RFC3041: Privacy Extensions for Stateless Address Autoconfiguration in IPv6
Download in PDF format Download in text format
Obsoleted By:
RFC4941
Network Working Group T. Narten
Request for Comments: 3041 IBM
Category: Standards Track R. Draves
Microsoft Research
January 2001
Privacy Extensions for Stateless Address Autoconfiguration in IPv6
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
Nodes use IPv6 stateless address autoconfiguration to generate
addresses without the necessity of a Dynamic Host Configuration
Protocol (DHCP) server. Addresses are formed by combining network
prefixes with an interface identifier. On interfaces that contain
embedded IEEE Identifiers, the interface identifier is typically
derived from it. On other interface types, the interface identifier
is generated through other means, for example, via random number
generation. This document describes an extension to IPv6 stateless
address autoconfiguration for interfaces whose interface identifier
is derived from an IEEE identifier. Use of the extension causes
nodes to generate global-scope addresses from interface identifiers
that change over time, even in cases where the interface contains an
embedded IEEE identifier. Changing the interface identifier (and the
global-scope addresses generated from it) over time makes it more
difficult for eavesdroppers and other information collectors to
identify when different addresses used in different transactions
actually correspond to the same node.
Narten & Draves Standards Track [Page 1]
RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001
Table of Contents
1. Introduction............................................. 2
2. Background............................................... 3
2.1. Extended Use of the Same Identifier................. 3
2.2. Address Usage in IPv4 Today......................... 4
2.3. The Concern With IPv6 Addresses..................... 5
2.4. Possible Approaches................................. 6
3. Protocol Description..................................... 7
3.1. Assumptions......................................... 8
3.2. Generation Of Randomized Interface Identifiers...... 9
3.3. Generating Temporary Addresses...................... 10
3.4. Expiration of Temporary Addresses................... 11
3.5. Regeneration of Randomized Interface Identifiers.... 12
4. Implications of Changing Interface Identifiers........... 13
5. Defined Constants........................................ 14
6. Future Work.............................................. 14
7. Security Considerations.................................. 15
8. Acknowledgments.......................................... 15
9. References............................................... 15
10. Authors' Addresses....................................... 16
11. Full Copyright Statement................................. 17
1. Introduction
Stateless address autoconfiguration [ADDRCONF] defines how an IPv6
node generates addresses without the need for a DHCP server. Some
types of network interfaces come with an embedded IEEE Identifier
(i.e., a link-layer MAC address), and in those cases stateless
address autoconfiguration uses the IEEE identifier to generate a 64-
bit interface identifier [ADDRARCH]. By design, the interface
identifier is likely to be globally unique when generated in this
fashion. The interface identifier is in turn appended to a prefix to
form a 128-bit IPv6 address.
All nodes combine interface identifiers (whether derived from an IEEE
identifier or generated through some other technique) with the
reserved link-local prefix to generate link-local addresses for their
attached interfaces. Additional addresses, including site-local and
global-scope addresses, are then created by combining prefixes
advertised in Router Advertisements via Neighbor Discovery
[DISCOVERY] with the interface identifier.
Not all nodes and interfaces contain IEEE identifiers. In such
cases, an interface identifier is generated through some other means
(e.g., at random), and the resultant interface identifier is not
globally unique and may also change over time. The focus of this
document is on addresses derived from IEEE identifiers, as the
Narten & Draves Standards Track [Page 2]
RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001
concern being addressed exists only in those cases where the
interface identifier is globally unique and non-changing. The rest
of this document assumes that IEEE identifiers are being used, but
the techniques described may also apply to interfaces with other
types of globally unique and/or persistent identifiers.
This document discusses concerns associated with the embedding of
non-changing interface identifiers within IPv6 addresses and
describes extensions to stateless address autoconfiguration that can
help mitigate those concerns for individual users and in environments
where such concerns are significant. Section 2 provides background
information on the issue. Section 3 describes a procedure for
generating alternate interface identifiers and global-scope
addresses. Section 4 discusses implications of changing interface
identifiers.
2. Background
This section discusses the problem in more detail, provides context
for evaluating the significance of the concerns in specific
environments and makes comparisons with existing practices.
2.1. Extended Use of the Same Identifier
The use of a non-changing interface identifier to form addresses is a
specific instance of the more general case where a constant
identifier is reused over an extended period of time and in multiple
independent activities. Anytime the same identifier is used in
multiple contexts, it becomes possible for that identifier to be used
to correlate seemingly unrelated activity. For example, a network
sniffer placed strategically on a link across which all traffic
to/from a particular host crosses could keep track of which
destinations a node communicated with and at what times. Such
information can in some cases be used to infer things, such as what
hours an employee was active, when someone is at home, etc.
One of the requirements for correlating seemingly unrelated
activities is the use (and reuse) of an identifier that is
recognizable over time within different contexts. IP addresses
provide one obvious example, but there are more. Many nodes also
have DNS names associated with their addresses, in which case the DNS
name serves as a similar identifier. Although the DNS name
associated with an address is more work to obtain (it may require a
DNS query) the information is often readily available. In such
cases, changing the address on a machine over time would do little to
address the concerns raised in this document, unless the DNS name is
changed as well (see Section 4).
Narten & Draves Standards Track [Page 3]
RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001
Web browsers and servers typically exchange "cookies" with each other
[COOKIES]. Cookies allow web servers to correlate a current activity
with a previous activity. One common usage is to send back targeted
advertising to a user by using the cookie supplied by the browser to
identify what earlier queries had been made (e.g., for what type of
information). Based on the earlier queries, advertisements can be
targeted to match the (assumed) interests of the end-user.
The use of a constant identifier within an address is of special
concern because addresses are a fundamental requirement of
communication and cannot easily be hidden from eavesdroppers and
other parties. Even when higher layers encrypt their payloads,
addresses in packet headers appear in the clear. Consequently, if a
mobile host (e.g., laptop) accessed the network from several
different locations, an eavesdropper might be able to track the
movement of that mobile host from place to place, even if the upper
layer payloads were encrypted [SERIALNUM].
2.2. Address Usage in IPv4 Today
Addresses used in today's Internet are often non-changing in practice
for extended periods of time, especially in non-home environments
(e.g., corporations, campuses, etc.). In such sites, addresses are
assigned statically and typically change infrequently. Over the last
few years, sites have begun moving away from static allocation to
dynamic allocation via DHCP [DHCP]. In theory, the address a client
gets via DHCP can change over time, but in practice servers often
return the same address to the same client (unless addresses are in
such short supply that they are reused immediately by a different
node when they become free). Thus, even within sites using DHCP,
clients frequently end up using the same address for weeks to months
at a time.
For home users accessing the Internet over dialup lines, the
situation is generally different. Such users do not have permanent
connections and are often assigned temporary addresses each time they
connect to their ISP (e.g., AOL). Consequently, the addresses they
use change frequently over time and are shared among a number of
different users. Thus, an address does not reliably identify a
particular device over time spans of more than a few minutes.
A more interesting case concerns always-on connections (e.g., cable
modems, ISDN, DSL, etc.) that result in a home site using the same
address for extended periods of time. This is a scenario that is
just starting to become common in IPv4 and promises to become more of
a concern as always-on internet connectivity becomes widely
available. Although it might appear that changing an address
regularly in such environments would be desirable to lessen privacy
Narten & Draves Standards Track [Page 4]
RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001
concerns, it should be noted that the network prefix portion of an
address also serves as a constant identifier. All nodes at (say) a
home, would have the same network prefix, which identifies the
topological location of those nodes. This has implications for
privacy, though not at the same granularity as the concern that this
document addresses. Specifically, all nodes within a home would be
grouped together for the purposes of collecting information. This
issue is difficult to address, because the routing prefix part of an
address contains topology information and cannot contain arbitrary
values.
Finally, it should be noted that nodes that need a (non-changing) DNS
name generally have static addresses assigned to them to simplify the
configuration of DNS servers. Although Dynamic DNS [DDNS] can be
used to update the DNS dynamically, it is not yet widely deployed.
In addition, changing an address but keeping the same DNS name does
not really address the underlying concern, since the DNS name becomes
a non-changing identifier. Servers generally require a DNS name (so
clients can connect to them), and clients often do as well (e.g.,
some servers refuse to speak to a client whose address cannot be
mapped into a DNS name that also maps back into the same address).
Section 4 describes one approach to this issue.
2.3. The Concern With IPv6 Addresses
The division of IPv6 addresses into distinct topology and interface
identifier portions raises an issue new to IPv6 in that a fixed
portion of an IPv6 address (i.e., the interface identifier) can
contain an identifier that remains constant even when the topology
portion of an address changes (e.g., as the result of connecting to a
different part of the Internet). In IPv4, when an address changes,
the entire address (including the local part of the address) usually
changes. It is this new issue that this document addresses.
If addresses are generated from an interface identifier, a home
user's address could contain an interface identifier that remains the
same from one dialup session to the next, even if the rest of the
address changes. The way PPP is used today, however, PPP servers
typically unilaterally inform the client what address they are to use
(i.e., the client doesn't generate one on its own). This practice,
if continued in IPv6, would avoid the concerns that are the focus of
this document.
A more troubling case concerns mobile devices (e.g., laptops, PDAs,
etc.) that move topologically within the Internet. Whenever they
move (in the absence of technology such as mobile IP [MOBILEIP]),
they form new addresses for their current topological point of
attachment. This is typified today by the "road warrior" who has
Narten & Draves Standards Track [Page 5]
RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001
Internet connectivity both at home and at the office. While the
node's address changes as it moves, however, the interface identifier
contained within the address remains the same (when derived from an
IEEE Identifier). In such cases, the interface identifier can be
used to track the movement and usage of a particular machine
[SERIALNUM]. For example, a server that logs usage information
together with a source addresses, is also recording the interface
identifier since it is embedded within an address. Consequently, any
data-mining technique that correlates activity based on addresses
could easily be extended to do the same using the interface
identifier. This is of particular concern with the expected
proliferation of next-generation network-connected devices (e.g.,
PDAs, cell phones, etc.) in which large numbers of devices are in
practice associated with individual users (i.e., not shared). Thus,
the interface identifier embedded within an address could be used to
track activities of an individual, even as they move topologically
within the internet.
In summary, IPv6 addresses on a given interface generated via
Stateless Autoconfiguration contain the same interface identifier,
regardless of where within the Internet the device connects. This
facilitates the tracking of individual devices (and thus potentially
users). The purpose of this document is to define mechanisms that
eliminate this issue, in those situations where it is a concern.
2.4. Possible Approaches
One way to avoid some of the problems discussed above is to use DHCP
for obtaining addresses. With DHCP, the DHCP server could arrange to
hand out addresses that change over time.
Another approach, compatible with the stateless address
autoconfiguration architecture, would be to change the interface id
portion of an address over time and generate new addresses from the
interface identifier for some address scopes. Changing the interface
identifier can make it more difficult to look at the IP addresses in
independent transactions and identify which ones actually correspond
to the same node, both in the case where the routing prefix portion
of an address changes and when it does not.
Many machines function as both clients and servers. In such cases,
the machine would need a DNS name for its use as a server. Whether
the address stays fixed or changes has little privacy implication
since the DNS name remains constant and serves as a constant
identifier. When acting as a client (e.g., initiating
communication), however, such a machine may want to vary the
addresses it uses. In such environments, one may need multiple
addresses: a "public" (i.e., non-secret) server address, registered
Narten & Draves Standards Track [Page 6]
RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001
in the DNS, that is used to accept incoming connection requests from
other machines, and a "temporary" address used to shield the identity
of the client when it initiates communication. These two cases are
roughly analogous to telephone numbers and caller ID, where a user
may list their telephone number in the public phone book, but disable
the display of its number via caller ID when initiating calls.
To make it difficult to make educated guesses as to whether two
different interface identifiers belong to the same node, the
algorithm for generating alternate identifiers must include input
that has an unpredictable component from the perspective of the
outside entities that are collecting information. Picking
identifiers from a pseudo-random sequence suffices, so long as the
specific sequence cannot be determined by an outsider examining
information that is readily available or easily determinable (e.g.,
by examining packet contents). This document proposes the generation
of a pseudo-random sequence of interface identifiers via an MD5 hash.
Periodically, the next interface identifier in the sequence is
generated, a new set of temporary addresses is created, and the
previous temporary addresses are deprecated to discourage their
further use. The precise pseudo-random sequence depends on both a
random component and the globally unique interface identifier (when
available), to increase the likelihood that different nodes generate
different sequences.
3. Protocol Description
The goal of this section is to define procedures that:
1) Do not result in any changes to the basic behavior of addresses
generated via stateless address autoconfiguration [ADDRCONF].
2) Create additional global-scope addresses based on a random
interface identifier for use with global scope addresses. Such
addresses would be used to initiate outgoing sessions. These
"random" or temporary addresses would be used for a short period
of time (hours to days) and would then be deprecated. Deprecated
address can continue to be used for already established
connections, but are not used to initiate new connections. New
temporary addresses are generated periodically to replace
temporary addresses that expire, with the exact time between
address generation a matter of local policy.
3) Produce a sequence of temporary global-scope addresses from a
sequence of interface identifiers that appear to be random in the
sense that it is difficult for an outside observer to predict a
Narten & Draves Standards Track [Page 7]
RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001
future address (or identifier) based on a current one and it is
difficult to determine previous addresses (or identifiers) knowing
only the present one.
4) Generate a set of addresses from the same (randomized) interface
identifier, one address for each prefix for which a global address
has been generated via stateless address autoconfiguration. Using
the same interface identifier to generate a set of temporary
addresses reduces the number of IP multicast groups a host must
join. Nodes join the solicited-node multicast address for each
unicast address they support, and solicited-node addresses are
dependent only on the low-order bits of the corresponding address.
This decision was made to address the concern that a node that
joins a large number of multicast groups may be required to put
its interface into promiscuous mode, resulting in possible reduced
performance.
3.1. Assumptions
The following algorithm assumes that each interface maintains an
associated randomized interface identifier. When temporary addresses
are generated, the current value of the associated randomized
interface identifier is used. The actual value of the identifier
changes over time as described below, but the same identifier can be
used to generate more than one temporary address.
The algorithm also assumes that for a given temporary address, an
implementation can determine the corresponding public address from
which it was generated. When a temporary address is deprecated, a
new temporary address is generated. The specific valid and preferred
lifetimes for the new address are dependent on the corresponding
lifetime values in the public address.
Finally, this document assumes that when a node initiates outgoing
communication, temporary addresses can be given preference over
public addresses. This can mean that all connections initiated by
the node use temporary addresses by default, or that applications
individually indicate whether they prefer to use temporary or public
addresses. Giving preference to temporary address is consistent with
on-going work that addresses the topic of source-address selection in
the more general case [ADDR_SELECT]. An implementation may make it a
policy that it does not select a public address in the event that no
temporary address is available (e.g., if generation of a useable
temporary address fails).
Narten & Draves Standards Track [Page 8]
RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001
3.2. Generation Of Randomized Interface Identifiers.
We describe two approaches for the maintenance of the randomized
interface identifier. The first assumes the presence of stable
storage that can be used to record state history for use as input
into the next iteration of the algorithm across system restarts. A
second approach addresses the case where stable storage is
unavailable and there is a need to generate randomized interface
identifiers without previous state.
3.2.1. When Stable Storage Is Present
The following algorithm assumes the presence of a 64-bit "history
value" that is used as input in generating a randomized interface
identifier. The very first time the system boots (i.e., out-of-the-
box), a random value should be generated using techniques that help
ensure the initial value is hard to guess [RANDOM]. Whenever a new
interface identifier is generated, a value generated by the
computation is saved in the history value for the next iteration of
the algorithm.
A randomized interface identifier is created as follows:
1) Take the history value from the previous iteration of this
algorithm (or a random value if there is no previous value) and
append to it the interface identifier generated as described in
[ADDRARCH].
2) Compute the MD5 message digest [MD5] over the quantity created in
the previous step.
3) Take the left-most 64-bits of the MD5 digest and set bit 6 (the
left-most bit is numbered 0) to zero. This creates an interface
identifier with the universal/local bit indicating local
significance only. Save the generated identifier as the
associated randomized interface identifier.
4) Take the rightmost 64-bits of the MD5 digest computed in step 2)
and save them in stable storage as the history value to be used in
the next iteration of the algorithm.
MD5 was chosen for convenience, and because its particular properties
were adequate to produce the desired level of randomization. IPv6
nodes are already required to implement MD5 as part of IPsec [IPSEC],
thus the code will already be present on IPv6 machines.
In theory, generating successive randomized interface identifiers
using a history scheme as above has no advantages over generating
them at random. In practice, however, generating truly random
numbers can be tricky. Use of a history value is intended to avoid
the particular scenario where two nodes generate the same randomized
Narten & Draves Standards Track [Page 9]
RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001
interface identifier, both detect the situation via DAD, but then
proceed to generate identical randomized interface identifiers via
the same (flawed) random number generation algorithm. The above
algorithm avoids this problem by having the interface identifier
(which will often be globally unique) used in the calculation that
generates subsequent randomized interface identifiers. Thus, if two
nodes happen to generate the same randomized interface identifier,
they should generate different ones on the followup attempt.
3.2.2. In The Absence of Stable Storage
In the absence of stable storage, no history value will be available
across system restarts to generate a pseudo-random sequence of
interface identifiers. Consequently, the initial history value used
above will need to be generated at random. A number of techniques
might be appropriate. Consult [RANDOM] for suggestions on good
sources for obtaining random numbers. Note that even though machines
may not have stable storage for storing a history value, they will in
many cases have configuration information that differs from one
machine to another (e.g., user identity, security keys, serial
numbers, etc.). One approach to generating a random initial history
value in such cases is to use the configuration information to
generate some data bits (which may remain constant for the life of
the machine, but will vary from one machine to another), append some
random data and compute the MD5 digest as before.
3.3. Generating Temporary Addresses
[ADDRCONF] describes the steps for generating a link-local address
when an interface becomes enabled as well as the steps for generating
addresses for other scopes. This document extends [ADDRCONF] as
follows. When processing a Router Advertisement with a Prefix
Information option carrying a global-scope prefix for the purposes of
address autoconfiguration (i.e., the A bit is set), perform the
following steps:
1) Process the Prefix Information Option as defined in [ADDRCONF],
either creating a public address or adjusting the lifetimes of
existing addresses, both public and temporary. When adjusting the
lifetimes of an existing temporary address, only lower the
lifetimes. Implementations must not increase the lifetimes of an
existing temporary address when processing a Prefix Information
Option.
2) When a new public address is created as described in [ADDRCONF]
(because the prefix advertised does not match the prefix of any
address already assigned to the interface, and the Valid Lifetime
in the option is not zero), also create a new temporary address.
Narten & Draves Standards Track [Page 10]
RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001
3) When creating a temporary address, the lifetime values are derived
from the corresponding public address as follows:
- Its Valid Lifetime is the lower of the Valid Lifetime of the
public address or TEMP_VALID_LIFETIME.
- Its Preferred Lifetime is the lower of the Preferred Lifetime
of the public address or TEMP_PREFERRED_LIFETIME -
DESYNC_FACTOR.
A temporary address is created only if this calculated Preferred
Lifetime is greater than REGEN_ADVANCE time units. In particular,
an implementation must not create a temporary address with a zero
Preferred Lifetime.
4) New temporary addresses are created by appending the interface's
current randomized interface identifier to the prefix that was
used to generate the corresponding public address. If by chance
the new temporary address is the same as an address already
assigned to the interface, generate a new randomized interface
identifier and repeat this step.
5) Perform duplicate address detection (DAD) on the generated
temporary address. If DAD indicates the address is already in
use, generate a new randomized interface identifier as described
in Section 3.2 above, and repeat the previous steps as appropriate
up to 5 times. If after 5 consecutive attempts no non-unique
address was generated, log a system error and give up attempting
to generate temporary addresses for that interface.
Note: because multiple temporary addresses are generated from the
same associated randomized interface identifier, there is little
benefit in running DAD on every temporary address. This document
recommends that DAD be run on the first address generated from a
given randomized identifier, but that DAD be skipped on all
subsequent addresses generated from the same randomized interface
identifier.
3.4. Expiration of Temporary Addresses
When a temporary address becomes deprecated, a new one should be
generated. This is done by repeating the actions described in
Section 3.3, starting at step 3). Note that, except for the
transient period when a temporary address is being regenerated, in
normal operation at most one temporary address corresponding to a
public address should be in a non-deprecated state at any given time.
Note that if a temporary address becomes deprecated as result of
processing a Prefix Information Option with a zero Preferred
Lifetime, then a new temporary address must not be generated. The
Prefix Information Option will also deprecate the corresponding
public address.
Narten & Draves Standards Track [Page 11]
RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001
To insure that a preferred temporary address is always available, a
new temporary address should be regenerated slightly before its
predecessor is deprecated. This is to allow sufficient time to avoid
race conditions in the case where generating a new temporary address
is not instantaneous, such as when duplicate address detection must
be run. It is recommended that an implementation start the address
regeneration process REGEN_ADVANCE time units before a temporary
address would actually be deprecated.
As an optional optimization, an implementation may wish to remove a
deprecated temporary address that is not in use by applications or
upper-layers. For TCP connections, such information is available in
control blocks. For UDP-based applications, it may be the case that
only the applications have knowledge about what addresses are
actually in use. Consequently, one may need to use heuristics in
deciding when an address is no longer in use (e.g., the default
TEMP_VALID_LIFETIME suggested above).
3.5. Regeneration of Randomized Interface Identifiers
The frequency at which temporary addresses should change depends on
how a device is being used (e.g., how frequently it initiates new
communication) and the concerns of the end user. The most egregious
privacy concerns appear to involve addresses used for long periods of
time (weeks to months to years). The more frequently an address
changes, the less feasible collecting or coordinating information
keyed on interface identifiers becomes. Moreover, the cost of
collecting information and attempting to correlate it based on
interface identifiers will only be justified if enough addresses
contain non-changing identifiers to make it worthwhile. Thus, having
large numbers of clients change their address on a daily or weekly
basis is likely to be sufficient to alleviate most privacy concerns.
There are also client costs associated with having a large number of
addresses associated with a node (e.g., in doing address lookups, the
need to join many multicast groups, etc.). Thus, changing addresses
frequently (e.g., every few minutes) may have performance
implications.
This document recommends that implementations generate new temporary
addresses on a periodic basis. This can be achieved automatically by
generating a new randomized interface identifier at least once every
(TEMP_PREFERRED_LIFETIME - REGEN_ADVANCE - DESYNC_FACTOR) time units.
As described above, generating a new temporary address REGEN_ADVANCE
time units before a temporary address becomes deprecated produces
addresses with a preferred lifetime no larger than
TEMP_PREFERRED_LIFETIME. The value DESYNC_FACTOR is a random value
(different for each client) that ensures that clients don't
Narten & Draves Standards Track [Page 12]
RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001
synchronize with each other and generate new addresses at exactly the
same time. When the preferred lifetime expires, a new temporary
address is generated using the new randomized interface identifier.
Because the precise frequency at which it is appropriate to generate
new addresses varies from one environment to another, implementations
should provide end users with the ability to change the frequency at
which addresses are regenerated. The default value is given in
TEMP_PREFERRED_LIFETIME and is one day. In addition, the exact time
at which to invalidate a temporary address depends on how
applications are used by end users. Thus the default value given of
one week (TEMP_VALID_LIFETIME) may not be appropriate in all
environments. Implementations should provide end users with the
ability to override both of these default values.
Finally, when an interface connects to a new link, a new randomized
interface identifier should be generated immediately together with a
new set of temporary addresses. If a device moves from one ethernet
to another, generating a new set of temporary addresses from a
different randomized interface identifier ensures that the device
uses different randomized interface identifiers for the temporary
addresses associated with the two links, making it more difficult to
correlate addresses from the two different links as being from the
same node.
4. Implications of Changing Interface Identifiers
The IPv6 addressing architecture goes to some lengths to ensure that
interface identifiers are likely to be globally unique where easy to
do so. During the IPng discussions of the GSE proposal [GSE], it was
felt that keeping interface identifiers globally unique in practice
might prove useful to future transport protocols. Usage of the
algorithms in this document may complicate providing such a future
flexibility.
The desires of protecting individual privacy vs. the desire to
effectively maintain and debug a network can conflict with each
other. Having clients use addresses that change over time will make
it more difficult to track down and isolate operational problems.
For example, when looking at packet traces, it could become more
difficult to determine whether one is seeing behavior caused by a
single errant machine, or by a number of them.
Some servers refuse to grant access to clients for which no DNS name
exists. That is, they perform a DNS PTR query to determine the DNS
name, and may then also perform an A query on the returned name to
verify that the returned DNS name maps back into the address being
used. Consequently, clients not properly registered in the DNS may
Narten & Draves Standards Track [Page 13]
RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001
be unable to access some services. As noted earlier, however, a
node's DNS name (if non-changing) serves as a constant identifier.
The wide deployment of the extension described in this document could
challenge the practice of inverse-DNS-based "authentication," which
has little validity, though it is widely implemented. In order to
meet server challenges, nodes could register temporary addresses in
the DNS using random names (for example a string version of the
random address itself).
Use of the extensions defined in this document may complicate
debugging and other operational troubleshooting activities.
Consequently, it may be site policy that temporary addresses should
not be used. Implementations may provide a method for a trusted
administrator to override the use of temporary addresses.
5. Defined Constants
Constants defined in this document include:
TEMP_VALID_LIFETIME -- Default value: 1 week. Users should be able
to override the default value.
TEMP_PREFERRED_LIFETIME -- Default value: 1 day. Users should be
able to override the default value.
REGEN_ADVANCE -- 5 seconds
MAX_DESYNC_FACTOR -- 10 minutes. Upper bound on DESYNC_FACTOR.
DESYNC_FACTOR -- A random value within the range 0 - MAX_DESYNC_FACTOR.
It is computed once at system start (rather than each time
it is used) and must never be greater than
(TEMP_VALID_LIFETIME - REGEN_ADVANCE).
6. Future Work
An implementation might want to keep track of which addresses are
being used by upper layers so as to be able to remove a deprecated
temporary address from internal data structures once no upper layer
protocols are using it (but not before). This is in contrast to
current approaches where addresses are removed from an interface when
they become invalid [ADDRCONF], independent of whether or not upper
layer protocols are still using them. For TCP connections, such
information is available in control blocks. For UDP-based
applications, it may be the case that only the applications have
knowledge about what addresses are actually in use. Consequently, an
implementation generally will need to use heuristics in deciding when
an address is no longer in use (e.g., as is suggested in Section
3.4).
Narten & Draves Standards Track [Page 14]
RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001
The determination as to whether to use public vs. temporary addresses
can in some cases only be made by an application. For example, some
applications may always want to use temporary addresses, while others
may want to use them only in some circumstances or not at all.
Suitable API extensions will likely need to be developed to enable
individual applications to indicate with sufficient granularity their
needs with regards to the use of temporary addresses.
7. Security Considerations
The motivation for this document stems from privacy concerns for
individuals. This document does not appear to add any security
issues beyond those already associated with stateless address
autoconfiguration [ADDRCONF].
8. Acknowledgments
The authors would like to acknowledge the contributions of the IPNGWG
working group and, in particular, Matt Crawford, Steve Deering and
Allison Mankin for their detailed comments.
9. References
[ADDRARCH] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[ADDRCONF] Thomson, S. and T. Narten, "IPv6 Address
Autoconfiguration", RFC 2462, December 1998.
[ADDR_SELECT] Draves, R. "Default Address Selection for IPv6", Work
in Progress.
[COOKIES] Kristol, D. and L. Montulli, "HTTP State Management
Mechanism", RFC 2965, October 2000.
[DHCP] Droms, R., "Dynamic Host Configuration Protocol", RFC
2131, March 1997.
[DDNS] Vixie, R., Thomson, S., Rekhter, Y. and J. Bound,
"Dynamic Updates in the Domain Name System (DNS
UPDATE)", RFC 2136, April 1997.
[DISCOVERY] Narten, T., Nordmark, E. and W. Simpson, "Neighbor
Discovery for IP Version 6 (IPv6)", RFC 2461, December
1998.
Narten & Draves Standards Track [Page 15]
RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001
[GSE] Crawford, et al., "Separating Identifiers and Locators
in Addresses: An Analysis of the GSE Proposal for
IPv6", Work in Progress.
[IPSEC] Kent, S., Atkinson, R., "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[MD5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC
1321, April 1992.
[MOBILEIP] Perkins, C., "IP Mobility Support", RFC 2002, October
1996.
[RANDOM] Eastlake 3rd, D., Crocker S. and J. Schiller,
"Randomness Recommendations for Security", RFC 1750,
December 1994.
[SERIALNUM] Moore, K., "Privacy Considerations for the Use of
Hardware Serial Numbers in End-to-End Network
Protocols", Work in Progress.
10. Authors' Addresses
Thomas Narten
IBM Corporation
P.O. Box 12195
Research Triangle Park, NC 27709-2195
USA
Phone: +1 919 254 7798
EMail: narten@raleigh.ibm.com
Richard Draves
Microsoft Research
One Microsoft Way
Redmond, WA 98052
Phone: +1 425 936 2268
EMail: richdr@microsoft.com
Narten & Draves Standards Track [Page 16]
RFC 3041 Extensions to IPv6 Address Autoconfiguration January 2001
11. Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Narten & Draves Standards Track [Page 17]