/64
|00:50:55:66:aa:bb | +------------------
| |
|fef0:0505:566a:abb0::/64 +-----------------------
| fef0:0409:8675:4234::/64
Figure 3: Sample Network
---------------------------------------------------------------------
The above diagram shows a sample network. Routers A and B are using
the ethernet MAC address (EUI-48) of each interface to generate
subnet addresses. Router C is using a single EUI-48 and using the
sub ID bits to generate subnet addresses.
Note that the link between A and B is multi-homed based on both A and
B's interfaces, while C's interface that links to A has been
configured not to serve an address. If this had not been done, C
would have allocated the prefix fef0:0409:8675:4232::/64 to that
link.
In the example above, the proposed fef0::/12 prefix has been used as
the conversion from EUI-48 to prefix is clearer.
5. IANA Considerations
A top level prefix is required for these addresses. There are two
options.
If this is to be nested within the existing site-local architecture,
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fef0::/12 is suitable. All 54 bits from 13 to 64 are used to define
the subnet identifer. This conflicts with [1] (16 bit SLA) but is in
accordance with [3].
Alternatively, a new 10 bit prefix could be allocated from within
fe00::/9 (e.g. fe00::/10).
6. Security Considerations
This proposal reveals the router's device identifier, thus
introducing similar concerns to those described in privacy addressing
[2]. While the link may be identified from within the site, site
based filtering should restrict these identifiers from leaking
outside the site.
Mechanisms for preventing site local addresses from leaking to the
public internet are beyond the scope of this memo.
Unless a router is intended to be installed as part of a site local
network, automatic allocation of site local subnet addresses SHOULD
be disabled by default.
References
[1] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[2] Narten, T. and R. Draves, "Privacy Extensions for Stateless
Address Autoconfiguration in IPv6", RFC 3041, January 2001.
[3] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", ID draft-ietf-ipngwg-addr-arch-v3-11, October
2002.
Authors' Addresses
Andrew White
Motorola Australian Research Centre
Locked Bag 5028
Botany, NSW 1455
AU
Phone: +61 2 9666 0500
EMail: Andrew.E.White@motorola.com
URI: http://www.motorola.com.au/marc/
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Aidan Williams
Motorola Australian Research Centre
Locked Bag 5028
Botany, NSW 1455
AU
Phone: +61 2 9666 0500
EMail: Aidan.Williams@motorola.com
URI: http://www.motorola.com.au/marc/
Appendix A. Acknowledgements
TBD.
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Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
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